How a thermonuclear reactor works and why it has not yet been built. Fusion reactor E.P. Velikhov, S.V. Putvinsky Low-energy nuclear reactions

ITER - International Thermonuclear Reactor (ITER)

Human energy consumption is growing every year, which pushes the energy sector towards active development. Thus, with the emergence of nuclear power plants, the amount of energy generated around the world increased significantly, which made it possible to safely use energy for all the needs of mankind. For example, 72.3% of the electricity generated in France comes from nuclear power plants, in Ukraine - 52.3%, in Sweden - 40.0%, in the UK - 20.4%, in Russia - 17.1%. However, technology does not stand still, and in order to meet the further energy needs of future countries, scientists are working on a number of innovative projects, one of which is ITER (International Thermonuclear Experimental Reactor).

Although the profitability of this installation is still in question, according to the work of many researchers, the creation and subsequent development of controlled thermonuclear fusion technology can result in a powerful and safe source of energy. Let's look at some of the positive aspects of such an installation:

• The main fuel of a thermonuclear reactor is hydrogen, which means practically inexhaustible reserves of nuclear fuel.
• Hydrogen can be produced by processing seawater, which is available to most countries. It follows from this that a monopoly of fuel resources cannot arise.
• The probability of an emergency explosion during the operation of a thermonuclear reactor is much less than during the operation of a nuclear reactor. According to researchers, even in the event of an accident, radiation emissions will not pose a danger to the population, which means there is no need for evacuation.
• Unlike nuclear reactors, fusion reactors produce radioactive waste that has a short half-life, meaning it decays faster. Also, there are no combustion products in thermonuclear reactors.
• A fusion reactor does not require materials that are also used for nuclear weapons. This eliminates the possibility of covering up the production of nuclear weapons by processing materials for the needs of a nuclear reactor.

Thermonuclear reactor - inside view

However, there are also a number of technical shortcomings that researchers constantly encounter.

For example, the current version of the fuel, presented in the form of a mixture of deuterium and tritium, requires the development of new technologies. For example, at the end of the first series of tests at the JET thermonuclear reactor, the largest to date, the reactor became so radioactive that the development of a special robotic maintenance system was further required to complete the experiment. Another disappointing factor in the operation of a thermonuclear reactor is its efficiency - 20%, while the efficiency of a nuclear power plant is 33-34%, and a thermal power plant is 40%.

Creation of the ITER project and launch of the reactor

The ITER project dates back to 1985, when the Soviet Union proposed the joint creation of a tokamak - a toroidal chamber with magnetic coils that can hold plasma using magnets, thereby creating the conditions required for a thermonuclear fusion reaction to occur. In 1992, a quadripartite agreement on the development of ITER was signed, the parties to which were the EU, the USA, Russia and Japan. In 1994, the Republic of Kazakhstan joined the project, in 2001 - Canada, in 2003 - South Korea and China, in 2005 - India. In 2005, the location for the construction of the reactor was determined - the Cadarache Nuclear Energy Research Center, France.

Construction of the reactor began with the preparation of a pit for the foundation. So the parameters of the pit were 130 x 90 x 17 meters. The entire tokamak complex will weigh 360,000 tons, of which 23,000 tons are the tokamak itself.

Various elements of the ITER complex will be developed and delivered to the construction site from all over the world. So in 2016, part of the conductors for poloidal coils was developed in Russia, which were then sent to China, which will produce the coils themselves.

Obviously, such a large-scale work is not at all easy to organize; a number of countries have repeatedly failed to keep up with the project schedule, as a result of which the launch of the reactor was constantly postponed. So, according to last year’s (2016) June message: “receipt of the first plasma is planned for December 2025.”

The operating mechanism of the ITER tokamak

The term "tokamak" comes from a Russian acronym that means "toroidal chamber with magnetic coils."

The heart of a tokamak is its torus-shaped vacuum chamber. Inside, under extreme temperature and pressure, the hydrogen fuel gas becomes plasma—a hot, electrically charged gas. As is known, stellar matter is represented by plasma, and thermonuclear reactions in the solar core occur precisely under conditions of elevated temperature and pressure. Similar conditions for the formation, retention, compression and heating of plasma are created by means of massive magnetic coils that are located around a vacuum vessel. The influence of magnets will limit the hot plasma from the walls of the vessel.

Before the process begins, air and impurities are removed from the vacuum chamber. Magnetic systems that will help control the plasma are then charged and gaseous fuel is introduced. When a powerful electric current is passed through the vessel, the gas is electrically split and becomes ionized (that is, electrons leave the atoms) and forms a plasma.

As the plasma particles are activated and collide, they also begin to heat up. Assisted heating techniques help bring the plasma to melting temperatures (150 to 300 million °C). Particles "excited" to this degree can overcome their natural electromagnetic repulsion upon collision, releasing enormous amounts of energy as a result of such collisions.

The tokamak design consists of the following elements:

Vacuum vessel

(“donut”) is a toroidal chamber made of stainless steel. Its large diameter is 19 m, the small one is 6 m, and its height is 11 m. The volume of the chamber is 1,400 m 3, and its weight is more than 5,000 tons. The walls of the vacuum vessel are double; a coolant will circulate between the walls, which will be distilled water. water. To avoid water contamination, the inner wall of the chamber is protected from radioactive radiation using a blanket.

Blanket

(“blanket”) – consists of 440 fragments covering the inner surface of the chamber. The total banquet area is 700m2. Each fragment is a kind of cassette, the body of which is made of copper, and the front wall is removable and made of beryllium. The parameters of the cassettes are 1x1.5 m, and the mass is no more than 4.6 tons. Such beryllium cassettes will slow down high-energy neutrons formed during the reaction. During neutron moderation, heat will be released and removed by the cooling system. It should be noted that beryllium dust formed as a result of reactor operation can cause a serious disease called beryllium and also has a carcinogenic effect. For this reason, strict security measures are being developed at the complex.

