USING RISK INFORMATION TO DETERMINE THE NEED FOR ADDITIONAL GEOCHEMICAL INFORMATION FOR THE PROPOSED YUCCA MOUNTAIN REPOSITORY

The U.S. Department of Energy (DOE) intends to apply for a license to construct a repository for high-level radioactive waste at Yucca Mountain, Nevada. The U.S. Nuclear Regulatory Commission (NRC) is an independent regulatory agency and has established a regulation for the proposed repository. The regulation uses a risk-informed and performance-based strategy that protects the public and the environment. The regulation requires the use of a performance assessment to judge the safety of the repository for 10,000 years. The DOE has completed performance assessments of the potential repository. The NRC has evaluated the assessments, including the supporting data. Where the NRC judges, based on a risk-informed, performance-based approach, the DOE's approach and technical bases are inadequate, the NRC requires the DOE provide additional information.

The DOE needs to use geochemical information in the performance assessment to describe how engineered barriers degrade and to describe radionuclide transport. What information is required to assess adequately the repository performance and how much information is required will vary dependent upon the information's importance to risk. Retardation of radionuclides in the saturated zone at Yucca Mountain is a fundamental part of the DOE's performance assessment. From a risk-informed perspective the NRC staff would focus on those radionuclides that DOE models show contribute to the dose to the reasonably maximally exposed individual within the period of compliance. The NRC staff would also assess whether the DOE had provided a technical basis for retarded radionuclides which did not contribute to dose within 10,000 years. Those geochemical species that are important to degradation of engineered barriers are also the focus of the NRC's risk-informed geochemical review of DOE's performance assessment. However, the NRC's review of the DOE's treatment of geochemical information is focused on whether degradation of the engineered barriers contributes to the dose to the reasonably maximally exposed individual within the period of compliance. Examples of calculations that use risk information to constrain what geochemical information is needed from the DOE will be discussed.

The regulation is available at http://www.nrc.gov/waste/hlw-disposal.htm