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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration 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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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration less intense than at Earth. Continuing outward to Pluto, sunlight is 99.94 percent less intense. RPS-powered Voyager, Galileo, Cassini, and New Horizons spacecraft have enabled the United States to explore every planet in this dark, outer region of the solar system. Much of their success has been due in large part to having a reliable power source that provides enough power to operate complex instruments at a data rate high enough to optimize the capabilities of the scientific instruments they carry. RPSs are also useful for missions to the surface of the Moon (especially during the long, cold lunar nights and in the permanently shadowed regions near the lunar poles); for missions to the surface of Mars (with its dust storms and extended winters); for extended missions below Venus’s cloud deck; and for other missions where solar power is not practical, for example, because the dynamic range of solar power would preclude the use of solar arrays.2 Space nuclear power reactors are another potential option for missions where solar power is not practical. However, the performance and reliability of space nuclear power reactor systems using current technology remains unproven, especially for missions with long lifetimes. In addition, the committee is not aware of any substantive effort currently under way anywhere in the world to develop space nuclear power reactor systems. The history of space nuclear power reactors suggests that space nuclear reactors, if successfully developed, could meet the needs of some missions and could enable other missions that are not now under consideration because of power limitations. For example, Project Prometheus, which was NASA’s most recent attempt to develop space nuclear power reactors, selected a nuclear electric propulsion reactor concept that was scalable from 20 kilowatts of electrical power (kWe) to 300 kWe. However, history also shows that the development of a high-power, long-life space nuclear power reactor would be very time-consuming and cost billions of dollars (see Appendix E). Since 1961, the United States has launched 45 RPSs on 26 spacecraft dedicated to navigation, meteorology, communications, and exploration of the Moon, Sun, Mars, Jupiter, Saturn, and elsewhere in the outer solar system (see Table 2.1). This critical work could not have been accomplished without RPSs. Current RPS-powered space missions include the Cassini spacecraft, with three RPSs, which is studying Saturn and its moons; and the New Horizons spacecraft, with one RPS, which is studying Pluto and the Kuiper Belt. The Mars Science Laboratory spacecraft is scheduled for launch in 2011 with an RPS-powered rover. Over the longer term, RPSs are expected to support continued exploration of extreme environments of the Moon, Mars, and Venus, as well as the dimly lit outer reaches of the solar system and beyond. Such missions will be severely constrained or eliminated unless RPSs are ready and available (see Table 2.2). FINDING. Importance of RPSs. RPSs have been, are now, and will continue to be essential to the U.S. space science and exploration program. WHY 238Pu? Plutonium-238, which does not occur in nature, is created by irradiating neptunium-237 (237Np) targets in a nuclear reactor. Although many studies over the past 50 years have assessed the advantages and disadvantages of using a wide variety of isotopes as a fuel for RPSs, every RPS launched into space by the United States has been fueled by 238Pu.3 Studies examined by the committee demonstrate that the longstanding decision by the Department of Energy (DOE) and NASA to rely on 238Pu is correct and well-justified. No other radioisotope meets or exceeds the safety and performance characteristics of 238Pu, particularly for long-duration, deep-space exploration missions (see Appendix D). Plutonium-238, which has a half-life of 88 years, is the only isotope that meets all of the general criteria for RPS fuels, as follows: It generates heat for a sufficient length of time (i.e., it has a radioactive decay half-life of sufficient length). The type and quantity of the emissions produced by the radioactive decay of the fuel allow it to be handled safely. It has high specific power (heat per mass) and high power density (heat per volume). It has a fuel form that is noncorrosive, water-insoluble, and chemically stable, and it demonstrates good engineering properties at high temperatures. It can be produced in sufficient quantity at an affordable cost. FINDING. Plutonium-238 Supply. Plutonium-238 is the only isotope suitable as an RPS fuel for long-duration missions because of its half-life, emissions, power density, specific power, fuel form, availability, and cost. An assured supply of 238Pu is required to sustain the U.S. space science and exploration program. NASA AND DOE ROLES AND RESPONSIBILITIES The Atomic Energy Act of 1954, as amended (Public Law 83-703, 1954), establishes comprehensive requirements regarding the possession, use, and production of nuclear 2 A specific example is a solar probe mission using Jupiter for a gravity assist in order to pass the Sun in an orbit highly inclined to the plane of the ecliptic. For a mission such as this, the spacecraft experiences such a wide range of solar intensity that current technology is unable to provide the spacecraft with a low-mass solar power system. 