Evaluation of a delayed hydride cracking in Zr–2.5Nb CANDU and RBMK pressure tubes

Delayed hydride cracking (DHC) has been identified as a cause for the failure of zirconium-based alloy pressure tubes in CANDU and RBMK reactors. In this paper, a delayed hydride cracking (DHC) was investigated using compact toughness specimens in order to determine the crack growth velocity in CANDU and RBMK Zr–2.5Nb pressure tubes under a constant loading, and different temperatures and different hydrogen contents. The fractured surface was examined and the velocity of a DHC for different materials was measured and analyzed. The results showed that the velocity of the RBMK pressure tube was lower and it had a longer incubation time at the same hydrogen content when compared to those of the CANDU pressure tube, which could be attributed to a lower yield stress in the RBMK tube. On the other hand, there was no continuous β-phase film in the grain boundaries of the CANDU tube. The formation of a striation on the fractured surface indicated that the crack growth of a DHC was discontinuous due to hydrogen diffusion and its subsequent accumulation at the tip of a crack.