For the radioisotopes with half-lives over a year, only eight appear to be obtainable in the foreseeable future. The fission products, strontium-90, cesium-137, and promethium-147, exist in wastes from reactor processing, diluted with enormous volumes of other elements and salts. Among those isotopes producible by irradiation of special target materials (cobalt-60, uranium-232, plutonium-238, and curium-244) cobalt-60, though easy to produce, requires a special design for the heat source generator because much of its emitted energy is penetrating gamma radiation. Cobalt-60 appears, therefore, to be rather limited in its prospects for use. Plutonium-238 is favored as a heat source because of its long half-line and no need for special shielding. However, its projected high cost, scarcity, and biological hazard encourages the search for a competitive material. When plutonium assumes a significant place as a recycled fuel in thermal reactors for power production, curium-244 can then become available at costs below that for plutonium-238. Curiunm-244 has five times the specific power of plutonium-238 and appears to be just as easy to handle. Promethium-147, although probably on the ''short end'' of the half-life scale, can be considered for some uses as a substitute for plutonium-238. Although the factors of availability, gamma activity, andmore » biological characteristics are unfavorable, the strongly points for uranium-232 (and thorium-228) are extremely high power densities, relatively low expected costs, and unusually long-life nearly constant heat output. The short life of thorium-228 (1.9 years) is a disadvantage. This study indicates that aged promethium-147 should be receiving more serious attention as a heat source.« less