Pierre-Clément Simon

This study focuses on the precipitation of nanoscale hydrides in polycrystalline zirconium as a first step to predicting the hydride morphology observed experimentally and investigating the mechanisms responsible for hydride reorientation at the mesoscale. A quantitative phase-field model, which includes the elastic anisotropy of the nanoscale zirconium hydride system, is developed to investigate the mechanism of hydride reorientation in which the presence of an applied hoop stress promotes hydride precipitation in grains with basal poles aligned with the circumferential direction. Although still elongated along the basal plane of the hexagonal matrix, nanoscale hydrides growing in grains oriented perpendicular to the applied stress appear radial at the mesoscale. Thus, a preferential hydride precipitation in grains with basal poles aligned parallel to the applied stress could account for mesoscale hydride reorientation. This mechanism is consistent with experimental observations performed in other studies.

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 (CSS) decreased, following the terminal solid solubility for precipitation (TSSP) curve, reflecting hydride precipitation in the matrix. However, when the samples were held for an isothermal anneal at a fixed temperature, the CSS continued to decrease below TSSP and approached the terminal solid solubility for dissolution (TSSD). This result suggests that TSSP is a kinetic limit and that a unique solubility limit TSSD 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 CSS is greater than TSSP while hydride growth occurs if preexisting hydride platelets are present and CSS is above TSSD. Combined with existing theory, these data were used to develop the hydride growth, nucleation, and dissolution model that can simulate hydrogen behavior in Zircaloy.