Effects of Temperature and Thermal Cycling on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes

CANDU™ reactor uses Zr-2.5 Nb alloy pressure tubes as the primary coolant containment. The pressure tubes are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, hydrided region formation and fracture at a crack-tip. The threshold stress intensity factor for DHC initiation from a crack, K_IH, is an important material parameter for assessing DHC initiation from crack-like or blunt flaws in the pressure tubes. Tests were performed on compact tension samples machined from hydrided unirradiated pressure tubes over the temperature range of 105°C and 286°C to determine the effect of test temperature on K_IH. The results indicated that K_IH values increased by about 10% from 105°C to 200°C. From 200°C to 286°C, a larger increase in the order of 30% was observed. The crack-tip hydrides when K_IH was reached at different test temperatures were examined by metallography which showed that the hydrides had a taper shape and increased in both thickness and length with increasing test temperature. The test results were discussed in terms of the temperature dependence of material properties of the zirconium matrix and zirconium hydrides, as well as the hydride formation process. Implication of the experimental observations on the hydride fracture criteria was discussed. Another set of K_IH tests was performed on C-shape hydrided unirradiated pressure tube samples to obtain K_IH values under thermal cycling conditions. The test results were compared to K_IH values obtained under isothermal condition at a test temperature of 200°C. The results indicated that the difference in K_IH values was small and can be accounted for by the dependence of K_IH on temperature.