The Nuclear Regulatory Commission has directed a three-tier approach as the performance assessment analysis for a low-level waste disposal site, including a time of compliance of 1,000 years, according to the 10 CFR Part 61 Staff Requirements Memorandum issued this week. Previously, the NRC staff’s draft rulemaking would require low-level radioactive waste disposal sites to perform a site-specific analysis to prove their site was protective of public health and safety for 10,000 years, down from a period of compliance of 20,000 years of previous drafts, with a two-tier analysis: the first period covering 10,000 years and the second period covering long-lived isotopes. Under the Commission’s direction, the first tier would cover the 1,000 year time of compliance followed by a further protective assurance analysis from the end of compliance period through 10,000 years. The SRM explained this new tier by saying, “Given the significant uncertainties inherent in these long timeframes, and to ensure a reasonable analysis, this performance assessment should reflect changes in features, events, and processes of the natural environment such as climatology, geology, and geomorphology only if scientific information compelling such changes from the compliance period is available. In general, this analysis should strive to minimize radiation dose with the goal of keeping doses below a 500 mrem/year analytical threshold,” the SRM said. The final and third tier of the analysis would measure the performance period quantitatively 10,000 years or more after site closure to “evaluate the ability of the disposal system to mitigate long-term risks.”
The SRM also said that the proposed rule should be published with a compatibility category B applied to it, thus enforcing the Agreement States follow these new requirements. The four Agreement States each enforce a various time periods of compliance so this would create a national standard for that issue. The SRM also included a 10,000 year intruder assessment analysis with a specific dose limit of 25 mrem/year. However, it did include a provision to make sure intruder assessments were reasonable. “The proposed rule should clearly indicate that the intruder assessment should be based on intrusion scenarios that are realistic and consistent with expected activities in and around the disposal site at the time of site closure,” the SRM said.
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