Modelling of the fission product release from irradiated overheated LWR fuel

This paper outlines the modelling work performed at BELGONUCLEAIRE in the frame of the COMETHE code, in order to describe the release of the fission products from LWR fuel, when the latter is overheated after operation at low temperature and high burnup. On basis of a review of high burnup data and power ramp tests, transient grain growth appears to be the most likely mechanism by which intragranular fission products can be transferred towards grain boundaries and fuel rod free volume; it also provides an evident interpretation of the denuded zones observed in irradiated fuel samples. A model is described where the grain growth rate is calculated as a function of fuel vapour pressure and grain boundary gas inventory. The main problem is to estimate the vapour pressure above irradiated fuel, as it results not only from the fuel, but also from the fuel-fission product compounds. Hence, grain growth and fission product release are expected to be quite sensitive to the irradiated fuel chemistry, stressing thus the need for the set-up of a comprehensive data base through ad-hoc international programmes. (author)