Technical Basis Document No. 3: 
Water Seeping into Drifts 
Revision 2 
Prepared for: 
U.S. Department of Energy 
Office of Civilian Radioactive Waste Management 
Office of Repository Development 
P.O. Box 364629 
North Las Vegas, Nevada 89036-8629 
Prepared by: 
Bechtel SAIC Company, LLC 
1180 Town Center Drive 
Las Vegas, Nevada 89144 
Under Contract Number 
DE-AC28-01RW12101 
QA: NA 
October 2003
Revision 2 
October 2003 No. 3: Water Seeping into Drifts
Revision 2 

1. INTRODUCTION 
1.1 OVERVIEW 
This technical basis document provides a summary of the conceptual understanding of water 
seepage into waste emplacement drifts, which is a process affecting the prediction of the 
postclosure performance of the high-level nuclear waste repository at Yucca Mountain. This 
document is one in a series that is being prepared for each component of the Yucca Mountain 
repository system relevant to predicting its likely postclosure performance. The relationship of 
drift seepage to the other components is illustrated in Figure 1-1. Drift seepage is affected by the 
processes that will be described in technical basis documents on climate and infiltration, 
unsaturated zone flow, mechanical degradation and seismic effects, low probability seismic 
events, and volcanic events. Seepage is affected by and affects the in-drift environment 
discussed in the technical basis document on in-drift chemical environment. The amount of 
seepage water entering the waste emplacement drifts and potentially contacting waste canisters 
impacts downstream repository system components, including corrosion, waste dissolution, and 
transport of radionuclides and colloids through the engineered barrier system, the unsaturated 
and saturated zones, and the biosphere. 
Figure 1-1. Components of the Postclosure Technical Basis for the License Application 
October 2003 1-1 No. 3: Water Seeping into Drifts Revision 2 
The information presented in this document, along with the associated references, forms an 
outline of the seepage-related studies supporting the ongoing development of the postclosure 
safety analysis that will comprise the license application (LA). This information is also used to 
respond to several open Key Technical Issue (KTI) agreements between the U.S. Nuclear 
Regulatory Commission (NRC) and the U.S. Department of Energy (DOE); detailed responses to 
these KTI agreements are provided in Appendices A through E, as shown in Table 1-1. 
TEF 2.08, 
GEN 1.01 (15) 
TSPAI 3.25 
USFIC 4.01 
USFIC 4.06 
RT 3.06, SDS 3.02 
Table 1-1. Seepage-Related Key Technical Issues Agreements Addressed in This Report 
Short Description KTI Agreement/AIN Appendix 
Effects of heterogeneity on thermal seepage 
Using test data to reduce uncertainty in TSPA seepage abstraction 
In situ field testing results and analyses 
Justification of continuum approach for seepage modeling 
Alcove 8–Niche 3 pretest predictions 
October 2003 1-2 
A
B
C
D
E 
Water is one of the principal agents determining corrosion of engineered barriers, waste 
dissolution, and radionuclide transport from the repository to the accessible environment. The 
amount and chemical composition of water seeping into waste emplacement drifts thus affects 
the long-term safety of the repository system at Yucca Mountain. 
In the unsaturated zone, percolating water encountering a large underground opening is partly 
diverted around the cavity. This effect reduces the amount of liquid water entering the waste 
emplacement drift or prevents dripping altogether (i.e., the seepage flux is expected always to be 
smaller than the local percolation flux). This effect is referred to as a capillary barrier; it is an 
attribute of the unsaturated zone at Yucca Mountain. Moreover, during the early stages after 
closure, the heat from decaying radionuclides will vaporize water that approaches the waste 
emplacement drift. Both mechanisms limit the amount of water that potentially contacts the 
waste packages. 
Characterizing and predicting seepage requires addressing a variety of processes and factors, 
including the distribution of water percolating through the mountain, the hydrogeologic 
properties of the fractured rock in the vicinity of a waste emplacement drift, the temperature and 
humidity conditions near and within the drift, the drift geometry, and the drift wall structure. 
These processes and factors are discussed in more detail in Section 2. A comprehensive testing 
program was designed and executed to understand seepage at Yucca Mountain and to obtain data 
supporting the models developed to predict drift seepage during the postclosure period. The 
testing program and data are presented in Section 3. Various process models were developed to 
analyze data from seepage experiments and to predict seepage into waste emplacement drifts 
under ambient and thermally elevated conditions. Experimental and modeling studies were 
performed to evaluate how seepage is affected by repository heat, mechanical deformation of the 
rock and the drift itself, changes in the chemical environment, and other perturbations. These 
modeling studies are summarized in Section 4. 
Uncertainties and variabilities in the predicted seepage rates are examined and accounted for in a 
simplified but reasonable representation of seepage in a probabilistic total system performance 
No. 3: Water Seeping into Drifts Revision 2 
assessment (TSPA) analysis. The testing and modeling program undertaken to understand and 
characterize seepage from fractured rock is considered suitable to provide an adequate basis for 
predicting seepage into waste emplacement drifts under ambient and thermally elevated 
conditions at Yucca Mountain. 
1.2 SIGNIFICANCE OF SEEPAGE FOR REPOSITORY PERFORMANCE 
The number of waste packages contacted by water, the corrosion performance of the engineered 
barriers, the dissolution and mobilization of radionuclides, and the release and migration of 
radionuclides to the accessible environment depend on the rate, chemical composition, and 
spatial and temporal distribution of water seeping into the emplacement drifts. 
Percolating water encountering a waste emplacement drift is partly diverted around the opening 
because of the capillary barrier effect, which refers to the tendency of water to be held in the 
pores of unsaturated rock rather than dripping into a large opening. As a result, the seepage flux 
is expected to be smaller than the local percolation flux. This capillary barrier effect is an 
attribute of the unsaturated zone at Yucca Mountain. 
While seepage is determined by features and processes occurring above the drift, the flow 
diversion phenomenon and reduction or prevention of water inflow into the drifts also affects the 
conditions below the repository, specifically the reduced saturation in the matrix and the 
fractures immediately beneath the drift. This is referred to as the drift shadow zone (see 
Figure 1-2). The saturation and flow conditions in the shadow zone impact diffusive releases at 
the interface between the drift invert and natural environment and the potential for the absence of 
fast advective transport of radionuclides through the fracture network. 
A scientific research program has been undertaken to understand and characterize the 
fundamental processes contributing to seepage into underground openings excavated in the 
unsaturated, fractured formations at Yucca Mountain. In addition, a research program was 
designed to characterize the seepage-related properties of the repository host rock, providing the 
basis for the site-specific seepage predictions. Seepage was examined using theoretical, 
experimental, numerical, and natural analog studies. The key factors affecting seepage were 
identified, and site-specific, seepage-relevant parameters were determined through in situ testing. 
Numerical models were developed and calibrated against data that contain seepage-relevant 
information. These models were extensively tested to gain confidence in their ability to make 
predictions of drift seepage. Natural analogs were studied to corroborate the water-exclusion 
concept under unsaturated conditions. 
Based on this research program, an understanding of the process of seepage into waste 
emplacement drifts has been gained. Seepage tests in the repository host units were performed to 
obtain site-specific characterization data related to seepage. A consistent and systematic 
approach to analyzing these data for use in subsequent seepage prediction models was 
developed. 
October 2003 1-3 No. 3: Water Seeping into Drifts Revision 2 
Figure 1-2. Schematic Showing Reduced Seepage and the Development of a Shadow Zone as a 
Result of the Flow Diversion Capability of the Capillary Barrier in the Unsaturated Zone 
While the seepage processes and controlling mechanisms are well understood, seepage 
predictions under both ambient and thermal conditions remain uncertain, primarily because of 
natural variability. These uncertainties were assessed through the abstraction process and will be 
propagated in a probabilistic TSPA calculation. 
1.3 PURPOSE AND SCOPE 
The purpose of this document is to provide an overview of the current understanding of drift 
seepage, including a summary description of seepage testing and modeling activities. It presents 
the technical basis for addressing seepage KTIs. The report summarizes the lines of reasoning 
used to arrive at the conclusion that (1) reduction or prevention of water seepage into 
underground openings under both ambient and elevated temperature conditions is an attribute of 
the unsaturated zone barrier at Yucca Mountain, (2) the extensive testing conducted in the 
Exploratory Studies Facility (ESF) and the Enhanced Characterization of the Repository Block 
(ECRB) Cross-Drift provided critical seepage-related characterization data, (3) the 
comprehensive data analysis and modeling approach yields reasonable, site-specific seepage 
predictions, and (4) remaining uncertainties and process understanding issues are recognized. 
The report describes the process of seepage from the fractured repository units in the Topopah 
Spring welded tuff into a waste emplacement drift and presents the factors affecting seepage 
(see Section 2). This discussion covers a range of spatial and temporal scales from the 
near-steady, mountain-scale distribution of percolation flux to episodic, small-scale flow events 
within individual fractures to film flow along the drift surface. In addition to describing seepage 
processes under ambient conditions, the report also includes a discussion of coupled thermal, 
October 2003 1-4 No. 3: Water Seeping into Drifts Revision 2 
hydrologic, chemical, and mechanical processes as they occur under increased temperature 
conditions and their impact on seepage. 
The report provides a summary description of field tests performed at Yucca Mountain to obtain 
site-specific data that characterize seepage-relevant features and processes (see Section 3). It 
also discusses the suite of models developed to analyze these data, to predict seepage into waste 
emplacement drift sections, and to abstract the simulation results for use in probabilistic TSPA 
calculations (see Section 4). 
This report does not describe in-drift processes that are affected by seepage water, which will be 
discussed in a technical basis document on the in-drift chemical environment. In-drift processes 
(specifically evaporation and condensation effects) are discussed here with respect to the 
interpretation of seepage test data. 
1.4 DEFINITIONS 
In the context of this report, seepage is defined as dripping of liquid water from the formation 
into an underground opening, such as a niche, alcove, access tunnel, or waste emplacement drift. 
According to this definition, seepage does not include advective or diffusive vapor flow into the 
opening or condensation of water vapor on surfaces, which may lead to drop formation and drop 
detachment. Some of the water entering an underground opening may evaporate or flow along 
the wall, thus not contributing to seepage in the sense defined here. In-drift moisture 
redistribution and potential condensate accumulation will be discussed in the technical basis 
document on in-drift chemical environment. 
The seepage threshold is defined as the minimal percolation flux required to initiate seepage. 
A nonzero seepage threshold indicates the existence of effective water exclusion (Philip et al. 
1989) capability of the formation. The presence of a seepage threshold can be directly observed 
from liquid-release tests conducted at different rates, which show that seepage ceases at a 
nonzero release rate. 
1.5 CAPILLARY AND VAPORIZATION BARRIERS 
In the unsaturated zone, percolating water encountering a waste emplacement drift tends to be 
diverted around the opening because of the capillary barrier effect. This effect is referred to as 
the water exclusion phenomenon (Philip et al. 1989), which reduces the amount of liquid water 
entering an underground opening or prevents dripping altogether, that is, the seepage flux 
averaged over a sufficiently large area (e.g., that of a drift section with the length of a waste 
package) will always be equal to or smaller than the local percolation flux. This effect is an 
attribute of the unsaturated zone barrier at Yucca Mountain. 
The water-exclusion phenomenon has been extensively described in the literature (see Philip 
et al. 1989). The related water diversion capability is utilized in practical applications for the 
protection of landfills and hazardous waste sites. These standard engineering applications 
consider porous materials rather than fractured rocks. However, since the key factors affecting a 
capillary barrier are permeability and capillarity, properties also relevant for fractured rocks, the 
same effect applies to Yucca Mountain. 
October 2003 1-5 No. 3: Water Seeping into Drifts Revision 2 
Water exclusion from drifts has been extensively tested through the in situ seepage experiments 
described in In Situ Field Testing of Processes (BSC 2001a, Sections 6.2 and 6.11) and Seepage 
into an Underground Opening Constructed in Unsaturated Fractured Rock Under Evaporative 
Conditions (Trautz and Wang 2002). The experiments show that the seepage rate is less than the 
injection rate. Because of storage effects and evaporation, the reduced seepage rate by itself 
does not conclusively prove that water is diverted around the opening, which would directly 
assess the barrier capability of the natural system at Yucca Mountain. However, a combination 
of observed and simulated contributions to the water balance during a seepage test indicates that 
a substantial amount of water is diverted around the opening, confirming this water-exclusion 
phenomenon in the unsaturated zone at Yucca Mountain under ambient conditions (see 
Section 3.1.5). 
After the emplacement of radioactive waste, the heat generated by the decay of the radionuclides 
will result in elevated rock temperatures for thousands of years after emplacement. For the 
current design, these temperatures are high enough to cause boiling in the rock. The thermally 
driven changes in water saturation affect the seepage potential. For above-boiling rock 
temperatures, vaporization of percolating water in the fractured rock overlying the repository 
will provide another effective process that greatly reduces (and most likely eliminates) seepage 
into the emplacement drifts during the thermal period, despite potentially increased reflux from 
the redistribution of water associated with vaporization and condensation of moisture above the 
drift. This additional process, referred to as the “vaporization barrier”, and its effect on seepage 
during the thermal period are further discussed in Sections 2.6.4 and 4.5. 
The term “capillary barrier” will be used throughout this technical basis document to refer to the 
flow diversion phenomenon occurring at the interface between two media with different capillary 
properties, where the medium with stronger capillarity overlies that of weaker capillarity. The 
term “vaporization barrier” will be used throughout this technical basis document to refer to the 
reduction of flow towards drifts as a result of vaporization occurring around a heated waste 
emplacement drift (BSC 2003a, Section 6.1.3). In these technical terms, the word “barrier” is 
used to describe the combined effect of a physical process occurring in a system with specific 
properties. This usage is slightly different from that in the definitions of natural and engineered 
barriers as given in Total System Performance Assessment—License Application Methods and 
Approach (BSC 2002a, Section 8.3.1). 
