2
Background

WHY SPACE EXPLORATION?

From its very beginning, the exploration of space has brought enormous gains to humanity. At one level it is about seizing the strategic initiative and using space technology for a broad array of activities that enhance our life on Earth. Indeed, weather, communications, reconnaissance, and navigation satellites have revolutionized many aspects of our lives. Spacecraft have also revolutionized our understanding of the solar system and beyond. They have investigated Earth’s relationship to the Sun and the larger cosmological system, the context of Earth in relation to other planets, and the fragility of our planet in ensuring our continued existence.

Understanding how and why Earth is an abode of life, understanding the potential for life elsewhere, advancing scientific knowledge of the origins and history of the solar system, and creating a sustainable long-term human presence on the Moon are vital components of the space exploration efforts of the United States. Why is Mars bone dry, virtually airless, and seemingly dead? Why is Venus a hostile world, hidden from view by a hot, heavy atmosphere and a dense layer of clouds? Is Titan an analogue for Earth-like meteorology and geological processes, albeit at frigid temperatures? What causes the dynamic and violent atmospheric conditions of Jupiter? What are the fundamental processes that shaped the origins and evolution of the solar system? Are we alone or is the universe teeming with undiscovered life beyond Earth? As John Glenn remarked, “Our spirit as a nation is reflected in our willingness to explore the unknown for the benefit of all humanity, and space is a prime medium in which to test our mettle” (Glenn, 1983).

WHY RADIOISOTOPE POWER SYSTEMS?

Through an investment of considerable resources—engineering and scientific knowledge, human capital, and public funds—the United States has gained undisputed leadership in the exploration of the solar system. This has been made possible since the 1950s by harnessing several core technologies that have enabled U.S. scientific spacecraft to travel for years on end, engage in extended scientific observations, and relay critical data back to Earth. Radioisotope power systems (RPSs) are one such technology.

RPSs convert the heat generated by the natural decay of radioactive material (specifically, 238Pu) to electrical energy. In a radioisotope thermoelectric generator (RTG), the heat flows through the thermocouples to a heat sink, generating direct current (dc) electricity in the process. The thermocouples are then connected through a closed loop that feeds an electrical current to the power management system of the spacecraft. All of the RPSs flown to date have been RTGs. They are compact, rugged, and extraordinarily reliable, but the energy conversion efficiency is low (~6 percent).

Advanced Stirling Radioisotope Generators (ASRGs) will have much higher efficiency (~29 percent), thereby greatly reducing the amount of 238Pu needed to support future missions. In the Stirling engine converter used by ASRGs, helium gas oscillates in a regenerator, one end of which is heated by radioactive decay of 238Pu, while the other end is cooled by a heat sink. This oscillating gas pushes a piston in a linear alternator that generates alternating current (ac) electricity. The ac is converted to dc electronically, and the current is fed to the power management system of the spacecraft. Although dynamic energy conversion systems have long been considered for RPSs, only recently have technological advances—and the need to minimize future demand for 238Pu—justified development of RPSs with a Stirling engine.

RPSs can provide power for multiyear missions to faraway places where either sunlight is lacking (e.g., missions beyond Jupiter) or solar power is unreliable (e.g., in Jupiter’s radiation belts).1 At Jupiter, sunlight is 96 percent

1

For example, the Juno mission to Jupiter will be powered by solar arrays, but it will be in a highly elliptical polar orbit; it will not orbit near the Jovian equatorial plane where the most intense portions of the belts are located. Thus, it will spend little time in the belts themselves.



