Nuclear reactor thermonuclear reactions. Everything you need to know about thermonuclear fusion. Nuclear weapons programs

The fusion reaction is as follows: two or more atomic nuclei are taken and, using a certain force, brought together so close that the forces acting at such distances prevail over the forces of Coulomb repulsion between equally charged nuclei, resulting in the formation of a new nucleus. It will have a slightly smaller mass than the sum of the masses of the original nuclei, and the difference becomes the energy that is released during the reaction. The amount of energy released is described by the well-known formula E=mc². Lighter atomic nuclei are easier to bring together to the desired distance, so hydrogen - the most abundant element in the Universe - is the best fuel for the fusion reaction.

It has been found that a mixture of two isotopes of hydrogen, deuterium and tritium, requires the least amount of energy for the fusion reaction compared to the energy released during the reaction. However, although deuterium-tritium (D-T) is the subject of most fusion research, it is by no means the only potential fuel. Other mixtures may be easier to produce; their reaction can be more reliably controlled, or, more importantly, produce fewer neutrons. Of particular interest are the so-called “Neutronless” reactions, since the successful industrial use of such fuel will mean the absence of long-term radioactive contamination of the materials and reactor design, which, in turn, could have a positive impact on public opinion and the overall cost of operating the reactor, significantly reducing the costs of its decommissioning. The problem remains that synthesis reactions using alternative fuels are much more difficult to maintain, so the D-T reaction is considered only a necessary first step.

Scheme of the deuterium-tritium reaction

Controlled fusion can use different types of fusion reactions depending on the type of fuel used.

Deuterium + tritium reaction (D-T fuel)

The most easily feasible reaction is deuterium + tritium:

2 H + 3 H = 4 He + n at an energy output of 17.6 MeV (megaelectronvolt)

This reaction is the most easily feasible from the point of view of modern technologies, provides a significant energy yield, and fuel components are cheap. Its disadvantage is the release of unwanted neutron radiation.

Two nuclei: deuterium and tritium fuse to form a helium nucleus (alpha particle) and a high-energy neutron.

²H + ³He = 4 He + . with an energy output of 18.4 MeV

The conditions for achieving it are much more complicated. Helium-3 is also a rare and extremely expensive isotope. It is not currently produced on an industrial scale. However, it can be obtained from tritium, which is produced in turn at nuclear power plants.

The complexity of carrying out a thermonuclear reaction can be characterized by the triple product nTt (density by temperature by confinement time). By this parameter, the D-3He reaction is approximately 100 times more complex than the D-T reaction.

Reaction between deuterium nuclei (D-D, monopropellant)

Reactions between deuterium nuclei are also possible, they are a little more difficult than reactions involving helium-3:

As a result, in addition to the main reaction in DD plasma, the following also occurs:

These reactions proceed slowly in parallel with the deuterium + helium-3 reaction, and the tritium and helium-3 formed during them are likely to immediately react with deuterium.

Other types of reactions

Some other types of reactions are also possible. The choice of fuel depends on many factors - its availability and low cost, energy output, ease of achieving the conditions required for the thermonuclear fusion reaction (primarily temperature), the necessary design characteristics of the reactor, etc.

"Neutronless" reactions

The most promising are the so-called. “neutron-free” reactions, since the neutron flux generated by thermonuclear fusion (for example, in the deuterium-tritium reaction) carries away a significant part of the power and generates induced radioactivity in the reactor design. The deuterium-helium-3 reaction is promising due to the lack of neutron yield.

Conditions

Nuclear reaction of lithium-6 with deuterium 6 Li(d,α)α

TCB is possible if two criteria are met simultaneously:

• Plasma temperature:

style="max-width: 98%; height: auto; width: auto;" src="/pictures/wiki/files/101/ea2cc6cfd93c3d519e815764da74047a.png" border="0">

• Compliance with Lawson's criterion:

style="max-width: 98%; height: auto; width: auto;" src="/pictures/wiki/files/102/fe017490a33596f30c6fb2ea304c2e15.png" border="0"> (for D-T reaction)

where is the density of high-temperature plasma, is the plasma retention time in the system.

It is on the value of these two criteria that the rate of occurrence of a particular thermonuclear reaction mainly depends.

At present, controlled thermonuclear fusion has not yet been carried out on an industrial scale. Construction of the international research reactor ITER is in its early stages.

Fusion energy and helium-3

Helium-3 reserves on Earth range from 500 kg to 1 ton, but on the Moon it is found in significant quantities: up to 10 million tons (according to minimum estimates - 500 thousand tons). Currently, a controlled thermonuclear reaction is carried out by the synthesis of deuterium ²H and tritium ³H with the release of helium-4 4 He and the “fast” neutron n:

However, the majority (more than 80%) of the released kinetic energy comes from the neutron. As a result of collisions of fragments with other atoms, this energy is converted into thermal energy. In addition, fast neutrons create significant amounts of radioactive waste. In contrast, the synthesis of deuterium and helium-3³He does not produce (almost) radioactive products:

Where p is proton

This allows the use of simpler and more efficient systems for converting the kinetic synthesis reaction, such as a magnetohydrodynamic generator.

Reactor designs

Two basic schemes for implementing controlled thermonuclear fusion are considered.

Research on the first type of thermonuclear reactor is significantly more developed than on the second. In nuclear physics, when studying thermonuclear fusion, a magnetic trap is used to contain plasma in a certain volume. The magnetic trap is designed to keep the plasma from contact with the elements of the thermonuclear reactor, i.e. used primarily as a heat insulator. The principle of confinement is based on the interaction of charged particles with a magnetic field, namely on the rotation of charged particles around magnetic field lines. Unfortunately, magnetized plasma is very unstable and tends to leave the magnetic field. Therefore, to create an effective magnetic trap, the most powerful electromagnets are used, consuming a huge amount of energy.

It is possible to reduce the size of a fusion reactor if it uses three methods of creating a fusion reaction simultaneously.

A. Inertial synthesis. Irradiate tiny capsules of deuterium-tritium fuel with a 500 trillion-watt laser:5. 10^14 W. This gigantic, very brief 10^-8 sec laser pulse causes the fuel capsules to explode, resulting in the birth of a mini-star for a split second. But a thermonuclear reaction cannot be achieved on it.

B. Simultaneously use the Z-machine with the Tokamak.

The Z-Machine operates differently than a laser. It passes through a web of tiny wires surrounding the fuel capsule a charge with a power of half a trillion watts 5.10^11 watts.

Next, approximately the same thing happens as with the laser: as a result of the Z-impact, a star is formed. During tests on the Z-Machine, it was already possible to launch a fusion reaction. http://www.sandia.gov/media/z290.htm Cover the capsules with silver and connect them with a silver or graphite thread. The ignition process looks like this: Shoot a filament (attached to a group of silver balls containing a mixture of deuterium and tritium) into a vacuum chamber. During a breakdown (discharge), form a lightning channel through them and supply current through the plasma. Simultaneously irradiate the capsules and plasma with laser radiation. And at the same time or earlier turn on the Tokamak. use three plasma heating processes simultaneously. That is, place the Z-machine and laser heating together inside the Tokamak. It may be possible to create an oscillatory circuit from Tokamak coils and organize resonance. Then it would work in an economical oscillatory mode.

Fuel cycle

First generation reactors will most likely run on a mixture of deuterium and tritium. Neutrons that appear during the reaction will be absorbed by the reactor protection, and the generated heat will be used to heat the coolant in the heat exchanger, and this energy, in turn, will be used to rotate the generator.

. .

The reaction with Li6 is exothermic, providing little energy for the reactor. The reaction with Li7 is endothermic - but does not consume neutrons. At least some reactions of Li7 are necessary to replace neutrons lost in reactions with other elements. Most reactor designs use natural mixtures of lithium isotopes.

This fuel has a number of disadvantages:

The reaction produces a significant number of neutrons, which activate (radioactively contaminate) the reactor and heat exchanger. Measures are also required to protect against a possible source of radioactive tritium.

Only about 20% of fusion energy is in the form of charged particles (the rest are neutrons), which limits the ability to directly convert fusion energy into electricity. The use of the D-T reaction depends on the available lithium reserves, which are significantly less than the deuterium reserves. Neutron exposure during the D-T reaction is so significant that after the first series of tests at JET, the largest reactor to date using this fuel, the reactor became so radioactive that a robotic remote maintenance system had to be added to complete the annual test cycle.

There are, in theory, alternative types of fuel that do not have these disadvantages. But their use is hampered by a fundamental physical limitation. To obtain sufficient energy from the fusion reaction, it is necessary to maintain a sufficiently dense plasma at the fusion temperature (10 8 K) for a certain time. This fundamental aspect of fusion is described by the product of the plasma density, n, and the heated plasma holding time, τ, required to reach the equilibrium point. The product, nτ, depends on the type of fuel and is a function of the plasma temperature. Of all types of fuel, the deuterium-tritium mixture requires the lowest nτ value by at least an order of magnitude, and the lowest reaction temperature by at least 5 times. Thus, the D-T reaction is a necessary first step, but the use of other fuels remains an important research goal.

