Hydride Reorientation in Zircaloy-4 under Different States of Stress as Studied with In Situ X-Ray Diffraction

Abstract

Hydride reorientation can occur as a result of vacuum drying or transportation of spent nuclear fuel rods prior to dry cask storage. The elevated temperatures generate high internal gas pressure in the fuel rods, causing δ-hydride platelets to precipitate perpendicular to the hoop stress during cooling. Because the loading causes multiaxial stresses, it is of interest to elucidate the role of stress state on the threshold stress for hydride reorientation. To that end, specially designed specimens were used with a range of stress biaxiality ratios (σ1/σ2) from uniaxial tension (σ1/σ2 = 0) to near-equibiaxial tension (σ1/σ2 = 0.8). The threshold stress was determined in each case by matching the major and minor stresses (and thus the local stress state) calculated by finite-element analysis to the hydride microstructures created by the thermomechanical treatment at that specific location. Using cold-worked stress-relieved Zircaloy-4, the results show that as the stress biaxiality ratio increased from uniaxial tension to near-equibiaxial tension, the threshold stress decreased from 155 to 75 MPa. To elucidate the hydride reorientation process, hydride precipitation and d-spacing behavior were investigated in situ using synchrotron radiation diffraction. The precipitation temperature for out-of-plane hydrides was lower than that for in-plane hydrides. The δ{111} d-spacing aligned with the hydride platelet face was greater than the d-spacing of planes aligned with platelet edges. Furthermore, δ{111} planes exhibited bilinear thermally induced expansion, but only for those planes aligned with hydride plate edges. In contrast, the hydride platelet face contracted upon heating. The experimental results were explained by a reversal of stress state associated with precipitating or dissolving hydrides within α-zirconium. In addition, irradiated cladding after thermomechanical treatments was examined by synchrotron radiation diffraction at ambient temperatures. Although the hydride intensity was low for accurately determining d-spacing, the diffraction patterns indicated that β-niobium peaks present in the un-irradiated cladding were diminished after irradiation.