Establishing safety goals on radiation protection for a space reactor through a case assessment of a loss-of-coolant accident in the International Space Station

Abstract

Power shortages caused by occlusion of solar cell arrays have been a major factor affecting the operation of the International Space Station (ISS). Although nuclear energy is a possible solution to ISS power shortages, current methods for evaluating nuclear plant safety goals in a space environment do not adequately address safety concerns. Here, we discuss this issue with respect to the SP-100, which is a typical candidate nuclear power source developed for space applications such as the ISS. An extreme core damage event due to loss of coolant is discussed to determine if the accident consequences are as serious as they would be under terrestrial conditions. An improved mathematical model based on the modified Rosin-Rammler equation, which describes distribution of released NaK coolant, is introduced. Then, the quantity and mass distribution of NaK droplets is simulated. The study chose I-131 for simulating short-term radioactive concentrations and Cs-137 for simulating long-term radioactive concentrations. Then, the study analyzed early and late exposure to the ISS crew. Thus, this study provides a new method for evaluating radioactive leakage applicable to the ISS. The results showed that even a critical core damage accident with loss of coolant would have little effect on the health of crew members aboard the ISS, providing support for the acceptability and justification of using nuclear energy in a space environment. Based on the case study, we discuss a rule for defining safety goals to protect ISS occupants from radiation due to accidents. The rule is that astronauts shall not bear significant additional risk due to a nuclear accident compared with the risks in the space environment where the reactor is operated. Hence, quantitative criteria for space reactor design can be deduced according to specific applications. By the results from this case study, we conclude that radioactive consequences due to accidents may not be a prohibitive limitation on the design, construction, and operation of space-based nuclear installations, which are different from terrestrial installations. This study may serve to alleviate concerns about reactor accidents in space.