In summary, partial or complete diversion of water around underground openings due to the 
capillary barrier effect in combination with vaporization during the thermal period will reduce 
seepage or even prevent water from dripping into waste emplacement drifts. In addition, the 
extent of a potential low-saturation and low-flux shadow zone beneath the drift (see Figure 1-2) 
and its potential effectiveness in delaying radionuclide transport from the invert of the waste 
emplacement drift to the accessible environment is related to the seepage-exclusion 
phenomenon. The capability of the capillary barrier to divert flow around underground openings 
in unsaturated fractured formations has been established theoretically, as well as through 
modeling and extensive field testing at Yucca Mountain. Natural analogs further confirm the 
existence and effectiveness of the capillary barrier effect. 
October 2003 1-6 No. 3: Water Seeping into Drifts Revision 2 
1.6 SUMMARY OF MODELING APPROACH 
Seepage into an opening in unsaturated fractured rock depends on the percolation flux, the 
formation characteristics, the geometric features of the opening, and the thermodynamic 
conditions in the rock and the opening itself (for details, see Section 2). Specifically, the 
processes that need to be combined in a comprehensive conceptual model of seepage include 
(1) the mountain-scale distribution of percolation flux; (2) the intermediate-scale channeling of 
flow in the fracture network; (3) the drift-scale capillary-barrier effect; and (4) the micro-scale 
phenomena of evaporation, film flow, drop formation, and drop detachment at the drift surface 
where water leaves the formation and enters the drift. In addition, the thermodynamic 
environment in the opening (e.g., temperature, relative humidity, preclosure ventilation regime) 
will also affect seepage. 
The analysis and modeling approach used to explain and reproduce seepage rates measured 
during liquid-release tests (see Section 3.1) and to predict seepage into waste emplacement drifts 
is described in detail in Section 4. The approach described in Section 4 is based on a process 
model of seepage in combination with effective parameters that are determined from in situ 
seepage experiments. Detailed predictions of drip frequency and individual seepage locations 
are not made; rather, calculated seepage rates are averaged in time and over a drift section of the 
approximate length of a waste package (i.e., they refer to the temporal and spatial scale of 
interest for a probabilistic assessment of seepage into a representative waste package). To 
support this approach, liquid-release tests were performed at rates below and above the seepage 
threshold. They covered the critical range of conditions expected under natural percolation 
conditions (which are most likely below the seepage threshold where no seepage occurs) and 
under localized, high flux conditions (as a result of wet climates combined with significant flow 
focusing) that may induce seepage. To summarize, the chosen approach focuses on the relevant 
drift- and waste-package scale and eliminates the need for auxiliary models and large spatial and 
temporal extrapolations from micro-scale models or low-resolution interpretation from site-scale 
models. 
A key advantage of this approach is that it relies directly on seepage-rate data that contain 
information about the relevant processes. Moreover, the type of data used for calibration 
(i.e., seepage rates from liquid-release tests collected in a drift section, see Section 3.1) are 
conceptually very similar to the variable of interest (i.e., average seepage rates from a natural 
percolation) that needs to be predicted with the calibrated model. This consistency between 
calibration and prediction data minimizes potential conceptual differences between the 
calibration and prediction model. The appropriateness of this approach is further discussed in 
Appendix D. Finally, uncertainties and variabilities inherent in the seepage process are 
addressed by a probabilistic treatment of seepage in TSPA (see Section 4). 
1.7 NOTE REGARDING THE STATUS OF SUPPORTING TECHNICAL 
INFORMATION 
This document was prepared using the most current information available at the time of its 
development. This technical basis document and its appendices providing KTI agreement 
responses that were prepared using preliminary or draft information reflect the status of the 
Yucca Mountain Project’s scientific and design bases at the time of submittal. In some cases this 
October 2003 1-7 No. 3: Water Seeping into Drifts Revision 2 
involved the use of draft analysis and model reports and other draft references whose contents 
may change with time. Information that evolves through subsequent revisions of the analysis 
and model reports and other references will be reflected in the LA as the approved analyses of 
record at the time of LA submittal. Consequently, the Project will not routinely update either 
this technical basis document or its KTI agreement appendices to reflect changes in the 
supporting references prior to submittal of the LA. 
October 2003 1-8 No. 3: Water Seeping into Drifts Revision 2 
2. PROCESSES AND FACTORS AFFECTING SEEPAGE 
2.1 SUMMARY DESCRIPTION OF SEEPAGE PROCESS 
The seepage process has been studied as part of the Yucca Mountain site characterization effort. 
The following section summarizes the processes and factors affecting seepage into waste 
emplacement drifts. 
Seepage is a process that occurs at the interface between the natural and engineered system. The 
repository design (drift geometry), and its construction (excavation effects, and drift surface 
roughness), and the operating conditions within the drifts, (e.g., heat load and ventilation) affect 
the amount and distribution of seepage. The following sections qualitatively discuss the seepage 
process and the important factors affecting seepage, which are schematically depicted in 
Figure 2-1 for both an intact and partially collapsed waste emplacement drift. 
Figure 2-1. Schematic Showing Seepage Processes and Factors Affecting Seepage 
October 2003 2-1 No. 3: Water Seeping into Drifts Revision 2 
The majority of the precipitation on Yucca Mountain is removed by runoff and 
evapotranspiration. The remaining water penetrates the ground surface and starts to percolate 
downwards, driven by gravity and capillary forces (details will be discussed in the upcoming 
technical basis document on climate and infiltration). As this water percolates to depth, it is 
spatially redistributed (for details see the upcoming technical basis document on unsaturated 
zone flow and transport). Seepage into a section of waste emplacement drift is given by the local 
percolation flux minus the water that is diverted around the drift due to the capillary and 
vaporization barrier effects, minus water that flows as a film along the drift surface or that 
evaporates. 
The detailed flow path is determined by the degree of fracturing, fracture geometry, orientation, 
connectivity, and the hydrogeologic properties of the fractures and the matrix. Depending on 
these factors, the water phase in the unsaturated fracture network will either disperse or 
concentrate along the flow path. Tilted contacts between hydrogeologic units (especially 
between welded and nonwelded tuffs) may affect the overall flow pattern or lead to a change in 
the frequency and spacing of flow channels. As flow concentration continues to occur, the 
distance between the individual channels carrying focused flow increases. As a result, the 
likelihood of two channels meeting and merging decreases with depth. Flow concentration and 
dispersion of flow paths also occurs within a rough-walled fracture where asperity contacts and 
locally larger fracture openings lead to small-scale redistribution of water within the fracture. 
A general discussion of channeling effects under unsaturated flow conditions can be found in 
Solute Channeling in Unsaturated Heterogeneous Porous Media (Birkholzer and Tsang 1997). 
Flow focusing is important for seepage because seepage depends on the local (rather than 
average) percolation flux. These issues are further discussed in Section 2.2. 
As water approaches an emplacement drift (at a distance of one to several meters above the drift 
ceiling), conditions change in several ways that affect the amount of water that can seep into the 
opening. At early times after repository closure, the water will first encounter a dryout zone 
(defined as the zone of reduced saturation caused by preclosure drift ventilation or by boiling due 
to radioactive decay heat). Under boiling conditions, the dryout zone will be surrounded by a 
two-phase zone in which vapor-liquid counterflow (heat-pipe effect) occurs and a condensation 
zone with increased saturation. Preclosure ventilation and elevated temperatures are limited in 
time and do not affect long-term seepage. 
Formation properties around the openings will be altered as a result of stress redistribution 
during drift excavation, which will lead to local opening or partial closing of fractures and 
potentially the creation of new fractures. Additionally, thermal expansion of the rock matrix will 
also induce changes in apertures. Finally, the local chemical environment, which is altered by 
evaporation and thermal effects, will lead to dissolution and precipitation of minerals, affecting 
flow properties of the fracture system and fracture-matrix interaction (see the technical basis 
document on in-drift chemical environment). These conditions lead to a flow pattern around a 
waste emplacement drift that is different from that in the undisturbed formation under ambient 
conditions (i.e., before excavation and waste emplacement). 
When water penetrates the boiling zone (BSC 2003b, Sections 6.2 and 6.3) and reaches the 
immediate vicinity of the drift wall, it will still be prevented from seeping into the drift because 
of the capillary barrier effect (Philip et al. 1989). This effect leads to a local saturation build-up 
and, thus, the development of a higher water potential in the formation immediately adjacent to 
October 2003 2-2 No. 3: Water Seeping into Drifts Revision 2 
the drift. If the permeability tangential to the drift wall (i.e., permeability in the direction along 
the ceiling) and the capillarity of the fracture network within this region are sufficiently high, all 
or a portion of the water will be diverted around the drift under partially saturated conditions. 
Alternatively, however, the water potential in the formation may be higher than that in the drift, 
allowing water to exit the formation. At the drift surface, the water will either evaporate or 
follow the inclined, rough wall in a film or form a drop that may grow and eventually detach (Or 
and Ghezzehei 2000). Only this last mechanism is considered drift seepage according to the 
definition of Section 1.4. 
The rate of water dripping into an opening in the unsaturated zone is expected to be significantly 
less than the local percolation rate because (1) the dryout zone around the drift reduces liquid 
water flow, potentially preventing water from reaching the drift surface, (2) the capillary barrier 
diverts water around the drift, thus bypassing the waste package, (3) water may flow along the 
drift surface without dripping into the opening, and (4) water may evaporate from the drift 
surface. Even if the seepage threshold were exceeded and seepage occurred, the seepage flux 
would be lower than the local percolation flux (i.e., the unsaturated zone at Yucca Mountain acts 
as a fully or partially effective barrier to water seeping into the waste emplacement drift). 
2.2 FACTORS AND PROPERTIES AFFECTING SEEPAGE 
Seepage is a process that occurs at the interface between the natural and engineered systems. 
Consequently, seepage is not only affected by hydrogeological factors (percolation flux, 
formation properties), but also by the design of the repository and waste emplacement drifts 
(location and geometry), the method of construction (excavation effects, drift surface roughness, 
ground support), and the conditions within the drifts (heat load, preclosure ventilation, and drift 
degradation). 
The following sections describe the key factors affecting drift seepage and how they are included 
in the base-case conceptual model. The most important factors are the magnitude of the local 
percolation flux in relation to the formation’s permeability, the strength of the capillary forces in 
the fractures, the connectivity of the fracture network in the immediate vicinity of the opening, 
the local topography of the rough drift wall, and the thermodynamic conditions in the drift. 
Percolation Flux–The magnitude of the percolation flux is a key factor determining seepage. 
Seepage is initiated if the local percolation flux in individual flow channels and the accumulation 
of water from these channels near the drift ceiling exceeds the diversion capacity of the capillary 
barrier, the evaporation potential of the atmosphere in the drift, and the capacity of films to carry 
water along the drift surface. It is the local (rather than average) percolation flux that controls 
the onset of seepage. 
The source of percolation flux at Yucca Mountain is net infiltration at the ground surface, 
stemming from precipitation events. Net infiltration is the fraction of precipitation that moves 
through the ground surface to a depth where the liquid water can no longer be removed by 
evaporation or transpiration. Net infiltration varies in space (as a result of several factors, such 
as vegetation, morphology, soil and bedrock, and runoff and runon conditions) and in time 
(USGS 2001). Time variations are short-term as a result of daily or seasonal fluctuations and 
long-term as a result of climatic changes. As infiltrating water percolates through the 
unsaturated zone, driven by gravity and capillary forces, the initial infiltration and flow patterns 
October 2003 2-3 No. 3: Water Seeping into Drifts Revision 2 
change depending on the hydrogeologic properties and their heterogeneities. On a large scale, 
several stratigraphic units of volcanic rock with significant differences in fracture frequency and 
matrix porosity can be distinguished at Yucca Mountain. Variations between units reflect the 
type of volcanic eruption, the rate of cooling, and the intensity of postdepositional processes. 
In general, percolation flux through the Tiva Canyon welded (TCw) hydrogeologic unit, the first 
fractured bedrock unit below alluvial deposits, is governed by the imposed distribution of net 
infiltration. Flow in this unit occurs mostly in the fractures before entering the underlying 
Paintbrush nonwelded (PTn) hydrogeologic unit. This unit corresponds to the lithostratigraphic 
units from the top of the Tiva Canyon Tuff crystal poor member vitric zone moderately welded 
subzone (Tpcpv2) to the bottom of the Topopah Spring Tuff crystal rich member vitric zone 
moderately welded subzone (Tptrv2). With its characteristics of high matrix porosity and low 
fracture frequency and the existence of tilted layers of nonwelded vitric and bedded tuff, the PTn 
can effectively divert percolating water into intercepting faults and fault zones (BSC 2003c). 
Also, the PTn unit dampens and homogenizes downward-moving transient pulses from surface 
infiltration events. Therefore, the percolation distribution below the PTn unit is considerably 
different from the distribution of net infiltration, both spatially and temporally. Additional 
information on flow diversion and dampening in the PTn will be presented in the technical basis 
document on unsaturated zone flow. The hydrogeologic unit below the PTn is the Topopah 
Spring welded (TSw) hydrogeologic unit, a thick, densely fractured unit corresponding to the the 
Topopah Spring Tuff upper lithophysal (Tptpul), middle nonlithophysal (Tptpmn), and lower 
lithophysal (Tptpll) zones, which will host the repository. The Tptpll will host the majority of 
the repository. 
