Nuclear fission is possible under the condition. Nuclear fission: the process of splitting an atomic nucleus

He began experiments on the irradiation of uranium with slow neutrons from a radium-beryllium source. The purpose of these experiments, which served as the impetus for numerous similar experiments carried out in other laboratories, was the discovery of transuranium elements unknown at that time, which were supposed to be obtained as a result of - -decay of uranium isotopes formed during neutron capture. New radioactive products were indeed found, but further research showed that the radiochemical properties of many of the "new transuranium elements" differed from those expected. The study of these unusual products continued until 1939, when the radiochemists Hahn and Strassmann proved that the new activities belong not to heavy elements, but to atoms of medium weight. The correct interpretation of the unusual nuclear process was given in the same year by Meitner and Frisch, who suggested that the excited nucleus of uranium is divided into two fragments of approximately equal mass. Based on an analysis of the binding energies of the elements of the periodic table, they came to the conclusion that in each fission event a very large amount of energy must be released, several tens of times greater than the energy released during -decay. This was confirmed by the experiments of Frisch, who registered pulses from fission fragments in the ionization chamber, and Joliot, who showed, on the basis of measurements of the distances of the fragments, that the latter have a large kinetic energy.

Figure 1 shows that cores with A = 40-120 have the highest stability, i.e. in the middle of the periodic table. The processes of joining (synthesis) of light nuclei and fission of heavy nuclei are energetically favorable. In both cases, the final nuclei are located in the region of values ​​A, where the specific binding energy is greater than the specific binding energy of the initial nuclei. Therefore, these processes must proceed with the release of energy. Using data on specific binding energies, one can estimate the energy that is released in one fission event. Let a nucleus with a mass number A 1 = 240 be divided into two equal fragments with A 2 = 120. In this case, the specific binding energy of the fragments increases by 0.8 MeV compared to the specific binding energy of the initial nucleus (from 1 7.6 MeV for a nucleus with A 1 = 240 to 2 8.4 MeV for a nucleus with A 2 = 120). In this case, energy must be released

E = A 1 1 - 2A 2 2 \u003d A 1 ( 2 - 1) 240 (8.4-7.6) MeV 200 MeV.

. Elementary theory of fission

Let us calculate the amount of energy released during the fission of a heavy nucleus. Substitute in (f.2) the expressions for the binding energies of the nuclei (f.1), assuming A 1 = 240 and Z 1 = 90. Neglecting the last term in (f.1) due to its smallness and substituting the values ​​of the parameters a 2 and a 3 , we get

From this we obtain that fission is energetically favorable when Z 2 /A > 17. The value of Z 2 /A is called the divisibility parameter. The energy E, released during fission, grows with an increase in Z 2 /A; Z 2 /A = 17 for nuclei in the region of yttrium and zirconium. It can be seen from the obtained estimates that fission is energetically favorable for all nuclei with A > 90. Why is the majority of nuclei stable with respect to spontaneous fission? To answer this question, let's see how the shape of the nucleus changes during fission.

In the process of fission, the nucleus successively passes through the following stages (Fig. 2): ball, ellipsoid, dumbbell, two pear-shaped fragments, two spherical fragments. How does the potential energy of the nucleus change at different stages of fission? After the fission has taken place, and the fragments are separated from each other by a distance much greater than their radius, the potential energy of the fragments, determined by the Coulomb interaction between them, can be considered equal to zero.

Let us consider the initial stage of fission, when the nucleus takes the form of an increasingly elongated ellipsoid of revolution with increasing r. At this stage of fission, r is a measure of the deviation of the nucleus from a spherical shape (Fig. 3). Due to the evolution of the shape of the nucleus, the change in its potential energy is determined by the change in the sum of the surface and Coulomb energies E"n + E"k. It is assumed that the volume of the nucleus remains unchanged during deformation. In this case, the surface energy E "p increases, since the surface area of ​​the nucleus increases. The Coulomb energy E" k decreases, since the average distance between nucleons increases. Let the spherical core, as a result of a slight deformation characterized by a small parameter, take the form of an axially symmetric ellipsoid. It can be shown that the surface energy E "p and the Coulomb energy E" k depending on change as follows:

