Zirconium Hydride Precipitation and Dissolution Kinetics in Zirconium Alloys

Hydride precipitation may impact the integrity of zirconium-based nuclear fuel cladding, both during normal operation and during extended dry storage. To better understand such degradation, a study of hydride precipitation of zirconium hydrides in Zircaloy-4 samples was performed. The samples were submitted to various thermomechanical cycles using both in situ synchrotron X-ray diffraction and differential scanning calorimetry. Results showed that as the hydrided samples were cooled at moderate to fast cooling rates, the hydrogen content in solid solution (C_SS) decreased, following the terminal solid solubility for precipitation (TSS_P) curve, reflecting hydride precipitation in the matrix. However, when the samples were held for an isothermal anneal at a fixed temperature, the C_SS continued to decrease below TSS_P and approached the terminal solid solubility for dissolution (TSS_D). This result suggests that TSS_P is a kinetic limit and that a unique solubility limit TSS_D governs zirconium hydride precipitation. Hydride precipitation rate and the degree of precipitation reaction completion between 280 and 350^◦C were obtained using differential scanning calorimetry. Using this data, a temperature-time transformation diagram for hydride precipitation in Zircaloy-4 was generated that showed that hydride precipitation is diffusion-driven under 310^◦C and reaction-driven above 310^◦C. The experimental data were fitted to the Johnson-Mehl-Avrami-Kolmogorov model and an Avrami parameter of 2.56 was obtained (2.5 is the theoretical value for the growth of platelets). Results imply that hydride nucleation occurs if C_SS is greater than TSS_P while hydride growth occurs if preexisting hydride platelets are present and C_SS is above TSS_D. Combined with existing theory, these data were used to develop the hydride growth, nucleation, and dissolution model that can simulate hydrogen behavior in Zircaloy.