Overview of the Safety Analyses and Risk Assessments of Nuclear Space Missions

Abstract

Space missions are conducted to gain an understanding of the universe and our solar system; to study the surface of planets other than earth; and to explore our extraterrestrial environment. Many of these missions travel to the far reaches of the solar system or explore regions that require a continuous source of electrical power that is more than what is available from the conversion of solar energy. For these space missions, electrical power is supplied by a radioisotope thermoelectric generator (RTG) that uses the heat generated by the decay radioactive material. The approval to launch and fly space vehicles using nuclear material is governed by Presidential Directive and requires authorization by the Executive Office of the President. As part of the launch approval process for these missions, a comprehensive safety analysis is conducted. This safety analysis employs a full-scope probabilistic risk assessment (PRA) to help identify improvement in launch and flight systems and quantify the risk associated with potential accidents and abort conditions during the mission. In general, the PRA follows a typical scenario-based assessment similar to PRAs that have been conducted for terrestrial hazardous facilities and operations. However, there are some significant differences when conducting a PRA of a space mission. This paper will provide a general overview of the PRA process as applied to nuclear space missions including; defining the analysis objective, system familiarization, accident sequence analysis, accident analysis, atmospheric transport, consequence analysis, and risk integration and uncertainty analysis.