advanced converters designed for the use of thermal neutrons and generally operating on the thorium cycle, and fast breeders designed for the use of fast neutrons that can generate more plutonium from ²³⁸U than they consume in generating power. Breeders can also generate ²³³U from thorium. Advanced converters using thorium and ²³³U can be designed to function as thermal breeders. With sufficiently careful design and frequent fuel reprocessing, they can operate without additional fissile isotopes from nature. However, these conditions are not likely to yield economical power generation.⁹

The breeder design closest to commercial status in the United States and elsewhere is the liquid-metal fast breeder reactor (LMFBR). In the most resource-efficient version, this reactor would be fueled with plutonium separated from the spent fuel of light water reactors and with depleted uranium left behind in the enrichment process for today’s light water reactor fuel. The energy available from uranium already mined and stored as depleted tails from domestic enrichment plants, if used in LMFBR’s, could provide one third to one half of the energy recoverable from domestic coal reserves and resources.

Advanced converters can also extend resources, but unless they are fueled with plutonium from the spent fuel of light water reactors, their operation will require some additional uranium feed. The amount of this required feed can be minimized by frequent reprocessing and by features in the converter designed to hold down the loss of neutrons to fission products, control rods, and structural materials. The advanced converter most widely used in the world is the natural-uranium, heavy water CANDU, developed in Canada. The advanced converters closest to commercial status in the United States are the high-temperature gas-cooled reactor and the light water breeder reactor (LWBR). They both use the thorium-uranium cycle with enriched ²³⁵U feed. Both require more uranium for their initial inventories of fuel than light water reactors.¹⁰ This uranium requirement can be reduced somewhat by mixing in plutonium from reprocessed light water reactor fuel. Advanced converters require far less uranium ore over their operating lives than light water reactors.

The thorium-²³³U fuel cycle can be used to greatest advantage in thermal advanced converters, and the uranium-plutonium fuel cycle can be used to greatest advantage in fast breeders. This suggests the possibility of using various integrated fuel cycles: combinations of fast breeders, advanced converters, and light water reactors.

These technical possibilities are unlikely to be realized unless nuclear power is publicly acceptable. Public opinion may show swings and trends in the future, as it has in the past. Public concern about nuclear power has centered on four issues: the safety of routine operation of the nuclear fuel