Chain reaction conditions necessary for the reaction. Nuclear chain reaction

Nuclear chain reaction- a self-sustaining fission reaction of heavy nuclei, in which neutrons are continuously produced, dividing more and more new nuclei. The uranium-235 nucleus under the influence of a neutron is divided into two radioactive fragments of unequal mass, flying at high speeds into different sides, and two or three neutrons. Controlled chain reactions carried out in nuclear reactors or nuclear boilers. Currently controlled chain reactions are carried out on the isotopes of uranium-235, uranium-233 (artificially obtained from thorium-232), plutonium-239 (artificially obtained from urium-238), as well as plutonium-241. A very important task is to isolate its isotope, uranium-235, from natural uranium. From the very first steps of the development of nuclear technology, the use of uranium-235, the production of which in pure form was, however, technically difficult, because uranium-238 and uranium-235 are chemically inseparable.

50.Nuclear reactors. Prospects for the use of thermonuclear energy.

Nuclear reactor is a device in which a controlled nuclear chain reaction occurs, accompanied by the release of energy. The first nuclear reactor was built and launched in December 1942 in the USA under the leadership of E. Fermi. The first reactor built outside the United States was ZEEP, launched in Canada on December 25, 1946. In Europe, the first nuclear reactor was the F-1 installation, which started working on December 25, 1946 in Moscow under the leadership of I.V. Kurchatov. By 1978, about a hundred nuclear reactors of various types were already operating in the world. The components of any nuclear reactor are: a core with nuclear fuel, usually surrounded by a neutron reflector, a coolant, a chain reaction control system, radiation protection, and a remote control system. The reactor vessel is subject to wear (especially under the influence of ionizing radiation). The main characteristic of a nuclear reactor is its power. A power of 1 MW corresponds to a chain reaction in which 3·1016 fission events occur in 1 second. Research into the physics of high-temperature plasma is carried out mainly in connection with the prospect of creating a thermonuclear reactor. The closest parameters to a reactor are tokamak type installations. In 1968, it was announced that the T-3 installation had reached a plasma temperature of ten million degrees; it is on the development of this direction that scientists from many countries have concentrated their efforts over the past decades. The first demonstration of a self-sustaining thermonuclear reaction should be carried out at the facility being built in France by the efforts of different countries tokamak ITER. Full-scale use of thermonuclear reactors in the energy sector is expected in the second half of the 21st century. In addition to tokamaks, there are other types of magnetic traps for confining high-temperature plasma, for example, so-called open traps. Due to a number of features, they can hold high-pressure plasma and therefore have good prospects as powerful sources of thermonuclear neutrons, and in the future as thermonuclear reactors.

Progress achieved in last years at the Institute of Nuclear Physics SB RAS in studies of modern axisymmetric open traps indicate the promise of this approach. These studies are ongoing, and at the same time, the BINP is working on a design for a next-generation facility, which will already be able to demonstrate plasma parameters close to those of a reactor.

Nuclear chain 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 the fission chain reaction of nuclei of heavy elements, in which the majority of fission events are initiated by neutrons produced by fission of nuclei in the previous generation.

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Nuclear physics. Nuclear reactions. Nuclear fission chain reaction. NPP

Nuclear forces Binding energy of particles in the nucleus Fission of uranium nuclei Chain reaction

Nuclear reactions

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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 microparticles of a substance are in a state with a rest energy greater than in another possible state to which a transition exists. A spontaneous transition is always prevented by an energy barrier, to overcome which the microparticle must receive a certain amount of energy from the outside - excitation energy. The exoenergetic reaction consists in the fact that in the transformation following 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 keep in mind the macroscopic scale of energy release, then all or initially at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvins, but in the case of nuclear reactions it is at least 10 7 K due to the very high altitude Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by joining particles does not require much 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 attractive forces. But to excite reactions, the particles themselves are necessary. And if we again mean 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 occurs when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Chain reactions

Chain reactions are common 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 environment causes a chain of reactions that follow one after another, which continues until the chain breaks due to the loss of the reaction carrier particle. There are two main reasons for losses: the absorption of a particle without the emission of a secondary one and the departure of the particle beyond the volume of the substance that supports 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 occurs, because one of the secondary particles continues the started chain, while the others give rise to new chains that branch again. True, processes that lead to chain breaks compete with the branching process, and the resulting situation gives rise to limiting or critical phenomena specific to branched chain reactions. If the number of broken 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 fades out. If the number of new chains formed exceeds the number of breaks, the chain reaction quickly spreads throughout the entire 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. The critical state is characterized by equality between the number of new circuits and the number of breaks.

