Why fission reaction? Lesson summary "Fission of uranium nuclei

>> Fission of uranium nuclei

§ 107 FISSION OF URANIUM NUCLEI

Only the nuclei of some can be divided into parts heavy elements. When nuclei fission, two or three neutrons and -rays are emitted. At the same time, a lot of energy is released.

Discovery of uranium fission. The fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn iF. Strassmann. They found that when uranium is bombarded with neutrons, elements of the middle part appear periodic table: barium, krypton, etc. However, the correct interpretation of this fact as the fission of a uranium nucleus that captured a neutron was given at the beginning of 1939 by the English physicist O. Frisch together with the Austrian physicist L. Meitner.

Neutron capture disrupts the stability of the nucleus. The nucleus becomes excited and becomes unstable, which leads to its division into fragments. Nuclear fission is possible because the rest mass of a heavy nucleus is greater than the sum of the rest masses of the fragments resulting from fission. Therefore, there is a release of energy equivalent to the decrease in rest mass that accompanies fission.

The possibility of fission of heavy nuclei can also be explained using a graph of the dependence specific energy connection from the mass number A (see Fig. 13.11). Specific binding energy of nuclei of atoms of elements occupying the periodic table last places(A 200), approximately 1 MeV less than the specific binding energy in the nuclei of elements located in the middle of the periodic table (A 100). Therefore, the process of fission of heavy nuclei into nuclei of elements in the middle part of the periodic table is energetically favorable. After fission, the system enters a state with minimal internal energy. After all, the greater the binding energy of the nucleus, the greater the energy that should be released upon the emergence of the nucleus and, consequently, the less the internal energy of the newly formed system.

During nuclear fission, the binding energy per nucleon increases by 1 MeV and the total energy released must be enormous - on the order of 200 MeV. No other nuclear reaction (not related to fission) releases such large energies.

Direct measurements of the energy released during the fission of a uranium nucleus confirmed the above considerations and gave a value of 200 MeV. Moreover, most of this energy (168 MeV) falls on the kinetic energy of the fragments. In Figure 13.13 you see the tracks of fissile uranium fragments in a cloud chamber.

The energy released during nuclear fission is of electrostatic rather than nuclear origin. The large kinetic energy that the fragments have arises due to their Coulomb repulsion.

Mechanism of nuclear fission. The process of fission of the atomic nucleus can be explained based on drip model kernels. According to this model, a bunch of nucleons resembles a droplet of charged liquid (Fig. 13.14, a). Nuclear forces between nucleons are short-range, like the forces acting between liquid molecules. Along with large forces electrostatic repulsion between protons, tending to tear the nucleus into pieces, is even greater nuclear forces attraction. These forces keep the nucleus from disintegrating.

The uranium-235 nucleus is spherical in shape. Having absorbed an extra neutron, it becomes excited and begins to deform, acquiring an elongated shape (Fig. 13.14, b). The core will stretch until the repulsive forces between the halves of the elongated core begin to prevail over the attractive forces acting in the isthmus (Fig. 13.14, c). After this, it breaks into two parts (Fig. 13.14, d).

Under the influence Coulomb forces repulsion, these fragments scatter at a speed equal to 1/30 the speed of light.

Emission of neutrons during fission. A fundamental fact of nuclear fission is the emission of two to three neutrons during the fission process. This is what made it possible practical use intranuclear energy.

Understand why emission occurs free neutrons, it is possible based on the following considerations. It is known that the ratio of the number of neutrons to the number of protons in stable nuclei increases with increasing atomic number. Therefore, the relative number of neutrons in fragments arising during fission is greater than is permissible for the nuclei of atoms located in the middle of the periodic table. As a result, several neutrons are released during the fission process. Their energy has different meanings- from several million electron volts to very small ones, close to zero.

Fission usually occurs into fragments, the masses of which differ by approximately 1.5 times. These fragments are highly radioactive, as they contain an excess amount of neutrons. As a result of a series of successive decays, stable isotopes are eventually obtained.

