Modeling and thermal analysis of Step 2 GPHS for a larger safe operating temperature range

The Step 2 General Purpose Heat Source (GPHS) is the most advanced modular isotope heat source currently used by NASA in the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) for last two Mars explo-ration missions. As it can be adapted to other advanced static and dynamic energy conversion systems, the primary concern is that the iridium (Ir) cladding must be within an acceptable temperature range for fuel containment. In this paper, a full-scale numerical model of Step 2 GPHS was developed to predict its temperature profile accurately according to the Ir cladding temperature limits. The calculated temperatures agree well with the reported values in various operating environments. The results indicate that the safe operating temperature range is -269.2-1138.7 degrees C, with Ir cladding temperatures in 900-1330 degrees C. In addition, the effects of the Ir cladding surface emissivity, insulator thermal conductivity and helium pressure in gaps are explored. The results show that when the cladding surface emissivity is doubled, the insulator thermal conductivity increases by half, and the helium pressure in gaps reaches 0.002 atm, the safe operating temperature range becomes 471.0-1177.8 degrees C, -269.2-1156.4 degrees C and 651.7-1188.3 degrees C, respectively.