Release of volatile fission products in the PWR primary coolant during power transients

Since several years, activities of short-lived fission products have been measured by EDF and CEA on French commercial PWRs running with cladding defects. More recently, investigations have been performed in order to understand the behaviour and to determine the release rate of volatile fission products in load follow conditions. These studies are mainly related to volatile fission products such as 133Xe, 135Xe, 131I, 133I, 134Cs, 137Cs. It has been shown that several parameters have an effect on the transient release rate: power increase/decrease rates; power levels; size and localization of the defect; repetition of the transients. At the moment, no comprehensive gap to primary circuit release model has been set up and data available for various experimental conditions do not allow to distinguish the separate effects of each parameter on the transient release rate. Nevertheless for a typical load follow transient, the effect of the defect size and repetition of the transient have been studied and values of the transient release rate have been obtained for gases, iodines and cesiums. The increase of gases activities during load follow is less than 20% whatever the defects sizes are. For small defects the iodine release is very low and not enhanced by power transient. For larger defects, the iodine transient release may be enhanced up to 50% compared with steady state release. Cesium release is negligible during load follow transients except for very large defects where 5% of the gap inventory has been released in the primary coolant. (author). 8 figs