Terminal cool-down temperature-dependent hydride reorientations in Zr-Nb Alloy claddings under dry storage conditions

In order to simulate high-burnup fuel cladding degradation under various interim dry storage conditions, 250 ppm and 500 ppm hydrogen-charged Zr-Nb alloy cladding tubes were used to investigate the effect of terminal cool-down temperature on hydride reorientations and subsequent mechanical property degradations under a tensile hoop stress of 150 MPa with two cooling rates of 2.0 and 7.0 °C/min from a peak temperature of 400 °C to three respective terminal cool-down temperatures of 300, 200 and 25 °C. The cool-down tests showed that the slower cooling rate, the lower terminal cool-down temperature and the higher hydrogen content generated the larger fraction of radial hydrides precipitated during the cool-down. This may be explained by hydrogen solid solubilities for precipitation at the respective terminal cool-down temperatures, by cooling rate-dependent residence times at a relatively high temperature during the cool-down and by remaining circumferential hydrides prior to the cool-down. Ultimate tensile strengths, plastic strains and fracture modes for the tensile-tested specimens are found to be well correlated to the amount of the radial and circumferential hydrides and hydride morphologies.