Low-Temperature Multiple-Reheat Closed Gas Power Cycles for the AHTR and LSFR

High Temperature Gas Cooled Reactors (HTGR) such as GT-MHR and PBMR with direct closed gas-turbine Brayton cycle can achieve efficiency between 44% to 48% with core outlet temperatures from 850 deg. C to 900 deg. C. The use of multiple reheat and inter-cooling stages can further improve thermal efficiency. Low-temperature multiple reheat cycles for the AHTR-MI and liquid-salt fast reactors (LSFR), with core outlet temperatures ranging from 620 deg. C to 750 deg. C, can reach similar efficiency as these direct-cycle HTGRs but with reduced technical risk due to lower temperatures. This paper discusses design optimization at these lower temperatures for multiple reheat closed gas cycles and vertical and horizontal arrangement options for power conversion units (PCU). Figures of merit such as specific power density, specific steel input, and specific helium inventory are estimated for different PCU arrangement configurations. With similar components parameters and reasonable arrangement, different configurations such as horizontal or vertical shaft, integrated system or distributed system, were compared. Among those configurations, integrated systems basing on the GT-MHR PCU design result in the highest specific power density and lowest specific steel input. Because the differences in these high-level performance parameters are not large enough to de-select any configurations, further detailed design and comparison must be performed to select optimal system designs. (authors)