Application of WANO Fuel Reliability Indicator in CGN Operating PWR Units

The fuel cladding is the first barrier to prevent the release of radioactive material into the environment in the reactor. The industry has focused on the improvement of fuel reliability in the past decades. The WANO fuel reliability indicator (FRI) is defined by the iodine volume activities in the primary loops adjusted by purification, the tramp uranium, and the power level of the unit. The WANO FRI has been widely used as the performance indicator of fuel integrity for peer review across the world for several decades. According to the CGN operation experience, the WANO FRI can reflect the fuel integrity at most of the operating cycles in CGN 1000 MWe PWR units. However, the FRI may become invalid at the following phenomenon: the calculated FRI may be less than the threshold when the defect size of failed rods is very tiny, or the calculated FRI may be higher than the threshold in the cycles without fuel failure but in which the primary loops is contaminated by the disseminated actinides in the previous cycles with fuel failure. It is found that the root cause of the phenomenon is that the release of Iodine-131 is significantly affected by the characteristics, e.g., defect size, the location of defects, and the linear power of failed rods which cannot be predicted before shutdown. One improvement of the current FRI has been proposed by CGN that way is to define FRI by the Xe-133 activity in the primary loops because the noble gas can release readily even though for the tiny fuel defect. It has been verified by the measured fission product in 58 cycles without fuel failure and 17 fuel cycles with fuel failure in CGN operating PWR units.