Achieving a critical state is determined by a number of factors. The 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 produced in one fission act is on average from 2 to 3). Consequently, the fission process can give rise to a branched chain reaction, the carriers of which will be neutrons. If the rate of neutron losses (captures without fission, escapes from the reaction volume, etc.) compensates for the rate of neutron multiplication in such a way that the effective neutron multiplication coefficient is exactly equal to unity, then the chain reaction proceeds in a stationary mode. The introduction of negative feedback between the effective multiplication factor and the rate of energy release allows for a controlled chain reaction, which is used, for example, in nuclear energy. If the multiplication factor is greater than one, the chain reaction develops exponentially; uncontrolled fission chain reaction is used in

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 the fission of other uranium nuclei, and the number of neutrons increases like an avalanche.

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

When k is less than 1, the reaction decays, because number of absorbed neutrons more number newly formed.
When k is greater than 1, an explosion occurs almost instantly.
When k equals 1, a controlled stationary chain reaction occurs.

The chain reaction is accompanied by the release large quantity energy.

To carry out a chain reaction, it is not possible to use any nuclei that 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, uranium-235 isotopes make up only 0.7% of the total uranium reserve, but they are the ones that are suitable for carrying out a chain reaction, because fission under the influence of slow neutrons.

Uranium-238 nuclei can fission only under the influence of high-energy neutrons (fast neutrons). Only 60% of the neutrons produced during the fission of the uranium-238 nucleus have this energy. Approximately only 1 in 5 neutrons produced causes nuclear fission.

Conditions for a chain reaction in uranium-235:

The minimum amount of fuel (critical mass) required to carry out a controlled chain reaction in nuclear reactor
- the speed of neutrons should cause fission of uranium nuclei
- absence of impurities that absorb neutrons

Critical mass:

If the mass of uranium is small, neutrons will fly outside 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 mass, a controlled chain reaction occurs

For uranium-235, the critical mass is 50 kg (this is, for example, a ball of uranium 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, and 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 describes two different phenomena in one equation: motor EMF, generated by the action of a magnetic force on a moving wire, and transformer EMF, generated by the action of electric force due to changes 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 gives a separate physical explanation for each of these two phenomena. Link to these two aspects electromagnetic induction available 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; see vector product of 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 right side loop similar reasoning shows that the emf is equal to v ℓ B(x C + w / 2). Two emfs opposite each other push the positive charge towards the bottom of the loop. In case 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 the current, is 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 intuitively attractive but flawed approach to using the flow rule expresses the flow through a circuit as Φ B = B wℓ, where w- width of the moving loop. This expression is independent of time, so it incorrectly follows that no emf is generated. The error in this statement is that it does not take into account the entire path of the current through the closed loop.

For correct use flow rules we must consider the entire current path, which includes the path through the rims on the upper and lower rims. We can choose an arbitrary closed path through the rims and the rotating loop, and using the flow law, find the emf along this path. Any path that includes a segment adjacent to a rotating loop takes into account the relative motion of the parts of the chain.

As an example, consider a path passing in the upper part of the chain in the direction of rotation of the upper disk, and in the lower part of the chain - in the opposite direction with respect to the lower disk (shown by arrows in Fig. 4). In this case, if the rotating loop has deviated by an angle θ from the collector loop, then it can be considered as part of a cylinder with an area A = rℓθ. This area is perpendicular to the field B, and its contribution to the flow is equal to:

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

in accordance with the formula of Lorentz's law.

Now consider another path, in which we choose to pass along the rims of the disks through opposite segments. In this case the associated thread will be decrease with increasing θ, but according to the right-hand rule, 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 produces 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 original 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 as temperature, and therefore, by heating the substance, a nuclear reaction can be achieved. It is this relationship between heating a substance and a nuclear reaction that is reflected by the term “thermonuclear reaction.”

