VOL 22-4 TECHNICAL PROGRESS REVIEW JUL • AUG 1981 423 General Safety Considerations Edited by J. R. Buchanan Overview of the U. S. Flight Safety Process for Space Nuclear Power By Gary L. Bennett* Abstract: The two current types of nuclear power sources used launch an orbiter and atmospheric probe to Jupiter and in U. S. spacecraft are described along with the jligh t safety the International Solar Polar Mission (ISPM) which will philosophies governing their use. In the case of radioisotope obtain scientific data on the sun and solar wind from thermoelectric generators, the design philosophy consists of containment, immobilization, and recovery of the nuclear high heliographic latitwdes. 2 materials. For reactors, the emphasis is on maintaining a As stated in a 1978 working paper submitted by subcritical configuration in all credible accident environments. the United States to the United Nations Committee on To document the safety activities, a safety analysis report is the Peaceful Uses of Outer Space: prepared for each mission. These reports, which are based on Since its inception, the U. S. space nuclear the probabilistic risk assessment methodology pioneered by the power program has placed great emphasis on safety space nuclear safety community, are subjected to an inter­ of people and protection of the environment. A agency safety review before a recommendation is made to approve the laundh of a nuclear-powered spacecraft. continuing primary objective has been to avoid undue risks by designing systems to safely contain the nuclear fuel under normal and potential acci­ The recent spectacular flights by Jupiter and Saturn of dent conditions. the National Aeronautics and Space Administration (NASA) spacecraft Voyager and Pioneer have also marked additional milestones in the continuing success­ *Gary L. Bennett is Chief of the Safety and Isotope Fuels ful and safe use of nuclear electric power in outer Branch in the Department of Energy Space and Terrestrial space. Since 1961, the United States has launched 22 Systems Division. He obtained the Ph.D. degree in physics from Washington State University in 1970. He has also worked NASA and military spacecraft having all or part of on several nuclear reactor safety research programs at the their power requirements supplied by nuclear power Idaho National Engineering Laboratory and the Nuclear sources. Twenty-one of these spacecraft were powered Regulatory Commission. He has worked on the nuclear rocket by radioisotope thermoelectric generators (RTGs) and program at the National Aeronautics and Space Administr<t­ one by a nuclear reactor. Table 1 summarizes the space tion's Lewis Research Center and was the government flight safety manager for the nuclear power sources used on the nuclear power sources launched by the United States USAF LES 819 communications satellites and the NASA to date. Electric-power output per individual nuclear Voyager spacecraft. In 1980 and 1981, he served as a member power source has ranged from 2.7 W(e) for SNAP-3A of the U.S. delegation to the United Nations Working Group to 500 W(e) for SNAP-lOA. The history of these on the Use of Nuclear Power Sources in Outer Space and has sources has shown that they can be safely and reliably served as an adviser to the U.S. delegation to the Scientific and Technical Subcommittee of the United Nations Committee on built and launched to meet a variety of mission the Peaceful Uses of Outer Space. He has written a number of 1 objectives. Future missions committed to nuclear papers on nuclear safety and the use of nuclear electric power power include NASA's Galileo mission which will in outer space, and one novel (The Star Sailors). NUCLEAR SAFETY, Vol. 22, No. 4, July-August 1981 • 1 424 GENERAL SAFETY CONSIDERATIONS Table 1 Summary of Space Nuclear Power Sources Launched by the United States (1961-1980) Power source* Spacecraft Mission type Launch date Status SNAP-3A Transit 4A Navigational June 29, 1961 Successfully achieved orbit SNAP-3A Transit 4B Navigational Nov. 15, 1961 Successfully achieved orbit SNAP-9A Transit-5BN-1 Navigational Sept. 28, 1963 Successfully achieved orbit SNAP-9A Transit-5BN-2 Navigational Dec. 5,1963 Successfully achieved orbit SNAP-9A Transit-5BN-3 Navigational Apr. 21, 1964 Mission aborted; burned up on reentry SNAP-lOA Snapshot Experimental Apr. 3, 1965 Successfully achieved orbit SNAP-19B2 Nimbus-B-1 Meteorological May 18, 1968 Mission aborted; heat source retrieved SNAP-19B3 Nimbus III Meteorological Apr. 14, 1969 Successfully achieved orbit SNA}'-27 Apollo 12 Lunar Nov. 14, 1969 SuccessfuiJy placed on lunar surface SNAP-27 Apollo 13 Lunar Apr. 11, 1970 Mission aborted on way to moon; heat source returned to South Pacific Ocean SNAP-27 Apollo 14 Lunar Jan. 31, 1971 Successfully placed on lunar surface SNAP-27 Apollo 15 Lunar July 26, 1971 Successfully placed on lunar surface SNAP-19 Pioneer 10 Planetary Mar. 2, 1972 Successfully operated to Jupiter and beyond SNAP-27 Apollo 16 Lunar Apr. 16, 1972 