A fusion reactor will heat the plasma to 150,000,000 degrees
This summer there were several good reasons to refresh the technical details of the ITER project. Firstly, a grandiose undertaking, the official start of which is the meeting of Mikhail Gorbachev and Ronald Reagan back in 1985, is taking on a material embodiment before our eyes. Designing a new generation reactor with the participation of Russia, the USA, Japan, China, India, South Korea and the European Union took more than 20 years. Today, ITER is no longer kilograms of technical documentation, but 42 hectares (1 km per 420 m) of perfectly flat surface of one of the largest man-made platforms in the world, located in the French city of Cadarache, 60 km north of Marseille. As well as the foundation of the future 360, 000-ton reactor, consisting of 150, 000 cubic meters of concrete, 16, 000 tons of reinforcement and 493 columns with rubber-metal antiseismic coating. And, of course, thousands of sophisticated scientific instruments and research facilities scattered across universities around the world.
The production of key reactor components is in full swing. In the spring, France reported on the manufacture of 70 frames for D-shaped coils of the toroidal field, and in June began winding the first coils of superconducting cables from Russia from the Institute of Cable Industry in Podolsk.
The second good reason to remember ITER right now is political. The new generation reactor is a test not only for scientists, but also for diplomats. This is such an expensive and technically difficult project that no country in the world can pull it alone. The ability of states to agree among themselves in both the scientific and financial spheres depends on whether it will be possible to complete the matter.
The ITER Council was scheduled for June 18 in St. Petersburg, but the US Department of State, as part of the sanctions, banned American scientists from visiting Russia. Taking into account the fact that the very idea of a tokamak (the toroidal chamber with magnetic coils that underlies ITER) belongs to the Soviet physicist Oleg Lavrentiev, the project participants treated this decision as a curiosity and simply transferred the advice to Kadarash on the same date. These events once again reminded the whole world that Russia (along with South Korea) is most responsible in fulfilling its obligations to the ITER project.
The phrase "thermonuclear reactor" in many people causes wariness. The associative chain is understandable: a thermonuclear bomb is worse than just a nuclear one, which means that a thermonuclear reactor is more dangerous than Chernobyl.
In fact, nuclear fusion, on which the tokamak principle is based, is much safer and more efficient than nuclear fission used in modern nuclear power plants. The synthesis is used by nature itself: the sun is nothing more than a natural thermonuclear reactor.
The reaction involves nuclei of deuterium and tritium - isotopes of hydrogen. The deuterium nucleus consists of a proton and a neutron, and the tritium nucleus consists of a proton and two neutrons. Under ordinary conditions, equally charged nuclei repel each other, but they can collide at very high temperatures.
In collisions, a strong interaction comes into play, which is responsible for combining protons and neutrons into nuclei. A core of a new chemical element arises - helium. In this case, one free neutron is formed and a large amount of energy is released. The strong interaction energy in the helium nucleus is less than in the nuclei of the starting elements. Due to this, the resulting core even loses in mass (according to the theory of relativity, energy and mass are equivalent). Recalling the famous equation E = mc2, where c is the speed of light, one can imagine the enormous energy potential of nuclear fusion.
In order to overcome the force of mutual repulsion, the initial nuclei must move very quickly, so temperature plays a key role in nuclear fusion. In the center of the sun, the process takes place at a temperature of 15 million degrees Celsius, but the colossal density of matter, due to the action of gravity, contributes to it. The colossal mass of the star makes it an effective thermonuclear reactor.
To create such a density on Earth is not possible. We can only increase the temperature. For hydrogen isotopes to give earthlings the energy of their nuclei, a temperature of 150 million degrees is needed, that is, ten times higher than on the Sun.
No solid material in the Universe can directly contact this temperature. So just building a stove for cooking helium will not work. The same toroidal chamber with magnetic coils, or tokamak, helps to solve the problem. The idea of creating a tokamak dawned on the bright minds of scientists from different countries in the early 1950s, while the championship is unequivocally attributed to the Soviet physicist Oleg Lavrentiev and his eminent colleagues Andrei Sakharov and Igor Tamm.
A vacuum chamber in the form of a torus (hollow donut) is surrounded by superconducting electromagnets that create a toroidal magnetic field in it. It is this field that holds the plasma heated up to ten suns at a certain distance from the walls of the chamber. Together with the central electromagnet (inductor), the tokamak is a transformer. By changing the current in the inductor, they generate a current flow in the plasma - the movement of particles necessary for synthesis.
Tokamak can rightfully be considered an example of technological sophistication. The electric current flowing in the plasma creates a poloidal magnetic field that encircles the plasma cord and maintains its shape. Plasma exists under strictly defined conditions, and at their slightest change the reaction immediately ceases. Unlike a nuclear power plant reactor, a tokamak cannot “peddle” and increase its temperature uncontrollably.
In the unlikely event of tokamak destruction, no radioactive contamination occurs. Unlike nuclear power plants, a thermonuclear reactor does not produce radioactive waste, and the only product of the synthesis reaction - helium - is not a greenhouse gas and is useful in the household. Finally, the tokamak uses fuel very carefully: during synthesis, only a few hundred grams of the substance are in the vacuum chamber, and the estimated annual fuel supply for an industrial power plant is only 250 kg.
