Long-lived fission product transmutation studies

摘要

A systematic study on long-lived fission products (LLFPs) transmutation has been performed with the aim of devising an optimal strategy for their transmutation in critical or subcritical reactor systems and evaluating impacts on the geologic repository. First, Tc-99 and I-129 were confirmed to have highest transmutation priorities in terms of transmutability and long-term radiological risk reduction. Then, the transmutation potentials of thermal and fast systems for Tc-99 and I-129 were evaluated by considering a typical pressurized water reactor (PWR) core and a sodium-cooled accelerator transmutation of waste system. To determine the best transmutation capabilities, various target design and loading optimization studies were performed It was found that both Tc-99 and I-129 can be stabilized (i.e., zero net production) in the same PWR core under current design constraints by mixing Tc-99 with fuel and by loading CaI2 target pins mixed with ZrH2 in guide tubes, but the PWR option appears to have a limited applicability as a burner of legacy LLFP. In fast systems, loading of moderated LLFP target assemblies in the core periphery (reflector region) was found to be preferable from the viewpoint of neutron economy and safety. By a simultaneous loading of Tc-99 and I-129 target assemblies in the reflector region, the self-generated Tc-99 and I-129 as well as the amount produced by several PWR cores could be consumed at a cost of similar to10% increased fuel inventory. Discharge burnups of similar to29 and similar to37% are achieved for Tc-99 and I-129 target assemblies with an similar to5-yr irradiation period. Based on these results, the impacts of Tc-99 and I-129 transmutation on the Yucca mountain repository were assessed in terms of the dose rate. The current Yucca Mountain release evaluations do not indicate a compelling need to transmute Tc-99 and I-129 because the resulting dose rates fall well below current regulatory limits. However, elimination of the LLFP inventory could allow significant relaxation of the waste form and container performance criteria, with associated economic benefits. Therefore, some development of either specialized waste form or transmutation target for the LLFP is prudent, especially considering the potential accumulation of large LLFP inventory with sustained use of nuclear energy into the future. [References: 22]