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WHAT to do with spent fuel is one of the nuclear industry鈥檚 big headaches. Reprocessing is risky, expensive and controversial, generating massive quantities of liquid radioactive waste that threatens the environment. Yet simply discarding spent fuel is very wasteful. Now a new reprocessing technique could dramatically cut the volumes of nuclear waste

Nuclear fuel rods have to be replaced every few years because fission products such as strontium build up inside them. At levels of just 5 to 10 per cent, these waste products block neutrons and significantly reduce the efficiency of the fission reaction. Some countries, including the US and Germany, simply store their used fuel rods, but others, including Britain, reprocess the spent rods to extract the unused uranium and plutonium.

Conventional reprocessing plants dissolve spent fuel in nitric acid and then remove the uranium, leaving large quantities of liquid radioactive waste. This is dried, converted to glass or 鈥渧itrified鈥�, and then sent for storage. The risk of contamination when dealing with these huge amounts of radioactive liquid is one reason why environmentalists oppose reprocessing.

Now, a group of researchers led by Chen Wai, a chemist at the University of Idaho, has developed an alternative technique that generates only one-hundredth as much waste and costs two-thirds as much as conventional reprocessing. Instead of dissolving fuel in nitric acid, the team uses carbon dioxide at such a high temperature and pressure that it becomes 鈥渟upercritical鈥�. 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 carbon dioxide. The same goes for plutonium dioxide, which is sometimes used as a fuel mixed in with the uranium. The fission products that gum up the fuel rods don鈥檛 react with the TBP compound, and get left behind when the uranium-TBP and plutonium-TBP dissolve in the CO₂. This creates a relatively small volume of fission-product waste compared with conventional reprocessing, and it can be filtered out. What鈥檚 more, 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 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鈥檚 no huge volume of radioactive liquid left sloshing around, and the uranium dioxide and plutonium dioxide can then be liberated from the TBP complex, though Wai is not saying how they do this.

Wai says 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. Wai and collaborator Youichi Enokida at Nagoya University in Japan have also found that blasting the uranium with ultrasound makes it dissolve 10 times faster. The Japanese government has just set aside $4 million for a project designed to assess the commercial viability of the new process.

Jim Riccio, a nuclear policy analyst with Greenpeace in Washington DC, says that while a reduction in the amount of reprocessing waste would be an obvious improvement, he would prefer to see nuclear energy phased out altogether and replaced with renewable sources such as solar power. He also points out that, like conventional reprocessing, the CO₂ technique isolates plutonium that can be used to make nuclear weapons.