Identification of safety-relevant radionuclides for performance assessment modeling

We propose, apply, and verify a screening approach for the selection of safety-relevant radionuclides that should be tracked in models assessing the performance of geologic repositories for the disposal of spent nuclear fuel and high-level radioactive wastes. Starting with a comprehensive list of radionuclides present in the waste form, a multi-step down-selection process evaluates each isotope’s potential relative contribution to the total peak exposure dose, which is a surrogate metric for overall repository safety. In the first screening step, only basic, readily available characteristics of a radionuclide are needed, such as its inventory, half-life, specific activity, and dose coefficient. In the second step, the radionuclide’s transport time from the repository to the accessible environment is estimated based on factors affecting its mobility and retardation. By adjusting the screening threshold, the number of radionuclides considered potentially safety-relevant can be changed, thus yielding a larger or smaller (more or less conservative) set of radioisotopes being tracked in the performance assessment model, as warranted by the stage of repository development. We exercise the proposed screening approach for a particular waste form—spent nuclear fuel assemblies—and two disposal pathways—deep horizontal and vertical borehole repositories. An integrated performance assessment model is then used to simulate the migration of a considerably larger set of radionuclides from the disposal canisters to the land surface. The acceptably small difference in peak dose calculated with the comprehensive and reduced set of radionuclides indicates the appropriateness of the proposed screening approach.