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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.
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.
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.