Pu, U redistribution in (U,Pu)O/sub 2/ fuels by temperature gradients

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Abstract
A predictive model of the time-dependent evolution of radial Pu/U + Pu gradients in operating nonmolten mixed oxide fuels is being developed. This model is based on preferential evaporation-condensation and vapor transport of metal-bearing species either along cracks and porosity channels within the fuel or inside closed pores migrating up the temperature gradient; equilibrium thermodynamics are applied to calculate the continually changing vapor composition over the hot fuel. Predictions of the model are used in combination with recent in-pile experimental data (from nonmolten mixed oxide fuel pins irradiated in EBR-II from 0.2 to about 11 at. % burnup at varying powers) to delineate the key parameters in radial actinide redistribution. A correlation between fuel stoichiometry (O-to-Pu ratio) and the degree of redistribution (described by an enrichment factor ..gamma../sub Pu/) identified, however, the degree of restructuring of the fuel - as determined by the peak linear power rating, time at this power, and the initial fuel density - also has a strong influence on the final value of ..gamma../sub Pu/. Certain kinetic features of the process and intended modifications to the present model are discussed, and evidence for axial actinide vapor transport in an irradiated annular fuel is also presented.

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