A Study on the Onset of Secondary Hydriding in Defective Zr-2 Fuel for Boiling Water Reactors

One of the most common threats to the integrity of reactor fuel cladding is the geometric imperfections such as the missing pellet surface (MPS) that produces a remarkable surge in the local fuel-clad gap. The cooling water could occupy this gap leading to secondary hydriding (SH) and hydrogen embrittlement. Most studies on this subject have identified extensive radiolysis in boiling water reactors (BWRs) to be responsible for hydrogen evolution during accident conditions. However, the quantitative determination of hydrogen and how it affects zircaloy-2 during normal reactor operation has not been given adequate attention. To bridge this gap and to better predict the onset of cladding failure, this study investigates secondary hydriding and its characteristic phenomenon in a fuel cladded with zircaloy-2. Multiphysics model is used to model diffusion of heat and hydrogen, then the effect of an intermediary porous/non-protective oxide layer and the impact of dose rate from different types of radiation are studied. The contributions of the source term due to radiolysis of neutrons (n), gamma (γ), alpha (α) and beta (β) radiations are also considered. Results showed that neutrons, having a maximum dose rate of 39.9 kGy/s accounts for over 99% of ZrH precipitation in a UO2 fuel with MPS. The effect of oxide (ZrO2) thickness in SH, and the derivative of oxide thickness are also discussed.