Fusion reaction as an industrial source of electricity

Fusion energy is considered by many researchers as a "natural" energy source in the long term. Proponents of the commercial use of fusion reactors for electricity production cite the following arguments in their favor:

• Virtually inexhaustible fuel reserves (hydrogen)
• Fuel can be extracted from sea water on any coast of the world, which makes it impossible for one or a group of countries to monopolize fuel
• Impossibility of an uncontrolled fusion reaction
• No combustion products
• There is no need to use materials that can be used to produce nuclear weapons, thus eliminating cases of sabotage and terrorism
• Compared to nuclear reactors, negligible amounts of radioactive waste are produced with a short half-life.
• A thimble filled with deuterium is estimated to produce energy equivalent to 20 tons of coal. A medium-sized lake can provide any country with energy for hundreds of years. However, it should be noted that existing research reactors are designed to achieve a direct deuterium-tritium (DT) reaction, the fuel cycle of which requires the use of lithium to produce tritium, while claims of inexhaustible energy refer to the use of deuterium-deuterium (DD) reaction in the second generation of reactors.
• Just like the fission reaction, the fusion reaction does not produce atmospheric carbon dioxide emissions, which is a major contributor to global warming. This is a significant advantage, since the use of fossil fuels to produce electricity results in, for example, the US producing 29 kg of CO 2 (one of the main gases that can be considered a cause of global warming) per US resident per day.

Cost of electricity compared to traditional sources

Critics point out that the economic feasibility of using nuclear fusion to produce electricity remains an open question. The same study commissioned by the British Parliament's Office of Science and Technology Records indicates that the cost of generating electricity using a fusion reactor would likely be at the higher end of the cost spectrum of conventional energy sources. Much will depend on future technology, market structure and regulation. The cost of electricity directly depends on the efficiency of use, the duration of operation and the cost of reactor decommissioning. Critics of the commercial use of nuclear fusion energy deny that hydrocarbon fuels are heavily subsidized by the government, both directly and indirectly, such as through the use of the military to ensure an uninterrupted supply; the Iraq War is often cited as a controversial example of this type of subsidization. Accounting for such indirect subsidies is very complex and makes accurate cost comparisons nearly impossible.

A separate issue is the cost of research. The countries of the European Community spend about €200 million annually on research, and it is predicted that it will take several more decades before the industrial use of nuclear fusion will be possible. Proponents of alternative sources of electricity believe that it would be more appropriate to use these funds to introduce renewable sources of electricity.

Availability of commercial fusion energy

Unfortunately, despite widespread optimism (since the 1950s, when the first research began), significant obstacles between today's understanding of nuclear fusion processes, technological capabilities and the practical use of nuclear fusion have not yet been overcome, it is unclear even to what extent there may be It is economically profitable to produce electricity using thermonuclear fusion. Although progress in research is constant, researchers are faced with new challenges every now and then. For example, the challenge is developing a material that can withstand neutron bombardment, which is estimated to be 100 times more intense than traditional nuclear reactors.

The following stages are distinguished in research:

1.Equilibrium or “pass” mode(Break-even): when the total energy released during the synthesis process is equal to the total energy spent on starting and maintaining the reaction. This relationship is marked with the symbol Q. The reaction equilibrium was demonstrated at JET (Joint European Torus) in the UK in 1997. (Having spent 52 MW of electricity to heat it up, the scientists obtained a power output that was 0.2 MW higher than what was expended.)

2.Blazing Plasma(Burning Plasma): An intermediate stage in which the reaction will be supported primarily by alpha particles that are produced during the reaction, rather than by external heating. Q ≈ 5. Still not achieved.

3. Ignition(Ignition): a stable reaction that maintains itself. Should be achieved at large values ​​of Q. Still not achieved.

The next step in research should be ITER (International Thermonuclear Experimental Reactor), the International Thermonuclear Experimental Reactor. At this reactor it is planned to study the behavior of high-temperature plasma (flaming plasma with Q ~ 30) and structural materials for an industrial reactor. The final phase of the research will be DEMO: a prototype industrial reactor in which ignition will be achieved and the practical suitability of the new materials will be demonstrated. The most optimistic forecast for the completion of the DEMO phase: 30 years. Considering the estimated time for construction and commissioning of an industrial reactor, we are ~40 years away from the industrial use of thermonuclear energy.

Existing tokamaks

In total, about 300 tokamaks were built in the world. The largest of them are listed below.

• USSR and Russia
  • T-3 is the first functional device.
  • T-4 - enlarged version of T-3
  • T-7 is a unique installation in which, for the first time in the world, a relatively large magnetic system with a superconducting solenoid based on tin niobate cooled by liquid helium is implemented. The main task of T-7 was completed: the prospect for the next generation of superconducting solenoids for thermonuclear power was prepared.
  • T-10 and PLT are the next step in world thermonuclear research, they are almost the same size, equal power, with the same confinement factor. And the results obtained are identical: both reactors achieved the desired temperature of thermonuclear fusion, and the lag according to the Lawson criterion is only two hundred times.
  • T-15 is a reactor of today with a superconducting solenoid giving a field strength of 3.6 Tesla.
• Libya
  • TM-4A
• Europe and UK
  • JET (English) (Joint Europeus Tor) is the world's largest tokamak, created by the Euratom organization in the UK. It uses combined heating: 20 MW - neutral injection, 32 MW - ion cyclotron resonance. As a result, the Lawson criterion is only 4-5 times lower than the ignition level.
  • Tore Supra (French) (English) - a tokamak with superconducting coils, one of the largest in the world. Located at the Cadarache research center (France).
• USA
  • TFTR (English) (Test Fusion Tokamak Reactor) - the largest tokamak in the USA (at Princeton University) with additional heating by fast neutral particles. A high result has been achieved: the Lawson criterion at a true thermonuclear temperature is only 5.5 times lower than the ignition threshold. Closed 1997
  • NSTX (English) (National Spherical Torus Experiment) is a spherical tokamak (spheromak) currently operating at Princeton University. The first plasma in the reactor was produced in 1999, two years after TFTR was closed.

The atom is the building block of the Universe. There are only about a hundred different types of atoms. Most elements are stable (for example, oxygen and nitrogen in the atmosphere; carbon, oxygen and hydrogen are the main components of our body and all other living organisms). Other elements, mostly very heavy ones, are unstable, meaning that they spontaneously decay to form other elements. This transformation is called a nuclear reaction.

Nuclear reactions are transformations of atomic nuclei when interacting with elementary particles, g-quanta or with each other.

Nuclear reactions are divided into two types: nuclear fission and thermonuclear fusion.

Nuclear fission reaction is the process of splitting an atomic nucleus into two (less often three) nuclei with similar masses, called fission fragments. As a result of fission, other reaction products can also arise: light nuclei (mainly alpha particles), neutrons and gamma rays. Division can be spontaneous (spontaneous) and forced.

Spontaneous (spontaneous) is nuclear fission, during which some fairly heavy nuclei decay into two fragments with approximately equal masses.

Spontaneous fission was first discovered for natural uranium. Like any other type of radioactive decay, spontaneous fission is characterized by a half-life (fission period). The half-life for spontaneous fission varies for different nuclei within very wide limits (from 1018 years for 93Np237 to several tenths of a second for transuranium elements).

Forced fission of nuclei can be caused by any particles: photons, neutrons, protons, deuterons, b-particles, etc., if the energy they contribute to the nucleus is sufficient to overcome the fission barrier. For nuclear energy, fission caused by neutrons is of greater importance. The fission reaction of heavy nuclei was carried out for the first time on uranium U235. In order for a uranium nucleus to decay into two fragments, it is given an activation energy. The uranium nucleus receives this energy by capturing a neutron. The nucleus comes into an excited state, becomes deformed, a “bridge” appears between parts of the nucleus, and under the influence of Coulomb repulsive forces, the nucleus divides into two fragments of unequal mass. Both fragments are radioactive and emit 2 or 3 secondary neutrons.

Rice. 4

Secondary neutrons are absorbed by neighboring uranium nuclei, causing them to fission. Under appropriate conditions, a self-developing process of mass nuclear fission, called a nuclear chain reaction, can occur. This reaction is accompanied by the release of colossal energy. For example, the complete combustion of 1 g of uranium releases 8.28·1010 J of energy. A nuclear reaction is characterized by a thermal effect, which is the difference between the rest masses of the nuclei entering into the nuclear reaction and those formed as a result of the reaction, i.e. The energy effect of a nuclear reaction is determined mainly by the difference in the masses of the final and initial nuclei. Based on the equivalence of energy and mass, it is possible to calculate the energy released or expended during a nuclear reaction if we know exactly the mass of all nuclei and particles participating in the reaction. According to Einstein's law:

• ?E=?mс2
• ?E = (mA + mx - mB - my)c2

where mA and mx are the masses of the target nucleus and the bombarding nucleus (particle), respectively;

mB and my are the masses of the nuclei formed as a result of the reaction.

The more energy released during the formation of a nucleus, the stronger it is. Nuclear binding energy is the amount of energy required to decompose the nucleus of an atom into its component parts - nucleons (protons and neutrons).

An example of an uncontrolled fission chain reaction is the explosion of an atomic bomb; a controlled nuclear reaction is carried out in nuclear reactors.

Thermonuclear fusion is a reaction inverse to atomic fission, a reaction of the fusion of light atomic nuclei into heavier nuclei, occurring at ultra-high temperatures and accompanied by the release of huge amounts of energy. The implementation of controlled thermonuclear fusion will give humanity a new environmentally friendly and practically inexhaustible source of energy, which is based on the collision of nuclei of hydrogen isotopes, and hydrogen is the most abundant substance in the Universe.

The fusion process occurs with noticeable intensity only between light nuclei that have a small positive charge and only at high temperatures, when the kinetic energy of the colliding nuclei is sufficient to overcome the Coulomb potential barrier. Reactions between heavy isotopes of hydrogen (deuterium 2H and tritium 3H) occur at an incomparably higher speed with the formation of strongly bound helium nuclei.