In the case of small ellipsoidal deformations, the increase in the surface energy occurs faster than the decrease in the Coulomb energy.
In the region of heavy nuclei 2En > Ek, the sum of the surface and Coulomb energies increases with increasing . From (f.4) and (f.5) it follows that at small ellipsoidal deformations, the increase in surface energy prevents further changes in the shape of the nucleus, and, consequently, fission. Expression (f.5) is valid for small values ​​(small strains). If the deformation is so great that the nucleus takes the form of a dumbbell, then the surface tension forces, like the Coulomb forces, tend to separate the nucleus and give the fragments a spherical shape. At this fission stage, an increase in strain is accompanied by a decrease in both the Coulomb and surface energies. Those. with a gradual increase in the deformation of the nucleus, its potential energy passes through a maximum. Now r has the meaning of the distance between the centers of future fragments. As the fragments move away from each other, the potential energy of their interaction will decrease, since the energy of the Coulomb repulsion E k decreases. The dependence of the potential energy on the distance between the fragments is shown in Fig. 4. The zero level of potential energy corresponds to the sum of the surface and Coulomb energies of two noninteracting fragments.
The presence of a potential barrier prevents instantaneous spontaneous nuclear fission. In order for the nucleus to instantly split, it must be given energy Q that exceeds the barrier height H. The maximum potential energy of the fissile nucleus is approximately equal to
e 2 Z 1 Z 2 /(R 1 +R 2), where R 1 and R 2 are the radii of the fragments. For example, when a gold nucleus is divided into two identical fragments, e 2 Z 1 Z 2 / (R 1 + R 2) \u003d 173 MeV, and the amount of energy E released during fission () is 132 MeV. Thus, during the fission of the gold nucleus, it is necessary to overcome a potential barrier with a height of about 40 MeV.
The barrier height H is the greater, the smaller the ratio of the Coulomb and surface energies E to /E p in the initial nucleus. This ratio, in turn, increases with an increase in the divisibility parameter Z 2 /A (). The heavier the core, the lower the barrier height H , since the divisibility parameter increases with increasing mass number:

Those. According to the drop model, nuclei with Z 2 /A > 49 should be absent in nature, since they spontaneously fission almost instantaneously (in a characteristic nuclear time of the order of 10 -22 s). The possibility of the existence of atomic nuclei with Z 2 /A > 49 ("island of stability") is explained by the shell structure. The dependence of the shape, the height of the potential barrier H, and the fission energy E on the value of the divisibility parameter Z 2 /А is shown in Fig. . 5.

Release of energy during nuclear fission. As in other nuclear reactions, the energy released during fission is equivalent to the difference in the masses of the interacting particles and the final products. Since the binding energy of a nucleon in uranium and the binding energy of one nucleon in fragments, during the fission of uranium, energy must be released

Thus, during the fission of the nucleus, huge energy is released, the overwhelming part of it is released in the form of the kinetic energy of the fission fragments.

Mass distribution of fission products. The uranium nucleus in most cases is divided asymmetrically. Two nuclear fragments have correspondingly different speeds and different masses.

The fragments fall into two groups according to their masses; one near krypton with the other near xenon. The masses of the fragments are related to each other on average as From the laws of conservation of energy and momentum, it can be obtained that the kinetic energies of the fragments should be inversely proportional to their masses:

The fission product yield curve is symmetrical with respect to the vertical straight line passing through the point. The significant width of the maxima indicates the diversity of fission paths.

Rice. 82. Mass distribution of uranium fission products

The listed characteristics refer mainly to fission under the action of thermal neutrons; in the case of fission under the action of neutrons with an energy of several or more, the nucleus breaks up into two fragments more symmetrical in mass.

Properties of fission products. During the fission of a uranium atom, very many shell electrons are shed, and the fission fragments are approximately -fold ionized positive ions, which, when passing through the substance, strongly ionize the atoms. Therefore, the paths of the fragments in the air are small and close to 2 cm.

It is easy to establish that the fragments formed during fission must be radioactive, prone to emitting neutrons. Indeed, for stable nuclei, the ratio of the number of neutrons and protons varies depending on A as follows:

(see scan)

Nuclei produced by fission lie in the middle of the table and therefore contain more neutrons than is acceptable for their stability. They can be freed from excess neutrons both by decay and by directly emitting neutrons.

delayed neutrons. In one of the possible variants of fission, radioactive bromine is formed. On fig. 83 shows a diagram of its decay, at the end of which are stable isotopes

An interesting feature of this chain is that krypton can be freed from an excess neutron either due to -decay, or if it was formed in an excited state due to the direct emission of a neutron. These neutrons appear 56 seconds after fission (the lifetime is relative to the transition to an excited state, although it itself emits neutrons almost instantly.

Rice. 83. Scheme of the decay of radioactive bromine formed in an excited state during the fission of uranium

They are called delayed neutrons. Over time, the intensity of delayed neutrons decreases exponentially, as in normal radioactive decay.

The energy of these neutrons is equal to the excitation energy of the nucleus. Although they make up only 0.75% of all neutrons emitted in fission, delayed neutrons play an important role in the implementation of a chain reaction.

Prompt neutrons. Over 99% of the neutrons are released within an extremely short time; they are called prompt neutrons.

When studying the fission process, the fundamental question arises, how many neutrons are produced in one fission event; this question is important because if their number is large on average, they can be used to divide subsequent nuclei, i.e., it becomes possible to create a chain reaction. Over the resolution of this issue in 1939-1940. worked in almost all major nuclear laboratories in the world.

Rice. 84. Energy spectrum of neutrons obtained from the fission of uranium-235

Fission energy distribution. Direct measurement of the energy of fragments and the energy carried away by other fission products gave the following approximate energy distribution

The study of the interaction of neutrons with matter led to the discovery of nuclear reactions of a new type. In 1939, O. Hahn and F. Strassmann investigated the chemical products resulting from the bombardment of uranium nuclei with neutrons. Among the reaction products, barium was found - a chemical element with a mass much less than that of uranium. The problem was solved by the German physicists L. Meitneroma and O. Frisch, who showed that when neutrons are absorbed by uranium, the nucleus is divided 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 fission of other uranium nuclei, - fission chain reaction . Thus, one neutron can give rise to a branched chain of nuclear fission, and the number of nuclei involved in the fission reaction will increase exponentially. The prospects for using a fission chain reaction have opened up in two directions:

· controlled nuclear fission reaction- creation of nuclear reactors;

· uncontrolled nuclear fission reaction- Creation of nuclear weapons.

