Molten salt reactor (MSR) is one of the six internationally recognized and recommended fourth generation reactors, which is different from conventional solid-state nuclear fuel reactors. It is necessary to analyze the relationship between ⁷Li abundance and nuclear critical parameters in order to manage MSR core design and nuclear safety supervision.

This study aims to model a molten salt reactor with reference to engineering practice, and analyze the impact of different ⁷Li abundance fuel salts on the reactivity of the MSR, as well as the changes in nuclear critical parameters by simulation.

Firstly, a MSR with engineering practice, i.e. the Molten Salt Reactor Experiment (MSRE) designed by Oak Ridge National Laboratory (ORNL), USA, was referenced to establish MSR model with mass abundance of nuclear fuel salt ²³⁵U assigned to 20% (enrichment of 20.2%) instead of the 33% enrichment designed by MSRE. Then, based on the established model, Standardized Computer Analyses for Licensing Evaluation (SCALE) code was applied to iterative calculation for quick and accurate obtaining of the ⁷Li abundance value at the critical state of the MSR core. Finally, in-depth exploration of calculation results was conducted from the perspective of applicable laws and regulations for the safety analysis of MSR.

Simulation results show that the reactivity of the MSR increases with the increase of fuel salt ⁷Li abundance, and the rate of reactivity variation of the MSR is also related to ⁷Li abundance. At the critical ⁷Li abundance (i.e. around 99.98%), for every 0.001% change in ⁷Li abundance, the reactivity changes by more than 0.05%.

Based on the analysis results of this study, the abundance of ⁷Li has a significant impact on the k_eff of MSR, hence it is necessary to choose an appropriate ⁷Li abundance for safety analysis of MSR criticality.