2D + 3T > 4He (3.5 MeV) + 1n (14.1 MeV)

These reactions are of greatest interest for the problem of controlled thermonuclear fusion. Deuterium is found in sea water. Its reserves are publicly available and very large: deuterium accounts for about 0.016% of the total number of hydrogen atoms that make up water, while the world's oceans cover 71% of the Earth's surface area. The reaction involving tritium is more attractive because it is accompanied by a large release of energy and proceeds at a significant speed. Tritium is radioactive (half-life 12.5 years) and does not occur in nature. Consequently, to ensure the operation of the proposed thermonuclear reactor using tritium as a nuclear fuel, the possibility of tritium reproduction must be provided.

The reaction with the so-called lunar isotope 3He has a number of advantages compared to the deuterium-tritium reaction, which is most achievable under terrestrial conditions.

2D + 3He > 4He (3.7 MeV) + 1p (14.7 MeV)

Advantages:

• 1. 3He is not radioactive.
• 2. Tens of times lower neutron flux from the reaction zone, which sharply reduces induced radioactivity and degradation of reactor structural materials;
• 3. The resulting protons, unlike neutrons, are easily captured and can be used for additional generation of electricity.

The natural isotopic abundance of 3He in the atmosphere is 0.000137%. Most of the 3He on Earth has been preserved since its formation. It is dissolved in the mantle and gradually enters the atmosphere. On Earth it is mined in very small quantities, amounting to several tens of grams per year.

Helium-3 is a byproduct of reactions occurring in the Sun. As a result, on the Moon, which does not have an atmosphere, there are up to 10 million tons of this valuable substance (according to minimal estimates - 500 thousand tons). During thermonuclear fusion, when 1 ton of helium-3 reacts with 0.67 tons of deuterium, energy is released equivalent to the combustion of 15 million tons of oil (however, the technical feasibility of this reaction has not been studied at the moment). Consequently, the lunar resource of helium-3 should be sufficient for the population of our planet for at least the next millennium. The main problem remains the reality of extracting helium from lunar soil. The helium-3 content in regolith is ~1 g per 100 tons. Therefore, to extract a ton of this isotope, at least 100 million tons of soil must be processed. The temperature at which the thermonuclear fusion reaction can occur reaches a value of the order of 108 - 109 K. At this temperature, the substance is in a completely ionized state, which is called plasma. Thus, the construction of a reactor involves: obtaining plasma heated to temperatures of hundreds of millions of degrees; maintaining the plasma configuration over time for nuclear reactions to occur.

Thermonuclear energy has important advantages over nuclear power plants: it uses absolutely non-radioactive deuterium and the helium-3 isotope and radioactive tritium, but in volumes thousands of times smaller than in nuclear energy. And in possible emergency situations, the radioactive background near the thermonuclear power plant will not exceed natural indicators. At the same time, per unit weight of thermonuclear fuel, approximately 10 million times more energy is obtained than during the combustion of organic fuel, and approximately 100 times more than during the fission of uranium nuclei. Under natural conditions, thermonuclear reactions occur in the depths of stars, in particular in the inner regions of the Sun, and serve as the constant source of energy that determines their radiation. The combustion of hydrogen in stars occurs at a low rate, but the gigantic size and density of stars ensure the continuous emission of huge streams of energy for billions of years.

All chemical elements of our planet and the Universe as a whole were formed as a result of thermonuclear reactions that occur in the cores of stars. Thermonuclear reactions in stars lead to a gradual change in the chemical composition of stellar matter, which causes the restructuring of the star and its advancement along the evolutionary path. The first stage of evolution ends with the depletion of hydrogen in the central regions of the star. Then, after an increase in temperature caused by compression of the central layers of the star, deprived of energy sources, thermonuclear reactions of helium combustion become effective, which are replaced by the combustion of C, O, Si and subsequent elements - up to Fe and Ni. Each stage of stellar evolution corresponds to certain thermonuclear reactions. The first in the chain of such nuclear reactions are hydrogen thermonuclear reactions. They proceed in two ways depending on the initial temperature at the center of the star. The first path is the hydrogen cycle, the second path is the CNO cycle.

Hydrogen cycle:

• 1H + 1H = 2D + e+ + v +1.44 MeV
• 2D + 1H = 3He + g +5.49 MeV

I: 3He + 3He = 4He + 21H + 12.86 MeV

or 3He + 4He = 7Be + g + 1.59 MeV

7Be + e- = 7Li + v + 0.862 MeV or 7Be + 1H = 8B + g +0.137 MeV

II: 7Li + 1H = 2 4He + 17.348 MeV 8B = 8Be* + e+ + v + 15.08 MeV

III. 8Be* = 2 4He + 2.99 MeV

The hydrogen cycle begins by the collision of two protons (1H, or p) to form a deuterium nucleus (2D). Deuterium reacts with a proton to form the light (lunar) isotope of helium 3He, emitting a gamma photon (g). The lunar isotope 3He can react in two different ways: two 3He nuclei collide to form 4He with the elimination of two protons, or 3He combines with 4He and gives 7Be. The latter, in turn, captures either an electron (e-) or a proton and another branching of the proton-proton chain of reactions occurs. As a result, the hydrogen cycle can end in three different ways I, II and III. To implement branch I, the first two reactions of V. c. must occur twice, since in this case two 3He nuclei disappear at once. In branch III, particularly energetic neutrinos are emitted during the decay of the 8B boron nucleus with the formation of an unstable beryllium nucleus in an excited state (8Be*), which almost instantly decays into two 4He nuclei. The CNO cycle is a set of three linked or, more precisely, partially overlapping cycles: CN, NO I, NO II. The synthesis of helium from hydrogen in the reactions of this cycle occurs with the participation of catalysts, the role of which is played by small admixtures of C, N and O isotopes in stellar matter.

The main reaction pathway of the CN cycle is:

• 12C + p = 13N + g +1.95 MeV
• 13N = 13C + e+ + n +1.37 MeV
• 13C + p = 14N + g +7.54 MeV (2.7 106 years)
• 14N + p = 15O + g +7.29 MeV (3.2 108 years)
• 15O = 15N + e+ + n +2.76 MeV (82 seconds)
• 15N + p = 12C + 4He +4.96 MeV (1.12 105 years)

The essence of this cycle is the indirect synthesis of a b particle from four protons during their successive capture by nuclei, starting from 12C.

In the reaction with the capture of a proton by the 15N nucleus, another outcome is possible - the formation of a 16O nucleus and a new NO I cycle is born.

It has exactly the same structure as the CN cycle:

• 14N + 1H = 15O + g +7.29 MeV
• 15O = 15N + e+ + n +2.76 MeV
• 15N + 1H = 16O + g +12.13 MeV
• 16O + 1H = 17F + g +0.60 MeV
• 17F = 17O + e+ + n +2.76 MeV
• 17O + 1H = 14N + 4He +1.19 MeV

The NO I cycle increases the rate of energy release in the CN cycle, increasing the number of catalyst nuclei in the CN cycle.

The last reaction of this cycle can also have a different outcome, giving rise to another NO II cycle:

• 15N + 1H = 16O + g +12.13 MeV
• 16O + 1H = 17F + g +0.60 MeV
• 17F = 17O + e+ + n +2.76 MeV
• 17O + 1H = 18F + g +5.61 MeV
• 18O + 1H = 15N + 4He +3.98 MeV

Thus, the CN, NO I and NO II cycles form a ternary CNO cycle.

There is another very slow fourth cycle, the OF cycle, but its role in energy production is negligible. However, this cycle is very important in explaining the origin of 19F.

• 17O + 1H = 18F + g + 5.61 MeV
• 18F = 18O + e+ + n + 1.656 MeV
• 18O + 1H = 19F + g + 7.994 MeV
• 19F + 1H = 16O + 4He + 8.114 MeV
• 16O + 1H = 17F + g + 0.60 MeV
• 17F = 17O + e+ + n + 2.76 MeV

During the explosive combustion of hydrogen in the surface layers of stars, for example, during supernova explosions, very high temperatures can develop, and the nature of the CNO cycle changes dramatically. It turns into the so-called hot CNO cycle, in which the reactions are very fast and confusing.

Chemical elements heavier than 4He begin to be synthesized only after complete combustion of hydrogen in the central region of the star:

4He + 4He + 4He > 12C + g + 7.367 MeV

Carbon combustion reactions:

• 12C + 12C = 20Ne + 4He +4.617 MeV
• 12C + 12C = 23Na + 1H -2.241 MeV
• 12C + 12C = 23Mg + 1n +2.599 MeV
• 23Mg = 23Na + e+ + n + 8.51 MeV
• 12C + 12C = 24Mg + g +13.933 MeV
• 12C + 12C = 16O + 24He -0.113 MeV
• 24Mg + 1H = 25Al + g

When the temperature reaches 5·109 K in stars under conditions of thermodynamic equilibrium, a large number of various reactions occur, resulting in the formation of atomic nuclei up to Fe and Ni.

The second half of the 20th century was a period of rapid development of nuclear physics. It became clear that nuclear reactions could be used to produce enormous energy from tiny amounts of fuel. Only nine years passed from the explosion of the first nuclear bomb to the first nuclear power plant, and when a hydrogen bomb was tested in 1952, there were predictions that thermonuclear power plants would come into operation in the 1960s. Alas, these hopes were not justified.

