Hydride-induced Embrittlement of Zircaloy-4 Cladding under Plane-strain Tension

Open Access
Author:
Daum, Robert Scott
Graduate Program:
Materials
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 21, 2007
Committee Members:
  • Arthur Thompson Motta, Committee Chair
  • Donald Albert Koss, Committee Member
  • Paul Raymond Howell, Committee Member
  • Clifford Jesse Lissenden Iii, Committee Member
  • Michael C Billone, Committee Member
Keywords:
  • hydrides
  • hydrogen
  • embrittlement
  • Zircaloy-4
  • phases
  • synchrotron diffraction
  • tension
  • ductility
  • fracture
Abstract:
The mechanical response of high-burnup Zircaloy-4 fuel cladding subjected to a postulated reactivity initiated accident (referred to as a rod ejection accident (REA) in a pressurized water reactor) can be affected by hydrogen embrittlement. This study addresses the hydrogen embrittlement of non-irradiated, stress-relieved Zircaloy-4 cladding under conditions (state of stress and temperature) relevant to those of a reactivity initiated accident. Specifically, the study has investigated the effects of a concentrated density of hydride particles (in the form of a rim at the outer surface of the cladding tube introduced by gas-charging) on the cladding ductility when tested under a near-plane-strain tension at 25, 300, and 375°C. The influence of the hydride-rim thickness and local hydrogen contents on cladding ductility is studied as a function of temperature and correlated with the hydride microstructure. Using synchrotron x-ray diffraction, this study has found that the δ-hydride phase (i.e., ZrHx, where x ≈ 1.66) is the predominant hydride phase to precipitate in stress-relieved Zircaloy-4 cladding for hydrogen contents up to 1250 wt ppm. At hydrogen contents above 2700 wt ppm, although δ-hydride is still the majority phase, both - and ε-hydride phases are also observed. The volume fraction of hydrides was estimated as a function of hydrogen content, using the diffracted x-ray intensities. These estimated values agree well with calculated values assuming hydride precipitates are δ-hydride. Under near-plane-strain hoop tension, the ductility and fracture of the cladding is highly dependent on both the hydride-rim thickness and the testing temperature. At room temperature, due to a high density of hydride particles within the rim, a Mode I crack is injected shortly after yielding. This limits cladding ductility, such that it decreases with increasing thickness of the hydride rim. Cladding containing hydride rims with a thickness of ≥100 μm was thus macroscopically brittle (the macroscopic failure strain was small) as the result of the initiation and propagation of a Mode I (i.e., tensile) crack through the thickness of the cladding. Crack growth occurred due to void initiation at fractured hydride particles and subsequent strain-induced coalescence. Mode I cracks were also observed at 300°C within the hydride rim, but the substrate failed by a mixed Mode I/II crack with no signs of void nucleation, as the hydride particles in the substrate resisted fracture. Macroscopically brittle behavior occurred for cladding with hydride rims thicker than ≈170-μm. In contrast, at 375°C, materials with rim thicknesses up to 260 μm were ductile and failed due to localized necking. As a result, the effect of hydrogen on ductility at this temperature is small. Also, at this highest temperature, small Mode I cracks were occasionally observed within the hydride rim; these cracks were associated with high local hydrogen contents (>4000 wt ppm) and the presence of the tetragonal ε-hydride phase near the outer surface, suggesting that this hydride phase is highly brittle at all temperatures of this study. This study also tested specimens with a uniform distribution of hydrides (containing 2200-wt-ppm hydrogen) in order to compare their behavior to that of hydride-rim specimens. Uniformly-hydrided specimens containing ≈2200-wt-ppm hydrogen tested at 300°C showed the initiation of Mode I cracks and macroscopically brittle behavior, similar to that of the hydride-rim cladding. However, when tested at 375°C, cladding with ≈2200-wt-ppm hydrogen showed comparable macroscopic ductility (>4% uniform hoop strain) and fracture (i.e., plastic instability) to that of non-hydrided cladding, suggesting that this cladding is macroscopically ductile. The difference between material behavior at 300 and 375°C indicates that the survivability of cladding during a reactivity initiated accident may depend critically on the exact cladding temperature at maximum load.