Burnup dynamics and reactivity change in proton induced Tc-99 transmutation fuel assembly

Neutron transmutation presents a viable method for the management and disposal of long-lived fission products. A newly published method describes a transmutation system that utilizes proton spallation in conjunction with a subcritical fuel assembly. This innovative approach has demonstrated the ability to reduce the transmutation half-life of technetium-99 (Tc-99) to less than one year. This study investigates the impact of Tc-99 concentrations on the subcriticality of the transmutation device during burnup using the Serpent 2 Monte Carlo neutron transport code. The burnup simulation demonstrates that (1) The number densities of U-235 and Tc-99 decrease steadily across all cases as burnup extends up to 600 days, (2) k eff decreases steadily and remains below 1.0 after 600 days of burnup. Specifically, k eff is below 0.7 in UO 2 fuels and below 0.1 in UF 4 fuels after 600 days. These findings provide valuable insights into optimizing and ensuring the safety of the transmutation device.