3 The Systems for Nuclear Auxiliary Power (SNAP)-3 Program used both polonium-210 and plutonium-238 as nuclear fuel for RTGs during ground tests (Dieckamp, 1967). Over the years, some papers have erroneously reported that SNAP-3 RTGs fueled with polonium-210 were operated in space. That is not the case.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration 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 responsibilities for regulating nuclear materials between the DOE and the Nuclear Regulatory Commission. In the United States, only the DOE is authorized to own space nuclear power systems. Therefore, NASA must team with the DOE to manufacture, launch, and operate RPSs in space. The DOE also owns and operates the nuclear facilities that are used to develop, fabricate, assemble, and test RPS systems and hardware that involve nuclear fuels. Although the DOE always retains ownership of RPSs, NASA may have custody. The nuclear fuel is integrated with other RPS 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. The NASA-DOE partnership to provide RPSs for space exploration has been extremely successful, with decades of mission success (see Appendix E). Scientific results of RPS missions have often greatly exceeded initial expectations because the RPSs powering those missions have far exceeded their design lifetimes.4 The DOE writes nuclear safety requirements applicable for the operations they perform. These requirements are similar to those established by the Nuclear Regulatory Commission and other agencies that regulate other types of nuclear operations. For example, regulations specify that safety should be engineered into systems during their design and development, and systems and processes should be designed and implemented with the goal of reducing radiation exposures to as low as reasonably achievable. 4 Voyager 1 and Voyager 2, originally designed for a 5-year mission to the 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, and both spacecraft are now out of the ecliptic plane. The Voyager RPSs are projected to provide enough power for these spacecraft to operate until approximately 2020.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration TABLE 2.2 RPS Contribution to Space Science and Exploration Missions 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 A memorandum of understanding between the secretary of energy and the NASA administrator defines NASA’s and DOE’s roles and responsibilities regarding research, technology development, design, production, delivery, space-vehicle integration, launch, and operation of RPSs (DOE, 1991). DOE’s responsibilities include the design, development, fabrication, evaluation, testing, and delivery of RPSs to meet NASA system-performance and schedule requirements. In accordance with the National Environmental Policy Act (NEPA, 1970), the DOE assesses potential environmental impacts from activities related to nuclear material operations, transportation, and storage. The DOE also provides nuclear risk assessments in support of environmental impact statements that NASA prepares to comply with NEPA for the launch of a spacecraft utilizing an RPS system. The DOE is also responsible for specifying minimum radiological, public-health, and safety criteria and procedures for the use of RPSs; providing safeguards and security guidance for NASA facilities and services; supporting NASA operational plans, mission definition, environmental analysis, launch
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration approval, and radiological contingency planning; affirming the flight readiness of RPSs with respect to nuclear safety; participating in the nuclear launch approval process; jointly investigating and reporting nuclear incidents; and assuming legal liability for damages resulting from nuclear incidents and accidents involving RPSs. NASA provides the DOE with overall system requirements, specifications, schedules, and interfaces; provides data to support DOE safety analyses in accordance with NEPA; supports nuclear launch approval (e.g., launch-vehicle databooks); complies with minimum radiological occupational and public health and safety criteria and procedures specified by the DOE; provides adequate facilities for safe and secure storage, assembly, and checkout of RPSs while in NASA custody; and provides tracking, command, and data services required to monitor RPSs during and subsequent to launch. The 2006 National Space Policy (OSTP, 2006) directs the United States to develop and use space nuclear power systems where such systems safely enable or significantly enhance space exploration. This policy reaffirms DOE’s role in maintaining nuclear infrastructure as well as the ability to conduct nuclear safety analyses to support the nuclear launch approval process. Historically, the DOE or its predecessor agencies (the Atomic Energy Commission and the Energy Research and Development Administration) bore the cost of establishing and maintaining the infrastructure to produce 238Pu and to develop RPS technology and systems. NASA would then reimburse the DOE for the incremental costs of producing the 238Pu that NASA used and for the flight hardware that it launched. Consistent with this historic precedent, NASA is reimbursing the DOE for the full cost of the 238Pu that the DOE is purchasing from Russia