It sounded great. We would have more energy by burning up radioactive rubbish. A Nobel prize-winning physicist would see to it. But so far it hasn't happened. Paul Guinnessy explains why
It's safe to say that the universe doesn't offer too many free lunches. Despite the best efforts of inventors, perpetual motion machines are doomed to failure by the Second Law of Thermodynamics. Thus it seems surprising when a Nobel physics laureate and former head of Cern, the European Laboratory for Particle Physics, suggests an idea for using up spent nuclear fuel with a process that, in essence, sounds like getting something for nothing - nuclear power without nuclear waste.

That is what Carlo Rubbia and his colleagues first proposed in 1993 - a new approach to nuclear energy production called the energy amplifier.

Rubbia, who won the Nobel Prize for Physics in 1984, claims that a stream of protons fired from a particle accelerator into a core of thorium would transform the thorium atoms into uranium-233, a highly fissile isotope of uranium. The decay of U-233 produces high-energy, or "fast" neutrons. When those hit other thorium atoms, they create more uranium atoms, and so on, until the thorium is used up.

According to Rubbia, the quantity of energy produced by the combination of these two processes is substantially greater than that needed to operate the accelerator, and so one can define an energy gain. Hence the choice of the name "energy amplifier".

Rubbia likes the idea of the thorium reactor because in theory it should produce between 1,000 and 10,000 times less plutonium than a conventional nuclear fission reactor for the same power output. A 1000-megawatt (MW) pressurised water reactor, for example, produces more than 200 kilograms of plutonium a year, increasing the risk of nuclear weapon proliferation as well as being a health hazard and storage problem.

Unlike all other present nuclear reactors, the energy amplifier nuclear fission reaction is not self-sustaining - it needs the continuous input of neutrons produced by the accelerator. This rules out Chernobyl-type accidents resulting from criticality. If the accelerator stops, the reaction stops.

Is this such a new idea? In 1949, Ernest Lawrence, the inventor of the cyclotron and an earlier Nobel prize-winner, and Luis Alvarez, one of his students at the Berkeley Radiation Laboratory, proposed a similar scheme as a way of generating uranium-233. At the time, a shortage of uranium was delaying the production of atomic bombs, and any way of increasing stocks was seized upon with great interest. It was thought that uranium bred from thorium would be easier to manufacture than plutonium-based weapons. But the cost of building a device, technical difficulties, the highly radioactive behaviour of uranium-233 (it emits high-energy gamma rays), and the discovery of new uranium deposits delayed any progress with the scheme.

Since then, Los Alamos Weapons Laboratory in New Mexico has done much research on the topic, particularly to see if the technology could be used to destroy plutonium and radioactive waste by using high-energy neutrons to change them into non-radioactive elements.

Neutrons are usually put into two classes - the slow, "thermal" ones and the "fast" ones, depending on the amount of energy they have. Nearly all nuclear reactors use slow neutrons, which are not energetic enough to transmute waste into non-radioactive elements. Fast neutrons, which can, are generated either by aiming protons generated in a particle accelerator at a heavy metal (such as thorium) to create neutrons - a process called spillation - or in a "fast breeder" reactor.

Many thorium-based prototype reactors have been built in the past, but without using the particle accelerator suggested by Lawrence. In the UK, the experimental Dragon reactor (1966-73) tested thorium fuel rods at temperatures exceeding 1,000C, but ran into serious technical difficulties. Attempts with reactors in the US and Germany suffered costly and highly publicised incidents which resulted in the plants being shut down.

Experiments carried out by Los Alamos and the Atomic Weapons Establishment at Aldermaston have also shown that any radioactive wastes placed in a fast neutron source are not completely destroyed. Between 2 and 3 per cent of the materials, such as plutonium, remained mixed up with transmuted elements in the core.

As this waste destruction process requires neutrons, it has a cost in the form of lower energy production, and that would significantly increase the price of any electricity produced. The suggestion from Rubbia's team is that the core of the energy amplifier should consist only of materials that have a very long half-life, such as caesium-135 (two million years), while elements with a short half-life, such as strontium-90 (about 30 years) should be buried for several hundred years in a underground chamber. (In comparison, Nirex expects its nuclear waste to require underground burial for 10 million years.)

Both Japan and Russia are working on similar technology, using a particle accelerator, but along with Los Alamos, their designs require linear accelerators that will certainly entail substantial technological developments, which the proponents admit will need a good 20 years' work.

Part of the great interest in Rubbia's project stems from the use of thorium. Thorium is more than twice as abundant in the Earth's crust than uranium and, unlike uranium, where only the isotope 235 is subjected to fission, all the thorium is burnt in the reactor, increasing the efficiency of the process. In these conditions, the energy reserves of the thorium cycle appear inexhaustible - at least in the short term. If the scheme works as advertised, it could be of great benefit to countries with large thorium deposits, such as India.

Rubbia claims that a prototype would be capable of generating 250MW of electricity, with a full-scale reactor producing 800MW. The accelerator would be a cyclotron, similar in design to an existing machine at the Paul Scherrer Institute in Zurich, and would use the experience obtained from the LEP collider at Cern. For example, using a proton beam energy of 7MW, produced by a state-of-the-art 1 gigaelectronvolt (GeV) cyclotron, a reactor would produce 280MW of thermal energy - which would then produce about 100MW of electrical power. Given that the power needed to operate the accelerator does not exceed about 20MW, there would thus be a net production of more than 80MW at the first estimates. Rubbia expects to obtain 600MW of thermal energy (about 200MW of electrical power) from the full-scale reactor, which is 12 times less than a standard industrial power station (2,400MW). Both these examples are based on technology available today.

Even assuming that the most optimistic figures of power generation by Rubbia are correct, replacing a standard power station would require at least three Rubbia-type power stations. The smallest prototype of such a station will cost more than pounds 1bn to build, using the most conservative estimates. Compare that to the cost of the last nuclear power station built in the UK, Sizewell B, which cost pounds 1.6bn for a 1,175MW-rated output.

For the moment, the energy amplifier so recently touted as the saviour of mankind will neither provide electricity too cheap to meter, nor destroy the legacy of 50 years of atomic power by-products without cost. Nor can it completely replace all other energy systems in production. It is still too expensive and produces too little electricity to be a completely viable proposition. And so far, Rubbia's experiments have produced in the order of a single watt of output energy. Not a gram of nuclear waste has been burnt. A free lunch? Not really.

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