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1. INTRODUCTION

At the request of the U.S. Department of Energy, Bechtel SAIC Company, LLC, formed the Waste Package Materials Performance Peer Review Panel (the Panel). The Panel's charge is to conduct a consensus peer review of the current technical basis and the planned experimental and modeling program for the prediction of the long-term performance of waste package materials being considered for use in a proposed repository at Yucca Mountain, Nevada.

1.1 Organization of the Peer Review

The Panel consists of a Chairman and six members who are technical experts in the materials science and engineering disciplines needed for the comprehensive peer review. A list of Panel members and brief biographies are attached (Appendices A and B).

During its review, the Panel called on the expertise of a group of Subject Matter Experts, engaged to provide technical advice to Panel members in the areas of corrosion, materials science, and geochemistry. These experts were given a general description of the proposed repository; however for the most part, they were not called upon to review the application, operating conditions or Project information in detail. The Subject Matter Experts provided advice to Panel members by participating in meetings, responding to questions, and preparing papers (see Compilation of Special Topics Reports, to be issued in March, 2002). The Panel retains full responsibility for assimilating any of the experts' work in the Panel's reports and recommendations.

This is the Panel's final report; the Panel issued an interim report on September 4, 2001. During their review, members of the Panel attended Project meetings, technical exchanges, and workshops. They met with and received technical information from Project personnel and others. Some exchanges worthy of special note include:

Waste Package Materials Performance Peer Review Kickoff Meeting, Las Vegas, Nevada, May 23, 2001

International Workshop on Long-Term Extrapolation of Passive Behavior, conducted by the U.S. Nuclear Waste Technical Review Board, Arlington, Virginia, July 19-20, 2001

Panel Sub-Group Meeting on Localized Corrosion, conducted by the Panel, Arlington, Virginia, July 21, 2001

State of Nevada and Nuclear Regulatory Commission Presentations to the Panel, conducted by the Panel, Cleveland, Ohio, July 24, 2001

Panel Sub-Group on Waste Package Fabrication, conducted by the Panel, Livermore, California, August 10, 2001.

Panel Sub-Group Meeting on Stress Corrosion Cracking Defensive Strategies, arranged by BSC with relevant Project staff, Las Vegas, Nevada, November 12, 2001

The Panel wishes to acknowledge the cooperative and open discussions and interactions that were experienced throughout this review period.

1.2 Objectives of the Review

The overall objectives of the review, as stated in the Peer Review Plan, Revision 1, June 6, 2001, were to:

Review the current basis for predicting long-term corrosion performance of Alloy 22 waste package and titanium Grade 7 drip shield materials and the on-going and planned experimental and modeling program to increase confidence in long-term performance projections.

Assess the adequacy of the experimental and modeling program and recommend any augmentations of existing and planned tasks or additional new tasks that would significantly strengthen the program.

Prepare an Interim and a Final Peer Review Report laying out the Panel's assessment of the current and planned program adequacy to assist the Department of Energy in prioritizing future work plans.

These overall objectives were broken down into sub-elements, each with an associated set of questions that constitute the focus of the peer review (Appendix C).

1.3 Content of the Final Report

This Final Report presents the overall findings, conclusions, and recommendations of the Panel. It is a comprehensive report and builds upon the material and recommendations from the Interim Report. In addition to an Executive Summary and brief Introduction, the Final Report contains:

Overview sections:

Section 2

Section 3

Detailed sections:

Sections 2

Contributing Factors:

Section 11

A "Compilation of Special Topic Reports", prepared for the Waste Package Materials Performance Peer Review will be released in March 2002.

2. MAIN FINDINGS

2.1 Perspective

The U.S. Department of Energy is analyzing a site at Yucca Mountain, Nevada, for development as a geologic repository for the disposal of high-level radioactive waste and spent nuclear fuel. The containment strategy for the disposal site is twofold: first, complete isolation of the waste until the first, full-thickness penetration of the waste package and, second, subsequent retardation of the egress of radionuclides from the penetrated waste package.

Although the waste packages, in all likelihood, will not be immersed in water because the repository is well above the water table, water does permeate the mountain from the ground surface down to the water table. Consequently, the waste packages will be exposed to air with high relative humidity. There is also a possibility that water droplets will fall onto the waste packages where water seeps into the emplacement drifts. The surrounding rock will increase in temperature from heat given off by the spent nuclear fuel, while the repository will remain at ambient, atmospheric pressure. Under these conditions, corrosion is the most significant potential degradation mode for waste package materials, although other potential degradation modes must also be considered.

Nickel-base Alloy 22 and titanium Grade 7, the principal alloys of interest, have excellent corrosion resistance over a wide range of aqueous solution compositions and temperatures. Two major considerations are the fabrication processes for the manufacture of waste packages and the time-temperature conditions of the repository. Fabrication processes, particularly welding, can have a major impact on corrosion resistance and performance. Temperature has major effects on the composition of the environment and the behavior of materials.

The Panel reviewed the approach, analysis, and methods used by the Yucca Mountain Project (the Project) and the current technical basis and the planned experimental and modeling program to support the evaluation of the long-term performance of waste package materials. The Panel’s task was to assess the level of confidence in the Project findings, to identify any deficiencies, and to make recommendations to strengthen the technical basis and reduce uncertainties.

The Panel is composed of scientists and engineers with expertise in materials performance and environmental effects on materials. Their framework for assessment followed the traditional methodology for materials selection and determination of expected performance: (a) identification of the performance requirements and environmental conditions for the waste packages, (b) consideration of alternative materials suitable for the conditions, (c) identification of potential degradation modes for the materials, and (d) assessment of the technical basis for determining resistance to the degradation modes.

2.2 Overall Findings

The Panel concludes, based on the body of technical information currently available, that Alloy 22 is a suitable material of construction for the outer barrier of a waste package: nevertheless, significant technical issues remain unsettled. Although the technical basis supporting the suitability of Alloy 22 is substantial and growing, there will always be uncertainty in the evaluation of the long-term performance of materials in the repository. This is primarily because of the extremely long life required for the waste packages. The Panel concludes that the confidence regarding the long-term performance of Alloy 22 can be substantially increased through further experiments and analyses, and the Panel recommends that this work be undertaken.

The Panel concludes that titanium Grade 7 might not be a suitable material of construction for the drip shield. Stress corrosion cracking of titanium Grade 7 has been observed in laboratory tests. If these tests are deemed to have occurred under realistic conditions for the drip shield over waste packages in the repository, then the benefits of titanium are severely diminished. The Panel does not recommend the use of titanium for drip shields, if it is found to be susceptible to stress corrosion cracking under realistic repository conditions.

The Panel has identified technical issues that have the potential to require a change in the waste package material or design and suggests approaches to the resolution of these issues. We believe that further comprehensive analysis and testing can resolve these issues. The issues are presented in Section 2.7.

The effective control of corrosion of waste package materials is essential to the satisfactory long-term performance of a Yucca Mountain repository. The proposed waste package material, nickel-base Alloy 22, has excellent corrosion resistance over a wide range of aqueous solution compositions and temperature. The Panel concludes that the Project staff is taking a sound approach to analyzing corrosion related issues.

Although the nominal waters at Yucca Mountain are fairly benign and not corrosive, water composition can change at the metal surface. The range of environments that could exist on these surfaces depends on the composition of waters that can enter the drifts and changes to the environment that can occur on hot metal surfaces and in crevices. Three conditions describe the surfaces of metal that will be subject to corrosion at Yucca Mountain: accumulated dust and particulate on the metal, deposits and scale on the metal, and tight areas of contact (crevices) between metals; moisture must be present for corrosion to occur under any of these conditions. The Panel recommends that a strong technical basis be developed for materials performance under each of these conditions. To help accomplish this, the Panel recommends the formation of a task group of Project technical experts in corrosion, materials science, geochemistry, and hydrology to work together to determine the range of the composition of the environments that could contact waste package surfaces and changes that could occur in the environment on hot metal surfaces and in crevices. The task group should have the authority and responsibility to direct the work.

Corrosion and mechanical behavior of waste packages must be considered in the selection of design and fabrication methods. The Panel notes a worrisome gap between the design/fabrication effort and the materials/corrosion effort. The former needs to recognize and control the effects of fabrication processes on metallurgical structure and residual stresses, and the latter needs to express findings in useable guidelines to direct design and fabrication decisions. The materials/corrosion work to date has focused on supporting performance assessment. The Panel concludes that it is time to balance this effort with work to support design and fabrication of the waste packages. In particular, work is required on weld and weld repair processes.

In summary, the Panel concludes that the current waste package design is likely to meet the performance criteria for the repository, if some technical issues are favorably resolved. Overview findings on three important corrosion modes are presented below. In addition, comments are made regarding two important issues: (a) the effects of higher-temperature versus lower-temperature operating mode on the expected long-term performance of waste packages and (b) the likelihood of long-term uniform corrosion rates remaining very small for hundreds and thousands of years.

2.3 Corrosion Degradation Modes

Three corrosion failure modes have been identified and are being evaluated by the Project staff: general corrosion of passive metals, localized corrosion, and stress corrosion cracking. The Panel concludes that there is a substantial and growing technical basis to support the evaluation of the long-term performance of waste package materials in each of these areas.