Thermonuclear reactions Of all the thermonuclear reactions, only four are of interest in the near future: deuterium + deuterium (products - tritium and proton, released energy 4.0 MeV), deuterium + deuterium (helium-3 and neutron, 3.3 MeV), deuterium + tritium (helium-4 and neutron, 17.6 MeV) and deuterium + helium-3 (helium-4 and proton, 18.2 MeV). The first and second reactions occur in parallel with equal probability. The resulting tritium and helium-3 “burn” in the third and fourth reactions

Igor Egorov

The main source of energy for humanity today is the combustion of coal, oil and gas. But their supplies are limited, and combustion products pollute the environment. A coal power plant produces more radioactive emissions than a nuclear power plant of the same power! So why haven't we switched to nuclear energy sources yet? There are many reasons for this, but the main one recently has been radiophobia. Despite the fact that a coal-fired power plant, even during normal operation, harms the health of many more people than emergency emissions at a nuclear power plant, it does so quietly and unnoticed by the public. Accidents at nuclear power plants immediately become the main news in the media, causing general panic (often completely unfounded). However, this does not mean that nuclear energy does not have objective problems. Radioactive waste causes a lot of trouble: technologies for working with it are still extremely expensive, and the ideal situation when all of it will be completely recycled and used is still far away.

Of all the thermonuclear reactions, only four are of interest in the near future: deuterium + deuterium (products - tritium and proton, released energy 4.0 MeV), deuterium + deuterium (helium-3 and neutron, 3.3 MeV), deuterium + tritium (helium -4 and neutron, 17.6 MeV) and deuterium + helium-3 (helium-4 and proton, 18.2 MeV). The first and second reactions occur in parallel with equal probability. The resulting tritium and helium-3 “burn” in the third and fourth reactions.

From fission to fusion

A potential solution to these problems is the transition from fission reactors to fusion reactors. While a typical fission reactor contains tens of tons of radioactive fuel, which is converted into tens of tons of radioactive waste containing a wide variety of radioactive isotopes, a fusion reactor uses only hundreds of grams, maximum kilograms, of one radioactive isotope of hydrogen, tritium. In addition to the fact that the reaction requires an insignificant amount of this least dangerous radioactive isotope, its production is also planned to be carried out directly at the power plant in order to minimize the risks associated with transportation. The synthesis products are stable (non-radioactive) and non-toxic hydrogen and helium. In addition, unlike a fission reaction, a thermonuclear reaction immediately stops when the installation is destroyed, without creating the danger of a thermal explosion. So why has not a single operational thermonuclear power plant been built yet? The reason is that the listed advantages inevitably entail disadvantages: creating the conditions for synthesis turned out to be much more difficult than initially expected.

Lawson criterion

For a thermonuclear reaction to be energetically favorable, it is necessary to ensure a sufficiently high temperature of the thermonuclear fuel, a sufficiently high density and sufficiently low energy losses. The latter are numerically characterized by the so-called “retention time”, which is equal to the ratio of the thermal energy stored in the plasma to the energy loss power (many people mistakenly believe that the “retention time” is the time during which hot plasma is maintained in the installation, but this is not so) . At a temperature of a mixture of deuterium and tritium equal to 10 keV (approximately 110,000,000 degrees), we need to obtain the product of the number of fuel particles in 1 cm 3 (i.e., plasma concentration) and the retention time (in seconds) of at least 10 14. It does not matter whether we have a plasma with a concentration of 1014 cm -3 and a retention time of 1 s, or a plasma with a concentration of 10 23 and a retention time of 1 ns. This criterion is called the Lawson criterion.
In addition to the Lawson criterion, which is responsible for obtaining an energetically favorable reaction, there is also a plasma ignition criterion, which for the deuterium-tritium reaction is approximately three times greater than the Lawson criterion. “Ignition” means that the fraction of thermonuclear energy that remains in the plasma will be enough to maintain the required temperature, and additional heating of the plasma will no longer be required.

Z-pinch

The first device in which it was planned to obtain a controlled thermonuclear reaction was the so-called Z-pinch. In the simplest case, this installation consists of only two electrodes located in a deuterium (hydrogen-2) environment or a mixture of deuterium and tritium, and a battery of high-voltage pulse capacitors. At first glance, it seems that it makes it possible to obtain compressed plasma heated to enormous temperatures: exactly what is needed for a thermonuclear reaction! However, in life, everything turned out, alas, to be far from so rosy. The plasma rope turned out to be unstable: the slightest bend leads to a strengthening of the magnetic field on one side and a weakening on the other; the resulting forces further increase the bending of the rope - and all the plasma “falls out” onto the side wall of the chamber. The rope is not only unstable to bending, the slightest thinning of it leads to an increase in the magnetic field in this part, which compresses the plasma even more, squeezing it into the remaining volume of the rope until the rope is finally “squeezed out.” The compressed part has a high electrical resistance, so the current is interrupted, the magnetic field disappears, and all the plasma dissipates.

The principle of operation of the Z-pinch is simple: an electric current generates an annular magnetic field, which interacts with the same current and compresses it. As a result, the density and temperature of the plasma through which the current flows increases.

It was possible to stabilize the plasma bundle by applying a powerful external magnetic field to it, parallel to the current, and placing it in a thick conductive casing (as the plasma moves, the magnetic field also moves, which induces an electric current in the casing, tending to return the plasma to its place). The plasma stopped bending and pinching, but it was still far from a thermonuclear reaction on any serious scale: the plasma touches the electrodes and gives off its heat to them.

Modern work in the field of Z-pinch fusion suggests another principle for creating fusion plasma: a current flows through a tungsten plasma tube, which creates powerful X-rays that compress and heat the capsule with fusion fuel located inside the plasma tube, just as it does in a thermonuclear bomb. However, these works are purely research in nature (the mechanisms of operation of nuclear weapons are studied), and the energy release in this process is still millions of times less than consumption.

The smaller the ratio of the large radius of the tokamak torus (the distance from the center of the entire torus to the center of the cross-section of its pipe) to the small one (the cross-section radius of the pipe), the greater the plasma pressure can be under the same magnetic field. By reducing this ratio, scientists moved from a circular cross-section of the plasma and vacuum chamber to a D-shaped one (in this case, the role of the small radius is played by half the height of the cross-section). All modern tokamaks have exactly this cross-sectional shape. The limiting case was the so-called “spherical tokamak”. In such tokamaks, the vacuum chamber and plasma are almost spherical in shape, with the exception of a narrow channel connecting the poles of the sphere. The conductors of magnetic coils pass through the channel. The first spherical tokamak, START, appeared only in 1991, so this is a fairly young direction, but it has already shown the possibility of obtaining the same plasma pressure with a three times lower magnetic field.

Cork chamber, stellarator, tokamak

Another option for creating the conditions necessary for the reaction is the so-called open magnetic traps. The most famous of them is the “cork cell”: a pipe with a longitudinal magnetic field that strengthens at its ends and weakens in the middle. The field increased at the ends creates a “magnetic plug” (hence the Russian name), or “magnetic mirror” (English - mirror machine), which keeps the plasma from leaving the installation through the ends. However, such retention is incomplete; some charged particles moving along certain trajectories are able to pass through these jams. And as a result of collisions, any particle will sooner or later fall on such a trajectory. In addition, the plasma in the mirror chamber also turned out to be unstable: if in some place a small section of the plasma moves away from the axis of the installation, forces arise that eject the plasma onto the chamber wall. Although the basic idea of ​​the mirror cell was significantly improved (which made it possible to reduce both the instability of the plasma and the permeability of the mirrors), in practice it was not even possible to approach the parameters necessary for energetically favorable synthesis.

Is it possible to make sure that the plasma does not escape through the “plugs”? It would seem that the obvious solution is to roll the plasma into a ring. However, then the magnetic field inside the ring is stronger than outside, and the plasma again tends to go to the chamber wall. The way out of this difficult situation also seemed quite obvious: instead of a ring, make a “figure eight”, then in one section the particle will move away from the axis of the installation, and in another it will return back. This is how scientists came up with the idea of ​​the first stellarator. But such a “figure eight” cannot be made in one plane, so we had to use the third dimension, bending the magnetic field in the second direction, which also led to a gradual movement of the particles from the axis to the chamber wall.

The situation changed dramatically with the creation of tokamak-type installations. The results obtained at the T-3 tokamak in the second half of the 1960s were so stunning for that time that Western scientists came to the USSR with their measuring equipment to verify the plasma parameters themselves. The reality even exceeded their expectations.

These fantastically intertwined tubes are not an art project, but a stellarator chamber bent into a complex three-dimensional curve.

In the hands of inertia

In addition to magnetic confinement, there is a fundamentally different approach to thermonuclear fusion - inertial confinement. If in the first case we try to keep the plasma at a very low concentration for a long time (the concentration of molecules in the air around you is hundreds of thousands of times higher), then in the second case we compress the plasma to a huge density, an order of magnitude higher than the density of the heaviest metals, in the expectation that the reaction will have time to pass in that short time before the plasma has time to scatter to the sides.

Originally, in the 1960s, the plan was to use a small ball of frozen fusion fuel, uniformly irradiated from all sides by multiple laser beams. The surface of the ball should have instantly evaporated and, expanding evenly in all directions, compressed and heated the remaining part of the fuel. However, in practice, the irradiation turned out to be insufficiently uniform. In addition, part of the radiation energy was transferred to the inner layers, causing them to heat up, which made compression more difficult. As a result, the ball compressed unevenly and weakly.

