Physicists claim to have invented a meltdown-free nuclear reactor that burns radioactive waste, writes Robin McKie in Geneva
SCIENTISTS have designed a nuclear reactor that they say can “burn” atomic waste. The breakthrough could solve the world’s nuclear waste crisis and save the beleaguered atomic energy industry.
Experiments carried out for two years at Cern, the European particle physics laboratory in Geneva, have demonstrated that these reactors would be safe and economical. And earlier this year, an international consortium was set up to consider building a prototype.
If successful, reactors of this type could deal with the hundreds of thousands of tons of highly radioactive waste products, including plutonium, that have been created by nuclear plants.
This material is so dangerous that only storage in deep underground silos for thousands of years is accepted as a safe way of dealing with it.
The proposed reactors are the idea of Professor Carlo Rubbia, winner of the 1984 Nobel Prize for Physics and a former director general of Cern. He produced his power plant plans three years ago.
Since then he has been directing experiments to demonstrate that his ideas work. “We have taken all sorts of crap churned out by reactors and put them into a mock-up of our machine,” he said. “Everything was broken down into harmless, low-level radioactive waste.”
Now Rubbia – who is to detail his experiments to a meeting of Cern’s council next month – is preparing to help engineers construct a first prototype reactor. “We could build one very quickly, but realistically I think it will take at least five years.”
Old reactors make plutonium, the prime ingredient of nuclear bombs, and civil and military nuclear programmes have become interchangeable in the public’s eyes as a result.
Then there is the question of controlling a nuclear reactor. Once a chain reaction has been started inside a core, it can be controlled only with difficulty. At Chernobyl, a test run ran wildly out of control. The end result was an explosion that irradiated half of Ukraine.
Finally there is the issue of waste. As well as making plutonium, current reactors churn out isotopes of uranium, caesium, strontium, iodine and other irradiated elements. Some are extremely radioactive and will remain so for thousands of years.
Storage, even in deep underground chambers, is a problem that plagues the nuclear industry and horrifies people living in areas earmarked as possible nuclear waste repositories.
Only a radical solution will solve such problems, and Carlo Rubbia claims has found it. He has spent his life using accelerators – such as the massive Lep machine, a circular tunnel 17 miles in circumference built under the Jura mountains by Cern – to collide beams of sub-atomic particles at colossal speeds, thus releasing energies that recreate the early, fiery conditions just after the “big bang” birth of the universe.
But instead of battering streams of protons into each other, Rubbia realised, they could be fired into a core of thorium, a relatively common element. These protons would transform thorium atoms into uranium- 233, whose atoms are unstable and disintegrate to produce neutrons. When these batter into more thorium atoms, they create more uranium. And so it goes. The next step is simply to use the excess heat from this reaction to drive a turbine.
Much of this technology may seem suspiciously similar to that used in conventional reactors. But there are crucial differences. For a start, says Rubbia, his energy amplifier can only maintain its reaction while it is battered by proton beams. It is not self-sustaining and therefore can be turned on or off simply by starting up or switching off the proton beam.
It is like a car. Take your foot off the accelerator and it slows to a halt. The prospect of a reaction like the one that wrecked Chernobyl is therefore precluded, says Rubbia.
Then there is the issue of the creation of plutonium. “We worked on this at length,” says Rubbia. “We have found that our machine simply cannot make plutonium.”
But the final advantage of Rubbia’s design is the most important. The proton beam he will use to control his amplifier’s reactions will be a highly energetic one, and the neutrons it will generate inside the reactor will also be highly energetic.
“Such neutrons are said to be fast,” says Rubbia. “By contrast, neutrons in normal reactors are not very energetic, so we call them slow neutrons.” Fast neutrons are important because they have the energy to break open the nucleus of any atoms they smack into.
Thus the neutrons that are the blood flow of any reactor will, in Rubbia’s case, have the special extra power of being able to break down any atoms that they encounter. Mix small amounts of nuclear waste with the reactor’s thorium fuel and it will be ground down and rendered harmless.
In one experiment, technetium 99 – a pernicious, radioactive nuclear residue with a half-life of 200 000 years – was broken down in seconds into a harmless isotope. Rubbia estimates that one of his reactors will be able to burn more than a kilogram of nuclear waste a day in this way. The average conventional reactor creates about 250kg of waste a year.
A Rubbia prototype would probably generate 250MW of electricity. After that, he envisages the construction of a series of 800MW plants.