THERMAL-TREATMENT OF URANIUM OXIDE IRRADIATED IN PRESSURIZED-WATER REACTOR - SWELLING AND RELEASE OF FISSION GASES

Unstressed samples of uranium oxide taken from pressurized water react or (PWR) fuel after two normal operating cycles, i.e., with a burn-up of 25 GWd/tU, were subjected to thermal treatment in a laboratory furn ace at temperatures between 1130 degrees C and 1715 degrees C for dura tions between 5 min and 10 h. The variation of the quantity of fission gas released over time was determined at each temperature. The experi mental results were found to compare with those of an existing numeric al model, and it would thus appear that the release of gas after forma tion in tunnels at the main boundaries is controlled by the diffusion oi these gases from the material towards the grain boundaries. These s amples were also subjected to a series of isothermal swelling measurem ents. Their comparison provides information on intergranular, intragra nular, open and closed porosity. The study carried out finally makes i t possible to interpret the behaviour of fission gases in the event of a temperature rise. Initially, swelling and release are controlled by a single mechanism: bubble coalescence. Subsequently, these phenomena are controlled by separate mechanisms: swelling continues through coa lescence and release is due to the diffusion of the fission gases from the matrix to the tunnels at the grain boundaries. (C) 1998 Elsevier Science B.V. All rights reserved.