Zirconium hydride precipitation kinetics in Zircaloy-4 observed with synchrotron X-ray diffraction

High-energy synchrotron X-ray diffraction was used to investigate the isothermal precipitation of δ-hydride platelets in Zircaloy-4 at a range of temperatures relevant to reactor conditions, during both normal operation and thermal transients. From an examination of the rate kinetics of the precipitation process, precipitation slows with increasing temperature above 200 °C, due to a reduction in the thermodynamic driving force. A model for nucleation rate as a function of temperature was developed, to interpret the precipitation rates seen experimentally. While the strain energy associated with the misfit between hydrides and the matrix makes a significant contribution to the energy barrier for nucleation, a larger contribution arises from the interfacial energy. Diffusion distance calculations show that hydrogen is highly mobile in the considered thermal range and on the scale of inter-hydride spacing and it is not expected to be significantly rate limiting on the precipitation process that takes place under reactor operating conditions.