Neutronics analysis of a stacked structure for a subcritical system with LEU solution driven by a D-T neutron source for ⁹⁹Mo production

The utilization of neutrons markedly affects the medical isotope yield of a subcritical system driven by an external D-T neutron source. The general methods to improve the utilization of neutrons include moderating, multiplying, and reflecting neutrons, which ignores the use of neutrons that backscatter to the source direction. In this study, a stacked structure was formed by assembling the multiplier and the low-enriched uranium solution to enable the full use of neutrons that backscatter to the source direction and further improve the utilization of neutrons. A model based on SuperMC was used to evaluate the neutronics and safety behavior of the subcritical system, such as the neutron effective multiplication factor, neutron energy spectrum, medical isotope yield, and heat deposition. Based on the calculation results, when the intensity of the neutron source was 5 × 10¹³ n/s, the optimized design with a stacked structure could increase the yield of ⁹⁹Mo to 182 Ci/day, which is approximately 16% higher than that obtained with a single-layer structure. The inlet H₂O coolant velocity of 1.0 m/s and initial temperature of 20 °C were also found to be sufficient to prevent boiling of the fuel solution.