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2 Background WhY space eXpLOratiOn? been made possible since the 1950s by harnessing several core technologies that have enabled U.S. scientific space- From its very beginning, the exploration of space has craft to travel for years on end, engage in extended scientific brought enormous gains to humanity. At one level it is about observations, and relay critical data back to Earth. Radio- seizing the strategic initiative and using space technology isotope power systems (RPSs) are one such technology. for a broad array of activities that enhance our life on Earth. RPSs convert the heat generated by the natural decay of Indeed, weather, communications, reconnaissance, and navi- radioactive material (specifically, 238Pu) to electrical energy. gation satellites have revolutionized many aspects of our lives. In a radioisotope thermoelectric generator (RTG), the heat Spacecraft have also revolutionized our understanding of flows through the thermocouples to a heat sink, generating the solar system and beyond. They have investigated Earth’s direct current (dc) electricity in the process. The thermo- relationship to the Sun and the larger cosmological system, the couples are then connected through a closed loop that feeds context of Earth in relation to other planets, and the fragility an electrical current to the power management system of the of our planet in ensuring our continued existence. spacecraft. All of the RPSs flown to date have been RTGs. Understanding how and why Earth is an abode of life, They are compact, rugged, and extraordinarily reliable, but understanding the potential for life elsewhere, advancing the energy conversion efficiency is low (~6 percent). scientific knowledge of the origins and history of the solar Advanced Stirling Radioisotope Generators (ASRGs) will system, and creating a sustainable long-term human presence have much higher efficiency (~29 percent), thereby greatly on the Moon are vital components of the space exploration reducing the amount of 238Pu needed to support future mis- efforts of the United States. Why is Mars bone dry, virtually sions. In the Stirling engine converter used by ASRGs, helium airless, and seemingly dead? Why is Venus a hostile world, gas oscillates in a regenerator, one end of which is heated by hidden from view by a hot, heavy atmosphere and a dense radioactive decay of 238Pu, while the other end is cooled by layer of clouds? Is Titan an analogue for Earth-like meteorol- a heat sink. This oscillating gas pushes a piston in a linear ogy and geological processes, albeit at frigid temperatures? alternator that generates alternating current (ac) electricity. What causes the dynamic and violent atmospheric conditions The ac is converted to dc electronically, and the current is fed of Jupiter? What are the fundamental processes that shaped to the power management system of the spacecraft. Although the origins and evolution of the solar system? Are we alone or dynamic energy conversion systems have long been consid- is the universe teeming with undiscovered life beyond Earth? ered for RPSs, only recently have technological advances— As John Glenn remarked, “Our spirit as a nation is reflected and the need to minimize future demand for 238Pu—justified in our willingness to explore the unknown for the benefit of development of RPSs with a Stirling engine. all humanity, and space is a prime medium in which to test our RPSs can provide power for multiyear missions to mettle” (Glenn, 1983). faraway places where either sunlight is lacking (e.g., mis- sions beyond Jupiter) or solar power is unreliable (e.g., in WhY radiOisOtOpe pOWer sYsteMs? Jupiter’s radiation belts).1 At Jupiter, sunlight is 96 percent Through an investment of considerable resources— 1For example, the Juno mission to Jupiter will be powered by solar arrays, engineering and scientific knowledge, human capital, and but it will be in a highly elliptical polar orbit; it will not orbit near the Jovian public funds—the United States has gained undisputed equatorial plane where the most intense portions of the belts are located. leadership in the exploration of the solar system. This has Thus, it will spend little time in the belts themselves. 