There are a number of modern stellarator configurations, all of which are close to a torus. One of the most common configurations involves the use of coils similar to the poloidal field coils of tokamaks, and four to six conductors twisted around a vacuum chamber with multidirectional current. The complex magnetic field created in this way allows the plasma to be reliably contained without requiring a ring electric current to flow through it. In addition, stellarators can also use toroidal field coils, like tokamaks. And there may be no helical conductors, but then the “toroidal” field coils are installed along a complex three-dimensional curve. Recent developments in the field of stellarators involve the use of magnetic coils and a vacuum chamber of a very complex shape (a very “crumpled” torus), calculated on a computer.

The problem of unevenness was solved by significantly changing the design of the target. Now the ball is placed inside a special small metal chamber (it is called “holraum”, from the German hohlraum - cavity) with holes through which laser beams enter inside. In addition, crystals are used that convert IR laser radiation into ultraviolet. This UV radiation is absorbed by a thin layer of hohlraum material, which is heated to enormous temperatures and emits soft X-rays. In turn, X-ray radiation is absorbed by a thin layer on the surface of the fuel capsule (ball with fuel). This also made it possible to solve the problem of premature heating of the internal layers.

However, the power of the lasers turned out to be insufficient for a noticeable portion of the fuel to react. In addition, the efficiency of the lasers was very low, only about 1%. For fusion to be energetically beneficial at such a low laser efficiency, almost all of the compressed fuel had to react. When trying to replace lasers with beams of light or heavy ions, which can be generated with much greater efficiency, scientists also encountered a lot of problems: light ions repel each other, which prevents them from focusing, and are slowed down when colliding with residual gas in the chamber, and accelerators It was not possible to create heavy ions with the required parameters.

Magnetic prospects

Most of the hope in the field of fusion energy now lies in tokamaks. Especially after they opened a mode with improved retention. A tokamak is both a Z-pinch rolled into a ring (a ring electric current flows through the plasma, creating a magnetic field necessary to contain it), and a sequence of mirror cells assembled into a ring and creating a “corrugated” toroidal magnetic field. In addition, a field perpendicular to the torus plane, created by several individual coils, is superimposed on the toroidal field of the coils and the plasma current field. This additional field, called poloidal, strengthens the magnetic field of the plasma current (also poloidal) on the outside of the torus and weakens it on the inside. Thus, the total magnetic field on all sides of the plasma rope turns out to be the same, and its position remains stable. By changing this additional field, it is possible to move the plasma bundle inside the vacuum chamber within certain limits.

A fundamentally different approach to synthesis is proposed by the concept of muon catalysis. A muon is an unstable elementary particle that has the same charge as an electron, but 207 times more mass. A muon can replace an electron in a hydrogen atom, and the size of the atom decreases by a factor of 207. This allows one hydrogen nucleus to move closer to another without expending energy. But to produce one muon, about 10 GeV of energy is spent, which means it is necessary to perform several thousand fusion reactions per muon to obtain energy benefits. Due to the possibility of a muon “sticking” to the helium formed in the reaction, more than several hundred reactions have not yet been achieved. The photo shows the assembly of the Wendelstein z-x stellarator at the Max Planck Institute for Plasma Physics.

An important problem of tokamaks for a long time was the need to create a ring current in the plasma. To do this, a magnetic circuit was passed through the central hole of the tokamak torus, the magnetic flux in which was continuously changed. The change in magnetic flux generates a vortex electric field, which ionizes the gas in the vacuum chamber and maintains current in the resulting plasma. However, the current in the plasma must be maintained continuously, which means that the magnetic flux must continuously change in one direction. This, of course, is impossible, so the current in tokamaks could only be maintained for a limited time (from a fraction of a second to several seconds). Fortunately, the so-called bootstrap current was discovered, which occurs in a plasma without an external vortex field. In addition, methods have been developed to heat the plasma, simultaneously inducing the necessary ring current in it. Together, this provided the potential for maintaining hot plasma for as long as desired. In practice, the record currently belongs to the Tore Supra tokamak, where the plasma continuously “burned” for more than six minutes.

The second type of plasma confinement installation, which has great promise, is stellarators. Over the past decades, the design of stellarators has changed dramatically. Almost nothing remained of the original “eight”, and these installations became much closer to tokamaks. Although the confinement time of stellarators is shorter than that of tokamaks (due to the less efficient H-mode), and the cost of their construction is higher, the behavior of the plasma in them is calmer, which means a longer life of the first inner wall of the vacuum chamber. For the commercial development of thermonuclear fusion, this factor is of great importance.

Selecting a reaction

At first glance, it is most logical to use pure deuterium as a thermonuclear fuel: it is relatively cheap and safe. However, deuterium reacts with deuterium a hundred times less readily than with tritium. This means that to operate a reactor on a mixture of deuterium and tritium, a temperature of 10 keV is sufficient, and to operate on pure deuterium, a temperature of more than 50 keV is required. And the higher the temperature, the higher the energy loss. Therefore, at least for the first time, thermonuclear energy is planned to be built on deuterium-tritium fuel. Tritium will be produced in the reactor itself due to irradiation with the fast lithium neutrons produced in it.
"Wrong" neutrons. In the cult film “9 Days of One Year,” the main character, while working at a thermonuclear installation, received a serious dose of neutron radiation. However, it later turned out that these neutrons were not produced as a result of a fusion reaction. This is not the director’s invention, but a real effect observed in Z-pinches. At the moment of interruption of the electric current, the inductance of the plasma leads to the generation of a huge voltage - millions of volts. Individual hydrogen ions, accelerated in this field, are capable of literally knocking neutrons out of the electrodes. At first, this phenomenon was indeed taken as a sure sign of a thermonuclear reaction, but subsequent analysis of the neutron energy spectrum showed that they had a different origin.
Improved retention mode. The H-mode of a tokamak is a mode of its operation when, with a high power of additional heating, plasma energy losses sharply decrease. The accidental discovery of the enhanced confinement mode in 1982 is as significant as the invention of the tokamak itself. There is no generally accepted theory of this phenomenon yet, but this does not prevent it from being used in practice. All modern tokamaks operate in this mode, as it reduces losses by more than half. Subsequently, a similar regime was discovered in stellarators, indicating that this is a general property of toroidal systems, but confinement is only improved by about 30% in them.
Plasma heating. There are three main methods of heating plasma to thermonuclear temperatures. Ohmic heating is the heating of plasma due to the flow of electric current through it. This method is most effective in the first stages, since as the temperature increases, the electrical resistance of the plasma decreases. Electromagnetic heating uses electromagnetic waves with a frequency that matches the frequency of rotation around the magnetic field lines of electrons or ions. By injecting fast neutral atoms, a stream of negative ions is created, which are then neutralized, turning into neutral atoms that can pass through the magnetic field to the center of the plasma to transfer their energy there.
Are these reactors? Tritium is radioactive, and powerful neutron irradiation from the D-T reaction creates induced radioactivity in the reactor design elements. We have to use robots, which complicates the work. At the same time, the behavior of a plasma of ordinary hydrogen or deuterium is very close to the behavior of a plasma from a mixture of deuterium and tritium. This led to the fact that throughout history, only two thermonuclear installations fully operated on a mixture of deuterium and tritium: the TFTR and JET tokamaks. At other installations, even deuterium is not always used. So the name “thermonuclear” in the definition of a facility does not at all mean that thermonuclear reactions have ever actually occurred in it (and in those that do occur, pure deuterium is almost always used).
Hybrid reactor. The D-T reaction produces 14 MeV neutrons, which can even fission depleted uranium. The fission of one uranium nucleus is accompanied by the release of approximately 200 MeV of energy, which is more than ten times the energy released during fusion. So existing tokamaks could become energetically beneficial if they were surrounded by a uranium shell. Compared to fission reactors, such hybrid reactors would have the advantage of preventing an uncontrolled chain reaction from developing in them. In addition, extremely intense neutron fluxes should convert long-lived uranium fission products into short-lived ones, which significantly reduces the problem of waste disposal.

Inertial hopes

Inertial fusion is also not standing still. Over the decades of development of laser technology, prospects have emerged to increase the efficiency of lasers by approximately ten times. And in practice, their power has been increased hundreds and thousands of times. Work is also underway on heavy ion accelerators with parameters suitable for thermonuclear use. In addition, the concept of “fast ignition” has been a critical factor in the progress of inertial fusion. It involves the use of two pulses: one compresses the thermonuclear fuel, and the other heats up a small part of it. It is assumed that the reaction that begins in a small part of the fuel will subsequently spread further and cover the entire fuel. This approach makes it possible to significantly reduce energy costs, and therefore make the reaction profitable with a smaller fraction of reacted fuel.

Tokamak problems

Despite the progress of installations of other types, tokamaks at the moment still remain out of competition: if two tokamaks (TFTR and JET) back in the 1990s actually produced a release of thermonuclear energy approximately equal to the energy consumption for heating the plasma (even though such a mode lasted only about a second), then nothing similar could be achieved with other types of installations. Even a simple increase in the size of tokamaks will lead to the feasibility of energetically favorable fusion in them. The international reactor ITER is currently being built in France, which will have to demonstrate this in practice.

However, tokamaks also have problems. ITER costs billions of dollars, which is unacceptable for future commercial reactors. No reactor has operated continuously for even a few hours, let alone for weeks and months, which again is necessary for industrial applications. There is no certainty yet that the materials of the inner wall of the vacuum chamber will be able to withstand prolonged exposure to plasma.