Uniform Corrosion. Failure of the waste package by uniform corrosion is unlikely. The long-term corrosion behavior is determined by the structure and composition of the passive film on Alloy 22. The passive film is influenced by thermal treatments during waste package fabrication, dry exposure periods in the repository, and finally the exposure to aqueous environments when the surfaces become wet. Work is underway to better understand the structure and composition of the passive film under these conditions and to relate that to the long-term corrosion behavior. The Panel concludes that if the environmental conditions do not cause film-breakdown and localized corrosion or if the underlying metal does not undergo detrimental metallurgical changes to destabilize the film, then it is highly likely that the passive film will remain stable and uniform corrosion rates will remain very low. Two phenomena that could jeopardize the desirable long-term behavior are worthy of study: (a) surface segregation of sulfur and (b) transpassive corrosion. Surface segregation of sulfur can occur as a result of thermal treatments (equilibrium segregation) and as a result of uniform corrosion (anodic segregation). Reducing the bulk sulfur concentration of the alloy can minimize both thermal segregation and anodic segregation of sulfur. Transpassive corrosion might be caused by radiolysis of water sitting on the surface of the waste package (a process that the Project states is unlikely to produce transpassive potentials) and/or by a combination of all of the following: low pH, Fe+3 (from corrosion of steel structures), some amount of radiolysis of the water, and an inhomogeneous alloy microstructure (i.e., grain boundary segregated phosphorus, precipitates rich in chromium and molybdenum).

Localized Corrosion. Nickel-base Alloy 22 and titanium Grade 7 are extremely resistant to localized corrosion; they have exhibited no evidence of pitting or crevice corrosion after four-year exposures to environments similar to those that may form on waste packages in a Yucca Mountain repository at temperatures up to 85 C. Nevertheless, these alloys are susceptible to crevice corrosion under extreme conditions of environment and potential. The framework used by the Project for the evaluation of resistance to crevice corrosion is appropriate. The Panel recommends a more rigorous approach for determination of the critical potentials. The critical potential should be the repassivation of an intentionally creviced electrode following significant crevice corrosion. The corrosion potential should be modeled and measured under a range of exposure conditions, taking into account the initial surface condition and the effects of high temperature exposure to air. Furthermore, the Panel recommends that the Project perform experiments under conditions beyond those thought to be relevant to Yucca Mountain in order to examine the margins of corrosion resistance.

Stress Corrosion Cracking. The mitigation method, experimental approach, and modeling efforts for stress corrosion cracking are consistent with the state-of-the-art, and appropriate work generally is underway to verify the models. Research to date shows that nickel base Alloy 22 is highly resistant to stress corrosion cracking in the environments expected in the repository. Modeling is an essential component of the stress corrosion cracking program, because the laboratory test techniques have inadequate sensitivity to crack growth for the prediction of acceptable long term performance of the waste package. The Panel recommends additional work to address some deficiencies in the current program including improving tests for establishing sensitivity to crack initiation and propagation, replacing the threshold stress component of the slip dissolution model with the threshold stress intensity factor model, and developing constants specific to Alloy 22 for the film rupture/slip dissolution model. The Panel recommends that alternative models for stress corrosion cracking be considered by the Project. Alternative models can provide validation and support for the Project approach. Alternative models also may be required, if the current models are found to be deficient.

The Panel recommends that the effectiveness of heat treatment for stress mitigation be thoroughly evaluated by finite-element thermal and stress analysis and by residual stress measurements on prototype containers. The Panel further recommends that the stress corrosion cracking behavior of Alloy 22 be evaluated with different stages of aging and in environments containing trace impurities, such as lead, that may influence cracking behavior.

2.4 Higher or Lower Temperature Operating Modes

There is considerable interest in the effect of operating temperature on the long-term performance of the repository. In the high temperature operating mode, the waste package surface temperature is limited to no more than 180°C and in the low temperature operating mode, to no more than 85°C. The maximum temperature and time-temperature behavior within the drifts can be controlled by operational parameters: distribution by waste package type, spacing between packages, and the amount and duration of ventilation prior to closure. From a materials standpoint, major incentives for operation at lower temperatures are reduced corrosion rates, a lower likelihood of localized corrosion or detrimental metallurgical aging processes, and less opportunity for evaporative condensation.

On the other hand, lower temperature operation would expose wet waste package surfaces to higher radiation fields the accompanying radiolysis effects because they become wet sooner. The potential effects of radiolysis on the environment and corrosion behavior need to be determined. Other disincentives for operation at lower temperature are the increased costs and risks associated with longer ventilation periods and a larger repository area required for a given amount of spent fuel. As with most design/operation decisions, there are tradeoffs associated with lowering the temperature and there may be diminishing returns for further temperature reductions.

Major benefit was gained in increased confidence and certainty of the hydrological-geological analysis and modeling when the Project discarded the very hot repository case, the base case for the Total System Performance Assessment conducted for the Viability Assessment. In the very hot case, the boiling/dry-out zones from adjacent drifts overlapped and the dry-out zone extended tens of meters into the rock near the repository. In both of the lower temperature cases currently under consideration, no dry-out zone overlap occurs, greatly reducing the volume of rock and water affected by temperature. In addition to the hydrological-geological benefits, the change from consideration of the very hot case also increased confidence in the control of damage to the waste package materials from corrosion and long-term metallurgical aging processes.

The Panel concludes that the benefits of moving from the high temperature operating mode, as currently defined, to the low temperature operating mode, are not clearly greater and might be offset by the radiolysis effects, in addition to the burdens of long-term ventilation and increased area for the repository. Work is required for testing and analysis at the higher operating temperatures before final conclusions can be drawn (see Technical Issues to be Resolved below).

2.5 Long-Term Uniform Corrosion of Passive Metal

Based on studies published in the open literature and on the results of investigations by the Project, the Panel has concluded that the waste package of Alloy 22 is unlikely to fail by uniform corrosion in the repository at Yucca Mountain. Nevertheless, important work remains to be done to confirm and enhance the resistance of Alloy 22 to uniform corrosion. Alloy 22 depends completely on its protective surface films for resistance to uniform corrosion. Research is needed (1) to the characterize the oxide films formed on Alloy 22 during high temperature heat treatments; (2) to characterize the changes that occur to the air-formed oxide films on Alloy 22 during exposure to the range of aqueous solutions relevant to Yucca Mountain.

Although failure of the waste packages by uniform corrosion is not likely, there are two possible causes of significant uniform corrosion of Alloy 22 that cannot be ruled out at this time: (1) transpassive corrosion and (2) corrosion caused by surface segregation of sulfur. In section 7, the Panel recommends specific experiments to alleviate concerns regarding uniform corrosion induced by transpassivity and surface segregation of sulfur. The outcomes of the experiments on surface segregation might indicate the need to reduce the bulk sulfur concentration of Alloy 22 to a value considerably less than the current specified maximum value and to limit the temperature of stress relief annealing of the waste package canisters to less than 1100°C.

2.6 Alloy Specification and Comparison

The Panel regards Alloy 22 as a representative member of a family of highly corrosion resistant nickel-chromium-molybdenum alloys. Today’s commercial alloys arise from a steady evolution in alloy development. It is unlikely that the evolution is complete. Due to either technical or commercial incentives, modifications within or around the chemical composition specified for Alloy 22 will emerge. The chemical composition of heats of commercially available Alloy 22 can vary within the specified range for each element. The Panel recommends that the Project staff determine the effects of alloy composition within the specified ranges of Alloy 22. It is possible that a more restrictive chemical composition specification for given elements would reduce uncertainty and increase confidence with respect to the stability of the alloy and the corrosion behavior of waste packages.

Conversely, there will be a strong economic incentive to use less expensive alloys. This should be evaluated most carefully. The Panel recommends that the Project not designate metals less corrosion resistant than Alloy 22 type metals, in view of confidence and uncertainties. Clearly, 304, 316, 825 and others would fail under much more benign conditions than would Alloy 22. It is prudent to use the most corrosion resistant alloy available unless strong, credible evidence is found that a less resistant alloy will work.

The Panel recommends that a back-up alloy be included in certain of the tests performed on Alloy 22 to address the risks associated with the possibility that, further into the Project, Alloy 22 is found to be inadequate. In addition, the Project should include a comparison alloy, such as 316L stainless steel, Alloy 825, Alloy 600 or Alloy 690, in a greater number of tests than is presently the case. This would provide a means for quantitatively comparing the performance of Alloy 22.

2.7 Technical Issues to be Resolved

The Panel has identified the following technical issues that have the potential to require a change in the waste package material or design. As noted above, the Panel concludes that the current waste package design is likely to meet the desired performance criteria in the repository, if these issues are resolved favorably. The Panel believes that further comprehensive analysis and testing, such as those we suggest below, can resolve the issues in a timely fashion.

The need to strengthen the technical basis for assessing waste package performance is clearly recognized by the Project staff. While extensive experimental and analytical work to support performance assessments has been completed, a large amount of necessary work remains in Project plans and has yet to be completed. Only a modest portion of the work on fabrication issues is complete.

The Panel is concerned that adequate resources might not be allocated to complete the work necessary for evaluating the long-term performance of waste package materials. The Panel’s perception is that a substantial effort is required to accomplish the needed work to support design and fabrication of durable waste packages. The Panel strongly recommends that adequate resources be provided to substantially build confidence in the long-term performance of waste packages. This area is particularly amenable to progress through experiments and modeling.

Design and fabrication procedures: Metallurgical structure, residual stresses, and metal surface condition are all affected by fabrication procedures and, in turn, these conditions can greatly affect corrosion behavior. The Panel recommends a coordinated analysis and testing program between the design/fabrication effort and the materials/corrosion effort. The inclusion of realistic mock-ups and prototypes, in addition to laboratory specimens is expensive, but necessary.

Develop and validate weld procedures: The closure weld and its postweld processing are critical to long-term performance of waste packages. The two primary issues regarding long-term integrity of the welds in the Alloy 22 corrosion barrier are the level and nature of residual stresses associated with the closure welds and the stability of the weld metal microstructure. The production and testing of full-scale mockups is required, and a weld repair procedure must be developed and validated.

