Investigation into reactivity feedback of fhr designs with alternative coolants

© 2018 American Nuclear Society. All rights reserved. Prismatic block type Fluoride-salt-cooled Hightemperature Reactors (FHRs) can benefit from Advanced Gas-cooled Reactor (AGR) technologies. To provide guidance for future AGR-like FHR design, this paper, based on systematic searches across a wide range of AGR-like assembly models and simplified single pin models, studies the beginning of cycle excess reactivity and coolant temperature coefficients of FHRs with different design parameters and alternative salt coolants. LiF-NaF-KF (FLiNaK) and non-Tritium-producing salt NaF-ZrF4 are studied as coolant options alternative to 2LiF-BeF2 (FLiBe) using unit cell models. The results suggest that the NaF-ZrF4 cooled, UC fuelled single pin models with 40% to 100% salt to graphite mass ratios and pitch to diameter ratios of 4.0 to 4.8 can achieve the best beginning of cycle neutronics performances among all designs options surveyed. While the assembly models and pin cell models capture the same reactor physics phenomena, designing FHR strictly within the physical constraint of AGR configuration limits the designs space and results in poorer neutronics performance comparing with the best performing unit cell models. Greater degrees of freedom could be considered in the future assembly level design process to best capture the desirable neutronics benefits of the recommended single pin designs.