The concept of a tokamak with a strong field can make the project less expensive. By increasing the field by two to three times, it is planned to obtain the required plasma parameters in a relatively small installation. This concept, in particular, is the basis for the Ignitor reactor, which, together with Italian colleagues, is now beginning to be built at TRINIT (Trinity Institute for Innovation and Thermonuclear Research) near Moscow. If the engineers’ calculations come true, then at a cost many times lower than ITER, it will be possible to ignite plasma in this reactor.

Forward to the stars!

The products of a thermonuclear reaction fly away in different directions at speeds of thousands of kilometers per second. This makes it possible to create ultra-efficient rocket engines. Their specific impulse will be higher than that of the best electric jet engines, and their energy consumption may even be negative (theoretically, it is possible to generate, rather than consume, energy). Moreover, there is every reason to believe that making a thermonuclear rocket engine will be even easier than a ground-based reactor: there is no problem with creating a vacuum, with thermal insulation of superconducting magnets, there are no restrictions on dimensions, etc. In addition, the generation of electricity by the engine is desirable, but It’s not at all necessary, it’s enough that he doesn’t consume too much of it.

Electrostatic confinement

The concept of electrostatic ion confinement is most easily understood through a setup called a fusor. It is based on a spherical mesh electrode, to which a negative potential is applied. Ions accelerated in a separate accelerator or by the field of the central electrode itself fall inside it and are held there by an electrostatic field: if an ion tends to fly out, the electrode field turns it back. Unfortunately, the probability of an ion colliding with a network is many orders of magnitude higher than the probability of entering into a fusion reaction, which makes an energetically favorable reaction impossible. Such installations have found application only as neutron sources.
In an effort to make a sensational discovery, many scientists strive to see synthesis wherever possible. There have been numerous reports in the press regarding various options for so-called “cold fusion.” Synthesis was discovered in metals “impregnated” with deuterium when an electric current flows through them, during the electrolysis of deuterium-saturated liquids, during the formation of cavitation bubbles in them, as well as in other cases. However, most of these experiments have not had satisfactory reproducibility in other laboratories, and their results can almost always be explained without the use of synthesis.
Continuing the “glorious tradition” that began with the “philosopher’s stone” and then turned into a “perpetual motion machine”, many modern scammers are now offering to buy from them a “cold fusion generator”, “cavitation reactor” and other “fuel-free generators”: about the philosophical Everyone has already forgotten the stone, they don’t believe in perpetual motion, but nuclear fusion now sounds quite convincing. But, alas, in reality such energy sources do not exist yet (and when they can be created, it will be in all news releases). So be aware: if you are offered to buy a device that generates energy through cold nuclear fusion, then they are simply trying to “cheat” you!

According to preliminary estimates, even with the current level of technology, it is possible to create a thermonuclear rocket engine for flight to the planets of the Solar System (with appropriate funding). Mastering the technology of such engines will increase the speed of manned flights tenfold and will make it possible to have large reserve fuel reserves on board, which will make flying to Mars no more difficult than working on the ISS now. Speeds of 10% of the speed of light will potentially become available for automatic stations, which means it will be possible to send research probes to nearby stars and obtain scientific data during the lifetime of their creators.

The concept of a thermonuclear rocket engine based on inertial fusion is currently considered the most developed. The difference between an engine and a reactor lies in the magnetic field, which directs the charged reaction products in one direction. The second option involves using an open trap, in which one of the plugs is deliberately weakened. The plasma flowing from it will create a reactive force.

Thermonuclear future

Mastering thermonuclear fusion turned out to be many orders of magnitude more difficult than it seemed at first. And although many problems have already been solved, the remaining ones will be enough for the next few decades of hard work of thousands of scientists and engineers. But the prospects that the transformations of hydrogen and helium isotopes open up for us are so great, and the path taken is already so significant that it makes no sense to stop halfway. No matter what numerous skeptics say, the future undoubtedly lies in synthesis.

Encyclopedic YouTube

1 / 5

✪ Nuclear Rocket Engine Latest Technologies 2016

✪ The world's first nuclear space engine was assembled in Russia.

✪ Atomic Horizons (03/26/2016): Nuclear safety technologies

✪ Nuclear reactor instead of a heart?

✪ Nuclear energy and technology

Subtitles

Physics

Atomic nuclei are made up of two types of nucleons - protons and neutrons. They are held together by the so-called strong interaction. In this case, the binding energy of each nucleon with others depends on the total number of nucleons in the nucleus, as shown in the graph on the right. The graph shows that for light nuclei, as the number of nucleons increases, the binding energy increases, and for heavy nuclei it decreases. If you add nucleons to light nuclei or remove nucleons from heavy atoms, this difference in binding energy will be released as the kinetic energy of the particles released as a result of these actions. The kinetic energy (energy of motion) of particles transforms into thermal motion of atoms after the collision of particles with atoms. Thus nuclear energy manifests itself in the form of heat.

A change in the composition of the nucleus is called a nuclear transformation or nuclear reaction. A nuclear reaction with an increase in the number of nucleons in the nucleus is called a thermonuclear reaction or nuclear fusion. A nuclear reaction with a decrease in the number of nucleons in the nucleus is called nuclear decay or nuclear fission.

Nuclear fission

Nuclear fission can be spontaneous (spontaneous) or caused by external influences (induced).

Spontaneous fission

Modern science believes that all chemical elements heavier than hydrogen were synthesized as a result of thermonuclear reactions inside stars. Depending on the number of protons and neutrons, the nucleus can be stable or tend to spontaneously divide into several parts. After the end of the stars' lives, stable atoms formed the world we know, and unstable atoms gradually decayed before the formation of stable ones. On Earth to this day, only two such unstable substances have survived in industrial quantities ( radioactive) chemical elements - uranium and thorium. Other unstable elements are produced artificially in accelerators or reactors.

Chain reaction

Some heavy nuclei easily attach an external free neutron, become unstable and decay, emitting several new free neutrons. In turn, these released neutrons can enter neighboring nuclei and also cause their decay with the release of further free neutrons. This process is called a chain reaction. For a chain reaction to occur, it is necessary to create specific conditions: to concentrate in one place a sufficiently large amount of a substance capable of a chain reaction. The density and volume of this substance must be sufficient so that free neutrons do not have time to leave the substance, interacting with nuclei with a high probability. This probability is characterized neutron multiplication factor. When the volume, density and configuration of the substance allow the neutron multiplication factor to reach unity, a self-sustaining chain reaction will begin, and the mass of the fissile substance will be called critical mass. Naturally, each decay in this chain leads to the release of energy.

People have learned to carry out chain reactions in special structures. Depending on the required rate of chain reaction and its heat generation, these structures are called nuclear weapons or nuclear reactors. In nuclear weapons, an avalanche-like uncontrolled chain reaction is carried out with the maximum achievable neutron multiplication factor in order to achieve maximum energy release before thermal destruction of the structure occurs. In nuclear reactors, they try to achieve a stable neutron flux and heat release so that the reactor performs its tasks and does not collapse from excessive thermal loads. This process is called a controlled chain reaction.

Controlled chain reaction

In nuclear reactors, conditions are created for controlled chain reaction. As is clear from the meaning of a chain reaction, its rate can be controlled by changing the neutron multiplication factor. To do this, you can change various design parameters: the density of the fissile substance, the energy spectrum of neutrons, introduce substances that absorb neutrons, add neutrons from external sources, etc.

However, the chain reaction is a very fast avalanche-like process; it is almost impossible to reliably control it directly. Therefore, to control the chain reaction, delayed neutrons are of great importance - neutrons formed during the spontaneous decay of unstable isotopes formed as a result of the primary decays of fissile material. The time from primary decay to delayed neutrons varies from milliseconds to minutes, and the share of delayed neutrons in the neutron balance of the reactor reaches a few percent. Such time values ​​already make it possible to regulate the process using mechanical methods. The neutron multiplication factor, taking into account delayed neutrons, is called the effective neutron multiplication factor, and instead of the critical mass, the concept of reactivity of a nuclear reactor was introduced.

The dynamics of a controlled chain reaction are also influenced by other fission products, some of which can effectively absorb neutrons (so-called neutron poisons). Once the chain reaction begins, they accumulate in the reactor, reducing the effective neutron multiplication factor and reactivity of the reactor. After some time, a balance occurs in the accumulation and decay of such isotopes and the reactor enters a stable mode. If the reactor is shut down, neutron poisons remain in the reactor for a long time, making it difficult to restart. The characteristic lifetime of neutron poisons in the decay chain of uranium is up to half a day. Neutron poisons prevent nuclear reactors from rapidly changing power.

Nuclear fusion

Neutron spectrum

The distribution of neutron energies in a neutron flux is usually called the neutron spectrum. The neutron energy determines the pattern of interaction of the neutron with the nucleus. It is customary to distinguish several neutron energy ranges, of which the following are significant for nuclear technologies:

• Thermal neutrons. They are named so because they are in energy equilibrium with the thermal vibrations of atoms and do not transfer their energy to them during elastic interactions.
• Resonant neutrons. They are named so because the cross section for the interaction of some isotopes with neutrons of these energies has pronounced irregularities.
• Fast neutrons. Neutrons of these energies are usually produced by nuclear reactions.