Realistic boundaries for environmental conditions: The determination of the realistic range of aqueous environments on metal surfaces is well underway. The Panel recommends that the Project complete a comprehensive experimental and analytical modeling program to establish the boundaries for three environmental conditions: moist dust, mineral scale and deposits, and crevices. The determination of environment should include temperature, oxidizing potential, pH, and composition of ionic species. Work on microbiologically influenced corrosion needs to refocus on the consequences of this corrosion-related process within the realistic boundaries.

Corrosion behavior within the range of realistic environmental conditions: Project staff should determine the performance boundaries of Alloy 22 in each of the identified corrosion modes: uniform corrosion, localized corrosion, and stress corrosion cracking. The testing should go beyond the range of realistic conditions in order to determine the margins of safety.

Radiolysis from gamma radiation: There is no evidence to suggest that radiation damage to the waste package canister material will alter its mechanical properties; therefore, radiation damage studies of alloy 22 are not warranted. In addition, there is no evidence that radiation damage of the passive film will alter its protective properties. However, the production of H₂O₂ and other products by radiolysis from gamma rays can result in a positive shift in the open circuit potential and possible degradation of passivity. The Panel recommends experiments and analysis of radiolysis effects at gamma radiation fluxes that will exist when condensed water is present on waste packages.

Proper measure of crevice corrosion susceptibility: The critical potential for localized corrosion of Alloy 22 should be determined by a repassivation potential procedure for intentionally creviced samples that undergo crevice corrosion.

Corrosion potential over long periods: Values for the corrosion potential are used in models of corrosion modes. The corrosion potential should be measured over long periods of time in a range of environments with varying chloride concentration, oxyanion concentration, pH, temperature, and radiation. Initial surface conditions must be considered. Theoretical approaches for predicting the corrosion potential over long periods of time and the influences of the various environmental parameters should supplement the experimental work.

Long-term metallurgical stability: Cr-Mo depletion and Long Range Ordering. Long term exposure of Alloy 22 to higher temperatures than those expected for the repository can lead to the degradation of corrosion resistance by Cr-Mo depletion and degradation of mechanical properties by long range ordering. The Panel recommends that Project staff determine whether chromium and molybdenum depletion occurs adjacent to the grain boundaries, and if so the time and temperature relationship for this depletion and its impact on corrosion and stress corrosion cracking. The Panel recommends that long range ordering be given equal importance to that for precipitation because of the implications of ordering for stress corrosion cracking and the potential effects of deformation adjacent to the induction heated and quenched zone accelerating the ordering kinetics.

Effects of sulfur and phosphorus: Sulfur and phosphorus are minor constituents in Alloy 22, and when enriched to high concentrations at grain boundaries and surfaces, they can have detrimental effects on corrosion and stress corrosion cracking resistance. The Panel recommends that Project staff: 1) measure the effects of sulfur on passive film stability in relevant repository environments; 2) model and/or measure the rate of sulfur accumulation on the surface as a function of corrosion rate, if sulfur affects the passive film stability; and 3) evaluate the potential for grain boundary impurity segregation as a function of time and temperature in the bulk and weld metal and in the heat affected zone of welds.

Effectiveness of tensile stress reduction for mitigating stress corrosion cracking: A principal component of the Project's stress corrosion cracking control strategy is to reduce the tensile stresses on waste package surfaces. Experience from large-diameter rolls in paper making machines demonstrates that this is not easily accomplished. There is insufficient experimental data and analytical modeling to support the position that there will be no significant tensile stresses on the waste packages. Experimental and modeling work to support this effort needs to consider the time-temperature constraints during processing to avoid detrimental metallurgical structures on cooling.

Stress corrosion crack growth and consequence: Because of the repository's long time frame, the stress corrosion crack growth rates of interest are less than the current experimental detection limits. The Panel recommends further work to demonstrate sufficiently slow crack growth through calibration and validation of stress corrosion cracking models. Stress corrosion cracks cover only a small fraction of a metal surface; they provide a tight, tortuous path through a metal thickness. The Panel recommends an analysis of the likely location, distribution, and geometry of stress corrosion cracks. This will provide useful input to the determination of water ingress through the waste package wall and the eventual egress of radionuclides from the waste package in the event that stress corrosion cracking does occur.

Hydrogen embrittlement of Alloy 22: The nickel-chromium-molybdenum family of alloys is known to be susceptible to hydrogen embrittlement. The degree of susceptibility is a function of alloy composition, cold working, and thermal history among other variables. Hydrogen can result as a by-product of corrosion processes. The Project has discounted hydrogen effects in Alloy 22 based on plans to use the alloy in an annealed condition and the known resistance of other nickel-chromium-molybdenum alloys in the annealed condition. The Panel recommends experimental work on Alloy 22 to support this position. While the fabrication strategy is designed to eliminate tensile stresses from the outer surface of the waste packages, industrial experience has shown this to be difficult to achieve completely. It is prudent to determine the affects of tensile stresses that can result from fabrication processes such as quenching and welding.

2.8 Organizational-Managerial Issues

Five fundamental elements make up the waste package design and performance portion of the overall Project: design, engineering, analysis, modeling, and testing. These five fundamental elements are organizationally separated in the current structure. The Panel considers that better integration of these elements is essential. The Panel has identified two areas that especially require further focus and integration of Project efforts: determination of the realistic range of aqueous environments on waste package surfaces and design and fabrication of waste packages for corrosion resistance. For the former, the Panel recommends the direct, collaborative participation of technical experts in corrosion, materials science, geochemistry, and hydrology in work on these conditions. For the latter, the Panel recommends closer integration between the design and fabrication engineers and the corrosion and materials experts working on performance assessment.

Increased involvement of technical experts from academia and industry in conceptual work, experimental method development, and analytical procedures could enhance the substance and level of confidence in the technical basis to support evaluation of the long-term performance of materials. Unfortunately, current management and administrative policies and procedures tend to limit broader involvement. A more effective means to engage a broader base of the corrosion science and engineering community in work relevant to the long-term performance of materials at Yucca Mountain would have significant benefits.

The Panel recommends the establishment of an External Advisory Board. Such a Board would be helpful in determining and maintaining focus and direction in the execution of the research agenda of the staff. As mentioned above, the five fundamental elements related to the design and performance of the waste package are organizationally separated at present. An Advisory Board should include both academics and industrial experts with credentials in areas important to the Project and should meet with the staff on a regular basis, perhaps twice a year on-site with interim phone conference updates. We note, moreover, that while the Project staff does include investigators who have important assets and skills, there is no visible senior, visionary leader with a deep materials science and engineering background and management credentials.

The Panel recommends that a back-up alloy and comparison alloy be included in the experiments and analysis. A back-up alloy should be included in certain of the tests performed on Alloy 22 to address the risks associated with the possibility that, further into the Project, Alloy 22 is found to be inadequate. In addition, a comparison alloy, such as, 316L stainless steel, Alloy 825, Alloy 600 or Alloy 690, should be included in a greater number of tests than is presently the case. Results obtained for the comparison alloys will provide a means for quantitatively describing the performance of Alloy 22. This recommendation also appears in the technical sections above; the Panel considers this to be both an organizational and a technical issue.

3. SUMMARY OF DEGRADATION MODES AND CONTRIBUTING FACTORS

3.1 Introduction

In the current design, the repository includes waste packages constructed of an outer barrier made of nickel-chromium-molybdenum Alloy 22 surrounding an inner container of Type 316 nuclear-grade stainless steel. When the repository is closed, the waste packages will be covered by a titanium Grade 7 (titanium-palladium) alloy drip shield. Under anticipated repository conditions, corrosion is expected to be the most significant degradation mode for the Alloy 22 and titanium Grade 7 alloy surfaces that will be exposed to the repository environment. Hence, the rate of corrosion will be a critical factor in determining the service life of the waste packages.

Alloy 22 and titanium Grade 7 are among the most corrosion resistant engineering materials that are available today. Nevertheless, like all engineering materials, these alloys are subject to environmental degradation under certain circumstances. The likelihood that a given type of corrosion phenomenon will occur is dependent upon a complex interplay among the metallurgy of the alloy, the repository environment, and the stress distribution of the waste package while in service. The identification of potential degradation modes and the evolution of corresponding control strategies are, therefore, crucial to the long-term stability of the waste package. Failure of the containment due to corrosion would ultimately allow release of radionuclides to the environment.

The Panel's views on potential degradation modes and descriptions of factors that are likely to affect anticipated corrosion phenomena are summarized in the following sections.

3.2 Repository Conditions: Overview of Time, Temperature, Environment

A particularly challenging aspect of the analysis of the durability of the waste packages in the Yucca Mountain Repository is the extraordinarily long time period for performance. During an operational phase of 50 years, emplacement of waste packages will be carried out. This will be followed by a monitoring phase to 300 years. After which, the repository will be closed beginning the closure phase. In the analysis of potential degradation modes for the waste packages, it is important not only to consider the conditions that could initiate a particular form of damage, but also to consider the time period over which those conditions persist.

Of particular importance are the temperature of the waste package surfaces and the chemical composition of wet environments in contact with the waste packages over time. The conditions in the repository will be determined by a combination of natural and man-made factors. The waste material gives off heat and radiation, which both decrease with time. Thermal effects diminish over several thousands of years, while radiation effects diminish after a few hundred years. At the repository level, waste packages will be isolated beneath 300 meters of rock and will be a couple hundred meters above the water table. At this level, the waste packages will sit in air on support pallets. Although the ambient air will be saturated with water equivalent to 100% relative humidity, it is highly unlikely that the waste packages will be fully immersed in water. Nonetheless, moisture will condense and seepage from the rock can drip onto metal surfaces. While the amounts of moisture will be small, there would be sufficient water for corrosion. Hence, corrosion resistant metals are required.

