SIZHYP

The oxidation of zirconium alloys occurring in light water nuclear power plants entails the adsorption of hydrogen that lead to hydrides precipitation when the terminal solid solubility (TSS) is reached. The heterogeneous material thus formed may have lower mechanical properties than the hydride-free material due the reputedly brittle character of hydrides. The well known hydrides microstructure is composed by needles aligned along dense directions for small hydrides (length below 1 micron). When an external stress is applied, hydrides precipitate preferentially perpendicularly to the stress direction which makes the composite material particularly brittle. Although having a longstanding knowledge of these facts, the scientific community lacked a global understanding on the microscopic scale mechanisms involved. This was due in our opinion to the partial knowledge of the precipitation sequence and to the thorough use of homogeneous elasticity models that assume that the matrix and precipitates have the same elastic constants. The recent discovery of the zheta Zr2H phase, coherent with the matrix and the demonstration that heterogeneous elasticity may provide a comprehensive description of the elastic interaction between precipitates and the matrix question old beliefs and may give rise to an improved prediction of the precipitation kinetics in a context of increased fuel burnup and safety concerns. The aim of this project is to gather industrial and academic active groups in the field in France and abroad through a collaboration with the group of M. Preuss at Manchester university in order to confront multi-scale modeling with state of the art in situ experiments of precipitation under a scattering beam ( Xray or electrons). Through a strong and willing collaboration we may expect to remove fundamental obstacles improving the overall comprehension on the subject. The project is divided in different tasks among which are , on the modeling side: development of atomic scale thermodynamics models to describe the Zr-H system atomic scale study of the precipitation using tight binding potentials atomic scale modeling of the coherency loss of hydrides and its consequence on elastic energy relaxation meso scale modeling of the precipitation using phase field techniques in the framework of the heterogeneous elasticity. Assessment of the influence of an applied stress. and on the experimental one Transmission Electron Microscopy study of the coherency loss and in situ precipitation study of precipitation with and without an applied stress in situ at a synchrotron source. comparisons between experiments and modeling will be performed during the whole project.