Nuclear chain reaction

Chain nuclear reaction- a sequence of single nuclear reactions, each of which is caused by a particle that appeared as a reaction product at the previous step of the sequence. An example of a nuclear chain reaction is a chain reaction of fission of nuclei of heavy elements, in which the majority of fission events are initiated by neutrons obtained from nuclear fission in the previous generation.

Energy release mechanism

The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energy. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. A spontaneous transition is always hindered by an energy barrier, to overcome which a microparticle must receive from the outside a certain amount of energy - excitation energy. The exoenergetic reaction consists in the fact that in the transformation following the excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of colliding particles, or due to the binding energy of the joining particle.

If we bear in mind the macroscopic scales of energy release, then the kinetic energy necessary for the excitation of reactions must have all or, first, at least some fraction of the particles of the substance. This is achievable only when the temperature of the medium rises to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvin, in the case of nuclear reactions it is a minimum of 10 7 K due to the very great height Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is realized in practice only in the synthesis of the lightest nuclei, for which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by attaching particles does not require large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the particles of attraction forces. But on the other hand, the particles themselves are needed to excite the reactions. And if we again have in mind not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Chain reactions

Chain reactions are widespread among chemical reactions, where the role of particles with unused bonds is played by free atoms or radicals. The chain reaction mechanism during nuclear transformations can be provided by neutrons that do not have a Coulomb barrier and excite nuclei upon absorption. The appearance of the necessary particle in the medium causes a chain of subsequent, one after another, reactions, which continues until the chain is terminated due to the loss of the carrier particle of the reaction. There are two main reasons for the losses: the absorption of a particle without the emission of a secondary one and the departure of the particle outside the volume of the substance supporting the chain process. If in each act of reaction only one carrier particle appears, then the chain reaction is called unbranched... An unbranched chain reaction cannot lead to energy release on a large scale.

If in each act of reaction or in some links of the chain more than one particle appears, then a branched chain reaction arises, because one of the secondary particles continues the chain that was started, while the others give new chains that branch again. True, the processes leading to chain breaks compete with the branching process, and the emerging situation gives rise to limiting or critical phenomena specific to branched chain reactions. If the number of open circuits is greater than the number of new circuits appearing, then self-sustaining chain reaction(SCR) turns out to be impossible. Even if it is excited artificially by introducing a certain amount of necessary particles into the medium, then, since the number of chains in this case can only decrease, the process that has begun quickly dies out. If the number of new chains formed exceeds the number of breaks, the chain reaction rapidly spreads throughout the volume of the substance when at least one initial particle appears.

The region of states of matter with the development of a self-sustaining chain reaction is separated from the region where a chain reaction is generally impossible, critical condition... A critical condition is characterized by the equality between the number of new circuits and the number of breaks.

The achievement of a critical state is determined by a number of factors. Fission of a heavy nucleus is excited by one neutron, and as a result of the fission act, more than one neutron appears (for example, for 235 U, the number of neutrons born in one fission act is on average 2.5). Consequently, the fission process can give rise to a branched chain reaction, which will be carried by neutrons. If the rate of neutron loss (captures without fission, departures from the reaction volume, etc.) compensates for the rate of neutron multiplication in such a way that the effective neutron multiplication factor is exactly unity, then the chain reaction proceeds in a stationary mode. The introduction of negative feedbacks between the effective multiplication factor and the rate of energy release allows for a controlled chain reaction, which is used, for example, in nuclear power. If the multiplication factor is greater than one, the chain reaction develops exponentially; unguided fission chain reaction is used in nuclear weapons.

Literature

A. N. Klimov Nuclear physics and nuclear reactors.- M. Atomizdat,.
V.E. Levin Nuclear physics and nuclear reactors/ 4th ed. - M .: Atomizdat,.
Petunin V.P. Heat power engineering of nuclear installations.- M .: Atomizdat,.

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See what "Nuclear Chain Reaction" is in other dictionaries:

Chain nuclear reaction a sequence of nuclear reactions excited by particles (for example, neutrons) generated in each reaction act. Depending on the average number of reactions following one previous less, equal or ... ... Nuclear power terms

nuclear chain reaction- The sequence of nuclear reactions, excited by particles (for example, neutrons), born in each act of reaction. Depending on the average number of reactions following one previous less, equal or greater than one reaction ... ...

nuclear chain reaction- grandininė branduolinė reakcija statusas T sritis fizika atitikmenys: angl. nuclear chain reaction vok. Kettenkernreaktion, f rus. nuclear chain reaction, f pranc. réaction en chaîne nucléaire, f; réaction nucléaire en chaîne, f… Fizikos terminų žodynas

The reaction of fission of atomic nuclei of heavy elements under the action of neutrons, in each act of the swarm the number of neutrons increases, so that a self-sustaining fission process can arise. For example, in the fission of one nucleus of the isotope of uranium 235U under the action of ... Big Encyclopedic Polytechnic Dictionary

Nuclear chain reaction- the reaction of fission of atomic nuclei under the action of neutrons, in each act of which at least one neutron is emitted, which ensures the maintenance of the reaction. It is used as a source of energy in nuclear charges (explosive central nuclear reactor) and nuclear reactors ... ... Dictionary of military terms

nuclear fission chain reaction- - [A.S. Goldberg. The English Russian Energy Dictionary. 2006] Topics energy in general EN divergent reaction ... Technical translator's guide

Self-sustaining nuclear chain reaction- 7. Self-sustaining nuclear chain reaction SCR A nuclear chain reaction characterized by an effective multiplication factor greater than or equal to one

It is a process in which one reaction carried out causes subsequent reactions of the same type.

During the fission of one uranium nucleus, the resulting neutrons can cause fission of other uranium nuclei, while the number of neutrons grows like an avalanche.

The ratio of the number of neutrons produced in one fission act to the number of such neutrons in the previous fission act is called the neutron multiplication factor k.

When k is less than 1, the reaction dies out, because the number of absorbed neutrons is greater than the number of newly formed ones.
When k is greater than 1, an explosion occurs almost instantly.
When k is equal to 1, a controlled stationary chain reaction occurs.

The chain reaction is accompanied by the release of a large amount of energy.

For the implementation of a chain reaction, it is impossible to use any nuclei fission under the influence of neutrons.