In 1942, the first nuclear reactor was built in the USA. In the USSR, the first reactor was launched in 1946. Currently, thermal and electrical energy is generated in hundreds of nuclear reactors operating in various countries of the world.

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

The consequence of this behavior of the specific binding energy depending on BUT is the existence of two processes - fusion and fission of nuclei . 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 turns out to be 13.6 eV less than the sum of the masses of a free electron and a proton. Similarly, the mass of two light nuclei exceeds the mass after their connection at D M. If they are connected, they will merge with the release of energy D MS 2. This process is called nuclear synthesis . The mass difference can exceed 0.5%.

If a heavy nucleus splits into two lighter nuclei, then their mass will be less than the mass of the parent nucleus by 0.1%. Heavy nuclei tend to division into two lighter nuclei with energy release. The energy of the atomic bomb and nuclear reactor is the energy , released during nuclear fission . H-bomb energy is the energy released during nuclear fusion. Alpha decay can be viewed 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 nuclei with Z> 82 (lead). Z> 92 (uranium) alpha decay half-lives are much 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 commonplace and costs only 15 dollars per 1 g. Until now, it has been possible to obtain elements up to Z= 118, but at a much higher price and, as a rule, in negligible quantities. It can be hoped that radiochemists will learn how to obtain, albeit 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 into nuclei with a stronger bond. In the process of restructuring, energy would be released. Spontaneous nuclear fission is allowed by the law of conservation of energy. However, the potential barrier in the fission reaction of 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, then it can go to a higher energy level near the top of the electrostatic potential barrier, as a result, the probability of fission will increase. The nucleus in an excited state can have a significant angular momentum and acquire an oval shape. Sites on the periphery of the nucleus penetrate the barrier more easily, since they are partially already behind the barrier. In an oval-shaped nucleus, the role of the barrier is even more weakened. When a nucleus or a slow neutron is captured, states are formed with very short lifetimes relative to fission. The difference between the masses of the uranium nucleus and typical fission products is such that, on average, 200 MeV energy is released during the fission of uranium. 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 2.2 × 10 5 MeV.

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

When a nucleus divides into two fragments, the surface energy and Coulomb energy change , with the surface energy increasing and the Coulomb energy decreasing. Fission is possible when the energy released during fission is 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. Value Z 2 /A called divisibility parameter . Energy E, released during division, increases with increasing Z 2 /A.

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

Due to the evolution of the shape of the nucleus, the change in its potential energy is determined by the change in the sum of the surface and Coulomb energies . It is assumed that the core volume remains unchanged during deformation. In this case, the surface energy increases, since 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 the surface energy occurs faster than the decrease in the Coulomb energy.

In the region of heavy nuclei, the sum of the surface and Coulomb energies increases with strain. At small ellipsoidal deformations, an increase in the surface energy prevents a further change in the shape of the nucleus, and hence fission. The presence of a potential barrier prevents instantaneous spontaneous nuclear fission. In order for the nucleus to instantly split, it must be supplied with energy exceeding the height of the fission barrier H.

barrier height H the larger, the smaller the ratio of the Coulomb and surface energies in the initial nucleus. This ratio, in turn, increases with increasing divisibility parameter Z 2 /BUT. The heavier the core, the lower the barrier height H, since the divisibility parameter increases with increasing mass number:

Heavier nuclei generally need to be supplied with less energy to cause fission. It follows from the Weizsäcker formula that the height of the fission barrier vanishes at . Those. According to the drop model, there should be no nuclei with in nature, since they spontaneously fission almost instantaneously (over a characteristic nuclear time of the order of 10–22 s). The existence of atomic nuclei with (" island of stability ”) is explained by the shell structure of atomic nuclei. Spontaneous nuclear fission with , for which the barrier height H is not equal to zero, from the point of view of classical physics it is impossible. From the point of view of quantum mechanics, such fission is possible as a result of the passage of fragments through a potential barrier and is called spontaneous fission . The probability of spontaneous fission increases with increasing fission parameter , i.e. with a decrease in the height of the fission barrier.

Forced nuclear fission 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 fragment's nucleons form a stable magical core. On fig. 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 for stable elements with 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 ~ 200 MeV is released. About 80% of it is accounted for by the fragment energy. In one act of fission, more than two fission neutrons with an average energy of ~ 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 costs much more than 1 g of coal, but the cost of 1 J of energy obtained by burning coal turns out to be 400 times higher than in the case of uranium fuel. Generating 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 emitted (Fig. 9.6). If one of these neutrons manages to cause the fission of another uranium nucleus, then the process will be self-sustaining.

