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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration Appendix D Comparison of 238Pu to Alternatives Numerous studies have been conducted over many years to determine the optimum isotope for use in radioisotope power systems (RPSs).1 After reviewing many of these studies, it is clear that plutonium-238 (238Pu) is the only technically credible isotope for powering RPSs. Selection of a suitable RPS fuel focuses mainly on three areas: radioactive decay half-life, radiation emissions, and power density/specific power. Secondary considerations include fuel form and availability/cost. HALF-LIFE CONSIDERATIONS Radioisotopes decay in a predictable and unalterable process that emits particles and/or photons, including alpha, beta, and gamma radiation. When this radiation is absorbed by the fuel or the fuel container, it is transformed into useful heat. The half-life of the fuel should be at least as long or longer than the mission lifetime. If the half-life is too short, the fuel decays too quickly, and a large amount of excess fuel is required at the beginning of life to provide adequate power at the end of life and to provide mission scheduling flexibility. However, if the half-life is too long, radioactive decay occurs so slowly that a large amount of fuel is required to provide adequate power throughout the mission. For projected NASA missions with lifetimes of 15 to 25 years, a half-life over 100 years is not required, and it would substantially reduce power density and specific power. Of more than 2,900 known radioisotopes, only the 22 listed in Table D.1 have half-lives in the range of 15 to 100 years. RADIATION EMISSION CONSIDERATIONS An RPS fuel should produce radiation that can easily be shielded to minimize shielding weight, to reduce worker exposure, to minimize risk of exposure to the general population in the event of a launch accident, and to avoid interference with sensitive particle and photon detectors used on the spacecraft. The first seven isotopes listed in Table D.1 decay purely by gamma radiation emissions. This is a highly penetrating form of radiation, and therefore these isotopes can be eliminated from consideration as an RPS fuel source. Although beta particle emissions are easily shielded, some of the beta particle energy is converted to bremsstrahlung radiation (x rays), which is difficult to shield. Beta decay also produces less heat energy than decay by highly energetic alpha emissions. This eliminates the nine beta emitting radioisotopes listed in Table D.1. The five remaining radioisotopes are alpha emitters. Gadolinium-148 (148Gd) is ideal in terms of emissions because it decays directly to a stable nuclide (samarium-144) and emits no secondary radiation.2 However, 148Gd can be produced only by using a proton accelerator, rather than a reactor. Even if an accelerator were devoted full-time to the production of 148Gd, the output would be only a few grams per year. There is no known or projected method for making kg quantities of this isotope in a year’s time. Curium-243 (243Cm) and the daughter products of uranium-232 (especially 1 The results of these studies are summarized in the following correspondence available from the Department of Energy’s Office of Nuclear Energy, Science, and Technology, Washington, D.C.: Information memorandum and associated transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, including the attachment “Radioisotope Fuel Selection for Outerplanetary Missions” prepared by Fairchild Space Company; and a letter from Arthur S. Mehner, Department of Energy, to Ronald F. Draper, Jet Propulsion Laboratory, dated February 14, 1989, including attachments prepared by the Fairchild Space Company, “Alternative Fuel Considerations” and response to the question “What radioisotope fuels can be used for space missions if Pu-238 can no longer be produced or procured?” 2 Information on gadolinium-148 was provided for the committee by Emil Skrabek, Orbital Sciences Corporation, in a paper “Gadolinium-148 as a Potential Fuel for Radioisotope Power Systems. A Synopsis for the National Research Council Radioisotope Power Systems Study Committee,” October 1, 2008.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration TABLE D.1 Primary Emissions Produced by Radioisotopes with Half-lives of 15 to 100 Years Isotope Half-Life (years) Type of Primary Emissions Promethium-145 (Pm-145) 18 gamma Halfnium-178m (Hf-178m) 31 gamma Bismuth-207 (Bi-207) 33 gamma Europium-150 (Eu-150) 37 gamma Titanium-44 (Ti-44) 47 gamma Platinum-193 (Pt-193) 50 gamma Terbium-157 (Tb-157) 99 gamma Actinium-227 (Ac-227) 22 beta, some alpha Niobium-93m (Nb-93m) 16 beta, gamma Lead-210 (Pb-210) 22 beta, some alpha Strontium-90 (Sr-90) 29 beta Cesium-137 (Cs-137) 30 beta, gamma Argon-42 (Ar-42) 33 beta Tin-121m (Sn-121m) 55 beta Samarium-151 (Sm-151) 90 beta Nickel-63 (Ni-63) 100 beta Curium-244 (Cm-244) 18 alpha, spontaneous fission Curium-243 (Cm-243) 29 alpha, gamma Uranium-232 (U-232) 72 alpha, spontaneous fission Gadolinium-148 (Gd-148) 75 alpha Plutonium-238 (Pu-238) 88 alpha, spontaneous fission SOURCE: Department of Energy, information memorandum and associated transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, Office of Nuclear Energy, Science, and Technology, Washington, D.C., Table 1, updated. thorium-228) emit a significant level of gamma radiation, resulting in dose rates that are higher than either 244Cm or 238Pu heat sources of comparable size. This leaves 238Pu and 244Cm as the only isotopes worthy of further consideration.3 Table D.2 compares the characteristics of 