Coulomb barrier[edit | edit wiki text]

Atomic nuclei have a positive electrical charge. At large distances, their charges can be shielded by electrons. However, in order for the fusion of nuclei to occur, they must approach each other to a distance at which the strong interaction operates. This distance is on the order of the size of the nuclei themselves and many times smaller size atom. At such distances, the electron shells of atoms (even if they were preserved) can no longer shield 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 on the order of the size of the nuclei, the magnitude of the strong interaction, which tends to bind them, begins to increase rapidly and becomes greater than the magnitude of the Coulomb repulsion.

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

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

· Firstly, temperature characterizes only the average kinetic energy; there are particles with both lower and higher energy. In fact, a thermonuclear reaction involves a small number of nuclei that have an energy much higher than the average (the so-called “tail of the Maxwellian distribution”

· Secondly, due to quantum effects, nuclei do not necessarily have an energy exceeding the Coulomb barrier. If their energy is slightly less than the barrier, they are more likely to 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

The 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 fuel atoms is somewhat smaller, which facilitates their approach and, in addition, increases the probability of tunneling of nuclei through the Coulomb barrier.

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 output of the catalyzed reaction.

So far, the amount of energy released is less than the energy costs for the production of the muon itself (5-10 GeV). Thus, muon catalysis is still an energetically unfavorable process. Commercially profitable production energy using muon catalysis is possible with X c ~ 10 4 .

Application[edit | edit wiki text]

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

At the same time, uncontrolled thermonuclear reaction has found its application in military affairs. The first thermonuclear explosive device was 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 (unlike 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.

TICKET 27 question 1

Self-induction phenomenon

We have already studied that a magnetic field arises near a conductor carrying current. We also studied that an alternating magnetic field generates a current (the phenomenon of electromagnetic induction). Let's consider electrical circuit. When the current strength changes in this circuit, the magnetic field will change, as a result of which an additional induced 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 occurrence of an EMF in a conducting circuit, created as a result of a change in 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 strength in the circuit.

Self-induced emf 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 a 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 a 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 strength quickly decreases, and the resulting self-induction emf prevents the decrease in magnetic flux. In this case, the induced current is directed in the same way as the original one. The self-induced emf can be many times greater than the external emf. Therefore, light bulbs very often burn out when the lights are turned off.

Magnetic field energy

Magnetic field energy of a current-carrying circuit:

Radioactive radiation is the radiation that an isotope releases during decay. It has three varieties: alpha rays (the flow of helium atomic nuclei), beta rays (the flow 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 mass. The absorption dose is designated by the letter D and is measured in grays.

In practice, the unit of measurement is also the roentgen (R), equal to 2.58 times 10 to the power of minus 4 coulomb, divided by kilogram.

Absorbed radiation can accumulate over time, and its dose increases the longer the irradiation lasts.

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

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

TICKET 28 Question 1

An elementary particle is a collective term that refers to micro-objects on a subnuclear scale that cannot be split into their component parts.

It should be borne in mind that some elementary particles ( electron, neutrino, quarks etc.) on this moment are considered structureless and are considered primary fundamental particles . Other elementary particles (so-called composite particles , including the particles that make up the nucleus 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.

In total 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 remaining elementary particles decay spontaneously in a time from approximately 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10 −24 to 10 −22, for resonances).

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

Free oscillations are called oscillations that occur in a system (capacitor and coil) after it is removed from an equilibrium position (when a charge is imparted to the capacitor). More precisely, free electromagnetic oscillations occur when a capacitor is discharged through an 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 oscillatory circuit. It consists of an inductor and a capacitor. This process will be repeated again and again. will arise electromagnetic vibrations due to energy conversion electric field capacitor.

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

· If the capacitor is shorted to a coil, then at this moment in time it will begin to discharge (Fig. b). 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. Electrical charges are again accumulated on the capacitor. The capacitor is recharged, i.e. The capacitor plate, previously positively charged, will become negatively charged. The energy of the capacitor becomes maximum. The current in this direction will stop, and the process will repeat in the opposite direction (Fig. d). This process will be repeated over and over again. will arise electromagnetic vibrations due to the conversion 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 law of cosine or sine are called harmonic. Equation of harmonic oscillation of charge: .

