Effect of Martian and Titan Atmospheres on Carbon Components in the General Purpose Heat Source

Radioisotope power systems (RPS) that are currently in use today are designed with a closed fuel cavity. Multiple proposed designs for future RPS have included the use of an open fuel cavity, which indicates that the fuel, and its associated hardware, will be exposed to gases that could be found outside the generator. Most missions that would utilize an RPS will take place in the vacuum of space, and for those missions the difference between an open or closed fuel cavity is inconsequential. A few missions, however, could take place at a site that has an atmosphere, such as Mars or Titan. In these cases, it is important to understand how the extraterrestrial atmosphere of these locations could impact the components within the RPS. This knowledge will then help RPS designs make informed decisions regarding the choice of an open or closed fuel cavity. One of the most important components within the fuel cavity in an RPS is the general purpose heat source (GPHS) module. The GPHS plays critical thermal, structural, and safety based roles within the RPS. In this paper, we will examine the potential impact of a Martian or Titan atmosphere on the GPHS in the fuel cavity. First, thermodynamic chemical modeling studies were performed. These studies indicated that nearly all of the Martian atmosphere would be able to react with and erode the GPHS carbon. Considering the very low pressure of the Martian atmosphere, however, it is recommended that reaction rate studies are performed on GPHS carbon to determine if the erosion will be significant over the life of the RPS. Modeling studies of Titan indicated that there are no predicted chemical reactions between the Titan atmosphere and the GPHS. It was noted, however, that components of the Titan atmosphere could decompose to form solid carbon and ammonia. While these products are not expected to be a problem for the GPHS, which is the focus of this study, they could create significant issues for other materials in the RPS. It is therefore recommended that any open fuel cavity designs consider the impact that solid carbon and ammonia could have on the whole RPS. Initial reaction rate studies were performed between a simulated Martian atmosphere and a carbon-carbon composite material that is a surrogate for GPHS carbon. It was interesting to note that there was no measurable erosion in the sample after 72 h at 700 °C. While this preliminary result is encouraging, it is not possible to provide a recommendation at this point regarding the use of an open fuel cavity on Mars. Additional studies will be required to evaluate the degree of erosion over much longer times and much higher temperatures. In addition to studying the erosion of the GPHS carbon, it is recommended that future studies also investigate changes in other GPHS carbon properties, including thermal conductivity and mechanical strength.