Influence of metallurgical variables on delayed hydride cracking in Zr–Nb pressure tubes

During service, Zr–2.5Nb pressure tubes of nuclear power reactors may be prone to suffer from crack growth by delayed hydride cracking (DHC). For a given hydrogen plus deuterium concentration there is a critical temperature (TC) below which DHC may occur. In this work, TC was measured for specimens cut from pressure tubes made in Canada (CANDU) and in Russia (RBMK). Hydrogen was added to the specimens to get concentrations ranging from 24 to 60 wt ppm. It was found that TC was higher than the corresponding precipitation temperature. The crack propagation velocity (VP), measured in axial direction, increases from a minimum at TC to a maximum at a temperature close but higher than the precipitation temperature. At lower temperatures, when hydride precipitates are present in the bulk, VP follows an Arrhenius law: VP = A exp(−Q/RT), with an activation energy Q of 66–68 kJ/mol for both tubes. The RBMK material presented lower velocities than CANDU one.