Method to Reduce Long-lived Fission Products by Nuclear Transmutations with Fast Spectrum Reactors

Transmutation of long-lived fission products (LLFPs: (79)Se, (93)Zr, (99)Tc, (107)Pd, (129)I, and (135)Cs) into short-lived or non-radioactive nuclides by fast neutron spectrum reactors without isotope separation has been proposed as a solution to the problem of radioactive wastes disposal. Despite investigation of many methods, such transmutation remains technologically difficult. To establish an effective and efficient transmutation system, we propose a novel neutron moderator material, yttrium deuteride (YD(2)), to soften the neutron spectrum leaking from the reactor core. Neutron energy spectra and effective half-lives of LLFPs, transmutation rates, and support ratios were evaluated with the continuous-energy Monte Carlo code MVP-II/MVP-BURN and the JENDL–4.0 cross section library. With the YD(2) moderator in the radial blanket and shield regions, effective half-lives drastically decreased from 10(6) to 10(2) years and the support ratios reached 1.0 for all six LLFPs. This successful development and implementation of a transmutation system for LLFPs without isotope separation contributes to a the ability of fast spectrum reactors to reduce radioactive waste by consuming their own LLFPs.