Prediction of pellet-to-gap escape and gap-to-coolant release rates of fission products

Some nuclear power plants in Korea were found to be leaking due to debris- and grid-to-rod fretting wear (GTRF)-induced fuel failures. Evaluation models for pellet-to-gap escape rates and gap-to-coolant release rates have been developed with the use of the primary coolant activity variations. It is found that the gap-to-coolant release rates for the debris fuel failure are different from those for the GTRF fuel failure. The gap-to-coolant release rate is controlled by both the primary defect and the secondary hydride-induced defect. The secondary hydride-induced through-cracks are more effective for the release of iodine and xenon than the primary defect. The fission product inventories in the gap and the gap-to-coolant release rates can be explained by the time-dependent defect progress, the water phase around the defects, the steam in the fuel gap, the pellet-to-clad contact, and the spacer grid springs/dimples contacting the defect.

► Coolant activities are found to be different for debris- and GTRF-induced fuel failure.
► Evaluation models for fission product inventory and escape and release rates are developed.
► Escape and release rates are generated using primary coolant activity variations.
► Release rates are dependent on primary defect and secondary defect progress with time.
► Secondary hydride-induced through crack is more effective than primary defect for debris.