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1. MAIN FINDINGS

The Performance Assessment Peer Review Panel (hereinafter "the Panel") conducted a phased review over a two-year period to observe the development and, ultimately, to review the Viability Assessment of a Repository at Yucca Mountain: Total System Performance Assessment, Volume 3 (DOE 1998a; hereinafter "TSPA-VA") that was conducted in support of the Viability Assessment of a Repository at Yucca Mountain (DOE 1998b; hereinafter "VA"). During the development of the TSPA-VA (DOE 1998a), the Panel submitted three Interim Reports (Whipple et al. 1997a, 1997b, and 1998) to the Management and Operating Contractor (M&O) with recommendations and comments on the process models, model abstractions, and draft documentation for the TSPA-VA (DOE 1998a).

The Panel’s final report on the TSPA-VA (DOE 1998a), Final Report Total System Performance Assessment Peer Review Panel (Whipple et al. 1999; hereinafter "the Final Report") is based primarily on the completed TSPA-VA (DOE 1998a), the Total System Performance Assessment-Viability Assessment (TSPA-VA) Analyses Technical Basis Document (CRWMS M&O 1998; hereinafter "TSPA-VA TBD"), and references cited in these reports. The Final Report (Whipple et al. 1999) includes the major points from the three Interim Reports (Whipple et al., 1997a, 1997b, and 1998), updated where appropriate, as well as new findings that the Panel developed during its review of the TSPA-VA (DOE 1998a).

The Panel provided a thorough and thoughtful review of the TSPA-VA (DOE 1998a). The Panel’s overarching strategy for improving the TSPA-VA (DOE 1998a) analysis is consistent with the U.S. Department of Energy’s (DOE) approach for developing the postclosure safety case for the potential Site Recommendation/License Application (SR/LA). In contrast, the Panel’s specific comments on the limitations of the component models and the associated databases were provided in the context of the TSPA-VA (DOE 1998a) and the VA design (DOE 1998b). The M&O has evaluated these comments in the context of the ongoing License Application Design Selection (LADS) process and efforts to re-prioritize future work. The relative importance of the Panel’s specific comments will depend on the DOE’s selection of a design for SR/LA and the evolution of the DOE’s postclosure safety case. The M&O will consider these specific comments for the development of the TSPA-SR/LA in the context of the design that is selected for SR/LA, if the site is found suitable, and the evolving postclosure safety case.

The following sections document the comments provided in the Final Report (Whipple et al. 1999) and the M&O response to these comments. The responses were based on input from a number of scientists from the M&O, several national laboratories, and the U.S. Geological Survey1. The comments and responses are organized to be consistent with the major headings in the Final Report (Whipple et al. 1999). Excerpts from the Final Report (Whipple et al. 1999) are in italics; the M&O responses to the comments are in straight text.

1.1 RELIABILITY OF THE TSPA-VA RESULTS

Comment 1 (pages 15–16): Because of the inadequacy of the supporting evidence, the Panel could not confirm whether a number of the TSPA-VA component models are representative of the systems, components, and processes they were designed to simulate. In addition, several of the component models are likely to be conservative and others non-conservative. For these reasons, it is unlikely that the TSPA-VA, taken as a whole, describes the long-term probable behavior of the proposed repository....

The objective for the TSPA-VA was to assess the probable behavior of the repository. In contrast, the objective for the TSPA-LA will be to determine whether it can be shown with reasonable assurance that the repository complies with the applicable regulatory limits. These are significantly different objectives, and recognition of this distinction should be an important element of a path forward to the TSPA-LA. This issue is discussed in more detail in Section III.

Response: The M&O does not agree with the first part of this comment. The TSPA-VA (DOE 1998a) was not meant to be a definitive prediction of probable repository behavior. Although the M&O agrees that the representations of the various component models are uneven in their degree of maturity, the analyses have incorporated the available scientific data and the VA design (DOE 1998b) concepts for the repository and the waste package. The M&O has also incorporated the most current understanding of key processes affecting the long-term behavior of the potential repository system. In this description of the probable behavior of the repository system, the M&O acknowledges that these evaluations are uncertain. Several factors lead to this uncertainty, including:

Although these uncertainties are recognized in the analyses and the interpretations of these analyses, they do not detract from the goal to objectively evaluate how the system is likely to behave. For the TSPA-VA (DOE 1998a), the M&O completed a deterministic case and a fully probabilistic case. Multiple realizations were used to define the range in the dose rates. The M&O believes that TSPA-VA (DOE 1998a) analyzed the long-term probable behavior of the repository system and that this behavior has likely been captured within the range of results presented. That range, however, is large. The goal for the upcoming cycle of TSPAs is to decrease the range of values and to gain more confidence in the median of the distribution.

The M&O agrees with the second part of this comment. The objectives of the TSPA-LA will be more stringent, focusing on demonstrating compliance with regulatory requirements. The M&O also agrees with the Panel’s suggested approach for a path forward towards a potential LA. This approach (Whipple et al. 1999) includes:

The M&O is implementing the suggested steps for a more robust TSPA-SR/LA. New data and refined analyses will be used to help constrain the range of values and increase confidence in the system performance. In addition, the selection of a design for SR/LA will focus on design changes and bounding analyses, where appropriate, to improve system performance. The M&O will also evaluate the design margin, defense-in-depth, and the performance confirmation program to enhance confidence that the potential repository system will meet regulatory requirements.

Comment 2 (page 15): With the benefit of hindsight, the Peer Review Panel finds that a credible assessment of the future probable behavior of the repository is beyond current analytical capabilities, given the complexity of the system and the nature of the data that now exist or that could be obtained within reasonable time and cost. The TSPA-VA team has performed well, has developed numerous analytical innovations, and has produced technical reports of exemplary clarity. The failure of the TSPA-VA to capture the probable future behavior of the proposed repository system is due in large part to the difficulty of the problem, including the long time scales over which performance is to be described and the large and heterogeneous physical setting that is addressed by the analysis. This difficulty was compounded by a failure, in many elements of the analyses, to initiate and complete the necessary research, develop the appropriate models, and collect and apply the needed data and information.

Response: The M&O does not agree that a credible assessment of the probable future behavior of the potential repository is beyond current analytical capabilities. Since the Yucca Mountain Site Characterization Project started, the M&O has obtained considerable information on the site and has gained a great deal of knowledge about how the natural systems operate. Many of the models that the M&O can now validate with laboratory and field data represent major steps forward in the development of those technical fields. However, the M&O agrees that more information is needed in some areas. The DOE is refining the postclosure safety case to help understand which information contributes the greatest uncertainty to the dose estimates in order to prioritize information-gathering activities. In this way, the M&O can most effectively determine when to obtain additional information and when to simply utilize the available data and develop appropriate models.

Comment 3 (page 15): Until there are improvements in the specific subsystem models for key elements of the system, in their supporting databases, in the coupling between certain aspects of the modeling, and in the use of tests, overall conclusions based on the analyses should be viewed skeptically, and decisions based on the analyses should be made cautiously.

Response: The M&O agrees and is working to collect additional data and to improve the models, the data, and their documented justification for the potential SR/LA.

Comment 4 (page 16): The Panel recognizes that substantial amounts of field and experimental data have been developed in support of the TSPA-VA; to be credible, however, many elements of the analysis require additional data. These data needs are of two types: fundamental data that are essential to the development and implementation of the models, and data sets designed to challenge conceptual models and test the coupled models used in the TSPA-VA.

Response: The M&O agrees and is actively collecting data that are essential for model development and to test models. The types of information to be obtained for the TSPA-SR/LA will be determined by the DOE’s evolving postclosure safety case for SR/LA. The needs identified in the Issue Resolution Status Reports issued by the U.S. Nuclear Regulatory Commission (NRC), formal comments from the Nuclear Waste Technical Review Board, and recommendations presented by this Panel will be considered in the development of the postclosure safety case for SR/LA. The objective of the ongoing prioritization is to establish the relative importance of the necessary data and analyses.

Comment 5 (pages 16–17): To be credible, the analysis would have needed to include:

Although the TSPA-VA offers many examples of partial, even substantial, success in each of these four areas, the Panel has also observed examples of important deficiencies in each.

Response: With respect to cladding, the M&O is currently reviewing the best estimates for cladding failure and the ranges that will be used in the potential LA, as the ranges for the cladding analysis may have been too narrow for TSPA-VA (DOE 1998a). For example, the fraction of cladding damaged during reactor operation was 0.1 percent, without considering the ranges. A more detailed review of the operating history for boiling water reactors shows that there is a 1 percent chance that fuel could have over 2.5 percent rod perforation, based on waste packages loaded with certain cores from the mid-1970s. Future analysis will provide a better basis for both the best estimates and ranges.

With respect to the waste package degradation analysis, where no process-level information was available, conservative (or bounding) assumptions were employed. For example, the M&O took conservative approaches in modeling the likelihood and penetration rate of crevice and pitting corrosion of the Alloy 22 inner barrier. Specific examples of the conservative approaches to the crevice and pitting corrosion modeling include:

Pit stifling or pit repassivation that is likely to occur during the penetration of a thick layer of a highly corrosion-resistant alloy, such as Alloy 22, was not considered. For general corrosion of the Alloy 22 inner barrier, the same penetration rates sampled for the patches were used for the entire simulation period. That is, the analysis did not consider the potential for decreasing general corrosion rates that could result from changes in the exposure (water chemistry) conditions on the waste package surface to less corrosive conditions over time. In reality, a decrease in the general corrosion rate is likely from a limited concentration of aggressive species from sustained dripping on the waste package surface and the high humidity in the drifts.

Potential non-conservatism in the waste package degradation analysis is acknowledged by not including: 1) stress corrosion cracking (SCC) of the Alloy 22 inner barrier, and 2) the effect of potential stress generation from volume expansion of carbon steel corrosion product oxides in the crevice between the two barriers (CRWMS M&O 1998). Their impacts are uncertain at the present time, and detailed analysis and testing are needed to quantify their impacts on waste package degradation. Also, additional data and analysis are needed to substantiate and/or update the modeling assumptions used for the individual degradation processes employed in the waste package degradation analysis.

Response: The M&O has utilized available information from the literature to support its position and to provide support for models for the corrosion of spent fuel. This approach was adequate for the VA (DOE 1998b). However, for the potential SR and LA, qualified data will be utilized as the basis for the development of predictive models. Data from the literature will be utilized as corroborative information. This has already been done for the dissolution rate of uranium oxide and spent fuel. For Alloy 22, where corrosion data are limited due to its relatively recent introduction, the corrosion performance will be compared to precursor nickel-based alloys that are not as corrosion resistant, but for which a larger database exists.

The Early Warning Drilling Program (EWDP) being undertaken by Nye County is expected to enhance the hydrogeologic data set needed for input to the site-scale saturated zone (SZ) flow and transport model in the down-gradient alluvial-aquifer segment of the expected pathway to the accessible environment. Laboratory experiments are being performed to provide additional data on neptunium speciation and solubility, and to improve assessments of colloid-facilitated transport. Laboratory experiments also are being performed, in addition to the collection of data from the ongoing Drift-Scale Heater Test, to enlarge the database by which to test and calibrate models simulating coupled thermal, hydrologic, and chemical processes within the rock mass enclosing the potential repository. The Busted Butte Field Test is expected to enhance knowledge of the flow and transport processes and the hydrologic- and transport-property data sets needed to evaluate radionuclide transport through the Calico Hills nonwelded hydrological unit in the unsaturated zone (UZ) beneath the repository.

