Introduction Within recent years, proposed U.S. space missions have shown the need for electric power levels well beyond those available through existing sources. The electric power used in space missions, such as the Voyager spacecraft launched by the National Aeronautics and Space Administration (NASA) and navigation satellites launched by the Department of Defense (DOD), has been generated by solar cells, chemical fuel cells, or low-power nuclear sources. Each of these sources offers unique advantages; however, they have practical limits to the amount of electrical power they can supply continuously and for long periods of time. During the 1950s to the early 1970s, the United States engaged in extensive space reactor development programs and even demonstrated a reactor power system in 1965 that produced 500 watts in space. Because firm missions that would use the nuclear reactor technology did not materialize and, because existing power sources were able to meet proposed space mission needs, the space nuclear power programs were terminated. Interest, however, has been renewed in developing space nuclear reactors because of the need for higher power levels and other requirements for future space missions. On May 20, 1986, the former Chairman of the House Committee on Sci- This chapter provides background information on space nuclear reactor systems, their advantages over other power systems, and future space missions for which their need has been identified. It also briefly describes DOE's two space reactor research and development programs. Finally, the chapter contains information on the objectives, scope, and methodology used to conduct our review on space reactor research and development. 1A watt is the electrical unit of power. A kilowatt is 1 thousand watts; a megawatt is 1 million watts. Introduction Space Nuclear Reactor Space nuclear reactor power systems are made up of several subsystems Power Systems and Their Advantages in addition to the reactor. The design of these power systems can vary and depends on factors such as mission type and duration, operating environment, electrical load demands, and other performance requirements. Nuclear reactors, however, because of their unique characteristics are considered the only power source option for many emerging civil and military space missions. A nuclear reactor power system consists of several subsystems: (1) a compact nuclear reactor, (2) shielding, (3) a heat transport system, (4) a power conversion system, (5) a radiator, and (6) a power conditioning and control system. Figure 1.1 illustrates a basic space nuclear reactor power system. The reactor power system operates as follows. When a reactor is operat ing, a chain reaction fissioning process of the uranium material in the reactor core is sustained. The process generates tremendous quantities of heat and also produces hazardous levels of radioactivity. The shielding provides protection for other flight system components from the Introduction radioactivity. To convert heat to electricity, coolant passes through the reactor core, absorbs the heat and is pumped to an energy converter that converts the heat to electricity. A power conditioning and control system regulates and delivers power to other flight system components. Residual waste heat from the converter is transported to and through radiator panels and dissipated into space. Alternative power system sources, such as solar cells, chemical fuel cells, and low-power nuclear radioisotopes, are available for space applications, but they cannot match the performance and characteristics of a nuclear power system for particular missions. Solar arrays, for example, made up of photovoltaic cells that convert the sun's light to electricity, are limited in the power they can deliver because the greater the power requirements, the larger will be the system's size, which will cause it to become very massive. In addition, the intensity of the power supplied by a solar cell system depends on its distance from the sun. A compact nuclear reactor, in comparison, has size advantages and can operate independently of the sun. A compact nuclear reactor also offers other advantages compared with either chemical fuel or radioisotope power sources. A chemical fuel power system, which has fuel cells that convert chemical energy directly into power, can supply high levels of power but only for short periods of time. Nuclear reactors, in comparison, can operate at high power levels for long durations. While a nuclear radioisotope system that converts heat from the spontaneous decay of radioactive material2 into electricity can also operate for long durations, it is generally limited to low power needs (less than 10 kilowatts) because of the quantity of fuel and weight that would be needed to supply higher levels of power. Nuclear reactors also have other advantages over existing power sources, depending on mission type. For example, where reducing space system vulnerability to military attack is a major objective, nuclearpowered space systems have advantages over other alternatives because of rugged construction utilizing high-temperature, high-strength materials that provide intrinsic hardness against hostile attack. Also, the compact size of nuclear power systems makes them more maneuverable for defense missions. 2The material is plutonium-238, a radioactive, man-made element. Introduction Space Nuclear Reactor DOD and NASA are currently considering future space missions whose power needs are in the tens, hundreds, or even thousands of kilowatts of electricity. Space nuclear reactor power technology has been identified for the power systems that will meet these missions' power needs. Among near-term military missions is the proposed Strategic Defense Initiative (SDI), popularly called "Star Wars." The SDI is a DOD research program to explore key technologies needed to assess the feasibility of building a defense system against the threat of nuclear ballistic missiles. The program includes research on sophisticated surveillance, sensing, orbital transfer vehicles, and intercept systems and weapons platforms that will need electrical power in the hundreds of kilowatts and tens to hundreds of megawatts. Other potential military applications that require high power levels, not necessarily SDI-related, include spacebased radar and a space-based submarine communications system. NASA has identified a number of potential civil missions, including unmanned science and exploration, manned space operations, and private commercial operations in space. According to a March 1987 report prepared by NASA's Jet Propulsion Laboratory, electrical power at greater levels than have been available seems essential to accomplishing civil mission objectives, and the availability of space nuclear power is an integral assumption in current U.S. planning for the next 60 years of space exploration, utilization, and settlement. The study states that various science and exploration missions-for example, spacecraft travel to Mars for on-site studies and sample analyses from its surface-will further the investigation of our solar system. While most of the principal science and exploration missions of the coming decades will require significant power levels for spacecraft, large space observatories are also being planned, some of which will require the same high power levels, many in the multihundred kilowatt range. The study also reports that space operations considered during the 1995 to 2050 time frame include space vehicles and outposts where humans would live and work. A wide variety of activities will be conducted from a proposed space station, such as (1) spacecraft servicing, (2) space technology and engineering research, and (3) life science research. 3Preliminary Survey of 21st Century Civil Mission Applications of Space Nuclear Power, prepared by the Jet Propulsion Laboratory, California Institute of Technology, March 1987. Introduction Commercial uses of space will probably continue to expand beyond present enterprises such as communication satellites. The report states that commercial enterprises will exploit the space environment for the benefit of private industry. One such enterprise being proposed is a materials-processing platform that would place a research and manufacturing facility in orbit. By eliminating gravitational effects, the facility would allow processing of glasses and fibers and biological materials under conditions different from those on earth. Two research and development programs are underway to develop tech- This report provides information in response to a request from the former Chairman of the House Committee on Science, Space and Technology and its current Ranking Minority Member to study the area of space nuclear reactor development. As requested in the May 20, 1986, letter and as amended by agreements reached in subsequent meetings with the Committee office, our objectives were to provide information on DOE's space nuclear reactor development programs; management strategies applied in the space nuclear reactor programs to ensure coordination among the funding agencies: DOE, Strategic Defense Initiative Organization (SDIO), and NASA; and resources projected to perform safety-related tasks. |