Radiation Effects on Nuclear Waste Forms—How Does the Crystallinity of a Glass-Ceramic Affect Radiation Tolerance?

Radioactive waste from the nuclear industry can contain molybdenum which can be immobilized by incorporating it into a borosilicate glass-ceramic. The molybdenum partitions into a durable crystal phase known as powellite (CaMoO₄) [1]. To be able to predict the durability of this waste over timescales of the many thousands of years relevant to geological disposal, the effects of radiation on high molybdenum content glass-ceramic nuclear wastes must be understood. The amorphisation of crystals accompanied by volume change could lead to cracking and a higher radionuclide leaching rate to the environment. This information is essential in the process of determining long-term safe disposal conditions and will be invaluable to the community, industry, and other stakeholders.<br/>Non-active nuclear waste simulant glass-ceramic samples with various compositions and different crystallinity were produced by National Nuclear Laboratory. X-ray diffraction and energy-dispersive X-ray spectroscopy measurements revealed the presence of five common crystal phases (powellite, zircon, ceria-zirconia, zincochromite and ruthenium dioxide). Electron microprobe analysis shows that rare earth elements (surrogates for radioactive actinides) mainly accumulate in powellite, zircon and ceria-zirconia. The size and morphology of powellite crystals varies based on their positions in the product container due to the different rates of cooling. Cracks formed in powellite and around zircon crystals, presumably due to a thermal expansion mismatch between the glass and these crystal phases.<br/>Nickel and gold ion irradiation experiments were performed and subsequent analysis using scanning electron microscopy and electron backscatter diffraction measurements reveals that ceria-zirconia, zincochromite and ruthenium dioxide are highly radiation-tolerant phases while powellite and zircon appear to amorphise and swell considerably. The swelling of crystals was more substantial in the gold irradiated sample. Powellite has previously been described as a highly radiation-tolerant material [2]; therefore, further ex-situ and in-situ transmission electron microscope analyses will be performed to fully understand the irradiation-induced microstructural effects on powellite. While these heavy ion irradiation experiments are representative of changes induced by the alpha recoil nucleus, ongoing Raman and electron paramagnetic resonance spectroscopy measurements on He-irradiated glass samples will examine alpha particle irradiation effects in the base glass. Overall, this research will provide new insights into the behaviour of glass-ceramic nuclear wastes under conditions of relevance to long term temporary storage and disposal.<br/><b>REFERENCES</b><br/>[1] M. T. Harrison, “Current challenges in the vitrification of nuclear wastes in the UK” Vitrogeowastes, Chapter 3, pp. 33-76, 2017.<br/>[2] X. Wang et al., “Irradiated rare-earth-doped powellite single crystal probed by confocal Raman mapping and transmission electron microscopy,” J. Raman Spectrosc., vol. 45, pp. 383-391, 2014.