Specialists from the Institute of Nuclear Physics named after. G. I. Budkera (BINP SB RAS) together with teams from other scientific organizations test boron carbide as a coating for the walls of the tokamak of the International Experimental Thermonuclear Reactor (ITER).
Potential Source of Energy with High Productivity
Research into thermonuclear fusion is a promising direction, since reactors based on it can become a new source of energy with high productivity. Plasma combustion during a thermonuclear reaction occurs at extremely high temperatures and researchers are faced with the task of finding a substance that can withstand these conditions without adversely affecting the plasma.
The results of tests carried out at the Institute of Nuclear Physics SB RAS showed the competitiveness of boron carbide coatings with tungsten and beryllium, which are often considered when choosing a protective material for the first wall and divertor of modern tokamaks.
In a fusion reactor, the plasma is contained in a vacuum chamber in a tokamak, a special device that holds the plasma using magnetic fields. Thus, the plasma almost does not touch the walls of the chamber and the main movement of hot plasma particles occurs in the center, which allows the plasma to maintain its temperature longer.
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The idea of a tokamak, which was proposed in Russia by A.D. Sakharov and I.E. Tamm back in the early 1950s, developed and reached an experimental thermonuclear reactor. A tokamak is a system with closed magnetic field lines, with magnetic confinement of plasma and ‘confinement’ in this case is the key word but a tokamak is not the only possible system,” said Alexander Burdakov, the chief researcher, advisor to the director of the BINP SB RAS.
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At the BINP, as in some other scientific centers, projects are also being developed based on open traps, where there is a magnetic field that rests against the walls of the chamber. Open traps still lag behind tokamaks in plasma retention, but at the same time they have a number of important advantages,” continued Burdakov.
Tokamak-based System
The tokamak-based system will be used in the large experimental fusion reactor - ITER. This is an international project in which, in addition to scientists from Russia, specialists from Japan, China, Korea, India, the USA and European countries take part. The main task of the ITER team is to create a reactor in which the plasma will support its combustion itself.
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As part of this project, new technologies are being developed around the world. Thanks to ITER, many useful technologies came to BINP. By participating in the project, we gain new experience and new knowledge that can be applied to domestic installations, for example, to the open trap GDML (Gas-dynamic multiple-mirror trap) currently being designed at the BINP,” explained Burdakov.
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The ITER project is experimental, because the questions it poses to us have never been resolved by humanity,” stated Burdakov. ITER is a necessary step towards fusion power plants. In a reactor, it is first necessary to ‘ignite’ the plasma and then achieve high efficiency. This technology will make it possible to obtain 10 times more energy from the invested 50 MW, that is, 500 MW.
Material for Tokamak Wall Coating
The plasma in a tokamak is located in a toroidal vacuum chamber. Despite the fact that it has little contact with the walls due to being held by a magnetic field, the load on them is still large. This includes heating and the radiation flux emanating from the plasma, that is, neutron and gamma radiation. The wall material may collapse under such conditions.
Particles of the wall coating will get into the plasma in any case, but heavy impurities are especially dangerous. Such substances in the plasma lead to its rapid cooling. Finding a material for the first wall that would meet all the requirements is very difficult.
Carbon has been widely used in research tokamaks, but its use in a reactor is difficult because it can capture and retain hydrogen isotopes, including radioactive tritium. Currently, tungsten and beryllium are used as materials for the first wall of the ITER chamber.
Tungsten is refractory and can withstand high temperatures well, but it is a heavy material and when it enters a plasma it quickly cools it. Beryllium is very light and even when it gets into plasma, it has almost no effect on its quality. But beryllium dust is toxic to humans and is a strong carcinogen.
Therefore, a team of scientists led by Anatoly Krasilnikov, head of the ITER Center, is looking for alternative options for coating the tokamak wall. They need heat-resistant and at the same time lightweight materials with high thermal and electrical conductivity, for example, special types of ceramics. Typically, ceramics are an insulator, but there are heat-resistant ceramic-grade materials that are sufficiently conductive.
Boron Carbide Neutron Protection
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We have been working together with the VIRIAL company (St. Petersburg) for a long time on the development of neutron protection made of boron carbide. This substance is very strong, has relatively good thermal conductivity and we test it under pulsed loads, which are typical for tokamaks,” added Alexander Burdakov.
Boron carbide, like beryllium, is light and when it enters the plasma it does not cause it to cool quickly and it is also an accessible material. There are two options for using boron carbide - it can completely replace tungsten or be applied to tungsten walls as a protective coating.
The problem of choosing a coating for the walls of a tokamak is common to all scientists working with plasma and many organizations are interested in solving it. In addition to high temperatures and radiation, in any tokamak there are sometimes ‘disruptions’, during which plasma flows out of the confinement zone and hits the wall.
It is precisely these most dangerous pulsed thermal loads that boron carbide is subjected to in the BETA installation. The results of the study showed that the threshold load values at which ceramics begin to fail are not inferior to tungsten.
Source: Institute of Nuclear Physics (INP)