The set of fissile material 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 campus of the University of Chicago. The first nuclear reactor was launched in 1946 under the leadership of I. Kurchatov in Moscow. The first nuclear power plant with a capacity of 5 MW was launched in the USSR in 1954 in the city of Obninsk (Fig. 9.7).

mass and you can also do supercritical . In this case, the neutrons produced 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 by means of an explosion. The subcritical mass is surrounded by chemical explosives. During its explosion, the plutonium or uranium mass is subjected to instantaneous compression. Since the density of the sphere in this case increases significantly, the rate of absorption of neutrons turns out to be higher than the rate of loss of neutrons due to their emission to the outside. This is the condition of supercriticality.

On fig. 9.8 shows a diagram of the atomic bomb "Kid" dropped on Hiroshima. Served as a nuclear explosive in a bomb, divided into two parts, the mass of which was less than critical. The critical mass necessary for the explosion was created by connecting both parts by the "cannon method" using conventional explosives.

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

Or it is almost 20,000 times more than in the explosion of 1 ton of TNT. Such a bomb is called a 20-kiloton bomb. Today's megaton bombs are millions of times more powerful than conventional TNT explosives.

Plutonium production is based on the irradiation of 238 U with neutrons, leading to the formation of the 239 U isotope, 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 235 U and 239 Pu isotopes undergo fission. Fission products are characterized by a stronger binding (~ 1 MeV per nucleon), due to which approximately 200 MeV of energy is released as a result of fission.

Each gram of spent plutonium or uranium gives rise to almost a gram of radioactive fission products, which have enormous radioactivity.

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How this process was discovered and described. Its use as a source of energy and nuclear weapons is disclosed.

"Indivisible" atom

The twenty-first century is replete with such expressions as "the energy of the atom", "nuclear technology", "radioactive waste". Every now and then in newspaper headlines flash messages about the possibility of radioactive contamination of the soil, oceans, ice of Antarctica. However, an ordinary person often does not have a very good idea of ​​what this field of science is and how it helps in everyday life. It is worth starting, perhaps, with history. From the very first question, which was asked by a well-fed and dressed person, he was interested in how the world works. How the eye sees, why the ear hears, how water differs from stone - this is what worried the wise men from time immemorial. Even in ancient India and Greece, some inquisitive minds suggested that there is a minimal particle (it was also called "indivisible") that has the properties of a material. Medieval chemists confirmed the guess of the sages, and the modern definition of the atom is as follows: an atom is the smallest particle of a substance that is the bearer of its properties.

Parts of an atom

However, the development of technology (in particular, photography) has led to the fact that the atom is no longer considered the smallest possible particle of matter. And although a single atom is electrically neutral, scientists quickly realized that it consists of two parts with different charges. The number of positively charged parts compensates for the number of negative ones, so the atom remains neutral. But there was no unambiguous model of the atom. Since classical physics still dominated at that time, various assumptions were made.

Models of the atom

At first, the model "bun with raisins" was proposed. The positive charge, as it were, filled the entire space of the atom, and negative charges were distributed in it, like raisins in a bun. The famous one determined the following: in the center of the atom there is a very heavy element with a positive charge (the nucleus), and much lighter electrons are located around. The mass of the nucleus is hundreds of times heavier than the sum of all the electrons (it makes up 99.9 percent of the mass of the entire atom). Thus, Bohr's planetary model of the atom was born. However, some of its elements contradicted the then accepted classical physics. Therefore, a new, quantum mechanics was developed. With its appearance, the non-classical period of science began.

Atom and radioactivity

From all of the above, it becomes clear that the nucleus is the heavy, positively charged part of the atom, which makes up its bulk. When the positions of electrons in the orbit of an atom were well studied, it was time to understand the nature of the atomic nucleus. The ingenious and unexpectedly discovered radioactivity came to the rescue. It helped to reveal the essence of the heavy central part of the atom, since the source of radioactivity is nuclear fission. At the turn of the nineteenth and twentieth centuries, discoveries rained down one after another. The theoretical solution of one problem necessitated new experiments. The results of the experiments gave rise to theories and hypotheses that needed to be confirmed or refuted. Often the greatest discoveries have come about simply because that is how the formula became easy to calculate (like, for example, Max Planck's quantum). Even at the beginning of the era of photography, scientists knew that uranium salts light up a photosensitive film, but they did not suspect that nuclear fission was the basis of this phenomenon. Therefore, radioactivity was studied in order to understand the nature of nuclear decay. Obviously, the radiation was generated by quantum transitions, but it was not entirely clear which ones. The Curies mined pure radium and polonium, working almost by hand in uranium ore, to answer this question.

Radiation charge

Rutherford did much to study the structure of the atom and made a contribution to the study of how the fission of the atomic nucleus occurs. The scientist placed the radiation emitted by a radioactive element in a magnetic field and got an amazing result. It turned out that radiation consists of three components: one was neutral, and the other two were positively and negatively charged. The study of nuclear fission began with the determination of its components. It was proved that the nucleus can divide, give up part of its positive charge.