Tokamak in section. Yellow - solenoid, orange - toroidal field (TF) and poloidal field (PF) magnets, blue - blanket, light blue - VV - vacuum vessel, purple - divertor

(“ashtray”) of the poloidal type is a device whose main task is to “cleanse” the plasma of dirt resulting from the heating and interaction of the blanket-covered chamber walls with it. When such contaminants enter the plasma, they begin to radiate intensely, resulting in additional radiation losses. It is located at the bottom of the tokomak and uses magnets to direct the upper layers of plasma (which are the most contaminated) into the cooling chamber. Here the plasma cools and turns into gas, after which it is pumped back out of the chamber. Beryllium dust, after entering the chamber, is practically unable to return back to the plasma. Thus, plasma contamination remains only on the surface and does not penetrate deeper.

Cryostat

- the largest component of the tokomak, which is a stainless steel shell with a volume of 16,000 m 2 (29.3 x 28.6 m) and a mass of 3,850 tons. Other elements of the system will be located inside the cryostat, and it itself serves as a barrier between the tokamak and the outside environment. On its inner walls there will be thermal screens cooled by circulating nitrogen at a temperature of 80 K (-193.15 °C).

Magnetic system

– a set of elements that serve to contain and control plasma inside a vacuum vessel. It is a set of 48 elements:

• Toroidal field coils are located outside the vacuum chamber and inside the cryostat. They are presented in 18 pieces, each measuring 15 x 9 m and weighing approximately 300 tons. Together, these coils generate a magnetic field of 11.8 Tesla around the plasma torus and store energy of 41 GJ.
• Poloidal field coils – located on top of the toroidal field coils and inside the cryostat. These coils are responsible for generating a magnetic field that separates the plasma mass from the chamber walls and compresses the plasma for adiabatic heating. The number of such coils is 6. Two of the coils have a diameter of 24 m and a mass of 400 tons. The remaining four are somewhat smaller.
• The central solenoid is located in the inner part of the toroidal chamber, or rather in the “donut hole”. The principle of its operation is similar to a transformer, and the main task is to excite an inductive current in the plasma.
• Correction coils are located inside the vacuum vessel, between the blanket and the chamber wall. Their task is to maintain the shape of the plasma, capable of locally “bulging” and even touching the walls of the vessel. Allows you to reduce the level of interaction of the chamber walls with the plasma, and therefore the level of its contamination, and also reduces the wear of the chamber itself.

Structure of the ITER complex

The tokamak design described above “in a nutshell” is a highly complex innovative mechanism assembled through the efforts of several countries. However, for its full operation, a whole complex of buildings located near the tokamak is required. Among them:

• Control, Data Access and Communication System – CODAC. Located in a number of buildings of the ITER complex.
• Fuel storage and fuel system - serves to deliver fuel to the tokamak.
• Vacuum system - consists of more than four hundred vacuum pumps, the task of which is to pump out thermonuclear reaction products, as well as various contaminants from the vacuum chamber.
• Cryogenic system – represented by a nitrogen and helium circuit. The helium circuit will normalize the temperature in the tokamak, the work (and therefore the temperature) of which does not occur continuously, but in pulses. The nitrogen circuit will cool the cryostat's heat shields and the helium circuit itself. There will also be a water cooling system, which is aimed at lowering the temperature of the blanket walls.
• Power supply. The tokamak will require approximately 110 MW of energy to operate continuously. To achieve this, kilometer-long power lines will be installed and connected to the French industrial network. It is worth recalling that the ITER experimental facility does not provide for energy generation, but operates only in scientific interests.

ITER funding

The international thermonuclear reactor ITER is a fairly expensive undertaking, which was initially estimated at $12 billion, with Russia, the USA, Korea, China and India accounting for 1/11 of the amount, Japan for 2/11, and the EU for 4/11 . This amount later increased to $15 billion. It is noteworthy that financing occurs through the supply of equipment required for the complex, which is developed in each country. Thus, Russia supplies blankets, plasma heating devices and superconducting magnets.

Project perspective

At the moment, the construction of the ITER complex and the production of all the required components for the tokamak are underway. After the planned launch of the tokamak in 2025, a series of experiments will begin, based on the results of which aspects that require improvement will be noted. After the successful commissioning of ITER, it is planned to build a power plant based on thermonuclear fusion called DEMO (DEMOnstration Power Plant). DEMo's goal is to demonstrate the so-called "commercial appeal" of fusion power. If ITER is capable of generating only 500 MW of energy, then DEMO will be able to continuously generate energy of 2 GW.

However, it should be borne in mind that the ITER experimental facility will not produce energy, and its purpose is to obtain purely scientific benefits. And as you know, this or that physical experiment can not only meet expectations, but also bring new knowledge and experience to humanity.

Humanity is gradually approaching the border of irreversible depletion of the Earth's hydrocarbon resources. We have been extracting oil, gas and coal from the bowels of the planet for almost two centuries, and it is already clear that their reserves are being depleted at tremendous speed. The leading countries of the world have long been thinking about creating a new source of energy, environmentally friendly, safe from the point of view of operation, with enormous fuel reserves.

Fusion reactor

Today there is a lot of talk about the use of so-called alternative types of energy - renewable sources in the form of photovoltaics, wind energy and hydropower. It is obvious that, due to their properties, these directions can only act as auxiliary sources of energy supply.

As a long-term prospect for humanity, only energy based on nuclear reactions can be considered.

On the one hand, more and more states are showing interest in building nuclear reactors on their territory. But still, a pressing problem for nuclear energy is the processing and disposal of radioactive waste, and this affects economic and environmental indicators. Back in the middle of the 20th century, the world's leading physicists, in search of new types of energy, turned to the source of life on Earth - the Sun, in the depths of which, at a temperature of about 20 million degrees, reactions of synthesis (fusion) of light elements take place with the release of colossal energy.

Domestic specialists handled the task of developing a facility for implementing nuclear fusion reactions under terrestrial conditions best of all. The knowledge and experience in the field of controlled thermonuclear fusion (CTF), obtained in Russia, formed the basis of the project, which is, without exaggeration, the energy hope of humanity - the International Experimental Thermonuclear Reactor (ITER), which is being built in Cadarache (France).

