• First remote automatic startup of a nuclear reactor in space; • First application of a high-temperature (810 K) liquid metal transfer system in space and the first application of a high-temperature spacecraft in space; • First use of a nuclear shadow shield in space; • Development and application of the highest powered thermoelectric power system to that time and the first use of a thermoelectric power system of that size in space; and • First thermoelectric powered liquid metal pump and the first use of such a pump in space. Space Nuclear Safety From the beginning, the US space nuclear power program has placed great emphasis on the safety of people and the protection of the environment. For RTGs, the safety philosophy is to contain or immobilize the radioisotope fuel to the maximum extent possible during all mission phases and postulated credible accidents. In the case of space nuclear reactor power systems, the current safety philosophy includes the launch of a nonoperating system so there is no buildup of radioactive fission products [25], The earlier NPS through SNAP-9A were designed to contain the fuel if the mission were aborted on the launch pad or during early ascent but to permit complete burnup of the fuel in the stratosphere. Worldwide dispersion and dilution of fine nuclear fuel particles would preclude local contamination. Transit 5BN-3, with a SNAP-9A power source, was launched on 21 April 1964 but failed to achieve orbit because of computer problems that affected the operation of the launch vehicle. The
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