A Windows computer code entitled as "CAAP (Coolant Activity Analysis Program)" has been developed to evaluate the number,;he degree of failures, and the location of failed fuel rods using primary coolant radioactivity data obtained from operating PWRS. New models are developed and incorporated into the CAAP program to improve some of the drawbacks of existing computer codes. The iodine and noble gas activities obtained by grab sampling and that obtained from the primary coolant radioactivity monitoring system can be used to estimate fuel rod failures with CAAP. For an on-line evaluation of the number of failed fuel rods and the degree of failures, CAAP has been combined with the primary coolant radioactivity monitoring system at Kori 3 & 4 operating nuclear power plants. The validity of the computational models of CAAP has been examined using nuclear power plant data collected. For 29 cycles of PWRs for which ultrasonic inspections were performed at the end of fuel cycles, the number of failed fuel rods has been estimated with CAAP and compared with the ultrasonic inspection results. The results show that CAAP gives a better agreement with the ultrasonic test data than existing models. In addition, CAAP calculations performed to estimate the region and the burnup of the failed fuel rods for 2 cycles, whose locations of failed fuel rods were known, show that CAAP can predict both the region and the burnup of the failed fuel rods accurately. (C) 1998 Elsevier Science Ltd.