History of thermonuclear fusion

The first thermonuclear research began in countries working on their atomic defense programs. This is not surprising, because at the dawn of the atomic era, the main purpose of the appearance of deuterium plasma reactors was the study of physical processes in hot plasma, knowledge of which was necessary, among other things, for the creation of thermonuclear weapons. According to declassified data, the USSR and the USA began almost simultaneously in the 1950s. work on UTS. But, at the same time, there is historical evidence that back in 1932, the old revolutionary and close friend of the leader of the world proletariat Nikolai Bukharin, who at that time held the post of chairman of the Supreme Economic Council committee and followed the development of Soviet science, proposed to launch a project in the country to study controlled thermonuclear reactions.

The history of the Soviet thermonuclear project is not without a fun fact. The future famous academician and creator of the hydrogen bomb, Andrei Dmitrievich Sakharov, was inspired by the idea of ​​magnetic thermal insulation of high-temperature plasma from a letter from a Soviet army soldier. In 1950, Sergeant Oleg Lavrentyev, who served on Sakhalin, sent a letter to the Central Committee of the All-Union Communist Party in which he proposed using lithium-6 deuteride instead of liquefied deuterium and tritium in a hydrogen bomb, and also creating a system with electrostatic confinement of hot plasma to carry out controlled thermonuclear fusion . The letter was reviewed by the then young scientist Andrei Sakharov, who wrote in his review that he “considers it necessary to have a detailed discussion of Comrade Lavrentiev’s project.”

Already by October 1950, Andrei Sakharov and his colleague Igor Tamm made the first estimates of a magnetic thermonuclear reactor (MTR). The first toroidal installation with a strong longitudinal magnetic field, based on the ideas of I. Tamm and A. Sakharov, was built in 1955 in LIPAN. It was called TMP - a torus with a magnetic field. Subsequent installations were already called TOKAMAK, after the combination of the initial syllables in the phrase “TORIDAL CHAMBER MAGNETIC COIL”. In its classic version, a tokamak is a donut-shaped toroidal chamber placed in a toroidal magnetic field. From 1955 to 1966 At the Kurchatov Institute, 8 such installations were built, on which a lot of different studies were carried out. If before 1969, a tokamak was built outside the USSR only in Australia, then in subsequent years they were built in 29 countries, including the USA, Japan, European countries, India, China, Canada, Libya, Egypt. In total, about 300 tokamaks have been built in the world to date, including 31 in the USSR and Russia, 30 in the USA, 32 in Europe and 27 in Japan. In fact, three countries - the USSR, Great Britain and the USA - were engaged in an unspoken competition to see who would be the first to harness plasma and actually begin producing energy “from water.”

The most important advantage of a thermonuclear reactor is the reduction in radiation biological hazard by approximately a thousand times in comparison with all modern nuclear power reactors.

A thermonuclear reactor does not emit CO2 and does not produce “heavy” radioactive waste. This reactor can be placed anywhere, anywhere.

A step of half a century

In 1985, academician Evgeniy Velikhov, on behalf of the USSR, proposed that scientists from Europe, the USA and Japan work together to create a thermonuclear reactor, and already in 1986 in Geneva an agreement was reached on the design of the installation, which later received the name ITER. In 1992, the partners signed a quadripartite agreement to develop an engineering design for the reactor. The first stage of construction is scheduled to be completed by 2020, when it is planned to receive the first plasma. In 2011, real construction began at the ITER site.

The ITER design follows the classic Russian tokamak, developed back in the 1960s. It is planned that at the first stage the reactor will operate in a pulsed mode with a power of thermonuclear reactions of 400–500 MW, at the second stage the continuous operation of the reactor, as well as the tritium reproduction system, will be tested.

It is not for nothing that the ITER reactor is called the energy future of humanity. Firstly, this is the world’s largest scientific project, because in France it is being built by almost the entire world: the EU + Switzerland, China, India, Japan, South Korea, Russia and the USA are participating. The agreement on the construction of the installation was signed in 2006. European countries contribute about 50% of the project's financing, Russia accounts for approximately 10% of the total amount, which will be invested in the form of high-tech equipment. But Russia’s most important contribution is the tokamak technology itself, which formed the basis of the ITER reactor.

Secondly, this will be the first large-scale attempt to use the thermonuclear reaction that occurs in the Sun to generate electricity. Thirdly, this scientific work should bring very practical results, and by the end of the century the world expects the appearance of the first prototype of a commercial thermonuclear power plant.

Scientists assume that the first plasma at the international experimental thermonuclear reactor will be produced in December 2025.

Why did literally the entire world scientific community begin to build such a reactor? The fact is that many technologies that are planned to be used in the construction of ITER do not belong to all countries at once. One state, even the most highly developed in scientific and technical terms, cannot immediately have a hundred technologies of the highest world level in all fields of technology used in such a high-tech and breakthrough project as a thermonuclear reactor. But ITER consists of hundreds of similar technologies.

Russia surpasses the global level in many thermonuclear fusion technologies. But, for example, Japanese nuclear scientists also have unique competencies in this area, which are quite applicable in ITER.

Therefore, at the very beginning of the project, the partner countries came to agreements about who and what would be supplied to the site, and that this should not just be cooperation in engineering, but an opportunity for each of the partners to receive new technologies from other participants, so that in the future develop them yourself.

Andrey Retinger, international journalist

Fusion reactor E.P. Velikhov, S.V. Putvinsky

E.P. Velikhov, S.V. Putvinsky.
Report dated October 22, 1999, carried out within the framework of the Energy Center of the World Federation of Scientists

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This article provides a brief overview of the current state of fusion research and outlines the prospects for fusion power in the 21st century energy system. The review is intended for a wide range of readers familiar with the basics of physics and engineering.

According to modern physical concepts, there are only a few fundamental sources of energy that, in principle, can be mastered and used by humanity. Nuclear fusion reactions are one such source of energy and... In fusion reactions, energy is produced due to the work of nuclear forces performed during the fusion of nuclei of light elements and the formation of heavier nuclei. These reactions are widespread in nature - it is believed that the energy of stars, including the Sun, is produced as a result of a chain of nuclear fusion reactions that convert four nuclei of a hydrogen atom into a helium nucleus. We can say that the Sun is a large natural thermonuclear reactor that supplies energy to the Earth's ecological system.

