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.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 43
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).1 After reviewing many of these have half-lives in the range of 15 to 100 years. studies, it is clear that plutonium-238 (238Pu) is the only 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, longer 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. Gadolinium-148 (148Gd) is ideal in terms of emissions to provide adequate power throughout the mission. For pro- jected NASA missions with lifetimes of 15 to 25 years, a because it decays directly to a stable nuclide (samarium-144) and emits no secondary radiation.2 However, 148Gd can be half-life over 100 years is not required, and it would substan- produced only by using a proton accelerator, rather than a reactor. Even if an accelerator were devoted full-time to the 1The results of these studies are summarized in the following correspon - production of 148Gd, the output would be only a few grams dence available from the Department of Energy’s Office of Nuclear Energy, 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 (243Cm) and the daughter products of uranium-232 (especially August 4, 1992, including the attachment “Radioisotope Fuel Selection for Outerplanetary Missions” prepared by Fairchild Space Company; and a 2Information on gadolinium-148 was provided for the committee by letter from Arthur S. Mehner, Department of Energy, to Ronald F. Draper, Jet 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. 

OCR for page 43
 RADIOISOTOPE POWER SYSTEMS TABLE D.2 Characteristics of 238Pu and 244Cm Isotope TABLE D.1 Primary Emissions Produced by 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 Gamma shield thicknessa (cm of uranium) Lead-210 (Pb-210) 22 beta, some alpha 0 5.6 Fast neutron flux @ 1m (n/cm2sec) Strontium-90 (Sr-90) 29 beta 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 pOWer densitY/specific pOWer to Plutonium-238 for Space Power Applications,” dated August 4, 1992, cOnsideratiOns Office of Nuclear Energy, Science, and Technology, Washington, D.C., 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 selection of RPSs powered by 238Pu to power many impor- 244Cm as the only isotopes worthy of further consideration.3 tant missions has demonstrated that its specific power and Table D.2 compares the characteristics of 238Pu and power density are acceptable.) Higher power density leads to 244Cm. Both produce gamma radiation (although the amount 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, Both characteristics are highly significant for space power 244Cm produces much more gamma radiation than 238Pu. 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 fueL fOrM cOnsideratiOns components. The shield weights would most likely be too 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 3Four 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), of the fuel form. All of the alpha emitting isotopes listed in all four also emit significant amounts of gamma rays.

OCR for page 43
 APPENDIX D 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. Also, 244Cm is more difficult to produce than 238Pu cations must be producible in sufficient quantities and on because the former requires extended irradiation of 239Pu a schedule to meet mission power needs. As a practical or americium-241 (241Am), with more neutron captures per matter, this means that it must be possible to produce the gram than are required to produce 238Pu from neptunium-237 radioisotope of interest by irradiation of target materials in (237Np).4 Ultimately, 244Cm would cost more and be less ben- a nuclear reactor, rather than using a particle accelerator. In 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 of 238Pu, particularly for long-duration, deep-space explora- port launch safety approvals. The fueled heat source and power 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. 4The 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.