Operational factors affect the time-temperature behavior in the repository and in the surrounding rock. These factors include the waste package loading, spacing of waste packages within the repository and duration and level of ventilation prior to closure. Two operational modes are now under consideration. In the higher-temperature operating mode, the waste package surface temperature could reach 180°C, the surrounding rock would be heated above boiling, and the rock would be dried out near the drifts. In the low-temperature operating mode, the waste package surface would be maintained below 85°C, the surrounding rock would not be heated above boiling, and there would be a minimal dry-out zone in the rock.

In addition to the challenges, there are several favorable aspects of the long-term storage. The waste packages will be exposed to one, long and slow, temperature cycle. There will be no moving parts. This favors good materials performance. The static exposure will not subject the waste packages to potentially detrimental cyclic loads. The low heat fluxes and extremely slow heating and cooling will not expose the waste packages to large thermal gradients or rapid thermal expansion and contraction. In a higher temperature operating mode, the waste packages would be exposed to dry conditions for long times (several hundred years) before the surfaces are wetted.

Time-Temperature-Relative Humidity

Presence of Moisture

Composition of Waters and Corrosive Environments

Atmospheric Conditions

Mechanical Loads and Stresses on Waste Packages

Damage Processes and Waste Package Penetrations

3.3 Composition of Aqueous Environments

In the view of the Panel, the most critical issues concerning the environment on the waste package involve the determination of the nature of the plausible extremes of environments. Temperature, composition of the environment, and the presence of microbes all can contribute to or affect these plausible extremes. A technically sound approach to the problem of determining the environment on the waste packages is to define the physical/chemical bounds of environments that can be expected. The Project staff should base such a determination upon known physical and chemical processes, inherent variability throughout the repository, and uncertainties in quantitative determination of the coupled processes that affect the environment. The Project should also make a similar determination of the performance bounds of waste package and drip shield materials. A comparison of the corrosion performance bounds to the environment bounds would allow the Project to estimate the likelihood that the waste package or drip shield would be exposed to environmental conditions under which corrosion failure would be likely to occur.

The Project staff's approach to determining the corrosion performance of proposed materials has focused on the use of concentrated simulated waters based on analyses of J-13 well water. The concentration levels in the test solutions have ranged up to 1,000 times the concentration of J-13. Project staff has also studied acidified solutions. In addition, some corrosion work has been performed in single salt solutions with concentrations up to 5 M. The majority of the work has been performed at temperatures below the boiling point of pure water at the elevation of Yucca Mountain (96°C). These solutions do not represent relevant, plausible extreme environments, particularly at the temperatures being used in the Long-Term Corrosion Test Facility. Concentrated aqueous solutions can exist in the liquid phase at temperatures well above the boiling point of water.

The Panel recommends that Project staff:

Expand the current work on understanding the effects of the interactions between seepage waters and hot (greater than 100°C) metal surfaces on both the solution composition and the corrosion of engineered materials. The goal is to provide information on both the nature of the deposits that form and the type and rate of corrosion underneath the deposits. Experiments at high temperatures should have higher priority than lower temperature (less than 100°C) studies. The Panel notes that Project staff has recently begun experimental work in this area and we support and encourage this new direction.
Continue to focus on developing a technical basis for environmental extremes that are realistic for each of the three surface scenarios (moist dust, scale, and crevices). These analyses must also include a delineation of the inherent uncertainties, such as the relative importance of transient vs. steady state environments.
Improve coupling between thermal-hydro-chemical, engineered barrier system, and environment calculations and the experimental results. Data needed for the environment on the waste packages must drive the nature of the computational and experimentation thermal hydrology work in the unsaturated zone.
Shift the focus in the study of microbiologically influenced corrosion. It is unlikely that the Project will be able to demonstrate unambiguously that microbes are not viable under expected repository conditions. Instead, the Project should characterize the possible metabolites and the effects of these on the engineered materials, including any steel components used in the repository.

3.4 Metallurgical Stability

There are three processes that can affect the metallurgical stability of Alloy 22. These are: 1) precipitation of intermetallic phases, 2) ordering of the nickel and solute atoms to have specific locations on the crystal lattice, and 3) enrichment of impurity atoms such as sulfur and phosphorus at crystal boundaries and free surfaces. The presence of intermetallic phases and grain boundary segregation can alter the corrosion and mechanical performance of Alloy 22, lattice ordering can promote hydrogen-induced cracking, and surface enrichment of sulfur can degrade passivity. Each of these processes is thermally activated and can be described by the appropriate kinetic equations to predict reaction rates as a function of temperature and time. While these metallurgical processes occur very slowly at proposed repository conditions, it must be demonstrated that either they will not occur during the life of the repository or, if they do occur, that they will not significantly affect the canister performance. Welding and stress mitigation process can both produce grain boundary precipitation that serves to initiate the process for further growth at repository conditions.

The Project is focusing on developing the necessary models to predict the precipitation and ordering in Alloy 22 as a function of repository relevant times and temperatures. The Project is making experimental measurements of precipitation using materials aged for extended periods (up to 40,000 hours at 260, 343, and 427°C) at Haynes Alloys. These materials have been shared with the Project and they serve as an excellent source of aged material. The extent of precipitation is being measured primarily by scanning electron microscopy and some by transmission electron microscopy analysis, with an emphasis on grain boundary precipitation. Phase identification is being done by transmission electron microscopy for P,

, carbide and Ni₂(Cr, Mo). The kinetics of ordering reactions in Alloy 22 is being treated by the Project in a manner very similar to that of precipitation. The sluggishness of these kinetics is a significant hindrance to obtaining sufficient data for a high confidence extrapolation to 10,000 years. Very small ordered domains have been observed by transmission electron microscopy, but the volume fraction was difficult to measure. Therefore, the Project has used some bounding arguments to establish the extrapolation to 10,000 years. The results show that long-range ordering will form in 10,000 years at 270° to 300°C. The Project has chosen not to evaluate either grain boundary or surface segregation effects. While the kinetics of thermally activated segregation will be very slow at repository temperatures, the potential effects of segregation, should it occur, are significant. Also, surface enrichment during anodic dissolution does not require much corrosion and is, again, potentially very significant should impurities such as sulfur degrade the passive film.

The Panel recommends that Project staff:

Determine whether chromium and molybdenum depletion occurs adjacent to the grain boundaries and, if so, determine the time and temperature relationship for this depletion and the impact of this depletion on corrosion and stress corrosion cracking.
Give long-range ordering importance equal to that given to precipitation because of the implications of ordering for stress corrosion cracking, hydrogen embrittlement, and the potential effects of deformation adjacent to the induction-heated and quenched zone accelerating the ordering kinetics.
Measure the effects of sulfur and phosphorus on passive film stability in relevant repository environments; model and/or measure the rate of sulfur and phosphorus accumulation on the surface as a function of corrosion rate, if sulfur and phosphorus affect the passive film stability; and evaluate the potential for grain boundary impurity segregation as a function of time and temperature and in the heat affected zone.

3.5 Long Term Uniform Corrosion

Experimental measurements of the uniform corrosion of Alloy 22 performed by the Project, as well as empirical studies of the uniform corrosion of nickel-chromium binary alloys and nickel-chromium-iron/molybdenum ternary alloys conducted by other researchers, strongly suggest that waste packages will not fail in the repository at Yucca Mountain as a result of the uniform corrosion of Alloy 22.

The resistance of Alloy 22 to uniform corrosion is the result of the highly protective nature of the films that form on the surface of Alloy 22. The oxide films formed on Alloy 22 in air and the passive films formed in aqueous solutions are multi layered. The most important component of the multi-layered films is the inner barrier layer, which resembles Cr₂O₃. The precise composition and structure of the entire film, and that of the inner layer is a function of the chemistry of the solution, in particular the pH of the solution, and the composition of the alloy.

The ability to predict the long-term, uniform corrosion behavior of Alloy 22 requires (1) complete knowledge of the structure and composition of Alloy 22's passive films and air-formed oxide films, and (2) complete understanding of the environmental and metallurgical factors that dictate the films' compositions and structures. The most important environmental factors are temperature, solution pH, and corrosion potential. The most important metallurgical factors are the bulk chromium and sulfur concentrations of Alloy 22.

The Project has begun to characterize the high temperature oxide films and the passive films that form on Alloy 22. Results to date are consistent with published results for films on nickel-chromium binary alloys and nickel-chromium-iron/molybdenum ternary alloys.

Collectively, the measurements of the uniform corrosion rate of Alloy 22 by changes in weight of test coupons in the Long Term Corrosion Test Facility, and by electrochemical techniques of samples immersed in a variety of electrolytes indicate the uniform corrosion rate of Alloy 22 in the passive state is extremely low and would result in losses in thickness of only several mm in 10,000 years of continuous exposure to environments that might contact the waste package in the repository.

The Project has demonstrated that large increases (

500 mV) occur in the corrosion potential of Alloy 22 in acidic, aqueous solutions at 90°C. Based on increases in oxidation rate of Alloy 22 in short time (

10 hr.) potentiostatic tests at high potentials, there was initial concern that high corrosion potentials would be accompanied by higher rates of uniform corrosion. Tests that are underway by the Project have shown that the ennoblement of the corrosion potential of Alloy 22 is, in fact, associated with very low rates of corrosion.

The ability of Alloy 22 to resist uniform corrosion depends on the stability of its passive film. Changes in the passive film might be caused by changes in the environment and/or changes in the alloy. Analyses by the Panel identified two possible processes that could lead to high rates of uniform corrosion of Alloy 22 in the repository at Yucca Mountain are (1) increases in corrosion potential caused by a combination of radiolysis of the water and the formation of oxidizing species, such as Fe+3, by the corrosion of steel structures proximate to the waste packages and (2) surface segregation of sulfur by either thermal treatments of anodic segregation.

