Hydrogen effects

During reactor operations, the cladding metals, mainly Zircaloy, corrode in water. This introduces hydrogen into the Zircaloy. Hydrogen can degrade the strength of Zircaloy through overall embrittlement caused by a disper­sion of radially oriented hydrides (perpendicular to the hoop stress) (Chung, 2004). The hydrides formed during reactor operations are mostly circum­ferential hydrides (parallel to the hoop stress). Circumferential hydrides may not affect the strength significantly, depending on the magnitude of severity. However, circumferential hydrides are known to become radially reoriented in the presence of appropriate applied stress and temperature (Chung, 2004). Figure 7.12 compares hydrides oriented circumferential or perpendicular to the hoop stress and Fig. 7.13 shows ductility loss with radial hydrides (Yagnik et al., 2004).

Another hydrogen effect is delayed-hydride cracking (DHC). Small cracks that develop on the inner or outer surface of cladding may lead to crack propagation when assisted by hydrogen diffusion to the crack tip, thus forming radially oriented hydrides at the crack tip. The mechanism has not been proven to exist under dry storage conditions. Figure 7.14 shows a schematic for the mechanism of the DHC process. The crack density and size from hydrogen embrittlement of hydride reorientation and DHC can be conservatively assessed like the SCC of stainless steel described in Section 7.4.2.

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