Thermonuclear reactions reactor briefly. Nuclear reactions

During the lesson, everyone will be able to get an idea about the topic “Thermonuclear reaction”. You will learn what a thermonuclear reaction, or fusion reaction, is. You will learn which elements and under what conditions can enter into this type of reaction, and get acquainted with developments in the use of thermonuclear reactions for peaceful purposes.

Thermonuclear reactions(or simply thermonuclear) is the reaction of fusion of light nuclei into one whole new nucleus, as a result of which a large amount of energy is released. It turns out that a lot of energy is released not only as a result of the fission of heavy nuclei, even more energy is released when light nuclei merge together and combine. This process is called synthesis. And the reactions themselves are thermonuclear fusion, thermonuclear reactions.

What elements are involved in these reactions? These are primarily hydrogen isotopes and helium isotopes. For example, the following reaction can be given:

Two isotopes of hydrogen (deuterium and tritium), when combined together, form a helium nucleus, and a neutron is also formed. When this reaction occurs, enormous energy is released E = 17.6 MeV.

Don't forget that this is just one reaction. And one more reaction. Two deuterium nuclei fuse together to form a helium nucleus:

In this case, a large amount is also released.

I draw your attention: for such reactions to occur, certain conditions are needed. First of all, it is necessary to bring the nuclei of these isotopes closer together. The nuclei have a positive charge; in this case, Coulomb forces act, which push these charges apart. This means that we need to overcome these Coulomb forces in order to bring one nucleus closer to another. This is only possible if the nuclei themselves have high kinetic energy, when the speed of these nuclei is quite high. To achieve this, it is necessary to create conditions where the isotope nuclei will have this speed, and this is only possible at very high temperatures. Only in this way will we be able to accelerate the isotopes to speeds that will allow them to approach each other to a distance of approximately 10 -14 m.

Rice. 1. The distance by which nuclei need to be brought together for a thermonuclear reaction to occur

This distance is exactly the distance from which nuclear forces begin to operate. The required temperature is about t° = 10 7 - 10 8° C. This temperature can be reached when a nuclear explosion is carried out. Thus, in order to produce a thermonuclear reaction, we must first produce a fission reaction of heavy nuclei. It is in this case that we will achieve a high temperature, and only then this temperature will make it possible to bring the nuclei of isotopes closer to a distance where they can combine. As you understand, this is precisely the principle of the so-called hydrogen bomb.

Rice. 2. Explosion of a hydrogen bomb

We, as peaceful people, are primarily interested in the use of thermonuclear reactions for peaceful purposes to create the same power plants, but of a new type.

Research is currently underway on how to create controlled thermonuclear fusion. Various methods are used for this, one of them is the use of lasers to obtain high energies and temperatures. With the help of lasers they are accelerated to high speeds, and in this case a thermonuclear reaction can occur.

As a result of a thermonuclear reaction, a huge amount of heat is released, the place in the reactor in which the isotopes interacting with each other will be located must be well insulated so that the substance that will be at a high temperature does not interact with the environment, with the walls of the object where it is located. For such insulation, a magnetic field is used. At high core temperatures, the electrons that are together form a new type of matter - plasma. Plasma is a partially or fully ionized gas, and once the gas is ionized, it is sensitive to a magnetic field. Plasma is electrically conductive; with the help of magnetic fields, it can be given a certain shape and held in a certain volume. However, the technical solution to control the thermonuclear reaction remains unresolved.

Rice. 3. TOKAMAK - toroidal installation for magnetic plasma confinement

In conclusion, I would also like to note: thermonuclear reactions play an important role in the evolution of our universe. First of all, we note that thermonuclear reactions flow into the Sun. We can say that it is the energy of thermonuclear reactions that is the energy that shaped the current appearance of our universe.

List of additional literature

1. Bronshtein M.P. Atoms and electrons. “Library “Quantum””. Vol. 1. M.: Nauka, 1980

2. Kikoin I.K., Kikoin A.K. Physics: Textbook for 9th grade of high school. M.: Enlightenment

3. Kitaygorodsky A.I. Physics for everyone. Book 4. Photons and nuclei. M.: Science

4. Myakishev G.Ya., Sinyakov A.Z. Physics. Optics. The quantum physics. 11th grade: textbook for in-depth study of physics. M.: Bustard

Lesson assignment.

1. As a result of the thermonuclear reaction of two protons combining, a deuteron and a neutrino are formed. What other particle appears?

2. Find the frequency γ - radiation generated during a thermonuclear reaction:

If α -the particle acquires an energy of 19.7 MeV

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 content of helium-3 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.

What elements are involved in these reactions? These are primarily hydrogen isotopes and helium isotopes. For example, the following reaction can be given:

Two isotopes of hydrogen (deuterium and tritium), when combined together, form a helium nucleus, and a neutron is also formed. When this reaction occurs, enormous energy is released E = 17.6 MeV.

Don't forget that this is just one reaction. And one more reaction. Two deuterium nuclei fuse together to form a helium nucleus:

In this case, a large amount is also released.

Rice. 1. The distance by which nuclei need to be brought together for a thermonuclear reaction to occur

Rice. 2. Explosion of a hydrogen bomb

We, as peaceful people, are primarily interested in the use of thermonuclear reactions for peaceful purposes to create the same power plants, but of a new type.