The M&O is evaluating the recommended approach to analyzing coupled processes. Lower thermal loads (which would reduce the effects of coupled processes) are being considered in the LADS process, though no decision has yet been made. First, rather than attempting to calculate specific thermal-hydrologic-mechanical-chemical effects at fine spatial and temporal scales, the M&O has used geostatistical methods and nested-scale models to bound the effects. Second, the LADS process has emphasized the use of engineered features that are designed to minimize coupled-process effects. The designs that are being considered include a reduced areal mass loading and enhanced preclosure thermal management features compared to the VA (DOE 1998b). These designs result in the boiling fronts penetrating only a few meters into the very wide pillars between drifts, and the duration of temperatures above the boiling point of water will only be several centuries. Thus, coupled processes in the rock would be much less significant to performance than for the VA design (DOE 1998b). Finally, the use of drip shields and optional backfill reduce the probability of crevice corrosion and pitting of the waste packages by avoiding the temperature–relative humidity windows of susceptibility for these modes and by diverting seepage water away from the waste packages.

The M&O expects that the Drift-Scale Heater Test will be quite valuable in terms of "verifying" the thermal response models. At the same time, the Panel’s suggestion to consider bounding analyses is being pursued as well. It may be difficult to truly "bound" the effects of coupled processes, but in the TSPA-VA (DOE 1998a), sensitivity analyses were "bounded," to some extent, where the effects of coupled processes were likened to the effects of larger or smaller fracture apertures for seepage into the drifts.

Response: The site-scale UZ flow and transport model is being tested in a number of ways, including predictions of and comparisons against ambient geochemistry and perched-water distributions within the UZ, pneumatic responses monitored in boreholes, temperature profiles measured in boreholes, and hydrologic conditions within the East-West Cross Drift (EWCD). Coupled thermal, hydrologic, and chemical models are being tested against the results from laboratory experiments, as well as against data acquired from the Large Block Test, the Single Heater Test, and the Drift-Scale Heater Test. Both natural and anthropogenic analogue data will be used to the extent feasible to test both conceptual and numerical models.

Regarding the waste packages, long-term testing is continuing under expected service conditions and short-term tests are underway under accelerated conditions. This information, along with any applicable analogue data, can be used to test model performance predictions.

Comment 6 (page 17): Depending on the degree of confidence that is necessary for licensing, the current analyses may not be adequate. As discussed in Section III, the Panel believes that, for many issues to be addressed in the TSPA-LA, use of simplified bounding analyses may be necessary to achieve the desired degree of confidence.

Response: All of the models in the TSPA have been simplified to some extent. No model, no matter how detailed, can capture all of the details needed to completely describe the natural or engineered components. The goal of the iterative TSPA is to determine how much simplification can be achieved without losing important details or interrelationships. Therefore, the M&O certainly agrees that it is useful to pursue simplification, but only where the simplification can: 1) be demonstrated to be truly bounding, and 2) is such that the M&O can be certain that all important processes and interactions have been adequately captured.

1.2 ADVANCES AND IMPROVEMENTS IN THE TSPA-VA ANALYSIS

Comment 7 (page 18): The Project team has incorporated a dramatic and needed improvement in numerical modeling in the area of transport in the saturated zone, where they have abandoned the previous finite-difference model in favor of a streamtube-based approach. Although the adoption of a streamtube approach based on an overall dilution factor is less desirable than a more detailed treatment of dispersion, it is appropriate, given the limitations in the data concerning the saturated zone. The new model eliminated numerical dispersion errors, inherent in the previous model, and may provide a more realistic prediction of dilution in the saturated zone. This model is not physically representative of the saturated zone transport for isolated waste package failures, however, although sensitivity analyses indicate that the model overestimates dilution for such cases by perhaps a factor of three. This factor is small in comparison to the other uncertainties in the assessment of the saturated zone.

Response: The TSPA-VA (DOE 1998a) analyses for the SZ do underestimate the concentrations in the accessible environment for those times in which a single waste package is contributing radionuclides to the system. These times are near the beginning of the simulations and possibly impact calculated doses at 10,000 years, but probably have no impact on calculated maximum doses in the 100,000-year and 1-million-year simulations.

The approach for future TSPA modeling will use streamline particle-tracking methodology in a three-dimensional (3-D) flow and transport model. Use of the 3-D model will not require the implicit homogenization of the streamtube approach. The source size will be conservatively assigned to a small region (approximately 100 m x 100 m) that will not result in the overestimates of dilution inherent in the TSPA-VA (DOE 1998a) approach. Simulating a source region at the water table of less than 100 m x 100 m would not result in higher concentrations at 20 km for a given value of transverse dispersivity, as confirmed in a sensitivity analysis by Arnold and Kuzio (Memorandum, 1998).

1.3 KEY ROLE OF THE WASTE PACKAGE

Comment 8 (page 19): A more rigorous treatment of the evaluation of the performance of the waste package material requires the determination of two important factors:

Response: There are ongoing efforts, both analytically and experimentally (the latter in the Drift-Scale Heater Test), to understand the change in water chemistry over time. The exposure conditions for the Long-Term Corrosion Test Facility tanks were established at the initiation of those experiments more than two years ago. At the time, a workshop consisting mainly of geochemists determined the expected composition range to be a 10 times J-13 water composition with pH values of 4.5 to 10. This was further expanded to pH 2.7 to 11 to take into account microbiological and concrete material impacts. The 1,000 times J-13 chemistry was selected as representative of very concentrated solutions due to evaporation. Recent work has focused on evaporation to dryness and the resultant equilibrium compositions at elevated temperature and high relative humidity. With these studies, the experimental program is attempting to bound the likely range of conditions and test the candidate waste package materials at these conditions.

A mechanistic process model was developed, based on observed phenomena published in the scientific literature and data developed by the M&O. The model can be used to predict the extent to which pH will be lowered and the extent to which the chloride anion concentration is increased. The predictions are based on experimentally determined hydrolysis equilibrium constants, experimentally determined cyclic polarization curves, and other experimental sources of data.

Crevice corrosion testing as a function of temperature, pH, oxidizing potential, and ionic species is underway. The kinetics of the relationships may or may not provide a sharp threshold, but should, under the conditions evaluated, indicate the temperatures below which crevice corrosion is negligible.

Comment 9 (page 20): For the TSPA-VA, the critical temperature for localized corrosion of C-22 was estimated to be 80°C and the critical temperature for moisture formation was estimated to be 100°C. An estimated probability function was used for estimating the onset of localized corrosion once the package reaches the critical temperature range. Estimates of corrosion rates as a function of temperature were used to determine the corrosion damage (penetration depth). While the approach is sound, once again it is important to recognize that all of the estimates of crucial parameters were based upon expert elicitation, not upon experimental data.

Response: The M&O is continuing to collect long-term and short-term data to improve the models to be used in subsequent TSPA analyses. Specific attention is being paid to the collection of the crucial parameters. At the time of the LA, however, only five-year data will be available.

Comment 10 (pages 20–21): The Panel considers the TSPA-VA to be a useful tool for better understanding the performance and the effects of individual components on the expected repository performance; however, the TSPA treats a highly complex system and is a work in progress. The Panel concludes that the results of the TSPA-VA should be used cautiously, and that they should not be used as the primary criterion for design selection. This is particularly relevant to the evaluation of engineering components and structures. For example, the outcomes of the TSPA-VA clearly show that preventing water coming into contact with the waste packages is highly beneficial. However, the projected efficacy of an additional engineered water barrier, be it a drip shield, backfill barrier, or ceramic coating, is driven by the assumptions made within the TSPA. The Panel concludes that many of these assumptions do not have an adequate analytical and experimental justification. In a similar manner, the credibility of the conclusions is dependent upon the underlying engineering and science that supports the presumed performance of the enhanced engineering features.

Response: The M&O agrees that any specific numerical results used to make decisions related to the repository system must be used judiciously. However, as the Panel has noted, running models under ranges of conditions and input values can be used in conjunction with data and expert judgement to gain intuition about how the system might work. Before any of the specific design features can be used as part of a safety argument, the M&O will need to develop better models with a mechanistic basis and exercise them within the context of the subsystem models and the total-system model.

The LADS work over the past year has used performance assessment as a means of gaining insight into processes that could cause a design or the natural system to fail to meet regulatory criteria or design goals. The insights gained were used to develop design solutions that reduce reliance on uncertain models or that mitigate the consequences of uncertainties. Examples may include limiting the extent and duration of above-100°C temperatures in the rock, separating the drifts to make them less coupled from a computational perspective, using line loading and blending to reduce temperature variation along the drifts, and using drip shields and optional backfill to reduce the need to accurately predict seepage and rockfall.

The M&O agrees that the potential SR/LA will require more detailed data and models, backed by mechanistic understanding wherever possible. Focusing of future efforts on the selected LADS design will direct resources to these key areas.

Comment 11 (page 21): For purposes of evaluating alternative design features, the Panel recommends that a concentrated effort be undertaken to collect and collate the available experimental data germane to an analysis of waste package performance.

Response: The M&O agrees with the Panel recommendation. A review of available germane experimental data has been completed on titanium (Ti) alloy corrosion and on radiolysis effects on the corrosion of nickel-based, titanium-based, and stainless steel alloys. In addition, literature surveys of material degradation modes have been completed on relevant nickel-based, titanium-based, and stainless steel alloys.

1.4 KEY ROLE OF INFILTRATION AND SEEPS ANALYSIS

Comment 12A (page 22): The Panel considers the analysis of seepage into drifts novel and informative. Given that it was only recently performed, however, it is understandable that the resulting analysis represents only a first-order approximation and that further improvements will be necessary before the accompanying estimates can be adopted with confidence. The following issues are of particular concern to the Panel.

The analysis relies on a conventional but questionable van Genuchten formalism applied to a fracture continuum. This approach ignored the unstable nature of gravity-driven infiltration in real fractures, the possibility of hysteretic (and chaotic) behavior during episodic flow, as documented in recent experiments in related systems (Faybishenko et al. 1998), the discrete nature of the fracture network, and a detailed characterization of the capillary barrier condition at the drift surface. Thus, it is questionable that the representation assumed for purposes of developing the model actually reflects the true physics of seepage in a fractured system. Furthermore, the analysts ignored the possibility of drift collapse as a result of thermomechanical or seismic events, except for the analysis of waste package damage from rockfall.

Response: The experimental seepage program is intended to address these concerns, and the M&O should have a firmer basis for the seepage model for TSPA-SR/LA because of this work. Drift-collapse effects on seepage will be included in TSPA-SR/LA. Note, also, that DOE is sponsoring a Drift Seepage Peer Review to evaluate the sufficiency of the testing and modeling approaches.