On an intermediate scale, there is also considerable heterogeneity within stratigraphic units. This 
kind of heterogeneity can focus water toward one drift location while diverting it away from 
another. Flow concentration on this scale (i.e., across the interface between a mountain-scale 
model and drift-scale models) is referred to as “flow focusing.” Flow channeling and diversion 
of flow paths also happen within each rough-walled fracture where asperity contacts and locally 
larger fracture openings lead to small-scale redistribution of water within the fracture. In 
addition, asperity-induced flow instabilities may cause small-scale episodic flow within 
fractures, leading to high-frequency fluctuations. These factors are considered in the 
development of a prediction and abstraction for seepage quantification. 
Flow concentration effects on various scales are schematically shown in Figure 2-2. Flow 
concentration, flow focusing, and channeling effects are uncertain. Flow simulations using a 
two-dimensional discrete fracture network model (see Figure 2-3) show that the average spacing 
between flow paths in a layered system tends to increase with depth (i.e., flow becomes more 
concentrated with the increase of depth under gravity-driven flow conditions). (The flow 
characteristic may change abruptly at interfaces between hydrogeologic units, leading to some 
local flow dispersion on a smaller scale in the unit with higher fracture density.) 
Temporal fluctuations superimpose the spatial flow-channeling effects described above. The 
flux in a flow channel may be near steady state or episodic with a wide frequency spectrum, 
ranging from high-frequency fluctuations triggered by small flow instabilities to intermediate 
variabilities in percolation fluxes in response to changing weather conditions to long-term 
variations from climate changes. 
October 2003 2-4 No. 3: Water Seeping into Drifts Revision 2 
Figure 2-2. Schematic of Flow-Channeling Effects on Various Scales 
October 2003 2-5 No. 3: Water Seeping into Drifts Revision 2 
measured fracture data from Yucca Mountain (nonconnected fractures excluded). 
NOTE: This two-dimensional fracture network was generated based on statistical information derived from the field 
Figure 2-3. Two-Dimensional Fracture Network at Yucca Mountain (a) and Simulated Steady-State 
October 2003 
Flux Distribution in the Fracture Network (b) 
The local (rather than average) percolation flux reaching the drift is the most important factor 
determining where and when seepage occurs, and the seepage rate. The spatial distribution of 
flow channels may change with the average percolation flux and may also change with time. 
Flow focusing could concentrate water onto a particular drift segment and lead to a flux that 
exceeds the seepage threshold. On the other hand, if flow is concentrated in one location, flow 
will be reduced in other areas leading to little or no seepage. The details of flow distribution on 
multiple scales are a major source of seepage prediction uncertainty, as discussed below. This 
uncertainty is accounted for in probabilistic TSPA calculations. 
The distribution of percolating water will be discussed in the technical basis document on 
unsaturated zone flow. The percolation flux used as a boundary condition for the seepage 
models is derived from UZ Flow Models and Submodels (BSC 2003c). Flow focusing factors 
estimated from an intermediate-scale flow focusing model (BSC 2001b, Section 6.4.2) are 
employed to account for intermediate-scale heterogeneity that is not represented in the 
layer-averaged UZ Flow Models and Submodels (BSC 2003c). Flow channeling due to 
small-scale heterogeneity is explicitly included, as discussed in Section 4.2. 
Formation Properties–The key formation properties determining the effectiveness of the 
capillary barrier are (1) the capillary strength and (2) the permeability tangential to the drift wall. 
Geologic formations with stronger capillarity and higher tangential permeability exhibit a higher 
seepage threshold (i.e., low seepage); whereas, a weaker capillary barrier effect (i.e., higher 
seepage) is expected if water retention is smaller or if the tangential permeability is insufficient 
to promote flow diversion. 
Porous formations with strong capillarity tend to have low permeability and vice versa, which is 
a correlation that reduces the probability of encountering parameter combinations conducive to 
extreme (low or high) seepage behavior, making seepage relatively uniform across different 
geologic units. However, this negative correlation between permeability and capillary strength 
2-6 No. 3: Water Seeping into Drifts Revision 2 
may not necessarily apply to a fractured system, specifically if considering the seepage process 
on the scale of a waste emplacement drift. A given permeability may result from a network 
consisting of a few large fractures or, alternatively, a network of many small, well-connected 
fractures. The first network with few large fractures would exhibit relatively weak capillarity, 
whereas the second network with many small fractures would have stronger capillarity. 
Moreover, if the predominant fracture orientation is aligned with the drift axis (see Section 4.2, 
Figure 4-2a), little or no tangential permeability is available and seepage is increased. For flow 
diversion to occur, the fracture system must have sufficient connectivity and permeability to 
provide the necessary effective diversion pathways in tangential direction around the drift. 
Heterogeneities in formation properties on the scale of a drift diameter or smaller may enhance 
seepage if they promote flow channeling and increase the probability of locally exceeding the 
seepage threshold. Intermediate-scale heterogeneities (from approximately 100 m to a drift 
diameter) may focus water into flow channels that either encounter or bypass the waste 
emplacement drifts. Large-scale heterogeneities may divert water (e.g., along contacts between 
hydrostratigraphic units) or concentrate it into local features (such as faults). Heterogeneities on 
these scales thus have roles in determining the fraction of waste packages encountering seepage 
and the seepage amount. The heterogeneous nature of the formation implies that certain regions 
are more prone to seepage than others, with some areas exhibiting properties that are strongly 
adverse to seepage. 
The impact of lithophysal cavities on flow and seepage is twofold: (1) lithophysal cavities are 
essentially obstacles to water flow because they act as capillary barriers, focusing the water that 
flows around them, and (2) lithophysal cavities intersected by the drift lead to a rough drift 
ceiling, potentially creating seepage points at local low points on the ceiling. Both effects tend to 
promote seepage. 
The capillarity and permeability of the fracture network alone do not determine seepage. Water 
emanating from the fracture network may not drip but be transported as film flow along the drift 
surface or evaporate (see Section 2.4). Hence, seepage is not only affected by the characteristics 
of the host rock but also by factors unrelated to the hydrogeologic properties of the fracture 
system. 
Drift Geometry and Drift-Wall Properties–The overall drift size and geometry can change the 
seepage threshold and the seepage amount. Generally, a large drift exhibits a significantly lower 
seepage threshold. More water accumulates in the high-saturation region near the drift boundary 
because it needs to move over a longer diversion distance around the wider opening. In a 
heterogeneous, fractured formation, the importance of drift shape and drift geometry may be 
diminished relative to that of flow channeling and local ponding (Birkholzer et al. 1999, pp. 372 
to 379). 
The effectiveness of a capillary barrier is greatest if the shape of the cavity follows an 
equipotential surface. In a homogeneous medium, parabolic cavities are more efficient in 
preventing seepage than circular or flat-roofed openings. Breakouts in the drift ceiling, caused 
by rockfall and drift degradation, may change the overall drift geometry and lead to local low 
points in the ceiling, which may trap water, reduce or prevent flow diversion, and initiate 
seepage. In addition, small-scale surface roughness tends to increase seepage if the amplitude of 
October 2003 2-7 No. 3: Water Seeping into Drifts Revision 2 
the irregularity is comparable to the boundary-layer thickness. The boundary-layer thickness is 
determined by the capillary rise in the fractures (i.e., it is on the order of a few centimeters). 
2.3 THERMAL, HYDROLOGIC, CHEMICAL, AND MECHANICAL IMPACTS ON 
SEEPAGE 
2.3.1 Introduction 
As outlined in Section 2.2, hydrogeologic properties (specifically permeability and capillary 
strength) and their spatial heterogeneity determine the flow diversion capacity of the fracture 
network and, thus, seepage behavior. Initially, the undisturbed rocks in the repository host units 
are likely to be altered as a result of stress redistribution during drift excavation, which typically 
leads to local opening of fractures and potentially the creation of new microfractures in the drift 
vicinity (BSC 2001a, Section 6.1.2.2; Wang and Elsworth 1999, pp. 752 to 756). These changes 
affect porosity, permeability, and capillarity of the fracture system in the vicinity of 
emplacement drifts. Later, the heat emanating from the radioactive material will induce changes 
as a result of coupled thermal-hydrologic-mechanical and thermal-hydrologic-chemical effects. 
Thermal expansion of the rock matrix will induce thermal stresses and associated aperture 
changes. Also, thermal effects will lead to dissolution and precipitation of minerals, affecting 
the porosity, permeability, and capillarity of the fracture system. 
2.3.2 Excavation Effects 
Because of excavation, stress is redistributed and fractures are generally expected to dilate near 
the crown of the drift. Such fracture dilation depends on the orientation of the fracture set and 
generally occurs within one drift radius. An increase in fracture aperture generally causes an 
increase in fracture permeability and a decrease in capillary strength—both key parameters 
affecting seepage. The measured increase in permeability from preexcavation to postexcavation 
values reflects this excavation effect (Wang et al. 1999). 
2.3.3 Drift Degradation 
It is possible that the initially circular-shaped open emplacement drifts will degrade with time as 
a result of thermal stress, seismic ground motion, and time-dependent degradation of rock 
strength (BSC 2003d). Thermal stresses are caused by the heat generated from the decaying 
nuclear waste. Significant stresses can also be caused by seismic ground motions. 
Time-dependent degradation of rock strength (joint mechanical properties) may be a result of 
over-stressing from thermal heating and of static fatigue of the rock resulting from stress 
corrosion mechanisms. These effects may lead to rock mass damage and rockfall into 
emplacement drifts, changing the drift shape and size. Depending on the type of rock, the stress 
conditions, and the time-dependent rock mechanical properties, damage to the drifts may be 
small, with sparse local rockfall from the ceiling of otherwise intact drift openings, or (in 
extreme cases) damage may result in partial or complete drift collapse, with rubble rock material 
filling the enlarged drifts. These changes affect the potential for drift seepage. Local breakouts 
in the drift ceiling may lead to geometry changes that can either increase seepage by reducing or 
preventing flow diversion around the opening (e.g., by creating low points) or reduce seepage by 
resulting in a drift geometry that promotes flow diversion (e.g., by creating a more parabolic drift 
shape through breakouts near the apex of the drift). The larger size and potentially different 
October 2003 2-8 No. 3: Water Seeping into Drifts Revision 2 
shape of collapsed drifts can also reduce the potential for flow diversion; furthermore, the larger 
footprint of the collapsed drift leads to an increase in the total amount of percolation flux arriving 
at the drifts, which, in turn, can affect the total amount of seepage. In addition, the 
capillary-barrier effect at the drift wall may be affected by the rubble rock particles filling the 
opening because the capillary strength inside the opening will be different from the 
zero capillarity condition in the initially open drift. 
2.3.4 Seepage under Thermal Conditions 
The heat generated by the decay of radioactive wastes will result in rock temperatures elevated 
for thousands of years after emplacement. For average rock properties, percolation flux, and 
heat load, the drift wall will be at above-boiling temperatures for approximately 1,000 years. 
Elevated temperatures (below boiling, but above ambient conditions) will prevail throughout the 
compliance period (BSC 2003b, Section 8). For the current repository design concept, these 
temperatures will be high enough to cause boiling conditions in the drift vicinity, giving rise to 
local water redistribution and altered flow paths. Key thermal-hydrologic processes occurring 
around a drift are shown schematically in Figure 2-4 for an idealized, circular-shaped drift. The 
figure indicates that heating of the rock will cause pore water in the rock matrix to boil and 
vaporize. The vapor will move away from the drift through the permeable fracture network, 
driven primarily by a pressure increase caused by boiling. In cooler regions away from the drift, 
the vapor will condense in the fractures, where it can drain either toward the heat source from 
above or shed around the drift into the zone below the heat source. Condensed water can also 
imbibe from fractures into the matrix, leading to increased liquid saturation in the rock matrix. 
With continuous heating, a superheated (above-boiling temperature) dryout zone will develop 
closest to the heat source, separated from the condensation zone by a nearly isothermal zone 
maintained nominally at the boiling temperature. This nearly isothermal zone is characterized by 
a continuous process of boiling, vapor transport, condensation, and migration of water back 
toward the heat source (vapor–liquid counter flow resulting from capillary forces or gravity 
drainage), a process often referred to as a heat pipe (Pruess et al. 1990). 
For the current repository design concept, the dryout zone around drifts will extend to a distance 
of approximately 5 to 10 m from the drift wall (BSC 2003b, Section 6.2). Boiling conditions in 
the rock are expected to exist for about 1,000 years after emplacement. While these values 
reflect average conditions, there may be significant spatial and temporal variability of the 
thermal-hydrologic conditions within the repository. One factor causing heterogeneity is the 
spatial and temporal variability directly related to the thermal load in different drift sections, 
stemming from heat output variations between individual waste packages and different 
emplacement times. Another factor is the heterogeneity of the formation properties and spatial 
distribution of the local percolation fluxes. Thermal rock properties, such as thermal 
conductivity, directly affect the conductive transport of heat. Hydrologic properties and local 
percolation fluxes, on the other hand, affect the significance of thermal-hydrologic coupling as 
they determine the effectiveness of convective heat transport. While heat conduction is the 
major component of energy transport in Yucca Mountain tuff, the impact of thermal-hydrologic 
coupling can be quite large. For example, a large percolation flux above a drift segment, 
combined with relatively high permeability, may cause heat-pipe effects that give rise to rock 
temperatures much lower and boiling periods much shorter than at average conditions (BSC 
2003b, Section 6.2.1.4). 
October 2003 2-9 No. 3: Water Seeping into Drifts Revision 2 
Repository Heating 
Figure 2-4. Schematic of Thermal-Hydrologic Processes Occurring in the Drift Vicinity as a Result of 
Heating of the rock in the drift vicinity can affect seepage in two different ways: 
1. Boiling of water in the rock directly affects the seepage-relevant flow processes close 
to the drift wall. For above-boiling conditions, vaporization of percolating water in the 
fractured rock overlying the repository further reduces the potential for seepage. 