Successfully placed on lunar surface Transit-RTG "Transit" Navigational Sept. 2, 1972 Successfully achieved orbit (TRIAD-Ol-1X) SNAP-27 Apollo 17 Lunar Dec. 7,1972 Successfully placed on lunar surface SNAP-19 Pioneer 11 Planetary Apr. 5, 1973 Successfully operated to Jupiter and Saturn and beyond SNAP-19 Viking 1 Mars Aug. 20, 1975 Successfully landed on Mars SNAP-19 Viking 2 Mars Sept. 9, 1975 Successfully landed on Mars MHW LES 8/9t Communications Mar. 14, 1976 Successfully achieved orbit MHW Voyager 2 Planetary Aug. 20, 1977 Successfully operated to Jupiter and Saturn and beyond MHW Voyager 1 Planetary Sept. 5, 1977 Successfully operated to Jupiter and Saturn and beyond *SNAP-lOA was powered by a nuclear reactor; the remainder were "powered by radioisotope thermoelectric generators. t LES = Lincoln experimental satellite. This article presents a brief overview of the U. S. RATIONALE FOR USING NUCLEAR philosophy in regard to space nuclear safety and the ELECTRIC POWER IN SPACE VEHICLES current flight safety review process. When the development risks and potential benefits Since its inception the U.S. program for the safe can be meaningfully identified in advance, cost­ use of nuclear power in outer space has involved benefit analyses are conducted before a major nuclear hundreds of scientists and engineers in several govern­ electric-power appHcation in outer space is undertaken. men tal agencies and private organizations. Space does These analyses include mission requirements; environ­ not permit the identification of individual contribu­ mental, health, and safety requirements; and other tors; however, the principal Department of :$nergy specific requirements (such as reliability, longevity, and (DOE)-supported organizations that have participated survivability) and consider nonnuclear as well as in the flight safety program in the past or are currently nuclear alternatives. 3 The benefits to be derived from participating are Los Alamos National Laboratory, the use of nuclear electric-power sources in space can Applied Physics Laboratory (Johns Hopkins Oniver­ be seen in the following conditions: 1 sity), NUS Corporation, Sandia National Laboratories, l . Lifetime: Nuclear power is the principal alterna­ General Electric Co~pany , Teledyne Energy s ystems, tive for spacecraft which must operate for a long and TRW, Inc. · : period of time. NUCLEAR SAFETY, Vol. 22, No. 4, Julv-August 1981 GENERAL SAFETY CONSIDERATIONS 425 2. Environment: Nuclear power sources are less device that directly converts the heat associated with vulnerable to external radiation damage (e .g., in the the decay of a radioisotope to electricity by means of Van Allen radiation belts) and to other potentially the Seebeck effect. Thus there are only two functional hostile environments [e.g., meteoroids, Martian dust parts of an RTG: a thermoelectric converter and a heat storms, and extreme temperatures (such as experienced source. For all U. S. space missions to date, the heat on the lunar and Martian surfaces)]. sources have been fueled with the radioisotope 2 3 8 Pu. 3. Self-sufficiency: Nuclear power sources enable This radioisotope has an appropriately long half-life the spacecraft to be more autonomous. In addition, (about 87.8 yr), which permits long operational life­ RTGs can be operated on the launch pad for systems times to be considered. Plutonium-238 decays pri­ checkouts prior to launch or from the orbiting space marily by emitting alpha particles, which are com­ shuttle. pletely absorbed in the heat source to produce the 4. Operational reliability: Nuclear power sources in heat; hence no special radiation shielding for these spacecraft have exhibited extremely high reliability. alpha particles is required. The principal safety objec­ They provide a compact source of electrical energy tive associated with the use· of 2 3 8 Pu is to keep it with a good power-to-mass ratio. The small exposed contained to prevent ingestion by humans and conse­ area of nuclear power sources can reduce the overall quent exposure of the unprotected internal organs'and size of the spacecraft, simplify attitude control, and bones to radiation. reduce structural interactions. A nuclear power source Figure 1 shows the two most recent RTGs : the can also be used to supply heat directly to selected multihundred-watt (MHW) and general-purpose heat spacecraft components without introducing electro­ source (GPHS) RTGs. The MHW RTG provided the magnetic interference. electric power for the NASA spacecraft Voyager (see Fig. 2). The GPHS RTGs are being designed and built TYPES OF NUCLEAR POWER to provide electric power for Galileo and the ISPM. SOURCES USED IN SPACECRAFT Figure 3 is a cutaway drawing showing the key parts of the GPHS. As in the MHW heat source, the fuel Two types of nuclear power sources have been used 38 e Pu02 ) is contained in a modular form, with each to generate electricity for spacecraft: RTGs and nu­ module multiply encased to ensure its survival under a clear reactors.