Performance indices optimization of long-lived fission products transmutation in fast reactors

An investigation on the nuclear transmutations of elemental long-lived fission products (LLFPs) in a fast reactor is being conducted in Japan, focusing on the Se-79, Zr-93, Tc-99, Pd-107, I-129, and Cs-135, to reduce the environmental burden. With their high neutron flux and adequate excessive neutrons, fast reactors are considered strong candidates for transmuting those LLFPs into stable isotopes or shorter effective half-lives under the partition and transmutation (P/T) strategy. In this investigation, the LLFP subassembly is assumed to be loaded into the radial blanket region of a Japanese MONJU class sodium-cooled fast reactor (710 MWth). This work focuses on two LLFPs, namely, I-129 and Tc-99. The LLFPs are mixed with YD₂ or YH₂ moderator material to enhance the LLFP transmutation rate. The results include the optimal position of the LLFP assembly in the radial blanket and the optimal moderator volume fraction to optimize the transmutation performance indices, that is, the transmutation rate (TR, %/year), the support factor (SF is defined as the ratio of transmuted to produced LLFP), and the effective half-life (T-half) of the LLFPs. A countermeasure is also devised to resolve the expected higher power peak at the fuel assembly adjacent to the LLFP assembly. The neutronics and burnup analyses are conducted using the continuous-energy Monte Carlo SERPENT2 code with the JENDL-4.0 nuclear data library.