The growth rate of postulated delayed hydride cracks in CANDU Zr-2.5%Nb pressure tubes is an important material property required for flaw evaluations and leak-before-break assessments. It is monitored using surveillance pressure tubes according to the requirements of the Canadian Standards Association (CSA) Standard N285.4 [1]. Radial growth rate and axial growth rate are used to calculate the propagation of delayed hydride cracks in the through-wall direction and along the pressure tube length, respectively. The axial delayed hydride cracking growth rate had been previously found to increase exponentially with inverse absolute test temperature. This dependence had been described by an Arrhenius-type regression model with one explanatory variable. As more experimental results were obtained from surveillance pressure tubes, it has become possible to assess whether there may be statistically significant effects of other variables, which should be incorporated into the representative relation for the axial delayed hydride cracking growth rate. In this paper, multi-variable regression analysis has been used to develop an improved representative model for the axial delayed hydride cracking growth rate of irradiated Zr-2.5%Nb pressure tube material. The developed model explains approximately 93% of overall observed variation in the experimental data, and therefore has better predictive capabilities than the reference regression model with test temperature as a sole predictor. The developed multi-variable model is proposed to be incorporated into the scheduled revision (2010 edition) of the CSA Standard N285.8 as the representative predictive model.

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