World’s largest nuclear reactor

 World’s largest nuclear reactor : Magnetic field 250,000 times stronger than Earth’s

ITER, the world’s largest fusion experiment, took two-decade-long process of the reactor design and fabrication extended over three continents to complete. As the world looks for better ways to produce carbon-free energy, nuclear fusion reactions offer a plausible solution which can be turned OFF and ON demand. Recent advances in the field have demonstrated that it is possible to gain energy from nuclear fusion, and more than 30 countries are collaborated in building the International Thermonuclear Experimental Reactor (ITER) in France. ITER’s plasma current will peak at 15 million amperes, a record for tokamaks built anywhere in the world till date.

The reactor is designed to heat the plasma to 200 million degrees Celsius and maintain it for about 100 seconds, much longer than previous large tokamaks. This will allow researchers to investigate how to control and optimize the plasma stability and performance, which are crucial for achieving fusion power. The ITER design also uses the tokamak approach, in which hydrogen fuel is injected into a torus or donut-shaped vacuum chamber and heated to make plasma and replicate conditions on the Sun. At extremely high temperatures of 150 million degrees, the fusion reaction begins to occur. However, the plasma must be contained inside the reactor’s walls, a job done by giant superconducting magnets. 

The ITER design of the tokamak uses niobium-tin and niobium-titanium as the material of choice for its magnets. The coils are energized with electricity and then cooled to temperatures of four degrees above absolute zero (-269 degrees Celsius) to make them superconducting. ITER will deploy magnets in three different ways to make the invisible magnetic cage that will contain the plasma. The outer donut shape is achieved with 18 D-shaped toroidal magnets. A set of six magnets will circle the tokamak horizontally to help control the shape of the plasma, while a central solenoid will use energy pulses to generate current in the plasma. In terms of the magnetic field, the total magnetic energy of the design will be 41 gigajoules or 250,000 times stronger than Earth’s. 

As a compromise for letting France host ITER, the world’s largest fusion experiment, Japan received the opportunity to build JT-60SA and two other smaller fusion facilities. This was part of a 2007 agreement between Japan and the EU, which also involved upgrading Japan’s old JT-60 reactor that had been running since the mid-1980s. The reactor was completely rebuilt from the ground up, but the cost was not disclosed. JT-60SA stands for “superadvanced” and is about half as tall as ITER. It can hold 135 cubic meters of plasma, one-sixth of what ITER can handle. Its plasmas are expected to provide helpful information for ITER.

The reactor took more than 15 years to complete, much longer than expected. It was supposed to start operating in 2016, but it faced many challenges. It had to be redesigned, deal with procurement issues, and recover from the Tohoku earthquake in March 2011. Then, in March 2021, a severe problem occurred during testing. One of the superconducting magnetic coils had a short circuit in its cable, which damaged the electrical connections and caused a helium leak that could have affected the cooling systems. The current in the circuit was shallow at that time. “It could have been much worse if the current had been higher,” said Hiroshi Shirai, who leads the project. “We were lucky.” The JT-60SA team had to fix the insulation in more than 100 electrical connections, which took 2.5 years. The incident also made the engineers at ITER more cautious about testing their coils.

More than 54,000 miles (87,000 km) of the niobium-tin strands were needed to make conductors for the 19 toroidal magnets, but that was perhaps the simplest task in the fabrication process of ITER. To make the D-shaped magnet, nearly 2,500 feet (750 m) of the conductor was bent into a double spiral trajectory and heated to 1200 Fahrenheit (650 degrees Celsius). It was then inserted into a D-shaped radial plate made of stainless steel. The conductor was wrapped and insulated using glass and Kapton tape and laser-welded with cover plates to make a double pancake structure using two conductor layers. The double pancake was then insulated. Further, its air pockets were removed and injected with resin for better strength. Seven such double pancakes were then used to make a winding pack, the core of the D-shaped magnet, and interconnected for electric flow. The winding pack was then insulated, heat-treated, and injected with resin. The winding pack was then inserted into a 200-tonne stainless steel case strong enough to withstand the forces of plasma movement and fusion energy generation. 

As far as ITER is concerned, each toroidal magnet is 55 feet (17 m) tall, nearly 30 feet (9 m) wide, and weighs a whopping 360 tons. Ten magnets were made in Europe by Fusion for Energy, ITER’s European wing, while eight of these coils plus a spare were manufactured by the National Institutes for Quantum Science and Technology (QST) in Japan. The production process began with a niobium-tin strand wound with copper strands into a rope-like structure and inserted into a steel jacket designed with a central conduit where helium can be forced to flow. This structure is called the conductor. When assembled, the ITER fusion reactor will generate 500 MW of thermal power at its peak. When connected to the grid, it will generate 200 MW of power continuously, sufficient to power 200,000 homes.

JT-60SA has a drawback that it will only use hydrogen and its isotope deuterium, not tritium, another form of hydrogen that is more powerful but also more costly, rare and radioactive. Tritium is the preferred fuel for energy production, so ITER plans to use deuterium-tritium from 2035. Japan also aims to build DEMO by 2050, a proposed power plant that would bridge the gap between the research of JT-60SA and ITER and the commercialization of fusion energy.