The application of van Genuchten parameter fitting, which was developed for alluvial material, to fractures is based on laboratory moisture-retention data and it produces results that are consistent. There are no currently accepted alternatives to van Genuchten parameters in this application of coupled matrix and fracture modeling. The M&O is approaching this issue by examining modifications to the van Genuchten relationships that will be based on physical measurements.

Alternative drift geometries will be evaluated in the UZ Drift-Scale model to estimate the effects of partial drift collapse. The collapse features will be based on probable rockfall distributions and will assume that an air gap remains between the drift wall and the waste package. Asperities along the drift wall will increase the probability of a dripping flux.

Comment 12B (pages 22–23): Because of the steady-state assumptions made, seep locations and rates are estimated to be time-independent, under conditions of constant climate. Specifically, the Project staff has assumed that water will come into contact with (drip onto) some patches some of the time, but water will not come into contact with other patches for periods as long as 1,000,000 years. Although a case was made in the TSPA-VA to support this assumption, the associated understanding of the features of the mountain, including the location and size of fractures, is not adequate. In addition, thermomechanical and thermochemical effects on the permeability and capillary structure of the fracture network will alter seepage patterns as a function of time, not only during the period of the thermal pulse, but also in a longer time horizon (recall that thermomechanical effects will last as long as the mountain is at a temperature higher than the ambient). This raises the possibility that seep locations and rates will shift with time. The consequences of this possibility should be investigated. Conversely, if precipitation caps develop (Hardin 1998), they may act to reduce the amount of seepage in drifts over which such a cap forms over a long time period. This effect was not considered in the TSPA-VA.

Response: In the TSPA-VA (DOE 1998a), the M&O assumed that seeps were stable over time with regard to which waste packages they contacted, but they were assumed to "wander" locally, so that the entire waste-package surface was wetted by the seep. The M&O agrees that this is an important assumption and that it needs to be investigated in the future. Similarly, if precipitate caps are determined to be a likely occurrence, their effects on seepage will need to be investigated.

Alternative drift geometries will be evaluated in the UZ Drift-Scale model to estimate the effects of partial drift collapse. The collapse features will be based on probable rockfall distributions and will assume that an air gap remains between the drift wall and the waste package.

Heterogeneity in the rock surrounding the drift wall is evaluated by stochastic variations in the hydraulic parameters describing flow in fractures. The distribution of air permeability values, which mainly characterize the fractures, will be used to describe the heterogeneity.

The overlying Paintbrush nonwelded unit will damp episodic infiltration from infiltration events. Episodic flow as a result of heating and reflux is believed to be a short-term condition that will take place between dry-out of the drift walls and repository cool-down that re-establishes percolation. This condition would cause some waste packages to be wetted earlier than others, but the time period for this process to be active is extremely short—only within the first 100 to 200 years after repository closure. After 200 years, some of the waste packages will have cooled to the boiling point of water and the overall re-establishment of percolation will begin. At 1,000 years, most of the repository rock will be below the boiling point of water.

Formation of a precipitation cap was simulated with the near-field model. This model assumed a very restrictive fracture porosity of 0.0001, whereas the fracture testing data indicate a value closer to 0.01. The more realistic porosity would allow some precipitation in the fractures without causing significant closure of the apertures. The reduction of porosity and closure of the fracture openings will also be considered for multiple constituents. These issues will be evaluated in a mountain-scale model to determine if the thermal-hydrologic-chemical (THC) reactions are sufficient to cause a change in the flow paths during and after repository heating.

Comment 12C (page 23): For these reasons, it is unclear to the Panel that the base case approach of the TSPA-VA correctly captures the behavior of seepage into drifts in the proposed repository and for the unprecedented periods of time considered in the TSPA-VA. Better characterization of the hydrologic properties near the drifts, improved modeling, consideration of coupled effects, and additional experimentation at the drift scale would add confidence to the approach taken. We note that efforts in these directions are currently under way.

Response: As noted, the M&O is working on refining the analysis of seepage into drifts.

1.5 POTENTIALLY NON-CONSERVATIVE ASPECTS OF THE ANALYSIS

Comment 13 (pages 23–24): The outcome of the TSPA-VA analysis depends to a considerable extent on the performance of the fuel cladding..., combined with an extended waste package lifetime. Despite the acknowledged corrosion resistance of Zircaloy cladding, this is a remarkably optimistic view of the long-term performance [for] this cladding. Zircaloy cladding is typically in the range of 600 to 900 microns thick (less than a millimeter) and, during its life in a reactor, has experienced high temperatures and neutron fluxes. Important changes in mechanical properties can also occur due to thermally induced chemical reactions (oxidation or hydride formation). Another concern is embrittlement.

To substantiate these comments, the Panel notes the following:

  1. ...insufficient information is currently available on stress corrosion cracking.

Response: The M&O recognized that the model developed for the TSPA-VA (DOE 1998a) was too preliminary and, therefore, it was not included in the analysis. The model for SCC is being enhanced to include both crack initiation and propagation kinetics. In addition, the SCC experimental effort has been greatly expanded and includes an evaluation of crack initiation and growth kinetics under realistic bounding aqueous environments. Further, the potential for reducing residual weld stress is being examined in detail.

  1. ...Zircaloy may be susceptible to corrosion under certain chemical conditions ...Additionally, stress corrosion cracking is sensitive to chemical conditions... . These chemical processes were explicitly not considered in the TSPA-VA.

Response: Local corrosion under extreme chemical environments was modeled in the TSPA-VA (DOE 1998a) by scaling the corrosion rate based on observed zirconium corrosion rates. An improved corrosion model is being developed for zirconium. A better analysis of the chemistry within the waste package is being developed to address both local corrosion and SCC. In addition, the data on SCC tests with various salts that were performed at Teledyne Wah Chang, Albany, Oregon, is being reviewed. Yau has reported no observed SCC in U-bend tests with boiling seawater (Yau 1983) and geothermal salts (Yau 1984). Tests with iodine have shown that SCC occurs at higher stresses than expected under potential repository conditions. Other analysts (Tasooji et al. 1984; Pescatore et al. 1989) have addressed this.

  1. The Panel notes, as it did in its third report, that additional mechanisms of failure remain to be investigated experimentally: (1) pitting and crevice corrosion; (2) hydride-induced embrittlement and cracking; and (3) "unzipping" of cladding due to secondary phase formation particularly uranyl oxy-hydroxides which form immediately as alteration products of UO2 under moist, oxidizing conditions.

Response: The M&O has concluded that it is unlikely that pitting and crevice corrosion or hydride-induced embrittlement and cracking will be a major cause of cladding degradation, particularly for waste packages that fail late in life and, hence, only see benign-chemistry water. Work has just been initiated to evaluate the potential for unzipping due either to high-temperature water vapor or to dripping silicate-containing water.

  1. Although the Panel strongly urges that the Project team initiate the necessary experimental programs, we note that time is limited.

Response: The M&O will utilize the available literature to define the operable mechanisms; however, models will be built upon qualified data. Data that are needed will be developed, although it is recognized that all of the required data may not be available until after the LA. Bounding analyses will be utilized where applicable.

To quote from the TSPA-VA, the "base case cladding model does not have a very wide uncertainty range, so the parameter does not show up in section 4.3 as a top rank-regression parameter." In the Panel’s view, this is an instance in which the TSPA-VA analysts have failed to identify the critical importance of a parameter because of optimistic assumptions in the analysis both in terms of performance and the uncertainty in that performance.

Response: Both the ranges and best estimates are currently being reviewed. A sensitivity study for the future LA will look at the absence of cladding.

Comment 14 (pages 24–25): The analysis of the biosphere dose conversion factors were conducted using the GENII-S model... . This model permits the user to specify the length of time that irrigation water is deposited on the soil prior to the intake period for which a dose is estimated. In TSPA-VA model runs, this time was assumed to be one year. Taking into account the fact that irrigation in some locations may continue for a period of hundreds or thousands of years, this one year assumption could lead to estimates of radionuclide concentrations in the soil that will significantly underestimate the radionuclide uptake by root crops.

The degree to which the failure to consider soil buildup leads to an underestimation of the dose rate depends on the specific radionuclides of concern. For technetium and iodine, the default assumption in GENII-S is that these radionuclides are rapidly washed through the soil column. This assumption appears to be inconsistent with measured iodine concentrations in surface soils near release sites for iodine... . Data from these studies indicate that iodine tends to remain in near-surface soils for extended periods. For radionuclides such as neptunium and plutonium, which are readily adsorbed by the soil, the degree to which the dose is underestimated could be significant.

Response: The initial assessment conducted in the TSPA-VA (DOE 1998a) was designed to address the release of radionuclides to the environment over a one-year period in order to be consistent with the regulatory concept of assessing an annual dose resulting from releases in a one-year period. The buildup of radionuclides in soil as a function of previous irrigation activities will be addressed in TSPA-SR/LA. Data on the retention of key radionuclides (e.g., iodine, technetium, and neptunium) will be considered in these analyses.

1.6 POTENTIALLY CONSERVATIVE ASPECTS OF THE ANALYSIS

1.6.1 Transport Through Penetrations in Waste Package

Comment 15 (page 25): The Panel concluded that the TSPA-VA treatment of the movement of water into a damaged waste package and the transport of radionuclides from such a package were highly conservative.

Response: The bounding approach was selected because of the uncertainties in the relative locations of package breaches and seeps. The M&O agrees, however, that this approach is conservative for several reasons. The worst pitting is unlikely to occur where the drip is. As the water film spreads over the container, it evaporates, leading to the precipitation of lower solubility materials (silica, calcium carbonate) near the drip and the formation of more soluble, hygroscopic salts (sodium chloride, sodium nitrate) farther from the drip as the last bit of water evaporates. Pitting is most likely to occur near the peak locations for chloride, away from the drip. Drip locations will also move over time. Locations receiving splatter from drips (assuming no backfill) are also more likely to be pit sources. Thus, water is unlikely to initially drip into the pit holes and lead to mass transport. Secondly, if only small amounts of water enter the container, all of the water will evaporate and be unavailable for transport for a significant period of time. Furthermore, leaching and transport from the package cannot begin until the drip rates exceed evaporation. Until then, flow is towards the waste since the waste is the source of heat. In future work, the M&O plans to take credit for evaporation.

Comment 16 (page 25): As the TSPA staff moves ahead, there is a need for an improved description of the progression of corrosion damage to waste packages, the size and shape of the assumed penetrations, and the distribution of penetrations across the inventory of waste packages. There is also a need for a more realistic conceptual description and treatment of the evolution of corrosion damage.

Response: The M&O agrees that there is a need to re-evaluate the progression of corrosion damage to waste package barriers. Experimental efforts are underway to assess the likelihood of pitting and crevice corrosion, and the kinetics, under expected aqueous conditions and to better define the expected localized corrosion morphology and progression. In addition, a literature review has been initiated to assess expected SCC morphology and crack-opening displacement based on the SCC experience of the nuclear industry.

Comment 17 (page 25): Should the corrosion-resistant metals fail by localized corrosion, the likely shapes of the penetrations will be small pits, tight cracks, or narrow channels. The size, shape, and distribution of penetrations in thick layers of corrosion-resistant metals were not analyzed as part of the TSPA-VA; the Panel recommends that this topic be examined in anticipation of the potential LA phase.