In conclusion, we note that there is also spontaneous fission of uranium nuclei. It was open Soviet physicists G.N. Flerov and K.A. Petrzhak in 1940. The half-life for spontaneous fission is 10 16 years. That's two million times longer period half-life during the decay of uranium.

The reaction of nuclear fission is accompanied by the release of energy.

The fission of uranium nuclei when bombarded with neutrons was discovered in 1939 by German scientists Otto Hahn and Fritz Strassmann.

Otto Hahn (1879-1968)
German physicist, pioneering scientist in the field of radiochemistry. Discovered the fission of uranium and a number of radioactive elements

Fritz Strassmann (1902-1980)
German physicist and chemist. The works relate to nuclear chemistry, nuclear fission. Gave chemical proof of the fission process

Let's consider the mechanism of this phenomenon. Figure 162a conventionally shows the nucleus of a uranium atom. Having absorbed an extra neutron, the nucleus is excited and deformed, acquiring an elongated shape (Fig. 162, b).

Rice. 162. The process of fission of a uranium nucleus under the influence of a neutron entering it

You already know that there are two types of forces at work in the nucleus: electrostatic repulsive forces between protons, which tend to tear the nucleus apart, and nuclear attractive forces between all nucleons, thanks to which the nucleus does not decay. But nuclear forces are short-range, so in an elongated nucleus they can no longer hold parts of the nucleus that are very distant from each other. Under the influence of electrostatic repulsive forces, the core breaks into two parts (Fig. 162, c), which fly apart into different sides at enormous speed and emit 2-3 neutrons.

It turns out that part internal energy the nucleus is converted into the kinetic energy of flying fragments and particles. The fragments are quickly decelerated in the environment, causing them to kinetic energy is converted into the internal energy of the medium (i.e., into the energy of interaction and thermal motion of its constituent particles).

With simultaneous division large quantity uranium nuclei, the internal energy of the environment surrounding the uranium and, accordingly, its temperature increase noticeably (i.e., the environment heats up).

Thus, the fission reaction of uranium nuclei occurs with the release of energy in environment.

The energy contained in the nuclei of atoms is colossal. For example, with the complete fission of all nuclei present in 1 g of uranium, the same amount of energy would be released as that released during the combustion of 2.5 tons of oil. To convert internal energy atomic nuclei in electrical power at nuclear power plants they use the so-called nuclear fission chain reactions.

Let us consider the mechanism of the chain reaction of fission of the uranium isotope nucleus. The nucleus of a uranium atom (Fig. 163) as a result of neutron capture split into two parts, emitting three neutrons. Two of these neutrons caused the fission reaction of two more nuclei, producing four neutrons. These, in turn, caused the fission of four nuclei, after which nine neutrons were produced, etc.

A chain reaction is possible due to the fact that the fission of each nucleus produces 2-3 neutrons, which can take part in the fission of other nuclei.

Figure 163 shows a chain reaction diagram in which total number free neutrons in a piece of uranium increases like an avalanche over time. Accordingly, the number of nuclear fissions and the energy released per unit time sharply increases. Therefore, such a reaction is explosive in nature (it occurs in an atomic bomb).

Rice. 163. Chain reaction of fission of uranium nuclei

Another option is possible, in which the number of free neutrons decreases with time. In this case chain reaction stops. Therefore, such a reaction also cannot be used to produce electricity.

IN for peaceful purposes It is possible to use the energy only of a chain reaction in which the number of neutrons does not change over time.

How can we ensure that the number of neutrons remains constant all the time? To solve this problem, you need to know what factors influence the increase and decrease in the total number of free neutrons in a piece of uranium in which a chain reaction occurs.

One such factor is the mass of uranium. The fact is that not every neutron emitted during nuclear fission causes the fission of other nuclei (see Fig. 163). If the mass (and, accordingly, the dimensions) of a piece of uranium is too small, then many neutrons will fly out of it, not having time to meet the nucleus on their way, cause its fission and thus generate a new generation of neutrons necessary to continue the reaction. In this case, the chain reaction will stop. In order for the reaction to continue, it is necessary to increase the mass of uranium to a certain value, called critical.