Why do we need ITER?
The classical tokamaks described above were built in the USA and Europe, Russia and Kazakhstan, Japan and China. With their help, it was possible to prove the fundamental possibility of creating a high-temperature plasma. However, the construction of an industrial reactor capable of delivering more energy than consuming is a fundamentally different task.
In a classical tokamak, the current flow in the plasma is created by changing the current in the inductor, and this process cannot be infinite. Thus, the plasma lifetime is limited, and the reactor can only operate in a pulsed mode. Enormous energy is required to kindle plasma - is it a joke to heat anything to a temperature of 150, 000, 000 ° C. This means that it is necessary to achieve a plasma lifetime that will produce energy that pays for ignition.
For example, in 2009, during an experiment on the Chinese EAST tokamak (part of the ITER project), it was possible to hold a plasma with a temperature of 107 K for 400 seconds and 108 K for 60 seconds.
To hold plasma for longer, additional heaters of several types are needed. All of them will be tested on ITER. The first method - injection of neutral deuterium atoms - assumes that the atoms will enter the plasma previously dispersed to a kinetic energy of 1 MeV using an additional accelerator.
This process is initially contradictory: only charged particles can be accelerated (an electromagnetic field acts on them), and only neutral particles can be introduced into the plasma (otherwise they will affect the current flow inside the plasma cord). Therefore, an electron is preliminarily taken from the deuterium atoms, and positively charged ions enter the accelerator. Then the particles enter the neutralizer, where they are reduced to neutral atoms, interacting with the ionized gas, and are introduced into the plasma. Currently, the ITER megavoltage injector is being developed in Padua, Italy.
The second heating method has something in common with heating products in the microwave. It involves exposure to plasma by electromagnetic radiation with a frequency corresponding to the particle velocity (cyclotron frequency). For positive ions, this frequency is 40-50 MHz, and for electrons - 170 GHz. To create powerful radiation of such a high frequency, a device called a gyrotron is used. Nine of the 24 ITER gyrotrons are manufactured at the Gycom facility in Nizhny Novgorod.
The classical concept of a tokamak suggests that the shape of the plasma cord is supported by a poloidal magnetic field, which itself forms when current flows in the plasma. For prolonged plasma confinement, this approach is not applicable. The ITER tokamak has special coils of the poloidal field, the purpose of which is to keep the hot plasma away from the walls of the reactor. These coils are among the most massive and complex structural elements.
In order to be able to actively control the shape of the plasma, timely eliminating vibrations along the edges of the cord, the developers provided for small low-power electromagnetic circuits located directly in the vacuum chamber, under the casing.
Fuel infrastructure for thermonuclear fusion is a separate interesting topic. Deuterium is found in almost any water, and its reserves can be considered unlimited. But the world’s reserves of tritium are calculated on the strength of tens of kilograms. 1 kg of tritium costs about $ 30 million. For the first launches of ITER, you will need 3 kg of tritium. In comparison, about 2 kg of tritium per year is needed to maintain the nuclear capabilities of the United States Army.
However, in the future, the reactor will provide itself with tritium. In the course of the main synthesis reaction, high-energy neutrons are formed, which are capable of converting lithium nuclei to tritium. The development and testing of the first lithium containing reactor wall is one of ITER's most important goals. In the first tests, beryllium-copper plating will be used, the purpose of which is to protect the reactor mechanisms from heat. According to calculations, even if we transfer all the planet’s energy to tokamaks, the world’s lithium reserves will last for a thousand years of operation.
To the Tokamak World
Accurate diagnostic tools are needed for the precise control of a fusion reactor. One of the key tasks of ITER is to choose the most suitable of the five dozen tools that are being tested today, and to start developing new ones.
At least nine diagnostic devices will be developed in Russia. Three - at the Moscow Kurchatov Institute, including a neutron beam analyzer. The accelerator sends a focused neutron flux through the plasma, which undergoes spectral changes and is captured by the receiving system. Spectrometry with a frequency of 250 measurements per second shows the temperature and density of the plasma, the strength of the electric field and the speed of rotation of the particles — parameters necessary for controlling the reactor with the goal of prolonged plasma confinement.
The three tools are being prepared by the Ioffe Research Institute, including a neutral particle analyzer that traps atoms from a tokamak and helps control the concentration of deuterium and tritium in the reactor. The remaining devices will be made at Trinity Institute, where diamond detectors for the ITER vertical neutron chamber are currently being manufactured. All of these institutes use their own tokamaks for testing. And in the heat chamber of the NIIEFA named after Efremov, fragments of the first wall and the divertor target of the future ITER reactor are being tested.
Unfortunately, the fact that many of the components of the future megareactor already exists in the metal does not necessarily mean that the reactor will be built. Over the past decade, the estimated cost of the project has grown from 5 to 16 billion euros, and the planned first launch was postponed from 2010 to 2020. The fate of ITER depends entirely on the realities of our present, above all economic and political. Meanwhile, every scientist involved in the project sincerely believes that his success is capable of unrecognizably changing our future.The article “Ten Suns in a Furnace” was published in the journal Popular Mechanics (No. 8, August 2014). I wonder how a nuclear reactor works and can robots build a house?
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