Response: Work is in progress to develop a consistent description for waste package degradation and radionuclide release from the waste package. Additional data and detailed analyses are required to develop: 1) an improved description of the morphology and size of penetrations from individual degradation processes (such as pits, cracks, patch openings, etc.), and 2) temporal and spatial distributions (top, bottom, and side) of the penetrations on individual waste packages. These results, with the information on drift seepage behavior and the interaction of water with the waste form inside a breached waste package, can be used to estimate water ingress and radionuclide release from breached waste packages. The processes or areas that are being considered are: 1) in-drift seepage behavior, 2) water (film) flow on the surface of a degrading waste package, 3) ingress of water into a waste package through the penetration openings, 4) in-waste-package water flow characteristics, mode of water contact with the waste form, and radionuclide mobilization from the waste form, and 5) transport of the radionuclides from the waste package through the penetration openings. The integrated representation of the above processes in the TSPA analysis will greatly improve the consistency and transparency of the source-term analysis.

Comment 18 (page 26): Regarding the base case conditions, the TSPA-VA staff assumed that the spent fuel and cladding would be instantly covered by a water film at the time a waste package was penetrated. Transport of moisture and air into the packages and the transport of products from the packages through such penetrations were judged not to provide any significant retardation to radionuclide releases. The Panel does not accept this view; we believe that it would have been more realistic to have assumed that the resulting penetrations will likely retard radionuclide releases from the waste packages. Although the task will be difficult, the Panel recommends that steps be taken to develop better methods for analyzing the movement of radionuclides into and from the waste packages.

Response: The bounding approach was selected because of large uncertainties associated with the geometry of the system as it evolves over time; however, the M&O plans to make improvements in this area. The M&O is planning a modeling effort to specifically address flow through the drip shields and waste packages to determine if the conservatism in this area can be reduced. Second, the M&O plans to take credit for water evaporation. Leaching can only occur when drip rates exceed evaporation. Until then, aqueous water flow is always towards the waste. Other types of credit are more difficult since it is necessary to know the geometry of failure, which will always be highly uncertain.

If the waste package is breached by small pit perforations or tight cracks that are the likely opening morphology of the waste packages failing initially by localized corrosion and SCC, only small amounts of water could ingress through such openings. This water would be consumed by reactions with the waste package internal materials and the waste form. Some of the water could be retained on the reaction (corrosion) products of those materials and may not be mobile, thus, not contributing to advective transport of radionuclides. The dominant radionuclide release mechanism in such cases would be diffusive transport through relatively stationary water films on reacting materials and corrosion products. The tortuosity of such a transport path would act as a significant barrier to radionuclide release.

Also, see the response to Comment 17.

1.6.2 Retention of Radionuclides in Spent Fuel Alternation Products

Comment 19 (page 26): At present, the TSPA-VA does not take credit for this type of radionuclide retardation; in this sense, the analysis is conservative. For some radionuclides (237Np and 79Se) some degree of co-precipitation is expected, and for other radionuclides (99Tc and 129I) this type of process is unlikely. For those radionuclides for which this is a likely retardation process, a well-defined experimental program (discussed in section IV.G of this report) may provide a substantive basis for increased retardation of key radionuclides, e.g., 237Np. The inclusion of this type of analysis in the TSPA would, however, increase the general level of complexity of the analysis of spent fuel corrosion and create new data needs which will require further experimental work.

Response: For the TSPA-93 (Wilson et al. 1994, pp. 9-13), expert judgment was used to define distributions for partition coefficients (Kd’s) for a number of elements and materials. The Kd on hematite was evaluated for Np and several other elements. Sorption on hematite is a conservative surrogate of rust since other mineral forms of iron oxide such as amorphous iron oxide and goethite have many more sorptive sites and, thus, higher sorption potential. Sorption of Np on rust from corrosion of the container was used in a Defense Spent Nuclear Fuel (DSNF) Performance Assessment conducted for the National Spent Nuclear Fuel Program (NSNFP) to evaluate the influence that sorption might have on peak release rates. Sorption at the container does delay releases of Np, but only on the order of many thousands of years, not tens of thousands of years (Rechard 1998, pp. 11-26). Hence, not enough decay of Np has occurred to noticeably reduce the peak release. Similar results were observed in preliminary TSPA runs where sorption was included in the UZ and SZ. As a result, sorption at the source term was not included in TSPA-VA (DOE 1998a). Certainly, high sorption will preclude any release of Np in the first 10,000 years; yet Np is not of concern in the first 10,000 years, even without retardation. Furthermore, past analysis for a million-year period indicates that only the time of the peak release can be influenced and that cannot be extended beyond 1 million years.

For TSPA-VA (DOE 1998a), Hardin (1998, Section 6.4.4) reported on sorption experiments using thermally altered concrete to simulate the degradation of the concrete liner. The apparent Kd values were very high. The M&O will consider whether some type of retardation should be included in or around the container in future long-term work.

The retention of radionuclides, particularly 237Np, is currently being examined utilizing spent fuel specimens from the unsaturated drip condition tests. Since concentrations lower than expected were found in the water effluent, it is likely that the Np is being held either in the secondary phases or on metallic surfaces. The effort is focusing on the development of a good mass balance for Np. A companion effort is examining the potential for co-precipitation of Np with uranyl phases.

1.6.3 Potential Sorption of Technetium and Iodine

Comment 20 (page 27): It seems likely that measurements taken of areas near the Chernobyl site should also provide relevant data on the retention or lack of retention of technetium in the soil. Regarding the retardation of iodine, additional data sets are likely to be available from environmental measurements taken at the Hanford site, where radioactive iodine was released during spent fuel processing.

Response: The M&O recognizes the need to examine field data from other sites regarding the retention of radionuclides in soils and in the substrate. Information from the literature on Chernobyl is being gathered and will be incorporated into a natural analogue synthesis report. In addition, information from Hanford, the Idaho National Engineering and Environmental Laboratory, Nevada Test Site (NTS), and other sites in the United States will be included in the synthesis report.

Comment 21 (page 27): Due to the difference between surface soils and the properties of the rock in the unsaturated zone and saturated zone flow paths, the retardation observed at the surface may not occur during underground transport. It is the Panel’s view that this question should be explored. It is also the Panel’s view that the TSPA-VA analysts over-emphasized laboratory Kd measurements and did not appropriately consider opportunities to observe the mobility of radionuclides in the environment.

Response: The M&O also agrees that the retardation of Tc and I needed further study to better understand their potential retardation in the UZ and SZ. Studies of the behavior of I in the environment indicate that organics play a large role in the retardation of this element in surface soils. In addition, SZ retardation/transport of Tc and I appears to be affected by organics (Beasley et al. 1998). Current laboratory studies and results from the German high-level waste (HLW) disposal program indicate that Tc can be reduced by low Eh waters and organics, thus enhancing its retardation. The Eh/pH and organic content studies in the Nye County alluvial system may prove to be very beneficial to the retardation of these elements once they escape the UZ.

There are differences between surface soils and the properties of the rock in the SZ flow paths. The SZ flow and transport model uses retardation factors obtained from the C-well complex tracer tests. A similar complex is planned in the alluvium to conduct tracer and hydraulic tests. These tests will provide data for obtaining the appropriate retardation factors for the saturated flow paths in the alluvium.

The M&O has placed primary emphasis on laboratory measurements of the sorptive properties of aquifer materials for the tuff units. These data were obtained from core samples from the site in both the UZ and the SZ and represent valid site-specific information. In addition, results from the Li-tracer tests at the C-holes confirm that in situ measurements of retardation are roughly consistent with laboratory-scale measurements of Kd’s. The C-holes data also suggest that laboratory measurements are generally conservative relative to the tracer test results. Evidence concerning the sorption of Tc from laboratory tests on rocks from Yucca Mountain and on clay materials indicates little or no sorption of this element under oxidizing conditions (Thomas 1987). Iodine is assumed to have no sorption due to its expected aqueous speciation as the negatively charged iodide ion. The assignment of zero Kd to these radionuclides is consistent with values reported in the scientific literature (Brandberg and Skagius 1991). The M&O plans to include a brief analysis of analogue systems with regard to radionuclide sorption and retardation in TSPA-LA.

1.7 POTENTIALLY IMPORTANT BUT OMITTED PROCESSES

Comment 22 (page 27): For waste package performance, the detrimental effect of the expansion of steel corrosion products on the inner barrier and canister internals has not been addressed.

Response: A detailed finite-element analysis was performed on the oxidation of the steel outer barrier and the resultant stress to the inner barrier at their interface. However, it was not advanced enough to be included in the TSPA-VA (DOE 1998a). A preliminary result indicated that, at partial wall oxidation, the resultant stress was high but insufficient to damage the inner barrier. However, work in this area was put on hold pending the completion of the LADS process.

Comment 23 (page 28): Damage due to hydrogen is a major threat to the integrity of zirconium cladding. When cladding is embrittled by hydrogen, it loses its mechanical strength and ductility and fails by through-wall cracks. One possible source of hydrogen in the proposed repository is the corrosion of dissimilar metals in contact with zirconium. Hydrogen formed by the corrosion of steels and stainless steels has damaged and led to failures of zirconium components in industrial applications. If the internal barrier of the waste package is penetrated, water can contact the package internals. The resulting corrosion and hydrogen production represent a significant threat to the integrity of the cladding. This degradation process was not addressed adequately in the TSPA-VA. As a result, the extent of the credit taken for cladding in the analysis is questioned.

Response: A review of the experiments on embrittlement is in process. Experiments show no tritium gas uptake into the zirconium when tritium was dissolved in water and zirconium oxidation occurred. Other experiments show hydrogen uptake occurs in a reducing environment when the oxide layer can be damaged, but does not occur in an oxidizing environment when the passivation layer is maintained.

Comment 24 (page 28): Stress corrosion cracking of the C-22 barrier is a realistic threat to waste package performance. The possibility of such a threat was not adequately addressed in the TSPA-VA. A proper evaluation will require more experimental data in realistic, repository environments. The Panel supports the recommendation to use double, U-bend specimens in stress corrosion cracking tests for realistic simulation of repository conditions.

Response: The M&O has initiated an experimental effort, including the double U-Bend specimen tests, to better understand the conditions under which SCC can occur in Alloy 22 and titanium alloys. The M&O is also initiating an outside literature review to bound the expected waste package residual stress distributions that affect SCC through-wall and lateral propagation. The M&O is also evaluating means to reduce the stress in the highly stressed weld regions utilizing newer techniques such as laser peening.

1.8 DATA NEEDS

Comment 25 (page 28): The Panel is [sic] recognizes that substantial amounts of field and experimental data were developed in support of the TSPA-VA; however, the future success of the Project depends even more critically on the acquisition of additional data, particularly as more sophisticated models are incorporated into the analysis.

Additional data needs are of two types:

Response: During the development of, and subsequent to, TSPA-VA (DOE 1998a), a major focus of the science program has been devoted to acquiring the data needed both to directly support TSPA and to support the natural-system process models that support the TSPA evaluations. Examples include data being derived from the Nye County EWDP in the SZ down-gradient from the site, field testing at the Busted Butte site, seepage and tracer testing in the Exploratory Studies Facility (ESF), and geochemical sampling in the ESF. Other examples are laboratory experiments on neptunium speciation and solubility and on colloid formation, stability, and sorptive properties.