Prompt and delayed neutrons

The chain reaction is a very fast process. The lifetime of one generation of neutrons (that is, the average time from the appearance of a free neutron to its absorption by the next atom and the birth of the next free neutrons) is much less than a microsecond. Such neutrons are called prompt. In a chain reaction with a multiplication factor of 1.1, after 6 μs the number of prompt neutrons and the energy released will increase by 10 26 times. It is impossible to reliably manage such a fast process. Therefore, delayed neutrons are of great importance for a controlled chain reaction. Delayed neutrons arise from the spontaneous decay of fission fragments remaining after primary nuclear reactions.

Materials Science

Isotopes

In the surrounding nature, people usually encounter the properties of substances determined by the structure of the electronic shells of atoms. For example, it is the electron shells that are entirely responsible for the chemical properties of the atom. Therefore, before the nuclear era, science did not separate substances by the mass of the nucleus, but only by its electric charge. However, with the advent of nuclear technology, it became clear that all well-known simple chemical elements have many - sometimes dozens - of varieties with different numbers of neutrons in the nucleus and, accordingly, completely different nuclear properties. These varieties came to be called isotopes of chemical elements. Most naturally occurring chemical elements are mixtures of several different isotopes.

The vast majority of known isotopes are unstable and do not occur in nature. They are obtained artificially for study or use in nuclear technology. The separation of mixtures of isotopes of one chemical element, the artificial production of isotopes, and the study of the properties of these isotopes are some of the main tasks of nuclear technology.

Fissile materials

Some isotopes are unstable and decay. However, decay does not occur immediately after the synthesis of the isotope, but after some time characteristic of this isotope, called half-life. From the name it is obvious that this is the time during which half of the existing nuclei of an unstable isotope decay.

Unstable isotopes are almost never found in nature, since even the longest-lived ones managed to completely decay in the billions of years that have passed since the synthesis of the substances around us in the thermonuclear furnace of a long-extinct star. There are only three exceptions: these are two isotopes of uranium (uranium-235 and uranium-238) and one isotope of thorium - thorium-232. In addition to them, in nature one can find traces of other unstable isotopes formed as a result of natural nuclear reactions: the decay of these three exceptions and the impact of cosmic rays on the upper layers of the atmosphere.

Unstable isotopes are the basis of almost all nuclear technologies.

Supporting the chain reaction

Separately, there is a group of unstable isotopes that is very important for nuclear technology and capable of maintaining a nuclear chain reaction. To maintain a chain reaction, the isotope must absorb neutrons well, followed by decay, resulting in the formation of several new free neutrons. Humanity is incredibly lucky that among the unstable isotopes preserved in nature in industrial quantities there was one that supports a chain reaction: uranium-235.

Construction materials

Story

Opening

At the beginning of the twentieth century, Rutherford made a huge contribution to the study of ionizing radiation and the structure of atoms. Ernest Walton and John Cockroft were able to split the nucleus of an atom for the first time.

Nuclear weapons programs

In the late 30s of the twentieth century, physicists realized the possibility of creating powerful weapons based on a nuclear chain reaction. This led to high government interest in nuclear technology. The first large-scale state atomic program appeared in Germany in 1939 (see German nuclear program). However, the war complicated the supply of the program and after the defeat of Germany in 1945, the program was closed without significant results. In 1943, a large-scale program codenamed the Manhattan Project began in the United States. In 1945, as part of this program, the world's first nuclear bomb was created and tested. Nuclear research in the USSR has been carried out since the 20s. In 1940, the first Soviet theoretical design for a nuclear bomb was developed. Nuclear developments in the USSR have been classified since 1941. The first Soviet nuclear bomb was tested in 1949.

The main contribution to the energy release of the first nuclear weapons was made by the fission reaction. Nevertheless, the fusion reaction was used as an additional source of neutrons to increase the amount of reacted fissile material. In 1952 in the USA and 1953 in the USSR, designs were tested in which most of the energy release was created by the fusion reaction. Such a weapon was called thermonuclear. In thermonuclear ammunition, the fission reaction serves to “ignite” the thermonuclear reaction without making a significant contribution to the overall energy of the weapon.

Nuclear energy

The first nuclear reactors were either experimental or weapons-grade, that is, designed to produce weapons-grade plutonium from uranium. The heat they created was released into the environment. Low operating powers and small temperature differences made it difficult to effectively use such low-grade heat to operate traditional heat engines. In 1951, this heat was used for the first time for power generation: in the USA, a steam turbine with an electric generator was installed in the cooling circuit of an experimental reactor. In 1954, the first nuclear power plant was built in the USSR, originally designed for electric power purposes.

Technologies

Nuclear weapon

There are many ways to harm people using nuclear technology. But states adopted only explosive nuclear weapons based on a chain reaction. The principle of operation of such weapons is simple: it is necessary to maximize the neutron multiplication factor in the chain reaction, so that as many nuclei as possible react and release energy before the weapon’s structure is destroyed by the generated heat. To do this, it is necessary either to increase the mass of the fissile substance or to increase its density. Moreover, this must be done as quickly as possible, otherwise the slow increase in energy release will melt and evaporate the structure without an explosion. Accordingly, two approaches to building a nuclear explosive device have been developed:

• A scheme with increasing mass, the so-called cannon scheme. Two subcritical pieces of fissile material were installed in the barrel of an artillery gun. One piece was fixed at the end of the barrel, the other acted as a projectile. The shot brought the pieces together, a chain reaction began and an explosive release of energy occurred. The achievable approach speeds in such a scheme were limited to a couple of km/sec.
• A scheme with increasing density, the so-called implosive scheme. Based on the peculiarities of metallurgy of the artificial isotope of plutonium. Plutonium is capable of forming stable allotropic modifications that differ in density. A shock wave passing through the volume of the metal is capable of converting plutonium from an unstable low-density modification to a high-density one. This feature made it possible to transfer plutonium from a low-density subcritical state to a supercritical state with the speed of shock wave propagation in the metal. To create a shock wave, they used conventional chemical explosives, placing them around the plutonium assembly so that the explosion squeezed the spherical assembly from all sides.

Both schemes were created and tested almost simultaneously, but the implosion scheme turned out to be more efficient and more compact.

Neutron sources

Another limiter on energy release is the rate of increase in the number of neutrons in the chain reaction. In subcritical fissile material, spontaneous disintegration of atoms occurs. The neutrons from these decays become the first in an avalanche-like chain reaction. However, for maximum energy release, it is advantageous to first remove all neutrons from the substance, then transfer it to a supercritical state, and only then introduce ignition neutrons into the substance in the maximum amount. To achieve this, a fissile substance with minimal contamination by free neutrons from spontaneous decays is selected, and at the moment of transfer to the supercritical state, neutrons are added from external pulsed neutron sources.

Sources of additional neutrons are based on different physical principles. Initially, explosive sources based on mixing two substances became widespread. A radioactive isotope, usually polonium-210, was mixed with an isotope of beryllium. Alpha radiation from polonium caused a nuclear reaction of beryllium with the release of neutrons. Subsequently, they were replaced by sources based on miniature accelerators, on the targets of which a nuclear fusion reaction with a neutron yield was carried out.

In addition to ignition neutron sources, it turned out to be advantageous to introduce additional sources into the circuit that are triggered by the beginning of a chain reaction. Such sources were built on the basis of synthesis reactions of light elements. Ampules containing substances such as lithium-6 deuteride were installed in a cavity in the center of the plutonium nuclear assembly. Streams of neutrons and gamma rays from the developing chain reaction heated the ampoule to thermonuclear fusion temperatures, and the explosion plasma compressed the ampoule, helping the temperature with pressure. The fusion reaction began, supplying additional neutrons for the fission chain reaction.

Thermonuclear weapons

Neutron sources based on the fusion reaction were themselves a significant source of heat. However, the size of the cavity in the center of the plutonium assembly could not accommodate much material for synthesis, and if placed outside the plutonium fissile core, it would not be possible to obtain the temperature and pressure conditions required for synthesis. It was necessary to surround the substance for synthesis with an additional shell, which, perceiving the energy of a nuclear explosion, would provide shock compression. They made a large ampoule from uranium-235 and installed it next to the nuclear charge. Powerful neutron fluxes from the chain reaction will cause an avalanche of fission of uranium atoms in the ampoule. Despite the subcritical design of the uranium ampoule, the total effect of gamma rays and neutrons from the chain reaction of the pilot nuclear explosion and the own fission of the ampoule nuclei will create conditions for fusion inside the ampoule. Now the size of the ampoule with the substance for fusion turned out to be practically unlimited and the contribution of the energy release from nuclear fusion many times exceeded the energy release of the ignition nuclear explosion. Such weapons began to be called thermonuclear.

.

Based on a controlled chain reaction of fission of heavy nuclei. Currently, this is the only nuclear technology that provides economically viable industrial generation of electricity at nuclear power plants.

Based on the fusion reaction of light nuclei. Despite the well-known physics of the process, it has not yet been possible to build an economically feasible power plant.

Nuclear power plant

The heart of a nuclear power plant is a nuclear reactor - a device in which a controlled chain reaction of fission of heavy nuclei is carried out. The energy of nuclear reactions is released in the form of kinetic energy of fission fragments and is converted into heat due to elastic collisions of these fragments with other atoms.

Fuel cycle

Only one natural isotope is known that is capable of a chain reaction - uranium-235. Its industrial reserves are small. Therefore, today engineers are already looking for ways to produce cheap artificial isotopes that support the chain reaction. The most promising is plutonium, produced from the common isotope uranium-238 by capturing a neutron without fission. It is easy to produce in the same energy reactors as a by-product. Under certain conditions, a situation is possible when the production of artificial fissile material completely covers the needs of existing nuclear power plants. In this case, they speak of a closed fuel cycle, which does not require the supply of fissile material from a natural source.

