FLiBe is commonly used as the coolant and carrier salt in liquid molten salt reactors (MSRs). Its certain moderating properties and thermal neutron scattering attributes affect the neutronic performance of the MSR, and this in turn influences the physical design and safe operation of the reactor. Consequently, studying FLiBe's thermal neutron scattering data is essential for MSRs.

This study aims to analyze the influence of of FLiBe thermal neutron scattering on neutronic performances of a 65-MW MSR.

First, according to the requirements, a core model of a 65-MW MSR was established by using the general Monte Carlo procedure. Then, the neutronics performance of the MSR was calculated by considering the scattering cross-section of the free gas model and FLiBe thermal neutron scattering data (e.g., the neutron spectrum, effective multiplication factor, and nuclide reactivity rate). Finally, the changes in the influence of FLiBe thermal neutron scattering effect on neutronic properties under different energy spectra were compared.

The computation results show that, by considering the thermal scattering effect of FLiBe molten salt, the neutron energy spectrum in the core of the MSR becomes harder, ²³⁵U fission rate decreases, the k_eff value of the reactor decreases, but the density coefficient in the temperature reaction coefficient of the fuel keeps almost unchanged, and the Doppler coefficient decreases by 0.28×10^-5 K^-1. With the hardening of the energy spectrum, the variation in the ²³⁵U fission rate reduction decreases, and the decrease in k_eff caused by thermal neutron scattering changs from 9.2×10^-4 to 2×10^-4.

Therefore, it is necessary to incorporate FliBe's thermal neutron scattering data into the physical calculations for the MSR core.