TRANSMUTATION of the elements was the goal of the medieval alchemists. They dreamed of the riches to be won by the man who could find the philosopher's stone—a substance that, among other wonderful properties, would convert base metals such as lead into gold. Actual transmutation, though, had to await those modern alchemists, the atomic physicists. Nuclear reactors transmute elements routinely. They break uranium atoms, which are heavy, into lighter so-called fission products, such as technetium. This releases energy, along with sub-atomic particles called neutrons. Some of these neutrons go on to hit further uranium nuclei so hard that they, too, shatter and release yet further neutrons. It is this chain reaction that sustains the process. Other neutrons, however, are captured by uranium nuclei. That makes those nuclei heavier still, converting them into neptunium, plutonium, americium and curium.

All these by-products of nuclear fission are radioactive, and many will remain so for thousands—sometimes millions—of years. They are thus difficult to dispose of; the most practical idea being to bury them deep underground in stable rock formations and just wait. On top of that, the plutonium could, in principle, be extracted to make nuclear bombs. But the organisers of the Kumatori Accelerator-driven Reactor Test Facility (KART), at Kyoto University in Japan, which starts up this month, have dusted off an old scheme that might help overcome the problems of nuclear waste. This is to transmute the by-products still further, into something that can be disposed of safely.

KARTs and horsepower

The idea behind the Kumatori project, which is led by Kaichiro Mishima, was originally championed by Carlo Rubbia, who was once head of CERN, a big subatomic-physics laboratory near Geneva. 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.

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. Indeed, Dr Rubbia's original name for the device was an energy amplifier.

Researchers in the field hope that results from KART (which should start to appear after September, when the machine becomes fully operational) and a number of other transmutation experiments around the world will contribute to the design of one or more large-scale transmuters. These facilities, which are expected to cost around $1 billion each, are being planned by physicists in Japan and Europe to come on stream some time after 2015—assuming, of course, that those physicists can persuade politicians of the merits of their work.

That is by no means a done deal. Researchers had hoped to start building a more ambitious version of KART in Italy last year. This project, however, was cancelled in 2004, and its demise had more to do with political disagreements than technical shortcomings.

Nor are all scientists fans of transmutation. The Institute for Energy and Environmental Research (IEER), based in Washington, DC, believes that if uranium is separated from spent fuel and then stored as low-level waste, it could pose a greater risk to the public than if it were placed in a repository deep underground. It also points out that some of the long-lived components of spent fuel cannot practicably be transmuted. For example, it would take more than a century to destroy half of the radioactive selenium present in spent fuel, because that element is very inefficient at capturing neutrons. The IEER describes evaluations in favour of transmutation as “seriously deficient” and made “mainly by those who would like to see a continuation of nuclear power”.

At the moment, though, nuclear power's supporters seem to have the upper hand, as many people think it is the only practical way to generate large amounts of electricity without producing climate-changing carbon dioxide. And when nuclear waste remains dangerous for tens of thousands of years, and repositories are estimated to cost tens of billions of dollars, transmutation is worth considering. It may not be lead-into-gold, but it could still be very worthwhile.