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 RADIOISOTOPE POWER SYSTEMS less intense than at Earth. Continuing outward to Pluto, sun- beyond. Such missions will be severely constrained or elimi- light is 99.94 percent less intense. RPS-powered Voyager, nated unless RPSs are ready and available (see Table 2.2). Galileo, Cassini, and New Horizons spacecraft have enabled FINDING. Importance of RPSs. RPSs have been, are now, the United States to explore every planet in this dark, outer region of the solar system. Much of their success has been and will continue to be essential to the U.S. space science due in large part to having a reliable power source that pro- and exploration program. vides enough power to operate complex instruments at a data rate high enough to optimize the capabilities of the scientific WhY 238pu? instruments they carry. RPSs are also useful for missions to the surface of the Plutonium-238, which does not occur in nature, is created by irradiating neptunium-237 (237Np) targets in a nuclear Moon (especially during the long, cold lunar nights and in the permanently shadowed regions near the lunar poles); for reactor. Although many studies over the past 50 years have missions to the surface of Mars (with its dust storms and assessed the advantages and disadvantages of using a wide extended winters); for extended missions below Venus’s variety of isotopes as a fuel for RPSs, every RPS launched into space by the United States has been fueled by 238Pu.3 cloud deck; and for other missions where solar power is not practical, for example, because the dynamic range of solar Studies examined by the committee demonstrate that the power would preclude the use of solar arrays.2 longstanding decision by the Department of Energy (DOE) and NASA to rely on 238Pu is correct and well-justified. No Space nuclear power reactors are another potential option for missions where solar power is not practical. However, other radioisotope meets or exceeds the safety and perfor- mance characteristics of 238Pu, particularly for long-dura- the performance and reliability of space nuclear power reactor systems using current technology remains unproven, tion, deep-space exploration missions (see Appendix D). especially for missions with long lifetimes. In addition, the Plutonium-238, which has a half-life of 88 years, is the only committee is not aware of any substantive effort currently isotope that meets all of the general criteria for RPS fuels, under way anywhere in the world to develop space nuclear as follows: power reactor systems. The history of space nuclear power reactors suggests that space nuclear reactors, if success- • It generates heat for a sufficient length of time (i.e., it fully developed, could meet the needs of some missions has a radioactive decay half-life of sufficient length). and could enable other missions that are not now under • The type and quantity of the emissions produced by consideration because of power limitations. For example, the radioactive decay of the fuel allow it to be handled Project Prometheus, which was NASA’s most recent attempt safely. to develop space nuclear power reactors, selected a nuclear • It has high specific power (heat per mass) and high electric propulsion reactor concept that was scalable from power density (heat per volume). 20 kilowatts of electrical power (kWe) to 300 kWe. However, • It has a fuel form that is noncorrosive, water-insoluble, history also shows that the development of a high-power, and chemically stable, and it demonstrates good engi- long-life space nuclear power reactor would be very time- neering properties at high temperatures. consuming and cost billions of dollars (see Appendix E). • It can be produced in sufficient quantity at an affordable Since 1961, the United States has launched 45 RPSs cost. on 26 spacecraft dedicated to navigation, meteorology, F INDING. P lutonium-238 Supply. P lutonium-238 is communications, and exploration of the Moon, Sun, Mars, Jupiter, Saturn, and elsewhere in the outer solar system (see the only isotope suitable as an RPS fuel for long-duration Table 2.1). This critical work could not have been accom- missions because of its half-life, emissions, power density, plished without RPSs. Current RPS-powered space missions specific power, fuel form, availability, and cost. An assured supply of 238Pu is required to sustain the U.S. space science include the Cassini spacecraft, with three RPSs, which is studying Saturn and its moons; and the New Horizons space- and exploration program. craft, with one RPS, which is studying Pluto and the Kuiper Belt. The Mars Science Laboratory spacecraft is scheduled nasa and dOe rOLes and respOnsiBiLities for launch in 2011 with an RPS-powered rover. Over the longer term, RPSs are expected to support continued explora- The Atomic Energy Act of 1954, as amended (Public tion of extreme environments of the Moon, Mars, and Venus, Law 83-703, 1954), establishes comprehensive requirements as well as the dimly lit outer reaches of the solar system and regarding the possession, use, and production of nuclear 2A specific example is a solar probe mission using Jupiter for a gravity 3The Systems for Nuclear Auxiliary Power (SNAP)-3 Program used both assist in order to pass the Sun in an orbit highly inclined to the plane of the polonium-210 and plutonium-238 as nuclear fuel for RTGs during ground ecliptic. For a mission such as this, the spacecraft experiences such a wide tests (Dieckamp, 1967). Over the years, some papers have erroneously range of solar intensity that current technology is unable to provide the reported that SNAP-3 RTGs fueled with polonium-210 were operated in spacecraft with a low-mass solar power system. space. That is not the case.

