Microstructure and radiation tolerance of molybdenum-rich glass composite nuclear waste forms

Non-active molybdenum-rich Ca/Zn glass composite materials (GCM) were prepared on the Vitrification Test Rig (VTR) in the UK. The GCM samples were subjected to Ni and Au ion irradiations to simulate the effects of alpha recoil damage and to determine how the crystallinity characteristics might affect the overall radiation tolerance of high-level wastes (HLW) generated during post-operational clean-out (POCO) operations at the Sellafield nuclear site. The typical crystal phases identified in the GCM were: powellite, ruthenium dioxide, zincochromite, zircon and cerianite. Gadolinium (a proxy for radioactive elements) accumulated mainly in powellite, zircon and cerianite crystals. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and grazing incidence X-ray diffraction (GIXRD) analyses showed that cerianite is a highly radiation-tolerant phase, whereas powellite and zircon became amorphous and swelled considerably after the Ni and Au ion irradiations. This research shows the first evidence of powellite amorphisation under heavy-ion irradiation and suggests that powellite is susceptible to amorphisation by alpha recoils in HLW materials, in contrast to previous findings. The evolution of cooling-related and irradiation-induced microcracks is also described. In HLW glass composite materials, the formation of microcracks is expected in the middle of the canister, where relatively large powellite and zircon crystals appear.