Effect of MOX Fuel Inhomogeneity on the Level of Fission Gas Release

Abstract

Two sections of OCOM fuel were investigated. In one fuel the MOX agglomerates contained 18 w% fissile plutonium, and had a low volume fraction of 0.17; in the other the agglomerates contained 9 wt% fissile plutonium, and had a high volume fraction of 0.34. Both fuels had been irradiated to a burn-up of approx. 44 GWd/t. It was found that the concentration of plutonioum in the MOX agglomerates had decreased by about 50% as a result of fission, whereas the plutonium content of the UO2 matrix had increased ten-fold to approx. 2 wt%, due to neutron capture. More than 80% of the fission gas had been released from the oxide lattice of the MOX agglomerates. A very high fraction of this gas precipitated and remained in the pore structure of the agglomerates. Consequently, for both fuels the percentage of gas released to the rod free volume increased from less than 0.5% at 10 GWd/t to a max. of 3.5% at 45 GWd/t. Caesium showed near complete retention in both the MOX agglomerates and te UO2 matrix.