Percolating water is expected to boil off in the superheated rock zone before reaching 
the drift crown. Therefore, seepage is unlikely as long as boiling conditions exist. On 
the other hand, condensation forms a zone of elevated water saturation above the 
dryout zone. Water from this zone may be mobilized to flow rapidly down toward the 
drift, in particular during later stages with reduced heat output from the waste 
packages when the effect of vaporization has diminished. This flow mobilization may 
promote seepage. 
2. Rock properties relevant for seepage may be affected from reversible and irreversible 
thermal, hydrologic, chemical, and mechanical effects. Thermal stress-induced 
changes tend to close existing fractures, leading to a general decrease in fracture 
permeability and porosity combined with an increase in fracture capillary strength. 
Aperture changes that occur during the thermal period could be fully reversible 
(i.e., these properties would recover to preemplacement values after the temperature 
has declined to ambient conditions). Thermal stresses could also induce permeability 
enhancement through fracture shear slip with accompanying shear dilation opening. 
Such permeability changes would be irreversible and remain after the temperature has 
declined to ambient conditions. Coupled thermal-hydrologic-chemical processes, such 
as mineral precipitation and dissolution in fractures and matrix, also have the potential 
to irreversibly modify permeability, porosity, and capillary strength of the system. 
The molar volumes of minerals created by hydrolysis reactions (i.e., anhydrous phases, 
such as feldspars, reacting with aqueous fluids to form hydrous minerals, such as 
October 2003 2-10 No. 3: Water Seeping into Drifts Revision 2 
zeolites or clays) are commonly larger than the molar volumes of the primary reactant 
minerals. Consequently, dissolution-precipitation reactions commonly lead to porosity 
and permeability reductions. The extent of mineral-water reaction is controlled by the 
surface areas of the mineral phases in contact with the aqueous fluid, as well as 
heterogeneity in the initial distribution of minerals in the fractures. Therefore, changes 
in porosity and permeability caused by these processes may also be heterogeneously 
distributed. Typically, chemical effects on hydrologic properties are irreversible. 
As mentioned in Section 2.3.3, emplacement drifts may completely collapse in extreme cases as 
a result of thermal and seismic stresses and degradation of joint mechanical properties. The 
thermal conditions in a collapsed drift will be different from those in an open drift, mainly 
because the thermal-hydrologic processes in a drift filled with rock fragments are different from 
those in an open, air-filled drift. The extent to which these differences can be important for 
seepage under thermally elevated conditions is governed by the time at which significant drift 
collapse occurs. Significant differences should only be expected when drift collapse occurs 
during the boiling period. 
The impact of repository heat on the hydrologic, chemical, and mechanical conditions was 
examined in a heater experiment referred to as the Drift Scale Test (DST). A short description of 
the DST can be found in Section 3.3.1. The related modeling documented in Drift-Scale 
Coupled Processes (DST) and TH Seepage Models (BSC 2003b) is mainly used to corroborate 
the abstraction of coupled processes on the evaluation of seepage under thermal conditions, as 
discussed in Sections 4.6 and 4.8. 
2.4 IN-DRIFT CONDITIONS 
Preclosure ventilation will cause partial rock drying in the drift vicinity and will also remove a 
significant amount of heat and moisture. While this relatively short ventilation period is 
expected to have minor impact on the postclosure performance of the repository, in-drift 
moisture conditions are important during seepage testing (see Sections 3.1 and 3.2) and the 
analysis of seepage-rate data (see Section 4.3). 
Reduced relative humidity in the underground opening leads to evaporation of water at the drift 
surface and the development of a dryout zone in the vicinity (5 to 10 m) of the cavity. In a 
liquid-release test at ambient conditions (or natural percolation during the ventilated period), part 
or all of the water reaching the ceiling of the opening may evaporate, depending on the 
evaporation potential in the drift and the wet area exposed to evaporation. The evaporation 
potential depends on the relative humidity in the opening and the thickness of a diffusive 
boundary layer at the drift surface, which in turn is governed by the air velocity in the ventilated 
drift. Total evaporation depends on the evaporation potential and the wetted area available for 
evaporative mass transfer. 
Wetting of the drift ceiling depends on the formation properties, the spreading mechanism along 
the drift surface, and evaporation itself. Under natural percolation conditions, evaporation in a 
ventilated drift prevents sufficient wetting at the drift surface and thus seepage. However, the 
development of a wet spot and its relation to in-drift conditions has been studied during the 
seepage tests conducted in several niches and along the ECRB Cross-Drift (see Section 3.1). 
October 2003 2-11 No. 3: Water Seeping into Drifts Revision 2 
Temporal reduction of the wet spot size during continued water release can be correlated to 
increased evaporation as a result of changes in the ventilation regime, highlighting the coupled 
nature of the processes. 
The importance of evaporation effects on the interpretation of seepage data from liquid-release 
tests conducted under ambient temperature conditions has been recognized. The issue has been 
addressed by (1) increasing relative humidity in the testing area (using bulkheads, end curtains, 
and humidifiers), (2) monitoring relative humidity and evaporation from a free water surface 
(pan experiments), and (3) inclusion of evaporation into the process model used to analyze 
seepage-rate data. 
If evaporation is significantly reduced, wetting of the drift surface is expected. Wet surfaces and 
film flow are an integral part of the capillary barrier concept, providing evidence that the 
capillary barrier is engaged. Wet surfaces were observed in the passive Cross-Drift hydrologic 
test (see Section 3.2). 
Additional discussion of evaporation issues can be found in Appendix C. 
October 2003 2-12 No. 3: Water Seeping into Drifts Revision 2 
3. FIELD OBSERVATIONS AND SEEPAGE TESTING DATA 
3.1 LIQUID-RELEASE TESTING 
3.1.1 Description of Liquid-Release Tests 
Drift-scale liquid-release tests were initiated in 1997 to investigate potential seepage into an 
underground opening representing a waste emplacement drift. Short drifts (ranging from 6.3 m 
to 15.0 m in length and referred to as niches) were constructed at various locations along the ESF 
and the ECRB Cross-Drift. Boreholes were installed prior to and after the niches were excavated 
to facilitate characterization of the rock using air-injection tests and investigation of seepage 
processes using liquid-release tests. The locations of the niches were chosen so that they 
represent different hydrogeologic units and rocks with different fracture statistics. A second 
study, referred to as the systematic borehole testing program, was initiated in 2000 to 
complement the niche seepage experiments. The purpose of the program is to provide broad, 
systematic coverage and characterization of the host rock by performing air-injection and 
liquid-release tests in approximately 20-m-long boreholes drilled at a 15° angle upward and 
parallel to the drift axis into the ceiling every 30 m along the ECRB Cross-Drift. Test locations 
are illustrated in Figure 3-1. 
The purpose of these experiments was to release water from borehole intervals located above 
drift sections or niches and to collect the water that seeps into the openings. These seepage-rate 
data contain information about the seepage process reflecting most of the factors discussed in 
Section 2. They have been analyzed to infer seepage-relevant, model-related effective 
parameters on the drift scale. The tests were generally performed by (1) injecting air to 
characterize the permeability distribution, (2) introducing water from a borehole interval above 
the opening, (3) recording the induced seepage, (4) monitoring temperature and relative 
humidity, (5) observing the formation and growth of the wetted spot at the ceiling and 
(6) studying the general seepage behavior. 
Multiple liquid-release tests were performed in the niches and along the ECRB Cross-Drift to 
observe, document, and quantify any water migrating to and seeping into the underground 
openings. The tests were performed by sealing a short section of the borehole above the opening 
using an inflatable packer system and then releasing water at a specified rate into the test 
interval. Any water that migrated from the borehole to the ceiling and dripped into the opening 
was captured and measured. Qualitative observations during the tests helped provide an 
understanding and corroboration of flow diversion and seepage concepts. Moreover, a 
quantitative analysis of the seepage-rate data provided calibrated values of seepage-relevant 
formation parameters (see Section 4.3). 
The effectiveness of the capillary barrier and the presence of a seepage threshold can be directly 
observed from liquid-release tests conducted at different rates, which show that seepage ceases at 
a nonzero injection rate, referred to as a seepage threshold. 
Seepage tests are further described in Appendices B and C. 
October 2003 3-1 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003e, Figure 4. 
NOTE: The shape of the openings is approximate. (In this and other figures, niches are labeled by their construction 
station number (i.e., Niche 2 is designated as Niche 3650, Niche 3 is Niche 3107, Niche 4 is Niche 4788, 
and Niche 5 is Niche 1620). ECRB = Enhanced Characterization of the Repository Block. 
Figure 3-1. Schematic Geologic Map Showing Approximate Location of Niches and Systematic Testing 
October 2003 3-2 
Boreholes SYBT-ECRB-LA#1–3 
3.1.2 Air-Permeability Testing 
The purpose of the air-injection tests was to estimate permeabilities as input data for the 
stochastic generation of heterogeneous permeability fields. The tests were performed by 
isolating a short section of a borehole using an inflatable packer system and then injecting 
compressed air at a constant rate into the isolated injection interval. The pressure buildup in the 
injection interval and in nearby similarly isolated observation intervals was monitored with time 
until steady conditions were reached, typically within a few minutes. Air injection was 
terminated after reaching steady pressures, and the decline in air pressure was then monitored as 
it recovered to its initial pretest condition. Air-permeability values were derived from the steady 
pressure data by means of a commonly used analytical solution (BSC 2001a, Section 6.1.2.1). 
No. 3: Water Seeping into Drifts Revision 2 
Permeabilities determined from air-injection tests are considered representative of the absolute 
permeability of the excavation-disturbed zone around the opening. Since air-injection tests are a 
standard method to obtain permeability values, the use of these values as the basis for model 
calibration, seepage prediction, and the development of probability distributions for TSPA 
ensures consistency and reduces a potential bias that would likely be introduced by relying on 
different measurement methods. 
Air permeabilities were measured before and after mining of the niches. The results showed that 
permeability around the niches and the ECRB Cross-Drift was affected by excavation (BSC 
2001a, Section 6.1.2.2; Wang and Elsworth 1999, pp. 752 to 756). Since seepage is determined 
by the formation properties and excavation effects in the immediate vicinity of the opening, it is 
reasonable to use postexcavation air-permeability data for seepage calculations. 
3.1.3 Relative-Humidity Monitoring 
Reduced relative humidity in the underground openings leads to partial evaporation of the water 
that reaches the cavity wall, potentially affecting seepage. To diminish, control, and quantify 
this effect, relative humidity in the closed-off niches was artificially increased to reduce the 
evaporation potential, and relative humidity was monitored. Figure 3-2 shows an example of 
temperature and relative humidity data measured in Niche 4 (also referred to as Niche 4788). 
Source: BSC 2001a, Figure 6.2.1-5. 
Figure 3-2. Temperature and Relative Humidity Measurements in Niche 4 
No. 3: Water Seeping into Drifts October 2003 3-3
Revision 2 
In a closed-off and humidified niche, potential evaporation at the wall or in the capture system is 
expected to be small compared to the amount of water being released. Seepage experiments in 
the middle nonlithophysal zone (Tptpmn) of the Topopah Spring welded unit were conducted in 
niches that were closed off by a bulkhead, which reduces air circulation and thus leads to 
comparatively high relative humidity. Moreover, a humidifier was used in some of the 
experiments to increase relative humidity to a value close to 100 percent. The measured 
evaporation rates and a related sensitivity analysis indicate that evaporation effects in the 
closed-off niches are insignificant and can be neglected in the analysis of seepage-rate data (BSC 
2003e, Section 6.7). On the other hand, relative humidity in the open ECRB Cross-Drift is 
significantly lower and exhibits relatively strong fluctuations depending on weather and 
ventilation conditions. Evaporation-rate data from pan experiments were used to estimate the 
thickness of the diffusive boundary layer, which affects the evaporation potential. Evaporation 
in the ventilated ECRB Cross-Drift was found to be a greater fraction of the observed seepage 
rate, compared to the niche tests and was taken into account for the analysis of seepage tests 
conducted in the open drift. 
The monitoring of relative humidity is further discussed in Appendix C. 
3.1.4 Liquid-Release Tests in Niches and along the Enhanced Characterization of the 
Repository Block 
Multiple liquid-release tests were performed in Niches 2, 3, 4, and 5 (also referred to as 
Niches 3650, 3107, 4788, and 1620, respectively), as well as in the systematic testing boreholes 
SYBT-ECRB-LA#1 to 3. Water was released at various rates from a short section of the 
borehole above the opening. Any water that migrated from the borehole to the ceiling and 
dripped into the opening was captured and measured. In many intervals, multiple liquid-release 
tests were conducted using different release rates with different durations of inactivity between 
individual test events. Only a very small amount of water was released in early tests conducted 
in Niche 2. To reduce the impact of storage effects and to test a more representative portion of 
the fracture network involved in flow diversion around the opening, later tests in Niches 3, 4, 
and 5 and along the ECRB Cross-Drift used significantly more water in an attempt to reach 
near-steady seepage conditions. These later tests were used for parameter estimation purposes, 
as described in Section 4.3. 
Figure 3-3 shows contours of the wetting front spreading across the niche ceiling during a 
liquid-release test. Water issued from the formation through fractures and micro-cracks and 
spread along the rough and dusty surface. Dripping was initiated at discrete points. As indicated 
in Figure 3-4, seepage rates measured during a representative liquid-release test appear to 
stabilize after about 4 days of continuous water injection. In this example, the high release rate 
resulted in a near-steady seepage rate that was approximately half the release rate. 