Used as fuel for nuclear reactors chemical element uranium naturally consists of two isotopes: uranium-235 and uranium-238.

In nature, isotopes of uranium-235 make up only 0.7% of the total uranium reserve, however, they are suitable for carrying out a chain reaction, since fission under the influence of slow neutrons.

Uranium-238 nuclei can only fission under the influence of high-energy neutrons (fast neutrons). Only 60% of the neutrons produced by the fission of the uranium-238 nucleus have such an energy. Only about 1 in 5 of the generated neutrons causes nuclear fission.

Conditions for the chain reaction in uranium-235:

The minimum amount of fuel (critical mass) required for a controlled chain reaction in a nuclear reactor
- the speed of neutrons should cause fission of uranium nuclei
- no impurities absorbing neutrons

Critical mass:

If the mass of uranium is small, neutrons will fly out of it without reacting
- if the mass of uranium is large, an explosion is possible due to a strong increase in the number of neutrons
- if the mass corresponds to the critical one, a controlled chain reaction proceeds

For uranium-235, the critical mass is 50 kg (this is, for example, a uranium ball with a diameter of 9 cm).

The first controlled chain reaction - USA in 1942 (E. Fermi)
In the USSR - 1946 (I.V. Kurchatov).

Faraday's law of electromagnetic induction is the basic law of electrodynamics concerning the principles of operation of transformers, chokes, many types of electric motors

And generators. The law states:

Faraday's law as two different phenomena [edit | edit wiki text]

Some physicists note that Faraday's law in one equation describes two different phenomena: motor emf generated by the action of a magnetic force on a moving wire, and transformer EMF generated by the action of an electric force due to a change magnetic field... James Clerk Maxwell drew attention to this fact in his work About physical lines of force in 1861. In the second half of Part II of this work, Maxwell provides a separate physical explanation for each of these two phenomena. There are references to these two aspects of electromagnetic induction in some modern textbooks. As Richard Feynman writes:

Lorentz's Law [edit | edit wiki text]

Charge q in the conductor on the left side of the loop experiences the Lorentz force q v × B k = −q v B (x C - w / 2) j (j, k - unit vectors in directions y and z; cm. cross product vectors), which causes EMF (work per unit charge) v ℓ B (x C - w / 2) along the entire length of the left side of the loop. On the right side of the loop, a similar reasoning shows that the EMF is equal to v ℓ B (x C + w / 2)... Two opposite EMFs push the positive charge towards the bottom of the loop. In the case when the field B increases along x, the force on the right side will be greater, and the current will flow clockwise. Using the rule right hand, we get that the field B created by a current opposite to the applied field. The EMF causing the current must increase in a counterclockwise direction (as opposed to the current). Adding the EMF in a counterclockwise direction along the loop, we find:

Faraday's Law [edit | edit wiki text]

An intuitive but flawed approach to using the flow rule expresses flow through a chain as Φ B = B wℓ, where w- the width of the moving loop. This expression does not depend on time, therefore it incorrectly follows from this that no EMF is generated. The mistake of this statement is that it does not take into account the entire path of the current through the closed loop.

To use the flow rule correctly, we must consider the entire current path, which includes the path through the rims on the top and bottom discs. We can choose an arbitrary closed path through the rims and a rotating loop, and, according to the law of flux, find the EMF along this path. Any path that includes a segment adjacent to a rotating loop allows for the relative movement of parts of the chain.

As an example, consider a path at the top of the chain in the direction of rotation of the upper disc, and at the bottom of the chain in the opposite direction to the lower disc (shown by arrows in Figure 4). In this case, if the rotating loop deviates by an angle θ from the collector loop, then it can be considered as a part of a cylinder with an area A = rℓ θ. This area is perpendicular to the field B, and its contribution to the stream is:

where the sign is negative because, according to the right-hand rule, the field B generated by the current loop is opposite in direction to the applied field B "... Since this is only a time-dependent part of the flow, according to the law of flow, the EMF is equal to:

in accordance with the Lorentz law formula.

Now let's consider another way, in which we choose the passage along the rims of the discs through the opposite segments. In this case, the associated thread will be decrease with increasing θ, but according to the rule of the right hand, the current loop adds attached field B, therefore the EMF for this path will be exactly the same value as for the first path. Any mixed return path leads to the same result for the EMF value, so it doesn't really matter which path you take.

A thermonuclear reaction is a type of nuclear reaction in which light atomic nuclei combine into heavier ones due to the kinetic energy of their thermal motion. Origin of the term [edit | edit wiki text]

In order for a nuclear reaction to occur, the initial atomic nuclei must overcome the so-called "Coulomb barrier" - the force of electrostatic repulsion between them. To do this, they must have high kinetic energy. According to kinetic theory, the kinetic energy of moving microparticles of a substance (atoms, molecules or ions) can be represented in the form of temperature, and therefore, by heating the substance, a nuclear reaction can be achieved. It is this relationship between the heating of matter and a nuclear reaction that is reflected in the term "thermonuclear reaction".

Coulomb barrier [edit | edit wiki text]

Atomic nuclei have a positive electrical charge. At large distances, their charges can be screened by electrons. However, in order for the nuclei to merge, they must approach each other at a distance at which a strong interaction acts. This distance is of the order of the size of the nuclei themselves and many times smaller atom. At such distances, the electron shells of atoms (even if they were preserved) can no longer screen the charges of the nuclei, so they experience strong electrostatic repulsion. The force of this repulsion, in accordance with Coulomb's law, is inversely proportional to the square of the distance between the charges. At distances of the order of the size of nuclei, the quantity strong interaction, which seeks to connect them, begins to increase rapidly and becomes greater than the value of the Coulomb repulsion.

Thus, in order to react, the nuclei must overcome a potential barrier. For example, for the deuterium-tritium reaction, this barrier is about 0.1 MeV. For comparison, the ionization energy of hydrogen is 13 eV. Therefore, the substance participating in the thermonuclear reaction will be almost completely ionized plasma.

