Studies of the radial distribution of Pu, Xe and Cs in irradiated thermal-reactor UO2 fuel, using an electron probe microanalyser

An electron probe microanalyser (EPMA) has been used to measure the radial distribution of plutonium, xenon and caesium in irradiated AGR and water reactor UO2 fuel pellets, with burn-ups in the range 4 to 35 GWd/t. In the cooler parts of the fuel (at 0.8 pellet radius) there was generally good agreement between measured and estimated values for the concentrations of retained fission products. In both types of fuel enhanced concentrations of plutonium and fission products were present in a narrow zone at the pellet surface, due to epithermal neutron capture effects. In the more highly rated fuel at proportional 35 W/g the central hotter region of fuel was depleted in xenon and to a lesser extent in caesium. This zone of Xe and Cs depletion corresponds to the region in which fission gas diffuses to grain boundaries to form bubbles which grow and interlink to allow gas release from the fuel. (orig.)