Calculation of Delayed Neutron Adjoint Flux in Molten Salt Reactor Based On the Coupling of Precursors Transport and Monte Carlo Particle Transport

[Background] The flow of molten salt fuel in molten salt reactors (MSRs) induces changes in the effective fraction of delayed neutrons, which exerts a significant impact on the operational control of MSRs. [Purpose] This study aims to use Monte Carlo method to obtain the adjoint flux distribution of delayed neutrons, which is used for the analysis of the flow-based point reactor model. [Methods] Based on the Monte Carlo method, the MCNP (Monte Carlo N Particle Transport Code) was employed in conjunction with a simplified flow field model. The adjoint flux of delayed neutron precursors in the flow model was calculated by incorporating the Iterated Fission Probability Method code, and the effective shares of delayed neutrons in different groups were thereby obtained. The MSRE (Molten Salt Reactor Experiment) model was utilized to verify the accuracy of the proposed method. Additionally, by calculating the axial adjoint flux distribution of each group of delayed neutrons, the effect of flow on the adjoint flux was investigated. [Results] Through calculations based on the MSRE model, it is verified that the computed values of flow-induced reactivity changes are in good agreement with the experimental results. The study reveals that there exist certain differences between the adjoint flux of delayed neutrons and that of prompt neutrons. Additionally, the flow causes a reduction in the axial adjoint flux of several groups of delayed neutrons, primarily attributed to the radial mixing inside the core of delayed neutron precursors returning from outside the core. The research further calculates the two-dimensional adjoint flux distribution of delayed neutrons, which is less affected by flow velocity. By using the ratio of delayed neutrons effective fraction and delayed neutrons fraction under statics condition in each group at rest to eliminate the statistical fluctuations in the calculation, the results obtained are good, and the deviation is within 2.9%, which proves the feasibility of the scheme. [Conclusion] This suggests that the flow point reactor model needs to be extended to two dimensions in terms of precursor transport within the core, and the adjoint flux differences of delayed neutrons in each group should also be considered. It laid the foundation for the systematic transient analysis of molten salt reactors.