Target Design and Optimizations for Spent Fuel Transmutation

There are six long-lived fission products (LLFPs) identified in nuclear spent fuel, which are responsible for at least 99% of the long-term radio-toxicity once actinide recycling has been completed. This paper discusses the feasibility of using proton beams to transmute the LLFPs into shorter-lived or stable isotopes. Although long-term storage for high-level waste would still be required, transmuting the LLFPs can reduce the volume of waste material stored. PHITS, a Monte Carlo transport code, is utilized to computationally model the interactions between the LLFP materials and the proton beam. PHITS is used in this paper to estimate the flux-energy spectrum and number of atoms irradiated in the LLFP target when interacting with the beam. This data is then post-processed in a 0-dimensional analysis in FISPACT to estimate the transmutation rate for each LLFP. Analysis of the performance of commercial cyclotrons with energies of 18-70 MeV has shown that transmutation rates will increase when the proton beam energy is increased. A cyclotron with a beam current of 10 mA and beam energy of 70 MeV running continuously can transmute 7.51 ± 1.19 g/year of technetium-99. However, technetium-99 is produced at around a rate of 1kg/year inside a 1 GW reactor, suggesting that commercial cyclotrons are not viable for transmutation purposes. Studies are ongoing on modeling 1000 MeV proton beams to cause spallation reactions inside the LLFP targets. The spallation process burns the LLFP target while producing many secondary neutrons that can be further used for transmuting the LLFPs. A preliminary analysis suggests that a 1000 MeV beam with a 10 mA current can transmute around 1kg/year of technetium-99 due to nuclear reactions without accounting for spallation reactions.