Currently, more than 85% of the energy produced by humans is obtained by burning organic fuels - coal, oil and natural gas. This cheap source of energy, mastered by man about 200 - 300 years ago, led to the rapid development of human society, its well-being and, as a result, to the growth of the Earth's population. It is assumed that due to population growth and more uniform energy consumption across regions, energy production will increase by about three times by 2050 compared to the current level and reach 10 21 J per year. There is no doubt that in the foreseeable future the previous source of energy - organic fuels - will have to be replaced by other types of energy production. This will happen both due to the depletion of natural resources and due to environmental pollution, which, according to experts, should occur much earlier than cheap natural resources are developed (the current method of energy production uses the atmosphere as a garbage dump, throwing out 17 million tons daily carbon dioxide and other gases accompanying the combustion of fuels). The transition from fossil fuels to large-scale alternative energy is expected in the middle of the 21st century. It is assumed that the future energy system will use a variety of energy sources, including renewable energy sources, more widely than the current energy system, such as solar energy, wind energy, hydroelectric power, growing and burning biomass and nuclear energy. The share of each energy source in the total energy production will be determined by the structure of energy consumption and the economic efficiency of each of these energy sources.

In today's industrial society, more than half of the energy is used in a constant consumption mode, independent of the time of day and season. Superimposed on this constant base power are daily and seasonal variations. Thus, the energy system must consist of base energy, which supplies energy to society at a constant or quasi-permanent level, and energy resources, which are used as needed. It is expected that renewable energy sources such as solar energy, biomass combustion, etc. will be used mainly in the variable component of energy consumption and. The main and only candidate for base energy is nuclear energy. Currently, only nuclear fission reactions, which are used in modern nuclear power plants, have been mastered to produce energy. Controlled thermonuclear fusion is, so far, only a potential candidate for basic energy.

What advantages does thermonuclear fusion have over nuclear fission reactions, which allow us to hope for the large-scale development of thermonuclear energy? The main and fundamental difference is the absence of long-lived radioactive waste, which is typical for nuclear fission reactors. And although during the operation of a thermonuclear reactor the first wall is activated by neutrons, the choice of suitable low-activation structural materials opens up the fundamental possibility of creating a thermonuclear reactor in which the induced activity of the first wall will decrease to a completely safe level thirty years after the reactor is shut down. This means that an exhausted reactor will need to be mothballed for only 30 years, after which the materials can be recycled and used in a new synthesis reactor. This situation is fundamentally different from fission reactors, which produce radioactive waste that requires reprocessing and storage for tens of thousands of years. In addition to low radioactivity, thermonuclear energy has huge, practically inexhaustible reserves of fuel and other necessary materials, sufficient to produce energy for many hundreds, if not thousands of years.

It was these advantages that prompted the major nuclear countries to begin large-scale research on controlled thermonuclear fusion in the mid-50s. By this time, the first successful tests of hydrogen bombs had already been carried out in the Soviet Union and the United States, which confirmed the fundamental possibility of using energy and nuclear fusion in terrestrial conditions. From the very beginning, it became clear that controlled thermonuclear fusion had no military application. The research was declassified in 1956 and has since been carried out within the framework of broad international cooperation. The hydrogen bomb was created in just a few years, and at that time it seemed that the goal was close, and that the first large experimental facilities, built in the late 50s, would produce thermonuclear plasma. However, it took more than 40 years of research to create conditions under which the release of thermonuclear power is comparable to the heating power of the reacting mixture. In 1997, the largest thermonuclear installation, the European TOKAMAK (JET), received 16 MW of thermonuclear power and came close to this threshold.

What was the reason for this delay? It turned out that in order to achieve the goal, physicists and engineers had to solve a lot of problems that they had no idea about at the beginning of the journey. During these 40 years, the science of plasma physics was created, which made it possible to understand and describe the complex physical processes occurring in the reacting mixture. Engineers needed to solve equally complex problems, including learning how to create deep vacuums in large volumes, selecting and testing suitable construction materials, developing large superconducting magnets, powerful lasers and X-ray sources, developing pulsed power systems capable of creating powerful beams of particles, develop methods for high-frequency heating of the mixture and much more.

§4 is devoted to a review of research in the field of magnetic controlled fusion, which includes systems with magnetic confinement and pulsed systems. Most of this review is devoted to the most advanced systems for magnetic plasma confinement, TOKAMAK-type installations.

The scope of this review allows us to discuss only the most significant aspects of research on controlled thermonuclear fusion. The reader interested in a more in-depth study of various aspects of this problem may be advised to consult the review literature. There is an extensive literature devoted to controlled thermonuclear fusion. In particular, mention should be made of both now classic books written by the founders of controlled thermonuclear research, as well as very recent publications, such as, for example, which outline the current state of thermonuclear research.

Although there are quite a lot of nuclear fusion reactions leading to the release of energy, for practical purposes of using nuclear energy, only the reactions listed in Table 1 are of interest. Here and below we use the standard designation for hydrogen isotopes: p - proton with atomic mass 1, D - deuteron, with atomic mass 2 and T - tritium, isotope with mass 3. All nuclei participating in these reactions with the exception of tritium are stable. Tritium is a radioactive isotope of hydrogen with a half-life of 12.3 years. As a result of β-decay, it turns into He 3, emitting a low-energy electron. Unlike nuclear fission reactions, fusion reactions do not produce long-lived radioactive fragments of heavy nuclei, which makes it possible in principle to create a “clean” reactor, not burdened with the problem of long-term storage of radioactive waste.

Table 1.
Nuclear reactions of interest for controlled fusion

All reactions shown in Table 1, except the last one, occur with the release of energy and in the form of kinetic energy and reaction products, q, which is indicated in brackets in units of millions of electron volts (MeV),
(1 eV = 1.6 ·10 –19 J = 11600 °K). The last two reactions play a special role in controlled fusion - they will be used to produce tritium, which does not exist in nature.