Why does a chain reaction become possible as mass increases? The greater the mass of the piece, the larger its dimensions and the more longer way, which neutrons pass through it. In this case, the probability of neutrons meeting nuclei increases. Accordingly, the number of nuclear fissions and the number of emitted neutrons increases.

At the critical mass of uranium, the number of neutrons produced during nuclear fission becomes equal to the number lost neutrons (i.e. captured by nuclei without fission and emitted outside the piece).

Therefore, their total number remains unchanged. In this case, a chain reaction can occur long time without stopping or becoming explosive.

• The smallest mass of uranium at which a chain reaction can occur is called critical mass

If the mass of uranium is greater than the critical mass, then as a result of a sharp increase in the number of free neutrons, the chain reaction leads to an explosion, and if it is less than the critical mass, then the reaction does not proceed due to a lack of free neutrons.

The loss of neutrons (which fly out of uranium without reacting with nuclei) can be reduced not only by increasing the mass of uranium, but also by using a special reflective shell. To do this, a piece of uranium is placed in a shell made of a substance that reflects neutrons well (for example, beryllium). Reflecting from this shell, neutrons return to uranium and can take part in nuclear fission.

There are several other factors on which the possibility of a chain reaction depends. For example, if a piece of uranium contains too many impurities of other chemical elements, then they absorb most of neutrons and the reaction stops.

The presence of a so-called neutron moderator in uranium also affects the course of the reaction. The fact is that uranium-235 nuclei most likely fission under the influence of slow neutrons. And when nuclei fission, they are formed fast neutrons. If fast neutrons are slowed down, then most of them will be captured by uranium-235 nuclei with subsequent fission of these nuclei. Substances such as graphite, water, heavy water (which includes deuterium, an isotope of hydrogen with mass number 2), and some others are used as moderators. These substances only slow down neutrons, almost without absorbing them.

Thus, the possibility of a chain reaction occurring is determined by the mass of uranium, the amount of impurities in it, the presence of a shell and moderator, and some other factors.

The critical mass of a spherical piece of uranium-235 is approximately 50 kg. Moreover, its radius is only 9 cm, since uranium has a very high density.

By using a moderator and a reflective shell and reducing the amount of impurities, it is possible to reduce the critical mass of uranium to 0.8 kg.

Questions

1. Why can nuclear fission begin only when it is deformed under the influence of a neutron absorbed by it?
2. What is formed as a result of nuclear fission?
3. What energy does part of the internal energy of the nucleus transform into during its division? kinetic energy of fragments of a uranium nucleus when they are decelerated in the environment?
4. How does the fission reaction of uranium nuclei proceed - with the release of energy into the environment or, conversely, with the absorption of energy?
5. Explain the mechanism of a chain reaction using Figure 163.
6. What is the critical mass of uranium?
7. Is it possible for a chain reaction to occur if the mass of uranium is less than the critical mass; more critical? Why?

2. What energy is called the energy output of the reaction? How to estimate the energy yield for a fission reaction?

3. What value characterizes the speed of a chain reaction? Write down the necessary condition for the development of a chain reaction.

4. What fission reaction is called self-sustaining? When does it occur?

5. Assess the critical core size and critical mass.

N is the concentration of nuclei. The number of collisions of a neutron with nuclei per unit time n.

Nuclear fission reactions- fission reactions, which consist in the fact that a heavy nucleus, under the influence of neutrons, and, as it later turned out, other particles, is divided into several lighter nuclei (fragments), most often into two nuclei of similar mass.