The temperature equivalent to 0.1 MeV is approximately 10 9 K, but there are two effects that lower the temperature required for a thermonuclear reaction:

· First, the temperature characterizes only the average kinetic energy, there are particles with both lower and higher energy. In fact, a small number of nuclei with energies much higher than the average (the so-called “tail of the Maxwellian distribution

Secondly, thanks to quantum effects, nuclei need not have an energy exceeding the Coulomb barrier. If their energy is slightly less than the barrier, they can most likely tunnel through it. [ source not specified 339 days]

Thermonuclear reactions [edit | edit wiki text]

Some of the most important exothermic thermonuclear reactions with large cross sections:

(1)

→

4He

(3.5 MeV)

(14.1 MeV)

(2)

→

(1.01 MeV)

(3.02 MeV)

(50 %)

(3)

→

3He

(0.82 MeV)

(2.45 MeV)

(50 %)

(4)

3He

→

4He

(3.6 MeV)

(14.7 MeV)

(5)

→

4He

+ 11.3 MeV

(6)

3He

→

4He

(7)

3He

→

4He

+ 12.1 MeV

(51 %)

(8)

→

4He

(4.8 MeV)

(9.5 MeV)

(43 %)

(9)

→

4He

(0.5 MeV)

(1.9 MeV)

(11.9 MeV)

(6 %)

(10)

6Li

→

4He

+ 22.4 MeV -

(11)

6Li

→

4He

(1.7 MeV)

3He

(2.3 MeV) -

(12)

3He

6Li

→

4He

+ 16.9 MeV

(13)

11B

→

4He

+ 8.7 MeV

(14)

6Li

→

4He

+ 4.8 MeV

Muon catalysis [edit | edit wiki text]

Main article: Muon catalysis

A thermonuclear reaction can be significantly facilitated by introducing negatively charged muons into the reaction plasma.

Muons µ - interacting with thermonuclear fuel form mesomolecules, in which the distance between the nuclei of the fuel atoms is somewhat smaller, which facilitates their approach and, in addition, increases the probability of nuclei tunneling through the Coulomb barrier.

The number of synthesis reactions X c initiated by one muon is limited by the value of the muon sticking coefficient. Experimentally, it was possible to obtain values of X c ~ 100, i.e., one muon is capable of releasing energy ~ 100 × X MeV, where X is the energy yield of the catalyzed reaction.

While the value of the released energy is less than the energy consumption for the production of the muon itself (5-10 GeV). Thus, muon catalysis is still an energetically unfavorable process. Commercially viable energy production using muon catalysis is possible with X c ~ 10 4 .

Application [edit | edit wiki text]

The use of a thermonuclear reaction as a practically inexhaustible source of energy is associated primarily with the prospect of mastering the technology of controlled thermo nuclear fusion(TCB). At present, the scientific and technological base does not allow the use of TCB on an industrial scale.

At the same time, uncontrolled thermonuclear reaction has found its application in military affairs. A thermonuclear explosive device was first tested in November 1952 in the United States, and already in August 1953, a thermonuclear explosive device in the form of an aerial bomb was tested in the Soviet Union. The power of a thermonuclear explosive device (as opposed to an atomic one) is limited only by the amount of material used to create it, which makes it possible to create explosive devices of almost any power.

SEASON 27 question 1

Self-induction phenomenon

We have already studied that a magnetic field arises near a conductor with current. They also studied that an alternating magnetic field generates a current (the phenomenon of electromagnetic induction). Consider electrical circuit... When the current strength in this circuit changes, the magnetic field will change, as a result of which an additional induction current... This phenomenon is called self-induction, and the current arising in this case is called self-induction current.

The phenomenon of self-induction is the emergence of an EMF in a conducting circuit, created as a result of a change in the current strength in the circuit itself.

The inductance of the circuit depends on its shape and size, on magnetic properties environment and does not depend on the current in the circuit.

EMF of self-induction determined by the formula:

The phenomenon of self-induction is similar to the phenomenon of inertia. Just as in mechanics it is impossible to instantly stop a moving body, so the current cannot instantly acquire a certain value due to the phenomenon of self-induction. If a coil is connected in series with the second lamp in a circuit consisting of two identical lamps connected in parallel to the current source, then when the circuit is closed, the first lamp lights up almost immediately, and the second with a noticeable delay.

When the circuit is opened, the current rapidly decreases, and the emerging EMF of self-induction prevents a decrease in magnetic flux... In this case, the induced current is directed in the same way as the initial one. The EMF of self-induction can exceed the external EMF many times. Therefore, light bulbs very often burn out when the light is turned off.

Magnetic field energy

The energy of the magnetic field of the circuit with current:

Radioactive radiation - radiation that an isotope emits during decay. It has three types: alpha rays (a stream of helium nuclei), beta rays (a stream of electrons) and gamma rays ( electromagnetic radiation). For humans, the most dangerous is gamma radiation.

The dose of absorbed radiation is equal to the ratio of the energy received by the body to the body weight. The absorption dose is indicated by the letter D and is measured in grays.

In practice, they also use the unit of measurement x-ray (P), equal to 2.58 multiplied by 10 to the minus 4 power of the coulomb, divided by the kilogram.

The absorbed radiation can accumulate over time, its dose is the greater, the longer duration irradiation.

The dose rate is determined by the ratio of the absorbed radiation dose to the exposure time. It is designated by the letter N and is measured in grays divided by a second.

For humans, the lethal dose of absorbed radiation is equivalent to 6 Gy. The maximum permissible radiation dose for humans is 0.05 Gy per year.

SEASON 28 Question 1

Elementary particle is a collective term referring to micro-objects on a subnuclear scale that cannot be split into their constituent parts.

It should be borne in mind that some elementary particles ( electron, neutrino, quarks etc.) on this moment are considered structureless and are considered as primary fundamental particles ... Other elementary particles (the so-called compound particles, including the particles that make up the core atom - protons and neutrons) have a complex internal structure but, nevertheless, according to modern ideas, it is impossible to divide them into parts due to the effect confinement.

Together with antiparticles more than 350 elementary particles have been discovered. Of these, the photon, electron and muon neutrino, electron, proton and their antiparticles are stable. The rest of the elementary particles spontaneously decay in a time from about 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10-24 to 10-22, for resonances).

With electromagnetic oscillations, there are periodic changes in the electric charge, current and voltage. Electromagnetic oscillations are divided into free decaying forced and self-oscillations.

Free vibrations are called the oscillations that arise in the system (capacitor and coil) after removing it from the equilibrium position (when the charge is imparted to the capacitor). More precisely, free electromagnetic oscillations occur when the capacitor is discharged through the inductor. Forced oscillations are called oscillations in a circuit under the influence of an external periodically changing electromotive force.

