Abstract
Delayed hydride cracking (DHC) has been a long studied failure mechanism of zirconium alloys used as pressure tubes and nuclear fuel cladding materials. However, challenges with DHC research have persisted with regard to testing realistic cracking directions (i.e., radial cracking caused by internal pressures). In this study, new testing procedures using a three-point bend setup alleviate geometric challenges by inducing a radial outside-in crack oriented in the axial direction of irradiated and unirradiated Zircaloy-2 claddings. One part of the irradiated samples stemmed from an inner liner cladding of a fuel rod that had been in use in the Swiss boiling water reactor at Leibstadt, and the other part came from Target-11 of the Swiss spallation neutron source. Radial DHC cracks were analyzed through high-resolution neutron imaging, metallography, fractography, and finite element modeling (FEM). When observed through the precipitation patterns in neutron imaging and metallography, irradiation damage appears to impact hydrogen diffusion, where diffusion seems reduced in irradiated material compared with unirradiated material. Hydrogen quantification around arrested crack tips shows the trend of hydride diffusion during DHC with respect to temperature and unveils the influence of the liner on source hydrogen for DHC. The combination of crack velocity measurements and hydrogen quantification through neutron imaging indicate that excess amounts of hydrogen do not drastically increase the crack velocity. FEM back-calculated the threshold stress intensity factor, KIH, showing a dependence on hydrogen concentration for optimum DHC conditions with a minimum value around 6 MPa√m.
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