Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

The first step for delayed hydride cracking in zirconium alloys

“…Puls [78] one of the purposes of which was to provide an updated version for the model for DHC as originally proposed by Dutton and Puls [16][17][18][19] by using the sign convention for tensile stress used in later versions of the model. This paper demonstrated the physical incorrectness of a model for DHC propagation proposed recently by Kim and co-workers [21,22,[71][72][73][74][75][76][77]. It also provides a rebuttal to their model in which it is suggested that hydrogen movement down a gradient of chemical potential is irrelevant and spontaneous and preferential precipitation of hydride at the crack tip upon load application is the first step for each successive stage of DHC.…”

“…In a series of papers Kim [21,22,[70][71][72][73][74][75][76][77] has criticized this diffusion first model on the following grounds:…”

Comment on ‘The first step for delayed hydride cracking in zirconium alloys’ by G.A. McRae et al., J. Nucl. Mater. 396 (2010) 130–143

“…Regarding the crack tip concentration, Puls has assumed that it would be lowered to the heating solvus or TSSD [22,23]. Kim's model also demonstrates that it is reduced to the TSSD [2][3][4][5][6][7]. However, by claiming that the TSSP would not change with stresses, the authors insist that the crack tip solubility would correspond to the TSSP, not to the TSSD [1].…”

“…McRae et al [1] submitted a paper entitled ''The first step for delayed hydride cracking (DHC) in zirconium alloys" claiming that the first step of DHC is hydrogen diffusion, not nucleation of hydrides as demonstrated by Kim's new model [2][3][4][5][6][7]. They have claimed that the driving force for hydrogen diffusion is the stress gradient and the crack growth rate (CGR) or DHC rate is governed only by hydrogen diffusion.…”

“…As it is later known that the molar volume of hydrogen is the same between zirconium and hydride [14], however, the crack tip solubility cannot be lowered due to the stress without precipitation of hydrides. This is the idea of Kim's model [2][3][4][5][6][7] that has been proposed for the first time. No one but Kim has come up with this new idea that precipitation of hydrides is the first step, resulting in the difference in hydrogen concentration between the bulk and a crack tip.…”