Published results of performance assessments for deep geologic disposal of high-level radioactive waste and spent nuclear fuel provide insight into those aspects of the waste form that are potentially important to the long-term performance of a repository system. Alternative waste forms, such as might result from new technologies for processing spent fuel and advances in nuclear reactor design, have the potential to affect the long-term performance of a geologic repository. This paper reviews relevant results of existing performance assessments for a range of disposal concepts and provides observations about how hypothetical modifications to waste characteristics (e.g., changes in radionuclide inventory, thermal loading, and durability of waste forms) might impact results of the performance assessment models. Disposal concepts considered include geologic repositories in both saturated and unsaturated environments. Specifically, we consider four recent performance assessments as representative of a range of disposal concepts. We examine the extent to which results of these performance assessments are affected by (i) thermal loading of the waste proposed for disposal; (ii) mechanical and chemical lifetime of the waste form; and (iii) radionuclide content of the waste. We find that peak subsurface temperature generally is a constraint that can be met through engineering solutions and that processing of wastes to reduce thermal power may enable more efficient use of repositories rather than improved repository performance. We observe that the rate of radionuclide release is often limited by geologic or chemical processes other than waste form degradation. Thus, the effects on repository performance of extending waste-form lifetime may be relatively small unless the waste form lifetime becomes sufficiently long relative to the period of repository performance. Finally, we find that changes to radionuclide content of waste (e.g., by separation or transmutation processes) do not in general correspond to proportional effects on repository performance. Rather, the effect of changes to radionuclide content depends on the relative mobility of various radionuclides through the repository system, and consequently on repository geology and geochemistry.
The safe management and disposition of used nuclear fuel and/or high level nuclear waste is a fundamental aspect of the nuclear fuel cycle. The United States currently utilizes a once-through fuel cycle where used nuclear fuel is stored on-site in either wet pools or in dry storage systems with ultimate disposal in a deep mined geologic repository envisioned. However, a decision not to use the proposed Yucca Mountain Repository will result in longer interim storage at reactor sites than previously planned. In addition, alternatives to the once-through fuel cycle are being considered and a variety of options are being explored under the U.S. Department of Energy's Fuel Cycle Technologies Program. These two factors lead to the need to develop a credible strategy for managing radioactive wastes from any future nuclear fuel cycle in order to provide acceptable disposition pathways for all wastes regardless of transmutation system technology, fuel reprocessing scheme(s), and/or the selected fuel cycle. These disposition paths will involve both the storing of radioactive material for some period of time and the ultimate disposal of radioactive waste. To address the challenges associated with waste management, the DOE Office of Nuclear Energy established the Used Fuel Disposition Campaign in the summer of 2009. The mission of the Used Fuel Disposition Campaign is to identify alternatives and conduct scientific research and technology development to enable storage, transportation, and disposal of used nuclear fuel and wastes generated by existing and future nuclear fuel cycles. The near-and long-term objectives of the Fuel Cycle Technologies Program and its ' Used Fuel Disposition Campaign are presented.
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.
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.
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.
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.