Nuclear fusion reactions 1-5 have a relatively high reaction rate, which is usually characterized by the reaction cross section, σ. The reaction cross sections from Table 1 are shown in Fig. 1 as a function of energy and colliding particles in the center of mass system.

Due to the presence of Coulomb repulsion between nuclei, the cross sections for reactions at low energy and particles are negligible, and therefore, at ordinary temperatures, a mixture of hydrogen isotopes and other light atoms practically does not react. In order for any of these reactions to have a noticeable cross section, the colliding particles need to have high kinetic energy. Then the particles will be able to overcome the Coulomb barrier, approach at a distance on the order of nuclear ones, and react. For example, the maximum cross section for the reaction of deuterium with tritium is achieved at a particle energy of about 80 KeV, and in order for a DT mixture to have a high reaction rate, its temperature must be on the scale of one hundred million degrees, T = 10 8 ° K.

The simplest way to produce energy and nuclear fusion that immediately comes to mind is to use an ion accelerator and bombard, say, tritium ions accelerated to an energy of 100 KeV, a solid or gas target containing deuterium ions. However, the injected ions slow down too quickly when colliding with the cold electrons of the target, and do not have time to produce enough energy to cover the energy costs of their acceleration, despite the huge difference in the initial (about 100 KeV) and energy produced in the reaction ( about 10 MeV). In other words, with this “method” of energy production and the energy reproduction coefficient and,
Q fus = P synthesis / P costs will be less than 1.

In order to increase Q fus, the target electrons can be heated. Then fast ions will decelerate more slowly and Q fus will increase. However, a positive yield is achieved only at a very high target temperature - on the order of several KeV. At this temperature, the injection of fast ions is no longer important; there is a sufficient amount of energetic thermal ions in the mixture, which themselves enter into reactions. In other words, thermonuclear reactions or thermonuclear fusion occur in the mixture.

The rate of thermonuclear reactions can be calculated by integrating the reaction cross section shown in Fig. 1 over the equilibrium Maxwellian particle distribution function. As a result, it is possible to obtain the reaction rate K(T), which determines the number of reactions occurring per unit volume, n 1 n 2 K(T), and, consequently, the volumetric density of energy release in the reacting mixture,

P fus = q n 1 n 2 K(T) (1)

In the last formula n 1 n 2- volumetric concentrations of reacting components, T- temperature of reacting particles and q- energy yield of the reaction given in Table 1.

At a high temperature characteristic of a reacting mixture, the mixture is in a plasma state, i.e. consists of free electrons and positively charged ions that interact with each other through collective electromagnetic fields. Electromagnetic fields, self-consistent with the motion of plasma particles, determine the dynamics of the plasma and, in particular, maintain its quasineutrality. With very high accuracy, the charge densities of ions and electrons in plasma are equal, n e = Zn z, where Z is the charge of the ion (for hydrogen isotopes Z = 1). The ion and electron components exchange energy due to Coulomb collisions and at plasma parameters typical for thermonuclear applications, their temperatures are approximately equal.

For the high temperature of the mixture you have to pay with additional energy costs. First, we need to take into account the bremsstrahlung emitted by electrons when colliding with ions:

The power of bremsstrahlung, as well as the power of thermonuclear reactions in the mixture, is proportional to the square of the plasma density and, therefore, the ratio P fus /P b depends only on the plasma temperature. Bremsstrahlung, in contrast to the power of thermonuclear reactions, weakly depends on the plasma temperature, which leads to the presence of a lower limit on the plasma temperature at which the power of thermonuclear reactions is equal to the power of bremsstrahlung losses, P fus /P b = 1. At temperatures below the threshold bremsstrahlung power losses exceed the thermonuclear release of energy and, and therefore in a cold mixture a positive energy release is impossible. The mixture of deuterium and tritium has the lowest limiting temperature, but even in this case the temperature of the mixture must exceed 3 KeV (3.5 10 7 °K). The threshold temperatures for the DD and DHe 3 reactions are approximately an order of magnitude higher than for the DT reaction. For the reaction of a proton with boron, bremsstrahlung radiation at any temperature exceeds the reaction yield, and, therefore, to use this reaction, special traps are needed in which the electron temperature is lower than the ion temperature, or the plasma density is so high that the radiation is absorbed by the working mixture.

In addition to the high temperature of the mixture, for a positive reaction to occur, the hot mixture must exist long enough for the reactions to occur. In any thermonuclear system with finite dimensions, there are additional channels of energy loss from the plasma in addition to bremsstrahlung (for example, due to thermal conductivity, line radiation of impurities, etc.), the power of which should not exceed the thermonuclear energy release. In the general case, additional energy losses can be characterized by the energy lifetime of the plasma t E, defined in such a way that the ratio 3nT / t E gives the power loss per unit plasma volume. Obviously, for a positive yield it is necessary that the thermonuclear power exceed the power of additional losses, P fus > 3nT / t E , which gives a condition for the minimum product of density and plasma lifetime, nt E . For example, for a DT reaction it is necessary that

nt E > 5 10 19 s/m 3 (3)

This condition is usually called the Lawson criterion (strictly speaking, in the original work, the Lawson criterion was derived for a specific thermonuclear reactor design and, unlike (3), includes the efficiency of converting thermal energy into electrical energy). In the form in which it is written above, the criterion is practically independent of the thermonuclear system and is a generalized necessary condition for a positive output. The Lawson criterion for other reactions is one or two orders of magnitude higher than for the DT reaction, and the threshold temperature is also higher. The proximity of the device to achieving a positive output is usually depicted on the T - nt E plane, which is shown in Fig. 2.

nt E

Fig.2. Region with a positive yield of a nuclear reaction on the T-nt E plane.
The achievements of various experimental installations for confining thermonuclear plasma are shown.

It can be seen that DT reactions are more easily feasible - they require a significantly lower plasma temperature than DD reactions and impose less stringent conditions on its retention. The modern thermonuclear program is aimed at implementing DT-controlled fusion.

Thus, controlled thermonuclear reactions are, in principle, possible, and the main task of thermonuclear research is the development of a practical device that could compete economically with other sources of energy and.