The M&O believes that the ongoing and currently planned heater tests, as well as analogue studies, will help challenge the conceptual models. The thermal tests are expected to provide additional data that can be used to analyze the capability of the coupled models.

Comment 26 (page 28): A substantial part of the knowledge base presently rests on expert elicitations ... for data on: flow in the unsaturated zone, the near-field environment, waste package degradation, waste form degradation, radionuclide mobilization and flow/ transport in the saturated zone. Additional site characterization in the unsaturated and saturated zones, as well as experimental programs in waste package and waste form degradation are required.

Response: The M&O agrees with this comment. As noted in the response to Comment 2, the M&O has ongoing field testing and experimental programs to increase the knowledge base in each of these areas. The M&O is also prioritizing future testing and experimental needs in light of the LADS activity and continued performance assessment analyses.

Comment 27 (page 28): Solubility limit distributions for the key radionuclides ... have only a limited experimental basis.

Response: It is true that the current solubility ranges/values for the key radionuclides of interest to performance assessment are lacking or not documented in accordance with quality assurance requirements in the TSPA-VA (DOE 1998a). The M&O is currently conducting literature surveys (including the Nuclear Energy Agency database) and experiments to produce qualified thermodynamic data for the GEMBOCHS (Geologic and Engineering Materials Bibliography of Chemical Species). These data will then be imported into a solubility model (e.g., EQ3/6) to calculate the solubilities of the radionuclides under a wide range of geochemical conditions.

Efforts to produce new thermodynamic data on the key transport radionuclides over the past several years were focused on Np solubility studies, Np co-precipitation experiments, and Tc solubility studies under reducing conditions. In addition to the Nuclear Energy Agency database for uranium, newer literature data are being incorporated into the GEMBOCHS database to help produce a more defensible data set for U.

Comment 28 (page 29): An experimental program should be developed to advance the spent fuel corrosion model beyond its present empirical representation by a response surface.

Response: The current approach is considered to be conservative. However, for TSPA-SR/LA, the approach will be reviewed. It is unlikely that new long-term tests will be initiated; however, information from the literature will be utilized for comparison with data generated by the M&O.

Comment 29 (page 29): There are similar lacks of data for Zircaloy cladding corrosion, secondary phase formation, colloid formation and transport, Kd, and saturated zone characteristics.

Response: The technical basis for cladding corrosion is extensive. Zircaloy degradation for the nuclear industry and zirconium alloy degradation for the chemical industry have been studied for over 40 years. This literature was utilized to determine which degradation modes are likely to be operable under Yucca Mountain conditions. The M&O has concluded that it is unlikely that pitting and crevice corrosion or hydride-induced embrittlement and cracking will be a major cause of cladding degradation, particularly for waste packages that fail late in life and, hence, only see benign-chemistry water. Work has been recently initiated to evaluate the potential for unzipping due either to high-temperature water vapor or to dripping silicate-containing water. Tests that address secondary phase formation and colloid formation and transport are ongoing. These data will provide the basis for the models to be utilized for TSPA-SR/LA.

Our understanding of the generation and stability of colloids formed during spent fuel dissolution is only in its infancy. Current collaborative studies are aimed at determining the stability of the colloids (reverse sorption "Kd" rates) formed during the spent fuel dissolution experiments. These studies will help determine if more information will need to be collected with regards to colloid formation/transport in the near-field environment (NFE).

As stated previously (Comment 5), the Nye County EWDP is expected to provide useful data in what has been identified as a data gap in the southern part of the site-scale SZ flow and transport model domain.

1.9 TESTING MODELS

Comment 30 (page 29): The Project staff should provide and regulators should require, where possible, demonstrations that the TSPA "works." This can be accomplished by designing experiments and field tests that are driven by the TSPA-VA analysis and that challenge the conceptual models used in the analysis. This is a standard approach in any scientific and engineering study, particularly one as complex as the TSPA.

An example of such a laboratory study was recently reported by Werme and Spahiu (1998). These authors illustrate the difficulty of modeling actinide concentrations in well-controlled experiments. They conclude, "There is a large body of data on the solubilities of pure actinide phases; however, it appears that the information available is insufficient to explain the experimental results." These conclusions speak directly to the uncertainty in the modeled results in the TSPA-VA. Such experiments cannot be replaced by sensitivity analyses, because the sensitivity analyses used in the TSPA-VA do not directly address the uncertainties associated with the experimental database or the selection of a conceptual model.

Response: The M&O agrees with the importance of demonstrations that the TSPA "works," and that experimental verification is an important part of this demonstration. However, it is not true that modeling analyses cannot be part of it as well. It may be true that the sensitivity analyses in TSPA-VA (DOE 1998a) do not address the kind of uncertainties that are referred to, but sensitivity analyses that do address them could be devised. Furthermore, the kind of system sensitivity analyses conducted for the TSPA-VA (DOE 1998a) can show whether or not more detailed studies are actually needed. If the particular model or parameter is shown not to have much effect on the TSPA results, then a lower level of confidence is acceptable. Also, note that the probability distributions for uncertain parameters are intended, in part, to address these kinds of issues.

1.10 INSIGHTS FROM THE TSPA-VA

Comment 31 (page 30): In the TSPA-VA analyses of the performance out to the time of peak doses, typically at several hundred thousand years, almost all of the protection was found to be provided by the engineered waste package, the cladding, and the dilution that occurs in the saturated zone. From this long-term perspective, the early thermal period during which liquid water does not contact the waste packages and the required times for transport of the radionuclides through the unsaturated or saturated zones were found not to be important to overall performance. This has led to criticism that the Project is relying principally on engineered features for protection, and that the natural features of the site contribute little to safety.

Response: The site is a major component of the waste management system that is intended to isolate radionuclides from the biosphere. The vast majority of radionuclides that provide a potential risk to postclosure health and safety are relatively insoluble or strongly sorbed by the minerals in the host rock, and are naturally inhibited from migrating away from the repository. For these radionuclides, the natural barriers at Yucca Mountain are capable of preventing migration into the ground water. The key aspects of the Yucca Mountain site are a very arid climate, low percolation flux, a thick UZ, and water chemistry that does not mobilize and transport the great majority of the radionuclides.

A small fraction of these radionuclides are potentially mobile or could be transported collodially if exposed to liquid water. For these radionuclides, the engineered barriers are designed to complement the site features. The engineered materials of the waste packages are intended to resist sporadic water contacts for extremely long periods, and the design geometry and components are intended to reduce the amount of water that does contact the waste packages. The combination of natural and engineered features will be required to isolate the radionuclides effectively. Neither type of feature can be or is relied on at the expense of the other.

Comment 32 (page 30): In the Panel’s view, the confidence that the public can have in the TSPA results will, to a large degree, depend on how the analyses of the major attributes of the repository system are conducted and presented.

Response: The M&O agrees that the method by which the analyses are constructed and presented is critical. In order to communicate the assumptions and the results, it must be possible to see the transparency of the relative contributions of the various components within the context of the total system. The M&O has benefited from the comments and guidance from the Panel, and from other oversight groups, and will continue to address this important issue by improving the clarity and transparency of the TSPA analyses and, also, by using other avenues such as visualization.

2. THE TSPA-VA METHODOLOGY

2.1 METHODOLOGY

Comment 33 (page 35): For several of the [model] components, however, it is not easy to review or evaluate the degree to which the model abstractions are equivalent to the underlying more complex analyses. For example, the TSPA-VA staff has stated that the modeling of the behavior of the near-field geochemical environment is based on a mixture of "abstracted models with some process level components"... . The document indicates that in at least one area (composition of gas in or around drifts), there is no process-based model. The Panel is unable, based on these comments, to determine what the basis is for the abstracted models. This matter needs to be clarified.

To help resolve these questions, the Panel recommends that the staff pay careful attention to its own definition of the model abstraction process. In each case, the abstraction should be a simplification of a more fundamental process-based model, and it should provide results consistent with the process-based models over the same range of parameter and input values as can be treated by the more complex process-based model.

Response: The M&O agrees that the process-model basis for all of the abstractions should be clearly defined. This will be addressed in the series of Process Model Reports (PMRs) that will form the basis for the TSPA-SR/LA. Each of these reports will clearly discuss the conceptual and numerical bases for each of the eight component models, along with data used and a comparison of the model results to experiments. Another section of each PMR will discuss TSPA abstractions based on the given component model and how the abstracted model compares with the process model.

Comment 34A (page 35): The TSPA-VA summarizes the results of extensive sensitivity analyses, conducted for different time periods (typically 10,000, 100,000, and 1,000,000 years), and provides estimates of the effects on isolated subsystems of changes in various performance parameters or site conditions. For example, estimates of the distribution of travel times for radionuclides through the unsaturated zone were based on three different climate regimes. This approach is informative and can provide helpful insights into the likely performance of the proposed repository system.

The degree to which such analyses reliably indicate which aspects of the system are more or less important is limited by the inconsistent degree of realism versus conservatism in the various analyses incorporated into the TSPA-VA. Because a mixture of both conservative bounding and more-or-less realistic analyses were used, the interpretation of the outcome of the sensitivity analyses is not straightforward. The Panel knows of no methodologically sound approach to quantify sensitivities for a given analysis that uses such an approach. This stems, in part, from the fact that the degree to which the actual performance of some aspect of the repository system differs from an estimate of that performance based on a bounding analysis is not known (if the actual performance were known, a bounding analysis would not be needed).

The Panel’s point in noting that the TSPA-VA will inevitably be an uneven mixture of bounding analyses and more realistic assessments is to caution against overconfidence in the validity of the results of the sensitivity analyses. Because the TSPA-VA incorporated many assumptions of varying validity, the results of these analyses need to be interpreted with judgment and their conditional status recognized.

Response: The M&O agrees that the results of the sensitivity analyses should be interpreted carefully by analysts who are aware of the limitations of the calculations. The fact that a parameter or subsystem has been replaced by some sort of "bounding" estimate can, indeed, affect the results of the sensitivity analysis—not only for that parameter or subsystem, but possibly for others as well. However, the sensitivity analyses are still useful. For one thing, they do indicate the sensitivities of the model system and, therefore, they provide information as to which parts of the model are most important and potentially most in need of improvement.

While problematic for determining "probable behavior," the use of conservative assumptions for some parts of the system is an acceptable approach for showing regulatory compliance, and is even recommended in some areas by the Panel (e.g., for effects of coupled processes).

Comment 34B (pages 35–36): ...the Panel notes that the TSPA-VA staff did not, at this stage, seek to use the sensitivity analyses to demonstrate that certain aspects and/or issues are unimportant and therefore need not be further considered. These judgments and/or decisions may be more appropriately made during the possible TSPA-LA phase.