A feature of nuclear fission is that it is accompanied by the emission of two or three secondary neutrons, called fission neutrons. Since for medium nuclei the number of neutrons is approximately equal to the number of protons ( N/Z ≈ 1), and for heavy nuclei the number of neutrons significantly exceeds the number of protons ( N/Z ≈ 1.6), then the resulting fission fragments are overloaded with neutrons, as a result of which they release fission neutrons. However, the emission of fission neutrons does not completely eliminate the overload of fragment nuclei with neutrons. This causes the fragments to become radioactive. They can undergo a series of β - -transformations, accompanied by the emission of γ quanta. Since β - decay is accompanied by the transformation of a neutron into a proton, then after a chain of β - transformations the ratio between neutrons and protons in the fragment will reach a value corresponding to a stable isotope. For example, during the fission of a uranium nucleus U

U+ n → Xe + Sr +2 n(265.1)

fission fragment Xe, as a result of three acts of β - decay, turns into stable isotope Lantana La:

Heh → Cs → Ba → La.

Fission fragments can be diverse, so reaction (265.1) is not the only one leading to the fission of U.

Most fission neutrons are emitted almost instantly ( t≤ 10 –14 s), and part (about 0.7%) is emitted by fission fragments some time after fission (0.05 s ≤ t≤ 60 s). The first of them are called instant, second – lagging. On average, each fission event produces 2.5 neutrons. They have a relatively wide energy spectrum ranging from 0 to 7 MeV, with an average energy of about 2 MeV per neutron.

Calculations show that nuclear fission must also be accompanied by the release of a large amount of energy. In fact, the specific binding energy for nuclei average weight is approximately 8.7 MeV, while for heavy nuclei it is equal to 7.6 MeV. Consequently, when a heavy nucleus divides into two fragments, an energy equal to approximately 1.1 MeV per nucleon should be released.

The theory of fission of atomic nuclei (N. Bohr, Ya. I. Frenkel) is based on the droplet model of the nucleus. The nucleus is considered as a drop of electrically charged incompressible liquid (with a density equal to the nuclear density and obeying the laws quantum mechanics), the particles of which, when a neutron hits the nucleus, come into oscillatory motion, as a result of which the core breaks into two parts, flying apart with enormous energy.

The probability of nuclear fission is determined by the energy of the neutrons. For example, if high-energy neutrons cause fission of almost all nuclei, then neutrons with an energy of several mega-electron-volts cause fission only of heavy nuclei ( A>210), Neutrons having activation energy(the minimum energy required to carry out a nuclear fission reaction) of the order of 1 MeV, causes fission of the nuclei of uranium U, thorium Th, protactinium Pa, plutonium Pu. Thermal neutrons fission the nuclei of U, Pu, and U, Th (the last two isotopes do not occur in nature, they are obtained artificially).

Secondary neutrons emitted during nuclear fission can cause new fission events, which makes it possible to fission chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of this reaction. The fission chain reaction is characterized by multiplication factor k neutrons, which equal to the ratio the number of neutrons in a given generation to their number in the previous generation. A necessary condition for the development of a fission chain reaction is requirement k ≥ 1.

It turns out that not all secondary neutrons produced cause subsequent nuclear fission, which leads to a decrease in the multiplication factor. Firstly, due to the finite dimensions core(the space where a valuable reaction occurs) and the high penetrating ability of neutrons, some of them will leave the active zone before being captured by any nucleus. Secondly, some neutrons are captured by the nuclei of non-fissile impurities, which are always present in the core. In addition, along with fission, competing processes of radiative capture and inelastic scattering can take place.

The multiplication coefficient depends on the nature of the fissile substance, and for of a given isotope– on its quantity, as well as the size and shape of the active zone. The minimum dimensions of the active zone at which a chain reaction is possible are called critical sizes. The minimum mass of fissile material located in a system of critical dimensions required to implement chain reaction, called critical mass.

