Analytical Report on Evaluation of DHC Models

CSA N285.4/N285.8 that was revised in 2005 by the Canadian government stipulates the DHC evaluation procedures to be conducted as one of in-service inspection of Zr-2.5Nb tubes with surface flaws operating in reactors. Given that they are set up 'based on an incomplete understanding of DHC and old delayed hydride cracking models which are known to be very defective, thus, there is a strong motive to develop a theoretically valid DHC model and further revise the DHC evaluation procedures based on it. During DHC, a crack grows when nucleation, growth and cracking of hydrides should occur consecutively at a crack tip. Nevertheless, all the old DHC models claimed that the DHC rate was governed only by hydrogen diffusion, yielding many unresolved issues related to DHC. In contrast, Kim's DHC model that is advanced lately can account for DHC in zirconium alloys in terms 0 kinetics, and the effects of hydrogen supersaturation or ΔC and crack tip stress state on DHC rate both, leading to a reduction of hydride nucleation rate. Furthermore, it explains first that the positive temperature dependence of DHC rate below 300 .deg. C occurs due to the hydride growth rate being the slowest process among the three processes involved in DHC but the negative temperature dependence of DHC rate occurs due to the rate of hydride cracking becoming the slowest above 300 .deg. C when compared to the other two processes. Hence, given that the old DHC models turn out to be imperfect and incorrect, the DHC evaluation procedures set up by the old DHC models, which are stipulated in CSA N285.4/N285.8 issued since 2005, should be revised in accordance with Kim's new DHC model. For example, the DHC evaluation procedures for 'a blunt flaw stipulate that DHC occurs when the crack tip concentration increases to the terminal solid solubility for hydride dissolution (or TSSD) due to the increment of hydrogen concentration caused by the stress effect despite the bulk concentration being below TSSD. However, this criterion is too conservative and unrealistic because the increment of hydrogen concentration at the crack tip due to the stress effect cannot occur in such a closed system as zirconium alloys where hydrogen cannot diffuse toward the stressed region from the unstressed region or the bulk due to the stress gradient. Hence, part of DHC evaluation procedures described in the CSA N285. 4 and N285.8 need to be revised in accordance with Kim's DHC model