Response: The next stage of development of the analyses for the TSPA-SR/LA will focus on developing a prioritization of their importance to system performance, consistent with the evolving postclosure safety case for SR/LA. The prioritization will take into account the NRC Issue Resolution Status Reports for the various components of the TSPA; the comments from the Nuclear Waste Technical Review Board and this Panel will also be considered. The objective of this prioritization will be not only to determine which analyses should not be carried further, but also to determine which elements are most important and, thus, should receive the most attention in the TSPA-SR/LA development.

Comment 35 (page 36): In its third interim report, the Panel recommended "...that the sensitivity analysis results not be used to identify key analytical uncertainties as the program progresses toward the TSPA-LA. Instead, the Panel recommends that the TSPA sensitivity analyses be viewed as an input to the collective judgment of the TSPA and other project staff. In addition, where sensitivity analyses produce results that are inconsistent with the intuitive judgments of the project staff or advisors, the underlying models and parameters should be examined to ensure that uncertainties in performance are appropriately represented." We continue to endorse this recommendation.

Response: The M&O is in complete agreement with this comment. It is very important that the analysts continue to question their results in the context of the entire system. The M&O also plans to have the results independently verified and checked by personnel with complementary backgrounds and, thus, expand the intuition and expertise applied to the problem.

Comment 36 (page 37): ...the Panel is concerned that expert elicitation could have been misused by the TSPA-VA staff through its application as a comparatively rapid and inexpensive way to synthetically generate "data" as inputs to the TSPA-VA, in place of actual laboratory or field measurements. Unfortunately, in several instances, noted in Section IV, this has occurred.

Response: The M&O structured expert elicitations for the TSPA-VA (DOE 1998a) to be generally consistent with guidance on the use of expert elicitation, such as Kotra et al. (1996) and Budnitz et al. (1997). The goals of these guidance documents are to draw criteria for when expert elicitation should be used from previous expert elicitation projects, both successes and failures, and to define credible procedures for the conduct of elicitations.

Section IV of the Panel’s Final Report (Whipple et al. 1999) discusses the use of expert elicitation for the component models for the UZ flow model, waste package degradation, SZ flow and transport, and waste form degradation. The topics covered in these elicitations were identified as significant to the calculated results and were subject to considerable uncertainty. In some cases, data collection activities related to the topics covered in these elicitations provided only a limited basis for quantifying uncertainties, selection among competing conceptual models, and estimating bounds or probability distributions for key parameters. The elicitations were intended to focus on the range of uncertainties that currently exist. By involving experts from outside the M&O who have their own databases and experience, a broader range of perspectives was obtained to support TSPA efforts to characterize uncertainty. In each of these elicitations, the experts were asked for their assessment of data collection or analysis activities that, in their opinion, could lead to a significant reduction in the uncertainties that were assessed. These recommendations will be used to assist in planning additional data collection and analyses to help reduce uncertainties in future iterations of the TSPA process.

If additional expert elicitation projects are planned, the M&O will continue to use existing guidance documents and careful judgment to avoid any unwarranted use of expert elicitation.

2.2 COMPLEXITIES OF THE SYSTEM AND OF ITS COMPONENTS

Comment 37 (page 39): While the Panel does not think that a fully-coupled, theoretically-defensible, first-principles analysis of coupled processes is possible, it believes that a considerable amount of data exists that could have been incorporated into the modeling approaches used in the TSPA-VA.

Response: It is true that a considerable amount of data exists and could be used in the determination of phenomena that occur in the UZ above and near the potential repository. Much of the data came from the thermal tests occurring in the ESF. In fact, the temperature results from the Single Heater Test were used by TSPA to perform a series of hydrologic property set comparisons to measured temperature data. With these data, an assessment was made regarding the application of hydrologic property sets for the performance assessment (developed by an ambient model and its governing assumptions) to a thermally perturbed system. Other thermal testing data related to thermal-mechanical and thermal-chemical processes were either just becoming available (as in the case of the Drift-Scale Heater Test) or were not applied due to the large uncertainties associated with a specific (thermal-hydrologic-mechanical or THC) process on affected flow properties. An example of this is the measured rock displacement, which is on the same scale as the Single Heater Test, and how it may be upscaled to a larger scale thermal-hydrological (TH) model with an appropriate constitutive relation governing the fluid flow. These processes are to be investigated further (with the use of thermal testing data) for the TSPA-SR/LA.

2.3 MANAGING COMPLEXITIES AND COMPONENT MODEL LIMITATIONS

Comment 38 (page 40): On the basis of its review, the Panel has concluded that there are two types of processes that should be analyzed as part of the possible upcoming TSPA-LA, particularly in terms of meeting the anticipated "reasonable assurance" requirements of the USNRC. These are (1) those for which analytical models are available, and (2) those that may be essentially intractable given current analytical capabilities, or intractable within the time constraints under which the TSPA staff is operating. Although both of these types of processes are complex and extremely difficult to analyze, each has distinct characteristics from the standpoint of the approaches that can be used to analyze them. These approaches include:

 Also to be considered is the incorporation of the "defense-in-depth" concept into the design of the overall repository system. Effective use of this concept, in concert with the approaches enumerated above, can enhance the confidence of the designers, the analysts, and the regulators that there is reasonable assurance that the proposed repository design will meet the regulatory requirements.

Response: The M&O agrees with the recommended approaches. The M&O intends to identify where the weaknesses lie in terms of being able to evaluate processes in an analytical manner or using some other simplified approach. The effort toward traceability and transparency should further enlighten where such decisions are made. In addition, the M&O and NRC are planning interactions to discuss appropriate defense-in-depth methodologies.

Comment 39 (pages 42–43): ...incorporation of the "defense-in-depth" concept into the design of the overall repository system can also provide increased confidence in the performance of the proposed repository system. ... In nuclear power plant regulation, the objective in applying this philosophy is to assure that the system will perform safely even if one or more individual barriers has failed. The analogous defense-in-depth requirements for a repository are not yet clear. While the assessment of defense-in-depth takes place outside the TSPA and, therefore, outside the Panel’s charge, we believe that the TSPA methodology can be a useful tool for assessing defense-in-depth. Just as in the case of the analyses of the performance of nuclear power plants, we believe that the TSPA methodology can provide a means of estimating how well a system of barriers within the proposed repository would perform, even when one or more of the barriers within the system is assumed to have failed... .

Response: During the LADS process, the M&O started to assess the appropriateness and the methodology for applying TSPA models as a tool for assessing defense-in-depth.

Comment 40 (page 43): While incremental improvements in the analyses of coupled processes can be made, the Panel has concluded that a detailed, technical defense of these analyses cannot be demonstrated at the present time. As a consequence, the Panel recommends that in the TSPA-LA phase, the processes be treated by the use of either bounding analyses or design changes, supported by the incorporation of the defense-in-depth philosophy into the overall design of the proposed repository system.

Response: In the LADS process, designs were considered that seek to minimize the effects of coupled processes on repository performance. The reduced importance of these effects would make bounding analyses more feasible and credible. (Bounding analyses are generally nothing more than analyses based on abstracted models.) As such, they are already standard within performance assessment. However, the term "bounding" also generally implies conservatism, which is presumed to require less of a process-model/experimental basis. This is difficult to demonstrate in most cases (especially for nonlinear processes), and in cases where it can be demonstrated, it is only because a very high degree of pessimism has been assumed in the bounding analysis. This sort of pessimism will be avoided wherever possible in the TSPA-SR/LA through comparing the TSPA-SR/LA models against the engineered-barrier system (EBS) 1/4-scale tests, preliminary results of the Drift-Scale Heater Test, and other laboratory experiments and analogues where available.

Comment 41 (page 43): In the case of thermal-hydrologic-mechanical-chemical coupled processes, the Panel has the following suggestion on how such a bounding analysis could be performed. The suggestion is based on the observation that the output of an ideal analysis of these processes would describe the duration of the thermal period, and the pattern and flow rates of water after the thermal period has ended. Although the Panel does not think that it is possible to analyze these coupled processes in detail, we have concluded that it may be possible to determine reasonable bounds for the following factors. The time curve over which the repository will heat up and then cool. This will enable the analysts to estimate when the waste packages will experience increasing humidity levels, subsequently followed by the flow of liquid water;

Other effects such as those on the chemistry of the water entering the drifts are likely to be of less importance, because the chemical conditions at the waste package surface are likely to be determined more by the local versus the far-field environment.

Response: The M&O appreciates the Panel’s suggestions on developing appropriate bounding analyses. However, in this particular example, alternative designs being considered for SR/LA may limit the need to perform this type of analysis. If the M&O is required to make more detailed models of coupled processes, an approach like this may be useful. However, it must be noted that the range of long-term average infiltration is considered by many reviewers to be unreasonably conservative. Also, allowing all waste packages to be wet at the same time is an extremely improbable assumption, given the volume of water available at one time in the mountain and current knowledge about water flow in this type of unsaturated media. However, it is possible to utilize such a construct in sensitivity studies, as discussed earlier, to help gain a better understanding of the coupled processes.

2.4 OVERALL CONCLUSIONS ABOUT THE TSPA-VA METHODOLOGY

Comment 42 (page 44): In the course of its review, the Panel has noted the inherent difficulty of several aspects of the performance assessment. Our purpose in doing so is to distinguish between those cases where refinements in the modeling and the acquisition of additional data will permit significant improvements to be made in the analysis, and those cases that may be essentially intractable within the time constraints under which the TSPA staff is operating. Our comments are not meant to excuse the Department of Energy from meeting its obligation of demonstrating with the required degree of confidence that the repository will meet or exceed the specified performance targets, should a license application be submitted the USNRC. Instead, they are to suggest that the approach to resolving deficiencies in the TSPA-VA, and the work toward preparation of the TSPA-LA, should be based on a clear understanding of the nature and cause of each deficiency.

For cases in which it is feasible to improve either the component models or their underlying data, the Panel recommends that efforts be made to implement such improvements wherever such changes would affect the overall assessment. Where conservative bounding analyses do not result in unduly pessimistic estimates of the total system performance, the Panel recognizes that it may not be cost-effective to spend additional time and effort refining the assessments and making them more realistic. For those issues for which, by virtue of their complexity, it is not feasible to produce more realistic models supported by data, the Panel recommends that a combination of bounding analyses and design changes be applied.

Our purpose in distinguishing between these situations is to acknowledge that there are some aspects of the analysis for which additional data collection and modeling will produce only small reductions in uncertainty. In such cases, we recommend that the TSPA staff demonstrate, where possible, either in the TSPA-VA reference design or in a revised design, that the cited uncertainties have only limited consequences with respect to the overall repository performance.

Response: A major effort is currently underway to identify and address data deficiencies in the TSPA-VA (DOE 1998a). See the responses to Comments 4 and 34B. The path forward for finalizing the process model and abstraction model development will be documented in the PMRs. The PMRs will describe which analyses will be completed and why, which areas will not be pursued and the expected impact, and which data and models would be developed further as construction and performance confirmation proceeds.

The M&O agrees that component models and the underlying data which are determined to be in need of improvement and which affect the overall performance of the system should be updated where feasible. The next iteration of the TSPA will provide updated models and data to the extent that the schedules and resources allow. Additional improvements may fall into the performance confirmation period, post LA. Additionally, the use of reasonable bounding analyses will be included as an approach in the TSPA-SR/LA for addressing certain models and data as appropriate. The SR design that will be selected later this summer will help render the analysis of some of the uncertainties in the system unnecessary.