The most important recommendations by the Panel regarding the uniform corrosion of Alloy 22 are related to (1) characterization of the passive films and air-formed oxide films of Alloy 22; (2) determination of the critical bulk sulfur concentration of Alloy 22 below which surface segregation of sulfur by either thermal treatments or anodic segregation will not occur; (3) determination of the risk that a combination of high oxidizing potentials (caused by radiolysis of the water and Fe+3 from corrosion of nearby steel components), high temperatures and adverse metallurgical processes (such as surface segregation of sulfur and the formation of chromium-rich second phases) would simultaneously exist and cause high rates of uniform corrosion of Alloy 22.

Finally, efforts to model the passive film and uniform corrosion rate of Alloy 22 should continue. The teaching of the history of engineering is that margins tend to get used up to enhance the ratio of performance to cost. In the context of the use of Alloy 22 for waste packages, the tendency will be to fully exploit the corrosion resistance of Alloy 22 by decreasing the wall thickness of the waste container. Modeling the uniform corrosion behavior of Alloy 22 will help to define the needed thickness plus safety margin required of the waste containers. In addition, models can help to explore the different corrosion rates that will result from the myriad of combinations of temperature, solution composition and corrosion potential over long periods of time.

The Panel recommends that the Project staff:

Surface segregation of sulfur and transpassivity are the two most likely factors that would cause significant rates of uniform corrosion of Alloy 22. Therefore, changes in the structure and composition of the passive film as a function of surface segregation of sulfur and as a consequence of highly oxidizing potentials should be investigated.
The Panel recommends that two causes of surface segregation of sulfur be investigated: equilibrium segregation due to high temperature heat treatments and anodic segregation resulting from uniform corrosion of Alloy 22. The Panel recommends the determination of critical bulk sulfur concentrations below which heat treatments and anodic segregation do not cause significant increases in the uniform corrosion rate of Alloy 22.
Determination of the likelihood of radiolysis of the water causing transpassive dissolution of fully homogenized Alloy 22. This should be done for the range of solution compositions relevant to the repository at Yucca Mountain and in particular, the full range of solution pH should be investigated.
The Panel recommends that changes in structure/composition of the high temperature oxide films as a result of immersion in the various solutions relevant to the repository at Yucca Mountain be investigated. Changes in the films need to be investigated at a range of potentials and temperatures. Such experiments have been proposed by the Project and some work is already underway.
The Panel recommends testing the uniform corrosion rate of coupons in the Long Term Corrosion Test Facility at temperatures higher than the present maximum of 90 °C. Higher test temperatures are warranted (1) by the increase in oxidation rate of Alloy 22 with increasing temperature, which was observed in short duration electrochemical tests, and (2) by the knowledge that salts with deliquescence points of 25% RH, might contact the surface of the waste package. The presence of salts with low deliquescence points would place the waste package in contact with an aqueous phase at temperatures as high as 160°C, albeit for a relatively short period of time.
The Panel recommends testing the uniform corrosion rate of Alloy 22 at high passive potentials for long periods of time. Short time electrochemical tests indicate the uniform corrosion rate of Alloy 22 increases with applied passive potential. High corrosion potentials might be established on waste canisters of Alloy 22 as a result of the combined effects of Fe⁺³ corrosion products from steel structures in the vicinity of the waste package and radiolysis of the water on the surface of the waste package.

Determination of the combinations of radiolysis of water, composition of the water, in particular, solution pH and concentration of Fe⁺³, and metallurgical factors, including surface segregation of sulfur, grain boundary segregation of phosphorus, and precipitates of chromium rich and molybdenum rich phases, that might interact synergistically to initiate transpassive dissolution at relatively low potentials. Results in the literature for Ni-Cr alloys suggest that such factors might initiate transpassivity locally, resulting in the formation of oxidizing corrosion products (e.g., Cr(VI) species) that lead to high rates of uniform dissolution of the alloy.

3.6 Localized Corrosion

Alloy 22 and titanium Grade 7 are extremely resistant to localized corrosion; they have exhibited no evidence of pitting or crevice corrosion after four-year exposures in the environments predicted to form on the waste package. However, these alloys are susceptible to crevice corrosion under extreme conditions of environment and potential. The Project is using the

E criterion for localized corrosion susceptibility, which is based on the difference between a critical potential, E_CRIT, and the corrosion or open circuit potential, E_OC. This approach assumes that localized corrosion will not occur if

E is positive, that is, if the E_OC is less than E_CRIT. If properly applied, this is an appropriate, viable, and conservative approach to prediction of long-term susceptibility to localized corrosion. This approach requires accurate knowledge of both E_CRIT and E_OC as a function of all the important environmental conditions (chloride, heavy metal, and inhibiting ion concentration, temperature, pH) and, in the case of E_OC, time.

Even though the

E criterion is valid and useful for the prediction of localized corrosion, the Project's approach for determination of the values of E_CRIT and E_OC, from which

E is calculated, is not appropriate. The Project has determined

E values from potentiodynamic polarization curves measured on uncreviced, freshly-prepared samples in a few different predicted waste package environments. Neither Alloy 22 nor titanium Grade 7 regularly exhibits localized corrosion under these conditions, but Alloy 22 does corrode transpassively at high potentials. Because

E is positive and large for all tested environments, Project staff predicts that localized corrosion will never occur. However, because the critical potentials measured are associated with the onset of transpassivity and not localized corrosion, it is inappropriate to use these data for the prediction of localized corrosion. Project staff has not explored other environments from which extrapolations could be made, including more aggressive environments. The Panel recommends that the Project follow an approach for determination of

Another major problem with the model developed by the Project is the use of E_OC values determined in potentiodynamic polarization curves. Such values are not representative of realistic waste package surfaces, which will be affected by the whole fabrication procedure, emplacement, and some period of exposure to hot air. The open circuit potential value is expected to vary with time as a result of continued improvement in passivity and changes in the environment. The Panel recommends that realistic and relevant E_OC values be measured and modeled, so that predictions can be made of how the open circuit potential will vary over long periods of time.

Temperature is a critical factor in localized corrosion with higher temperatures being more likely to cause localized corrosion. The Panel recommends that E_R,CREV and E_OC values used for

E be determined at higher temperatures and, most particularly, at all temperatures to which the waste packages might be exposed in the presence of an aqueous layer.

The Panel further recommends that the Project staff:

Undertake studies of localized corrosion propagation and arrest to supplement work on initiation because these factors will control the accumulated extent of damage from localized corrosion should it occur. Experiments in aggressive environments provide the opportunity to study growth and arrest behavior. Since there is a chance that the waste package surface will be exposed only to a moist dust environment rather than to dripping or full immersion, the possible influence of such an exterior environment on limiting localized corrosion growth should be studied.
Examine the effects of metal surface condition on localized corrosion. The metal surface condition can have a large influence on the localized corrosion behavior. In general, the susceptibility to localized corrosion increases as the roughness of the surface finish increases. On the other hand, protective surface films can reduce the likelihood for localized corrosion.
Address the influence of structural changes on the susceptibility to localized corrosion. The structure of Alloy 22 might change during long-term aging at moderate temperatures; it will certainly change locally as a result of the multipass weld required to seal the canister. The Panel recommends that experiments for determination of E_R,CREV described above be performed on welded samples.
Investigate environments containing possible oxidizing agents, such as peroxide or ferrous/ferric. The possible effects of radiolysis should be reevaluated for the current waste package design and expected condition of spent fuel.
Examine localized corrosion behavior for the most relevant conditions that can result in wet environments in contact with metal surfaces: moist dust surface associated with humid air exposure or scale and deposits formed by evaporated drips. Full immersion of metal surfaces is a highly unlikely condition in the repository. It is unlikely that the thin aqueous layer on a waste package will be able to support the cathodic reactions needed to maintain localized corrosion to the same extent as possible in bulk solutions. This has not been addressed by anyone in the field of localized corrosion. The Panel recommends that the Project complete analysis of the initiation, propagation, and arrest of crevice corrosion and pitting under the conditions listed above.

3.7 Stress Corrosion Cracking

Stress corrosion cracking is a corrosion damage mode during which slow crack growth in a metal or metal alloy is caused by the conjoint action of a tensile stress and a cracking environment. Stress corrosion cracking may compromise the integrity of the waste package. Project staff is addressing this integrity threat in the design of the waste package and in the research program. The Project plans to mitigate stress corrosion cracking of the waste package through the use of the highly stress corrosion cracking resistant Alloy 22 and by control of residual tensile stresses. Research to date shows that nickel base Alloy 22 is highly resistant to stress corrosion cracking in the environments expected in the repository. Residual tensile stresses introduced during fabrication will be relieved by a stress anneal of the entire canister prior to filling and final closure. In the vicinity of the final closure welds, beneficial residual compressive stresses will be introduced into the outer surfaces of the waste package by laser peening or local induction annealing treatments.

The mitigation method, experimental approach, and modeling efforts for stress corrosion cracking are consistent with the state of the art and appropriate research work generally is underway to verify the models. However, the Panel has identified a few deficiencies with the current program.

The Project staff has proposed two stress corrosion cracking models: an initiation model based on a threshold stress intensity factor and a propagation model based on film rupture/slip dissolution. Sensitivity of the crack growth measurement techniques is a critical issue for experiments designed to verify both of the models. In the current experimental procedures used by the Project staff, the sensitivity to crack growth has not been established and appears to be inadequate. The Panel recommends that the maximum sensitivity to crack growth in the threshold stress intensity factor and crack propagation tests be established and improved. If necessary, a combination of accelerated testing, longer exposure periods, and fractography could be used.

