Impedance spectroscopy characterization of neutron irradiated thermoelectric modules for space nuclear power

The European Space Agency is currently supporting the research and development of advanced radioisotope power systems utilising thermoelectric modules. The performance of thermoelectric modules following exposure to neutron radiation is of significant interest due to the likely application of radioisotope thermoelectric generators in deep space exploration or planetary landers requiring prolonged periods of operation. This study utilises impedance spectroscopy to characterise the effects of neutron irradiation on the performance of complete thermoelectric modules, as opposed to standalone material. For a 50 We americium-241 radioisotope thermoelectric generator design, it is estimated that the TE modules could be exposed to a total integrated flux of approximately 5 × 1013 neutrons cm-2 (>1 MeV). In this study, an equivalent neutron dose was simulated experimentally via an acute 2-hour exposure in a research pool reactor. Bi2Te3-based thermoelectric modules with different leg aspect ratios and microstructures were investigated. Gamma-ray spectroscopy was initially used to identify activated radionuclides and hence quantify irradiation induced transmutation doping. To evaluate the thermoelectric properties pre- and post-irradiation, impedance spectroscopy characterization was employed. Isochronal thermal annealing of defects imparted by the irradiation process, revealed that polycrystalline based modules required significantly higher temperature than those with a monolithic microstructure. Whilst this may indicate a greater susceptibility to neutron irradiation, all tested modules demonstrated sufficient radiation hardness for use within an americium-241 radioisotope thermoelectric generator. Furthermore, the work reported demonstrates that impedance spectroscopy is a highly capably diagnostic tool for characterising the in-service degradation of complete thermoelectric devices

Recent Joint Studies Related to the Development of Space Radioisotope Power Systems

Over the last several years there has been a mutually beneficial ongoing technical interchange between the U.K and the U.S. related to various aspects of space radioisotope power systems (RPS). While this interchange has been primarily focused on materials based activities, it has also included some aspects related to safety, environmental, and lessons learned during the application of RPSs by the U.S. during the last fifty years. Recent joint technical RPS endeavors have centered on the development of a possible “cold” ceramic surrogate for 238PuO2 and 241AmOx and the irradiation of thermoelectrics and other materials at expected RPS related neutron fluences. As the U.S. continues to deploy and Europe develops RPS capability, on-going joint RPS technical interfaces will continue to enhance each entities’ endeavors in this nuclear based power technology critical for deep space exploration