Thermal analyses of heat source assembly for a dual loop, Turbo-Brayton Radioisotope power system

•3D-thermal analyses of radioisotope heat source assembly for Turbo-Brayton power system.•Heat source assembly is thermally coupled to helical coils heat exchange with Xe working fluid.•Investigated effects of vacuum or helium in all internal voids of GPHS modules.•Temperatures are below specified or suggested limits for 238Pu pellets, Iridium alloy cladding, and aeroshell. This paper presents the results of 3-D thermal analyses of the heat source assembly (HSA) for a dual-loop, Turbo-Brayton Radioisotope Power System (RPS) for generating in excess of 300 We. The HSA with 6 Step 2 General Purpose Heat Source (GPHS) modules, is thermally coupled to two Brayton loops, each with a 300 We Turbo-Brayton unit. Results show that with either vacuum or helium gas in all internal gaps and voids of the GPHS modules the temperatures of the 238PuO2 fuel pellets, the Iridium (Ir) alloy cladding, and the Fine Weave Pierced Fabric (FWPF) aeroshell are largely within safe margins. With vacuum in all internal gaps and voids, the highest aeroshell is 1271 K which is 102 K below the NASA specified maximum of 1373 K to limit potential heating of the Ir-alloy cladding during an unlikely reentry. The Ir alloy cladding temperature is ≥300 K above the suggested low limit of 1173 K, and ≥112 K below its recrystallization temperature of 1603 K. With helium gas in all internal gaps and voids, the highest surface temperature of the aeroshell is ≥145 K below 1373 K, while that of the Ir-alloy cladding is 80 K above the suggested minimum temperature (1173 K) to maintain ductility and 326 K below that for recrystallization and the onset of grain growth.