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 BACKGROUND TABLE 2.1 U.S. Spacecraft Using Radioisotope Power Systems Spacecraft Power Source No. of RPSs Mission Type Launch Date Location Transit 4A SNAP-3B7 1 Navigational 06/29/1961 Currently in orbit Transit 4B SNAP-3B8 1 Navigational 11/15/1961 Currently in orbit Transit 5BN-1 SNAP-9A 1 Navigational 09/28/1963 Currently in orbit Transit 5BN-2 SNAP-9A 1 Navigational 12/05/1963 Currently in orbit Transit 5BN-3 SNAP-9A 1 Navigational 04/12/1964 Reentered; burned up Nimbus B-1 SNAP-19B2 2 Meteorological 05/18/1968 Aborted; retrieved Nimbus III SNAP-19B3 2 Meteorological 04/14/1969 Currently in orbit Apollo 12 SNAP-27 1 Lunar/ALSEP 11/14/1969 On lunar surface Apollo 13 SNAP-27 1 Lunar/ALSEP 04/11/1970 Reentered in South Pacific Apollo 14 SNAP-27 1 Lunar/ALSEP 01/31/1971 On lunar surface Apollo 15 SNAP-27 1 Lunar/ALSEP 07/26/1971 On lunar surface Pioneer 10 SNAP-19 4 Planetary/Sun escape 03/02/1972 Heliosheath Apollo 16 SNAP-27 1 Lunar/ALSEP 04/16/1972 On lunar surface Triad-01-1X Transit-RTG 1 Navigational 09/02/1972 Currently in orbit Apollo 17 SNAP-27 1 Lunar/ALSEP 12/07/1972 On lunar surface Pioneer 11 SNAP-19 4 Planetary/Sun escape 04/05/1973 Heliosheath Viking 1 SNAP-19 2 Mars Lander 08/20/1975 On martian surface Viking 2 SNAP-19 2 Mars Lander 09/09/1975 On martian surface LES 8, LES 9 MHW-RTG 2, 2 Communication 03/14/1976 Currently in orbit Voyager 2 MHW-RTG 3 Planetary/Sun escape 08/20/1977 Heliosheath Voyager 1 MHW-RTG 3 Planetary/Sun escape 09/05/1977 Heliosheath Galileo GPHS-RTG 2 Planetary (Jupiter) 10/18/1989 Intentionally deorbited into Jupiter Ulysses GPHS-RTG 1 Solar and space physics 10/06/1990 Heliocentric, polar orbit Cassini GPHS-RTG 3 Planetary (Saturn) 10/15/1997 Operating at Saturn New Horizons GPHS-RTG 1 Planetary/Sun escape 01/19/2006 En route to Pluto NOTE: ALSEP, Apollo Lunar Surface Experiments Package; GPHS, general purpose heat source; LES, Lincoln Experimental Satellite; MHW, Multi-hundred Watt; RTG, radioisotope thermoelectric generator; SNAP, Systems for Nuclear Auxiliary Power. SOURCES: Data from G.L. Bennett, J.J. Lombardo, and B.J. Rock, “Development and use of nuclear power sources for space applications,” Journal of the Astronautical Sciences 29 (October-December):321-342, 1981; N.L. Johnson, “Nuclear power supplies in orbit,” Space Policy 2:223-233, 1986; G.L. Bennett, “Space Nuclear Power: Opening the Final Frontier,” AIAA 2006-4191, p. 2, presentation at 4th International Energy Conversion Engineering Conference and Exhibit, San Diego, Calif., June 26-29, 2006. materials and facilities. Other federal legislation allocates because the RPSs powering those missions have far exceeded their design lifetimes.4 responsibilities for regulating nuclear materials between the DOE and the Nuclear Regulatory Commission. In the United The DOE writes nuclear safety requirements applicable States, only the DOE is authorized to own space nuclear for the operations they perform. These requirements are simi- power systems. Therefore, NASA must team with the DOE lar to those established by the Nuclear Regulatory Commis- to manufacture, launch, and operate RPSs in space. sion and other agencies that regulate other types of nuclear The DOE also owns and operates the nuclear facilities operations. For example, regulations specify that safety that are used to develop, fabricate, assemble, and test RPS should be engineered into systems during their design and systems and hardware that involve nuclear fuels. Although development, and systems and processes should be designed the DOE always retains ownership of RPSs, NASA may and implemented with the goal of reducing radiation expo- have custody. The nuclear fuel is integrated with other RPS sures to as low as reasonably achievable. components at DOE facilities located at several DOE sites. In addition, DOE regulations apply to the RPS storage, handling, and checkout facility at NASA Kennedy Space Center. 4Voyager 1 and Voyager 2, originally designed for a 5-year mission to the The NASA-DOE partnership to provide RPSs for space Saturn system, are still sending back scientific data 31 years after launch. Voyager 2 became the first and only spacecraft to fly by Uranus and Neptune, exploration has been extremely successful, with decades of and both spacecraft are now out of the ecliptic plane. The Voyager RPSs mission success (see Appendix E). Scientific results of RPS are projected to provide enough power for these spacecraft to operate until missions have often greatly exceeded initial expectations approximately 2020.