The structure of the nucleus

Later it turned out that the atomic nucleus consists not only of positively charged particles of protons, but also of neutral particles of neutrons. Together they are called nucleons (from the English "nucleus", the nucleus). However, scientists again ran into a problem: the mass of the nucleus (that is, the number of nucleons) did not always correspond to its charge. In hydrogen, the nucleus has a charge of +1, and the mass can be three, and two, and one. Helium next in the periodic table has a nuclear charge of +2, while its nucleus contains from 4 to 6 nucleons. More complex elements can have many more different masses for the same charge. Such variations of atoms are called isotopes. Moreover, some isotopes turned out to be quite stable, while others quickly decayed, since they were characterized by nuclear fission. What principle corresponded to the number of nucleons of the stability of nuclei? Why did the addition of just one neutron to a heavy and quite stable nucleus lead to its splitting, to the release of radioactivity? Oddly enough, the answer to this important question has not yet been found. Empirically, it turned out that stable configurations of atomic nuclei correspond to certain amounts of protons and neutrons. If there are 2, 4, 8, 50 neutrons and/or protons in the nucleus, then the nucleus will definitely be stable. These numbers are even called magic (and adult scientists, nuclear physicists, called them that). Thus, the fission of nuclei depends on their mass, that is, on the number of nucleons included in them.

Drop, shell, crystal

It has not yet been possible to determine the factor responsible for the stability of the core. There are many theories of the model. The three most famous and developed ones often contradict each other on various issues. According to the first, the nucleus is a drop of a special nuclear liquid. Like water, it is characterized by fluidity, surface tension, coalescence and decay. In the shell model, there are also certain energy levels in the nucleus, which are filled with nucleons. The third asserts that the nucleus is a medium that is capable of refracting special waves (de Broglie), while the refractive index is. However, not a single model has yet been able to fully describe why, at a certain critical mass of this particular chemical element, the splitting of the nucleus begins.

What is the decay

Radioactivity, as mentioned above, was found in substances that can be found in nature: uranium, polonium, radium. For example, freshly mined, pure uranium is radioactive. The splitting process in this case will be spontaneous. Without any external influences, a certain number of uranium atoms will emit alpha particles, spontaneously converting into thorium. There is an indicator called the half-life. It shows for what period of time from the initial number of the part about half will remain. Each radioactive element has its own half-life - from fractions of a second for California to hundreds of thousands of years for uranium and cesium. But there is also forced radioactivity. If the nuclei of atoms are bombarded with protons or alpha particles (helium nuclei) with high kinetic energy, they can "split". The mechanism of transformation, of course, is different from how mother's favorite vase is broken. However, there is a certain analogy.

Atom energy

So far, we have not answered a practical question: where does the energy come from during nuclear fission. To begin with, it must be clarified that during the formation of a nucleus, special nuclear forces act, which are called the strong interaction. Since the nucleus is made up of many positive protons, the question remains how they stick together, because the electrostatic forces must push them away from each other quite strongly. The answer is both simple and not at the same time: the nucleus is held together by a very fast exchange between nucleons of special particles - pi-mesons. This connection lives incredibly short. As soon as the exchange of pi-mesons stops, the nucleus decays. It is also known for certain that the mass of a nucleus is less than the sum of all its constituent nucleons. This phenomenon is called the mass defect. In fact, the missing mass is the energy that is expended to maintain the integrity of the nucleus. As soon as some part is separated from the nucleus of an atom, this energy is released and converted into heat in nuclear power plants. That is, the energy of nuclear fission is a clear demonstration of the famous Einstein formula. Recall that the formula says: energy and mass can turn into each other (E=mc 2).

Theory and practice

Now we will tell you how this purely theoretical discovery is used in life to produce gigawatts of electricity. First, it should be noted that controlled reactions use forced nuclear fission. Most often it is uranium or polonium, which is bombarded by fast neutrons. Secondly, it is impossible not to understand that nuclear fission is accompanied by the creation of new neutrons. As a result, the number of neutrons in the reaction zone can increase very quickly. Each neutron collides with new, still intact nuclei, splits them, which leads to an increase in heat release. This is the nuclear fission chain reaction. An uncontrolled increase in the number of neutrons in a reactor can lead to an explosion. This is exactly what happened in 1986 at the Chernobyl nuclear power plant. Therefore, in the reaction zone there is always a substance that absorbs excess neutrons, preventing a catastrophe. It is graphite in the form of long rods. The rate of nuclear fission can be slowed down by immersing the rods in the reaction zone. The equation is drawn up specifically for each active radioactive substance and the particles bombarding it (electrons, protons, alpha particles). However, the final energy output is calculated according to the conservation law: E1+E2=E3+E4. That is, the total energy of the original nucleus and particle (E1 + E2) must be equal to the energy of the resulting nucleus and the energy released in free form (E3 + E4). The nuclear reaction equation also shows what kind of substance is obtained as a result of decay. For example, for uranium U=Th+He, U=Pb+Ne, U=Hg+Mg. The isotopes of chemical elements are not given here, but this is important. For example, there are as many as three possibilities for the fission of uranium, in which different isotopes of lead and neon are formed. In almost one hundred percent of cases, the nuclear fission reaction produces radioactive isotopes. That is, the decay of uranium produces radioactive thorium. Thorium can decay to protactinium, that to actinium, and so on. Both bismuth and titanium can be radioactive in this series. Even hydrogen, which contains two protons in the nucleus (at the rate of one proton), is called differently - deuterium. Water formed with such hydrogen is called heavy water and fills the primary circuit in nuclear reactors.

