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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration 1 The Problem For nearly 50 years, the United States has led the world in the scientific exploration of space. U.S. spacecraft have circled Earth; landed on the Moon and Mars; flown to and beyond Jupiter, Saturn, Uranus, and Neptune; and traveled beyond our solar system. The spectacular images and data sent back to Earth by these spacecraft have greatly expanded human knowledge. Even so, there is much yet to learn from continued space exploration. Spacecraft require electrical energy. This energy must be available in the outer reaches of the solar system where sunlight is very faint. It must be available through lunar nights that last for 14 days, through long periods of dark and cold at the higher latitudes on Mars, and in high-radiation fields such as those around Jupiter. Radioisotope power systems (RPSs) are the only available power source that can operate unconstrained in these environments for the long periods of time needed to accomplish many missions (see Box 1.1). RPSs generate electricity by converting heat from the natural decay of the plutonium-238 (238Pu ) radioisotope into electricity. Plutonium-238 has been produced inquantity only for the purpose of fueling RPSs; unlike 239Pu, it is unsuitable for use in nuclear weapons. In the past, the United States had an adequate supply of 238Pu, which was produced in facilities that existed to support the U.S. nuclear weapons program. The problem is, no 238Pu has been produced in the United States since the Department of Energy (DOE) shut down those facilities in the late 1980s. Since then the U.S. space program has had to rely on the inventory of 238Pu that existed at that time, supplemented by the purchase of 238Pu from Russia. However, Russian 238Pu production facilities were also shut down many years ago, and the DOE will soon take delivery of its last shipment of 238Pu from Russia. The committee does not believe that there is any additional 238Pu (or any operational 238Pu production facilities) available anywhere in the world. The total amount of 238Pu available for NASA is fixed, and essentially all of it is already dedicated to support several pending missions.1 Reestablishing domestic production of 238Pu will be expensive; the cost will likely exceed $150 million. Previous proposals to make this investment have not been enacted, and cost seems to be the major impediment. However, the day of reckoning has arrived. NASA has been making mission-limiting decisions for some time because of the short supply of 238Pu. Moreover, NASA has been eliminating RPSs as an option for some missions and delaying other missions that require RPSs until more 238Pu becomes available. Unless and until a new source of 238Pu is established, the restricted supply of 238Pu will increasingly limit both the quality and the quantity of U.S. space science in many mission areas, and continued U.S. leadership in these areas will be at risk. The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is the only specific RPS currently available. Like all prior RPSs, MMRTGs convert the thermal energy produced by the radioactive decay of 238Pu to electricity using thermocouples. This is a proven technology. RPSs that use thermocouples have no moving parts and have demonstrated high reliability and long life, albeit with low energy-conversion efficiency. The Advanced Stirling Radioisotope Generator (ASRG) is a new type of RPS, and it is still being developed. It uses a Stirling engine (with moving parts) to convert thermal energy to electricity. Stirling engine converters are much more efficient than thermocouples. As a result, ASRGs produce more electricity than MMRTGs, even though they require only one-fourth as much 238Pu. ASRG development efforts have made good progress thus far, but it remains to be seen when a flight-qualified ASRG will be available. 1 This report focuses on large quantities of 238Pu (measured in kilograms) necessary to fuel RPSs. It is not concerned with small quantities of 238Pu (measured in grams, milligrams, or micrograms) that are produced for research or other purposes.
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Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration BOX 1.1 What Is a Radioisotope Power System? Radioisotope power systems (RPSs) are compact, rugged spacecraft power systems that provide reliable, long-lived power in harsh environments where other power systems such as solar arrays are not practical. RPSs are not nuclear reactors. They do not use nuclear fission or fusion to produce energy. Instead, they produce heat through the natural radioactive decay of plutonium-238 (238Pu). All U.S. RPSs launched to date have used solid-state thermoelectric converters to convert this heat into electricity. Such RPSs have supported 26 NASA and Department of Defense missions since 1961. Advanced Stirling Radioisotope Generators, which are still under development, use a more efficient dynamic energy conversion system to generate electricity. U.S. RPSs have an outstanding safety and reliability record. RPSs have never caused a spacecraft failure, and almost 50 years of effort have been invested in the engineering, safety, analysis, and testing of RPSs. Safety features are incorporated into the design of RPSs, extensive testing has demonstrated that they can withstand severe conditions associated with a wide spectrum of credible accidents, and mission experience has demonstrated that they can operate continuously for decades. FINDING. Production of 238Pu. The United States has not produced 238Pu since the Department of Energy shut down its nuclear weapons production reactors in the late 1980s. Chapter 2 provides background information on space exploration, the case for using RPSs and 238Pu, NASA and DOE roles and responsibilities, and nuclear safety. Chapter 3 examines 238Pu supply and demand and the importance of immediate action to reestablish domestic production of 238Pu. Chapter 4 reviews the performance of various RPSs, related research and development, and the importance of completing the development of ASRGs with all deliberate speed.