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0 RADIOISOTOPE POWER SYSTEMS TABLE 2.2 RPS Contribution to Space Science and Exploration Missions Discovery New Frontiers Flagship Lunar Mars Exploration ATHLETE PR #1,2,3 SPABSR S/M NET ILN 1&2 ILN 3&4 Mercury Major Questions and Objectives ( Adapted from 2006 NASA Solar Venus CSSR Moon V ISE MSR V ME MSL NTE EAL GO System Exploration Roadmap) SB EE TE IO How did the Sun's family of planets and minor bodies originate? Understand the initial stages of planetary and satellite formation Study the processes that determine the original characteristics of bodies in the solar system How did the solar system evolve to its current diverse state? Determine how the processes that shape planetary bodies operate and interact Understand why the terrestrial planets are so different from one another Learn what our Solar System can tell us about extrasolar planetary systems What are the characteristics of the solar system that led to the origin of life? Determine the nature, history, and distribution of volatile and organic compounds in the Solar System Determine evidence for a past ocean on the surface of Venus Identify the habitable zones in the outer solar system How did life begin and evolve on Earth, and has it evolved elsewhere in the solar system? Identify the sources of simple chemicals important to prebiotic evolution and the emergence of life Evidence for life on Europa, Enceladus, and Titan Evidence for past life on Venus Identify environmental hazards and resources enabling human presence in space. Determine the inventory and dynamics of objects that may pose an impact hazard to Earth Inventory and characterize planetary resources that can sustain and protect human explorers Science Contribution RPS Dependence Major return Not possible without RPS Secondary return RPS use enhances science return NOTE: ATHLETE, All-Terrain Hex-Legged Extra-Terrestrial Explorer (rover); CSSR, Comet Surface Sample Return; EAL, Europa Astrobiology Lander; EE, Europa Explorer; GO, Ganymede Observer; ILN, International Lunar Network; IO, Io Observer; MSL, Mars Science Laboratory; MSR, Mars Sample Return; NTE, Neptune-Triton Explorer; PR, Pressurized Rover; SB, small bodies; S/M NET, seismological/meteorological network science; SPABSR, South Pole-Aitken Basin Sample Return; TE, Titan/Enceladus Explorer; VISE, Venus In-Situ Explorer; VME, Venus Mobile Explorer. SOURCE: T.J. Sutliff, NASA, “Space Science and RPSs, What Missions Cannot Be Accomplished without RPSs,” presentation to the Radioisotope Power Systems Committee, January 12, 2009, Irvine, California. agreements between nasa and the dOe (NEPA, 1970), the DOE assesses potential environmental impacts from activities related to nuclear material operations, A memorandum of understanding between the secretary transportation, and storage. The DOE also provides nuclear of energy and the NASA administrator defines NASA’s and risk assessments in support of environmental impact state- DOE’s roles and responsibilities regarding research, technol- ments that NASA prepares to comply with NEPA for the ogy development, design, production, delivery, space-vehicle launch of a spacecraft utilizing an RPS system. The DOE integration, launch, and operation of RPSs (DOE, 1991). is also responsible for specifying minimum radiological, DOE’s responsibilities include the design, development, public-health, and safety criteria and procedures for the use fabrication, evaluation, testing, and delivery of RPSs to meet of RPSs; providing safeguards and security guidance for NASA system-performance and schedule requirements. In NASA facilities and services; supporting NASA operational accordance with the National Environmental Policy Act plans, mission definition, environmental analysis, launch

