Measurement of Hydride Precipitation and Dissolution Kinetics Using Synchrotron X-Rays

Structural components of nuclear reactors made from zirconium alloys are subject to degradation mechanisms associated with hydrogen pickup during their operating life. Even small amounts of hydrogen isotopes (∼tens of wt. ppm) can significantly reduce the material’s fracture toughness and make it susceptible to a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC). The mitigation of potentially costly failures of these components requires assessments based upon not only the bulk concentration of hydrogen, but the concentration of hydrogen precipitated as hydrides, and in solution. The behaviour of the zirconium-hydrogen system near ∼100 ppm hydrogen concentration continues to be the subject of research, primarily due to its otherwise complex behaviour and critical role in nuclear plant operations. There is an anisotropic volumetric expansion associated with the precipitation of zirconium hydride, and this precipitation event results in plasticity in the surrounding matrix and compressive strain in the hydride phase. This leads to both a pronounced solubility hysteresis upon dissolution and precipitation, as well as variances in solubility behaviour depending on the prior thermal and mechanical history. In the present study, high-energy synchrotron x-ray diffraction is used to study the evolution of hydrogen solubility in Zr-2.5 wt% Nb pressure tube material with different hydrogen concentrations in situ during thermal cycling between 100 and 400°C. This technique provides the ability to directly measure the amount of hydride in a given sample at different temperatures, and to evaluate zirconium-hydrogen precipitation and dissolution kinetics.