Safety Studies for the ESA Space Nuclear Power Systems: Accident Modelling and Analysis

Abstract

Within the framework of the ESA radioisotope power system (RPS) program, the University of Leicester is currently developing radioisotope heater unit (RHU) and radioisotope thermoelectric generator (RTG) systems for future space missions, with americium-241 as radioactive fuel. An important aspect of the overall program is safety, and this involves ensuring that the design of these systems, in particular of the heat source (i.e. fuel and containment layers), meets a set of stringent requirements: it is fundamental to properly design both the RTG and RHU in order to avoid inadvertently releasing radioactive material into the environment in the event of an accident. The inner containment, or cladding, made of a platinum-rhodium alloy, is the first line of defense surrounding the americium-based fuel; additional layers of carbon-based insulators and carbon-carbon composites for the aeroshell ensure that the heat source can survive all possible accident conditions, from launch failures to Earth re-entry. Validated heat source accident models are necessary to inform the design iteration of the RHU and RTG heat sources, and to construct a safety case for their launch. The goal of the activity here described, performed in collaboration with ArianeGroup in France and ESA, is to start the process of understanding the behavior of the fuel containment systems under the most relevant accident conditions by computer modelling, to validate them experimentally given the infrastructure, test means and expertise of ArianeGroup in this field, and to characterize the different materials at ESTEC. The data obtained will help to iterate and improve the design of the European RPS heat sources by focusing on the fuel containment.