The simplest system in which free electromagnetic oscillations are observed is an oscillatory circuit. consists of an inductor and a capacitor. This process will be repeated over and over. Will arise electromagnetic vibrations due to transformation of energy electric field capacitor.

· The capacitor, being charged from the battery, at the initial moment of time will acquire the maximum charge. His energy W e will be maximum (Fig. a).

· If the capacitor is closed to the coil, then at this moment in time it will begin to discharge (Fig. B). A current will appear in the circuit. As the capacitor discharges, the current in the circuit and in the coil increases. Due to the phenomenon of self-induction, this does not happen instantly. Coil energy W m becomes maximum (Fig. c).

· The induction current flows in the same direction. Electric charges build up on the capacitor again. The capacitor is being recharged, i.e. the capacitor plate, previously charged positively, will be charged negatively. The energy of the capacitor becomes maximum. The current in this direction will stop, and the process will be repeated in the opposite direction (Fig. D). This process will be repeated over and over again. Will arise electromagnetic vibrations due to the transformation of the energy of the electric field of the capacitor into the energy of the magnetic field of the current coil, and vice versa. If there are no losses (resistance R = 0), then the current strength, charge and voltage change over time according to a harmonic law. Oscillations that occur according to the cosine or sine law are called harmonic. The equation harmonic oscillation charge: .

A circuit in which there is no energy loss is an ideal oscillating circuit. Period of electromagnetic oscillations in an ideal oscillatory circuit depends on the inductance of the coil and the capacitance of the capacitor and is located along Thomson's formula where L is the inductance of the coil, C is the capacitance of the capacitor, T is the period of the e / m oscillations.
In a real oscillatory circuit, free electromagnetic oscillations will be decaying due to energy losses when heating wires. For practical application it is important to obtain continuous electromagnetic oscillations, and for this it is necessary to replenish the oscillatory circuit with electricity in order to compensate for the energy losses from the continuous oscillation generator, which is an example of a self-oscillating system.

Ticket 29 Question 1

Antiparticle - particle-twin of some other elementary particle possessing the same mass and the same spin, which differs from it in signs of all other characteristics of interaction (charges such as electric and color charges, baryonic and lepton quantum numbers).

The very definition of what to call a "particle" in a particle-antiparticle pair is largely arbitrary. However, with this choice"Particles" its antiparticle is determined unambiguously. Conservation of the baryon number in the processes of weak interaction allows one to determine the "particle" in any baryon-antibaryon pair by the baryon decay chain. The choice of an electron as a "particle" in an electron-positron pair is fixed (due to the conservation of the lepton number in the processes weak interaction) determination of the state of the "particle" in a pair of electron neutrino-antineutrino. Transitions between leptons of different generations (type) have not been observed, so that the definition of a "particle" in each generation of leptons, generally speaking, can be made independently. Usually, by analogy with the electron, "particles" are called negatively charged leptons, which, while conserving the lepton number, determines the corresponding neutrino and antineutrino... For bosons the concept of "particle" can be fixed by a definition, for example, hypercharge.

A chain reaction is a self-sustaining chemical reaction in which initially appearing products take part in the formation of new products. Chain reactions usually proceed at high speed and often have the character of an explosion.

Chain reactions go through three main stages: nucleation (initiation), development and chain termination.

Rice. 9.13. The energy profile of the reaction (a graph of the potential energy versus the reaction coordinate) showing a minimum that corresponds to the formation of a reaction intermediate.

Initiation stage. At this stage, the formation of intermediates (intermediate products) occurs. Intermediates can be atoms, ions, or neutral molecules. Initiation can be carried out by light, nuclear radiation, thermal (thermal) energy, anions or catalysts.

Development stage. At this stage intermediate products react with the starting reagents, forming new intermediates and final products. The developmental stage in chain reactions is repeated many times, which leads to the formation of a large number of final and intermediate products.

Chain break stage. At this stage, the final consumption of intermediate products or their destruction occurs. As a result, the reaction stops. The chain reaction can break off spontaneously or under the influence of special substances - inhibitors.

Chain reactions play important role in many branches of chemistry, in particular in photochemistry, combustion chemistry, nuclear fission and nuclear fusion reactions (see Ch. 1), in organic chemistry (see Ch. 17-20).

Photochemistry

This section of chemistry covers the chemical processes associated with the action of light on a substance. Photosynthesis is an example of photochemical processes.

Many chain reactions are initiated by light. In this case, the initiating particle is a photon, which has energy (see Section 1.2). A classic example is the reaction between hydrogen and chlorine in the presence of light

This reaction is explosive. It includes the following three stages.

Initiation. At this stage, the covalent bond in the chlorine molecule is broken, as a result of which two atoms are formed, each with an unpaired electron:

This type of reaction is homolysis, or hemolytic division (see Section 17.3). It is also an example of photolysis. The term "photolysis" means photochemical degradation. The two resulting chlorine atoms are intermediate products (intermediates). They are radicals. A radical is an atom (or group of atoms) with at least one unpaired electron. It should be noted that although the initiation step is the slowest step in the chain reaction, it does not determine the rate of the entire chain reaction.

Development stage. At this stage, chlorine atoms react with hydrogen molecules, forming the final product - hydrogen chloride, as well as hydrogen radicals. Hydrogen radicals react with chlorine molecules; as a result, new portions of the product and new chlorine radicals are formed:

These two reactions, together making up the developmental stage, are repeated millions of times.

Chain break stage. The chain reaction finally ends as a result

reactions such as

To absorb the energy that is released during these chain termination reactions, it is necessary that some other third body takes part in them. This third body is usually the walls of the vessel in which the reaction is carried out.

Quantum output

The absorption of one photon of light by a chlorine molecule in the chain reaction described above can lead to the formation of millions of hydrogen chloride molecules. The ratio of the number of product molecules to the number of light quanta (photons) that initiate the reaction is called quantum yield. The quantum yield of photochemical reactions can range from one to several million. A high quantum yield indicates a chain-like nature of the reaction taking place.

