Effect of burn-up on the thermal conductivity of light water reactor fuel: Results of investigations employing the laser flash technique

Analysing data drawn from three of EPRI`s NFIR Programmes and the High Burn-up Rim Project (HBRP) managed by CRIEPI of Japan, expressions for the degradation of thermal conductivity with burn-up up to 103.5 MWd/kgHM are presented. These indicate that at high burn-up, above 70 MWd/kgHM, the thermal conductivity of the matrix of the high burn-up structure (HBS) increases by 25 - 30% concomitant with recrystallization of the fuel grains. This increase is partially or completely off-set, however, by a decrease in conductivity associated with the porosity of the HBS which contains the fission gas gathered from the fuel matrix during transformation of the microstructure. Below 70 MWd/kgHM, in the absence of the HBS, the conductivity is about 15% lower than that currently accepted. It is also found that the conductivity of UO ₂ fuel falls sharply immediately when irradiation begins. This is attributed to radiation damage rather than the incorporation of fission products in the fuel lattice. The data further reveal that at burn-ups below 70 MWd/kgHM the conductivity of UO ₂ containing 10 wt% Gd ₂O ₃ is markedly lower than that of UO ₂, but once the HBS has formed its conductivity is indistinguishable from that of conventional UO ₂, UO ₂ additive fuel or MOX. It is concluded that for accurate determination of the radial temperature profile in a high burn-up fuel pellet, knowledge of the volume fraction of the HBS in the pellet and the radial distribution of the porosity in the affected region is required.