Mitigating corrosion by liquid tin could lead to better cooling in fusion reactors
Researchers at Tokyo Institute of Technology and the National Institute for Fusion Science have clarified the chemical compatibility between high temperature liquid metal tin (Sn) and reduced activation ferritic martensitic, a candidate structural material for fusion reactors. This discovery has paved the way for the development of a liquid metal tin divertor, which is an advanced heat-removal component of fusion reactors. A device called a divertor is installed in the fusion reactors to maintain the purity of the plasma. For divertors, there has been demand for liquid metals that can withstand extremely large heat loads from high-temperature plasma.
Background
Fusion reactors are being actively developed throughout the world as a form of sustainable zero-carbon energies because their fuel can be extracted from an inexhaustible supply of seawater. Also, they do not emit greenhouse gases. In addition to the construction of the tokamak (ITER), which is being constructed through collaboration of seven of the world’s leading countries and regions (Japan, EU, United States, South Korea, China, Russia, and India), fusion development by the private sector is also accelerating.
One of the most important components in these fusion reactors is the divertor, a component which gasifies impurities in the plasma and sends the gas to an exhaust pump. During operation of a fusion reactor, some of the structural components of the divertor are exposed to extremely large heat loads at the same level as the “space shuttle when entering the atmosphere.” Researchers are working to develop a solid divertor in which a block of heat-resistant material such as tungsten is placed in contact with the plasma and cooled with high-temperature, high-pressure water. This solid divertor system is also used in the ITER project and the prototype fusion reactors. Conversely, as an innovative mechanism to withstand the large heat load from plasma, researchers have also considered the concept of a liquid metal divertor which protects the divertor from plasma by covering the structural material of the divertor with a liquid metal that possesses excellent cooling performance.
Tin (Sn) is a metal that has been used in various ways in our daily lives; for example, as a material for tableware and as a component of solder. Tin has a relatively low melting point of 232°C and is suitable for use in a liquid state. Another property of tin is that its vapor pressure at high temperatures is lower than thatof other liquid metals. When liquid metal tin is used as a coolant to cover and protect the structural material surface of the liquid metal divertor of a fusion reactor, it is difficult to evaporate even if it is heated by plasma and reaches a high temperature. It also possesses the advantage of the evaporated metal being less likely to mix with the plasma. However, the corrosion of structural materials is a technical issue that has been concerned by researchers.
Research results
Kondo’s laboratory has focused on chemical coexistence with various structural and functional materials. The laboratory has given particular attention to liquid metal coolants attracting attention in the field of next-generation energy such as fusion reactors. Researchers concentrated on liquid metal tin, which reveals the inconvenient property of being highly reactive at high temperatures. They worked to clarify the corrosion mechanism of fusion reactor structural materials and to discover materials that exhibit corrosion resistance. More