In the film rupture/slip dissolution model, crack propagation is assumed to occur above a threshold stress. The threshold stress criterion used in the model is not conservative. Threshold stresses for the initiation of cracking on surfaces containing defects are generally much lower than those of smooth surfaces on which the threshold values used in the model are based. The Panel recommends that the threshold stress component of the slip dissolution model be replaced with the threshold stress intensity factor model for the welded portions of the waste package which will contain preexisting flaws.

The calculations of the constants in the film rupture/slip dissolution model for Alloy 22 are based on relationships developed for 304 stainless steel in 288°C water. It is likely that these constants are not applicable because of differences in the deformation behavior between 304 stainless steel and Alloy 22. The Panel recommends that Project staff develop constants specific to Alloy 22 for the film rupture/slip dissolution model.

The Panel further recommends that alternative models for stress corrosion cracking be considered by the Project. Alternative models can provide validation and support for the Project approach. Alternative models also may be required if the current models are found to be deficient.

The Project plans to reduce residual tensile stresses in the waste package to mitigate the initiation of stress corrosion cracking by means of a stress anneal and water quench of the entire Alloy 22 waste canister, followed by an induction anneal or laser peening of the final closure welds. Based on experience with suction roll shells in the pulp and paper industry, the quenching process after solution annealing could produce unacceptably high tensile residual stresses. On the other hand, slow cooling could lead to the initiation and growth of deleterious grain boundary precipitates. The Panel recommends that the issue of heat treatment for stress mitigation and alloy stability be thoroughly evaluated.

Two approaches can be employed to minimize tensile residual stresses induced by cooling after solution annealing. The first is to cool the fabricated containers slowly. This approach may require the use of an alternative nickel base alloy (to Alloy 22) that is not susceptible to sensitization during the slow cooling process. The second is to carefully design and control the quenching process so that minimal levels of tensile residual stress are produced at or near free surfaces during the quenching operation. The selected approach should be verified by finite-element thermal and stress analysis and by experimental measurements on prototype containers.

The Project's future corrosion test plans include the evaluation of the resistance to stress corrosion cracking of aged and welded-and-aged microstructures in simulated repository environments in the long-term corrosion test facility. The specimens will be aged to produce grain boundary precipitation, but currently there are no plans to evaluate the different stages of aging. The early stages may be more deleterious than later stages. For example, the potential for chromium and molybdenum depletion near the precipitates may be greater early in the aging process. This depletion may be healed later in the process. The Panel recommends that the effects of early stages of ordering, grain boundary segregation, and impurity segregation on stress corrosion cracking be evaluated. These processes may be dependent on the composition of the alloy, so the Project staff should evaluate the role of heat-to-heat variation on stress corrosion cracking behavior for the different stages of aging.

Minor constituents in the repository environment may influence cracking behavior. In a study of secondary-side intergranular stress corrosion cracking in Alloy 600 tubing from one-through and recirculating steam generators, lead was found in the leading edges of cracks in several instances including three where lead contamination in the aqueous environment was not reported. The Panel recommends that the issue of the effects of minor constituents, such as lead, in the repository environment on stress corrosion cracking be evaluated in the testing program.

3.8 Hydrogen Effects

The nickel-chromium-molybdenum family of alloys has a well-known history of hydrogen embrittlement susceptibility. The degree of susceptibility is a function of alloy composition, cold working, and thermal history among other variables. Hydrogen is potentially available during repository operation as a product of the cathodic partial reaction in a general corrosion process or, perhaps more likely, in a localized corrosion cell associated with crevice geometries or microbial activity. It appears that the Project has discounted hydrogen effects in this nickel-based alloy since the structural requirements for the waste package outer barrier at the Yucca Mountain site are such that Alloy 22 could be used in the annealed condition which, given the history of other nickel-chromium-molybdenum alloys, is likely to be resistant to hydrogen embrittlement. There is, however, no known research on the hydrogen susceptibility of Alloy 22 that would support that position. While the fabrication strategy is designed to eliminate tensile stresses from the outer surface of the waste packages, industrial experience has shown this to be difficult to achieve completely. It is prudent to determine the affects of tensile stresses that can result from fabrication processes such as quenching and welding.

The Panel recommends that experimental work be undertaken to explore the hydrogen embrittlement susceptibility of cold worked and cold worked and aged Alloy 22 in repository environments to simulate service conditions associated with residual stresses derived from welding and fabrication.

3.9 Fabrication of Waste Packages

Design and fabrication details can have a great effect upon corrosion and mechanical behavior of waste packages. The Panel notes a gap between the design/fabrication effort and the materials/corrosion effort on the Project. The former needs to recognize and control the effects of fabrication processes on metallurgical structure and residual stresses, and the latter needs to express findings in useable guidelines to direct design and fabrication decisions. For example, thermal treatments need to be selected with awareness of their effects upon metallurgical structure. Waste packages can be fabricated to meet the relevant specification regarding mechanical performance but have a detrimental microstructure for corrosion performance. The condition of the metal surface, i.e. surface roughness, residual stresses, oxide films and surface contaminants, can adversely affect corrosion performance.

The materials performance and corrosion work to date has focused primarily on supporting performance assessment. The Panel concludes that it is time to balance this effort with work to support design and fabrication of the waste packages. In particular, work is required on weld and weld repair processes. This is not a recommendation to end the science program, but rather to redirect and balance the science effort to support design and manufacture/fabrication for long-term performance. Many of the same scientist/engineers and experimental/modeling procedures are appropriate to address the important issues with regard to fabrication of durable waste packages.

Fabrication processes affect the surface condition, metallurgical condition, and stress patterns of the waste packages. Welds are a primary area of concern. The effects of other fabrication processes, such as solution annealing, assembling of dual cans, and induction heating are also important. Tight areas of contact (crevices) between metals are potential trouble spots for corrosion.

The Panel recommends that the Project staff:

Undertake a weld procedures development and validation program. The closure weld and its postweld processing are critical to long-term performance of waste packages. The two primary issues regarding long-term integrity of the welds in the Alloy 22 corrosion barrier are the level and nature of residual stresses associated with the closure welds and the stability of the weld metal microstructure. The production and testing of full-scale mockups is required and a weld repair procedure must be developed and validated.
Carry out a weld filler metal qualification program. The ERNiCrMo-10 filler metal has been used in the early development studies because it is the matching composition to the Alloy 22 base metal. It is likely that better long-term performance can be achieved by the use of an alternate filler metal. The effect of postweld annealing on the weld microstructure is not clear, nor is the long-term stability of the weld metal relative to long range ordering.
Evaluate alternate welding processes for the closure weld. While the GTAW process is reliable and dependable, it is slow and results in high levels of residual stress. Electron beam welding is attractive because it eliminates the need for a weld joint and can be conducted in a single pass, thus reducing the weld time significantly. A "single shot" process such as friction welding is also very attractive from both a metallurgical, residual stress, and productivity standpoint. Because friction welding is a solid-state process, many of the concerns regarding long term metallurgical stability resulting from segregation during weld solidification are minimized. There may also be other "hybrid" processes that are applicable.
Determine the composition effects within the chemical specification for Alloy 22. Composition specifications for Alloy 22 are relatively broad, and the microstructure and properties of Alloy 22 are likely to be sensitive to chemistry and processing history within the specified composition. The goal is to narrow the chemistry ranges so that detrimental phases are kept to a minimum while maintaining satisfactory mechanical properties and corrosion resistance. A set of vacuum melted ingots weighing on the order of 50 pounds could be hot rolled to plate stock, and thus provide a substantial supply of test material. The effects of thermal exposures could then be determined, along with spot checks of the mechanical properties and corrosion behavior. In addition, autogenous GTA weld beads could be made on test coupons and examined before and after thermal exposures to provide information on how to optimize the filler metal chemistry. It would also useful to make several heats of weld wire with modified compositions to check the weldability.

3.10 Radiation Effects

The waste canister and surrounding environment will be subjected to a flux of neutrons and gamma rays from the stored radioactive waste. These fluxes can cause the following damage: 1) neutrons will produce atomic displacement damage in the metal, 2) neutrons will produce atomic displacement damage and gamma rays will cause electron-hole pairs in the passive film and 3) gamma rays will cause radiolysis of the surrounding environment. The peak neutron flux has been calculated to be about 5 x 10⁴ n/cm²-s The total neutron fluence, taking the most conservative estimate with no nuclear decay of the waste, will be 1.5 x 10¹⁶ n/cm² in 10,000 years. The peak gamma flux is about 1000 rad/hr at the time of emplacement (DOE Report-Dose calculation); decreases to approximately 10 rad/hr after 200 years; and decreases to approximately 0.1 rad/hr after 400 years. Radiolysis from gamma radiation is only a factor in the environment on the waste package surfaces when the waste packages are wet.

There is no evidence to suggest that radiation damage to the waste package canister will alter its mechanical properties; therefore, radiation damage studies of alloy 22 are not warranted. Also, there is no evidence that radiation damage of the passive film will alter its protective properties; although, this is based on studies by Saito et al. (1997) of Type 304 SS at 280°C. There are differences in the passive film formed on the iron based Type 304 SS at 280°C and the lower temperature (less than 170°C) film formed on the nickel based Alloy 22.

Results suggest that the production of H₂O₂ by radiolysis is the primary cause of a shift in the open circuit potential, but that this effect has not been sufficiently studied for Alloy 22 at gamma fluxes that will exist when condensed water is present.

The Project staff has calculated nitrogen oxide, nitrogen acids and ammonia production during the period when the waste package is exposed only to moist air. Also, little consideration has been given to the effect of radiolysis in condensed water and its affect on the corrosion behavior of Alloy 22. Glass et al. (1986) have measured the effect of radiolysis of J-13 well water on the corrosion behavior of Type 304 and Type 316 SS and found a 150 to 250 mV shift in the open circuit potential. Gamma radiation effects have not been conducted on alloy 22 although an approximation of these effects have been conducted by measuring the open circuit potential as a function of H₂O₂ concentration in both simulated acidified water and simulated concentrated water. These measurements were made at 25°C and a shift in the open circuit potential of about 225 mV was noted for simulated acidified water and about 300 mV for simulated concentrated water at an H₂O₂ concentration of 72 ppm. The critical issue is the actual concentration of H₂O₂ expected in the repository environment, at the time that liquid water is formed, and the corresponding shift in the open circuit potential.