Pulse photolysis

This is the name of the technique used to obtain radicals with a concentration high enough to detect them. In fig. 9.14 shows a simplified diagram of the setup used for pulsed photolysis. The reaction mixture is affected

Rice. 9.14. Pulsed photolysis.

a powerful flash of light from a special pulsed source. Such a source makes it possible to create flashes of light with energies up to 105 J and with a duration of the order of s or less. Modern techniques pulsed photolysis uses pulsed lasers with a flash duration of the order of a nanosecond (10-9 s). The reaction resulting from such a flash of light can be followed by recording a sequence of optical absorption spectra of the reaction mixture. The first flash is followed by a series of flashes from a low-power pulsed source. These flashes follow one another at intervals of the order of milliseconds or microseconds and allow the absorption spectra of the reaction mixture to be recorded at such time intervals.

Combustion

The reaction with oxygen, leading to the release of heat energy and light, is called combustion. Combustion usually proceeds as a complex sequence of radical reactions.

Let's take the combustion of hydrogen as an example. Under certain conditions, this reaction proceeds with an explosion. In fig. 9.15 presents experimental data for the reaction of a stoichiometric mixture of hydrogen and oxygen in a Pyrex reactor. The shaded section of the diagram corresponds to the explosive region of this reaction. For the hydrogen combustion reaction, this section of the diagram has the shape of an explosive peninsula. The area of the explosion is limited by the boundaries of the explosion.

Rice. 9.15. Conditions for the explosive occurrence of the hydrogen combustion reaction:

Consider the fission chain reaction mechanism. When heavy nuclei fission under the action of neutrons, new neutrons are produced. For example, with each fission of the uranium 92 U 235 nucleus, 2.4 neutrons appear on average. Some of these neutrons can again cause nuclear fission. Such an avalanche-like process is called chain reaction .
The fission chain reaction takes place in a medium in which the process of neutron multiplication takes place. This environment is called active zone ... The most important physical quantity characterizing the intensity of neutron multiplication is medium neutron multiplication factor k ∞. The multiplication factor is equal to the ratio of the number of neutrons in one generation to their number in the previous generation. The ∞ index indicates that it comes about an ideal environment of infinite dimensions. Similarly to the value of k ∞, we define neutron multiplication factor in a physical system k. The k factor is a characteristic of a specific installation.
In a fissile medium of finite dimensions, some of the neutrons will escape from the core to the outside. Therefore, the coefficient k also depends on the probability P for a neutron not to leave the core. By definition

k = k ∞ P.

(1)

The value of P depends on the composition of the core, its size, shape, and also on the extent to which the substance surrounding the core reflects neutrons.
Important concepts of critical mass and critical dimensions are associated with the possibility of neutrons leaving the core. Critical size is the size of the core at which k = 1. Critical mass called the mass of the core of critical dimensions. Obviously, at a mass below the critical one, the chain reaction does not occur, even if> 1. On the contrary, a noticeable excess of the mass over the critical one leads to an uncontrollable reaction - an explosion.
If there are N neutrons in the first generation, then in the nth generation there will be Nk n. Therefore, for k = 1, the chain reaction proceeds stationary, for k< 1 реакция гаснет, а при k >1, the intensity of the reaction increases. For k = 1, the reaction mode is called critical , for k> 1 - supercritical and for k< 1 – subcritical .
The lifetime of one generation of neutrons strongly depends on the properties of the medium and is on the order of 10 –4 to 10 –8 s. Due to the smallness of this time, for the implementation of a controlled chain reaction, it is necessary to maintain the equality k = 1 with great accuracy, since, say, at k = 1.01, the system will explode almost instantly. Let's see what factors determine the coefficients k ∞ and k.
The first quantity that determines k ∞ (or k) is the average number of neutrons emitted in one fission event. The number depends on the type of fuel and on the energy of the incident neutron. Table 1 shows the values of the main isotopes nuclear power for both thermal and fast (E = 1 MeV) neutrons.

The energy spectrum of fission neutrons for the 235 U isotope is shown in Fig. 1. Spectra of this kind are similar for all fissile isotopes: there is a strong spread in energies, with the bulk of neutrons having energies in the range of 1-3 MeV. The neutrons generated during fission are slowed down, diffuse over a certain distance, and are absorbed either with or without fission. Depending on the properties of the medium, the neutrons have time to slow down to different energies before absorption. In the presence of a good moderator, the bulk of the neutrons has time to slow down to thermal energies of the order of 0.025 eV. In this case, the chain reaction is called slow, or, which is the same, thermal... In the absence of a special moderator, neutrons have time to slow down only to energies of 0.1–0.4 MeV, since all fissile isotopes are heavy and therefore slow down poorly. The corresponding chain reactions are called fast(We emphasize that the epithets "fast" and "slow" characterize the speed of neutrons, not the speed of the reaction). Chain reactions in which neutrons are slowed down to energies from tens to one keV are called intermediate .
When a neutron collides with a heavy nucleus, radiative capture of a neutron (n, γ) is always possible. This process will compete with fission and thus reduce the multiplication factor. Hence, it follows that the second physical quantity affecting the coefficients k ∞, k is the probability of fission when a neutron is captured by a fissile isotope nucleus. This probability for monoenergetic neutrons is obviously equal to

(2)

where nf, nγ are the fission and radiative capture cross sections, respectively. To simultaneously take into account both the number of neutrons per fission act and the probability of radiative capture, a coefficient η is introduced, which is equal to the average number of secondary neutrons per neutron capture by a fissile nucleus.

(3)

the value of η depends on the type of fuel and on the neutron energy. The values of η for the most important isotopes for thermal and fast neutrons are given in the same table. 1. The quantity η is the most important characteristic of fuel nuclei. A chain reaction can only take place at η> 1. The higher the value of η, the higher the quality of the fuel.

