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NOTICE This document contains information of a preliminary nature. It is subject to revision or correction and therefore does not represent a final report.

Thermal-gradient-induced redistribution of porosity and fuel components during irradiation of (U,Pu)02 will alter the fuel thermal conductivity, melting point, mechanical properties, and radial heat generation profile sufficiently to influence fast breeder reactor fuel pin performance. Analytical models, which should prove useful in design and analysis of such fuel pins, were developed for predicting radial porosity and Pu:(U + Pu) profiles. The interrelated porosity and actinide redistribution models are kinetic and based on the evaporationcondensation mechanism of material transport. The models were shown to yield predictions in accord with experimentally measured porosity and actinide profiles for an irradiated pin containing stoichiometric fuel.The volume-averaged porosity of the columnar grain region of irradiated pins was 5.9 and 23.8% after burnups of 0.7 and 4,,2% FIMA, respectively.The columnar grains are thus more porous than previously believed.

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