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Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr-2.5Nb tubes that had previously been investigated with Dutton and Puls’s DHC model. At temperatures over 180 °C along with a hydrogen concentration of over 15 ppm H, the DHC initiation in a CANDU Zr-2.5Nb tube was suppressed, which required a cooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures. With the number of the thermal cycle increasing, the DHC initiation temperatures or Tcs gradually shifted towards the TSSD. At a hydrogen concentration as low as 7 ppm H and temperatures lower than 180 °C, a DHC initiation occurred at temperatures near the TSSD with little ΔT. Different DHC initiation temperatures with hydrogen concentrations are discussed in view of precipitation of hydrides in the zirconium matrix either by a cooling or by a stress-induced γ- to δ-hydride transformation. The DHC arrest temperatures were governed by the critical supersaturated hydrogen concentration or ΔC regardless of the thermal cycle treatment. By correlating the DHC cracking and arrest temperatures with the supersaturated hydrogen concentration or ΔC for the DHC cracking and arrest, we conclude that the ΔC arising from the hysteresis of the terminal solid solubility of hydrogen on a heat-up and on a cool-down is the driving force for the DHC.

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Threshold stress intensity factor or KIH required to initiate a delayed hydride crack in the axial direction of a Zr-2.5Nb tube was determined at temperatures of 160 oC to 280 oC. KIH remained constant at temperatures from 160 to 250 oC, increased with an increasing temperature in excess of 280 oC using a load decreasing method. To correlate KIH and dislocation twins, tensile tests were conducted on the Zr-2.5Nb tube and textural changes during tensile tests were investigated using an X-ray diffractometer. The extent of the twins increased from 150 to 300 oC and then decreased at temperatures in excess of 300 oC with no twins occurring at 350 oC. Temperature dependencies of KIH and a temperature limit for delayed hydride cracking were discussed with cracking of hydrides by the twins. This study provides supportive evidence to the feasibility of Kim’s DHC model.