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 BACKGROUND 238Pu approval, and radiological contingency planning; affirming and advanced flight-qualified RPS technology under the flight readiness of RPSs with respect to nuclear safety; the existing organizational structures and allocation of roles participating in the nuclear launch approval process; jointly and responsibilities. investigating and reporting nuclear incidents; and assuming legal liability for damages resulting from nuclear incidents rps nucLear safetY and accidents involving RPSs. NASA provides the DOE with overall system require- Safety is an integral part of any nuclear system, and it ments, specifications, schedules, and interfaces; provides encompasses the entire system life cycle. Nuclear safety for data to support DOE safety analyses in accordance with RPSs encompasses design, development, assembly, check- NEPA; supports nuclear launch approval (e.g., launch- out, testing, handling, transport, storage, ground checkout, vehicle databooks); complies with minimum radiological integration with payload, mating with launch vehicle, pre- occupational and public health and safety criteria and pro - launch activities, launch, ascent, orbital insertion, trajectory cedures specified by the DOE; provides adequate facilities insertion, in-flight checkout, mission operations, and final for safe and secure storage, assembly, and checkout of RPSs disposition. RPS safety includes the protection of the public, while in NASA custody; and provides tracking, command, the environment, workers, property, and other resources from and data services required to monitor RPSs during and sub- undue risk of injury or harm. To achieve these goals, three sequent to launch. objectives must be met: (1) design safety into each RPS at The 2006 National Space Policy (OSTP, 2006) directs the outset, (2) demonstrate the safety of RPSs through testing the United States to develop and use space nuclear power and analysis, and (3) assess the level of risk for each RPS- systems where such systems safely enable or significantly powered space mission as required to support the launch enhance space exploration. This policy reaffirms DOE’s role approval process. in maintaining nuclear infrastructure as well as the ability to Processes have been established to address all of these conduct nuclear safety analyses to support the nuclear launch objectives. The DOE has well-established rules, specifica- approval process. tions, and procedures for the safe design, development, Historically, the DOE or its predecessor agencies (the testing, transport, and handling of RPSs. The DOE also Atomic Energy Commission and the Energy Research and has developed sophisticated tools to conduct safety and Development Administration) bore the cost of establishing risk analyses to support the flight safety review and launch and maintaining the infrastructure to produce 238Pu and to approval process. Because 238Pu emits alpha particles, U.S. RPSs pose a develop RPS technology and systems. NASA would then biological hazard only if the 238Pu is somehow released reimburse the DOE for the incremental costs of producing the 238Pu that NASA used and for the flight hardware that it into the environment and is then either ingested or inhaled. launched. Consistent with this historic precedent, NASA is Ingestion is only plausible through the food chain, where reimbursing the DOE for the full cost of the 238Pu that the foods contaminated with 238Pu are consumed. This requires DOE is purchasing from Russia because all of that 238Pu is that the 238Pu be released and vaporized or pulverized into being used for NASA missions. small particles (less than ~100 microns in diameter) and then If the United States is to continue using RPSs for space transported through the atmosphere so they can deposit on science and exploration, it is appropriate for the DOE to or within food stuffs. Similarly, inhalation is only plausible if 238Pu is released and vaporized or pulverized into respi- continue the maintenance and operation of the nuclear facili- ties required for the fabrication and testing of fueled RPS rable particles (less than ~3 microns in diameter) and then components and systems. Because of the DOE’s statutory transported through the atmosphere where it can be inhaled. U.S. RPSs are fueled with 238Pu in the form of a ceramic responsibilities, it is also appropriate for the funding of these oxide (238PuO2) that has a high melting point and very low facilities to be included in the DOE budget rather than pass- ing these funds through NASA’s budget. These facilities are solubility to (1) minimize fuel vaporization and transport in required to operate according to DOE rules and regulations. the atmosphere and (2) minimize fuel retention within the The DOE’s budget has funding to continue the maintenance human body, if it should occur. and operations of the nuclear facilities required to support the RPSs are designed with multiple fuel containment barriers fabrication of RPSs—but no funds are included for produc- (i.e., defense in depth) to prevent release and, if a release tion of 238Pu. If the production of 238Pu is not reestablished, should occur, to limit the dispersal of 238Pu into the biosphere these DOE facilities could be shut down after they process in credible accident scenarios that could occur during a space the last available 238Pu. mission. For U.S. RPSs on the Galileo mission to Jupiter (October 1989) and on all subsequent missions to date, FINDING. Roles and Responsibilities. Roles and respon- each 238PuO2 fuel pellet is encapsulated in a ductile, high- temperature iridium-based alloy. Two encapsulated 238PuO2 sibilities as currently allocated between NASA and the Department of Energy are appropriate, and it is possible fuel pellets are packaged within a cylindrical graphite to address outstanding issues related to the short supply of impact shell constructed of a carbon-carbon composite. Two

