ITER (International Thermonuclear Experimental Reactor) is one of the projects more ambitious and complex which humanity is facing. Its purpose is to imitate the processes that allow stars to obtain energy through the fusion of their fuel nuclei, which is made up of approximately 70% protium, which is the isotope of hydrogen that lacks neutrons, and therefore , it has only one proton and one electron; between 24 and 26% helium, and between 4 and 6% chemical elements heavier than helium.

The problem is that mimicking the nuclear fusion processes that occur naturally in the nuclei of stars is not easy. And it is not, among many other reasons, because we do not have a very valuable ally who makes it much easier for the stars: gravitational confinement. And it is that its mass is so enormous that gravity manages to compress the gases of the stellar nucleus as necessary to naturally recreate the conditions in which the hydrogen nuclei begin to spontaneously fuse. This is how the stars get their energy.

A challenge like this requires a good plan, and we have it

On Earth we cannot recreate those same conditions because we do not have the knowledge and technology to manipulate gravitational fields. Nothing seems to indicate that something like this will be possible in the future, much less that we will be able to generate a gravitational field minimally close to that of a star.

For this reason, to trigger nuclear fusion we have no choice but to heat the fuel in our reactors until it reaches a temperature between 150 and 300 million degrees centigrade, which, curiously, is ten times greater than that of the Sun’s nucleus. Only in this way do deuterium and tritium nuclei, which are the hydrogen isotopes that we use as fuel, manage to acquire the kinetic energy necessary to overcome their natural repulsion and fuse.

This is ITER’s goal: to produce 500 megawatts for no less than 500 s using only 1 g of tritium as part of the fuel and after investing about 50 megawatts of energy in the ignition of the reactor

The ITER nuclear fusion reactor has been designed to show that nuclear fusion on the scale that man can handle works. And also what is profitable from an energy point of view because it generates more energy than it is necessary to invest to start the process.

Its aim is to produce around 500 megawatts of power for no less than 500 seconds using only 1 gram of tritium as part of the fuel and after investing about 50 megawatts of energy in igniting the fusion reactor.

The machine that an international consortium is developing in the French town of Cadarache it is extraordinarily complex. In fact, probably only CERN’s particle detectors rival the ITER nuclear fusion reactor for engineering complexity.

A project of this magnitude is only possible gathering resources of the major powers on the planet, prompting China, Japan, Russia, the European Union, the United States, India and South Korea to come together to bring to fruition the amazing machine we are about to dive into.

The Tokamak reactor, in detail

The heart of ITER is its Tokamak-type reactor. This design was devised in the 1950s by Soviet physicists Igor Yevgenievich Tamm and Andrei Sakharov, which reminds us that we have been working on nuclear fusion, at least from a theoretical point of view, for almost seven decades. The defining characteristic of Tokamak reactors and allowing anyone to identify one at a glance is its donut shape.

The choice of this geometry, as we can guess, is not accidental; responds to the need to confine extremely hot fuel (in a plasma state) inside to recreate the conditions necessary for it to controlled fusion reactions take place.

Everything in ITER is colossal. Not only its complexity; also its figures. It will weigh 23,000 tons, and the chamber in which the plasma is confined will have a radius of 6.2 meters and a volume of 840 cubic meters.

Everything in ITER it’s colossal. Not only its complexity; also its figures. And it is that when it is finished it will weigh no less than 23,000 tons. More shocking facts: the radius of the section of the “donut” in which the plasma is confined measures 6.2 meters, and the volume of the vacuum chamber that contains the fuel at the monstrous temperature that I mentioned in the first paragraphs of the item is 840 m3.

This is the largest Tokamak reactor that humanity has built so far, and will possibly only be surpassed by DEMO, whose construction according to the timetable set by EUROfusion should be completed by the end of the next decade.

The cryostat

This component is a huge 29 x 29 meter stainless steel chamber that weighs 3,850 tons and has a volume of 16,000 m3. You have the responsibility to provide high vacuum necessary so that the conditions required for the fusion of the deuterium and tritium nuclei that make up the plasma at high temperature to occur inside the chamber.