The speed of development of chain reactions is different. Let T - average time

life of one generation, and N- the number of neutrons in a given generation. In the next generation their number is equal kN,T. e. increase in the number of neutrons per generation dN = kN – N = N(k – 1). The increase in the number of neutrons per unit time, i.e., the rate of growth of the chain reaction,

. (266.1)

Integrating (266.1), we obtain

,

Where N 0– number of neutrons in starting moment time, and N- their number at a time t. N determined by the sign ( k– 1). At k>1 is coming developing reaction, the number of fissions continuously increases and the reaction can become explosive. At k=1 goes self-sustaining reaction in which the number of neutrons does not change over time. At k <1 идет fading reaction

Chain reactions include controlled and uncontrollable ones. The explosion of an atomic bomb, for example, is an uncontrolled reaction. To prevent an atomic bomb from exploding during storage, U (or Pu) in it is divided into two parts distant from each other with masses below critical. Then, with the help of an ordinary explosion, these masses come closer together, the total mass of the fissile substance becomes greater than the critical one and an explosive chain reaction occurs, accompanied by the instant release of a huge amount of energy and great destruction. The explosive reaction begins due to available neutrons from spontaneous fission or neutrons from cosmic radiation. Controlled chain reactions occur in nuclear reactors.

The study of the interaction of neutrons with matter led to the discovery of a new type of nuclear reactions. In 1939, O. Hahn and F. Strassmann investigated the chemical products resulting from the bombardment of uranium nuclei by neutrons. Among the reaction products, barium was discovered, a chemical element with a mass much less than the mass of uranium. The problem was solved by German physicists L. Meitner and O. Frisch, who showed that when neutrons are absorbed by uranium, the nucleus splits into two fragments:

Where k > 1.

During the fission of a uranium nucleus, a thermal neutron with an energy of ~0.1 eV releases an energy of ~200 MeV. The essential point is that this process is accompanied by the appearance of neutrons capable of causing the fission of other uranium nuclei - fission chain reaction . Thus, one neutron can give rise to a branched chain of nuclear fissions, and the number of nuclei participating in the fission reaction will increase exponentially. Prospects for using the fission chain reaction have opened up in two directions:

· managed nuclear reaction divisions- Creation nuclear reactors;

· runaway nuclear fission reaction- creation of nuclear weapons.

In 1942, the first nuclear reactor. In the USSR, the first reactor was launched in 1946. Currently, thermal and electrical energy produced in hundreds of nuclear reactors operating in different countries around the world.

As can be seen from Fig. 4.2, with increasing value A specific binding energy increases up to A» 50. This behavior can be explained by a combination of forces; The binding energy of an individual nucleon increases if it is attracted not by one or two, but by several other nucleons. However, in elements with mass number values ​​greater A» 50 specific binding energy gradually decreases with increasing A. This is due to the fact that nuclear attractive forces are short-range, with a radius of action on the order of the size of an individual nucleon. Outside this radius, electrostatic repulsion forces predominate. If two protons are separated by more than 2.5 × 10 - 15 m, then the forces of Coulomb repulsion rather than nuclear attraction prevail between them.

A consequence of this behavior of the specific binding energy depending on A is the existence of two processes - nuclear fusion and fission . Let's consider the interaction of an electron and a proton. When a hydrogen atom is formed, an energy of 13.6 eV is released and the mass of the hydrogen atom is 13.6 eV less than the sum of the masses free electron and proton. Similarly, the mass of two light nuclei exceeds the mass after their combination on D M. If you connect them, they will merge releasing energy D Ms 2. This process is called nuclear fusion . The mass difference can exceed 0.5%.

If a heavy nucleus splits into two lighter nuclei, their mass will be 0.1% less than the mass of the parent nucleus. Heavy nuclei tend to division into two lighter nuclei with the release of energy. Energy atomic bomb and a nuclear reactor represents energy , released during nuclear fission . Energy hydrogen bomb is the energy released when nuclear fusion. Alpha decay can be considered as a highly asymmetric fission in which the parent nucleus M splits into a small alpha particle and a large residual nucleus. Alpha decay is possible only if the reaction

weight M turns out to be greater than the sum of the masses and the alpha particle. All cores with Z> 82 (lead) .At Z> 92 (uranium) alpha decay half-lives turn out to be significantly longer than the age of the Earth, and such elements do not occur in nature. However, they can be created artificially. For example, plutonium ( Z= 94) can be obtained from uranium in a nuclear reactor. This procedure has become common and costs only 15 dollars per 1 g. So far, it has been possible to obtain elements up to Z= 118, however at a much higher price and, as a rule, in negligible quantities. One can hope that radiochemists will learn to obtain, although in small quantities, new elements with Z> 118.