because all of that 238Pu is being used for NASA missions. If the United States is to continue using RPSs for space science and exploration, it is appropriate for the DOE to continue the maintenance and operation of the nuclear facilities required for the fabrication and testing of fueled RPS components and systems. Because of the DOE’s statutory responsibilities, it is also appropriate for the funding of these facilities to be included in the DOE budget rather than passing these funds through NASA’s budget. These facilities are required to operate according to DOE rules and regulations. The DOE’s budget has funding to continue the maintenance and operations of the nuclear facilities required to support the fabrication of RPSs—but no funds are included for production of 238Pu. If the production of 238Pu is not reestablished, these DOE facilities could be shut down after they process the last available 238Pu. FINDING. Roles and Responsibilities. Roles and responsibilities as currently allocated between NASA and the Department of Energy are appropriate, and it is possible to address outstanding issues related to the short supply of 238Pu and advanced flight-qualified RPS technology under the existing organizational structures and allocation of roles and responsibilities. RPS NUCLEAR SAFETY Safety is an integral part of any nuclear system, and it encompasses the entire system life cycle. Nuclear safety for RPSs encompasses design, development, assembly, checkout, testing, handling, transport, storage, ground checkout, integration with payload, mating with launch vehicle, prelaunch activities, launch, ascent, orbital insertion, trajectory insertion, in-flight checkout, mission operations, and final disposition. RPS safety includes the protection of the public, the environment, workers, property, and other resources from undue risk of injury or harm. To achieve these goals, three objectives must be met: (1) design safety into each RPS at the outset, (2) demonstrate the safety of RPSs through testing and analysis, and (3) assess the level of risk for each RPS-powered space mission as required to support the launch approval process. Processes have been established to address all of these objectives. The DOE has well-established rules, specifications, and procedures for the safe design, development, testing, transport, and handling of RPSs. The DOE also has developed sophisticated tools to conduct safety and risk analyses to support the flight safety review and launch approval process. Because 238Pu emits alpha particles, U.S. RPSs pose a biological hazard only if the 238Pu is somehow released into the environment and is then either ingested or inhaled. Ingestion is only plausible through the food chain, where foods contaminated with 238Pu are consumed. This requires that the 238Pu be released and vaporized or pulverized into small particles (less than ~100 microns in diameter) and then transported through the atmosphere so they can deposit on or within food stuffs. Similarly, inhalation is only plausible if 238Pu is released and vaporized or pulverized into respirable particles (less than ~3 microns in diameter) and then transported through the atmosphere where it can be inhaled. U.S. RPSs are fueled with 238Pu in the form of a ceramic oxide (238PuO2) that has a high melting point and very low solubility to (1) minimize fuel vaporization and transport in the atmosphere and (2) minimize fuel retention within the human body, if it should occur. RPSs are designed with multiple fuel containment barriers (i.e., defense in depth) to prevent release and, if a release should occur, to limit the dispersal of 238Pu into the biosphere in credible accident scenarios that could occur during a space mission. For U.S. RPSs on the Galileo mission to Jupiter (October 1989) and on all subsequent missions to date, each 238PuO2 fuel pellet is encapsulated in a ductile, high-temperature iridium-based alloy. Two encapsulated 238PuO2 fuel pellets are packaged within a cylindrical graphite impact shell constructed of a carbon-carbon composite. Two
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration graphite impact shells are packaged within a reentry aeroshell that is also constructed of a carbon-carbon composite. This assembly, which is approximately 4 in. × 4 in. × 2 in., is called a general purpose heat source (GPHS) module. This is the standard RPS fuel module now used in all U.S. RPSs, and it reflects many improvements in materials and packaging that have been introduced over time.5 Testing and analysis must be performed to determine the response of all RPSs to credible accident environments. Testing validates analysis models and establishes and demonstrates the level of safety built into the design. A tremendous amount of testing has been conducted on the GPHS fuel, materials, and hardware since its original design and development in the mid-to-late 1970s. The efficacy of the U.S. RPS design safety approach was demonstrated during the launch of the Nimbus B-1 meteorological satellite, with two Systems for Nuclear Auxiliary Power (SNAP)-19B2 RPSs on board, from Vandenberg Air Force Base, California, on May 18, 1968. During this launch, range safety destruct of the launch vehicle and upper stage was initiated by the range safety officer because the launch vehicle was ascending erratically. Although the launch vehicle and payload were totally destroyed by the explosion, the RPSs were recovered intact. No release of 