The TSPA analyses are just one factor in determining whether or not additional data collection and modeling will be conducted. Other factors are considered when determining the potential value and need for additional data collection and modeling. In general, the TSPA analyses attempt to show the very aspect pointed out by the Panel, but these analyses are not the only inputs for the selection of future work.

3. COMPONENT MODELS OF TSPA-VA

3.1 THE UNSATURATED ZONE UNDER INITIAL CONDITIONS

Comment 43 (page 46): The accuracy of the current infiltration maps was subject to some criticism by the UZ expert elicitation panel. The validity of future Projections is also questionable, given that they are based on present-day values for the various model parameters (including vegetation, cloudiness, etc.).

Response: Future climate and infiltration modeling for the PMRs, and the potential SR and LA, will be based on future projections of temperature, precipitation, vegetation, and other aspects. While uncertainty exists, the M&O intends to use a conservative range of infiltration values based on more realistic models of climate and infiltration.

Comment 44 (page 47): With respect to upscaled flow and capillary properties, the approach taken in the TSPA-VA is to use the van Genuchten model derived for UZ flow in homogeneous soils. Although convenient, use of this model is not justifiable in the present context. Ignored in this approximation are a multitude of processes: the unstable nature of gravity-driven infiltration in real fractures; the possibility of hysteretic (and chaotic) behavior during episodic flow, as documented in recent experiments in related systems (Faybishenko et al., 1998); the effect of sub-grid-scale heterogeneities, including correlated structures, anisotropy in fracture permeability, and saturation gradients; the effect of the connectivity of the fracture and matrix continua; and the effect of abrupt changes in properties on transport fluxes expected along stratigraphic discontinuities. The last item has already been shown to be sensitive to the particular flux-weighting scheme used in the simulations. Also ignored are the differences between wetting and drying cycles, which are expected to develop during the heating period. A similarly questionable approach is used in the modeling of heat pipes in thermal hydrology, where recent findings have shown a complex flow behavior (Hardin and Chestnut, 1997).

Response: Although approximate, the M&O hopes to capture the global behavior of flow through fractured media with the calibrations and the fact that the models demonstrate a consistency with observed data at the site. The alternatives (e.g., discrete fracture-flow models) suffer similar approximations and assumptions as the dual-continuum models. Discrete fracture-flow models often consider simplified representations of the fractured system, and they usually neglect matrix interaction. In addition, confirmation and testing of these discrete-fracture models would be even more difficult at the scale of the site.

Comment 45 (page 47): With respect to the representation of the fracture-continuum/ matrix-continuum coupling, the increase in the estimated infiltration rate has forced the introduction in the dual continuum (DKM) model of an adjustable fracture-matrix interaction factor. In this way, a non-trivial fraction of the infiltration is forced to partition in the fracture continuum. Using the inverse modeling calibration procedure, reducing this interaction factor by as much as four orders of magnitude, has enabled the TSPA team to accommodate changes in the revised infiltration rate, without producing unphysical changes in other hydrological properties.

The introduction of a reduction factor is reasonable and appropriate in order to account for a variety of processes, which are not included currently in the description of physics at the various scales, such as the scale of a single fracture and the scale of a numerical grid block, as mentioned above. However, in the current approach of the TSPA, this reduction factor is simply an adjustable parameter, devoid of convincing physical meaning and often taking values as small as 0.0001. This is not satisfactory and reflects a lack of understanding of the actual physics of the process and, more generally, the lack of progress in the scale-up of two-phase flow in the fractured system, as also noted above.

Response: The fracture/matrix reduction factor has physical meaning. It represents the effects of micro-scale processes such as fingering, coatings, channeling, and film flow. Because it is not possible to fully characterize these processes at the site, the reduction factor is used as a fitting parameter for calibration to the site data. This calibration of the geometric reduction factor was recommended by Gerke and van Genuchten (1993). In addition, the M&O is implementing a new fracture/matrix interaction model that defines the reduction factor as a function of fracture saturation with rigorous derivations of the characteristic curves, based on the effective saturation of active fractures.

Comment 46 (page 48): The difficulties in the above two issues are compounded by the lack of convincing field data to support the representations taken, inasmuch as reliable flow data have only been gathered from core studies. As a result, the Panel is skeptical of the validity of the base case set of hydrologic parameters and particularly of the van Genuchten-type capillary and flow properties of the fracture network and of the fracture-matrix reduction factor. These are all key variables in the partition of flow between fractures and matrix. Given the significance to other TSPA components (seepage fluxes into drifts, thermohydrology, and UZ transport), the Panel believes that efforts should be made to reduce the existing uncertainties, using analytical studies and field tests. Although acknowledging that the upscaling of UZ flow in a fractured system is a non-trivial task, the Panel believes that such a step is also necessary in order to conclusively and unambiguously determine the relevant hydrologic response of the site in developing the TSPA-VA.

Response: Although van Genuchten properties are associated with porous media, they have physical meaning for discrete fractures as well. The estimates of hydrologic properties yield results that are consistent with a physically meaningful depiction of the system, thereby furthering M&O efforts to have correlated data. See, also, the response to Comment 47.

Physically, the fracture/matrix reduction factor is a result of sub-grid scale gravitational fingering and coating effects. It should also be noted that although the active fractures correspond to a small portion of the total connected fractures, their number is still very large for a site-scale model. The M&O will provide better documentation of the physical meaning of this factor in the future and develop a two-dimensional (2-D) grid block-scale fracture network model to further explore sub-grid physics. This will provide a means to address issues regarding upscaling, constitutive relations, and confirming continuum approaches for unsaturated flow in fractures. In a dual-continuum approach, fracture flow will occur when the matrix is at less than full saturation.

The M&O agrees with the Panel that better treatment of fracture/matrix interaction is needed to simulate the transport processes more accurately. The decision to use the dual-continuum approaches with a fracture/matrix reduction factor is based on the following considerations:

  1. Mathematically, it is not a problem to simulate fracture/matrix interaction more accurately by subdividing the matrix blocks using the TOUGH2 code. However, for a site-scale model, it is not computationally practical.

  2. The use of the dual-continuum approach may underestimate the transport into the matrix block. This is because the actual concentration gradients near fracture/matrix interfaces may be larger than those calculated due to the fact that reactions occur near fractures. Since the matrix transport processes correspond to long travel times to the water table, compared to those in fractures, the dual-continuum approach should give conservative predictions of the nuclide transport.

  3. In the site-scale model, the M&O does not intend to give predictions of transport at small scales. Instead, the focus is on the large-scale, long-term, overall behavior of the transport processes.

The Panel suggests the use of an analytical solution to deal with transport processes between fractures and matrix. However, the M&O is unaware of an analytical solution for this purpose where the dual-permeability approach is used that is available in the literature. Some solutions exist for matrix diffusion processes under saturated conditions with no advection in the matrix. These solutions would not be applicable to transport between two continua when advection is present in the matrix continuum. Its use would be highly questionable and not defensible.

From an analytical standpoint, the "fracture/matrix coupling factor" used in TSPA-VA (DOE 1998a) is being replaced by an "active fracture" model described by Liu et al. (1998). The authors contend that this approach provides a more defensible representation of flow through a variably saturated fractured rock mass than was previously obtainable through introduction of an ad hoc "fracture/matrix coupling factor." Field tests to further examine this "active fracture" approach are being conducted as part of the seepage tests in the ESF.

The current conceptual model of the UZ flow system considers that the overall behavior of site-scale flow and transport processes are controlled mainly by relatively large-scale heterogeneities associated with the geological structures of the mountain. The complexity of a model incorporating heterogeneity needs to be consistent with the availability of relevant data. This approach is also supported by field observations, such as matrix saturation distributions. For a given geological unit, measured matrix saturation distributions are very similar from different boreholes, indicating that large-scale matrix flow behavior and effective fracture and matrix hydraulic parameters should be similar within the unit. A site-scale model based on a layered approach can be calibrated relatively easily with multiple data sets and would provide a means of incorporating a significant amount of the available site data. It is straightforward to deal with upscaling issues using inverse modeling where a layered approach is employed. This is because effective parameters are inferred directly by matching the large-scale simulation results with grid-scale observations that are averaged from small-scale in situ measurements.

A two-step approach to upscaling is used. Simple averaging schemes are used to estimate block-scale effective parameters and these estimations are then refined, based on inverse modeling. It is important to note that the currently available stochastic theories have been developed for single-continuum systems and, therefore, cannot be directly applied to dual-continuum systems. For example, these theories indicate that large-scale effective permeability can be considerably larger than those at small scales. However, this may not be true for matrices in unsaturated fractured rock due to the fracture/matrix interaction. Large fractures can act as capillary barriers for flow between matrix blocks separated by these fractures, even when the matrix is essentially saturated (capillary pressure is close to the air entry value). Consequently, the presence of fractures may reduce the effective permeability of the matrix continuum.

Comment 47 (page 48): The Panel believes that at this stage there is still a considerable need for further improvement of the hydrologic characterization of the UZ. This can be obtained by collecting additional data, where possible, by revisiting and relaxing the questionable assumptions made in modeling, and by proceeding with 3-D inverse modeling.

Response: Additional testing to improve hydrologic characterization of the UZ and to refine conceptual models of fundamental hydrologic process within the UZ, such as fracture/matrix-interaction processes, are ongoing in the North Ramp and Main Drift of the ESF and are planned to be conducted in the EWCD. Testing to collect additional data on the hydrologic and transport properties of the Calico Hills nonwelded unit and to test hypotheses of flow and transport within this unit also is underway at the Busted Butte Field Test site. The problems associated with the simplification of one-dimensional flow in performing the model inversions are recognized, and this restriction is being relaxed; both 2-D and 3-D inversions will be completed to support the future TSPA evaluations.

Comment 48 (page 49): The Panel believes that the analysis of seepage is an important new contribution. However, because the sensitivity of the seepage estimates depends on the permeability and capillary structure of the fracture continuum in the immediate neighborhood of the drifts, they are subject to the large uncertainty in our knowledge of the heterogeneity, spatial correlation and anisotropy of these properties.

Response: The Drift Seepage Model will be improved in future efforts. First, the upper model boundary value (determined by site-scale percolation flux) will be revisited, using a more physically based infiltration model. Second, the hydraulic property sets will be recalibrated to recently collected test data from the ESF. This provides hydraulic calibration information to better describe flow at the drift scale and make the model less dependent on small-scale laboratory and site-scale information. Third, additional air permeability information from the ESF will be used to give a better description of the stochastic distribution of the fracture permeabilities. The model will also be revised to consider alternative drift geometries representing partial collapse features. This will simulate asperities along the drift wall that would facilitate a dripping flux. Together, these activities will reduce the uncertainty of the Drift-Scale Model.

