Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration (2009)

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Appendix D Comparison of 238Pu to Alternatives Numerous studies have been conducted over many years tially reduce power density and specific power. Of more than to determine the optimum isotope for use in radioisotope 2,900 known radioisotopes, only the 22 listed in Table D.1 power systems (RPSs). After reviewing many of these have half-lives in the range of 15 to 100 years. ­ tudies, it is clear that plutonium-238 (238Pu) is the only s technically credible isotope for powering RPSs. Radiation Emission Considerations Selection of a suitable RPS fuel focuses mainly on three areas: radioactive decay half-life, radiation emissions, and An RPS fuel should produce radiation that can easily be power density/specific power. Secondary considerations shielded to minimize shielding weight, to reduce worker include fuel form and availability/cost. exposure, to minimize risk of exposure to the general popula- tion in the event of a launch accident, and to avoid interfer- ence with sensitive particle and photon detectors used on Half-life considerations the spacecraft. Radioisotopes decay in a predictable and unalterable The first seven isotopes listed in Table D.1 decay purely process that emits particles and/or photons, including alpha, by gamma radiation emissions. This is a highly penetrating beta, and gamma radiation. When this radiation is absorbed form of radiation, and therefore these isotopes can be elimi- by the fuel or the fuel container, it is transformed into useful nated from consideration as an RPS fuel source. heat. The half-life of the fuel should be at least as long or Although beta particle emissions are easily shielded, l ­ onger than the mission lifetime. If the half-life is too short, some of the beta particle energy is converted to bremsstrah- the fuel decays too quickly, and a large amount of excess lung radiation (x rays), which is difficult to shield. Beta fuel is required at the beginning of life to provide adequate decay also produces less heat energy than decay by highly power at the end of life and to provide mission scheduling energetic alpha emissions. This eliminates the nine beta emit- flexibility. However, if the half-life is too long, radioactive ting radioisotopes listed in Table D.1. decay occurs so slowly that a large amount of fuel is required The five remaining radioisotopes are alpha emitters. to provide adequate power throughout the mission. For pro- ­ adolinium-148 (148Gd) is ideal in terms of emissions G jected NASA missions with lifetimes of 15 to 25 years, a because it decays directly to a stable nuclide (samarium-144) half-life over 100 years is not required, and it would substan- and emits no secondary radiation. However, 148Gd can be produced only by using a proton accelerator, rather than a The results of these studies are summarized in the following correspon- reactor. Even if an accelerator were devoted full-time to the dence available from the Department of Energy’s Office of Nuclear Energy, production of 148Gd, the output would be only a few grams Science, and Technology, Washington, D.C.: Information memorandum per year. There is no known or projected method for ­making and associated transmittal memorandum to S-1 from NE-1 on the subject kg quantities of this isotope in a year’s time. Curium-243 of “Alternatives to Plutonium-238 for Space Power Applications,” dated August 4, 1992, including the attachment “Radioisotope Fuel Selection for (243Cm) and the daughter products of uranium-232 ­(especially Outerplanetary Missions” prepared by Fairchild Space Company; and a l ­etter from Arthur S. Mehner, Department of Energy, to Ronald F. Draper, Jet Information on gadolinium-148 was provided for the committee by Propulsion Laboratory, dated February 14, 1989, including attachments pre- Emil Skrabek, Orbital Sciences Corporation, in a paper “Gadolinium-148 pared by the Fairchild Space Company, “Alternative Fuel Considerations” as a Potential Fuel for Radioisotope Power Systems. A Synopsis for the and response to the question “What radioisotope fuels can be used for space National Research Council Radioisotope Power Systems Study Commit- missions if Pu-238 can no longer be produced or procured?” tee,” October 1, 2008. 43 