The panel recommends that the Project complete the following analysis: 1) perform a calculation of the H₂O₂ concentration expected in the repository water, 2) measure the open circuit potential at the repository relevant H₂O₂ concentration, 3) directly measure the shift in the open circuit potential of alloy 22 in the presence of gamma irradiation, 4) assess the stability of the open circuit potential as a function of time for Alloy 22 exposed to gamma irradiation. Radiolysis measurements should be conducted using repository relevant waters in an autoclave system that allows continuous measurement of the open circuit potential to be made up to 170°C. High temperature measurements are desirable because of the need to measure film stability and because this stability is temperature dependent. It would be desirable to measure the current-voltage response of the material in the gamma flux but this is of a lesser priority then measuring the stability of the open circuit potential as a function of time. It would also be desirable to measure the film chemistry following long-term exposure to the ionizing gamma irradiation field, but this is a lower priority because of the complexity of measuring film chemistry when samples are transported through air. Available gamma sources generally produce at least a factor of 100 times higher flux than that expected in the repository. Therefore, tasks numbered 1 and 2 are needed to provide specific data on open circuit potential shifts at repository relevant conditions. The current-voltage curves should be determined as a function of H₂O₂ concentration at repository relevant temperatures.

3.11 Ennoblement

During the past year, Project personnel have made open circuit potential measurements on Alloy 22 samples exposed in the Long Term Corrosion Test Facility at Lawrence Livermore National Labs for as long as four years. The open circuit potentials were considerably higher than those found for fresh samples immersed in fresh solution. Details of these observations can be found in recent Project reports. The corrosion potential of aged Alloy 22 samples in the old solution from the Long Term Corrosion Test Facility was found to be over 500 mV higher than that of new samples in new solution. Measurements on new samples in the old solution and on aged samples in new solution indicated that the ennoblement was caused partly by enhanced passivity and partly from an increase in the oxidation potential of the solution.

Despite the increase in corrosion potential observed in the samples exposed in the Long Term Corrosion Test Facility, no evidence of localized corrosion has been found on any of the samples. These observations underscore the resistance of Alloy 22 to localized corrosion. Furthermore, potentiodynamic polarization experiments have shown that the open circuit potential remained well below the potentials associated with transpassivity.

It is clear that

E determined from a potentiodynamic polarization curve on a fresh sample in fresh solution is not representative of the situation that can develop with time. The surface will change as a result of the improved passivity and the oxidizing power of the solution may increase.

The Panel recommends that the electrochemical measurements on samples exposed in the Long Term Corrosion Test Facility continue, as should measurements on fresh samples in old solution and on aged samples in fresh solution. These experiments will provide improved understanding of the ennoblement process.

4. REPOSITORY CONDITIONS AND POTENTIAL DEGRADATION MODES

4.1 Repository Conditions: Overview of Time, Temperature, Environment

A particularly challenging aspect of the analysis of waste packages in the Yucca Mountain Repository is the extraordinarily long time period required for performance. During an operational phase of 50 years, emplacement of waste packages will be carried out. This will be followed by a monitoring phase to 300 years. At that time, the repository will be closed beginning the closure phase. The Panel’s main objective was to assess the technical basis for the determination of waste package performance over 10,000 years. In order to do this, it was necessary to consider the time-temperature-environment conditions in the repository over this long time. In the analysis of potential degradation modes for the waste packages, it is important not only to consider the conditions that could initiate a particular form of damage, but also to consider the time period over which those conditions persist.

The exposure conditions for waste packages in the repository are subject to the initial repository configuration and natural processes over time. Change over time can be affected by engineered processes, such as waste package loadings and distribution of waste packages in the repository and by natural processes, such as the amount of precipitation on the mountain. Of particular importance are the temperature of waste package surfaces and the chemical composition of wet environments in contact with the waste packages.

The waste material gives off heat and radiation at rates that decrease with time. Thermal effects diminish over several thousands of years, while radiation effects diminish over a few hundred years. At the repository level, the waste packages will be isolated beneath 300 meters of rock and are a few hundred meters above the water table. At this level, the waste packages will sit in air on support pallets. The ambient air will be saturated with water equivalent to 100% relative humidity. Although it is highly unlikely that the waste packages will be fully immersed in water, however, moisture will condense and seepage from the rock could drip onto metal surfaces. While the amounts of moisture will be small, there would be sufficient water for corrosion, and corrosion resistant metals are required.

Operational factors in the repository affect the time-temperature behavior of the waste packages and the surrounding rock. These factors include the waste package loading, spacing between waste packages, and the duration and level of ventilation prior to closure. Two operational modes are now under consideration. In the higher-temperature operating mode, the waste package surface temperature could reach 180°C, the surrounding rock would be heated above boiling, and the rock would be dried out near the drifts. In the lower-temperature operating mode, the waste package surface would be maintained below 85°C, the surrounding rock in not heated above boiling, and there would be a minimal dry-out zone in the rock.

There are several favorable aspects of the long-term storage. The waste packages will be exposed to one, long and slow, temperature cycle. There will be no moving parts. This favors good materials performance. The static exposure will not subject the waste packages to potentially detrimental, cyclic loads. The low heat fluxes and extremely slow heating and cooling will not expose the waste packages to large thermal gradients or rapid thermal expansion and contraction. In a higher temperature operating mode, the waste packages would be exposed to dry conditions for long times (several hundred years) before the surfaces are wetted.

It is broadly accepted that dry waste packages will not undergo significant corrosion damage. When the metal surfaces are wet, there is a potential for corrosion damage. Great emphasis has rightfully been placed upon the determination of times-of-wetness and the corresponding temperature and chemistry of the wet environments. These results are summarized below, and more details on the chemistry of wet environments are presented in a separate section of this report. Other aspects of interest, e.g. the mechanical loads and stresses on waste packages and the form and distribution of penetrations in waste packages, are also presented here.

4.1.1 Time-Temperature-Relative Humidity

The duration and level of ventilation prior to closure will have a significant effect upon the time-temperature response of the rock near the repository. With ventilation for 50 years, the mountain-scale thermal-hydrological model predicts completely dry drifts with temperatures rising above boiling after 500 years and cooling back to near boiling after 1000 years. The dry zone would extend approximately 10 meters into the rock surrounding each drift, and the drifts would be spaced so the dry zones from adjacent drifts would not overlap. With 300 years of ventilation, temperatures would not reach 85°C in the rock, with highest temperature being reached after 500 years. These data are for a thermal loading of 1.35 kW/m. Lower thermal loadings would result in lower temperatures and smaller dry zones.

Drift-scale, analytical models for a range of thermal loadings have determined the waste package temperature and relative humidity in the air surrounding the packages. There is good agreement between the Project results and those from the Center for Nuclear Waste Regulatory Analysis. A similar description of the behavior emerges for the time-temperature-relative humidity conditions.

During the emplacement and ventilation period, the waste package temperature and relative humidity in the drifts will be low as heat and moisture are removed from the repository. On closure, the temperature will rise to a peak temperature within 10-20 years, and the relative humidity will be further reduced. This will be followed by a long period over which the waste packages cool and the relative humidity increases.

For 50 years of ventilation prior to closure, a peak temperature of waste packages of 160 to 180 °C would be reached after approximately 15 years. At the time of the peak temperature, the relative humidity in the drifts would be less than 20% and the packages would be dry. The packages would likely remain dry for hundreds of years. The following summary statements were determined by taking data for temperature-relative humidity behavior from a Project presentation (YMBlink_08/02/01.ppt). On cooling, the following is a set of matching temperature-relative humidity values:

Waste Package Temp, °C	Relative Humidity, %	Condition
100	55-90	Wet
110	45-60	Wet
120	35-45	(Dry)
130	30-35	Dry
140	25	Dry
150	20	Dry

Correspondingly, 200 years after emplacement, the temperature would be 120-140 °C and relative humidity would be about 40%, and after 500 years, the temperature would be 110-120 °C and relative humidity is less than 60%. After 900 years, the temperature would be 100°C with a relative humidity of 80% and condensation of moisture would be present. The relative humidity level at which sufficient moisture forms to support corrosion processes varies with local chemistry, but it is likely that 20% is dry, 80% is wet and sufficient moisture forms around 60%.

For 300 years of ventilation prior to closure, the peak waste package temperature would not exceed 75 to 85 °C. The relative humidity would decrease on closure as temperature increases, but because the rock temperatures never exceed boiling and there would be no dry zone created. For this case, the packages would be dry for 300 years if the ventilation removes sufficient moisture. After closure at 300 years, there may be a dry period but the relative humidity would be high enough for moisture to form on waste package surfaces.

Over very long times as the repository returns to ambient, the conditions during the operation phase are not significant. The following set of waste package temperatures and time were determined by taking data from a Project presentation (YMBlink_08/02/01.ppt).

Waste Package Temp, °C	Higher Temp Conditions	Lower Temp Conditions
120 C	500 years	N.A.
100 C	1000 years	N.A.
80 C	3000 years	At closure to 1000 years
60 C	10,000 years	5000 years
40 C	25,000 years	25,000 years
Ambient (~25 C)	100,000 years	100,000 years

A conclusion from this data set is that it is crucial to get the first several hundred to one thousand years correct i.e. have high confidence that the waste packages are durable for this time period. During this period, the gamma radiation levels have dropped dramatically and temperature has decreased. As the exposure time increases, the conditions become more benign, the likelihood of localized corrosion and stress corrosion cracking decrease, and the uniform corrosion rates decrease.

