Microstructure and mechanical properties of Zircaloy-4 cladding hydrogenated at temperatures typical for loss-of-coolant accident (LOCA) conditions

The series of single rod tests was performed at KIT in framework of the new QUENCH-LOCA programme to investigate the properties of Zircaloy-4 claddings hydrogenated at temperatures of 900, 1000, 1100, and 1200 K to hydrogen contents between 600 and 10,000 wppm H. The impact of simultaneous annealing, phase transformation and hydriding processes on the material properties was investigated. Changes in microhardness revealed the distinct transition from annealing softening to hydrogen hardening. The intermediate stages of structure transformation during hydrogenation phase were registered. The X-ray diffractometry (XRD) analysis was applied to observe the existing phases in the tested samples including possibly precipitated hydrides as well as change in the lattice parameters a and c. The presence of γ- and δ-hydrides was clearly indicated by this method; however the dimensions of hydrides correspond rather to the nano-scale and could not be observed by optical microscopy. The evolution of XRD peak intensities and peak shift was analysed to estimate the texture change and concentration of dissolved hydrogen correspondingly. Tensile test examination of tube samples at room temperatures showed significant reduction of their residual ductility already at 700 wppm H.