Toshiba : Dry Process Applied for Reductive Cleavage of Vitrified Wastes (Toshiba Energy Systems & Solutions Corporation)

Toshiba Energy Systems & Solutions Corporation
Japan Science Technology Agency (JST)
Director General for Science, Technology and Innovation Policy, Cabinet Office

Points

・Reversible and retrievable process established regarding long-term geological disposal
・Studies under completely different conditions from high-level-radiation waste disposal
・Dissolve network structures of vitrified radioactive wastes to separate reusable elements
・Can ease constraints on geological disposal sites when used with nuclear transmutation

Kawasaki, Japan - Toshiba Energy Systems & Solutions Corporation today announced that it has established redox treatment technology for metal-oxide to minimize secondary radioactive wastes from nuclear power plants(*). The technology takes advantage of molten salt¹⁾ to isolate four elements containing long-lived fission products (LLFPs²⁾) into metal from vitrified radioactive waste³⁾.

The research was conducted under the ImPACT Program led by the Council for Science, Technology and Innovation, Cabinet Office for the 'Reduction and Resource Recycling of High-Level Radioactive Wastes⁴⁾ through Nuclear Transmutation⁵⁾' (Program Manager: Reiko Fujita, PhD). In the ImPACT Program led by Fujita, reduction of LLFPs and recycling resources from radioactive waste are being investigated along with conversion of LLFPs into stable or short-lived nuclides. The vitrified radioactive wastes contain palladium, selenium, cesium, zirconium and other LLFPs, which have a half-life of around one million years. Furthermore, the LLFPs must be separated from high-level radioactive wastes in order to reuse and minimize the wastes.

On the other hand, reversibility and retrievability become critical issues when it comes to long-term geological disposal. The research team demonstrated that reusable elements can be separated from disposed vitrified wastes by using molten salt technology. This technology, when combined with other technologies being developed under the ImPACT program, can possibly make the geological disposal site smaller, or less deep than before.

The research team successfully retrieved mock LLFP nuclides as solids, molten salts and gasses by reducing mock vitrified waste in the molten salt where Si-O network structures of the silicon dioxide were dissolved. The radiation-tolerant molten salt can be used repeatedly resulting in decreased amounts of secondary wastes, and the team will continue research to provide a practical system to reuse and minimize high-level radioactive waste.

The outcome of this research will be presented at the 2019 Annual Meeting of the Atomic Energy Society of Japan (Mito, Japan, March 20, 2019), and at the 86th Annual Meeting of the Electrochemical Society of Japan (Kyoto, Japan, March 27, 2019).

*In the current research, glass structures were dissolved using a special chemical treatment. It is important to note that vitrified radiation wastes stored at disposal sites will never dissolve naturally.

Technical terms:

1) Molten salt
A high temperature salt that is also a fluid. Often used to dissolve various chemical species with sufficiently high concentration.

2) Long-lived fission products (LLFPs)
Fission products, such as Sr-90 and Cs-137, are nuclides created through the fission of U-235 in nuclear reactors. Those that are long-lived are called long-lived fission products, or 'LLFPs.' High-level radioactive waste contains products such as 79Se (half-life: 2.95105 years), 93Zr (1.53106 years), 99Tc (2.11105 years), 107Pd (6.5106 years), 126Sn (1.0105 years), 129I (1.57107 years), and 135Cs (2.3106 years).

3) Vitrified radioactive waste
To produce vitrified radioactive waste, high-level wastes are mixed with glass-forming chemicals, heated to a melting point, and then poured into a stainless steel containment vessel where it cools to form glass. As a solid, the waste becomes easier to store and handle.

4) High-level radioactive waste
High-level radioactive waste is waste containing very highly radioactive nuclides and needs to be kept cool for many years before it can be disposed. Usually, it is in the form of nitric acid solution after spent fuel is partitioned from nuclear plants.

5) Nuclear transmutation
Nuclear transmutation is the process of changing an LLFP into another nuclide, such as short-lived fission products or stable fission products.

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