If a massive uranium nucleus could be divided into two groups of nucleons, then these groups of nucleons would rearrange themselves into nuclei with a stronger bond. During the restructuring process, energy would be released. Spontaneous nuclear fission is permitted by the law of conservation of energy. However, the potential barrier to fission reactions in naturally occurring nuclei is so high that the probability of spontaneous fission is much less than the probability of alpha decay. The half-life of 238 U nuclei relative to spontaneous fission is 8×10 15 years. This is more than a million times the age of the Earth. If a neutron collides with a heavy nucleus, it can move to a higher energy level near the top of the electrostatic potential barrier, the probability of fission will increase as a result. A nucleus in an excited state can have a significant angular momentum and acquire an oval shape. Areas on the periphery of the nucleus penetrate the barrier more easily, since they are partially already behind the barrier. At the core oval shape the role of the barrier is further weakened. When captured by a core or slow neutron states with very for short periods of time life relative to division. The difference in mass between the uranium nucleus and typical fission products is such that, on average, the fission of uranium releases an energy of 200 MeV. The rest mass of the uranium nucleus is 2.2×10 5 MeV. About 0.1% of this mass is converted into energy, which is equal to the ratio of 200 MeV to the value of 2.2 × 10 5 MeV.

Energy rating,released by division,can be obtained from Weizsäcker formulas :

When a nucleus divides into two fragments, the surface energy and Coulomb energy change , and the surface energy increases, and the Coulomb energy decreases. Fission is possible when the energy released during fission E > 0.

.

Here A 1 = A/2, Z 1 = Z/2. From this we obtain that fission is energetically favorable when Z 2 /A> 17. Magnitude Z 2 /A called divisibility parameter . Energy E, released during division, increases with increasing Z 2 /A.

During the process of division, the nucleus changes shape - it sequentially passes through the following stages (Fig. 9.4): a ball, an ellipsoid, a dumbbell, two pear-shaped fragments, two spherical fragments.

After fission has occurred, and the fragments are located at a distance from each other much greater than their radius, the potential energy of the fragments, determined by Coulomb interaction between them can be considered equal to zero.

Due to the evolution of the shape of the nucleus, its change potential energy determined by the change in the sum of surface and Coulomb energies . It is assumed that the volume of the core remains unchanged during deformation. In this case, the surface energy increases as the surface area of ​​the nucleus increases. The Coulomb energy decreases as the average distance between nucleons increases. In the case of small ellipsoidal deformations, the increase in surface energy occurs faster than the decrease in Coulomb energy.

In the region of heavy nuclei, the sum of surface and Coulomb energies increases with increasing deformation. At small ellipsoidal deformations, an increase in surface energy prevents further changes in the shape of the nucleus and, consequently, fission. The presence of a potential barrier prevents the instantaneous spontaneous fission of nuclei. In order for a nucleus to instantly split, it must be given an energy exceeding the height of the fission barrier N.

Barrier height N the more than less attitude Coulomb and surface energy in the initial nucleus. This ratio, in turn, increases with increasing divisibility parameter Z 2 /A. The heavier the core, the lower the height of the barrier N, since the fissibility parameter increases with increasing mass number:

Heavier nuclei generally need to impart less energy to cause fission. From the Weizsäcker formula it follows that the height of the fission barrier vanishes at . Those. according to the droplet model, there should be no nuclei with in nature, since they almost instantly (for the characteristic nuclear time about 10–22 s) spontaneously divide. The existence of atomic nuclei with (" island of stability ") is explained by the shell structure of atomic nuclei. Spontaneous fission of nuclei with , for which the barrier height N not equal to zero, from the point of view classical physics impossible. From the point of view of quantum mechanics, such division is possible as a result of fragments passing through a potential barrier and is called spontaneous fission . The probability of spontaneous fission increases with increasing fissibility parameter, i.e. with decreasing fission barrier height.