The cryostat also takes care of preserve ultra-cold environment necessary for the superconducting magnets that we will talk about later to carry out their work. A few of the more than two hundred holes that we can see in its cylindrical surface are used for maintenance, but most of them are used to access the cooling system, diagnostic equipment or the blanket. that covers the inside of the reactor, among other applications.

The vacuum chamber

Like the cryostat, this 8,000-ton chamber is made of stainless steel, although its composition also contains a small amount of boron (around 2%). In its interior, the fusion of deuterium and tritium nuclei takes place, so one of its most important functions is to act as first containment barrier of residual radiation that may not be retained by the blanket, a crucial component that we will explore a little later.

The vacuum chamber is hermetically sealed, and its interior preserves the high vacuum necessary for the fusion of the plasma nuclei to take place. Its toroidal shape contributes to gas stabilization, so that the nuclei rotate at high speed around the central hole of the chamber, but without touching the walls of the torus at any time.

The temperature to which this chamber is subjected is very high, so it is necessary to introduce circulating water into a compartment housed between its internal and external walls to cool it and prevent it from reaching your maximum temperature threshold.

The magnets

The superconducting magnets placed on the outside of the vacuum chamber have the responsibility of generating the magnetic field necessary to confine the plasma inside. They are also responsible for controlling and stabilizing it to prevent it from touching the walls of the container. These magnets weigh 10,000 tons and are made of an alloy of niobium and tin, or niobium and titanium, which acquires superconductivity when cooled with supercritical helium to a temperature of -269ºC.

The structure that you can see above this paragraph is the heart of the complex magnetic motor of ITER. Its cylindrical shape allows this superconducting solenoid to be placed inside the central hole of the vacuum chamber, thus inducing a huge electric current in the plasma.

In addition, this very powerful magnet is used to optimize the shape of the plasma, stabilize it, and also helps warm it thanks to a mechanism known as the Joule Effect, helping to raise its temperature above the 150 million degrees Celsius necessary for the nuclear fusion reaction to take place. It is 18 meters high, 4 meters in diameter and weighs 1,000 tons.

The divertor

The huge component that we can see in this photograph is just one of the 54 identical pieces that make up the base of the reactor’s vacuum chamber. It is made of stainless steel, although incorporates tungsten shields which are responsible for supporting the bombardment of the high-energy neutrons from the plasma, transforming their kinetic energy into heat.

The water circulating inside is responsible for releasing this thermal energy and cooling the diverter. Tungsten has been chosen to fine-tune shields exposed to plasma because this is the metal with the highest melting point: no less than 3,422 ° C. In addition, the divertor is responsible for purifying the plasma, allowing the extraction of the ashes and impurities resulting from the nuclear fusion reaction and the interaction of the plasma with the most exposed layer of the mantle.

The mantle (‘blanket’)

The structure that we can see in this image is the mantle that covers the inside of the reactor’s vacuum chamber. Is a critical component which is on the front line of battle because it is exposed to the direct impact of the high energy neutrons resulting from the fusion of the deuterium and tritium nuclei.

In addition, it will be used to regenerate tritium that you need to use as fuel. To achieve this, it is necessary to coat the inner layer of the mantle with lithium, a chemical element that allows us to obtain tritium nuclei when lithium nuclei receive the impact of high-energy neutrons.

The mantle protects the vacuum chamber, cryostat, and magnets from heat and the direct impact of high-energy neutrons

The mantle also has the responsibility of protect the structure stainless steel vacuum chamber, cryostat and magnets from the heat and the direct impact of high-energy neutrons, which would degrade them in a short time.

The kinetic energy of neutrons is transformed into thermal energy when colliding with the mantle, and, again, the water from the cooling system is responsible for evacuating that heat, which will be used by power plants to produce electricity through a mechanism very similar to that used by current fission nuclear power plants.

One last interesting point to conclude the article: the chemical element that will constitute the most superficial layer of the mantle is beryllium because its physicochemical properties allow it to withstand the stress imposed by the impact of neutrons better than other metals.

The deeper layers of the mantle are made of copper and stainless steel, although it is possible that the elements used to manufacture both the mantle and the diverter of the future DEMO reactor will change if the technicians involved in the IFMIF-DONES project find materials capable of supporting better direct exposure to plasma to which these components are subjected.

Images | ITER | Rswilcox
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