Investigation on the impact of thermal-kernel effect of lithium hydride on the reactivity of nuclear propulsion particle bed reactor

Highlights: • The free gas approximation approach introduced significant error by about 2000 pcm in PBR reactor. • The systematic temperature effect study was made through LiH thermal effects of kernel, expansion and dissociation. • The mixed moderator method was designed to reduce or resolve the positive moderator temperature effect. - Abstract: Due to the thermalization of lithium hydride (LiH) in nuclear propulsion particle bed reactor (PBR), thermal-kernel effect of LiH and its impact on the reactivity of PBR was studied. The phonon properties of LiH have been analyzed by the first-principles simulations using the VASP and PHONON codes. Applying the generated phonon density of states, the ACE-format thermal-neutron cross-section library of LiH has been generated by NJOY code. Neutronic modeling and simulation of PBR were done in this paper. The impact of LiH thermalization on the reactivity feedback of PBR was studied and analyzed. From the numerical results, it can be observed that the thermal effect of LiH was positive to the reactivity of PBR. The approximation of the free-gas model introduces significant error over 2000 pcm (10⁻⁵). In order to find the thermal mechanism of LiH, detailed research and analysis towards the thermal expansion and thermal dissociation have been also done. It was found that the thermal-kernel effect finally made the most contribution. In order to improve the safety of PBR reactor, the mixed moderator method was designed to reduce or resolve the positive moderator temperature effect.