The plan is to build a “sub-critical” nuclear reactor. Such a reactor would not be able to sustain a chain reaction. Instead, the nucleus-transmuting subatomic particles would be supplied from outside, using a particle accelerator.
Facilities, which will probably cost around $1 billion each, are being planned by physicists in Japan and Europe to come on stream some time after 2015.
About 95% of the mass of a piece of used nuclear fuel is unconverted uranium, so the first step is to extract the 5% that is waste. This is done chemically. The radioactive elements to be transmuted are then turned into a target for protons fired out of a particle accelerator. Neutrons cannot be speeded up in an accelerator because they have no electric charge to grab hold of. But the main role of the protons is to knock neutrons free from nuclei in the target.
These neutrons should, if all goes well, be absorbed by the technetium and other fission products, transmuting them into new elements. They will also break up the elements heavier than uranium into products similar to those from uranium fission. Although, initially, the new elements will be more radioactive than the spent nuclear waste was, that radioactivity will last only a few hundred years. This means that the dumps into which they are put need not be as secure (or as expensive) as those envisaged for long-term waste-storage. And, as a bonus, the whole process should generate more energy than it consumes