Appendices B and C contain additional information on seepage testing. 
October 2003 3-4 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2001a, Figure 6.2.1-7. 
NOTE: Blue contours are outlines of wetting fronts. Red numbers indicate ordering of the wetting fronts in time. 
The pink bar indicates the approximate position of the release interval in the borehole above the niche, 
projected onto the crown. 
Figure 3-3. Wetting-Front Sequences Overlying Fracture Map of Niche 4 Crown for a Representative 
Liquid-Release Test 
October 2003 3-5 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2001a, Figure 6.2.1-6. 
Figure 3-4. Seepage Rates Observed during a Representative Liquid-Release Test Conducted in 
October 2003 3-6 
Niche 4 
3.1.5 Mass Balance in Niche 5 
The flow-diversion capability of the capillary barrier is the mechanism of interest for the 
estimation of reduced seepage into waste emplacement drifts. The fate of water during a 
transient liquid-release test is difficult to assess (i.e., the difference between the release rate and 
seepage rate cannot be attributed to flow diversion with certainty unless all the water used in a 
liquid-release test is accounted for in a full mass balance). Obtaining a complete mass balance in 
a transient test requires estimation of the amount of water that (1) is injected, (2) seeps into the 
opening, (3) evaporates from the rock surface and the collection system, (4) is stored in the 
formation between the injection point and the opening, (5) bypasses the opening through known 
and unknown geologic features, and (6) is diverted around the opening on account of the 
capillary-barrier effect. 
Components 1 and 2 can be directly measured with sufficient accuracy. Reliable estimates of 
Component 3 can be obtained from relative-humidity or evaporation-rate measurements. 
Alternatively, the evaporation potential can be minimized, as described in Section 3.1.3. For 
Component 4, the amount of water stored in the formation, is difficult to obtain. However, the 
tests can be run to near-steady-state conditions, and a rate balance (as opposed to a mass balance) 
can be performed. Finally, measuring Components 5 and 6 requires capturing the water in a 
secondary water collection system. 
No. 3: Water Seeping into Drifts Revision 2 
Performing such a water mass balance was considered for the Alcove 8–Niche 3 test (see 
Section 3.1.6) but was not performed due to the large scale of the test and the correspondingly 
large secondary collection system that would be required. Instead, an attempt was made to 
obtain a mass balance in the Niche 5 seepage tests. The main element of this attempt was the 
construction of a slot on the side of the niche to capture the water that is diverted around the 
opening (i.e., Component 6). Relative humidity in Niche 5 was increased to reduce the 
contribution of Component 3, and a long-term liquid-release test was performed to minimize 
storage effects (Component 4). The relatively close proximity of the injection point to the drift 
ceiling reduces the risk that water bypasses the niche through unknown, unique geologic features 
(Component 5). Since near seepage conditions were achieved, a rate balance (rather than total 
mass balance) was performed, which is more appropriate in light of the eventual estimation of 
near steady seepage rates under repository conditions. 
Water was collected at the side of the niche during two seepage tests conducted in Niche 5. 
However, because of difficulties in excavating the slot and installing the capture system, it was 
not possible to visually confirm that the water collected in the slot near the wall was derived 
entirely from seepage into the slot. While qualitative evidence was obtained that the capillary 
barrier was capable of diverting water around the niche, a quantitative mass or rate balance based 
on the measured data alone could not be obtained. The seepage predictions, therefore, continue 
to rely on (1) established understanding of the physics underlying flow diversion around a 
capillary barrier, (2) extensive site-specific and seepage-related characterization data, 
(3) qualitative evidence demonstrating flow diversion and water exclusion, (4) calibration and 
validation of a physically based process model that captures all components of the mass balance 
(storage, evaporation, flow diversion, and seepage), and (5) propagation of uncertainties and 
variabilities and probabilistic treatment of seepage in TSPA calculations. 
The mass-balance issue is further described in Appendix C. 
3.1.6 Alcove 8–Niche 3 Testing 
Evaluation of seepage is among the multiple objectives of the ongoing testing at 
Alcove 8-Niche 3. Alcove 8 (located in the ECRB Cross-Drift, see Figure 3-5) was excavated 
and prepared to perform liquid-releases tests into a fault and a network of fractures. 
Approximately 20 m below Alcove 8, Niche 3 (located in the main drift of the ESF at 
construction station 3107) serves as the site for monitoring of seepage. The interior of the niche 
was instrumented with trays for seepage collection. In addition, a series of boreholes 
surrounding Niche 3 was instrumented with sensors to detect the arrival of the wetting front. 
Liquid water and tracers were released at the floor of Alcove 8 (located in the upper lithophysal 
zone, Tptpul) along approximately 5 m of an exposed fault. The amount of water seeping from 
the fault into Niche 3 (which is located in the middle nonlithophysal zone, Tptpmn) and tracer 
concentrations were monitored as functions of time. An example is presented in Figure 3-6, 
showing the seepage rate is approximately 10 percent of the injection rate. 
Seepage-related issues are further discussed in Appendix C. Pretest predictions are described in 
Appendix E. 
October 2003 3-7 No. 3: Water Seeping into Drifts Revision 2 
Figure 3-5. Schematic Illustration of the Alcove 8–Niche 3 Test Configuration 
October 2003 3-8 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003c, Figures 7.6-1 and 7.6-2. 
Figure 3-6. Infiltration Rate in Alcove 8 (a) and Seepage Rate in Niche 3 Test (b) 
3.2 MOISTURE MONITORING IN NONVENTILATED DRIFT SECTIONS 
3.2.1 Description of Passive Cross-Drift Hydrologic Tests and Alcove 7 
The terminal section of the ECRB Cross-Drift was closed off by a series of bulkheads to 
minimize ventilation effects (see Figure 3-7). The 918-m-long drift section is located in the 
Topopah Spring lower lithophysal (Tptpll) and the lower nonlithophysal (Tptpln) tuff units; the 
drift section is intersected by the Solitario Canyon fault. Within the isolated sections between 
bulkheads, barometric pressure, relative humidity, and temperature are currently being measured 
at various stations to provide information on moisture dynamics. Psychrometers were installed 
along seven boreholes to measure the water potential and, thus, the initial extent and later 
rewetting of the dryout zone in the fractured rock. Electrical resistance probes were laid out at 
0.5 m intervals to measure saturation changes along the drift wall. Six water collection units 
were installed. Periodically, the bulkhead doors sealing the nonventilated sections have been 
opened for observations. These observations have included sampling of liquid water from these 
water-collection units. Although the quantity of water has not been measured, water samples 
have been collected and subsequently analyzed to evaluate whether the water originated from 
seepage or condensation (see Section 3.2.4). 
An experiment similar to that described above has been conducted in Alcove 7 since 1998. 
Bulkhead doors were installed 64 m and 132 m from the entrance of the alcove. Ambient 
temperature, barometric pressure, and relative humidity were monitored. The bulkheads were 
opened after being closed for extended periods of time. Evidence of moisture was observed, 
including drip spots on the drip collection sheets, moisture drops on the utility lines and on the 
shotcrete around the bulkheads, and moisture spots in the dust on one instrument enclosure. The 
rock in the crown had a dark, moist appearance, and the fractures in the rib appeared wet. 
Although moisture was present, it was not possible to collect water for analysis. The Alcove 7 
test results are summarized in Appendix C. 
3-9 No. 3: Water Seeping into Drifts October 2003
Revision 2 
NOTE: TBM = tunnel boring machine; ERP = electrical resistance probe. 
Figure 3-7. Schematic Illustration of the Location of Monitoring Stations in the Enhanced 
Characterization of the Repository Block and the Tunnel Boring Machine 
Results of in situ moisture monitoring since the construction of the ESF and ECRB Cross-Drift 
and the DST (see Section 3.3.1) indicate no direct observations of seepage. However, moisture 
has been observed in nonventilated sections of the ECRB Cross-Drift and Alcove 7. The origin 
of these moisture observations is uncertain. It is expected that moisture will be released from the 
partially saturated rockmass in contact with the drift wall by evaporation processes. Given the 
drifts are ventilated, the relative humidity in the drifts prior to the ventilation being turned off is 
significantly less than 100 percent; therefore, a potential gradient exists to allow moisture vapor 
to be transported from the rock matrix into the drift. This moisture vapor flux, once it enters the 
drift, can be transported along the drift and condense on cooler surfaces. This gradient and the 
resultant net influx of rock moisture into the drifts are transient phenomena related to the change 
in moisture and thermal conditions in the drift. At steady state, after the drifts have been 
removed from transient humidity and pressure conditions, the relative humidity is expected to 
stabilize at 100 percent. At steady state, the principal source of liquid water is expected to be 
derived from seepage water. Prior to steady state, when in-drift conditions are less than 
100 percent humidity, moisture in the drift derived from evaporation in the rock wall may exceed 
the seepage influx. This moisture may condense on cooler surfaces, leading to liquid water. 
October 2003 3-10 No. 3: Water Seeping into Drifts Revision 2 
3.2.2 Evidence of Ventilation Effects 
The observations of changes in water potential, saturation, and in-drift atmospheric conditions 
after closure of the bulkheads confirm the strong impact of ventilation and reduced relative 
humidity on the hydrologic conditions in the immediate vicinity of the drift. The matrix is 
partially dried out up to a few meters from the drift surface. Even though the relative humidity 
increases rapidly after closing of the bulkheads, the water potential data indicate that the 
rewetting of the matrix is a slow process. Figure 3-8 shows water potentials as a function of time 
and distance from the drift surface. Low water potentials up to a distance of about 1.5 m from 
the borehole collar indicate the extent of the dryout zone as a result of drift ventilation. After 
installation of the bulkheads, water potentials increase over a period of 1.5 years. 
In-drift conditions (temperature, relative humidity, and barometric pressure) not only show 
temporal variations but also spatial differences along the tunnel section, particularly when heat 
sources are present. The resulting gradients constitute driving forces for in-drift moisture 
redistribution. Figure 3-9 shows temperature fluctuations as a function of time at four stations 
along the nonventilated section of the ECRB Cross-Drift. 
Source: BSC 2003f, Figure 6.10.2-2b. 
Figure 3-8. Water Potential Measurement as a Function of Time and Distance from the Drift Surface 
October 2003 3-11 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003f, Figure 6.10.2-4. 
Figure 3-9. Temporal and Spatial Variations in Temperature within Nonventilated Section of the 
October 2003 
Enhanced Characterization of the Repository Block 
3.2.3 Observations Related to Wet Zones 
Wet zones observed in the closed-off sections of the ECRB Cross-Drift provide information 
regarding potential seepage and in-drift moisture redistribution. During the entries behind the 
bulkheads, droplets and other moisture conditions were observed on rock bolts, ventilation ducts, 
utility conduits, wire meshes, and painted patches of tunnel walls. Puddles of water were 
observed on the conveyor belt, and drip cloths showed evidence of dripping. Observed rust spots 
and organic growths indicate the prolonged presence of moisture. 
The observations of moisture on impervious (nonporous) surfaces suggest that, at least partially, 
the moisture originated from condensation. The near–100 percent relative humidity in the ECRB 
Cross-Drift and local thermal gradients (partly induced by electrical equipment in the closed-off 
drift sections) are the likely causes for the observed condensation. Although it is possible that 
the dripping may have resulted from seepage of formation water, the geochemical analyses 
presented below support the conclusion that this is most likely condensation water. 
3-12 No. 3: Water Seeping into Drifts Revision 2 
3.2.4 Chemical and Isotopic Water Analyses 
Determining the origin of the moisture observed in the closed-off sections of the ECRB Cross- 
Drift is of interest because it relates to the process inducing dripping (condensation or seepage) 
and affects the chemistry of the water potentially contacting waste packages. Chemical analyses 
and isotopic measurements were conducted on the water samples collected during the periodic 
entries into the nonventilated sections of the ECRB Cross-Drift. The chemical analyses included 
major anionic and cationic constituents (such as bromide, chloride, and lithium). 
The initial water samples collected from the puddles on the conveyor belt were dark in color. 
Because the surface of the conveyor belt was covered by dust and rock fragments, the chemical 
analyses of these samples were not expected to provide meaningful signature about the source of 
water found in the puddles. Subsequent samples were taken from clean containers placed on top 
of the conveyor belt. These samples were found to be low in chloride and silica content, 
characteristic of condensates. The samples were also found to lack the chemical signature of the 
construction water that was spiked with 20 mg/L of lithium bromide. While these data clearly 
show that part of the observed moisture originated from condensation, they do not rule out the 
possibility that some of these waters are of mixed origins. 
The hydrogen (äD) and oxygen (ä18O) isotope compositions were also analyzed. The äD values 
range from .48‰ to .9‰, and the ä18O values range from .3‰ to .10.7‰. These values are 
higher than those found in construction water. The lag time between opening of bulkheads and 
sample collection (3 to 4 hours) is sufficient to result in a significant degree of evaporation of the 
samples. As seen in Figure 3-10, all samples from the ECRB Cross-Drift are shifted from the 
global meteoric water line. The offset is characteristic of waters that have undergone some 
degree of evaporation. The same degree of shifts for both the contaminated samples and the 
relatively clean samples may indicate that approximately the same amount of evaporation 
occurred. 
The geochemical and isotopic analyses of water collected in the nonventilated ECRB Cross-Drift 
seem to indicate that condensation is a source of the dripping water. 
3.2.5 Summary 
Moisture monitoring in nonventilated sections of the ECRB Cross-Drift and ESF is an ongoing 
part of the field testing program. Observations of hydrologic and thermodynamic conditions in 
the drift itself and in the nearby rock lead to the following seepage-related findings: 
1. Ventilation effects in ventilated drift sections have a significant impact on seepage, 
mainly through evaporation of potential seepage water at the drift surface but also 
through the development of a dryout zone around the opening. 