All devices invented over 50 years can be divided into two large classes: 1) stationary or quasi-stationary systems based on magnetic confinement of hot plasma; 2) pulse systems. In the first case, the plasma density is low and the Lawson criterion is achieved due to good energy retention in the system, i.e. long energy plasma lifetime. Therefore, systems with magnetic confinement have a characteristic plasma size of the order of several meters and a relatively low plasma density, n ~ 10 20 m -3 (this is approximately 10 5 times lower than the atomic density at normal pressure and room temperature).

In pulsed systems, the Lawson criterion is achieved by compressing fusion targets with laser or x-ray radiation and creating a very high-density mixture. The lifetime in pulsed systems is short and is determined by the free expansion of the target. The main physical challenge in this direction of controlled fusion is to reduce the total energy and explosion to a level that will make it possible to make a practical fusion reactor.

Both types of systems have already come close to creating experimental machines with a positive energy output and Q fus > 1, in which the main elements of future thermonuclear reactors will be tested. However, before moving on to a discussion of fusion devices, we will consider the fuel cycle of a future fusion reactor, which is largely independent of the specific design of the system.

1) TOKAMAK T-15 has so far only operated in the mode with ohmic plasma heating and, therefore, the plasma parameters obtained with this installation are quite low. In the future, it is planned to introduce 10 MW of neutral injection and 10 MW of electron cyclotron heating.

2) The given Q fus was recalculated from the parameters of the DD plasma obtained in the setup to the DT plasma.

And although the experimental program on these TOKAMAKs has not yet been completed, this generation of machines has practically completed the tasks assigned to it. TOKAMAKs JET and TFTR for the first time received high thermonuclear power of DT reactions in plasma, 11 MW in TFTR and 16 MW in JET. Figure 6 shows the time dependences of thermonuclear power in DT experiments.

Fig.6. Dependence of thermonuclear power on time in record deuterium-tritium discharges at the JET and TFTR tokamaks.

This generation of TOKAMAKs reached the threshold value Q fus = 1 and received nt E only several times lower than that required for a full-scale TOKAMAK reactor. TOKAMAKs have learned to maintain a stationary plasma current using RF fields and neutral beams. The physics of plasma heating by fast particles, including thermonuclear alpha particles, was studied, the operation of the divertor was studied, and modes of its operation with low thermal loads were developed. The results of these studies made it possible to create the physical foundations necessary for the next step - the first TOKAMAK reactor, which will operate in combustion mode.

What physical restrictions on plasma parameters are there in TOKAMAKs?

Maximum plasma pressure in TOKAMAK or maximum value β is determined by the stability of the plasma and is approximately described by Troyon's relation,

Where β expressed in %, Ip– current flowing in the plasma and β N is a dimensionless constant called the Troyon coefficient. The parameters in (5) have the dimensions MA, T, m. Maximum values ​​of the Troyon coefficient β N= 3÷5, achieved in experiments, are in good agreement with theoretical predictions based on calculations of plasma stability. Fig.7 shows the limit values β , obtained in various TOKAMAKs.

Fig.7. Comparison of limit values β achieved in Troyon scaling experiments.

If the limit value is exceeded β , large-scale helical disturbances develop in the TOKAMAK plasma, the plasma quickly cools and dies on the wall. This phenomenon is called plasma stall.

As can be seen from Fig. 7, TOKAMAK is characterized by rather low values β at the level of several percent. There is a fundamental possibility to increase the value β by reducing the plasma aspect ratio to extremely low values ​​of R/ a= 1.3÷1.5. Theory predicts that in such machines β can reach several tens of percent. The first ultra-low aspect ratio TOKAMAK, START, built several years ago in England, has already received values β = 30%. On the other hand, these systems are technically more demanding and require special technical solutions for the toroidal coil, divertor and neutron protection. Currently, several larger experimental TOKAMAKs than START are being built with a low aspect ratio and plasma current above 1 MA. It is expected that over the next 5 years, experiments will provide enough data to understand whether the expected improvement in plasma parameters will be achieved and whether it will be able to compensate for the technical difficulties expected in this direction.

Long-term studies of plasma confinement in TOKAMAKs have shown that the processes of energy and particle transfer across the magnetic field are determined by complex turbulent processes in the plasma. And although plasma instabilities responsible for anomalous plasma losses have already been identified, the theoretical understanding of nonlinear processes is not yet sufficient to describe the plasma lifetime based on first principles. Therefore, to extrapolate plasma lifetimes obtained in modern installations to the scale of the TOKAMAK reactor, empirical laws—scalings—are currently used. One of these scalings (ITER-97(y)), obtained using statistical processing of an experimental database from various TOKAMAKs, predicts that the lifetime increases with plasma size, R, plasma current I p, and elongation of the plasma cross section k = b/ A= 4 and decreases with increasing plasma heating power, P:

t E ~ R 2 k 0.9 I р 0.9 / P 0.66

The dependence of the energy lifetime on other plasma parameters is rather weak. Figure 8 shows that the lifetime measured in almost all experimental TOKAMAKs is well described by this scaling.

Fig.8. Dependence of the experimentally observed energy lifetime on the one predicted by ITER-97(y) scaling.
The average statistical deviation of experimental points from scaling is 15%.
Different labels correspond to different TOKAMAKs and the projected TOKAMAK reactor ITER.

This scaling predicts that a TOKAMAK in which self-sustaining thermonuclear combustion will occur should have a large radius of 7-8 m and a plasma current of 20 MA. In such a TOKAMAK, the energy lifetime will exceed 5 seconds, and the power of thermonuclear reactions will be at the level of 1-1.5 GW.

In 1998, the engineering design of the TOKAMAK reactor ITER was completed. The work was carried out jointly by four parties: Europe, Russia, the USA and Japan with the aim of creating the first experimental TOKAMAK reactor designed to achieve thermonuclear combustion of a mixture of deuterium and tritium. The main physical and engineering parameters of the installation are given in Table 3, and its cross-section is shown in Fig. 9.

Fig.9. General view of the designed TOKAMAK reactor ITER.

