"How to Endure 100 Million Degrees?"... The Secret of the Artificial Sun Project [Reading Science]

[Asia Economy Reporter Kim Bong-su] "Endure a temperature of 100 million degrees." South Korea has once again broken the world record in research on the 'Artificial Sun' (nuclear fusion reactor), which is gaining attention as one of the next-generation energy sources. What is the Artificial Sun, and how and why is it being created? Let's take a closer look.

◇ What is the Artificial Sun?

The Artificial Sun is a project concept to reproduce the nuclear fusion reactions occurring in the sun here on Earth and generate electricity from the heat produced. To trigger the same nuclear fusion reactions as the sun on Earth, a temperature exceeding 100 million degrees Celsius must be maintained. In the sun, hydrogen atoms combine under high pressure and a temperature of 15 million degrees Celsius to form helium, releasing enormous thermal energy in the process. However, since it is impossible to replicate such high pressure on Earth, a much higher temperature of over 100 million degrees must be maintained to enable the fusion of hydrogen atoms.

The Korea Institute of Fusion Energy recently succeeded in maintaining ultra-high-temperature plasma above 100 million degrees for 30 seconds in the Korean Artificial Sun (KSTAR). The core of KSTAR research is to sustain this for a long time and eventually convert it into electricity generation. Because it uses hydrogen, a cheap and clean energy source, it is regarded as a key technology that can replace nuclear and fossil fuel power generation. South Korea was the first among countries worldwide to achieve a temperature of 100 million degrees in 2018, and it has been breaking records with maintenance times of 20 seconds last year and 30 seconds this year, boasting world-class technology. The plan is to surpass the 300-second barrier by 2026 and begin the construction and commercialization of the first power plant in the 2040s. South Korea is also participating in the international nuclear fusion reactor (ITER) construction project with six other countries, with KSTAR serving as the basic model.

◇ How do you reach 100 million degrees?

KSTAR uses a fully superconducting tokamak (a sealed container that confines ultra-high-temperature plasma with superconducting magnets). There is no material in this world that can contain a flame of 100 million degrees. Therefore, superconducting magnets generate a magnetic field, which is then used to suspend the flame in mid-air. The temperature is raised using a neutral particle beam heating device. Ion particles are accelerated to high energy to generate heat, which is then transferred inside the plasma. This is why the device has a doughnut shape with a hollow center.

Another method is the 'stellarator' device, which is twisted like a pretzel. The tokamak method seals both ends of the plasma container with a strong magnetic field, but it has the disadvantage of significant magnetic field loss at both ends. The stellarator device twists the magnetic field so that plasma particles rotate while moving between the inside and outside of the doughnut-shaped toroid, reducing the escape rate. In other words, it can confine plasma without controlling the magnetic field individually.

Yoon Si-woo, head of KSTAR research, explained, "Other countries can only maintain 100 million degrees for 7 to 8 seconds," adding, "Our country fundamentally has excellent performance in superconducting magnets and other components, so we expect to achieve the goal of maintaining over 300 seconds by 2026."

◇ What materials can withstand 100 million degrees?

The temperature of magma flowing beneath the Earth's surface is about 1,200 degrees Celsius, and the inside of a blast furnace that melts steel is only around 1,500 degrees. So, what materials make up the interior of a nuclear fusion reactor that exceeds 100 million degrees? In the case of KSTAR, since the flame floats in the air, the fusion reactor facility does not come into direct contact with it. Still, the internal facilities must withstand temperatures well above several thousand degrees. The element with the highest melting point on Earth is carbon, which can endure up to about 3,642 degrees Celsius. Accordingly, KSTAR currently uses a carbon-made diverter installed inside to protect the equipment. However, carbon has the disadvantage that, when used over time, it combines with deuterium, the fuel of the fusion reactor, to form methane impurities. This means soot is generated like candle smoke.

Recently, tungsten has emerged as an alternative. Tungsten has a melting point slightly lower than carbon at 3,422 degrees Celsius but is the highest among metallic elements. It also has advantages such as less damage from neutrons emitted by plasma at ultra-high temperatures, high erosion resistance, and low radioactivation. As a result, it has already been selected for several nuclear fusion devices worldwide. The international nuclear fusion reactor (ITER), being constructed by South Korea and six other countries, has adopted tungsten for some parts, as have Germany's ASDEX-Upgrade and China's EAST. South Korea's KSTAR is currently replacing its carbon diverter with a tungsten diverter.

Tungsten means "heavy stone" in Swedish and has been used in various applications due to its excellent heat resistance, low thermal expansion, low vapor pressure, and the highest tensile strength among elements. The filament of incandescent light bulbs is made of tungsten. When alloyed with other metals, it boasts tremendous strength and is used in abrasives, drills, and cutting tools, as well as rocket and missile engine nozzles that must withstand ultra-high temperatures and pressures, and radiation shielding materials. It is also widely used for military purposes such as tanks, cannons, and missiles. Many countries classify tungsten as a strategic material and restrict its export and use.