Matrix swelling rate and cavity volume balance of UO2 fuels at high burn-up

The low temperature matrix swelling of UO2 fuels was analysed as a function of burn-up, taking into account changes in the fuel density, porosity and retained Xe concentration as measured by EPMA. The evaluation of these data combined with an analytical solution to a rate-theory model for gas-driven swelling leads to the conclusion that at average pellet burn-ups ⩽ 60 GWd/t M the total matrix swelling rate is gradually reduced from ≈1% per 10 GWd/t M to ≈0.3% per 10 GWd/t M; the latter value being reached at an average burn-up above 110 GWd/t M. The decrease in the swelling rate is due to the progressive depletion of fission gas in the fuel matrix by thermal and athermal processes, such that at very high burn-up the matrix swelling becomes dominated by the precipitation of solid fission products. Owing to volume conservation, the volume occupied by the depleted gas is replaced by porosity.