The current DOE General Purpose Heat Source (GPHS) was developed specifically for a SiGe Radioisotope Thermoelectric Generator (RTG) which was to be launched on a Space Shuttle. In such a launch, if the final stage of the deployed spacecraft failed to ignite, the spacecraft and RTG would reenter the atmosphere after about three months in a Shuttle parking orbit. The iridium alloy clad containing the radioisotope fuel would have then been at about 1300 degrees C for the entire time on orbit. This would lead to some grain growth in the clad material, which would tend to decrease its ductility. Due to these constraints, it was concluded by the heat source community that safety considerations would require that the clads be maintained at a temperature of at least 930 degrees C through impact. The safety testing for the GPHS RTG programs was therefore conducted with these conditions as guides. Within this set of guidelines, the heat source was shown to be able to safely handle all credible launch and reentry accident events in these missions. Currently proposed missions plan to use different power conversion systems and launch scenarios that are able to utilize much lower operating temperatures. Imposing the high operating and impact temperatures required by the SiGe RTG onto these new systems would lead to unnecessary added design complexity and probably a less robust system. If the heat source is operated so that the iridium alloy clads are generally maintained at less than 1000 degrees C, essentially no grain growth would be experienced over normal mission lifetimes, and the clads would stay more ductile and able to safely withstand the resulting lower impact temperatures. Therefore DOE has embarked on a study to determine the lowest safe operating and impact temperatures for the GPHS. This paper presents the historical background and some preliminary results of this study.
