Interstage Heating and Cooling Options for High Temperature Helium Brayton Cycles

This report summarizes work to examine interstage heating and cooling options for multiple reheat helium Brayton cycle power conversion system (PCS) designs. Previous work has demonstrated that thermal powers in the range of 2400 MWt can be readily derived from combining multiple single-expansion units derived from the 600-MWt, vertical shaft gas-turbine module helium reactor (GT-MHR). This PCS design approach was optimized for the 2400 MWt Advanced High Temperature Reactor (AHTR) system, which uses liquid salt as coolant. Because this original AHTR PCS used an integrated design where all the heat exchangers except for the recuperator are put into PCU vessels, the resulting multiple-reheat PCS is very compact and has very high power density. This new study further extends the earlier design to examine horizontal, distributed PCS configurations. Four variants of horizontal shaft designs were developed for analysis. The power densities, specific steel inputs, and specific helium inventories for all these designs are compared. It was found that differences in the figures of merit are not sufficiently different to deselect any systems, that is, that both horizontal and vertical configurations can achieve attractive efficiency and power density. Therefore, detailed studies and other considerations must be provided to further compare these designs. Modular designs in the 600 MWt class are also considered in this study. Although the power density and specific steel input for the 600 MWt design are lower than those large systems, the difference again is not so large that smaller designs have a substantial cost penalty relative to large systems. The results show that the multiple-reheat indirect cycle can achieve the same thermodynamic efficiency as the non-reheat Brayton cycle commonly used for direct-cycle power conversion. Because the multiple-reheat cycle can still reject heat easily to low-quality heat sinks, it provides a potentially attractive option for indirect-cycle power conversion for 600 MWt-class gas-cooled reactors.