Table 1. Values of ν, η for fissile isotopes

Core		92 U 233	92 U 235	94 Pu 239
Thermal neutrons (E = 0.025 eV)	ν	2.52	2.47	2.91
Thermal neutrons (E = 0.025 eV)	η	2.28	2.07	2.09
Fast neutrons (E = 1 MeV)	ν	2.7	2.65	3.0
Fast neutrons (E = 1 MeV)	η	2.45	2.3	2.7

The quality of nuclear fuel is determined by its availability and the coefficient η. In nature, there are only three isotopes that can serve as nuclear fuel or raw materials for its production. This is the isotope of thorium 232 Th and isotopes of uranium 238 U and 235 U. Of these, the first two do not give a chain reaction, but can be processed into isotopes on which the reaction takes place. The 235 U isotope itself gives rise to a chain reaction. V earth crust thorium is several times more than uranium. Natural thorium practically consists of only one isotope, 232 Th. Natural uranium mainly consists of the 238 U isotope and only 0.7% of the 235 U isotope.
In practice, the question of the feasibility of a chain reaction on a natural mixture of uranium isotopes is extremely important, in which there are 140 238 U nuclei per 235 U nucleus. Let us show that a slow reaction is possible in a natural mixture, but a fast one is not. To consider the chain reaction in a natural mixture, it is convenient to introduce a new value - the average neutron absorption cross section, referred to one nucleus of the 235 U isotope. By definition

For thermal neutrons, = 2.47, = 580 barn, = 112 barn, = 2.8 barn (note the smallness of the last cross section). Substituting these numbers in (5), we find that for slow neutrons in a natural mixture

This means that 100 thermal neutrons, absorbed in the natural mixture, will create 132 new neutrons. It follows directly from this that a chain reaction on slow neutrons is, in principle, possible on natural uranium. In principle, because for the real implementation of a chain reaction, one must be able to slow down neutrons with low losses.
For fast neutrons ν = 2.65, 2 barn, 0.1 barn. If we take into account fission only on the 235 U isotope, we obtain

235 (fast) 0.3.

(7)

But one must also take into account that fast neutrons with energies above 1 MeV can also fission nuclei of the 238 U isotope with a noticeable relative intensity, of which there is a lot in a natural mixture. For division by 238 U, the factor is approximately 2.5. In the fission spectrum, about 60% of neutrons have energies above the effective threshold of 1.4 MeV for fission by 238 U. But of these 60%, only one out of 5 neutrons has time to produce fission without slowing down to an energy below the threshold due to elastic and especially inelastic scattering. Hence, for the coefficient 238 (fast), we obtain the estimate

Thus, a chain reaction in a natural mixture (235 U + 238 U) cannot proceed with fast neutrons. It has been experimentally established that for pure uranium metal, the multiplication factor reaches unity at an enrichment of 5.56%. In practice, it turns out that the fast neutron reaction can be maintained only in an enriched mixture containing at least 15% of the 235 U isotope.
A natural mixture of uranium isotopes can be enriched with the isotope 235 U. Enrichment is a complex and expensive process due to the fact that Chemical properties both isotopes are almost the same. We have to take advantage of small differences in the rates of chemical reactions, diffusion, etc., arising from the difference in the masses of the isotopes. The chain reaction for 235 U is almost always carried out in an environment with a high content of 238 U. A natural mixture of isotopes is often used, for which η = 1.32 in the range of thermal neutrons, since 238 U is also useful. The 238 U isotope is fissioned by neutrons with energies above 1 MeV. This fission results in a small additional multiplication of neutrons.
Let us compare fission chain reactions on thermal and fast neutrons.
For thermal neutrons, the capture cross sections are large and vary greatly when passing from one nucleus to another. On the nuclei of some elements (for example, on cadmium), these cross sections are hundreds and more times higher than the cross sections by 235 U. Therefore, high purity requirements are imposed on the core of thermal neutron installations with respect to some impurities.
For fast neutrons, all capture cross sections are small and do not differ so much from each other, so that the problem of high purity of materials does not arise. Another advantage of fast reactions is a higher reproduction rate.
An important distinctive feature of thermal reactions is that the fuel in the core is much more diluted, that is, there are significantly more nuclei that do not participate in fission per one fuel nucleus than in a fast reaction. For example, in a thermal reaction on natural uranium, 140 nuclei of 238 U raw material fall on the core of the 235 U fuel, and in a fast reaction, no more than five to six 238 U nuclei can fall on the 235 U nucleus. and the same energy is released in a thermal reaction in a much larger volume of matter than in a fast one. Thus, it is easier to remove heat from the active zone of the thermal reaction, which makes it possible to carry out this reaction with a greater intensity than a fast one.
The lifetime of one generation of neutrons for a fast reaction is several orders of magnitude shorter than for a thermal one. Therefore, the rate of the rapid reaction can change noticeably through a very a short time after a change in the physical conditions in the core. During normal operation of the reactor, this effect is insignificant, since in this case the operating mode is determined by the lifetimes of delayed rather than prompt neutrons.
In a homogeneous medium consisting only of fissile isotopes of the same type, the multiplication factor would be equal to η. However, in real situations, in addition to fissile nuclei, there are always other non-fissionable ones. These foreign nuclei will capture neutrons and thereby affect the multiplication factor. Hence it follows that the third quantity that determines the coefficients k ∞, k is the probability that a neutron will not be captured by one of the non-fissioning nuclei. In real installations, “extraneous” capture occurs on the moderator cores, on the cores of various structural elements, as well as on the nuclei of fission products and capture products.
To carry out a chain reaction on slow neutrons, special substances are introduced into the core - moderators, which convert fission neutrons into thermal ones. In practice, the chain reaction on slow neutrons is carried out on natural or slightly enriched in the 235 U isotope uranium. The presence of a large amount of the 238 U isotope in the core complicates the deceleration process and makes it necessary to impose high requirements on the quality of the moderator. The life of one generation of neutrons in a moderated core can be roughly divided into two stages: deceleration to thermal energies and diffusion c. thermal rates before absorption. In order for the main part of neutrons to have time to slow down without absorption, it is necessary to satisfy the condition

