Fuel cladding is the first barrier to confine the release of fission products to the environment during the normal operation of reactors. Since the fuel assemblies have been washed many times in the manufacturing plant, the content of tramp uranium in the primary circuit of commercial pressurized water reactors has been reduced to a negligible level. However, when a fuel failure with large defects occurs during normal operation, the fuel pellets will be eroded locally and some amount of actinide nuclides may be released from the defective fuel rod into the primary coolant, which will deposit in the primary system and cause radioactive contamination in several subsequent cycles. This paper introduces the release mechanism of fission products by diffusion and recoil in defective fuel rods of pressurized water reactors. A model is established to analyze the relationship between the release-to-birth and half-life of fission products in the same chemical forms. Based on the measured radiochemical data in the primary coolant, the quantity of fuel washout can be estimated by recoil release. The results show that severe secondary degradation may result in the dissemination of several dozens of grams of fuel washout into the coolant. It reminds utilities of reactors to try to reduce power disturbances to avoid secondary deterioration of defective fuel rods.