Experimental investigations of caesium and iodine release from irradiated UO2

The modelling of the behaviour of fuel rods whose temperature is rising requires a knowledge of fission product release under such conditions. For this purpose an experimental apparatus has been developed and built for the determination of vapour pressures by the Knudsen method. The results obtained on UO2 samples of light-water reactor fuel with a burnup of 32 MWd tU-1 shows that a noticeable release of iodine and caesium occurs only at fuel temperature above 1400-15000C. Thus the influence of iodine on the mechanical behaviour of fuel rods under loss-of-coolant-accident conditions can be neglected. The onset temperature for iodine release matches excellently the pellet-clad-interaction (PCI) threshold identified in ramping experiments. The relative release is nearly equal for iodine and caesium, and increases rapidly with temperature and burnup. (author)