Comment 49 (pages 49–50): Given the importance of seepage, its sensitivity to the flow properties around the drifts, but also the short time that has been available for its study, the Panel believes that further work should be done to increase confidence in the TSPA-VA predictions. The following issues require further attention:

  1. Since capillary barriers essentially reflect boundary effects, seepage would be sensitive to the particular geometric and wetting conditions in a small region around the drift wall. The need exists, therefore, for an accurate characterization of the heterogeneity and fracture capillary properties in this area. The effects of the geometrical changes that may result from the collapse of the drift roof in response to thermomechanical or seismic processes need to be analyzed. In addition, given that the actual fracture spacing is of the same order as the grid block size used in the continuum DKM model for the seepage study (grid spacing of 0.5 m), the use of discrete fracture models of transient flow would be more appropriate and should be pursued.

Response: The model geometry will be revised to consider alternative drift conditions representing partial collapse features. This will simulate asperities along the drift wall that would facilitate a dripping flux. An air gap will be assumed between the drift wall and the waste package. Fracture heterogeneity is included at the drift scale by statistically considering the variations in air permeability.

The current approach is to assume all flow is in the fractures within the dual continuum model. Continuum steady fracture flow is simulated both with homogeneous properties and with stochastic variations in fracture permeability and the van Genuchten alpha. Fractures that drain exclusively into the drifts were also simulated. This work has illustrated that heterogeneity causes a broad range of seepage rates per unit area at low percolation rates (less than 100 mm/yr) and similar seepage rates at higher percolation.

The discrete fracture model could be imposed on two scales. The first scale would be a single fracture extending from surface, through the PTn, and into the drift. This model would examine the maximum effect of transient infiltration bringing water into a drift. The second model would be a discrete fracture model at the drift scale. The test data are currently collected at a 0.3-meter frequency from horizontal bores in the niches. These data could be applied to the inverse method to determine effective fracture characteristics and statistical properties of the fractures. Stochastic flow representations could be compared to heterogeneous DKM modeling to determine the sensitivity of the conceptualization of flow at the drift scale. Discrete fracture modeling will be considered for FY00.

  1. Although recent experimental data are reported to be consistent with the seepage analysis, further testing is needed.

Response: Ongoing testing in the ESF and new seepage testing to be initiated in the EWCD will extend the knowledge of fracture permeabilities and seepage. A large-scale seepage test is planned between Niche 3 in the ESF Main Drift and the Cross-Over Alcove in the overlying EWCD.

  1. In the TSPA base case, seepage into the drifts is decoupled from TH, and is assumed to only take place following the end of the thermal period and under ambient flow conditions. The model also assumes that seep locations and rates are time-independent, under conditions of constant climate, and that the drift geometry remains constant. Many of these assumptions will not be valid.

Response: See the response to Comment 12B.

3.2 THERMOHYDROLOGY

Comment 50 (page 53): It is important to realize that the process of dissolution is much slower than the process of precipitation, and as a result, a cap of significant thickness is projected to still be present after thousands of years (indicated in Hardin [1998] to last as long as 900,000 years). Thus, the properties of the cap in controlling seepage into the EBS, and eventually onto the canisters is of critical importance to the problem of establishing how the environment of the repository will change over time and how this will effect the distribution and quantity of water flowing through the repository.

Response: Although natural analogues were not specifically mentioned by the Panel as a means of addressing this problem, the M&O will assemble data from geothermal fields that have experienced self-sealing over thousands of years, and will assess the factors that have contributed to the formation of the caps, their longevity, and their effects on permeability of the system over long time frames. This information will be used to help evaluate the conditions under which silica caps would form and remain at Yucca Mountain.

Comment 51 (pages 53–55): Even though the Panel is unable to determine from the available information whether the results are sufficient to provide the level of information and support needed in the TSPA-VA, the coupled effects and their absence from the TSPA-VA are a significant cause of concern. In the opinion of the Panel, the assumptions made in the TSPA-VA that the effects of THC activities will be short-lived and can be neglected are not warranted... .

Response: Coupled THC effects in the host rock that may cause changes in the hydrologic conditions that persist beyond the thermal period will be more directly evaluated in the next TSPA using abstracted results of the coupled process-level models. The TSPA-VA (DOE 1998a) analyses attempted to capture some of the transient coupled effects in the models for the gas and water chemistries perturbed by thermal effects, but these were derived only in a loosely coupled way, based on the thermal hydrology process-level model results. The coupled THC models being developed should allow further improvements in the consideration of these processes.

The site-scale UZ flow and transport model has been enhanced to provide the capability of modeling coupled THC effects within the UZ rock mass involving the assemblage of minerals known to be present within the tuff (e.g., silica, feldspar, calcite, zeolites, and clays) in the presence of both water and air, including explicit consideration of the partial pressure of carbon dioxide. This simulation capability will be applied to evaluate the effects of durable changes in both hydrologic and transport properties within the rock mass above and below the repository as a result of chemical reactions induced by repository heat release. This reactive-chemistry modeling capability can be similarly applied at the drift scale to examine near-field effects in the vicinity of the waste emplacement drifts and is currently being used to predict the expected THC effects in the rock mass affected by the Drift-Scale Heater Test.

Comment 52 (pages 55–57): ...the Panel believes there are some complications in analyzing the thermal processes that need further attention.

In this regard, the Panel offers the following comments:

  1. The abstraction methodology presented is a commendable attempt to provide answers to a complex problem and represents a significant improvement over the previous TSPA reports. In order to avoid detailed three-dimensional (3-D) thermohydrologic calculations, which are computationally expensive or possibly intractable, the approach taken is to use a quasi two-dimensional (2-D) scheme to analyze TH behavior in drifts in symmetric elements. Variability due to waste-package type, 3-D and mountain-scale considerations are incorporated by conducting less expensive, conduction-only calculations at the mountain-scale. Thermohydrologic and conduction-only models are made compatible by a creative, but heuristic, scheme which maps 3-D conduction-only temperatures to quasi 2-D TH temperatures. Quantities important for other TSPA components, such as temperature, RH and air mass fraction in the drift, are estimated using this approach.

    In addition to the various problems of modeling the actual physical processes, to which we will refer below, one conceptual problem with such an approach is that detailed information on the temporal and spatial dependence of various properties is sought by using a combination of various approximate methods, which by nature are best suited for estimating average quantities. Another problem is the assumption that the relation between TH properties, such as RH and air mass fraction, and the drift temperature, remain the same as calculated for an isolated symmetric drift, regardless of its environment, the history and sequence of the loading, or the possible lack of symmetry around the drift. As a result, proper account has not been taken of the TH interactions between adjacent drifts and the effects of natural convection. The possibility of heat pipe instability in some locations, with a resulting condensate seepage (as observed in the Large Block Test,     Hardin, 1998) cannot be predicted. The method also over predicts the performance of drifts at the repository edge. Coarse scale factors, such as a reduction of the actual heat load to an effective value, are introduced to handle such problems. Although the methodology is a creative approach, it needs to be tested against real data (perhaps in conjunction with the Drift Scale Test) to validate the results of the analyses.

Response: The Panel’s comments on the TH abstraction methodology consists of several points: 1) the influence of the assumption of symmetry in the 2-D TH submodels (employed in the abstraction methodology) on the prediction of lateral condensate migration and heat-pipe instabilities, 2) the influence of natural convection, 3) the performance of drifts close to the repository edge, 4) the use of the Drift-Scale Heater Test to validate the abstraction methodology, and 5) the influence of drift-scale variability. The following paragraphs successively address these five points.

The Panel’s comment about the abstraction methodology assuming an isolated symmetric drift is partially accurate. The 2-D TH submodels used in the abstraction methodology do not assume that the drifts are isolated from each other. These 2-D TH submodels assume a finite spacing between drifts and are run for a range of thermal-loading conditions spanning those applicable to the edge of the potential repository and those applicable to the potential repository center. However, because the 2-D TH models assume that heating conditions and property distributions are symmetrical about the drift axis, the possibility of a large-scale lateral migration of the condensate (from drift to drift) is not considered in the abstraction methodology. The Panel’s concern about the possibility of heat-pipe instability resulting in condensate seepage into drifts was a valid concern for the VA design (DOE 1998b).

The LADS process developed an enhanced design alternative that was specifically designed to prevent the coalescence of the boiling zones between drifts and, thereby, facilitate the drainage of condensate between the drifts. In the enhanced design, preliminary thermal-hydrological calculations indicate that less than approximately 20 percent of the repository horizon reaches the boiling point of water. Because 80 percent of the rock pillar separating drifts never reaches the boiling point of water, it is extremely unlikely that the condensate will migrate laterally from drift to drift. The limited lateral (as well as vertical) extent of the boiling zones also limits the volume of rock in which fractures may undergo substantial changes in porosity and permeability as a result of coupled THC processes. In the VA design (DOE 1998b), the ponding of condensate above the boiling zones was the major factor leading to the development of a very thick heat-pipe zone above the repository horizon. If this enhanced design alternative is selected, it will greatly limit the ponding of condensate above the repository horizon, thereby restricting the vertical extent of the heat-pipe zone to only a few meters.

The Panel’s comment about the effects of natural convection being neglected by the abstraction methodology is partially accurate. The effects of thermal buoyancy (i.e., natural convection) are not accounted for at the mountain scale; however, the effects of thermal buoyancy are accounted for at the drift scale. Because the 2-D TH submodels used in the abstraction methodology have lateral no-flow boundaries, and because the water table is a barrier to vapor flow, these models force almost all of the vapor (generated during the boiling period) to flow upward above the repository horizon. This biasing of vapor flow causes almost all of the condensate generation to occur above the repository horizon, thereby over-predicting the condensate drainage flux back into the emplacement drifts. The biasing of vapor and condensate above the repository is equivalent to exaggerating the influence of thermal buoyancy; therefore, the influence of drift-scale thermal buoyancy is over-predicted by the abstraction methodology for TSPA-VA (DOE 1998a). Note that the biasing of vapor and condensate flow only occurs if the boiling zones coalesce between the drifts. For a repository design that does not result in coalescence of the boiling zones, the above-mentioned biasing of vapor and condensate flow does not occur in the drift-scale TH models. For some of the TSPA-SR/LA calculations, the M&O plans to replace the mountain-scale, conduction-only heat-flow model in the abstraction methodology with a mountain-scale TH model and, thus, be able to assess the influence of mountain-scale, buoyant gas-phase convection in the predictions of drift-scale TH conditions.

The Panel’s comment about the abstraction methodology over-predicting the performance of drifts at the repository edge is only accurate if the influence of mountain-scale buoyant gas-phase convection is found to be strong enough to significantly enhance the rate of cooling at drift locations close to the repository edge. Otherwise, the abstraction methodology does not over-predict the performance of drifts (with respect to temperature increase and duration of dry-out zone) at the repository edge. The Panel’s comment that the abstraction methodology uses coarse-scale factors to represent the influence of edge cooling at the repository edges is incorrect. The abstraction methodology uses a finely gridded mountain-scale, conduction-only model and functional relations between drift-scale TH models and drift-scale conduction-only models to determine, in a highly resolved manner, the entire spectrum of waste package conditions. These conditions range from the coolest waste package locations on the repository edge to the hottest locations in the center of the repository. Once the abstraction methodology is modified to employ a mountain-scale TH model (rather than a mountain-scale, conduction-only model), the possibility of enhanced cooling by natural convection will be fully addressed.

The Panel’s suggestion about testing the abstract