2. Once ventilation effects are reduced or eliminated, the relative humidity of the initially 
dry in-drift environment generally increases in a short time period. 
3. The high relative humidity in closed-off drift section leads to conditions that facilitate 
in-drift moisture redistribution processes, which may be associated with condensation 
October 2003 3-13 No. 3: Water Seeping into Drifts Revision 2 
of water on surfaces, inducing dripping. These processes are likely to occur even in 
the presence of only small temperature gradients. 
4. The presence of water on nonporous surfaces and the chemistry and isotopic 
composition of water samples suggest that the water accumulations in the ECRB 
Cross-Drift originated from condensate. However, minor contributions from seepage 
water, masked by condensate, are possible based on currently available information. 
The observations in the nonventilated section of the ECRB Cross-Drift are presented in 
Appendices B and C. Appendix C also summarizes results from the tests in Alcove 7. 
Source: BSC 2003f, Figure 6.10.2-9. 
NOTE: Also plotted is the isotopic composition of construction water, two pore water samples extracted from core 
samples from Alcove 5 and the Global Meteoric Water Line. 
Figure 3-10. Plot of the Hydrogen and Oxygen Isotope Compositions of Water Samples Collected from 
October 2003 
the Enhanced Characterization of the Repository Block 
3-14 No. 3: Water Seeping into Drifts Revision 2 
3.3 COUPLED PROCESSES TESTING 
3.3.1 Drift Scale Test 
As discussed in Sections 2.4 and 2.6, seepage is partly determined by the properties of the 
fractured rock. The properties may be altered as a result of coupled effects induced by heating of 
the repository. For example, increased temperatures lead to mechanical deformations that 
change the hydrologic properties of the fracture network (BSC 2003g). Moreover, mineral 
dissolution and precipitation during the thermal period may temporarily or permanently affect 
the apertures of fractures in the immediate vicinity of the waste emplacement drifts (BSC 
2003h). These coupled effects during repository heating have been studied using numerical 
simulation and the DST, which is the largest underground thermal test carried out to date in the 
ESF. 
The purpose of the DST is to evaluate the coupled thermal, hydrologic, chemical, and 
mechanical processes that take place in unsaturated fractured tuff over a range of temperatures 
(approximately 25°C to 200°C). Details regarding the DST layout, borehole orientations, 
operation of the test, and measurements performed (as well as their uncertainties) are discussed 
in the Thermal Testing Measurements Report (BSC 2002b, Section 6.3) and Drift Scale Test 
As-Built Report (CRWMS M&O 1998). In brief, the DST consists of an approximately 
50-m-long drift that is 5 m in diameter. Nine electrical canister heaters were placed in this drift 
to simulate nuclear-waste-bearing containers. Electrical heaters were also placed in a series of 
horizontal boreholes (wing heaters) drilled perpendicularly outward from the central axis to both 
sides of the drift. These heaters were emplaced to simulate the effect of adjacent emplacement 
drifts. The DST heaters were activated on December 3, 1997, with a planned period of 4 years 
of heating, followed by 4 years of cooling. After just over 4 years, the heaters were switched off 
on January 14, 2002, and since that time the test area has been slowly cooling. Figure 3-11 
shows a schematic view of the test layout with the main heater tunnel, the wing heaters, and the 
array of observation boreholes monitoring temperature, as well as chemical, mechanical, and 
hydrologic variables. Data on the evolution of gas phase, liquid phase and solid phase 
compositions, changes in water content and air permeabilities, and rock deformations collected 
during the DST were used to validate the related coupled models on thermal, hydrologic, 
chemical, and mechanical processes, as discussed in Sections 4.6 and 4.8. 
No seepage has been observed during the DST. This observation is consistent with the effect of 
the vaporization barrier described in Section 1.5. However, the absence of observed seepage 
should not be directly used to evaluate seepage into repository drifts because the potential for 
seepage in the DST may have been reduced as a result of vapor losses through the bulkheads 
(BSC 2003b, Section 6.4.3.2). 
Property changes induced by elevated temperatures as inferred from direct observations and 
associated modeling of the DST were examined for their impact on seepage, as discussed in 
Sections 4.6 and 4.8. 
October 2003 3-15 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2002b, Figure 6.3-2. 
Note: ERP = electrical resistance probe; ESF = Exploratory Studies Facility 
Figure 3-11. Three-Dimensional Perspective of the As-Built Borehole Configuration of the Drift Scale 
October 2003 3-16 
Test 
3.3.2 Fracture Sealing Experiment 
A laboratory fracture sealing experiment was designed to evaluate the effects of condensate 
reflux through a fracture network and into a boiling environment. Two saw-cut blocks of welded 
rhyolite ash-flow tuff, measuring 31.7 cm tall, 16.2 cm wide, and 3.2 cm thick, were separated to 
create a vertical planar fracture. Heating was accomplished through the use of electrical 
resistance heaters mounted directly on the sides of the blocks. A vertical temperature gradient 
was established within the block assembly, with a temperature of 80°C at the top and 130°C at 
the bottom. Temperatures were monitored throughout the experiment. 
After steady-state chemical conditions were attained in a plug-flow reactor, a portion of the 
water generated from the reactor was flowed at a rate of 10.8 mL/hr into the top of the fracture. 
After 5 days, the fracture began to seal, as evidenced by a declining outflow rate, leaks in the 
inlet side of the fracture, and the need for increased pressure to maintain a constant inlet flow 
rate. The aperture was effectively sealed. 
No. 3: Water Seeping into Drifts Revision 2 
After cooling, the fracture was opened and examined to determine the location and nature of 
secondary mineral formation (Figure 3-12). The precipitate (identified as mainly amorphous 
silica) was deposited almost exclusively at temperatures exceeding 100°C. Bridging structures 
of amorphous silica that formed during the experiment in both the saw-cut fracture and a natural 
fracture appear to obstruct fluid flow within the fracture system. These same processes could 
result in the development of a precipitation cap above the heated drift, potentially reducing 
seepage (see also discussion in Section 4.8). The fast and complete sealing seen in this 
laboratory experiment may be a result of the fact that a single fracture has been tested. The high 
thermal gradient imposed on the sample may also have accelerated the sealing phenomenon. 
Source: BSC 2002b, Figure 7.3-2. 
NOTE: Scale bars are 0.5 mm. 
Figure 3-12. Bridging Structures (a) in an Opened Planar Fracture and (b) in a Crosscutting Fracture 
3.4 MINERAL DEPOSITIONS IN LITHOPHYSAL CAVITIES 
Observations of naturally occurring calcite and opal precipitation in lithophysal cavities (BSC 
2002c, Section 6.10.1; Paces et al. 2001; Marshall et al. 2003) can be used to estimate long-term 
seepage rates into these small openings. Calcite precipitates from downward-percolating 
meteoric water because of (1) evaporation, (2) CO2 outgassing as a result of the geothermal 
gradient, and (3) interaction with a gas phase containing less CO2 than the gas with which the 
water was last equilibrated. Considering these calcite-precipitation mechanisms and using 
certain water-to-calcite ratios, seepage into lithophysal cavities was estimated from 
calcite-deposition data. 
October 2003 3-17 No. 3: Water Seeping into Drifts Revision 2 
The calcite depositions on lithophysal cavity floors may not originate from dripping water 
(i.e., seepage); in fact, there is a lack of evidence of dripping from cavity ceilings (absence of 
stalactites or stalagmites at any scale), even where fractures containing coatings intersect 
lithophysae ceilings (Whelan et al. 2002, p. 744). This observation is consistent with the 
capillary barrier concept. Even if making the assumption that all calcite deposited in lithophysal 
cavities originated from seepage, the data indicate that (1) not all lithophysal cavities 
encountered seepage, and (2) seepage flux derived from mineral deposits is a very small fraction 
of percolation flux. Both conclusions corroborate the general concept of a capillary barrier 
reducing seepage below the value of the percolation flux. 
Additional discussion of the analysis of secondary minerals in Appendix C. 
3.5 ABSENCE OF SEEPAGE IN VENTILATED DRIFTS 
So far, no dripping of natural percolation water has been observed in any of the ventilated 
openings at Yucca Mountain (Trautz and Wang 2002). While this finding can be considered 
consistent with the current understanding of the capillary diversion capacity of the fractured 
formation, the absence of seepage can also be explained by evaporation effects. The significant 
evaporation potential of the dry drift atmosphere is evident not only from theoretical 
considerations, but also from temporal observations of wet spots observed at the drift ceiling 
during liquid-release tests conducted in the ECRB Cross-Drift (see discussion of Figure 3-3 
above) and the fact that a damp looking feature observed immediately after the dry excavation of 
Niche 1 (also referred to as Niche 3566) dried up before a bulkhead could be installed to increase 
the relative humidity in the opening. The damp looking feature was observed along a vertical 
wall; that is, it did not induce seepage. The feature did not reappear after sealing the niche with a 
bulkhead. 
Because the evaporation potential in ventilated openings at Yucca Mountain evidently exceeds 
the seepage potential, an effort has been made to maximize relative humidity in the niches 
designated for seepage testing (see Section 3.1.3) in Alcove 7 and in the ECRB Cross-Drift (see 
Section 3.2). These tests and the related modeling studies confirm the impact of evaporation on 
the determination of seepage. 
3.6 NATURAL ANALOGS 
Natural analogs, such as those reported in Natural Analogue Synthesis Report (BSC 2002d, 
Section 8), provide evidence that the concept of water exclusion from underground openings is 
consistent with the process that actually occurs in caves, lava tubes, rock shelters, and buildings. 
The qualitative evidence for water exclusion and flow diversion was substantiated by 
quantitative seepage measurements in limestone caves. These studies show that seepage is 
considerably smaller than the pertinent percolation flux (BSC 2002d, Section 8.2), corroborating 
the seepage testing and modeling results at Yucca Mountain. Calcite-deposition data in 
lithophysal cavities (see Section 3.4) can be considered a natural analog at Yucca Mountain itself 
that further corroborates the concept. 
October 2003 3-18 No. 3: Water Seeping into Drifts Revision 2 
4. SEEPAGE MODELING AND ABSTRACTION 
4.1 PURPOSE AND GENERAL APPROACH 
A number of conceptual, analytical, and numerical models have been developed to help 
understand seepage-related processes and phenomena and to determine site-specific, 
seepage-relevant formation parameters. Some of the models are concerned with simulating 
in situ seepage tests (BSC 2003e), while others were developed for the prediction of seepage into 
intact and degraded waste emplacement drifts under ambient and thermal conditions (BSC 2003i; 
BSC 2003h; BSC 2003a). These drift-scale seepage models are linked to other models on 
different scales, specifically the mountain-scale model of the unsaturated zone at Yucca 
Mountain (BSC 2003c), models considering the coupled thermal-hydrologic-chemical (BSC 
2003h) and thermal-hydrologic-mechanical (BSC 2003g) processes, scale-transition models 
providing flow-focusing factors (BSC 2001b, Section 6.4.2), as well as small-scale models 
evaluating flow within individual fractures (BSC 2003h). Quantitative and qualitative results 
from these process models are used in the seepage abstraction step (BSC 2003a) to arrive at a 
reasonable approach to handling seepage in TSPA calculations. 
The general approach followed to analyze data from seepage experiments conducted at Yucca 
Mountain and to arrive at a calibrated and validated predictive seepage model is based on the 
recognition that (1) detailed simulation of individual seeps is not necessary to estimate average 
seepage rates into waste emplacement drifts, (2) certain factors affecting seepage can be lumped 
into effective parameters, (3) calibrating and validating a model against data from seepage 
experiments provides confidence that the model captures the relevant processes, (4) estimating 
effective parameters partly compensates for processes and features that are not explicitly 
considered in the model, and (5) the estimated parameters can be directly used in a predictive 
model that uses a consistent conceptualization. 
The main advantage of this approach is that it relies directly on seepage-rate data, which 
inherently contain information about the relevant processes. Also, the calibration data (seepage 
rates on the scale of a drift section) are very similar to the quantity of interest for the subsequent 
predictions. The consistency between the calibration model used to derive seepage-relevant 
parameters and the predictive model used to forecast seepage minimizes conceptual differences. 
The appropriateness of the selected method and its potential advantages over alternative 
approaches are further discussed in Appendix D. 
As discussed above, the seepage models are based on data showing seepage-relevant processes 
under in situ conditions; these data were described in Section 3. Some of these data were 
directly used to develop and calibrate the models. For example, air permeabilities (see 
Section 3.1.2) were used to generate and condition heterogeneous permeability fields for the 
seepage model. They were also used to derive statistical measures representing uncertainty and 
variability of this important, seepage-relevant parameter (see Section 4.9). Evaporation-rate and 
relative-humidity data (see Section 3.1.3) were used to determine the evaporative boundary layer 
thickness, which is part of a submodel used for the interpretation of seepage tests conducted in 
the ventilated ECRB Cross-Drift. Seepage-rate data measured during liquid-release tests (see 
Section 3.1.4) were the main data used for calibration of a drift-scale seepage model. Additional 
seepage-rate data were used for testing the capability of the calibrated model to make predictions 
October 2003 4-1 No. 3: Water Seeping into Drifts Revision 2 
under different conditions and at different locations. Data from the DST were used to 
corroborate the conceptual model supporting the abstraction of thermal-hydrologic-mechanical 
and thermal-hydrologic-chemical effects (see Sections 4.6 and 4.8 below). 
The development of the seepage models involves the following steps: 
1. Geostatistical parameters of the permeability field are determined from the results 
of air-injection test data. 
