Background During the operation of a nuclear power plant, fuel rod failure will lead to an increase in the radioactive activity in the primary coolant, increases the difficulty of storing, transporting and disposing of spent fuel. If the fuel failure has deteriorated to some degree, the reactor should be shut down within several hours to limit the release of fission products to the environment.

Purpose Therefore, it is necessary to assess the status of fuel failure for the utilities as soon as possible.

Methods In the paper, the generation and migration mechanism of fission products in the primary loops in pressurized water reactors has been introduced, and then one analytical method for diagnosing the status of fuel failure based on the measured fission product volume activity has been established. The characteristics of iodine and noble gas release from failed with tiny defects has been identified through one typical fuel failure cases. The evolution of fuel defect size can also be determined by the regression of release-to-birth (R/B) and the decay constant (λ) of fission products.

Results The model diagnosis results have been confirmed by the sipping examination. The analysis of a typical failure case successfully reveals the characteristic release behavior of iodine and noble gases from small defects.

Conclusions The proposed analytical method can help quickly identify and locate failed fuel assemblies, thereby reducing the impact of fuel failure treatment on the reactor overhaul period. This approach provides with a practical tool for timely assessment of fuel failure status and informed decision-making regarding reactor operation.