Long-Lived Fission Product Transmutation Studies

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, ⁹⁹Tc and ¹²⁹I 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 ⁹⁹Tc and ¹²⁹I 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 ⁹⁹Tc and ¹²⁹I can be stabilized (i.e., zero net production) in the same PWR core under current design constraints by mixing ⁹⁹Tc 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 ⁹⁹Tc and ¹²⁹I target assemblies in the reflector region, the self-generated ⁹⁹Tc and ¹²⁹I as well as the amount produced by several PWR cores could be consumed at a cost of ~10% increased fuel inventory. Discharge burnups of ~29 and ~37% are achieved for ⁹⁹Tc and ¹²⁹I target assemblies with an ~5-yr irradiation period.

Based on these results, the impacts of ⁹⁹Tc and ¹²⁹I 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 ⁹⁹Tc and ¹²⁹I 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.