The influence of stress state on the reorientation of hydrides in a zirconium alloy

Abstract

Hydride reorientation can occur in spent nuclear fuel cladding when subjected to a tensile hoop stress above a threshold value during cooling. Because in these circumstances the cladding is under a multiaxial stress state, the effect of stress biaxiality on the threshold stress for hydride reorientation is investigated using hydrided CWSR Zircaloy-4 sheet specimens containing ~180 wt. ppm of hydrogen and subjected to a two-cycle thermo-mechanical treatment. The study is based on especially designed specimens within which the stress biaxiality ratios range from uniaxial (σ 2 /σ 1 =0) to "near-equibiaxial" tension (σ 2 /σ 1 =0.8). The threshold stress is determined by mapping finite element calculations of the principal stresses and of the stress biaxiality ratio onto the hydride microstructure obtained after the thermo-mechanical treatment. The results show that the threshold stress (maximum principal stress) decreases from 155 to 75 MPa as the stress biaxiality increases from uniaxial to "near-equibiaxial" tension.

In situ synchrotron X-ray diffraction study of hydrides in Zircaloy-4 during thermomechanical cycling

“…ppm of hydrogen; see ref. [13] for details. Two locations along the specimen gauge length were examined after a thermo-mechanical treatment: one in which only in-plane hydrides were present and another location in which out-of-plane hydrides formed.…”

“…It should be noted that plane-stress conditions exist in the thickness direction because of the thin sheet geometry of the specimens; however, a plane-strain condition can exist as a result of biaxial tension within the plane of the sheet. Finite element analyses (FEA) were performed using ANSYS to calculate the magnitude of the local principal stresses and the corresponding stress biaxiality ratios (ζ 2 /ζ 1 ) [13] . These calculations were performed under assumptions of (a) plane stress through the thickness and (b) isotropic mechanical properties at 400 °C.…”

“…For example, the observation of thermal contraction of the hydride d-spacing during heating as hydrides dissolve remains unexplained [8]. In addition, recent results indicate that stress state affects the threshold stress at which out-of-plane hydrides precipitate during cooling [13], but previous synchroton radiation studies have been limited to uniaxial tension during the thermo-mechanical cycling to induce hydride reorientation [10,31].…”

“…The purpose of this study is to analyze the d-spacing behavior for both hydride and Zr matrix phases using high energy synchrotron radiation diffraction during dissolution and precipitation of either in-plane or out-of-plane hydrides during thermomechanical treatments under an imposed multi-axial stress state. The study uses hydrided Zircaloy-4 sheet material fabricated into double-edge notched tensile specimens that create a range of stress states within the specimen when under load [13]. Particular attention is devoted to the details of δ-hydride {111} d-spacings of hydride platelet planes aligned with the platelet faces and edges during both precipitation and dissolution.…”

Delayed hydride cracking of Zr-2.5%Nb pressure tube material due to partially constrained precipitates

“…However, stress generated in the pressure tube near the oxide nodule due to its formation is multiaxial as discussed in section 2.3. Cinbiz et al [44] concluded in their work, that the state of stress influences the hydride reorientation in zirconium alloys. The radial hydrides in the pressure tube end up with the formation of through thickness cracks by DHC mechanism as discussed.…”