Non-peaceful atom

Expressions such as "arms race", "cold war", "nuclear threat" may seem historical and irrelevant to modern man. But once upon a time, every news release almost all over the world was accompanied by reports about how many types of nuclear weapons were invented and how to deal with them. People built underground bunkers and stocked up in case of a nuclear winter. Entire families worked to build the shelter. Even the peaceful use of nuclear fission reactions can lead to disaster. It would seem that Chernobyl taught humanity to be careful in this area, but the elements of the planet turned out to be stronger: the earthquake in Japan damaged the very reliable fortifications of the Fukushima nuclear power plant. The energy of a nuclear reaction is much easier to use for destruction. Technologists only need to limit the force of the explosion, so as not to accidentally destroy the entire planet. The most "humane" bombs, if you can call them that, do not pollute the surroundings with radiation. In general, most often they use an uncontrolled chain reaction. What they strive to avoid at nuclear power plants by all means is achieved in bombs in a very primitive way. For any naturally radioactive element, there is a certain critical mass of pure substance in which a chain reaction is born by itself. For uranium, for example, it is only fifty kilograms. Since uranium is very heavy, it is only a small metal ball 12-15 centimeters in diameter. The first atomic bombs dropped on Hiroshima and Nagasaki were made exactly according to this principle: two unequal parts of pure uranium simply combined and generated a terrifying explosion. Modern weapons are probably more sophisticated. However, one should not forget about the critical mass: there must be barriers between small volumes of pure radioactive material during storage, preventing the parts from connecting.

Sources of radiation

All elements with a nuclear charge greater than 82 are radioactive. Almost all lighter chemical elements have radioactive isotopes. The heavier the nucleus, the shorter its lifetime. Some elements (such as California) can only be obtained artificially - by colliding heavy atoms with lighter particles, most often in accelerators. Since they are very unstable, they do not exist in the earth's crust: during the formation of the planet, they very quickly disintegrated into other elements. Substances with lighter nuclei, such as uranium, can be mined. This process is long, uranium suitable for extraction, even in very rich ores, contains less than one percent. The third way, perhaps, indicates that a new geological epoch has already begun. This is the extraction of radioactive elements from radioactive waste. After fuel is spent at a power plant, on a submarine or aircraft carrier, a mixture of the original uranium and the final substance, the result of fission, is obtained. At the moment, this is considered solid radioactive waste and there is an acute question of how to dispose of them so that they do not pollute the environment. However, it is likely that in the near future ready-made concentrated radioactive substances (for example, polonium) will be extracted from these wastes.

In 1934, E. Fermi decided to obtain transuranium elements by irradiating 238 U with neutrons. The idea of ​​E. Fermi was that as a result of the β - decay of the 239 U isotope, a chemical element with the serial number Z = 93 is formed. However, it was not possible to identify the formation of the 93rd element. Instead, as a result of the radiochemical analysis of radioactive elements performed by O. Hahn and F. Strassmann, it was shown that one of the products of uranium irradiation with neutrons is barium (Z = 56) - a chemical element of medium atomic weight, while, according to the assumption of the Fermi theory transuranium elements should have been obtained.
L. Meitner and O. Frisch suggested that as a result of the capture of a neutron by a uranium nucleus, the compound nucleus breaks up into two parts

92 U + n → 56 Ba + 36 Kr + xn.

The process of uranium fission is accompanied by the appearance of secondary neutrons (x > 1) that can cause the fission of other uranium nuclei, which opens up the potential for a fission chain reaction to occur - one neutron can give rise to a branched chain of fission of uranium nuclei. In this case, the number of separated nuclei should increase exponentially. N. Bohr and J. Wheeler calculated the critical energy required for the 236 U nucleus, formed as a result of the capture of a neutron by the 235 U isotope, to split. This value is 6.2 MeV, which is less than the excitation energy of the 236 U isotope formed during the capture of a thermal neutron 235 U. Therefore, when thermal neutrons are captured, a fission chain reaction of 235 U is possible. For the most common isotope 238 U, the critical energy is 5.9 MeV, while when a thermal neutron is captured, the excitation energy of the resulting 239 U nucleus is only 5.2 MeV. Therefore, the chain reaction of fission of the most common in nature isotope 238 U under the action of thermal neutrons is impossible. In one act of fission, an energy of ≈ 200 MeV is released (for comparison, in chemical combustion reactions, an energy of ≈ 10 eV is released in one act of reaction). The possibility of creating conditions for a fission chain reaction opened up prospects for using the energy of a chain reaction to create atomic reactors and atomic weapons. The first nuclear reactor was built by E. Fermi in the USA in 1942. In the USSR, the first nuclear reactor was launched under the leadership of I. Kurchatov in 1946. In 1954, the world's first nuclear power plant began operating in Obninsk. Currently, electrical energy is generated in about 440 nuclear reactors in 30 countries around the world.
In 1940, G. Flerov and K. Petrzhak discovered the spontaneous fission of uranium. The following figures testify to the complexity of the experiment. The partial half-life with respect to spontaneous fission of the 238 U isotope is 10 16 –10 17 years, while the decay period of the 238 U isotope is 4.5∙10 9 years. The main decay channel for the 238 U isotope is α-decay. In order to observe the spontaneous fission of the 238 U isotope, it was necessary to register one fission event against the background of 10 7 –10 8 α-decay events.
The probability of spontaneous fission is mainly determined by the permeability of the fission barrier. The probability of spontaneous fission increases with an increase in the charge of the nucleus, since. this increases the division parameter Z 2 /A. In Z isotopes< 92-95 деление происходит преимущественно с образованием двух осколков деления с отношением масс тяжёлого и лёгкого осколков 3:2. В изотопах Z >100, symmetrical fission predominates with the formation of fragments of the same mass. As the charge of the nucleus increases, the proportion of spontaneous fission increases in comparison with α-decay.

