Using Glass for Nuclear Waste Vitrification

For over 60 years nuclear power has been used to generate electricity and is responsible for around 16% of the electrical energy produced globally. This percentage is presumably going to increase, as countries depart from the use of fossil fuels in the pursuit of an energy source that will make up any shortfall this will cause.

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With more and more nuclear reactors becoming active and increases in fuel consumption, an issue is raised: there is no conclusive method to handle the spent fuel. Vitrification of nuclear waste in glass is one accepted solution, yet there is still room for improvement in this method.

The Issue of Nuclear Waste

One major issue with nuclear power is what to do with the waste products when the fuel has been exhausted. Although no longer suitable to generate power, this waste is extremely dangerous if stored incorrectly.

Highly radioactive and the most problematic waste, known as ‘high level’ nuclear waste (HLW), has an extremely long half-life that necessitates cooling and containment due to the elemental decay, which significantly emits heat and radiation.

Furthermore, numerous radioactive isotopes such as Tc-99, Se-79, and I-129 are mobile in water, which requires additional measures so that their ability to move into the groundwater is reduced. Secondary waste streams can also present issues as this waste can contain large amounts of molybdenum and noble metals.

Nuclear Vitrification in Glass

One method of long-term storage and disposal is using glass for nuclear waste vitrification. HLW immobilization 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.

The relative insolubility of glass makes it desirable as a form for long-term storage as well as it being a compact and solid material. This form allows for easier storage and handling, thus saving space and reducing costs. Glass also possesses high chemical durability, which means that it can remain in a corrosive environment for thousands or even millions of years without being compromised. Often thought of as a fragile material, a properly treated block of borosilicate glass is incredibly resilient.

Process of Nuclear Waste Vitrification in Glass

Vitrification as a process is quite simple but the execution can prove to be difficult. Firstly, waste is dried, then heated to convert the nitrates to oxides. Glass-forming additives are blended with the waste material and heated again to around 1000 °C. Molten liquid is then poured into an appropriate containment vessel that will cool and form the glass. Once solidified, the final vitreous product has consolidated the waste contaminants in its macro- and micro-structure, immobilizing the hazardous waste constituents.

Borosilicate and aluminophosphate are the two main types of glass currently used to immobilize nuclear waste. Both of these materials allow for high waste loadings and can render large amounts of actinides inactive. For example, borosilicate glasses can house up to 7.2 mass pct of PuO₂.

Advantages and Limitations of Nuclear Waste Vitrification in Glass

Despite the fact that it is often the preferred method of waste storage, the current glass vitrification technique has its drawbacks, both with the requisite setup and materials used. The initial investment cost is high as well as increased operational costs and complex technology requiring qualified personnel for the process of vitrification.

This makes it most economically viable in locations where the availability of radioactive waste with stable composition is in relatively large volumes, such as HLW from nuclear power plants.

The current generation of glasses cannot deal with large amounts of MoO₃ and noble metals that come from secondary waste streams. The amount of waste that can be loaded into the material is limited due to these compounds being poorly soluble in borosilicate glasses; increasing process time and material costs.

Mo-Sci Leading Glass Manufacturer

For the long-term success of nuclear power it is clear that vitrification is of vital importance. Recognizing that there must also be improvement in the materials, Mo-Sci has started working on new types of glass that can immobilize a higher percentage of waste, as well as developing methods within the processing that can speed up the vitrification.

This includes an iron phosphate waste form with the ability to contain 40 wt% of the simulated molybdenum-rich nuclear waste. This nuclear waste vitrified glass is prepared by melting the mixture of simulated waste components and iron phosphate glass additives in a commercial-scale cold crucible induction melter (CCIM). When the waste form was measured for chemical durability, it was determined to be as good as or better than that of borosilicate glass.

The CCIM melting technology can also accelerate the processing of waste forms: as it removes the metal electrodes that directly contact the molten glass and refractory used to contain the melt, it is also safer and less costly than other melting technologies.

This innovative CCIM-processed iron phosphate waste form could instigate great savings in time and money relative to the industry’s need to remediate nuclear waste, and strengthen the appeal of nuclear power as a choice for the future.

References

• Thompson, L. (2010) Vitrification of Nuclear Waste http://large.stanford.edu/courses/2010/ph240/thompson2/
• Criscenti, L. et al. (2013). An international initiative on long-term behavior of high-level nuclear waste glass. Materials Today, 16 https://doi.org/10.1016/j.mattod.2013.06.008
• Ojovan, M. I., & Lee, W. E. (2011). Glassy Wasteforms for Nuclear Waste Immobilization, 42 https://doi.org/10.1007/s11661-010-0525-7
• Ojovan, M. I. (2007). Glasses for Nuclear Waste Immobilization, WM’07 Conference. https://www.researchgate.net/publication/267700284_Glasses_for_Nuclear_Waste_Immobilization/download
• Cheol-Woon, K. (2018) Iron Phosphate Waste Forms for Nuclear Waste Disposal https://mo-sci.com/iron-phosphate-nuclear-waste-disposal

This information has been sourced, reviewed and adapted from materials provided by Mo-Sci Corp.

For more information on this source, please visit Mo-Sci Corp.