where σ el, σ capture are the energy-averaged cross sections for elastic scattering and capture, respectively, and n is the number of collisions of a neutron with moderator nuclei required to achieve thermal energy. The number n grows rapidly with an increase in the mass number of the moderator. For uranium 238 U, the number n is of the order of several thousand. And the ratio σ el / σ capture for this isotope, even in a relatively favorable region of energies of fast neutrons, does not exceed 50. The so-called resonance region from 1 keV to 1 eV is especially “dangerous” with respect to neutron capture. In this region, the total cross section for the interaction of a neutron with 238 U nuclei has a large number of intense resonances (Fig. 2). At low energies, the radiation widths exceed the neutron ones. Therefore, in the region of resonances, the ratio σ el / σ capture becomes even less than unity. This means that when it enters the region of one of the resonances, the neutron is absorbed with almost one hundred percent probability. And since the deceleration on such a heavier nucleus as uranium proceeds in “small steps,” when passing through the resonance region, the moderating neutron will surely “stumble” onto one of the resonances and be absorbed. Hence it follows that a chain reaction cannot be carried out on natural uranium without impurities: the reaction does not take place on fast neutrons due to the smallness of the coefficient η, and slow neutrons cannot be formed.In order to avoid resonant neutron capture, very light nuclei should be used to slow down , at which the deceleration takes “large steps”, which sharply increases the probability of a successful “slip” of a neutron through the resonance energy region. The best moderating elements are hydrogen, deuterium, beryllium, and carbon. Therefore, the moderators used in practice are mainly reduced to heavy water, beryllium, beryllium oxide, graphite, as well as ordinary water, which slows down neutrons no worse than heavy water, but absorbs them in much larger quantities. The retarder must be well cleaned. Note that for a slow reaction to occur, the moderator must be tens or even hundreds of times larger than that of uranium in order to prevent resonant collisions of neutrons with 238 U nuclei.

The decelerating properties of the active medium can be approximately described by three quantities: the probability for a neutron to avoid absorption by the moderator during deceleration, the probability p to avoid resonance capture by 238 U nuclei, and the probability f for a thermal neutron to be absorbed by the fuel core, and not by the moderator. The f value is usually called the coefficient of thermal utilization. The exact calculation of these quantities is difficult. Usually, approximate semiempirical formulas are used to calculate them.

The p and f values depend not only on the relative amount of the moderator, but also on the geometry of its placement in the core. The active zone, consisting of a homogeneous mixture of uranium and moderator, is called homogeneous, and the system of their alternating blocks of uranium and moderator is called heterogeneous (Fig. 4). A qualitatively heterogeneous system is distinguished by the fact that in it the fast neutron formed in uranium manages to escape into the moderator without reaching resonance energies. Further deceleration takes place in a pure moderator. This increases the probability p of avoiding resonant trapping.

p het> p hom.

On the other hand, on the contrary, having become thermal in the moderator, the neutron must diffuse to participate in the chain reaction, without being absorbed in the pure moderator, to its boundary. Therefore, the thermal utilization factor f in a heterogeneous environment is lower than in a homogeneous one:

f gett< f гом.

To estimate the multiplication factor k ∞ of a thermal reactor, an approximate formula of four factors

k ∞ = η pfε .

(11)

We have already considered the first three factors earlier. The quantity ε is called fast neutron multiplication factor ... This coefficient is introduced in order to take into account that some of the fast neutrons can produce fission without having time to slow down. By its meaning, the coefficient ε always exceeds one. But this excess is usually small. Typical for thermal reactions is ε = 1.03. For fast reactions, the formula of four factors is inapplicable, since each coefficient depends on energy and the spread in energies during fast reactions is very large.
Since the value of η is determined by the type of fuel, and the value of ε for slow reactions almost does not differ from unity, the quality of a specific active medium is determined by the product pf. Thus, the advantage of a heterogeneous medium over a homogeneous one is quantitatively manifested in the fact that, for example, in a system in which there are 215 graphite nuclei per nucleus of natural uranium, the product pf is 0.823 for a heterogeneous medium and 0.595 for a homogeneous one. And since η = 1.34 for a natural mixture, we get that for a heterogeneous medium k ∞> 1, and for a homogeneous k ∞< 1.
For practical implementation a stationary current chain reaction must be able to control this reaction. This control is greatly simplified due to the emission of delayed neutrons during fission. The overwhelming majority of neutrons are emitted from the nucleus almost instantaneously (i.e., in a time that is many orders of magnitude shorter than the lifetime of a neutron generation in the core), but several tenths of a percent of neutrons are delayed and are emitted from fragment nuclei after a rather long time interval - from fractions seconds to several or even tens of seconds. The effect of delayed neutrons can be qualitatively explained as follows. Let the multiplication factor instantly increase from a subcritical value to such a supercritical value that k< 1 при отсутствии запаздывающих нейтронов. Тогда, очевидно, цепная реакция начнется не сразу, а лишь после вылета запаздывающих нейтронов. Тем самым процесс течения реакции будет регулируемым, если время срабатывания регулирующих устройств будет меньше сравнительно большого времени задержки запаздывающих нейтронов, а не очень малого времени развития цепной реакции. Доля запаздывающих нейтронов в ядерных горючих колеблется от 0.2 до 0.7%. Среднее время жизни запаздывающих нейтронов составляет приблизительно 10 с. При небольшой степени надкритичности скорость нарастания интенсивности цепной реакции определяется только запаздывающими нейтронами.
The capture of neutrons by nuclei not participating in the chain reaction reduces the intensity of the reaction, but can be useful in relation to the formation of new fissile isotopes. Thus, when neutrons are absorbed by the isotopes of uranium 238 U and thorium 232 Th, isotopes of plutonium 239 Pu and uranium 233 U are formed (through two successive β-decays), which are nuclear fuel:

(12)

(13)

These two reactions present a real opportunity reproduction of nuclear fuel in the course of a chain reaction. In the ideal case, that is, in the absence of unnecessary losses of neutrons, an average of 1 neutrons can be spent on reproduction for each act of absorption of a neutron by a fuel nucleus.

Nuclear (atomic) reactors

A reactor is a device in which a controlled fission chain reaction is maintained. During operation of the reactor, heat is released due to the exothermicity of the fission reaction. The main characteristic of a reactor is its power - the amount of thermal energy released per unit of time. The reactor power is measured in megawatts (10 6 W). A power of 1 MW corresponds to a chain reaction in which 3 × 10 16 fission events occur per second. There are a large number of different types of reactors. One of the typical schemes of a thermal reactor is shown in Fig. 5.
The main part of the reactor is the core, in which the reaction takes place and thus energy is released. In thermal and intermediate neutron reactors, the core consists of a fuel, usually mixed with a non-fissile isotope (usually 238 U), and a moderator. There is no moderator in the core of fast reactors.
The core volume varies from tenths of a liter in some fast reactors to tens of cubic meters in large thermal reactors. To reduce the neutron leakage, the core is spherical or nearly spherical (for example, a cylinder with a height approximately equal to the diameter, or a cube).
Depending on the relative position of the fuel and the moderator, homogeneous and heterogeneous reactors are distinguished. An example of a homogeneous core is a solution of a uranyl sulfate salt and U 2 SO 4 in ordinary or heavy water. Heterogeneous reactors are more common. In heterogeneous reactors, the core consists of a moderator in which cassettes containing fuel are placed. Since the energy is released precisely in these cassettes, they are called fuel elements or abbreviated fuel rods... The reflector core is often enclosed in a steel casing.

