World of pane for nuclear waste - vitrification for safe disposal

Materials World magazine
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25 Sep 2012

Austere-looking cement containers are the current storage method for intermediate level nuclear waste in the UK.   

This isn’t the only method available. Several processes that turn radioactive intermediate level waste into glass have been developed to an inactive pilot-scale level in the past decade, and high level waste is processed using this technology. Inspired by this approach, a research team led by the University of Sheffield, and funded by the UK’s Nuclear Decommissioning Authority and Royal Academy of Engineering, claims to have developed a way to vitrify radioactive waste for safe, long-term disposal that is compatible with various UK industrial level waste streams.    

Lead researcher Professor Neil Hyatt explains, ‘By heating radioactive wastes and glass forming chemicals at high temperature, we can lock the radioactive species within the glass structure, making a durable material which can be safely disposed of.’    

The present process involves mixing specially formulated cement with waste materials and sealing them. ‘The existing method of cementation effectively dilutes the waste by simply encapsulating it, so the overall waste volume increases,’ he adds.    

Hyatt says vitrification, which creates a glass similar in composition to windowpanes or pyrex cookware, is more efficient for several reasons. ‘The high temperatures employed in vitrification mean that we destroy any organic component in the waste, remove water, oxidise any reactive metals, and the radionuclides become trapped in the glass at the atomic level. This allows us to concentrate the radionuclides, meaning the overall waste volume should be reduced.’    

His team has used a materials informatics approach to select the best glass composition for any given type of waste. ‘By understanding the chemistry of the waste stream, we can use waste materials themselves, such as contaminated sand or radioactive liquids, to supply some of the glass-forming components, such as silicon and sodium, to achieve a lower product volume.’    

The vitrification process itself is simple. The waste is combined with the appropriate additives, and is then heated at 1,000–1,200°C. The resultant glass traps radionuclides in the structure at an atomic level.    

‘In principle, we could apply this approach to almost any type of waste by modifying the glass composition or processing conditions to match the waste characteristics,’ Hyatt says. ‘For example, let’s say we were dealing with plutonium contaminated materials, which typically have a high metallic component such as glove boxes and fuel cladding – we could design the process to scavenge the plutonium and incorporate it into the glass, leaving a decontaminated metal fraction that could be disposed of as low level waste, or even reused.’ 

 

 

The researchers have used iron to fortify the glass further. According to Hyatt, the presence of iron can protect the glass against the build-up of defects caused by y-radiation interaction by flipping the oxidation state.    

While a full-scale inactive demonstration of the process has been carried out, Hyatt admits there is still much to do. ‘We need to understand the mechanisms controlling radionuclide solubility in glasses and volatilisation in more detail, and we need to understand the long-term stability of these materials in anticipated disposal environments.’    

However, he adds, ‘This is a technology which could really help accelerate the timescale of nuclear waste clean up, decommissioning and disposal’.