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

Ticket 29 question 1

Antiparticle - a twin particle of some other elementary particle , having the same mass and the same spin, differing from it in the signs of all other interaction characteristics (charges such as electric And color charges, baryon and lepton quantum numbers).

The very definition of what to call a “particle” in a particle-antiparticle pair is largely arbitrary. However, for a given choice of “particle,” its antiparticle is determined uniquely. The conservation of the baryon number in weak interaction processes makes it possible to determine the “particle” in any baryon-antibaryon pair from the chain of baryon decays. The choice of an electron as a “particle” in the electron-positron pair fixes (due to the conservation of the lepton number in processes weak interaction) determination of the state of a “particle” in a pair of electron neutrino-antineutrino. Transitions between leptons of different generations (type ) have not been observed, so the definition of a “particle” in each generation of leptons, generally speaking, can be made independently. Usually, by analogy with an electron, “particles” are called negatively charged leptons, which, while preserving the lepton number, determines the corresponding neutrino And antineutrino. For bosons the concept of “particle” can be fixed by definition, for example, hypercharge.

Device diagram nuclear bomb

Fission chain reaction

Secondary neutrons emitted during nuclear fission (2.5 per fission act) can cause new fission acts, which makes a chain reaction possible. The fission chain reaction is characterized by the neutron multiplication factor K, which is equal to the ratio of the number of neutrons in a given generation to their number in the previous generation. A necessary condition the development of a fission chain reaction is . With less, the reaction is impossible. When the reaction occurs at a constant number of neutrons (constant power of energy released). This is a self-sustaining reaction. At - damped reaction. The multiplication factor depends on the nature of the fissile material, the size and shape of the core. Minimum weight The fissile substance necessary for a chain reaction to occur is called critical. For the critical mass is 9 kg, while the radius of the uranium ball is 4 cm.

Chain reactions can be controlled or uncontrollable. The explosion of an atomic bomb is an example of an uncontrolled reaction. The nuclear charge of such a bomb is two or more pieces of almost pure or. The mass of each piece is less than critical, so a chain reaction does not occur. Therefore, for an explosion to occur, it is enough to combine these parts into one piece, with a mass greater than the critical one. This must be done very quickly and the connection of the pieces must be very tight. Otherwise, the nuclear charge will fly apart before it has time to react. An ordinary explosive is used for connection. The shell serves as a neutron reflector and, in addition, keeps the nuclear charge from sputtering until the maximum number of nuclei releases all the energy during fission. Chain reaction in atomic bomb runs on fast neutrons. During an explosion, only part of the neutrons of a nuclear charge have time to react. The chain reaction leads to the release of colossal energy. The temperature that develops reaches degrees. The destructive force of the bomb dropped on Hiroshima by the Americans was equivalent to the explosion of 20,000 tons of trinitrotoluene. The power of the new weapon is hundreds of times greater than that of the first. If we add to this that an atomic explosion produces a huge number of fission fragments, including very long-lived ones, then it becomes obvious what a terrible danger these weapons pose to humanity.

By changing the neutron multiplication factor, a controlled chain reaction can be achieved. The device in which it is carried out controlled response, is called a nuclear reactor. The fissile material is natural or enriched uranium. To prevent the radiative capture of neutrons by uranium nuclei, relatively small blocks of fissile material are placed at some distance from each other, and the gaps are filled with a substance that moderates the neutrons (moderator). Neutron moderation occurs due to elastic scattering. In this case, the energy lost by the particle being slowed down depends on the ratio of the masses of the colliding particles. Maximum amount energy is lost if the particles have the same mass. Deuterium, graphite and beryllium satisfy this condition. The first uranium-graphite reactor was launched in 1942 at the University of Chicago under the leadership of the outstanding Italian physicist Fermi. To explain the operating principle of the reactor, consider a typical diagram of a thermal neutron reactor in Fig. 1.

Fig.1.