4.1.2 Presence of Moisture

Yucca Mountain is located in a desert climate; however, there is water that moves from the ground surface through the rock and past the repository level to the water table below. The waste packages sit on support pallets in air at the repository and in a zone of unsaturated rock a couple of hundred meters above the water table. Full immersion of metal surfaces is a highly unlikely condition in the repository due to the low infiltration rates of water and the sufficiently high rate of transport away from the drifts. The two forms of water of interest are the water that can seep from the rock and drip onto waste packages and the water that can condense from the air onto the waste packages.

Condensation: At sufficiently high temperatures and low relative humidity, no aqueous phase will be present on the waste package or drip shield surfaces. As the waste packages cool and relative humidity rises, moisture will condense on the surface. For pure water on a clean surface, the critical relative humidity for the formation of several monolayers of water is approximately 70%. Deliquesce and capillary forces can lead to the formation of moisture at lower relative humidity.

Deliquescence is important for the proposed repository because soluble salts are expected to be present on the metallic surfaces. These salts can arise from airborne dust and scales from seepage water evaporation, and they can result in the formation of an aqueous environment at relative humidity less than 70%. In the proposed repository, the most likely scenario is that there will be a mixture of salts on the metallic surfaces.

Capillary condensation represents another mechanism by which an aqueous environment can exist at relative humidity less than saturation. As the radius of a pore decreases, the free energy of the liquid water decreases, allowing it to form at a lower relative humidity. The practical importance of capillary condensation in terms of the corrosion of waste packages is two-fold: fine porosity in deposits will act as reservoirs for water and areas of near-intimate contact between surfaces will condense water and hold it.

Seepage and Dripping Water: The amount of water that can seep into the drifts is not large. The expected seepage rate into drifts over the first couple of thousand years is 0.1 m³/yr (YMBlink_09/10-12/01.ppt). The amount of seepage into drifts and drips onto waste packages will vary both spatially throughout the repository and temporally for a given area of drip shield or waste package. Factors could focus seepage water from a larger area onto a smaller region, but these same factors would divert that water from other areas. The volume and patterns of dripping are likely to change with time. Many drifts may never see any seepage water. While these amounts of moisture are small, the waste packages will not remain dry, and the chemical composition of the environments that can form on the drip shield and/or waste packages must be considered.

When the rock fractures are unsaturated, the capillary forces are quite high and rapid flow or dripping of water from the rock is unlikely. If the rock is saturated, capillary forces become quite small and water can flow more rapidly through fractures. Portions of the water moving across the repository front will remain in the rock and flow around the drifts.

Three conditions describe the surfaces of metal that will be subject to corrosion at Yucca Mountain:

Condensation leading to moist dust: The moist dust condition is the most likely condition for the vast majority of waste package and drip shield surfaces. These are areas where no drips fall upon the surface. In the moist dust condition, the metal surface is not contacted by dripping waters; the formation of an aqueous environment on the surface is controlled by surface temperature, relative humidity in the air around package, and the composition of dust and deposits on the surface.
Dripping seepage water forming mineral scale and deposits: Mineral scale and deposits will form on surfaces where drips fall onto the waste package or drip shield. In the wet scale/deposit condition, dripping water from the rock contacts the alloy directly. Drips can fall onto the upper surface of drip shield and could fall onto waste packages where the drip shield is not effective. It is expected that in these areas, a mineral scale will accumulate due to evaporation of the water on the hot surfaces.
Crevice areas at metal-to-metal contact surfaces: Crevices are formed at designed structural contact areas (e.g., where the waste package rests upon a support pallet). Depending upon the design and fabrication details, crevices may also be present at other (unintended) areas on waste packages and drip shields. Drips or condensation can wet the crevices formed at areas of metal-to-metal contact. Crevices must be considered as a different environmental condition because reaction at the metal surfaces can result in the formation of products. Limitations in the transport of products out of crevices results in changes in the local crevice environment. Crevice environments are typically more aggressive than the corresponding environment outside the crevice.

4.1.3 Composition of Waters and Corrosive Environments

Chemical species and water compositions: Before considering any chemical interactions with the waste package and drip shield materials, the aqueous environments can be bounded as saturated brines dominated by Na-CO₃-HCO₃-NO₃, Na-Ca-Mg-Cl-NO₃, or Na-Mg-SO₄-Cl-NO₃. The main conclusion of relevance from this analytical treatment is that the solutions on the waste package will evolve into either an alkaline solution (pH ca. 11-12) containing high concentrations of sulfate, carbonate, nitrate, and chloride, or a near-neutral solution (pH ca. 6) containing high concentrations of chloride and nitrate with or without sulfate. For localized corrosion, the high chloride solutions would be considered the most aggressive, with higher ratios of chloride concentration to the sum of the concentrations of other anions being more deleterious. As the temperature of the metal surface decreases, the relative humidity increases, more water condenses on the surface, and dilution of the aqueous solution on the waste package occurs. The relative concentrations of the dissolved salts change as less deliquescent salts become increasingly soluble.

Low melting point metals such as mercury and lead are known to exacerbate corrosion in a wide range of materials. In addition, some species containing elements such as arsenic can be potent accelerators of hydrogen uptake into metals, leading to increased risk of hydrogen cracking. These types of species can have effects at low levels (ppm). Other species such as bromine have been introduced during the excavation process as LiBr, which has been used for dust reduction and tracer studies. In addition to their presence at Yucca Mountain or their introduction during excavation or ventilation, the amount of the elements present, their chemical form, and the extent to which they can access the surfaces of the waste package and drip shield need to be considered.

Mixed ionic solutions: Mitigating some of the corrosion effects of having an aqueous environment at lower relative humidity (and thus higher temperatures) due to the presence of mixed salts is the fact that most anions tend to inhibit the localized corrosion of metals in the presence of chloride. Thus, although the salt with the lowest deliquescent point will initially dominate the ionic content of the solution, the solution will be comprised of a mixture of all of the salts present on the surface, even those with high deliquescent points. As the relative humidity continues to increase, the fluid will contain increasing proportions of the salts with high deliquescent points. The specifics of the composition evolution will depend on the composition of the soluble species on the surface. The Center for Nuclear Waste Regulatory Analysis staff has done analyses to show that a mixture of NaCl, NaNO₃, and KNO₃ can have a deliquescence point as low as 30%.

pH of Environment: Based on geochemistry, the lowest pH of the incoming water would be near-neutral (pH 6), and the highest pH would be near 10 or 11. Several species in the expected seepage water and rock have buffering effects on the pH. In addition to the carbon dioxide system, the silica present in the rocks will tend to keep the pH below about 10. The pH of solution at any site on a waste package will be a complex function of temperature, time, position in the repository, and seepage water composition. Nonetheless it is highly unlikely that a water of initial pH of less than 6 or more than 10 will drip onto the waste package.

It is highly unlikely that a low pH aqueous solution can be maintained on the waste package in contact with the gaseous environment surrounding it due to the limits on the system pressure (fixed at 0.89 atm). The acid gases (e.g., HCl, HNO₃) would need to maintained at pressures well above those attainable in the mountain in order to maintain the pH of the solutions at low values.

Interactions between the waste package and the environment can affect the pH. If the cathodic and anodic reactions of the corrosion couple are separated physically by substantial distances as in crevice corrosion, the hydrolysis of the metal ions formed from the dissolution will act to lower the local pH at the anode within the crevice. The presence of rock dust, along with the natural waters, will provide a sink for hydrogen ions that will impede lowering of the pH below 6. At the cathodic areas, increases in pH may occur, as the natural waters and groundwater are not strongly buffered in that direction. However the presence of silica in the rock and rock dust will tend to buffer the pH below 10.

Oxidizing potential: A thermodynamic argument can be made that places the upper bound for the open circuit potential of the waste package and drip shield as the reversible potential for dissolved oxygen gas reduction (for pH below 7). No other reducible species expected in the repository has higher reversible potentials. Although dissolved oxygen gas has the most positive reversible potential, the kinetics are quite sluggish on almost all materials, and materials with oxide films tend to have slow kinetics. Therefore, the thermodynamic boundary of the water line as the highest oxidizing potential for the waste package and the drip shield is likely extremely conservative. Nonetheless any argument for a lower bound on the oxidizing potential of the waste package and drip shield must involve kinetic arguments based upon the electrochemistry of the materials involved.

Dust and scale deposits: During the operation phase and prior to closure, rock dust and material entrained in the ventilation air can deposit on the waste package surfaces. Fine particulate dust (< 2.5

m diameter) deposit on all surfaces independent of gravity, but controlled by convection and thermophoretic effects. Coarse particles (diameter 2.5 to 15

m) are likely to dominate the dust in the repository and tend to settle on horizontal surfaces. It is likely that at least the top 1/3 of the waste package and drip shield will be covered with a dust layer of significant thickness. The dust layer will consist of fine and coarse particulate from the excavation and emplacement processes, entrained material from the ventilation system, and other matter carried in on the waste package themselves from production.

As the temperature decreases and relative humidity increases, a moist dust condition will develop. When and where seepage occurs, waters can drip onto metal surfaces. The incoming waters contain dissolved salts, and evaporation of water from the metal surface can lead to the formation of mineral scales and deposits. For the Alloy 22 waste package, the aqueous environments originating from the seepage are relevant at locations where the drip shield is not functioning as designed, whereas the condensation of water from gas can apply to areas of waste package surface even if the drip shield is fully functional. If condensation of water is the only source, then the aqueous environment will be determined by the hydration of the dust and other matter on the waste package. If seepage is an additional source of water, then any dissolved salts in the seepage water contribute to the environment on the metal surface.