238Pu occurred, and the 238PuO2 fuel was used on a later mission. Nevertheless, the use of RPSs does create some risk that 238Pu could be released into the biosphere, however low that risk may be. To assess this risk, the Unites States has established a flight safety review and launch approval process for RPS-powered missions. This process is structured to ensure that the radiological risk for each mission is characterized in detail and independently evaluated so that an informed launch decision can be made, based on sound risk-benefit considerations. The U.S. flight safety review and launch approval process for space nuclear power systems is established by Presidential Directive/National Security Council Memorandum 25 (PD/NSC-25, 1977). As part of this process, the DOE prepares a series of increasingly detailed Safety Analysis Reports that characterize the radiological risk for the each mission. For each NASA mission, the NASA administrator requests establishment of an Interagency Nuclear Safety Review Panel (INSRP) comprised of coordinators from the Department of Defense, the Environmental Protection Agency, NASA, and the DOE, with a technical advisor from the Nuclear Regulatory Commission. The INSRP coordinators and the technical advisor are appointed by senior management from within each agency’s safety oversight office. They are, therefore, independent of the mission and associated RPS development efforts, and they have the responsibility and authority to identify and address issues at any level. Each INSRP is supported by technical experts, as needed, typically in six working groups: Launch Abort, Reentry, Power Systems, Meteorology, Biomedical and Environmental Effects, and Risk Integration and Uncertainty. The Final Safety Analysis Report is reviewed in great depth by the INSRP, which often performs additional independent analyses. The INSRP then prepares a Safety Evaluation Report. These reports identify and characterize credible accident scenarios, including the probabilities that 238Pu will be released and the postulated health effects for each accident scenario, to determine overall mission risk and the uncertainties associated with that risk. NASA uses the Final Safety Analysis Report and the Safety Evaluation Report to determine whether it will formally request launch approval from the White House. If it does, both reports are provided to the director of the Office of Science and Technology Policy (within the Executive Office of the President), who may grant launch approval, deny launch approval, or defer the decision to the President. The entire launch approval process typically takes 3 years (including the resolution of legal challenges that are sometimes raised), although it could take as long as 8 years. The process usually takes longer than average if a mission uses a launch vehicle, upper stage, launch complex, launch trajectory, and/or spacecraft combination that has not previously been characterized and analyzed. In such cases, extra effort is needed to prepare the Launch Vehicle Databook, which identifies and characterizes accident sequences and environments that could occur during pre-launch, launch, ascent, and trajectory insertion. FINDING. RPS Nuclear Safety. The U.S. flight safety review and launch approval process for nuclear systems comprehensively addresses public safety, but it introduces schedule requirements that must be considered early in the RPS system development and mission planning process. REFERENCES DOE (Department of Energy). 1991. Memorandum of Understanding between the Department of Energy and the National Aeronautics and Space Administration Concerning Radioisotope Power Systems for Space Missions, as amended. Signed by VADM Richard H. Truly, NASA administrator, and ADM James D. Watkins, secretary of energy, dated July 26, 1991. Dieckamp, H.M. 1967. Nuclear Space Power Systems. Atomics International. Unpublished book. September. Glenn, J., Jr., 1983. The Next 25: Agenda for the U.S. IEEE Spectrum (September):91. NEPA (National Environmental Policy Act). 1970. National Environmental Policy Act of 1969, as amended, 42 USC Sections 4321-4347. Available at http://ceq.hss.doe.gov/Nepa/regs/nepa/nepaeqia.htm. OSTP (Office of Science and Technology Policy). 2006. “U.S. National Space Policy,” National Security Presidential Directive 49, unclassified version released on October 6, 2006. Executive Office of the 5 It is possible to conceive of an RPS design that uses a different approach to packaging the 238Pu fuel. However, any new approach would require demonstrating, through analysis and testing, that the new approach will be safe during normal operating conditions and credible accident scenarios. This would be very expensive and time-consuming, in part because some of the facilities used to develop the current fuel system no longer exist.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration President, Washington, D.C. Available at http://www.ostp.gov/cs/issues/space_aeronautics. PD/NSC-25 (Presidential Directive/National Security Council-25). 1977. Scientific or Technological Experiments with Possible Large-Scale Adverse Environmental Effects and Launch of Nuclear Systems into Space (as amended). December 14. Public Law 83-703. 1954. Atomic Energy Act of 1954, as amended in NUREG-0980. U.S. Nuclear Regulatory Commission, Washington, D.C. Available at http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0980/ml022200075-vol1.pdf#pagemode=bookmarks&page=14.