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How to Line a Thermonuclear Reactor
16 August 2012 3:51 pm
One of the biggest question marks hanging over the ITER fusion reactor project—a giant international collaboration currently under construction in France—is over what material to use for coating its interior wall. After all, the reactor has to withstand temperatures of 100,000°C and an intense particle bombardment.
Researchers have now answered that question by refitting the current world's largest fusion device, the Joint European Torus (JET) near Oxford, U.K., with a lining akin to the one planned for ITER. JET's new "ITER-like wall," a combination of tungsten and beryllium, is eroding more slowly and retaining less of the fuel than the lining used on earlier fusion reactors, the team reports. "This was very good news, because it means that our choice of materials for ITER was the right one," says physicist Peter de Vries, task force and session leader at JET.
Fusion is the process that powers the sun and stars, and, potentially, it's the perfect energy source. The necessary fuels are easily accessible and virtually inexhaustible, and the process doesn't produce any greenhouse gases or long-lived nuclear waste. For fuel, it requires deuterium and tritium (forms of hydrogen with one and two extra neutrons, respectively, in their nuclei). These have to be heated so that they form plasma—an ionized gas—and when they reach about 150 million°C, the nuclei collide with such force that they overcome their mutual repulsion and fuse into a new, larger nucleus. The products of the reaction are a helium nucleus and a very energetic neutron, whose energy is later harvested in the form of heat.
But the harsh truth is it's not at all easy to run this fusion process in a controlled way. The current favored technique is to use a reactor called a tokamak, which employs powerful electromagnets to confine the plasma inside a doughnut-shaped reactor vessel. The magnets aim to hold the plasma away from the walls of the vessel long enough for the nuclei to fuse but plasma can often shift around in unpredictable ways. If the plasma touches the wall, this can cool it to below reaction temperature and also scour off atoms of the lining material that poison the fusion reaction. And tritium is a radioactive isotope that reactor operators have to account for very carefully. Any tritium that embeds itself in the reactor wall has to be painstakingly extracted.
No fusion reactor has yet produced more energy than was put in to heat the plasma in the first place. But researchers have high hopes for ITER, the massive reactor with an estimated price tag of as much as $20 billion that is now being built in the south of France by China, the European Union, India, Japan, Russia, South Korea, and the United States.
The most common reactor lining, known as the first wall, in earlier fusion reactors was carbon because it is extremely resistant to high temperatures and erosion and doesn't pollute the plasma if atoms do get into it. Carbon's big drawback is that it's very happy to absorb deuterium and tritium. For ITER, the first reactor to use tritium on a regular basis, absorption of tritium has to be kept to a minimum, so carbon is out.
Since no perfect material exists, the plan is to compromise and use two different materials. Most of the first wall would be coated with beryllium, which is the least plasma-polluting metal but has a low melting point if it comes into contact with the plasma. At the bottom of the torus is a structure called the divertor, which is like the reactor’s exhaust pipe because it extracts helium from the plasma. The divertor is deliberately in contact with the plasma and so needs a tougher coating. For this, the plan is to use tungsten, which can withstand the heat in the divertor region—lower than in the bulk of the plasma—but if some does get eroded away, it poisons the plasma pretty badly.
The tungsten elements of the divertor "are designed to handle steady heat flows twice as large as those experienced by the nose cone of the Space Shuttle on reentry into the Earth’s atmosphere," says physicist Richard Pitts, leader of the Plasma-Wall Interaction and Divertor Physics Group at ITER. The reactor designers want the divertor to survive many years of plasma operation before replacement, which is a major operation. "Having to replace a divertor means that you'd have to stop making plasma and then send in robots, because the inside of the vessel has become radioactive. This remote handling is an arduous and slow process that will require 6 to 12 months on ITER," says Pitts, who was not involved in the new study.
This is why the ITER team wanted to make absolutely sure that their proposed lining would work. To do that, they enlisted the help of JET, a reactor built in the 1980s and the current fusion record-holder for energy production—16 megawatts. "As a matter of fact, JET is super important for ITER," Pitts says. It is a key experimental environment to test materials and processes for the reactor. During JET's most recent overhaul, which lasted from May 2010 to May 2011, the components for the inside wall of its vessel as well as the divertor—previously made mainly out of carbon—were replaced by those planned for ITER: thousands of beryllium tiles for the wall and tungsten elements for the divertor.
The results gained during operation of this upgraded JET machine have been very positive. The beryllium wall eroded much more slowly under the influence of the plasma than the previous carbon wall, the team reported at a conference last month. But even more important: It retained fuel at one-tenth the rate. "Fuel retention was a big problem. When tritium from the plasma was absorbed by the carbon it may be released later. This makes it very difficult to control the fuel in the plasma," says JET's de Vries. "Even more so, the total amount of tritium retained in ITER should be limited. Otherwise that can be a safety hazard and the reactor will have to be stopped."
There are, however, major differences of scale between JET and ITER, such as in the duration of each plasma pulse. In JET, the plasma is being sustained for only about 40 seconds—enough to gather loads of data. ITER will operate in pulses of at least 10 minutes, which means a bigger impact on the materials facing the plasma. Both the larger size of ITER as well as these longer pulses will inevitably lead to divertor materials being bombarded by many more particles during their lifetime. "The divertor in ITER will catch more particles in one day of operation than the same component in JET has in a decade," says ITER's Pitts. For this reason, the Dutch Institute For Fundamental Energy Research has built a device called Magnum-PSI. This is the only machine in the world in which one can expose a test surface to the continuous stream of particles expected in the ITER divertor, with the presence of a very strong magnetic field, like in ITER.
JET is now temporarily out of service while tiles of beryllium from the general lining and tungsten from the divertor are removed by a robot arm to be meticulously studied for erosion patterns. It will start up again in early 2013. Then researchers hope to try deliberately melting some of the tungsten to see what happens. "We hope that low levels of damage to the divertor can be tolerated by the plasma. This is our biggest unknown in planning to start ITER up with tungsten divertor targets," Pitts says.