A study on the effects of hydrogen content and peak temperature on threshold stress for hydride reorientation in Zircaloy-4 cladding

Hydride reorientation is one of the crucial degradation mechanisms of cladding integrity under dry storage conditions. Recently, it has been experimentally reported that the threshold stress triggering the reorientation depends on various factors synergistically such as temperature, thermal history, hydrogen content, thermal cycling, and stress biaxiality. In this study, the combined effects of the hydrogen content and peak temperature on the threshold stress were studied using ring tension tests with a Zircaloy-4 cladding tube. To simulate the thermo-mechanical history of the cladding during interim dry storage, pre-hydrided specimens with hydrogen concentrations up to 585 wppm were tested following a transient temperature from a peak temperature, 350 °C or 400 °C, to room temperature. Results show that the threshold stress reaches a minimum when the hydrogen content is the terminal solid solubility of dissolution (TSSD) at a given peak temperature. The minimum stress was found to be 52 MPa and 75 MPa at 400 °C and 350 °C, respectively. With increasing hydrogen concentration, the threshold stress decreases in the regime of the solid solution state, whereas the stress steadily increases and then tends to saturate to a certain value in the regime of the supersaturated state. A qualitative model is presented to explain these findings on the basis of the mechanistic understanding of stress-driven hydrogen transport, memory effect, and radial hydride re-precipitation.