Fuel safety criteria for space nuclear reactor: the case of TRISO and UZrH fuels

Space agencies showed a renewed interest in human settlements on the Moon or Mars, mainly to support further space exploration and test new technologies. In this context, nuclear power can be a strategic choice for providing the necessary energy. After a brief review of space reactor history, their requirements and key components, a review of possible fuels and safety concept is made. Among the safety aspects of a space nuclear reactor, a particular emphasis is given to fuel safety criteria: indeed, this thesis aims to define safety limits under normal operation that will ensure the fuel’s integrity and the proper implementation of the radioactive material confinement safety function. The focus is on two candidate fuels, TRISO (TRi-structural ISOtropic) particle fuel and Uranium Zirconium Hydride (UZrH) fuels, that have been considered suitable due to their maturity and safety characteristics. To draw up these fuel safety criteria, firstly the behaviour of these fuels under irradiation is presented; then, from the identification of the main causes of fuel failures, the development of potential safety criteria is done, with an explicit reference to the concept of margin, that must always be present in a safety criterion to ensure conservative choices. The analysis of identified fuel failures in TRISO and UZrH fuels has led to the identification of twelve and nine fuel safety criteria, respectively, that depend mainly on operational conditions and some manufacturing characteristics. The fuel safety criteria presented are intended as a guide for fuel designers in developing appropriate fuels for space reactors, emphasising the most critical dependencies and guiding to specifications for fabrication purposes. While not exhaustive, these criteria can offer a basis for ensuring fuel integrity and safety in space reactor applications.