The parameters that influence the thermal conductivity of high burnup uranium dioxide fuel are discussed and the thermal conductivity of uranium dioxide fuel with a burnup of approximately 39 M Wd kgUO_2~(-1) has been analysed. The main parameters are changes in the pore structure, dissolution of fission products in the matrix, formation of precipitates, and the formation of cracks. The influence of dissolution or precipitation of fission products can be approximated by the use of data for SIMFUEL (Lucuta et al). The influence of porosity can be estimated by the use of the finite element method. The influence of cracks depends strongly on the width of the fuel-to-cladding gap.
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