Nuclear power is one of the most controversial forms of alternative energy, partly because of the challenges of storing and disposing the radioactive waste it produces. But what if there was a way to turn this waste into a durable and stable material that can be safely buried underground for thousands of years?
That’s the idea behind vitrification, a process that converts liquid and chemical waste into solid glass form. Vitrification has been used for over 40 years in most countries that have a nuclear power program, including France, UK, Germany, Belgium, Russia, USA and Japan. Vitrification is the processing and transformation of the spent fuel into a glass.
To produce the glass, the waste is dried, heated to convert the nitrates to oxides, and then mixed with glass-forming chemicals and heated again to very high temperatures (approximately 1000 °C) to produce the melt. This is then poured into a stainless steel canister where it cools to form a glass. The canister can then be sealed, decontaminated, and placed into a long-term storage facility.
But how do we know that the glass will remain intact and not release harmful radionuclides to the environment over time? That’s where standard methods to assess the durability of vitrified radioactive waste come in. These methods were first developed in the 1980’s and have evolved to yield a range of responses depending on experimental conditions and glass composition.
One of these methods is the product consistency test (PCT), which measures how much of the glass dissolves in water over a certain period of time. Another method is the vapor hydration test (VHT), which exposes the glass to water vapor at high temperatures and pressures. A third method is the EPA Method 1313 test, which simulates different environmental scenarios such as acidic or alkaline conditions.
With the implementation of subsurface disposal for vitrified radioactive waste drawing closer, it is timely to review the available standard methodologies and reflect upon their relative advantages, limitations, and how the data obtained can be interpreted to support the post-closure safety case for radioactive waste disposal.
One way to do that is to compare the results from laboratory tests with the actual corrosion behavior of vitrified materials in natural environments. For example, researchers have studied vitrified archeological samples from a ~1500-year-old Iron Age Swedish hillfort as an analog for nuclear waste glass disposal. They found that some features of the surface layer formed by VHT were similar to those formed over longer timescales in the environment.
However, using archeological samples as analogues has its limitations, as their precise corrosion environment and composition are unknown and may differ from modern industrial glasses. Therefore, more research is needed to understand the mechanisms of glass dissolution, develop better models and tests, and explore alternative glass compositions.
One promising option is iron phosphate glasses, which have been proposed as an alternative to borosilicate glasses for nuclear waste vitrification. They can accommodate higher waste loadings and have comparable or better chemical durability. This results in a higher waste loading than that achievable with borosilicate glasses.
Vitrification is not a perfect solution for nuclear waste management, but it is a proven and effective one that can reduce the volume, toxicity, and mobility of radioactive waste. By using glass as a medium for nuclear waste immobilization, we can ensure a safer and cleaner future for ourselves and generations to come.
Relevant articles:
– Forty years of durability assessment of nuclear waste glass by standard methods, npj Materials Degradation, 20 December 2021
– Applying laboratory methods for durability assessment of vitrified material to archaeological samples, npj Materials Degradation, 12 November 2021
– Using Glass for Nuclear Waste Vitrification, AZoM.com, 18 June 2019