The Effect of Load Reduction on Crack Initiation Behavior of Hydrides From Flaws in Zr-2.5Nb Pressure Tube Material

Flaws in Zr-2.5Nb alloy pressure tubes in CANDU^® nuclear reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC). DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region, and fracture of the hydrided region at the flaw tip. One scenario of crack initiation is that the flaw-tip hydrides are formed and cracked at the same stress under constant reactor operating pressure. This is known as crack initiation under constant-load condition. Another scenario of crack initiation is that the flaw-tip hydrides are formed at the operating pressure and then cracked during a transient over-pressure. This is known as crack initiation under overload condition as the hydrides are subjected to a stress higher than the hydride formation stress. In some CANDU reactors, a 20% reduction in pressure is implemented during reactor cool-down. This paper examines the effect of pressure reduction, and hence load reduction, on flaw-tip hydride morphology and crack initiation behavior under constant-load and overload conditions. Experiments were performed on specimens of an unirradiated Zr-2.5Nb pressure tube, with 57 wt. ppm hydrogen concentration. The specimens contained machined V-notches with a root radius of 0.015 mm to simulate service-induced debris fretting flaws. The results indicate that the 20% load reduction increases the threshold stresses for crack initiation under constant-load and overload conditions. Finite element stress analyses were performed to determine the notch-tip stress distribution under constant-load and 20% load-reduction conditions. The load reduction lowers the notch-tip peak stress and shifts its location away from the notch surface. This is consistent with the notch-tip hydride morphologies observed using optical and scanning electron microscopy.