A Method to Evaluate Defect Size of Failed Fuel Rods in CGN 1000MWe PWRs

Abstract

Abstract Fuel cladding is the first barrier to confine the release of fission products to the environment during normal operation. Once the fuel rods defect, the fission product accumulated in the pellet-cladding gap of failed fuel rods will release into the primary loop and lead to high activity levels in a pressurized water reactor. It is necessary to determine the status of fuel failure to avoid its degradation in the operation. This paper introduces the mechanism of the generation and release of fission products in fuel rods and the primary loops in a pressurized water reactor, and provides the theoretical method of the regression between release-to-birth R/B and decay constant λ of radio-iodine. It has been verified by one typical case of fuel failure in CGN 1000 MWe PWR, but the post-irradiation examinations in hot cell reveal that six defects exist in one failed rod and the actinides have an obvious release from the failed rods due to secondary hydriding. The actual status of defect size of failed rods is much more complicated than expected in most analysis codes and modelings and it is suggested to introduce some parameters, e.g. the equivalent defect size, to describe the status of fuel failure in future research. The related operation experience in CGN has also been presented to help to better understand the status of fuel failure in PWR units.