Characterisation of high burnup spent mixed oxide fuel after leaching at hyperalkaline pH in a reducing environment

Microstructural investigations of a Zircaloy-4 clad mixed oxide fuel rod segment with a burnup of 50.3 GWd/tHM after exposure to a synthetic young cementitious water at a hyperalkaline pH of 13.7(2) under reducing atmosphere for about 3.5 years were conducted. A deposit on surfaces exposed to the leachate was observed and attempts were made to identify its composition. Initial scanning electron microscopy/energy dispersive X-ray spectroscopy results indicate that it contains several leachate constituents, as well as Zr and Ti. The presence of Ti indicates corrosion of the autoclave liner during the leaching experiment. Surfaces which were not directly exposed to the leachate revealed additional features, such as distinct ε-particles. Remarkably, these could not be identified on surfaces exposed to the leachate, which may indicate that dissolution of the ε-particles took place as well.