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 reactor (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 furnace at temperatures between 1130°C and 1715°C for durations between 5 min and 10 h. The variation of the quantity of fission gas released over time was determined at each temperature. The experimental results were found to compare with those of an existing numerical model, and it would thus appear that the release of gas after formation in tunnels at the grain boundaries is controlled by the diffusion of these gases from the material towards the grain boundaries. These samples were also subjected to a series of isothermal swelling measurements. Their comparison provides information on intergranular, intragranular, open and closed porosity. The study carried out finally makes it 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 coalescence and release is due to the diffusion of the fission gases from the matrix to the tunnels at the grain boundaries.