238Pu and 244Cm. Both produce gamma radiation (although the amount produced is much smaller than the amount from isotopes that produce gamma radiation as a primary emission). As shown, 244Cm produces much more gamma radiation than 238Pu. Also, the fast neutron radiation level from 244Cm is nearly 450 times that of 238Pu. These high gamma and neutron radiation levels would require shielding during handling and use of the 244Cm heat sources to protect personnel and sensitive components. The shield weights would most likely be too heavy for deep-space applications. Nearly all of the gamma dose from 238Pu is attributable to the decay chain of the 236Pu isotope impurity in the fuel, which is limited to very small amounts by 238Pu fuel quality specifications. TABLE D.2 Characteristics of 238Pu and 244Cm Isotope Fuels Isotope Plutonium-238 Curium-244 Half-life 87 18.1 Type of emission Alpha Alpha Activity (curies/watt) 30.73 29.12 Fuel form PuO2 Cm2O3 Melting point (°C) 2,150 1,950 Specific power (watt/g) 0.40 2.42 Power density (watt/cc) 4.0 26.1 Radiation levels Gamma dose rate (mR/hr @ 1m) ~5 ~900 Gamma shield thicknessa (cm of uranium) 0 5.6 Fast neutron flux @ 1m (n/cm2sec) 260 116,000 NOTE: mR, milliroentgen. a Gamma shielding to reduce dose rates to ~5 mR/hr @ 1m (equivalent to Pu-238) SOURCE: Department of Energy, information memorandum and associated transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, Office of Nuclear Energy, Science, and Technology, Washington, D.C., Table 2. POWER DENSITY/SPECIFIC POWER CONSIDERATIONS The power density (watts/cubic centimeter) and specific power (watts/gram) of radioisotope fuel is directly proportional to the energy absorbed per disintegration and is inversely proportional to half-life. (As shown in Table D.2, 244Cm has a higher specific power and power density than 238Pu, because the former has a shorter half-life, but the selection of RPSs powered by 238Pu to power many important missions has demonstrated that its specific power and power density are acceptable.) Higher power density leads to smaller volume heat sources for comparable power levels and higher specific power leads to lighter weight heat sources. Both characteristics are highly significant for space power heat sources. For radioisotope fuels with comparable half-lives, a beta emitting heat source will be larger and heavier than an alpha emitter. FUEL FORM CONSIDERATIONS The radioisotope fuel must be used in a fuel form that has a high melting point and remains stable during credible launch accidents and accidental reentries into Earth’s atmosphere. The fuel form must also be noncorrosive and chemically compatible with its containment material (metallic cladding) over the operating lifetime of the power system. It is desirable that the fuel form have a low solubility rate in the human body and the natural environment. Daughter products and the decay process must not affect the integrity of the fuel form. All of the alpha emitting isotopes listed in 3 Four additional alpha emitters have half-lives between 100 and 500 years (polonium-209, americium-242m, californium-249, and americium-241). In addition to the problem of low specific power (caused by their long half-life), all four also emit significant amounts of gamma rays.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration Table D.1 form very stable, high-melting-temperature oxides which are acceptable for space applications. AVAILABILITY AND COST CONSIDERATIONS Any radioisotope fuel selected for space power applications must be producible in sufficient quantities and on a schedule to meet mission power needs. As a practical matter, this means that it must be possible to produce the radioisotope of interest by irradiation of target materials in a nuclear reactor, rather than using a particle accelerator. In addition, appropriate types and amounts of target materials and facilities for processing them are needed. Chemical processing technology to produce the power fuel compound is required, as well as fuel form fabrication processes and facilities. The proposed fuel form must be extensively tested to support launch safety approvals. The fueled heat source and power system must undergo an extensive analysis and test program to qualify them for use in space applications. Development of a fuel production and fuel form fabrication capability for a new fuel is very costly and time-consuming. To qualify a new fuel form and heat source for flight use is also a large effort in terms of cost and time. More than $40 million has been spent on safety qualification of the 238Pu-fueled general purpose heat source. Similar work has not been done for 244Cm oxide fuel form, heat source, or power system. Also, 244Cm is more difficult to produce than 238Pu because the former requires extended irradiation of 239Pu or americium-241 (241Am), with more neutron captures per gram than are required to produce 238Pu from neptunium-237 (237Np).4 Ultimately, 244Cm would cost more and be less beneficial to NASA for long-duration, deep-space missions. SUMMARY In the final analysis, no other radioisotope is available that meets or exceeds the safety and performance characteristics of 238Pu, particularly for long-duration, deep-space exploration missions. Plutonium-238 stands alone in terms of its half-life, emissions, power density, specific power, fuel form, availability, and cost. 4 The availability of target materials is not a key discriminating factor. The Department of Energy already has a large supply of 237Np on hand, and 241Am, which is commonly used in smoke detectors, can be produced in kilogram quantities.