Nuclear fission. Story

1934- E. Fermi, irradiating uranium with thermal neutrons, found radioactive nuclei among the reaction products, the nature of which could not be established.
L. Szilard put forward the idea of ​​a nuclear chain reaction.

1939− O. Hahn and F. Strassmann discovered barium among the reaction products.
L. Meitner and O. Frisch announced for the first time that under the action of neutrons, uranium was fissioned into two fragments comparable in mass.
N. Bohr and J. Wheeler gave a quantitative interpretation of nuclear fission by introducing the fission parameter.
Ya. Frenkel developed the drop theory of nuclear fission by slow neutrons.
L. Szilard, E. Wigner, E. Fermi, J. Wheeler, F. Joliot-Curie, Ya. Zeldovich, Yu. Khariton substantiated the possibility of a nuclear fission chain reaction occurring in uranium.

1940− G. Flerov and K. Petrzhak discovered the phenomenon of spontaneous fission of U uranium nuclei.

1942− E. Fermi carried out a controlled fission chain reaction in the first atomic reactor.

1945− The first test of nuclear weapons (Nevada, USA). Atomic bombs were dropped on the Japanese cities of Hiroshima (August 6) and Nagasaki (August 9).

1946− Under the leadership of I.V. Kurchatov, the first reactor in Europe was launched.

1954− The world's first nuclear power plant was launched (Obninsk, USSR).

Nuclear fission.Since 1934, E. Fermi began to use neutrons to bombard atoms. Since then, the number of stable or radioactive nuclei obtained by artificial transformation has increased to many hundreds, and almost all places in the periodic table have been filled with isotopes.
The atoms arising in all these nuclear reactions occupied the same place in the periodic table as the bombarded atom, or neighboring places. Therefore, the proof by Hahn and Strassmann in 1938 of the fact that when neutrons bombard the last element of the periodic system−uranium−decay into elements that are in the middle parts of the periodic system. There are various types of decay here. The atoms that arise are mostly unstable and immediately decay further; some have half-lives measured in seconds, so Hahn had to use the analytical Curie method to prolong such a fast process. It is important to note that the elements in front of uranium, protactinium and thorium, also show similar decay under the action of neutrons, although higher neutron energy is required for the decay to begin than in the case of uranium. Along with this, in 1940, G. N. Flerov and K. A. Petrzhak discovered spontaneous fission of the uranium nucleus with the longest half-life known until then: about 2· 10 15 years; this fact becomes clear due to the neutrons released in the process. So it was possible to understand why the "natural" periodic system ends with the three named elements. Transuranium elements are now known, but they are so unstable that they quickly decay.
The fission of uranium by means of neutrons now makes it possible to use atomic energy, which has already been imagined by many as "the dream of Jules Verne."

M. Laue, History of Physics

1939 O. Hahn and F. Strassmann, irradiating uranium salts with thermal neutrons, discovered among the reaction products barium (Z = 56)

Otto Gunn (1879 – 1968)

Nuclear fission is the splitting of a nucleus into two (rarely three) nuclei with similar masses, which are called fission fragments. During fission, other particles also arise - neutrons, electrons, α-particles. As a result of fission, an energy of ~200 MeV is released. Fission can be spontaneous or forced under the action of other particles, most often neutrons.
A characteristic feature of fission is that fission fragments, as a rule, differ significantly in mass, i.e., asymmetric fission predominates. Thus, in the case of the most probable fission of the uranium isotope 236 U, the fragment mass ratio is 1.46. A heavy fragment has a mass number of 139 (xenon), and a light fragment has a mass number of 95 (strontium). Taking into account the emission of two prompt neutrons, the considered fission reaction has the form

Nobel Prize in Chemistry
1944 - O. Gan.
For the discovery of the fission reaction of uranium nuclei by neutrons.

Fission Shards

Dependence of the average masses of light and heavy groups of fragments on the mass of the fissile nucleus.

