...:: Days of Our Future ::...

An international team of researchers is building a machine to recreate the sun. This will happen in Balmy, south of France. It will take tens of thousands of tonnes of steel and concrete, plus a whole host of more unusual materials: beryllium, niobium, titanium and tungsten; frigid liquid nitrogen and helium. Oh, and a supply of burnt coconuts.

This eclectic mix of ingredients will be turned into ITER, the International Thermonuclear Experimental Reactor - the next big thing in nuclear fusion research. When completed in 2018, the reactor will fuse together two heavy isotopes of hydrogen to release vast quantities of energy. In theory, the result will be clean electricity galore with no carbon emissions and far less radioactive waste than today's nuclear fission reactors leave behind.

While the concept of nuclear fusion is simple, the practicalities are not. That's because the nuclei themselves are reluctant participants: each carries a positive electrical charge and these repel one another, so forcing two nuclei together is almost impossible. Only at stupendously high temperatures do the nuclei acquire enough energy to overcome their mutual aversion, smash into one another, and fuse.

It is much the same picture in the sun. There, heat is generated from the fusion of hydrogen nuclei. But the fuel barely smoulders even at 15 million kelvin, the temperature of the sun's core. It is consumed so slowly that the supply lasts for billions of years.

At a fusion power plant, the fuel needs to be burned on human, not cosmological, timescales. The heavier isotopes deuterium and tritium are a little easier to burn than ordinary hydrogen, but even so, to get a good blaze going inside ITER the temperature will have to be racked up to a hellish 150 million kelvin. That brings a mountain of engineering problems. Not least is how to contain a plasma of electrons and atomic nuclei that is 10 times as hot as the sun's core.

Even the most hardy of construction materials cannot withstand temperatures of more than a few thousand kelvin. So the solution is to weave a cage for the plasma from magnetic fields.

ITER follows the design of several smaller experimental reactors where physicists have already achieved the temperatures required for fusion. The nuclear fuel is held inside a ring-shaped reactor called a tokamak.

Magnets outside the ITER ring combine to generate a spiralling field that holds the superhot plasma in place. To make its magnetic cage, ITER will use superconducting coils of niobium-alloy wire weighing a total of 10,000 tonnes and cooled by a supply of liquid helium. Outside the magnetic cage, a vacuum isolates the confined plasma from the reactor's inner wall - and this is where the coconut comes in.

Trapped in its cage, the fusion fuel is simultaneously cooked in three different ways. While electrical circuits force a current through the plasma, it is blasted with microwaves and bombarded by high-energy atoms generated by small particle accelerators dotted around the ring. Even under this triple-pronged attack, so far no tokamak has yielded much fusion energy. ITER should do better by firing up a much bigger, denser ring of plasma. A lot of power will have to be pumped in to start the plasma sizzling, but if all goes to plan, 10 times as much will emerge.

All that power poses a threat to ITER because the magnetic cage is not impregnable. The violet-hot plasma will radiate X-rays, a trickle of charged particles will always escape, and the fusion reaction will create high-energy neutrons, which are electrically neutral and can't be contained by magnetism. So despite the magnetic cage, ITER's plasma will blast the surrounding walls with several megawatts of heat per square metre, far more than in previous tokamaks or conventional nuclear fission reactors.

The solution is simple: use water to carry the heat away. "Of course this is exactly what we want from fusion in the end - to extract the heat," says Mario Merola, head of the ITER division responsible for internal components of the reactor.

Once again, it's the practicalities that are the problem. The main reactor wall, known as the blanket, will be made from 440 stainless steel blocks nearly half a metre thick and riddled with high-pressure water pipes. This steel blanket should absorb most of the neutrons, which will heat the blanket from within. Near the inner wall, the water pipes can be no more than 2.5 centimetres apart, otherwise the steel between them would become dangerously warm and soft.

For the innermost surface facing the plasma, steel is no good. Incoming plasma particles would chip iron atoms out of the steel and back into the chamber, where they would pollute the fuel and damp down the fusion reactions. So the ITER team has chosen to face the wall with tiles made of beryllium. While beryllium is toxic to humans, it is quite palatable to the plasma because it is such a light element, close in atomic weight to deuterium and tritium. So although some beryllium will get blasted off the walls, it won't quench the reactor's fire.

The steel and beryllium plates will also be battered by mechanical forces generated by the interaction of the electric currents and magnetic fields passing through them. Each 4-tonne plate will experience forces of up to the weight of 100 tonnes, so they will have to be firmly locked in place - and sturdily built, even though they are punctured with holes for the pipes. "The design of the blanket modules is one of the most technically challenging parts of the whole machine," says Merola.

A different kind of armour plate is needed around the bottom of the chamber. Here, a device called the divertor is used to keep the plasma pure. The main by-product of the fusion reaction is helium nuclei, which would eventually build up and stifle the nuclear fire. The divertor's job is to skim off the outermost layer of plasma, which can then be cooled and siphoned off to have the helium "ash" and other impurities removed. The surface of the divertor will get hot enough to melt beryllium, so it will be covered in tungsten and carbon fibre, both materials with melting points above 3000 kelvin.

Fusion remains a controversial goal, not least because of the expense of the research still required. ITER alone will cost more than $10 billion. Sceptics also like to point out that ever since the idea was first touted in the 1950s, fusion's promise of clean power has receded endlessly into the future.

The ITER team are now hoping to drag it closer to the present. If they can successfully hold a slice of the sun at the reactor's heart, we might finally be on the verge of getting usable energy out of this electric dream.

Source newscientist.com