An improved methodology for estimating the number of failed fuel rods based on the primary coolant activity in operating PWRs has been developed. This method deals with both the diffusion and the kinetic models. In case of small, medium, and large cladding failures, the diffusion model which can consider different sizes of failure is used, whereas for gross cladding failures the kinetic model is used. In the kinetic model, the release-to-birth rate ratio (R/B) is represented as a linear function of the number of failed fuel rods. This has been done by expressing the gap escape rate coefficient in terms of the slope of log(R/B) versus log λ.
Incorporating these models, the CAAP (Coolant Activity Analysis Program) computer program, running under a graphical user interface, has been developed to estimate the number of failed fuel rods based on the primary coolant activity and has been applied to the cases of 26 cycles, including 21 cycles of foreign PWRs and 5 cycles of domestic PWRs, for which ultrasonic testings were performed at end of cycle (EOC). The results show that the present method gives better predictions than the existing computer codes such as IODYNE and CADE.