A model for estimating the number of failed fuel rod and defect size based on the primary coolant activity is presented. Transport phenomena in the gap is represented by a diffusion process to attain a link with the defect size. The gap attenuation factor dependent on the defect size and the radioactive decay constant is introduced. It is shown that if the effective defect size is larger than the thickness of the clad, the gap attenuation factor is weekly dependent on the defect size. From the values of the gap attenuation factors, the rod escape rate coefficients can be evaluated. On the basis of the present model, the ENOFS computer program has been developed and applied to cycles 1 and 2 of KORI-1. The number of fuel failures deduced from ENOFS calculations was between four and five with small defect size for cycle 1. On the other hand, about 15 failed fuel rods with small defect size and one with more large defect size were predicted for cycle 2. The end of cycle 1 sipping test results showed that there were two assemblies with failed fuel rods, and for cycle 2 there were two fuel assemblies which included a few large defective fuels.