44 RADIOISOTOPE POWER SYSTEMS TABLE D.1  Primary Emissions Produced by TABLE D.2  Characteristics of 238Pu and 244Cm Isotope Radioisotopes with Half-lives of 15 to 100 Years Fuels Half-Life Plutonium- Curium- Isotope (years) Type of Primary Emissions Isotope 238 244 Promethium-145 (Pm-145) 18 gamma Half-life 87 18.1 Halfnium-178m (Hf-178m) 31 gamma Type of emission Alpha Alpha Bismuth-207 (Bi-207) 33 gamma Activity (curies/watt) 30.73 29.12 Europium-150 (Eu-150) 37 gamma Fuel form PuO2 Cm2O3 Titanium-44 (Ti-44) 47 gamma Melting point (°C) 2,150 1,950 Platinum-193 (Pt-193) 50 gamma Specific power (watt/g) 0.40 2.42 Terbium-157 (Tb-157) 99 gamma Power density (watt/cc) 4.0 26.1 Actinium-227 (Ac-227) 22 beta, some alpha Radiation levels Niobium-93m (Nb-93m) 16 beta, gamma Gamma dose rate (mR/hr @ 1m) ~5 ~900 Lead-210 (Pb-210) 22 beta, some alpha Gamma shield thicknessa (cm of uranium) 0 5.6 Strontium-90 (Sr-90) 29 beta Fast neutron flux @ 1m (n/cm2sec) 260 116,000 Cesium-137 (Cs-137) 30 beta, gamma NOTE: mR, milliroentgen. Argon-42 (Ar-42) 33 beta a Gamma shielding to reduce dose rates to ~5 mR/hr @ 1m (equivalent to Tin-121m (Sn-121m) 55 beta Pu-238) Samarium-151 (Sm-151) 90 beta SOURCE: Department of Energy, information memorandum and associated Nickel-63 (Ni-63) 100 beta transmittal memorandum to S-1 from NE-1 on the subject of “Alternatives to Curium-244 (Cm-244) 18 alpha, spontaneous fission Plutonium-238 for Space Power Applications,” dated August 4, 1992, Office Curium-243 (Cm-243) 29 alpha, gamma of Nuclear Energy, Science, and Technology, Washington, D.C., Table 2. 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, Power density/Specific Power Office of Nuclear Energy, Science, and Technology, Washington, D.C., Considerations Table 1, updated. The power density (watts/cubic centimeter) and specific power (watts/gram) of radioisotope fuel is directly pro- portional to the energy absorbed per disintegration and is inversely proportional to half-life. (As shown in Table D.2, thorium-228) emit a significant level of gamma radiation, 244Cm has a higher specific power and power density than resulting in dose rates that are higher than either 244Cm or 238Pu, because the former has a shorter half-life, but the 238Pu heat sources of comparable size. This leaves 238Pu and 244Cm as the only isotopes worthy of further consideration. selection of RPSs powered by 238Pu to power many impor- tant missions has demonstrated that its specific power and Table D.2 compares the characteristics of 238Pu and 244Cm. Both produce gamma radiation (although the amount power density are acceptable.) Higher power density leads to smaller volume heat sources for comparable power levels and produced is much smaller than the amount from isotopes that higher specific power leads to lighter weight heat sources. produce gamma radiation as a primary emission). As shown, 244Cm produces much more gamma radiation than 238Pu. Both characteristics are highly significant for space power heat sources. For radioisotope fuels with comparable half- Also, the fast neutron radiation level from 244Cm is nearly lives, a beta emitting heat source will be larger and heavier 450 times that of 238Pu. These high gamma and neutron radia- than an alpha emitter. tion 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 Fuel Form Considerations heavy for deep-space applications. The radioisotope fuel must be used in a fuel form that Nearly all of the gamma dose from 238Pu is attributable has a high melting point and remains stable during cred- to the decay chain of the 236Pu isotope impurity in the fuel, ible launch accidents and accidental reentries into Earth’s which is limited to very small amounts by 238Pu fuel quality atmosphere. The fuel form must also be noncorrosive and specifications. chemically compatible with its containment material (metal- lic cladding) over the operating lifetime of the power system. It is desirable that the fuel form have a low solubility rate Four additional alpha emitters have half-lives between 100 and 500 years in the human body and the natural environment. Daughter (polonium-209, americium-242m, californium-249, and americium-241). In products and the decay process must not affect the integrity addition to the problem of low specific power (caused by their long half-life), all four also emit significant amounts of gamma rays. of the fuel form. All of the alpha emitting isotopes listed in 

APPENDIX D 45 Table D.1 form very stable, high-melting-temperature oxides new fuel is very costly and time-consuming. To qualify a new which are acceptable for space applications. 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 Availability and Cost Considerations heat source. Similar work has not been done for 244Cm oxide Any radioisotope fuel selected for space power appli- fuel form, heat source, or power system. cations must be producible in sufficient quantities and on Also, 244Cm is more difficult to produce than 238Pu a schedule to meet mission power needs. As a practical because the former requires extended irradiation of 239Pu matter, this means that it must be possible to produce the or americium-241 (241Am), with more neutron captures per radio­isotope of interest by irradiation of target materials in gram than are required to produce 238Pu from neptunium-237 a nuclear ­reactor, rather than using a particle accelerator. In (237Np). Ultimately, 244Cm would cost more and be less ben- addition, appropriate types and amounts of target materials eficial to NASA for long-duration, deep-space missions. and facilities for processing them are needed. Chemical processing technology to produce the power fuel compound Summary is required, as well as fuel form fabrication processes and facilities. In the final analysis, no other radioisotope is available that The proposed fuel form must be extensively tested to sup- meets or exceeds the safety and performance characteristics port launch safety approvals. The fueled heat source and power of 238Pu, particularly for long-duration, deep-space explora- system must undergo an extensive analysis and test program tion missions. Plutonium-238 stands alone in terms of its to qualify them for use in space applications. Development half-life, emissions, power density, specific power, fuel form, of a fuel production and fuel form fabrication ­capability for a availability, and cost. 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.