A group of researchers at the University of Idaho claims to have developed an alternative technique that generates only 1% of the waste and costs two-thirds as much as conventional radwaste reprocessing.

Instead of dissolving the fuel in nitric acid, the process uses CO2 at supercritical temperatures. Supercritical fluids hover between being a gas and a liquid, with some of the properties of each.

The key to the process is a complex that forms between the unused uranium dioxide fuel and a chemical based on a solvent called tributyl phosphate (TBP). This complex dissolves in the supercritical CO2. The same goes for plutonium dioxide, used in MOX fuel.

The fission products that gum up the fuel rods do not react with the TBP compound, and get left behind when the uranium-TBP and plutonium-TBP dissolve in the CO2. This action creates a relatively small volume of fission-product waste compared with that produced by conventional reprocessing, and it can be filtered out. In addition, uranium oxide needs only a small volume of supercritical CO2 to dissolve, compared with the amount of nitric acid that would be needed in conventional reprocessing.

The solution containing uranium and plutonium is subsequently transferred to a second vessel, where the pressure is lowered. This allows the CO2 to evaporate, leaving behind the uranium-TBP and plutonium-TBP. There is no huge volume of radioactive liquid left, and the uranium dioxide and plutonium dioxide can then be liberated from the TBP complex, although the university has not yet released details on how this is done.

The pressures needed to operate a supercritical reprocessing plant are no higher than those commonly used in nuclear reactors. The plant would be able to recycle most of its CO2, and any gas that was vented could be filtered to remove radioactive particles.

It has been found that blasting the uranium with ultrasound makes it dissolve 10 times faster.

The Japanese government has set aside $4 million for a project designed to assess the commercial viability of the new process.
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