Discovery of nuclear fission. 1939

I came to Sweden, where Lise Meitner suffered from loneliness, and as a devoted nephew, I decided to visit her at Christmas. She lived in the small hotel Kungälv near Gothenburg. I caught her at breakfast. She considered the letter she had just received from Han. I was very skeptical about the content of the letter, which reported the formation of barium by irradiating uranium with neutrons. However, she was attracted by this opportunity. We walked in the snow, she walked, I skied (she said that she could do this way without falling behind me, and she proved it). By the end of the walk we were already able to formulate some conclusions; the nucleus did not split, and pieces did not fly off from it, but it was a process that rather resembled the drop model of the Bohr nucleus; like a drop, the nucleus could elongate and divide. I then investigated how the electric charge of the nucleons reduces the surface tension, which, as I was able to establish, drops to zero at Z = 100, and possibly very low for uranium. Lise Meitner was engaged in determining the energy released during each decay due to a mass defect. She had a very clear idea of ​​the mass defect curve. It turned out that due to electrostatic repulsion, fission elements would acquire an energy of about 200 MeV, and this just corresponded to the energy associated with a mass defect. Therefore, the process could proceed purely classically without involving the concept of passing through a potential barrier, which, of course, would be useless here.
We spent two or three days together over Christmas. Then I returned to Copenhagen and barely had time to tell Bohr about our idea at the very moment when he was already boarding the steamer for the USA. I remember how he slapped his forehead as soon as I began to speak and exclaimed: “Oh, what fools we were! We should have noticed this sooner." But he did not notice, and no one noticed.
Lise Meitner and I wrote an article. At the same time, we constantly kept in touch by long-distance telephone Copenhagen - Stockholm.

O. Frisch, Memoirs. UFN. 1968. T. 96, issue 4, p. 697.

Spontaneous nuclear fission

In the experiments described below, we used the method first proposed by Frisch for recording nuclear fission processes. An ionization chamber with plates coated with a layer of uranium oxide is connected to a linear amplifier tuned in such a way that α particles emitted from uranium are not registered by the system; the impulses from the fragments, which are much larger than the impulses from the α-particles, unlock the output thyratron and are considered a mechanical relay.
An ionization chamber was specially designed in the form of a multilayer flat capacitor with a total area of ​​15 plates of 1000 cm. 2 .
In the very first experiments with an amplifier tuned to count the fragments, it was possible to observe spontaneous (in the absence of a neutron source) pulses on a relay and an oscilloscope. The number of these impulses was small (6 in 1 hour), and it is quite understandable, therefore, that this phenomenon could not be observed with cameras of the usual type ...
We tend to think that the effect we observe must be attributed to the fragments resulting from the spontaneous fission of uranium ...

Spontaneous fission should be attributed to one of the unexcited U isotopes with half-lives derived from an evaluation of our results:

U 238 – 10 16 ~ 10 17 years,
U 235 – 10 14 ~ 10 15 years,
U 234 – 10 12 ~ 10 13 years.

Isotope decay 238 U

Spontaneous nuclear fission

Half-lives of spontaneously fissile isotopes Z = 92 - 100

The first experimental system with a uranium-graphite lattice was built in 1941 under the direction of E. Fermi. It was a graphite cube with a rib 2.5 m long, containing about 7 tons of uranium oxide, enclosed in iron vessels, which were placed in the cube at equal distances from each other. A RaBe neutron source was placed at the bottom of the uranium-graphite lattice. The multiplication factor in such a system was ≈0.7. The uranium oxide contained from 2 to 5% impurities. Further efforts were directed towards obtaining purer materials, and by May 1942, uranium oxide was obtained, in which the impurity was less than 1%. To ensure a fission chain reaction, it was necessary to use a large amount of graphite and uranium - on the order of several tons. The impurities were to be less than a few parts per million. The reactor, assembled by the end of 1942 by Fermi at the University of Chicago, had the shape of an incomplete spheroid cut off from above. It contained 40 tons of uranium and 385 tons of graphite. On the evening of December 2, 1942, after the neutron absorber rods were removed, it was discovered that a nuclear chain reaction was taking place inside the reactor. The measured coefficient was 1.0006. Initially, the reactor operated at a power level of 0.5 W. By December 12, its power was increased to 200 watts. Subsequently, the reactor was moved to a safer place, and its power was increased to several kW. In this case, the reactor consumed 0.002 g of uranium-235 per day.

The first nuclear reactor in the USSR

The building for the first F-1 research nuclear reactor in the USSR was ready by June 1946.
After all the necessary experiments were carried out, the reactor control and protection system was developed, the dimensions of the reactor were established, all the necessary experiments were carried out with reactor models, the neutron density was determined on several models, graphite blocks were obtained (the so-called nuclear purity) and (after neutron-physical checks) uranium blocks, in November 1946 began the construction of the F-1 reactor.
The total radius of the reactor was 3.8 m. It required 400 tons of graphite and 45 tons of uranium. The reactor was assembled in layers, and at 3 pm on December 25, 1946, the last, 62nd layer was assembled. After the extraction of the so-called emergency rods, the control rod was lifted, the neutron density began to count, and at 18:00 on December 25, 1946, the first reactor in the USSR came to life. It was an exciting victory for the scientists - the creators of the nuclear reactor and for the entire Soviet people. A year and a half later, on June 10, 1948, the industrial reactor with water in the channels reached a critical state and soon began the industrial production of a new type of nuclear fuel - plutonium.