One of the suggested ways of permanently disposing of spent nuclear fuel and other highly radioactive wastes is to mix them with sand and chemicals and heat the mixture until it turns into glass logs. This is referred to as vitrification. A seventeen-billion dollar vitrification plant is being constructed at Hanford to deal with the toxic soup of radioactive materials and toxic chemicals stored in underground tanks. Elements in the radioactive logs will include palladium, selenium, cesium and zirconium, and other long-lived fission products (LLFP) with a half-life of about one million years.
The Japanese Cabinet Office’s Council for Science, Technology and Innovation works with Japanese corporations through their Paradigm Change through Disruptive Technologies (ImPACT) Program to explore new technologies for Japan’s energy sector. Under this government program a team of researchers from Toshiba Energy Systems & Solutions Corporation have been working on developing a way to recover useful elements from vitrified radioactive waste. Their work involves investigating the reduction of LLFPs into stable and short-lived nuclides. It also involves the recycling of resources from nuclear waste.
The Toshiba researchers in collaboration with the Japan Science and Technology Agency have successfully demonstrated that reusable elements can be extracted from vitrified waste by the use of a molten salt technology. The research team released a joint statement that said that when combined with other technologies developed under the ImPACT program including transmutation from long lived radioactive isotopes to short lived radioactive isotopes, their research might make it possible to reduce the size and or depth of geological nuclear waste repositories.
The researchers successfully recovered dummy LLFP nuclides as solids, molten salts and gases by reducing mock vitrified waste in the molten salt. The silicon monoxide network structures of the silicon dioxides had to be dissolved in the molten salt to permit this extraction. The molten salt is radiation tolerant and can be reused. This results in the reduction of secondary wastes produced by the new process. The team will continue to research practical systems to reuse and minimize high-level radioactive waste.
Making use of such processes presupposes that the vitrified logs of waste are accessible for recycling. Some designs for permanent geological repositories might make this difficult. The Waste Isolation Pilot Plant (WIPP) geological repository for high-level nuclear waste produced by nuclear weapons production near Carlsbad, New Mexico illustrates the problem. The repository has been operating for about fifteen years. It is carved out of an old salt mine. There are big rooms that are filled with waste that are supposed to be permanently sealed when they are full. Originally, huge steel and concrete doors were going to be welded shut.
If there was interest in recycling vitrified waste from the WIPP, the big doors would have to be breached in order to access the vitrified waste. In addition, there have been hydrological studies that indicate that the assumption that the salt mine was safe from migrating ground water is not accurate. If the mine was flooded, crews might face flooded chambers in their attempts to recover the waste. This would greatly increase the cost.
Ultimately, new geological repositories would have to be constructed with recovery and recycling in mind. If the market failed to adopt the recycling technology for any reason, geological repositories constructed on the assumption that the nuclear waste they contain would be recovered might prove to be unsafe if the waste was not ultimately recovered. The Toshiba research is interesting but a lot of factors other than scientific feasibility will govern whether or not it is ever widely implemented.