Forced fission of nuclei with can be caused by any particles: photons, neutrons, protons, deuterons, α-particles, etc., if the energy they contribute to the nucleus is sufficient to overcome the fission barrier.

The masses of fragments formed during fission by thermal neutrons are not equal. The nucleus tends to split in such a way that the main part of the nucleons of the fragment forms a stable magical core. In Fig. Figure 9.5 shows the mass distribution during division. The most likely combination of mass numbers is 95 and 139.

The ratio of the number of neutrons to the number of protons in the nucleus is 1.55, while stable elements, having a mass close to the mass of fission fragments, this ratio is 1.25 - 1.45. Consequently, fission fragments are heavily overloaded with neutrons and are unstable to β-decay - they are radioactive.

As a result of fission, energy of ~200 MeV is released. About 80% of it comes from the energy of fragments. During one fission act more than two are formed fission neutrons with average energy~ 2 MeV.

1 g of any substance contains . The fission of 1 g of uranium is accompanied by the release of ~ 9 × 10 10 J. This is almost 3 million times greater than the energy of burning 1 g of coal (2.9 × 10 4 J). Of course, 1 g of uranium is much more expensive than 1 g of coal, but the cost of 1 J of energy obtained by burning coal is 400 times higher than in the case of uranium fuel. Producing 1 kWh of energy cost 1.7 cents at coal-fired power plants and 1.05 cents at nuclear power plants.

Thanks to chain reaction nuclear fission process can be done self-sustaining . With each fission, 2 or 3 neutrons are released (Fig. 9.6). If one of these neutrons manages to cause the fission of another uranium nucleus, then the process will be self-sustaining.

A collection of fissile matter that satisfies this requirement is called critical assembly . The first such assembly, called nuclear reactor , was built in 1942 under the direction of Enrico Fermi on the grounds of the University of Chicago. The first nuclear reactor was launched in 1946 under the leadership of I. Kurchatov in Moscow. First nuclear power plant with a capacity of 5 MW was launched in the USSR in 1954 in Obninsk (Fig. 9.7).

Mass and you can also do supercritical . In this case, the neutrons generated during fission will cause several secondary fissions. Because neutrons travel at speeds in excess of 10 8 cm/s, a supercritical assembly can fully react (or fly apart) in less than a thousandth of a second. Such a device is called atomic bomb . A nuclear charge made of plutonium or uranium is transferred to a supercritical state, usually with the help of an explosion. The subcritical mass is surrounded by chemical explosives. When it explodes, the plutonium or uranium mass undergoes instant compression. Since the density of the sphere increases significantly, the rate of absorption of neutrons turns out to be higher than the rate of loss of neutrons due to their escape outward. This is the condition for supercriticality.

In Fig. Figure 9.8 shows a diagram of the Little Boy atomic bomb dropped on Hiroshima. The nuclear explosive in the bomb was divided into two parts, the mass of which was less than the critical mass. The critical mass required for the explosion was created by connecting both parts “by the gun method” using conventional explosives.

The explosion of 1 ton of trinitrotoluene (TNT) releases 10 9 cal, or 4 × 10 9 J. The explosion of an atomic bomb that consumes 1 kg of plutonium releases about 8 × 10 13 J of energy.

Or this is almost 20,000 times more than the explosion of 1 ton of TNT. Such a bomb is called a 20-kiloton bomb. Modern bombs megatons of explosives are millions of times more powerful than conventional TNT explosives.

The production of plutonium is based on the irradiation of 238 U with neutrons, leading to the formation of the isotope 239 U, which, as a result of beta decay, turns into 239 Np, and then after another beta decay into 239 Pu. When a low-energy neutron is absorbed, both isotopes 235 U and 239 Pu undergo fission. Fission products are characterized by stronger binding (~1 MeV per nucleon), due to which approximately 200 MeV of energy is released as a result of fission.

Every gram of plutonium or uranium consumed produces almost a gram of radioactive fission products, which have enormous radioactivity.

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