satellite reentered the atmosphere over the ocean east of Africa. The RTG burned up on reentry, as it was designed to do. The burnup of SNAP-9A added about 4% to the total amount of plutonium in the environment. Subsequent studies by Italy, Japan, the UK, and the US have shown no measurable health effects from this reentry [25-30]. All US RTGs following SNAP-9A were designed to contain or immobilize the fuel through all credible accident conditions, including reentry and impact on Earth. The implementation of the new reentry philosophy was verified in two subsequent reentries: • Abort of the launch of the Nimbus-Bl satellite on 18 May 1968 by the range safety officer because of a guidance error. The two SNAP-19B RTGs were recovered intact as designed. • Damage of the Apollo 13 spacecraft from an oxygen tank explosion after a successful launch on 11 April 1970 leading to the intact reentry (as designed) of the SNAP-27 fuel cask over the South Pacific Ocean on 17 April 1970. The US Government employs an independent, formal multi-agency safety and environmental review of all NPS designs before the first launch. This process is illustrated in Fig. 25. The overall US approach is consistent with a UN working group report [31, 32]. In fact, the US has been an active participant in UN discussions on the safe use of NPS in outer space [33]. The US has supported the conclusion reached by the UN technical experts: ‘The Working Group reaffirmed its previous conclusion that NPS can be used safely in outer space, provided that all necessary safety requirements are met' [31]. Conclusion Space nuclear power sources have proved to be reliable, long-lived sources of electrical power that have enabled the conduct of a number of important US space missions, including the first long-term study of the surfaces of the Moon and Mars and the first exploratory visits to Jupiter, Saturn, and Uranus. In general, the NPS, from SNAP-3B to the MHW-RTG, met or exceeded their design requirements by providing power at or above that required and beyond the planned lifetime. All of the power sources met their safety requirements. This successful performance has laid a secure foundation for future US missions that will use nuclear power. ACKNOWLEDGEMENTS The author acknowledges with thanks the contributions made by members of the staffs of Teledyne Energy Systems, Rockwell International, General Electric Company, TRW Space and Defense, Fairchild Space Company, NUS Corporation, Applied Physics Laboratory, Battelle Columbus Laboratories, 3M Company, Sandia National Laboratories, Los Alamos National Laboratory, Savannah River Plant and Laboratory, the Mound Plant, and Oak Ridge National Laboratory. In particular, the author would like to thank John Dassoulas, Paul J. Dick, James C. Hagan, Richard B. Harty, C. E. Kelly, Frank D. Postula, E. A. Skrabek, and C. W. Whitmore for supplying information over the years.
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