2. Multiple realizations of the permeability field are generated, each of which is 
consistent with the geostatistical properties of the measured air permeabilities 
representing the excavation-disturbed zone in the drift vicinity and are mapped onto 
the computational grid. 
3. A numerical model is developed for the simulation of liquid-release tests conducted 
in niches and along the ECRB Cross-Drift. 
4. Seepage-relevant, model-related capillary-strength parameters are determined by 
calibrating the models against data from liquid-release tests. 
5. The model is tested by comparing predicted seepage rates to observed data from 
seepage experiments not used for model calibration. This step provides confidence 
that the model is capable of predicting the seepage behavior above and below the 
seepage threshold. 
6. A conceptually consistent model is developed for simulating seepage into waste 
emplacement drifts. Drift seepage is evaluated for ranges of percolation flux and 
other seepage-relevant hydrogeologic properties. 
7. Sensitivity analyses are performed to examine the impact of drift degradation, 
ground support, above-boiling conditions during the thermal period, and 
thermal-hydrologic-chemical effects. 
8. The results from Steps 6 and 7, along with other information on uncertainty and 
variability, are combined in the abstraction, providing the basis for a probabilistic 
evaluation of seepage in TSPA calculations. 
Issues related to the general modeling approach are addressed in Appendix D. 
The following sections describe the drift-scale models developed to estimate seepage-relevant 
formation parameters and to make predictive seepage calculations for TSPA. 
4.2 CONCEPTUAL AND NUMERICAL MODELS 
The drift-scale numerical seepage models consider three-dimensional flow through the 
unsaturated fracture network and seepage into the niches or the ECRB Cross-Drift. The 
fractured rock is conceptualized as a heterogeneous continuum with effective fracture 
permeabilities assigned to each grid block. A number of processes affecting seepage (such as 
film flow along the drift surface, impact of drift-wall roughness, and small-scale effects of 
October 2003 4-2 No. 3: Water Seeping into Drifts Revision 2 
discrete fractures terminating at the drift ceiling) are not explicitly modeled but are lumped into 
an effective capillary-strength parameter determined by calibrating the model against 
seepage-rate data, which contain the relevant information about these effects. This 
conceptualization and approach are considered a suitable basis for a model designed to make 
predictions of seepage averaged over a drift section of the length of a waste package. 
The numerical process model used for reproducing seepage-rate data from liquid-release tests 
and to predict seepage into waste emplacement drifts solves the Richards equation (Richards 
1931) for saturated-unsaturated flow through porous materials. The van Genuchten-Mualem 
constitutive relations (van Genuchten 1980) describe the capillary pressure and relative liquid 
permeability in the fracture continuum as a function of liquid saturation. This approach captures 
the main driving forces (gravity, viscous, and capillary forces) and physical phenomena (phase 
interference) relevant for modeling unsaturated flow and seepage in a geologic medium. 
To appropriately include small-scale heterogeneity, the spatial structure of the air-permeability 
data was analyzed, and the resulting geostatistical parameters were used to generate multiple 
realization of a spatially correlated permeability field, which were conditioned on the 
permeabilities measured in borehole intervals. The permeability fields were eventually mapped 
onto the numerical grid; an example is shown in Figure 4-1. The numerical grids created for the 
simulation of liquid-release tests represent an appropriate section of the formation around the 
injection interval, including the underground opening. 
Evaporation from the drift surface is accounted for by specifying a time-dependent 
water-potential boundary condition based on Kelvin’s equation, where the thickness of the 
diffusive boundary layer was determined from evaporation experiments at the seepage test 
location. 
October 2003 4-3 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003e, Figures 15b and 14. 
NOTE: In Figure 4-1a, the mesh is split into two parts to expose the boreholes (indicated by a thick black lines) and 
the injection interval (thick white line). 
Figure 4-1. Examples of the Numerical Grid and One Realization of the Underlying Heterogeneous 
Permeability Field for the Simulation of Liquid-Release Tests (a) in a Niche, and (b) in the 
October 2003 
Enhanced Characterization of the Repository Block 
Although the permeability structure comprises a network of discrete fractures, the continuum 
approach is appropriate for simulating unsaturated flow and seepage because it is capable of 
predicting observed seepage behaviors and seepage rates. In a network of randomly oriented 
fractures, flow diversion around drift openings occurs primarily within fracture planes. 
Diversion of water through multiple fractures arises only if a fracture is too short and the flow 
path within a fracture plane is interrupted. In this case, water is diverted into the next connected 
fracture, if available. This fracture is again unlikely to be perfectly parallel to the drift axis, 
allowing the in-plane flow-diversion process to continue. The situation is schematically 
illustrated in Figure 4-2, which shows two fractures intersected by a drift. In Figure 4-2a, the 
two fractures are aligned with the drift axis. As an artifact of this specific and unlikely fracture 
orientation, in-plane flow diversion is prevented, and the resulting impact of discreteness on 
seepage is accentuated. In Figure 4-2b, the fractures are approximately perpendicular to the drift 
axis. Flow diversion occurs within the fracture plane, a process that is appropriately captured by 
4-4 No. 3: Water Seeping into Drifts Revision 2 
a heterogeneous fracture continuum model even for a single fracture (which can be considered 
most discrete case). In-plane flow occurring in multiple fractures can be readily combined and 
described by an effective fracture continuum. Given the significance of in-plane flow diversion 
around the drift in combination with relatively high fracture density, a three-dimensional, 
heterogeneous fracture continuum model is an appropriate conceptualization and is, thus, used as 
the basis for the numerical models developed to analyze seepage data from liquid-release tests 
and to predict seepage into waste emplacement drifts. The appropriateness of using a continuum 
model for seepage prediction and its comparison to an approach based on a discrete fracture 
network model are further discussed in Appendix D. 
NOTE: (a) A two-dimensional fracture network model assumes that all fractures are parallel to the drift axis, 
perpendicular to the drift axis. 
preventing flow diversion within the fracture plane; (b) a two-dimensional (and three-dimensional) fracture 
continuum model considers flow diversion occurring within multiple fracture planes that are approximately 
October 2003 
Figure 4-2. Schematic Showing Two Fractures Intersecting a Drift 
In summary, the process model described above captures the forces, processes, and features 
relevant for simulating unsaturated flow and seepage into underground openings at Yucca 
Mountain. The processes are captured either explicitly by solving the physically based 
governing equations or implicitly through an effective parameter that is determined from data 
that include the respective seepage process. The degree of complexity of the model is 
appropriate for its purpose, (i.e., for the prediction of average seepage on the scale of an 
approximately 5-m-long drift segment). Small-scale features and processes, including discrete 
fracture-flow behavior, surface roughness, and film flow, are captured in effective model-related 
parameters determined from site-specific data that reflect the seepage process on the appropriate 
scale (see Section 4.3). Calibration and validation against seepage-rate data and the consistent 
conceptualization in the downstream models make this a valid and reasonable approach to 
characterizing and predicting seepage at Yucca Mountain. 
4-5 No. 3: Water Seeping into Drifts Revision 2 
4.3 MODEL CALIBRATION AND VALIDATION 
Models of niches and the ECRB Cross-Drift (summarily referred to as the seepage calibration 
model) are calibrated against late-time seepage-rate data from numerous liquid-release tests 
conducted in several boreholes at locations in both the middle nonlithophysal zone and the lower 
lithophysal zone (see Figure 3-1). Early-time seepage data are discarded because they are 
affected by storage effects and the properties of a few fractures connecting the injection interval 
with the opening. These fractures are not necessarily representative of the fracture network that 
is activated in flow diversion around the entire opening under steady-state conditions. Late-time 
data are more representative of near-steady conditions and are less influenced by storage effects. 
Moreover, the relatively large amount of released water at late time has likely encountered a 
larger portion of the capillary barrier. Consequently, the late-time seepage data better reflect 
average conditions on the scale of interest. 
Eighty-one liquid-release tests conducted in Niches 2, 3, 4, and 5 as well as the systematic 
testing area in the ECRB Cross-Drift were simulated with the seepage calibration model, a 
numerical process model that captures transient unsaturated flow through a heterogeneous 
fracture continuum and seepage into the underground openings. In addition, evaporation effects 
were accounted for when deemed significant (i.e., when simulating seepage into the open, 
ventilated ECRB Cross-Drift). The simulated saturation distribution at the end of liquid-release 
tests conducted in Niche 5 and the ECRB Cross-Drift are depicted in Figure 4-3. 
Source: BSC 2003e, Figures 26a and 21d. 
Figure 4-3. Saturation Distributions at the End of a Liquid-Release Test Conducted in (a) Niche 5 and 
(b) the Enhanced Characterization of the Repository Block as Simulated with the Calibrated 
Seepage Process Model 
No. 3: Water Seeping into Drifts October 2003 4-6
Revision 2 
Measured seepage-rate data from 22 liquid-release tests performed in boreholes above Niches 3, 
4, and 5 and along the ECRB Cross-Drift were used to calibrate the seepage calibration model 
and to estimate the seepage-relevant, model-related van Genuchten capillary-strength 
parameter 1/á. The remainder of the seepage-rate data was used to validate the seepage 
calibration model, (i.e., to determine whether it is appropriate and adequate for its intended use). 
The choice of the tests used for calibration and validation is discussed in Seepage Calibration 
Model and Seepage Testing Data (BSC 2003e, Section 6.6.3.2) 
Examples of calibrated liquid-release tests conducted along the ECRB Cross-Drift and in 
Niche 5 are shown in Figure 4-4. For the tests conducted along the ECRB (Figure 4-4a, b, 
and c), a significant component of the fluctuations in both the simulated and observed seepage 
rates are a result of variations in the relative humidity and, thus, evaporation potential in the 
ventilated drift; the effect is appropriately captured by the model. No such fluctuations are 
observed in the Niche 5 test, where relative humidity was approximately constant at 85 percent. 
Figure 4-5 shows examples of validation runs, in which seepage rates for liquid-release tests 
conducted in Niche 3 were predicted with the calibrated model using a single parameter set and 
compared to measured data that had not been used for model calibration. Linear error 
propagation analysis yielded the uncertainty of the model prediction as a result of uncertainty in 
the input parameters. The data fall within the range of predicted seepage rates. Based on the 
acceptance criteria outlined in Seepage Calibration Model and Seepage Testing Data (BSC 
2003e, Section 7.2), seven of the eight test predictions are considered acceptable. In 
Test UM 4.88–5.18, the observed late-time seepage rates are slightly larger than the relatively 
narrow uncertainty band. The prediction would be considered acceptable if the chosen 
uncertainty in the input parameters were marginally increased (e.g., to account for uncertainty as 
a result of the stochastic nature of the underlying heterogeneous permeability field). This 
uncertainty will be included in TSPA calculations. 
October 2003 4-7 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003e, Figures 19, 22, 23, and 25. 
NOTE: The gray line is the measured release rate, which is approximated in the model by the black line. Blue 
symbols represent measured seepage-rate data; the red line is calculated with the calibrated models. The 
green line is the relative humidity used to prescribe the evaporation boundary condition. Relative humidity in 
Niche 5 was constant at approximately 85 percent. 
Figure 4-4. Calibration of Seepage-Rate Data from Liquid-Release Tests in Zone 2 of Boreholes 
(a) SYBT-ECRB-LA#1, Zone 2, (b) SYBT-ECRB-LA#2, Zone 2, (c) SYBT-ECRB-LA#2, 
Zone 3, and (d) Borehole #4 in Niche 5 
October 2003 4-8 No. 3: Water Seeping into Drifts Source: BSC 2003e, Figure 43. 
NOTE: Linear uncertainty propagation analysis was used to calculate the uncertainty band of the model predictions. 
Figure 4-5. Validation of Seepage Model and Tptpmn Seepage-Relevant Parameters Using Data from 
Niche 3 
No. 3: Water Seeping into Drifts 
Revision 2 
October 2003 4-9
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The capillary-strength parameter for each tested borehole interval was determined by calibrating 
the model against multiple tests using different liquid-release rates. Some of these release rates 
induced a local percolation flux that was above the seepage threshold (i.e., water dripped into the 
opening and yielded seepage-rate data valuable for calibration). However, tests were also 
performed below the seepage threshold. Moreover, the model was validated against tests 
conducted above and below the seepage threshold. The system was probed and the model was 
validated for the critical range of percolation rates about the seepage threshold. Seepage 
predictions for natural percolation fluxes that are even less than the low fluxes (below the 
seepage threshold) induced during the low-rate tests will yield the correct result, namely, 
zero seepage. A high-infiltration climate or strong flow focusing can cause the natural local 
percolation flux to exceed the seepage threshold. 
In summary, the capillary strength parameters determined from relatively high-rate liquid-release 
tests provide a solid basis for seepage predictions under low and high natural percolation fluxes. 
The validation approach demonstrates that the seepage process model is capable of predicting the 
behavior above and below the seepage threshold. 
The analysis of seepage-rate data for the determination of seepage-relevant parameters can be 
summarized as follows: 
1. The testing and modeling approach is considered adequate for providing the 
conceptual basis and parameters for predictive seepage models. The approach consists 
of analyzing seepage by means of a numerical process model that is calibrated against 
seepage-rate data from liquid-release tests conducted within the repository host units. 
2. The estimation of seepage-relevant, model-related, effective parameters on the scale of 
interest is an appropriate methodology provided that the structure of the prediction 
model (see Sections 4.4 and 4.5) is consistent with the model used for calibration (the 
seepage calibration model). 