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 RADIOISOTOPE POWER SYSTEMS graphite impact shells are packaged within a reentry aero- independent of the mission and associated RPS develop- shell that is also constructed of a carbon-carbon composite. ment efforts, and they have the responsibility and authority This assembly, which is approximately 4 in. × 4 in. × 2 in., to identify and address issues at any level. Each INSRP is is called a general purpose heat source (GPHS) module. This supported by technical experts, as needed, typically in six is the standard RPS fuel module now used in all U.S. RPSs, working groups: Launch Abort, Reentry, Power Systems, and it reflects many improvements in materials and packag- Meteorology, Biomedical and Environmental Effects, and ing that have been introduced over time.5 Risk Integration and Uncertainty. Testing and analysis must be performed to determine The Final Safety Analysis Report is reviewed in great the response of all RPSs to credible accident environ - d epth by the INSRP, which often performs additional ments. Testing validates analysis models and establishes independent analyses. The INSRP then prepares a Safety and demonstrates the level of safety built into the design. Evaluation Report. These reports identify and characterize A tremendous amount of testing has been conducted on the credible accident scenarios, including the probabilities that 238Pu will be released and the postulated health effects for GPHS fuel, materials, and hardware since its original design and development in the mid-to-late 1970s. each accident scenario, to determine overall mission risk and The efficacy of the U.S. RPS design safety approach was the uncertainties associated with that risk. demonstrated during the launch of the Nimbus B-1 meteo- NASA uses the Final Safety Analysis Report and the rological satellite, with two Systems for Nuclear Auxiliary Safety Evaluation Report to determine whether it will for- Power (SNAP)-19B2 RPSs on board, from Vandenberg Air mally request launch approval from the White House. If it Force Base, California, on May 18, 1968. During this launch, does, both reports are provided to the director of the Office of range safety destruct of the launch vehicle and upper stage Science and Technology Policy (within the Executive Office was initiated by the range safety officer because the launch of the President), who may grant launch approval, deny vehicle was ascending erratically. Although the launch launch approval, or defer the decision to the President. vehicle and payload were totally destroyed by the explosion, The entire launch approval process typically takes 3 years the RPSs were recovered intact. No release of 238Pu occurred, (including the resolution of legal challenges that are some- and the 238PuO2 fuel was used on a later mission. times raised), although it could take as long as 8 years. The Nevertheless, the use of RPSs does create some risk that process usually takes longer than average if a mission uses a 238Pu could be released into the biosphere, however low that launch vehicle, upper stage, launch complex, launch trajec- risk may be. To assess this risk, the Unites States has estab - tory, and/or spacecraft combination that has not previously lished a flight safety review and launch approval process for been characterized and analyzed. In such cases, extra effort RPS-powered missions. This process is structured to ensure is needed to prepare the