In the reactor core there are fuel elements 1 and moderator 2, which slows down neutrons to thermal speeds. Fuel elements (fuel rods) are blocks of fissile material enclosed in a sealed shell that weakly absorbs neutrons. Due to the energy released during nuclear fission, the fuel elements are heated, and therefore, for cooling, they are placed in the coolant flow (3-5 - coolant channel). The core is surrounded by a reflector that reduces neutron leakage. The chain reaction is controlled by special control rods made of materials that strongly absorb neutrons. The reactor parameters are calculated so that when the rods are fully inserted, the reaction obviously does not occur. As the rods are gradually removed, the neutron multiplication factor increases and at a certain position reaches unity. At this moment the reactor begins to operate. As the reactor operates, the amount of fissile material in the core decreases and it becomes contaminated with fission fragments, which may include strong neutron absorbers. To prevent the reaction from stopping, the control rods are gradually removed from the core using an automatic device. Such control of reactions is possible due to the existence of delayed neutrons emitted by fissile nuclei with a delay of up to 1 minute. When the nuclear fuel burns out, the reaction stops. Before the reactor is restarted, the burnt-out nuclear fuel is removed and new fuel is loaded. The reactor also has emergency rods, the introduction of which immediately stops the reaction. A nuclear reactor is a powerful source of penetrating radiation, approximately times higher than sanitary standards. Therefore, any reactor has biological protection - a system of screens made of protective materials (for example, concrete, lead, water) - located behind its reflector, and a remote control.

For the first time nuclear energy was used for peaceful purposes in the USSR. In Obninsk in 1954, under the leadership of Kurchatov, the first nuclear power plant with a capacity of 5 MW was put into operation.

However, uranium thermal neutron reactors can solve the problem of power supply on a limited scale, which is determined by the amount of uranium.

The most promising way to develop nuclear energy is the development of fast neutron reactors, the so-called breeder reactors. Such a reactor produces more nuclear fuel than it consumes. The reaction proceeds with fast neutrons, so not only but can also participate in it, which turns into. The latter can be chemically separated from. This process is called nuclear fuel breeding. In special breeder reactors, the nuclear fuel breeding factor exceeds one. The core of breeders is an alloy of isotope-enriched uranium with a heavy metal that absorbs little neutrons. Breeder reactors do not have a moderator. Control of such reactors by moving the reflector or changing the mass of fissile material.

Nuclear chain reaction

Nuclear chain 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 main number of fission events is initiated by neutrons obtained during fission of nuclei in the previous generation.

Energy release mechanism

If we keep in mind the macroscopic scale of energy release, then all or initially at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvins, but in the case of nuclear reactions it is at least 10 7 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by joining 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 attractive forces of particles. But to excite reactions, the particles themselves are necessary. And if we again mean 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 occurs 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 environment causes a chain of reactions that follow one after another, which continues until the chain breaks due to the loss of the reaction carrier particle. There are two main reasons for losses: the absorption of a particle without the emission of a secondary one and the departure of the particle beyond the volume of the substance that supports 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.

Achieving a critical state is determined by a number of factors. The 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 produced in one fission act is on average 2.5). Consequently, the fission process can give rise to a branched chain reaction, the carriers of which will be neutrons. If the rate of neutron losses (captures without fission, escapes from the reaction volume, etc.) compensates for the rate of neutron multiplication in such a way that the effective neutron multiplication factor is exactly equal to unity, then the chain reaction proceeds in a stationary mode. The introduction of negative feedback 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; runaway fission chain reaction is used in nuclear weapons.

Literature

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

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Chain nuclear reaction is a sequence of nuclear reactions excited by particles (for example, neutrons) born in each reaction event. Depending on the average number of reactions following one previous one is less than, equal to or... ... Nuclear energy terms

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

nuclear chain reaction- grandininė branduolinė reakcija statusas T sritis fizika atitikmenys: engl. 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

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

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

nuclear fission chain reaction with neutrons- - [A.S. Goldberg. 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 unity

50.Nuclear reactors. Prospects for the use of thermonuclear energy.

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Energy release mechanism

Chain reactions

Energy release mechanism

Chain reactions

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Literature

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