Radioisotope power systems in space missions: Overview of the safety aspects and recommendations for the European safety case

Nuclear power sources can offer a potential solution to some of the challenges related to space applications, such as the use of photovoltaics in opaque atmospheres or in a dark environment for a long time. Thanks to their high energy densities, certain isotopes are capable of generating considerable amounts of heat for long periods of time, independently of insolation levels; this heat can then be converted into electrical power, or used to keep suitable temperatures inside the spacecraft. However, the presence of radioactive material implies new sets of requirements, to properly manage all the aspects related to safety (heat source design, risk assessments, launch approval process etc.). This paper provides an overview of the international agreements, the existing NASA/US and Russian policies for the management of space nuclear power systems, as well as some updates and recent considerations for the ESA Radioisotope Power Systems (RPS) programme, which aims to provide a European capability for the independent design, management and launch of RPS. A significant amount of programmatic effort in many domains and by different entities will be required in order to create a common European safety framework, but the long and wide-ranging experience of other countries in dealing with space nuclear power systems could be an appropriate starting point. Recommendations are also made which could lead to an international standard linked to key stages of the mission lifecycle.