The impacts of thermal neutron scattering effect and resonance elastic scattering effect on FHRs

Two effects in Fluoride-salt-cooled High-temperature Reactors (FHRs) are analyzed based on Monte Carlo simulation. Firstly, the thermal neutron scattering effect of fluoride salt (2LiF-BeF₂) is considered by utilization of the newly generated Thermal neutron Scattering Library (TSL) files. It is found that the neutron spectrum becomes harder and the fission reaction rate of ²³⁵U decreases at thermal energy range due to the up-scattering introduced by thermal neutron scattering effect. Secondly, the resonance elastic scattering effect of heavy nuclides in epithermal energy range is covered by implementation of the Doppler Broadening Rejection Correction (DBRC) method in MCNP. It is shown that neutron up-scattering is enhanced in the low energy wing of the resonance peak and neutron down-scattering is increased in the high energy wing of the resonance peak. This phenomenon leads to an increase in the neutron capture rate of ²³⁸U by about 1.0%. For the analyzed FHR pebble unit cells at 1200 K depending on TRISO packing factor, the thermal neutron scattering effect of 2LiF-BeF₂ and the resonance elastic scattering effect result in a decrease in k_inf of 93–290 pcm and 131–591 pcm, respectively. By taking into account the two effects simultaneously, the k_inf of FHR pebble unit cells decreases by 248–881 pcm.