EXECUTIVE SUMMARY
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) to review the technical basis for evaluating the long-term performance of waste package materials in a proposed repository at Yucca Mountain, Nevada. The Panel was formed in March 2001; this Executive Summary accompanies their Final Report of February 28, 2002. In that report, the Panel identifies important issues for assessing the performance of waste package materials, evaluates the technical basis available for understanding these issues, and comments on approaches and plans to resolve unsettled issues. The report presents the Panel’s findings, conclusions, and recommendations. Substantial technical information is included to provide background and perspective for the Panel’s work. Although the focus of this peer review was the assessment of the technical basis for evaluating the long-term performance of materials exposed to the conditions of the proposed repository, the Panel’s findings are relevant to a broad range of important matters, including overall performance assessment, site recommendation, license application, and repository design and operation. These, however, were beyond the scope of the Panel. All are multi-faceted and require both technical and non-technical considerations to be taken into account in any decisions. Panel comments regarding these matters should be understood to have the qualifier "… from the perspective of long-term materials performance." Throughout the peer review, the Panel interacted with Department of Energy staff and contractors responsible for evaluating the performance of waste package materials. Subject Matter Experts prepared helpful background reports to assist the Panel in its deliberations. The staff of the U.S. Nuclear Regulatory Commission and their Center for Nuclear Waste Regulatory Analyses, members and staff of the U.S. Nuclear Waste Technical Review Board, and staff of the State of Nevada Agency for Nuclear Projects and its contractors, and others provided valuable assistance. Their assistance is gratefully acknowledged. 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. Although the technical basis supporting the suitability of Alloy 22 for the outer barrier of the waste package is substantial and growing, there will always be uncertainty in the evaluation of the long-term performance of materials in the repository. This uncertainty 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. Technical Findings The effective control of corrosion of waste package materials is essential to the satisfactory long-term performance of a Yucca Mountain repository and the Panel concludes that the Project staff is taking a sound approach to analyzing corrosion related issues. 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. 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 proposed waste package material, nickel-base Alloy 22, has excellent corrosion resistance over a wide range of aqueous solution compositions and temperature. However, corrosion related failures are possible and three potential corrosion failure modes have been identified and are being evaluated by the Project staff: long-term uniform corrosion of passive metals, localized corrosion, and stress corrosion cracking. The Panel has identified several technical issues related to these failure modes that have the potential to require a change in the waste package material or design. Many of these issues are associated with the determination of the realistic boundaries for the environmental conditions and the corrosion behavior of the waste package within these boundaries. Included in the determination of the realistic environmental boundaries are the effects of temperature, radiation, pH, oxidizing potential, ionic species, and microbiological activity. The surface condition and metallurgy of the alloy, which are, in turn, affected by design and fabrication procedures and the composition of the alloy within the specification range of Alloy 22 will affect resistance of the waste package to failure by these corrosion modes. Of particular importance is the long-term metallurgical stability of the alloy under repository conditions. The Panel does not recommend the use of titanium Grade 7 for construction of the drip shield if it is found to be susceptible to stress corrosion cracking under realistic repository conditions. Stress corrosion cracking of this alloy has been observed in laboratory tests performed by the Project. The benefits of this design are severely diminished if these tests are deemed to have been conducted under realistic repository conditions. 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. 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 greater radiolysis effects because they become wet sooner. With lower temperature operation, there also would be increased costs and risks associated with longer ventilation periods and a larger repository area would be 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. Organizational-Managerial Issues 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. This is not a recommendation to terminate the science program, but rather to redirect and balance the science effort to support design and manufacture/fabrication for long-term performance. 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. 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 the Project’s technical experts in corrosion, materials science, geochemistry, and hydrology in work on these conditions. For the latter, the Panel recommends closer integration between the Project’s 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. 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 to help determine and maintain focus and direction in the execution of the research agenda of the staff. 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. Moreover, 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.