Nuclear waste

Spent nuclear fuel (SNF) and reactor structural materials with induced radioactivity are powerful sources of dangerous ionizing radiation. Technologies for working with them are being intensively improved in the direction of minimizing the amount of landfilled waste and reducing the period of its danger. SNF is also a source of valuable radioactive isotopes for industry and medicine. SNF reprocessing is a necessary step in closing the fuel cycle.

Nuclear safety

Use in medicine

In medicine, various unstable elements are commonly used for research or therapy.

You already know that in the middle of the 20th century. the problem arose of finding new sources of energy. In this regard, thermonuclear reactions attracted the attention of scientists.

• Thermonuclear reaction is the fusion reaction of light nuclei (such as hydrogen, helium, etc.), occurring at temperatures from tens to hundreds of millions of degrees.

Creating a high temperature is necessary to give the nuclei a sufficiently large kinetic energy - only under this condition will the nuclei be able to overcome the forces of electrical repulsion and get close enough to fall into the zone of action of nuclear forces. At such small distances, the forces of nuclear attraction significantly exceed the forces of electrical repulsion, due to which synthesis (i.e., fusion, association) of nuclei is possible.

In § 58, using the example of uranium, it was shown that energy can be released during the fission of heavy nuclei. In the case of light nuclei, energy can be released during the reverse process - during their fusion. Moreover, the reaction of fusion of light nuclei is energetically more favorable than the reaction of fission of heavy nuclei (if we compare the released energy per nucleon).

An example of a thermonuclear reaction is the fusion of hydrogen isotopes (deuterium and tritium), resulting in the formation of helium and the emission of a neutron:

This is the first thermonuclear reaction that scientists have managed to carry out. It was implemented in a thermonuclear bomb and was of an uncontrollable (explosive) nature.

As already noted, thermonuclear reactions can occur with the release of large amounts of energy. But in order for this energy to be used for peaceful purposes, it is necessary to learn how to conduct controlled thermonuclear reactions. One of the main difficulties in carrying out such reactions is to contain high-temperature plasma (almost completely ionized gas) inside the installation, in which nuclear fusion occurs. The plasma should not come into contact with the walls of the installation in which it is located, otherwise the walls will turn into steam. Currently, very strong magnetic fields are used to confine plasma in a confined space at an appropriate distance from the walls.

Thermonuclear reactions play an important role in the evolution of the Universe, in particular in the transformation of chemical substances in it.

Thanks to thermonuclear reactions occurring in the depths of the Sun, energy is released that gives life to the inhabitants of the Earth.

Our Sun has been radiating light and heat into space for almost 4.6 billion years. Naturally, at all times, scientists have been interested in the question of what is the “fuel” due to which the Sun produces a huge amount of energy for such a long time.

There were different hypotheses on this matter. One of them was that energy in the Sun is released as a result of a chemical combustion reaction. But in this case, as calculations show, the Sun could exist for only a few thousand years, which contradicts reality.

The original hypothesis was put forward in the middle of the 19th century. It was that the increase in internal energy and the corresponding increase in the temperature of the Sun occurs due to a decrease in its potential energy during gravitational compression. It also turned out to be untenable, since in this case the lifespan of the Sun increases to millions of years, but not to billions.

The assumption that the release of energy in the Sun occurs as a result of thermonuclear reactions occurring on it was made in 1939 by the American physicist Hans Bethe.

They also proposed the so-called hydrogen cycle, i.e. a chain of three thermonuclear reactions leading to the formation of helium from hydrogen:

where is a particle called a “neutrino”, which means “little neutron” in Italian.

To produce the two nuclei needed for the third reaction, the first two must occur twice.

You already know that, in accordance with the formula E = mс 2, as the internal energy of a body decreases, its mass also decreases.

To imagine the colossal amount of energy the Sun loses as a result of the conversion of hydrogen into helium, it is enough to know that the mass of the Sun decreases by several million tons every second. But, despite the losses, the hydrogen reserves on the Sun should last for another 5-6 billion years.

The same reactions occur in the interiors of other stars, the mass and age of which are comparable to the mass and age of the Sun.

Questions

1. What reaction is called thermonuclear? Give an example of a reaction.
2. Why are thermonuclear reactions only possible at very high temperatures?
3. Which reaction is energetically more favorable (per nucleon): the fusion of light nuclei or the fission of heavy ones?
4. What is one of the main difficulties in carrying out thermonuclear reactions?
5. What is the role of thermonuclear reactions in the existence of life on Earth?
6. What is the source of solar energy according to modern ideas?
7. How long should the supply of hydrogen on the Sun last, according to scientists’ calculations?

This is interesting...

Elementary particles. Antiparticles

The particles that make up the atoms of various substances - electron, proton and neutron - are called elementary. The word "elementary" implied that these particles are primary, simplest, further indivisible and unchangeable. But it soon turned out that these particles are not immutable at all. They all have the ability to transform into each other when interacting.

Therefore, in modern physics, the term “elementary particles” is usually used not in its exact meaning, but to name a large group of smallest particles of matter that are not atoms or atomic nuclei (the exception is the proton, which is the nucleus of a hydrogen atom and at the same time belongs to the elementary particles).

Currently, more than 350 different elementary particles are known. These particles are very diverse in their properties. They may differ from each other in mass, sign and magnitude of the electric charge, lifetime (i.e., the time from the moment the particle is formed until the moment it is transformed into some other particle), penetrating ability (i.e., the ability to pass through matter ) and other characteristics. For example, most particles are “short-lived” - they live no more than two millionths of a second, while the average lifetime of a neutron outside the atomic nucleus is 15 minutes.

The most important discovery in the field of elementary particle research was made in 1932, when the American physicist Carl David Anderson discovered a trace of an unknown particle in a cloud chamber placed in a magnetic field. Based on the nature of this trace (radius of curvature, direction of bending, etc.), scientists determined that it was left by a particle, which is like an electron with a positive electric charge. This particle was called a positron.

It is interesting that a year before the experimental discovery of the positron, its existence was theoretically predicted by the English physicist Paul Dirac (the existence of just such a particle followed from the equation he derived). Moreover, Dirac predicted the so-called processes of annihilation (disappearance) and the birth of an electron-positron pair. Annihilation is that an electron and a positron disappear upon meeting, turning into γ-quanta (photons). And when a γ-quantum collides with any massive nucleus, an electron-positron pair is born.

Both of these processes were first observed experimentally in 1933. Figure 166 shows the tracks of an electron and a positron formed as a result of the collision of a γ-quantum with a lead atom during the passage of γ-rays through a lead plate. The experiment was carried out in a cloud chamber placed in a magnetic field. The same curvature of the tracks indicates the same mass of particles, and curvature in different directions indicates opposite signs of the electric charge.

Rice. 166. Tracks of an electron-positron pair in a magnetic field

In 1955, another antiparticle was discovered - the antiproton (the existence of which also followed from Dirac's theory), and a little later - the antineutron. An antineutron, like a neutron, has no electrical charge, but it undoubtedly belongs to antiparticles, since it participates in the process of annihilation and the birth of a neutron-antineutron pair.

The possibility of obtaining antiparticles led scientists to the idea of ​​​​creating antimatter. Antimatter atoms should be built in this way: in the center of the atom there is a negatively charged nucleus, consisting of antiprotons and antineutrons, and positrons revolve around the nucleus. In general, the atom is neutral. This idea also received brilliant experimental confirmation. In 1969, at the proton accelerator in Serpukhov, Soviet physicists obtained nuclei of antihelium atoms.

At present, antiparticles of almost all known elementary particles have been experimentally discovered.

Chapter summary. The most important

Below are physical concepts and phenomena. The sequence of presentation of definitions and formulations does not correspond to the sequence of concepts, etc.

Transfer the names of the concepts into your notebook and enter the serial number of the definition (wording) corresponding to this concept in square brackets.

• Radioactivity;
• nuclear (planetary) model of the structure of the atom;
• atomic nucleus;
• radioactive transformations of atomic nuclei;
• experimental methods for studying particles in atomic and nuclear physics;
• nuclear forces;
• nuclear binding energy;
• mass defect of the atomic nucleus;
• chain reaction ;
• nuclear reactor ;
• environmental and social problems arising from the use of nuclear power plants;
• absorbed dose of radiation.

1. Registration of particles using a Geiger counter, studying and photographing particle tracks (including those involved in nuclear reactions) in a cloud chamber and a bubble chamber.
2. The forces of attraction acting between nucleons in the nuclei of atoms and significantly exceeding the forces of electrostatic repulsion between protons.
3. The minimum energy required to split a nucleus into individual nucleons.
4. Spontaneous emission of radioactive rays by atoms of certain elements.
5. A device designed to carry out a controlled nuclear reaction.
6. Consists of nucleons (i.e. protons and neutrons).
7. Radioactive waste, the possibility of accidents, promotion of the proliferation of nuclear weapons.
8. An atom consists of a positively charged nucleus located at its center, around which electrons orbit at a distance significantly greater than the size of the nucleus.
9. The transformation of one chemical element into another through α- or β-decay, as a result of which the nucleus of the original atom undergoes changes.
10. The difference between the sum of the masses of the nucleons forming a nucleus and the mass of this nucleus.
11. A self-sustaining fission reaction of heavy nuclei, in which neutrons are continuously produced, dividing more and more new nuclei.
12. The energy of ionizing radiation absorbed by the emitted substance (in particular, body tissues) and calculated per unit mass.

check yourself