3. Seepage properties are spatially variable. The variability has been examined by 
performing liquid-release tests at various sites along the ESF and the ECRB. Spatial 
variability in the estimated van Genuchten capillary-strength parameter á / 1 is 
relatively large compared to the estimation uncertainty at a given location. The main 
contribution to the estimation uncertainty is small-scale heterogeneity that can only be 
described stochastically. Random fluctuations in seepage-rate data lead to 
insignificant uncertainty in the parameter estimates. 
4.4 PREDICTION OF AMBIENT SEEPAGE 
While the seepage calibration model described in Section 4.3 simulates liquid-release tests and 
seepage into niches and the ECRB for calibration and validation purposes, the seepage model for 
performance assessment was developed to predict total seepage into a section of a waste 
emplacement drift under ambient percolation conditions (BSC 2003i). Isothermal flow 
simulations were performed over wide ranges of selected key parameters. The key parameters 
affecting ambient seepage are the effective capillary-strength parameter, the reference 
permeability, and the local percolation flux imposed at the upper model boundary. Conceptually 
October 2003 4-10 No. 3: Water Seeping into Drifts Revision 2 
consistent with the seepage calibration model, the predictive seepage model for performance 
assessment is a three-dimensional drift-scale model, employing a stochastic continuum 
representation of the small-scale heterogeneity of fractured rock in the drift vicinity. Applying a 
percolation flux at the top of the model, the steady-state seepage flux is obtained. As an 
example, Figure 4-6 shows the resulting saturation distribution for one realization. The 
calculation is repeated for different parameter combinations and different realizations of the 
underlying stochastic permeability field. Results are provided in the form of a lookup table, that 
gives seepage rates and related seepage estimation uncertainty as a function of these key 
parameters. During a probabilistic TSPA calculation, values of input parameters will be sampled 
from their respective probability distributions (see Section 4.6), and the corresponding seepage 
rate will be extracted from the lookup table. Sensitivity analyses were performed to examine the 
impact of the stochastic parameters (standard deviation and correlation) describing the 
small-scale heterogeneity of the permeability field. 
Source: BSC 2001b, Figure 6.4-8. 
NOTE: The development of a shadow zone is shown. 
Figure 4-6. Simulated Liquid Saturation Distribution in Heterogeneous Fracture Continuum in the 
October 2003 
Vicinity of a Waste Emplacement Drift 
4-11 No. 3: Water Seeping into Drifts Revision 2 
The seepage model assumes that the relative humidity within the drift is constant and 
100 percent. This assumption is appropriate for times significantly following the cessation of 
ventilation that steady state humidities in the drift are appropriate. This assumption effectively 
forces all incoming moisture into the drifts to be liquid water (i.e., seepage). Observations made 
in the ECRB (presented in Section 3.2) described the possible effects associated with transient 
moisture flux from the drift wall into the drift and the subsequent redistribution of that moisture 
due to small temperature differences along the drift. 
The systematic simulations performed by the seepage model performance assessment cover a 
wide range of capillary-strength values 1/á (from 100 Pa to 1,000 Pa), mean permeability values 
log10(k[m2]) (from -14 to -10), and local percolation flux (from 1 to 1,000 mm/yr). For each 
parameter combination, 20 realizations of the heterogeneous permeability fields were simulated. 
The range of results from the 20 realizations provides information about the estimation 
uncertainty in the predicted seepage rates because of uncertainty in the stochastic small-scale 
heterogeneity. 
Example results from the seepage model performance assessment are illustrated in Figure 4-7. 
The figure gives contours of the simulated seepage percentage into a drift with a diameter of 
5.5 m as a function of the capillary-strength parameter 1/á and the mean fracture permeability 
for selected percolation fluxes between 1 mm/yr and 1,000 mm/yr. (These percolation fluxes are 
local; they include potential flow focusing effects.) The seepage percentage (the ratio between 
the seepage rate and percolation flux over the drift area) indicates the flow diversion capability 
of the capillary barrier at the drift surface. As expected, the seepage percentage is large for small 
capillary strength, small permeability, and large percolation flux. In these cases, seepage may 
approach 100 percent (i.e., there is no flow diversion at the drift wall), and the entire percolation 
flux seeps into the drift. In contrast, the seepage percentage is small for all cases with strong 
capillarity, large permeability, and small percolation flux. In many of these cases, there is no 
seepage at all; the entire percolation flux is diverted around the drift by capillary forces because 
the percolation flux is below the seepage threshold for the given parameters. 
October 2003 4-12 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003i, Figure 6-8. 
Figure 4-7. Mean Seepage Percentage as a Function of Capillary-Strength Parameter and Mean 
Permeability for a Percolation Flux of 1,10, 50, 200, 400, 600, 800, and 1,000 mm/yr 
4.5 PREDICTION OF SEEPAGE UNDER THERMAL CONDITIONS 
A thermal-hydrologic seepage model (BSC 2003b) was developed to evaluate the coupled 
thermal-hydrologic processes; and their impact on seepage; in the vicinity of waste emplacement 
drifts during the heating phase of the repository. This drift-scale process model is designed to 
analyze the combined effect of the two barriers that may prevent seepage into drifts at elevated 
temperatures: (1) the capillary barrier, which is independent of the thermal conditions, and 
(2) the vaporization barrier (see Section 1.5), which is effective while temperature is elevated 
above the boiling point. While incorporating the conceptual framework for ambient seepage, the 
thermal-hydrologic seepage model accounts for important flow and energy transport processes in 
response to the emplacement of heat-generating waste, including the movement of both gaseous 
and liquid phases, transport of latent and sensible heat, phase transition between liquid and 
vapor, and vapor pressure lowering. The fractured rock is treated as a dual-permeability domain, 
accounting for the fractures and the rock matrix as two separate, overlapping continua. The 
active fracture model (Liu et al. 1998, p. 2,636) is employed to account for the fact that 
October 2003 4-13 No. 3: Water Seeping into Drifts Revision 2 
unsaturated flow may be restricted to a limited number of (active) fractures and that flow within 
a fracture is likely to be channelized. A stochastic continuum model is implemented for fractures 
near the drift that considers the small-scale variability of permeability to account for flow 
channeling. The capillary-strength parameter close to the drift wall was derived from the 
properties provided by the calibration against seepage-rate data (see Section 4.3). 
Transient simulations in a two-dimensional cross section (extending from the ground surface to 
the water table; see Figure 4-8) were performed explicitly to calculate percolation to the drift 
during the heating phase of the repository, and to calculate directly transient seepage rates into 
the drift under elevated temperature conditions. Relevant parameters varied in the evaluation of 
thermal seepage are the thermal operating mode, the local percolation flux, and selected rock 
properties. Results of this model were used in the seepage abstraction to develop an appropriate 
methodology of adjusting the seepage model performance assessment results to account for 
thermally perturbed conditions. 
The thermal-hydrologic seepage model was validated by comparison with in situ heater tests 
conducted at Yucca Mountain (BSC 2003b, Section 7), with particular focus on the DST. The 
model validation included quantitative evaluation of continuously measured temperature data 
with a detailed analysis of subtle temperature signals indicative of thermal-hydrologic coupling, 
as well as qualitative evaluation of periodic measurements that monitored moisture redistribution 
processes using geophysical methods, air-injection data, and withdrawal of liquid water in 
packed-off boreholes. 
The thermal hydrologic seepage model predictions regarding the effectiveness of the 
vaporization barrier were also tested by comparison with an alternative conceptual model of 
water flow in the superheated rock environment (BSC 2003b, Section 6.3). In this alternative 
model, the thermally perturbed downward flux from the condensation zone toward the 
superheated rock zone is conceptualized to form episodic preferential-flow events (finger flow). 
The effectiveness of the vaporization barrier was then tested for the extreme conditions where 
downward flux in small fingers is fast and large in magnitude compared to average flow. 
A semi-analytical solution (Birkholzer 2003) was employed to simulate the complex flow 
processes of episodic finger flow in a superheated fracture. With this solution, the maximum 
penetration distance into the superheated rock was determined for specific episodic flow events 
and thermal conditions, and the amount of water arriving at the drift crown was calculated. It 
was shown that finger flow is not likely to penetrate through the superheated rock during the first 
several hundred years of heating, when rock temperature is high and boiling conditions exist in a 
sufficiently large region above the drifts. Only later, when the boiling zone is small and the 
impact of vaporization is limited, may channelized water arrive at the drift crown. This issue is 
further discussed in Appendix A. 
The thermal-hydrologic seepage model was applied to simulate the thermal-hydrologic coupled 
processes for a period of 4,000 years after waste emplacement. A series of selected simulation 
cases was conducted for the Tptpmn and Tptpll submodels, comprising different thermal loads, 
various percolation flux scenarios, different capillary-strength parameters, and different 
conceptual models for fracture-matrix interaction (see overview of simulation cases in BSC 
2003b, Section 6.2.1.6). The simulation results relevant for seepage abstraction are briefly 
discussed below. 
October 2003 4-14 No. 3: Water Seeping into Drifts Revision 2 
Source: BSC 2003b, Figure 6.2.1.2-1. 
NOTE: The emplacement drift is located in the middle nonlithophysal zone Tptpmn, which is bounded by the upper 
lithophysal (Tptpul) and lower lithophysal (Tptpll) zones. 
Figure 4-8. Example of Numerical Grid for the Thermal-Hydrologic Seepage Model 
The predicted thermal-hydrologic conditions are strongly driven by the thermal load placed into 
the drifts and by the local percolation flux. For the reference thermal mode, the heat generated 
from the waste canisters results in maximum rock temperatures at the drift wall between 120°C 
and 140°C, depending on the hydrogeologic unit and the amount of percolation considered. The 
period of above-boiling rock temperature is about 1,000 years for the base-case flux scenario, 
and rock temperature at the end of the simulation period is still as high as 65°C. Elevated 
percolation leads to cooler temperatures and a shorter boiling period with stronger flux activity 
(liquid-vapor counter flow in the boiling-temperature zone). 
October 2003 4-15 No. 3: Water Seeping into Drifts Revision 2 
In general, seepage under thermal conditions (thermal seepage) is possible only when (a) water 
arrives at the drift wall (depending on the vaporization impact), and (b) the saturation at the drift 
wall exceeds a given threshold value, defined by the capillary barrier effect at the rock-drift 
interface. The modeling results consistently demonstrate that the thermal perturbation of the 
flow field, causing increased downward flux from the condensation zone toward the drifts, is 
strongest during the first few hundred years after closure, corresponding to the time period when 
rock temperature is highest and the vaporization barrier is most effective. Even for high 
percolation fluxes into the model domain and strong flow channeling as a result of fracture 
heterogeneity, water cannot penetrate far into the superheated rock during the time that rock 
temperature is above boiling. Thus, the potential for seepage is small. The majority of the 
mobilized water is diverted around the dryout zone and drains away from the drift, as shown in 
Figure 4-9a. At the time when temperature has returned to below-boiling conditions (see 
Figure 4-9b), fractures start to be rewetted at the drift wall. However, while the vaporization 
barrier has become less effective, the capillary barrier at the drift wall will continue to reduce (or 
prevent) water seepage into the drift. 
Transient seepage rates were explicitly calculated by the thermal-hydrologic seepage model to 
quantify the potential for seepage during the thermally perturbed time period. These transient 
seepage rates were compared with results from ambient (steady-state) simulations to evaluate the 
vaporization barrier. Example results illustrating the evolution of thermal seepage for 
one realization and a flow-focusing factor of 10 are given in Figure 4-10. No water arrives at the 
drift during the boiling period (i.e., even without heating of the repository). The capillary barrier 
at the drift wall is predicted to be fully effective during the first 600 years after waste 
emplacement. As rock temperature decreases during cool down and the first stepwise change in 
infiltration occurs at 600 years, the saturation values build up strongly. The change to 
monsoonal climate at 600 years (from 60 to 160 mm/yr) brings the saturation up to the threshold 
value for seepage at about 1,400 years after emplacement while still in the monsoon climate 
period. With the stepwise increase of infiltration at 2,000 years, the seepage percentage 
increases strongly, and water starts to seep at a second location along the drift wall in the model. 
At the end of the simulation period, the thermal seepage percentage is at about 17 percent, only 
slightly smaller than the long-term ambient value of 19.5 percent for a 250 mm/yr percolation 
flux. This provides additional confidence because as two simultaneously acting barriers prevent 
seepage. 
October 2003 4-16 No. 3: Water Seeping into Drifts Source: BSC 2003b, Figures 6.2.2.2-3a and 6.2.2.2-4a. 
NOTE: SL = liquid saturation. 
Figure 4-9. Fracture Saturation and Liquid Flux for Tptpmn Submodel with Heterogeneous Permeability 
Field at (a) 100 Years and (b) 1,000 Years 
4-17 No. 3: Water Seeping into Drifts 
Revision 2 
October 2003
Revision 2 
Source: BSC 2003b, Figure 6.2.2.2-7b. 
Figure 4-10. Seepage Percentage for Tptpmn Submodel at Reference Thermal Mode and Tenfold 
October 2003 4-18 
Percolation Flux 
The different simulation cases studied with the thermal-hydrologic seepage model show 
considerable variability with respect to the thermal-hydrologic conditions in the rock. Despite 
this variability, there were several important observations with respect to thermal seepage that 
are common to all cases (BSC 2003b, Sections 6.2.4 and 8.1): 
1. Thermal seepage was never observed in simulation runs where the respective ambient 
seepage was zero. 
2. Thermal seepage never occurred during the period of above-boiling temperatures in 
the rock close to the emplacement drifts. 
3. In simulation cases where ambient seepage was obtained, thermal seepage was 
initiated at several hundred to a few thousand years after rock temperature has returned 
below boiling. 
4. Thermal-seepage rates were always smaller than the respective ambient reference 
values. Thus, the ambient seepage values provide an as