The United States has not launched a nuclear reactor into space since the SNAP 10A in 1965. No nuclear reactors have completed the current launch approval process, although numerous Radioisotope Power Systems (RPS) have been approved and launched. Recent investigations indicate that launch approval may be straightforward for a small fission power system that is designed to be subcritical prior to orbit. For small, simple designs the probability of criticality can be made small, and furthermore, the potential radioactive releases given a criticality are minimal.
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