SP-100 Space Reactor Risk Assessment: Lessons Learned

摘要

SP-100 was a space reactor design developed under U. S. Department of Energy contract in the late 1980's for a variety of potential missions. In 1989 a probabilistic risk assessment 1 of this design was completed for one particular mission, involving operation in a 300- year earth orbit. The purpose of this paper is not to summarize this assessment, but to draw lessons from it that would be useful for risk assessments of future space reactor missions. The SP-100 design was a lithium cooled uranium nitride fueled reactor with rotating external reflector control drums and internal safety rods. The design had a number of key safety features including: 1) uranium fuel; 2) no operation before achieving 300-year orbit; 3) internal safety rods to maintain sub-criticality on surface impact; 4) reentry cone. The first two safety features assured that the reactor would have had negligible radioactivity on launch. Features 3 and 4 assured that even if there is a mishap resulting in inadvertent reentry, the core will remain intact and enter the ocean, or bury itself, minimizing release of radioactivity. Nevertheless, there would be a small residual risk arising from failure of safety features, or from mishaps causing conditions exceeding the design capability. The total probability of all mishaps resulting in radiological consequences was estimated to be less than 0.0007 per mission. Lessons described in the paper include: 1) It is important to define the purposes of the risk assessment, the risk criteria, and metrics of interest before analysis. 2) Risk is highly dependent on the mission trajectory. 3) Risk-oriented safety analysis may rule out certain categories of potential accidents.