Hydride induced deferred fractures in Zr-2.5% Nb pressure tubes

In the presence of hydrogen and tension gradients Zr alloys suffer a fracturing process from the precipitation of hydrides known as hydride induced deferred fracture (HIDF). This type of failure can be produced in pressure tubes of nuclear centers, built with Zr-Nb alloys, while operating (temperatures of around 250oC-315oC and pressures of 10 mpa) or when stopped. For a given concentration of H2 plus D2 there is a critical temperature (Ct) below which HIDF can occur. This work evaluated that temperature in test tubes extracted from CANDU (Canadian) and RBMK (Russian) pressure tubes. The hydrogen concentration was increased through an electrolytic charge up to values of 35 to 72 ppm. The critical temperatures (Ct) are higher than the hydride precipitation temperatures for each concentration. The fissure propagation speed (Sp) in the tube's axial direction was also measured. For temperatures below Ct the speed increases until it reaches its maximum value at precipitation temperature. Below these an Arrhenius type behavior was corroborated: Vp=A.exp.(-Q/RT). The propagation speed was less with the Russian tube than with the Canadian one for the same concentration, in all temperature ranges studied. The parameter Q was similar in both and was close to the sum of the activation energy for the precipitation of hydrides plus the activation energy for the diffusion, reported by the CANDU material (cw)