Reflector temperature coefficient of molten salt fast reactor using moderating reflector

This work investigates the reflector temperature coefficient (RTC) in a 100 MWth long-life molten salt fast reactor (MSFR) employing a BeO moderating reflector. The design retains advantages of MSFR while greatly reducing fuel inventory, but localized moderation in the separated reflector leads to a positive RTC, raising safety concerns. To elucidate the underlying mechanisms of this positive RTC, neutronic analyses were performed with a simplified analysis model excluding control drums or B 4 C shield. Quantitative evaluations were conducted to characterize local spectrum shifts, variations in cross-sections within the fuel salt and reflector, neutron flux, neutron current, and reaction rates under elevated reflector temperatures. In addition, region-wise RTCs were evaluated to quantify the contribution of each reflector subregion to overall reactivity changes. Parametric studies were carried out to investigate key influencing factors—including the active core size, and inner barrier thicknesses—with a further investigation for micro-scale MSR design. Furthermore, the RTC behavior in various moderating reflectors (BeO, Be, graphite, and MgO) was analyzed in terms of neutron spectrum and component-wise thermal neutron absorption characteristics, further supported by a fine region-wise analysis of thermal neutron net current under increasing reflector temperatures. All neutronic calculations were performed using the Serpent 2 continuous-energy Monte Carlo code.