ITER will already have all the main features of the TOKAMAK reactor. It will have a fully superconducting magnetic system, a cooled blanket and protection from neutron radiation, and a remote maintenance system for the installation. It is assumed that neutron fluxes with a power density of 1 MW/m 2 and a total fluence of 0.3 MW × yr/m 2 will be obtained on the first wall, which will allow nuclear technology tests of materials and blanket modules capable of reproducing tritium.

Table 3.
Basic parameters of the first experimental thermonuclear TOKAMAK reactor, ITER.

ITER is planned to be built in 2010-2011. The experimental program, which will continue on this experimental reactor for about twenty years, will make it possible to obtain plasma-physical and nuclear-technological data necessary for the construction in 2030-2035 of the first demonstration reactor - TOKAMAK, which has already will produce electricity. The main task of ITER will be to demonstrate the practicality of the TOKAMAK reactor for generating electricity and.

Along with TOKAMAK, which is currently the most advanced system for implementing controlled thermonuclear fusion, there are other magnetic traps that successfully compete with TOKAMAK.

In addition to TOKAMAKs and STELLARATORs, experiments, although on a smaller scale, continue on some other systems with closed magnetic configurations. Among them, field-reversed pinches, SPHEROMAKs and compact tori should be noted. Field-reversed pinches have a relatively low toroidal magnetic field. In SPHEROMAK or compact tori there is no toroidal magnetic system at all. Accordingly, all these systems promise the ability to create plasma with a high parameter value β and, therefore, may in the future be attractive for the creation of compact fusion reactors or reactors using alternative reactions, such as DHe 3 or rB, in which a low field is required to reduce magnetic bremsstrahlung. The current plasma parameters achieved in these traps are still significantly lower than those obtained in TOKAMAKS and STELLARATORS.

A study of the interaction of laser radiation with matter showed that laser radiation is well absorbed by the evaporating substance of the target shell up to the required power densities of 2÷4 · 10 14 W/cm 2 . The absorption coefficient can reach 40÷80% and increases with decreasing radiation wavelength. As mentioned above, a large thermonuclear yield can be achieved if the bulk of the fuel remains cold during compression. To do this, it is necessary that the compression be adiabatic, i.e. It is necessary to avoid preheating the target, which can occur due to the generation of energetic electrons, shock waves, or hard X-rays by laser radiation. Numerous studies have shown that these unwanted effects can be reduced by profiling the radiation pulse, optimizing the tablets, and reducing the radiation wavelength. Figure 16, borrowed from the work, shows the boundaries of the region on the plane power density - wavelength lasers suitable for target compression.

Fig. 16. The region on the parameter plane in which lasers are capable of compressing thermonuclear targets (shaded).

The first laser installation (NIF) with laser parameters sufficient to ignite targets will be built in the USA in 2002. The installation will make it possible to study the physics of compression of targets that will have a thermonuclear output at the level of 1-20 MJ and, accordingly, will allow obtaining high values Q>1.

Although lasers make it possible to carry out laboratory research on the compression and ignition of targets, their disadvantage is their low efficiency, which, at best, so far reaches 1-2%. At such low efficiencies, the thermonuclear yield of the target must exceed 10 3, which is a very difficult task. In addition, glass lasers have low pulse repeatability. In order for lasers to serve as a reactor driver for a fusion power plant, their cost must be reduced by approximately two orders of magnitude. Therefore, in parallel with the development of laser technology, researchers turned to the development of more efficient drivers - ion beams.

Ion beams

Currently, two types of ion beams are being considered: beams of light ions, type Li, with an energy of several tens of MeV, and beams of heavy ions, type Pb, with an energy of up to 10 GeV. If we talk about reactor applications, then in both cases it is necessary to supply an energy of several MJ to a target with a radius of several millimeters in a time of about 10 ns. It is necessary not only to focus the beam, but also to be able to conduct it in the reactor chamber at a distance of about several meters from the accelerator output to the target, which is not at all an easy task for particle beams.

Beams of light ions with energies of several tens of MeV can be created with relatively high efficiency. using a pulse voltage applied to the diode. Modern pulsed technology makes it possible to obtain the powers required to compress targets, and therefore light ion beams are the cheapest candidate for a driver. Experiments with light ions have been carried out for many years at the PBFA-11 facility at Sandywood National Laboratory in the USA. The setup makes it possible to create short (15 ns) pulses of 30 MeV Li ions with a peak current of 3.5 MA and a total energy of about 1 MJ. A casing made of large-Z material with a target inside was placed in the center of a spherically symmetric diode, allowing for the production of a large number of radially directed ion beams. The ion energy was absorbed in the hohlraum casing and the porous filler between the target and the casing and was converted into soft x-rays that compressed the target.

It was expected to obtain a power density of more than 5 × 10 13 W/cm 2 necessary for compressing and igniting targets. However, the achieved power densities were approximately an order of magnitude lower than expected. A reactor using light ions as a driver requires colossal flows of fast particles with a high particle density near the target. Focusing such beams onto millimeter targets is a task of enormous complexity. In addition, light ions will be noticeably inhibited in the residual gas in the combustion chamber.

The transition to heavy ions and high particle energies makes it possible to significantly mitigate these problems and, in particular, to reduce the particle current densities and, thus, alleviate the problem of particle focusing. However, to obtain the required 10 GeV particles, huge accelerators with particle accumulators and other complex accelerating equipment are required. Let us assume that the total beam energy is 3 MJ, the pulse time is 10 ns, and the area on which the beam should be focused is a circle with a radius of 3 mm. Comparative parameters of hypothetical drivers for target compression are given in Table 6.

Table 6.
Comparative characteristics of drivers on light and heavy ions.

*) – in the target area

Beams of heavy ions, as well as light ions, require the use of a hohlraum, in which the energy of the ions is converted into X-ray radiation, which uniformly irradiates the target itself. The design of the hohlraum for a heavy ion beam differs only slightly from the hohlraum for laser radiation. The difference is that the beams do not require holes through which the laser beams penetrate into the hohlraum. Therefore, in the case of beams, special particle absorbers are used, which convert their energy into X-ray radiation. One possible option is shown in Fig. 14b. It turns out that the conversion efficiency decreases with increasing energy and ions and increasing the size of the region on which the beam is focused. Therefore, increasing the energy and particles above 10 GeV is impractical.

