Simulation of Outside-in Cracking in Boiling Water Reactor Fuel Cladding Tubes under Power Ramp

In power ramp tests, some high burn-up boiling water reactor (BWR) fuels failed by outside-in cracking. After detailed post irradiation examinations, it was deduced that fracture of radially oriented hydrides precipitated at the outer surface had resulted in the formation of incipient radial cracks that propagated towards the inner surface. Since crack propagation is a key process that dominates the outside-in failure, experimental data of outside-in cracking are obviously required. The objective of the present study is to evaluate the threshold conditions and the velocity of the outside-in cracking during a power ramp to assess the integrity of BWR fuels during a power ramp. In the experiments, by using unirradiated Zry-2 fuel cladding tubes, the outside-in cracking was examined both with and without the thermal gradient. Without the radial thermal gradient, there was no radial delayed hydride cracking in all experimental conditions examined. On the other hand, with the radial thermal gradient, the velocity of the outside-in cracking was significantly higher than that expected from the data obtained under the isothermal condition in our previous study. The threshold depth of the incipient crack was found to be ∼0.1 mm at the hoop stress of 300 MPa. It was suggested that the outside-in cracking during the power ramp is strongly dependent on the distribution of dissolved hydrogen as a result of the thermal diffusion.