For light water reactor (LWR), if fuel rods failure occurs, the fission products (FP) will be released into the primary coolant, which will affect radiation safety, cause adverse environmental impact and economic loss. When fuel defects occur in nuclear power units, the prediction of iodine peak usually refers to the results of NRC research of the US Nuclear Regulatory Commission: the peak effect during the shutdown can make the activity value of primary loop FP (iodine and RIG) be 10–1000 times of that during normal operation This method is evaluated from a statistical point of view and has little significance for the actual operation of the nuclear power plants. The main reason is that the release rate from the cladding to the primary circuit is not considered in the calculation of the iodine peak purification time. Combined with the actual power operation data of the WWER units, the difference between RIG escape and iodine release is analyzed from the mechanism. Based on the actual power operation data, the calculation model of fission nuclide activity concentration in the primary circuit is established, and a method to predict the iodine peak size and purification time is finally obtained. In the case of fuel rods failure, the peak value of iodine-131 during the shutdown was estimated, and the purification time and final concentration from purification to reactor opening were estimated, which ensured the overhaul progress of the nuclear power plant.