Specialists at the GI Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (INP SB RAS) together with teams from other scientific organisations are testing boron carbide as a coating for the walls of the International Experimental Thermonuclear Reactor (ITER) under construction in France. Plasma burning during a thermonuclear reaction occurs at extremely high temperatures which can damage the walls of the reactor. Research is underway to find a substance that can protect against this damage.
The plasma in the tokamak is in a toroidal vacuum chamber. Despite the fact that it has little contact with the walls due to the retention of the magnetic field, the load on them is still large. This is both heating and radiation flux emanating from plasma, that is, neutron and gamma radiation. The material of the wall in such conditions can be destroyed. In any case, the wall cover particles will fall into the plasma, but heavy impurities are especially dangerous. Such substances in the plasma lead to its rapid cooling. Finding material for the first wall that would meet all the requirements is very difficult.
Carbon was widely used in research tokamaks to protect the walls, but its use was problematic as it can capture and retain hydrogen isotopes, including radioactive tritium. Currently, tungsten and beryllium are used as material for the first wall of the camera in ITER. Tungsten is refractory and withstands high temperatures well, but it is heavy, and when it enters the plasma, it quickly cools it. Beryllium is very light, and even when it enters the plasma, it does not affect its quality. However, the dust from beryllium is toxic to humans and is a strong carcinogen.
Therefore, a team of scientists led by Anatoly Krasilnikov, head of the ITER centre (Russia’s national agency for the construction of ITER) looked for alternative options for covering the wall of the tokamak. It needed to be heat-resistant and at the same time a light material with high thermal conductivity and electrical conductivity such as some special types of ceramics. Typically, ceramics are an insulator, but there are heat-resistant materials of the ceramic class that have sufficient conductivity.
The study also involves the Lavrentyev Institute of Hydrodynamics SB RAS, Khristianovich Institute of Theoretical & Applied Mechanics (ITAM SB RAS), and Tomsk State University of Management Systems & Radio Electronics. They applied a coating of special material with a thickness of only tens of microns. Tests are being carried out at the BETA installation in INP SB RAS, where the material is subjected to thermonuclear pulse loads. BETA is a material testing complex where it is possible to observe the parameters of the substance directly during the experiment. During testing, the material is submitted to a laser-powered thermal load from the plasma. Using a diagnostic system, temperature, absorbed heat and the degree of erosion can be tracked. Surface damage causes roughness also to change. At the BETA complex, the exact moment erosion begins can be identified with the subsequent loss of matter. “The purpose of the tests was to characterise the limit of the loads that our test materials can withstand during pulsed heating,” said research engineer Dmitry Cherepanov.
“We have been developing neutron protection from boron carbide with Virial (St Petersburg) for a long time. Virial company is a manufacturer of equipment components of ceramic and cera-metallic materials. This substance is very durable, has relatively good thermal conductivity, and we test it under the impulse loads that are characteristic of tokamaks,” explained researcher Alexander Burdakov.
Boron carbide is similar to light beryllium and does not cause the walls to cool quickly and it is also a readily available material. There are two options for using boron carbide – it can completely replace tungsten or applied to tungsten walls as a protective coating.
So far, the results from testing at the BETA complex show that the threshold values of loads at which ceramics begin to collapse are similar to tungsten. Tests suggest boron carbine is competitive with tungsten carbide and beryllium coatings.
Image: The BETA installation at the GI Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (courtesy of INP SB RAS)
