Publications

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The Department of Energy's 2008 Yucca Mountain Performance Assessment represents the culmination of more than two decades of analyses of post-closure repository performance in support of programmatic decision making for the proposed Yucca Mountain repository. The 2008 performance assessment summarizes the estimated long-term risks to the health and safety of the public resulting from disposal of spent nuclear fuel and high-level radioactive waste in the proposed Yucca Mountain repository. The standards at 10 CFR Part 63 request several numerical estimates quantifying performance of the repository over time. This paper summarizes the key quantitative results from the performance assessment and presents uncertainty and sensitivity analyses for these results.

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Published analyses of geologic repositories indicate potential for excellent long-term performance for a range of disposal concepts. Estimates of peak dose may be dominated by different radionuclides in different disposal concepts. Thermal loading issues can be addressed by design and operational choices. Impact of waste form lifetime on estimates of peak dose varies for different disposal concepts.

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This paper provides a summary of observations drawn from twenty years of personal experience in working with regulatory criteria for the permanent disposal of radioactive waste for both the Waste Isolation Pilot Plant repository for transuranic defense waste and the proposed Yucca Mountain repository for spent nuclear fuel and high-level wastes. Rather than providing specific recommendations for regulatory criteria, my goal here is to provide a perspective on topics that are fundamental to how high-level radioactive waste disposal regulations have been implemented in the past. What are the main questions raised relevant to long-term disposal regulations? What has proven effective in the past? Where have regulatory requirements perhaps had unintended consequences? New regulations for radioactive waste disposal may prove necessary, but the drafting of these regulations may be premature until a broad range of policy issues are better addressed. In the interim, the perspective offered here may be helpful for framing policy discussions.

The presentation briefly addresses three topics. First, science has played an important role throughout the history of the WIPP project, beginning with site selection in the middle 1970s. Science was a key part of site characterization in the 1980s, providing basic information on geology, hydrology, geochemisty, and the mechanical behavior of the salt, among other topics. Science programs also made significant contributions to facility design, specifically in the area of shaft seal design and testing. By the middle 1990s, emphasis shifted from site characterization to regulatory evaluations, and the science program provided one of the essential bases for certification by the Environmental Protection Agency in 1998. Current science activities support ongoing disposal operations and regulatory recertification evaluations mandated by the EPA. Second, the EPA regulatory standards for long-term performance frame the scientific evaluations that provide the basis for certification. Unlike long-term dose standards applied to Yucca Mountain and proposed repositories in other nations, the WIPP regulations focused on cumulative releases during a fixed time interval of 10,000 years, and placed a high emphasis on the consequences of future inadvertent drilling intrusions into the repository. Close attention to the details of the regulatory requirements facilitated EPA's review of the DOE's 1996 Compliance Certification Application. Third, the scientific understanding developed for WIPP provided the basis for modeling studies that evaluated the long-term performance of the repository in the context of regulatory requirements. These performance assessment analyses formed a critical part of the demonstration that the site met the specific regulatory requirements as well as providing insight into the overall understanding of the long-term performance of the system. The presentation concludes with observations on the role of science in the process of developing a disposal system, including the importance of establishing the regulatory framework, building confidence in the long-term safety of the system, and the critical role of the regulator in decision making.

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Despite decades of international consensus that deep geological disposal is the best option for permanent management of long-lived high-level radioactive wastes, no repositories for used nuclear fuel or high-level waste are in operation. Detailed long-term safety assessments have been completed worldwide for a wide range of repository designs and disposal concepts, however, and valuable insights from these assessments are available to inform future decisions about managing radioactive wastes. Qualitative comparisons among the existing safety assessments for disposal concepts in clay, granite, salt, and unsaturated volcanic tuff show how different geologic settings can be matched with appropriate engineered barrier systems to provide a high degree of confidence in the long-term safety of geologic disposal. Review of individual assessments provides insights regarding the release pathways and radionuclides that are most likely to contribute to estimated doses to humans in the far future for different disposal concepts, and can help focus research and development programs to improve management and disposal technologies. Lessons learned from existing safety assessments may be particularly relevant for informing decisions during the process of selecting potential repository sites.

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This paper summarizes the historical development of the United States Department of Energy's (DOE's) 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, and explains how the methods and results meet regulatory requirements specified by the United States Nuclear Regulatory Commission (NRC) and the United States Environmental Protection Agency (EPA). Topics covered include (i) screening of features, events and processes, (ii) development of scenario classes, (iii) descriptions of barrier capability, and (iv) compliance with applicable quantitative standards for individual protection, individual protection following human intrusion, and ground water protection. © 2009 Materials Research Society.

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