they use. They can eventually make use of more than 70 percent of the energy potential of uranium ore. There are also conceptual reactors and fuel cycles capable of converting fertile thorium-232 (232Th) to another fissile isotope of uranium, 233U. These could in principle make use of nearly 70 percent of the energy in thorium, which is believed to be 4 times as abundant as uranium in the earth’s crust.

Thus, the ability to unlock the energy potential of the fertile isotopes 238U and 232Th has a tremendous multiplying effect on available resources—much more than the approximate factor of 100 implied by the numbers just quoted. This is because the use of breeder reactors reduces the contribution of resource prices to the price of electricity by a factor of 100, thus making available ores that are too low in grade, and thus too expensive, to be used as fuel for conventional reactors. For practical purposes, the resource costs for breeders make a negligible contribution to the cost of electricity. Thus, the economics of breeders are closer to those of renewable resources than to those of nonrenewable resources.

As explained earlier, the present generation of light water reactors can be relied on as an energy source only until the early twenty-first century, even if optimized for fuel efficiency. The resource base may be extended 20–30 percent by working enrichment plants harder (to recover a larger fraction of the 235U in the natural uranium). Another 35–40 percent extension could be achieved by reprocessing spent fuel in a chemical separation process to recover fissile plutonium and uranium for refabrication into new fuel elements. Either measure, however, would significantly extend the life of a nuclear industry based on light water reactors only if electricity growth leveled off after 2000.

Unfortunately, during fuel reprocessing, plutonium appears briefly in a form that can be converted into nuclear weapons much more readily than can the fissile and fertile material in the spent fuel elements themselves. This gives rise to the fear that a nation in possession of fuel reprocessing facilities might be tempted to manufacture clandestine nuclear weapons, or that a determined and well-organized terrorist group could steal enough material to manufacture a nuclear bomb, It is possible that the recycling process could be modified to make it much less vulnerable in this respect, but both the desirability and the effectiveness of such modifications are still matters of debate. (See chapter 5 under the heading “Reprocessing Alternatives.”) These considerations bear heavily on decisions to deploy advanced, more efficient reactors, because all advanced reactors require reprocessing and refabrication of fuel to realize their maximum potential for more efficient resource use. (However, there are several advanced converter designs that could realize substantial, though not the greatest possible, resource savings over improved light water reactors even with a once-through fuel cycle.)

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