The Role of Delayed Neutrons in Nuclear Reactor Control

Controlled chain reaction.

If the chain reaction is limited in its development so that the number of neutrons generated per unit time, reaching a certain of great importance, then would cease to increase, then a calmly proceeding self-sustaining chain reaction of fission will take place. It will be possible to control the reaction only if it turns out to be possible to regulate the coefficient k eff of neutron multiplication rather slowly and smoothly, and for an optimal system, k eff should exceed unity by only 0.5%. Soviet physicists Ya.B. Zeldovich and Yu.B. Khariton theoretically showed (1939) that a controlled chain reaction can be carried out on natural uranium.

For the development of a chain process in natural uranium, neutrons must be slowed down to thermal velocities, since in this case the probability of their capture by U nuclei with subsequent fission sharply increases. For this purpose, special substances are used - retarders.

The control of a stationary flowing chain reaction (k eff = 1) is greatly simplified due to the presence delayed neutrons(see clause 3.6). It turns out that the time T of "acceleration" of the reaction (the time during which the number of divisions increases by e "2.71 times) with a small degree of supercriticality (k eff - 1<< 1) определятся только запаздывающими нейтронами:

T = t s × b / (k eff - 1),

where t s is the average lifetime of delayed neutrons (t s ~ 14.4 s),

b is the fraction of delayed neutrons (b ~ 0.68% for U).

Since the value of t z × b is of the order of ~ 5 × 10 -2 s., The intensity of the reaction will grow rather slowly, and the reaction is well regulated.

The value of k eff can be controlled by automatically introducing into the active zone substances that strongly absorb neutrons - absorbers.

12.3.1. Nuclear reactor

The device in which a stationary nuclear fission reaction is carried out and maintained is called a nuclear reactor, or atomic boiler.

The first nuclear reactor was built under the leadership of E. Fermi at the end of 1942 (USA). The first European reactor was built in 1946 in Moscow under the leadership of I. V. Kurchatov.

Currently, there are about a thousand nuclear reactors of various types in the world, which differ:

· By the principle of operation (reactors on thermal, fast, etc. neutrons);

· By the type of moderators (for heavy water, graphite, etc.);

· Fuel used (uranium, thorium, plutonium);

For the intended purpose (research, medical, energy, for the reproduction of nuclear fuel, etc.)

The main parts of a nuclear reactor (see Fig. 4.5) are:

· The core (1), where the nuclear fuel is located, the fission chain reaction proceeds, energy is released;

· A neutron reflector (2) surrounding the core;

· Control system of the chain process in the form of rods-absorbers (3) of neutrons;

· Radiation protection (4) from radiation;

Heat carrier (5).

V homogeneous In reactors, the nuclear fuel and the moderator are mixed and form a homogeneous mixture (for example, actinouran salts and heavy water). V heterogeneous reactors (Fig. 4.6), nuclear fuel is placed in the core in the form of fuel rods ( fuel elements) - block-rods (1) of small cross-section, enclosed in a hermetic shell that weakly absorbs neutrons. There is a moderator (2) between the fuel rods.

The neutrons formed during nuclear fission, not having time to be absorbed in the fuel elements, enter the moderator, where they lose their energy, slowing down to thermal velocities. Then falling back into one of the fuel rods, thermal neutrons have a high probability of being absorbed by fissionable nuclei (U, U, Pu). Those neutrons that are captured by U nuclei also play a positive role, replenishing to some extent the consumption of nuclear fuel.

Light nuclei are good moderators: deuterium, beryllium, carbon, oxygen. The best neutron moderator is the combination of deuterium with oxygen - heavy water... However, due to its high cost, carbon is more often used in the form of very pure graphite... Beryllium and its oxide are also used. Fuel rods and a moderator usually make up a regular lattice (for example, uranium-graphite).

Due to the fission energy, the fuel rods are heated. For cooling, they are placed in the stream coolant(air, water, steam, He, CO 2, etc.).

Due to the fact that neutrons are lost in the moderator and in fission fragments, the reactor must have supercritical dimensions and generate an excess of neutrons. The chain process is controlled (i.e., the elimination of excess neutrons) is carried out by control rods (3) (see Fig. 4.5 or 4.6) made of materials that strongly absorb neutrons (boron steel, cadmium).

The parameters of the reactor are calculated so that when the absorber rods are completely inserted into the core, the reaction does not take place. With the gradual withdrawal of the rods, the neutron multiplication factor increases, and at a certain position, k eff reaches unity, the reactor begins to operate. The absorber rods are moved from the control panel. Regulation is simplified by the presence of delayed neutrons.

The main characteristic of a nuclear reactor is its power. A power of 1 MW corresponds to a chain process in which 3 × 10 16 fission events occur in 1 second. The reactor contains emergency rods, the introduction of which, with a sudden increase in the reaction power, immediately drops it.

During the operation of a nuclear reactor, a gradual burnup of nuclear fuel, fission fragments accumulate, transuranic elements are formed. The accumulation of fragments causes a decrease in k eff. This process is called poisoning reactor (if the fragments are radioactive) and slagging(if the shards are stable). In case of poisoning, k eff decreases by (1-3)%. So that the reaction does not stop, special (compensating) rods are gradually (automatically) removed from the core. When the nuclear fuel completely burns out, it is removed (after the termination of the reaction) and loaded with a new one.

Among nuclear reactors, a special place is occupied by breeder reactors on fast neutrons - breeders... In them, the generation of electricity is accompanied by the reproduction of secondary nuclear fuel (plutonium) due to reaction (3.5), due to which not only the isotope U, but also U is used effectively (see §3.6). This makes it possible to radically solve the problem of providing nuclear fuel: for every 100 used nuclei in such a reactor, 150 new ones capable of fission are produced. Fast reactor technology is in the process of seeking the best engineering solutions. The first experimental industrial station of this type (Shevchenko) is used for the production of electricity and desalination of sea water (Caspian Sea).

Chain reaction. §266. Fission chain reaction