efficient in its use of uranium. This is particularly significant if fuel is not recycled, as the fissile isotopes in spent fuel could replace some of the 235U in freshly mined uranium. It is possible to use uranium more efficiently. The key is to use reactors with higher conversion ratios and long fuel lifetimes. Higher conversion ratios can substitute for recycling to the extent that the fissile atoms formed undergo fission in place. With recycle, the higher conversion ratio permits more fissile atoms to be substituted for natural 235U.

Although light water reactors do not now have high conversion ratios, a great deal of the plutonium created in their operation undergoes fission in place. About one third of all the energy in LWR’s is obtained from plutonium fission, and at the end of fuel life, more than 60 percent of the fissions occur in 239Pu. LWR’s could, in principle, be designed for higher conversion ratios and better use of natural uranium, a fact that should be remembered in comparing them to other reactors. Such designs would have lower enrichments and burnups than existing LWR cycles and could only achieve better use of natural uranium through plutonium recycle.31 The required rate of reprocessing might be twice as high, per unit of electrical energy generated, as that for the standard LWR recycle mode estimated in Table 5–4, but lifetime uranium consumption would be less than 3000 tons. For all reactors, the conversion ratio varies with the composition of the fuel loaded and with fuel management. Differences among reactors often correspond to differences in the conversion ratio that can be readily achieved for fuel loading and management practices permitting economical power generation.

The reactors proposed to achieve greater efficiency in the use of fissile resources fall into two classes: advanced converters and breeders. Advanced converters can be designed to achieve conversion ratios ranging from 0.7 to slightly more than 1. (Light water reactors operate at conversion ratios of 0.6 or less.) Breeder reactors can be designed to achieve conversion significantly greater than 1, although they could obviously be designed and operated at lower conversion ratios. For some breeders, such as the molten-salt breeder reactor (MSBR), the reduction in fissile inventory could be sufficient for greater economy (i.e., the savings in charges against inventory could be greater than the loss of income from product sale and the extra cost of feed material).

Prototypes and designs for various types of advanced converters and breeder reactors have been developed in the United States and other countries. The functional and practical points that must be considered to evaluate the relative merits of these reactors and fuel cycles cannot all be assessed equally for the designs and prototypes. Some reactor designs are only conceptual, others have been tested through small pilot plants, and others are close to commercial status. A complete safety assessment, for



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