Precipitation of crack tip hydrides in zirconium alloys

Hydride Reorientation and Delayed Hydride Cracking of Spent Fuel Rods in Dry Storage

“…A model experiment [5] was conducted to provide evidence for the suggestion that prior creep deformation promotes precipitation of the reoriented hydrides in the radial direction. When the load was applied at room temperature or from the beginning of the thermal cycle (point A in Figure 2), the Zr-2.5Nb CB specimen experienced considerable creep deformation during a hold at 380°C for 50 hours and showed large radial hydrides all the over the cross section, as shown in Figure 5(a).…”

“…If that assumption were true, the similar distribution of reoriented hydrides would appear irrespective of the loading time changing from RT through 380°C to 250°C, and furthermore their size would be the same independent of the peak temperature changing from 310°C to 380°C, both of which are in contrast with the results shown in Figures 5 and 6. However, if the applied stress could enhance precipitation of the reoriented hydrides, thus increasing the TSSP1 temperature close to or above the terminal solid solubility for hydride dissolution (TSSD) (corresponding to point A in Figure 7) according to Kim's DHC model, [5] then the reoriented hydrides would nucleate in stressed regions such as a crack tip starting from a higher temperature close to or above the TSSD temperature (corresponding to point A in Figure 7) and grow on cooling to 250°C. This explains why many larger radial hydrides appeared on the crept Zr-2.5Nb specimen, as shown in Figure 5(a).…”

“…[4] Thus, it seems that thermal creep deformation of spent fuel rods will not affect their mechanical integrity. However, given that prior plastic deformation would promote nucleation of hydrides, [5] the creep deformation during vacuum drying would affect hydride reorientation, which thus far has not been appreciated. Furthermore, little attention has been paid to delayed hydride cracking (DHC) of spent fuel rods.…”

“…To these ends, we analyzed Tsai's creep results [4] of the spent fuel rods in dry storage for 15 years, for the former, and Simpson and Ells' observation [7] where a zirconium alloy cladding tube failed during long-term storage at room temperature, for the latter. As concrete evidence for the effect of prior plastic deformation on hydride reorientation, a model experiment was conducted using a cold-worked Zr-2.5Nb tube with 60 ppm H. [5] II. EXPERIMENTAL Tsai [4] conducted thermal creep tests with defueled Zircaloy-4 cladding segments from two pressurized water reactors-Surry at~36 GWd/MTU burnup and H.B.…”

Influence de l'hydrogène sur les mécanismes de déformation et d'endommagement des alliages de titane et de zirconium

“…Si les processus d'endommagement ductile associés aux hydrures sont relativement bien compris il n'en est pas de même pour la propagation de fissure fragile lorsque le plan d'habitat des hydrures et perpendiculaire à la sollicitation. Enfin, lors de sollicitations mécaniques à hautes températures, les hydrures peuvent être amenés à se dissoudre et à reprécipiter dans des zones de forte concentration de contrainte (en pointe de fissure) [148]. Ainsi les couplages diffusion-contrainte sont plus que jamais d'actualité dans le domaine de la déformation et l'endommagement de matériaux présentant des hydrures.…”