Rice. 3. TOKAMAK - toroidal installation for magnetic plasma confinement

List of additional literature

1. Bronshtein M.P. Atoms and electrons. “Library “Quantum””. Vol. 1. M.: Nauka, 1980

2. Kikoin I.K., Kikoin A.K. Physics: Textbook for 9th grade of high school. M.: Enlightenment

3. Kitaygorodsky A.I. Physics for everyone. Book 4. Photons and nuclei. M.: Science

4. Myakishev G.Ya., Sinyakov A.Z. Physics. Optics. The quantum physics. 11th grade: textbook for in-depth study of physics. M.: Bustard

Lesson assignment.

1. As a result of the thermonuclear reaction of two protons combining, a deuteron and a neutrino are formed. What other particle appears?

2. Find the frequency γ - radiation generated during a thermonuclear reaction:

If α -the particle acquires an energy of 19.7 MeV

Thermonuclear reactions
Thermonuclear reactions

Thermonuclear reactions− reactions of fusion (synthesis) of light nuclei occurring at high temperatures. These reactions usually involve the release of energy, since in the heavier nucleus formed as a result of the merger the nucleons are more strongly bound, i.e. have, on average, a higher binding energy than in the original merging nuclei. The excess total binding energy of nucleons is released in the form of kinetic energy of reaction products. The name “thermonuclear reactions” reflects the fact that these reactions occur at high temperatures ( > 10 7 –10 8 K), since for fusion light nuclei must come together to distances equal to the radius of action of nuclear attractive forces, i.e. to distances of ≈10 -13 cm. And outside the zone of action of these forces, positively charged nuclei experience Coulomb repulsion. Only nuclei flying towards each other at high speeds can overcome this repulsion, i.e. included in highly heated environments, or specially accelerated.
Below are several main reactions of nuclear fusion and the energy release values Q are indicated for them. d means deuteron - 2 N nucleus, t means triton - 3 N nucleus.

d + d → 3 He + n + 4.0 MeV,
d + d → t + p + 3.25 MeV,
t + d → 4 He + n + 17.6 MeV,
3 He + d → 4 He + p + 18.3 MeV.

The nuclear fusion reaction begins when the colliding nuclei are in the region of their mutual nuclear attraction. In order to get so close, the colliding nuclei must overcome their mutual long-range electrostatic repulsion, i.e. Coulomb barrier. The rate of the fusion reaction is extremely low at energies below a few keV, but it increases rapidly with increasing kinetic energy of the nuclei entering into the reaction. The corresponding effective reaction cross sections depending on the deuteron energy are shown in Fig. 1.

Rice. 1. Dependence of effective cross sections for the fusion reaction
from the energy of the deuteron.

Self-sustaining thermonuclear reactions are an effective source of nuclear energy. However, it is difficult to implement them on Earth, since this requires maintaining high concentrations of nuclei at enormous temperatures. The necessary conditions for the occurrence of self-sustaining thermonuclear reactions are present in stars, where they are the main source of energy. So inside the Sun, where hydrogen nuclei are located at a density of ≈100 g/cm 3 and a temperature of 10 7 K, there is a chain of thermonuclear reactions transforming four protons (hydrogen nuclei) into a helium-4 nucleus (4 He). Each such transformation releases an energy of 26.7 MeV. This chain of reactions (called proton-proton) begins with reaction (1) and is shown in the figure.

Proton-proton chain.

On Earth, self-sustaining thermonuclear reactions with the release of enormous energy were carried out within a very short time (10 -7 -10 -6 sec) during explosions of hydrogen bombs. One of the main thermonuclear reactions that provide energy release during such explosions is the fusion of two heavy isotopes of hydrogen (deuterium and tritium) into a helium nucleus with the emission of a neutron.

A thermonuclear reaction belongs to the category of nuclear reactions, but, unlike the latter, it involves a process of formation, not destruction.
To date, two variants of thermonuclear fusion have been developed - explosive thermonuclear fusion and controlled thermonuclear fusion.

The Coulomb barrier or why people haven’t blown up yet

Atomic nuclei carry a positive charge. This means that when they come closer, a repulsive force begins to act, which is inversely proportional to the square of the distance between the nuclei. However, at a certain distance, which is equal to 0.000 000 000 001 cm, strong interaction begins to act, leading to the fusion of atomic nuclei.

As a result, a colossal amount of energy is released. The distance that prevents the fusion of nuclei is called the Coulomb barrier, or potential barrier. The condition under which this occurs is high temperature, about 1 billion degrees Celsius. In this case, any substance turns into plasma. The main substance for carrying out a thermonuclear reaction is tritium.

Explosive thermonuclear fusion

This method of conducting a thermonuclear reaction arose much earlier than the controlled one and was first used in the hydrogen bomb. The main explosive substance is lithium deuteride.

The bomb consists of a trigger - a plutonium charge with an amplifier and a container with thermonuclear fuel. First, the trigger explodes, emitting a pulse of soft x-rays. The shell of the second stage, together with the plastic filler, absorbs these radiations, heating up to a high-temperature plasma, which is under high pressure.

A jet thrust is created, which compresses the volume of the second stage, reducing the internuclear distance by thousands of times. In this case, a thermonuclear reaction does not occur. The final stage is the nuclear explosion of the plutonium rod, which starts the nuclear reaction. Lithium deuteride with neutrons to form tritium.

Controlled thermonuclear fusion

Controlled thermonuclear fusion is possible because special types of reactors are used. The fuel is deuterium, tritium, helium, lithium, boron-11.

Reactors:
1) A reactor based on the creation of a quasi-stationary system in which plasma is confined by a magnetic field.
2) Reactor based on a pulse system. In these reactors, small targets containing deuterium and tritium are briefly heated by a high-power particle stream or laser.