Launch Vehicle Databook, which that the radiological risk for each mission is characterized identifies and characterizes accident sequences and environ- in detail and independently evaluated so that an informed ments that could occur during pre-launch, launch, ascent, and launch decision can be made, based on sound risk-benefit trajectory insertion. considerations. FINDING. RPS Nuclear Safety. The U.S. flight safety The U.S. flight safety review and launch approval process for space nuclear power systems is established by Presi- review and launch approval process for nuclear systems dential Directive/National Security Council Memorandum comprehensively addresses public safety, but it introduces 25 (PD/NSC-25, 1977). As part of this process, the DOE schedule requirements that must be considered early in the prepares a series of increasingly detailed Safety Analysis RPS system development and mission planning process. Reports that characterize the radiological risk for the each mission. references For each NASA mission, the NASA administrator requests DOE (Department of Energy). 1991. Memorandum of Understanding establishment of an Interagency Nuclear Safety Review Panel between the Department of Energy and the National Aeronautics and (INSRP) comprised of coordinators from the Department of Space Administration Concerning Radioisotope Power Systems for Defense, the Environmental Protection Agency, NASA, and Space Missions, as amended. Signed by VADM Richard H. Truly, the DOE, with a technical advisor from the Nuclear Regula- NASA administrator, and ADM James D. Watkins, secretary of energy, tory Commission. The INSRP coordinators and the technical dated July 26, 1991. Dieckamp, H.M. 1967. Nuclear Space Power Systems. Atomics Interna - advisor are appointed by senior management from within tional. Unpublished book. September. each agency’s safety oversight office. They are, therefore, Glenn, J., Jr., 1983. The Next 25: Agenda for the U.S. IEEE Spectrum (September):91. 5It is possible to conceive of an RPS design that uses a different approach NEPA (National Environmental Policy Act). 1970. National Environmental to packaging the 238Pu fuel. However, any new approach would require Policy Act of 1969, as amended, 42 USC Sections 4321-4347. Available at http://ceq.hss.doe.gov/Nepa/regs/nepa/nepaeqia.htm. demonstrating, through analysis and testing, that the new approach will be OSTP (Office of Science and Technology Policy). 2006. “U.S. National safe during normal operating conditions and credible accident scenarios. Space Policy,” National Security Presidential Directive 49, unclas- This would be very expensive and time-consuming, in part because some of sified version released on October 6, 2006. Executive Office of the the facilities used to develop the current fuel system no longer exist.

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 BACKGROUND President, Washington, D.C. Available at http://www.ostp.gov/cs/issues/ Public Law 83-703. 1954. Atomic Energy Act of 1954, as amended in space_aeronautics. NUREG-0980. U.S. Nuclear Regulatory Commission, Washington, D.C. PD/NSC-25 (Presidential Directive/National Security Council-25). 1977. Available at http://www.nrc.gov/reading-rm/doc-collections/nuregs/ Scientific or Technological Experiments with Possible Large-Scale staff/sr0980/ml022200075-vol1.pdf#pagemode=bookmarks&page=14. Adverse Environmental Effects and Launch of Nuclear Systems into Space (as amended). December 14.