Design and Development of the ESA Am-Fueled Radioisotope Power Systems

Radioisotope heater units (RHU) and radioisotope thermoelectric generators (RTG) are currently being developed for the ESA radioisotope power system program. The state-of-the-art for the USA and Russian systems is to use plutonium-238 as the radioisotope fuel; however, for the ESA applications americium-241 has been selected due to its availability and relatively cost-effective production in the European context. The proposed designs implement a multi-layer containment approach for safety reasons, with a platinum-rhodium alloy for the inner containment of the fuel and carbon-based materials for the outer layers. The Am-fueled RHU provides 3 W of thermal power, and makes this design competitive with existing models in relation to specific power. The heat source for the RTG has a 6-side polygonal shape, with a distributed 3-fuel pellet architecture: this configuration allows to maximize the specific power of the RTG, since Am-based fuels have a lower power density than Pu-based fuels. The heat supplied by the fuel is 200 W, with an expected electrical power output of 10 W provided by six Bi-Te thermoelectric modules. Finite element structural and thermal analyses have been performed to assess the theoretical feasibility of the components as initially conceived. Mechanical and electrically-heated prototypes for the systems have already been tested in a representative lab environment at the University of Leicester; these tests have provided initial estimates for the efficiency of the systems. Both the RHU and RTG architectures are currently undergoing a new design iteration process. This paper reports on the overall architecture and design of the Am-fueled RTG and RHU, the modelling results and the experimental data obtained so far.