The same U enrichment is used throughout a given PWR fuel assembly, but the core usually contains several levels of enrichment arranged to give uniform power distribution. In contrast, BWR fuel rods have

several axial segments with different enrichments and a BWR fuel assembly has several different rods with different enrichments. Thus, there are a variety of UO2 pellets with different U enrichments depending on reactor design; the enrichments are within 5% which is due to the limits of fuel fabrication facilities and fuel shipments.

For current LWR MOX fuels, depleted uranium (0.2—0.3% 235U), which is obtained in the form of tails from the enrichment process, is coupled with plutonium because there are economic incentives to concentrate as much plutonium in as few fuel assemblies as possible as it conserves the expensive fabrication cost of MOX fuel. As the quality of plutonium, from a neutronic aspect, varies with the isotope composition of plutonium, the specification of the plutonium content of LWR MOX fuel is affected by the quality ofplutonium. Total plutonium concentrations of 7.5% are considered to be equiva­lent to U enrichments of 4.0-4.3% for the current usual plutonium that is recycled from spent LWR UO2 fuel.2

To determine plutonium content of FBR MOX fuel, equivalent 239Pu (239Pu/(U + Pu)) is used. The actual plutonium content for a given batch is obtained by a calculation that uses the neutronic equivalent coefficient of each isotope and the isotope composition of plutonium to be used for the batch. 241Am, a daughter product of 241Pu, is considered in the calculation as well. The specification for equiva­lent 239Pu (239Pu/(U + Pu)) is relatively low for a large size core; equivalent 239Pu is 12-15% for the SUPERPHENIX (1200 GWe),28 1 4 -22% forMONJU (280 GWe).