components for durable long lasting systems. These components include seals, membranes, bearings, and insulators.

Safety and environmental considerations are inherent in the design of all power systems. Both concerns are subject to U.S. Department of Energy procedures, as well as state and local requirements. In fact, satisfying the myriad requirements has become a major cost factor. The nuclear industry associated with small power sources has an impeccable record of addressing environmental and safety issues through extensive testing programs. Disposal is not a problem for deep space and planetary probes. But if terrestrial use increases significantly, disposal will become a critical issue that must be addressed in advance.

The selected fuel must not pose a threat to the environment or to human health. Unfortunately, the most desirable fuels are not available in quantities necessary for power applications. Hence, fuel type is also a critical issue. For terrestrial applications to date, the fuels have tended to be ⁶⁰Co and ⁹⁰Sr, both of which were available from reprocessed nuclear reactor fuels. Some fuel-grade materials are in storage, but they are decaying rapidly and will be of limited use in another 10 years. Because the United States does not produce suitable quantities of fuel, they must be purchased from countries that routinely reprocess nuclear materials or specialize in isotope production. The United States recently purchased ²³⁸Pu from Russia to meet NASA's projected needs. China, Canada, Japan, and France are other potential sources.

Many potential terrestrial applications for radiothermal generator technology are not being pursued, primarily because of public perception, the cost of materials, and environmental and safety issues. Most of the nuclear systems used to date have used thermoelectric converters. Conversion efficiency is expected to improve by a factor of two to five in the near future for converters on thermodynamic cycles and for technologies like AMTEC and thermophotovoltaics. But even if the use of nuclear materials were acceptable, the isotope manufacturing industry in this country is bordering on collapse. Before radiothermal generators can be widely used, the public perception of nuclear systems will have to change significantly.

The enormous specific energy of nuclear fuels makes them very attractive candidates for Army and civilian use. But the practicality of using nuclear fuels is low because of concerns about safety, environmental impact, cost, fuel infrastructure, public image, and the poor shelf-life of more powerful isotopes. Indeed, an isotope that could be used in a power system begins to generate energy the moment it is made in a reactor and cannot be turned off. Therefore, the key issues deal with system studies rather than fundamental research. The Army should: