Mixed-oxide fuel, isotopic composition

The effective multiplication factor for a full loading of 112 initial FTR driver hiel assemblies (26.4 wt% Pu) was calculated to be 0,73 0.04. The isotopic composition of the plutonium in the driver fuel is 0.864, 0.117, 0.017, and 0.002 for the isotopes 239, 240, 241, and 242, respectively. Based on this isotopic composition, it was calculated that mixed-oxide fuel containing up to 35 wl% Pu could be stored in the IDS facility with the effective multiplication factor remaining below 0.9 at the 95% confidence level. In all cases, the diluent was natural uranium oxide. 

This paper presents a set of isoreactivity curves, applicable only to mixed-oxide rods of a certain design, which permit the determination by simple inspection, rather than laborious calculation, of whether a different plutonium isotopic composition in the same rod design is more or less reactive than a base design of known reactivity for criticality safety purposes. In effect, the curves incorporate reactivity equivalency factors lor the three main isotopes of plutonium which are expected to vary as a function of recycling in reactors. An equation that predicts the percent reactivity increase of the mixed-oxide fuel rods where a certain percentage of the Pu isotope is replaced by Pu is also given. Criticality safety limits for new plutonium isotopic composition of fuel rods of the same design, therefore, may be determined more rapidly. 

The isotopic composition of the plutonium fraction of mixed-oxide fuels strongly depends on the production history (previous fuel enrichment and irradiation) of the individual plutonium batch supplied from the reprocessing plant. For... 

Before discussing the sustainability of Gen-IV systems, a reminder about natural uranium and the composition of spent nuclear fuel (SNF) is necessaryNatural uranium is composed of 0.005% U, 0.720% U%, and 99.275% U. The fuel used in a standard LWR relies on the fissile isotope U, which is typically enriched to U concentrations in the range of 4%. However, it should be noted that some 40% of the energy produced in the course of a nuclear fuel cycle in an LWR comes from Pu, which is thus an excellent fissile fuel material. Moreover, ceramic-mixed oxide fuel (MOX, which is UO2 + PUO2), consisting of about 7—10% Pu mixed with depleted uranium ( U), is equivalent to UO2 fuel enriched to approximately 4.5% U, assuming that the Pu contains approximately two-thirds fissile isotopes. 

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