Currently, both in Europe and in the USA, it has been decided to focus the main efforts on the development of drivers based on heavy ion beams. It is expected that these drivers will be developed by 2010-2020 and, if successful, will replace lasers in next-generation NIF installations. So far, the accelerators required for inertial fusion do not exist. The main difficulty in their creation is associated with the need to increase particle flux densities to a level at which the spatial charge density of ions already significantly affects the dynamics and focusing of particles. In order to reduce the effect of space charge, it is proposed to create a large number of parallel beams, which will be connected in the reactor chamber and directed towards the target. The typical size of a linear accelerator is several kilometers.

How is it supposed to conduct ion beams over a distance of several meters in the reactor chamber and focus them on an area several millimeters in size? One possible scheme is self-focusing of beams, which can occur in a low-pressure gas. The beam will cause ionization of the gas and a compensating counter electric current flowing through the plasma. The azimuthal magnetic field, which is created by the resulting current (the difference between the beam current and the reverse plasma current), will lead to radial compression of the beam and its focusing. Numerical modeling shows that, in principle, such a scheme is possible if the gas pressure is maintained in the desired range of 1-100 Torr.

And although heavy ion beams offer the prospect of creating an effective driver for a fusion reactor, they face enormous technical challenges that still need to be overcome before the goal is achieved. For thermonuclear applications, an accelerator is needed that will create a beam of 10 GeV ions with a peak current of several tens of spacecraft and an average power of about 15 MW. The volume of the magnetic system of such an accelerator is comparable to the volume of the magnetic system of the TOKAMAK reactor and, therefore, one can expect that their costs will be of the same order.

Pulse reactor chamber

Unlike a magnetic fusion reactor, where high vacuum and plasma purity are required, such requirements are not imposed on the chamber of a pulsed reactor. The main technological difficulties in creating pulsed reactors lie in the field of driver technology, the creation of precision targets and systems that make it possible to feed and control the position of the target in the chamber. The pulse reactor chamber itself has a relatively simple design. Most projects involve the use of a liquid wall created by an open coolant. For example, the HYLIFE-11 reactor design uses molten salt Li 2 BeF 4, a liquid curtain from which surrounds the area where the targets arrive. The liquid wall will absorb neutron radiation and wash away the remains of the targets. It also dampens the pressure of micro-explosions and evenly transfers it to the main wall of the chamber. The characteristic outer diameter of the chamber is about 8 m, its height is about 20 m.

The total flow rate of the coolant liquid is estimated to be about 50 m 3 /s, which is quite achievable. It is assumed that in addition to the main, stationary flow, a pulsed liquid shutter will be made in the chamber, which will open synchronized with the supply of the target with a frequency of about 5 Hz to transmit a beam of heavy ions.

The required target feeding accuracy is fractions of millimeters. Obviously, passively delivering a target over a distance of several meters with such precision in a chamber in which turbulent gas flows caused by explosions of previous targets will occur is a practically impossible task. Therefore, the reactor will require a control system that allows tracking the position of the target and dynamically focusing the beam. In principle, such a task is feasible, but it can significantly complicate reactor control.

“Lockheed Martin has begun developing a compact thermonuclear reactor... The company’s website says that the first prototype will be built within a year. If this turns out to be true, in a year we will live in a completely different world,” this is the beginning of one of “The Attic.” Three years have passed since its publication, and the world has not changed that much since then.

Today, in nuclear power plant reactors, energy is generated by the decay of heavy nuclei. In thermonuclear reactors, energy is obtained during the process of fusion of nuclei, during which nuclei of less mass than the sum of the original ones are formed, and the “residue” is lost in the form of energy. Waste from nuclear reactors is radioactive, and its safe disposal is a big headache. Fusion reactors do not have this drawback, and also use widely available fuel such as hydrogen.

They have only one big problem - industrial designs don't exist yet. The task is not easy: for thermonuclear reactions, the fuel must be compressed and heated to hundreds of millions of degrees - hotter than on the surface of the Sun (where thermonuclear reactions occur naturally). It is difficult to achieve such a high temperature, but it is possible, but such a reactor consumes more energy than it produces.

However, they still have so many potential advantages that, of course, not only Lockheed Martin is involved in development.

ITER

ITER is the largest project in this area. It involves the European Union, India, China, Korea, Russia, the USA and Japan, and the reactor itself has been built on French territory since 2007, although its history goes much deeper into the past: Reagan and Gorbachev agreed on its creation in 1985. The reactor is a toroidal chamber, a “donut”, in which the plasma is held by magnetic fields, which is why it is called a tokamak - That roidal ka measure with ma rotten To atushki. The reactor will generate energy through the fusion of hydrogen isotopes - deuterium and tritium.

It is planned that ITER will receive 10 times more energy than it consumes, but this will not happen soon. It was initially planned that the reactor would begin operating in experimental mode in 2020, but then this date was postponed to 2025. At the same time, industrial energy production will begin no earlier than 2060, and we can only expect the spread of this technology somewhere at the end of the 21st century.

Wendelstein 7-X

Wendelstein 7-X is the largest stellarator-type fusion reactor. The stellarator solves the problem that plagues tokamaks - the “spreading” of plasma from the center of the torus to its walls. What the tokamak tries to cope with due to the power of the magnetic field, the stellarator solves due to its complex shape: the magnetic field holding the plasma bends to stop the advances of charged particles.

Wendelstein 7-X, as its creators hope, will be able to operate for half an hour in 21, which will give a “ticket to life” to the idea of ​​thermonuclear stations of a similar design.

National Ignition Facility

Another type of reactor uses powerful lasers to compress and heat fuel. Alas, the largest laser installation for producing thermonuclear energy, the American NIF, was unable to produce more energy than it consumes.

It is difficult to predict which of all these projects will really take off and which will suffer the same fate as NIF. All we can do is wait, hope and follow the news: the 2020s promise to be an interesting time for nuclear energy.

“Nuclear Technologies” is one of the profiles of the NTI Olympiad for schoolchildren.