Nuclear reactions: simple and clear. Nuclear reactions: types, laws

Rachek Maria, Yesman Vitalia, Rumyantseva Victoria

This research project was completed by 9th grade students. It is a leading task in the study by schoolchildren of the topic "The structures of the atom and the atomic nucleus. The use of the energy of atomic nuclei" in the 9th grade physics course. The aim of the project is to clarify the conditions for the occurrence of nuclear reactions and the principles of operation of nuclear power plants.

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Municipal budgetary educational institution

Secondary school No. 14

Name of the Hero of the Soviet Union

Anatoly Perfilyev

G . Alexandrov

Research work in physics

"Nuclear Reactions"

Completed

pupils

9B class:

Rachek Maria,

Rumyantseva Victoria,

Yesman Vitalia

teacher

Romanova O.G.

2015

Project plan

Introduction

Theoretical part

• Nuclear power.

Conclusion

Bibliography

Introduction

Relevance :

One of the most important problems facing humanity is the energy problem. Energy consumption is growing so rapidly that currently known fuel reserves will be exhausted in a relatively short time. The problem of "energy hunger" is not solved by the use of energy from so-called renewable sources (energy of rivers, wind, sun, sea waves, deep heat of the Earth), since they can provide at best only 5-10% of our needs. In this regard, in the middle of the 20th century, it became necessary to search for new sources of energy.

Currently, the real contribution to the energy supply is made by nuclear energy, namely, nuclear power plants (abbreviated NPP). Therefore, we decided to find out whether nuclear power plants are useful to mankind.

Goals of the work:

1. Find out the conditions for the occurrence of nuclear reactions.
2. Find out the principles of operation of nuclear power plants, as well as find out whether it has a good or bad effect on the environment and on humans.

In order to achieve the goal, we have set the following tasks:

1. Learn the structure of the atom, its composition, what is radioactivity.
2. Explore the uranium atom. Explore a nuclear reaction.
3. Explore the principle of operation of nuclear engines.

Research methods:

1. Theoretical part - reading literature on nuclear reactions.

Theoretical part.

History of the atom and radioactivity. The structure of the atom.

The assumption that all bodies are composed of the smallest particles was made by the ancient Greek philosophers Leucippus and Democritus about 2500 thousand years ago. These particles are called "atom", which means "indivisible". An atom is the smallest particle of matter, the simplest, having no constituent parts.

But around the middle of the 19th century, experimental facts began to appear that cast doubt on the idea of ​​the indivisibility of atoms. The results of these experiments suggested that atoms have a complex structure and that they contain electrically charged particles.

The most striking evidence of the complex structure of the atom was the discovery of the phenomenonradioactivitytaken by the French physicist Henri Becquerel in 1896. He discovered that the chemical element uranium spontaneously (i.e. without external interactions) emits previously unknown invisible rays, which were later namedradioactive radiation. Since radioactive radiation had unusual properties, many scientists began to study it. It turned out that not only uranium, but also some other chemical elements (for example, radium) also spontaneously emit radioactive rays. The ability of atoms of some chemical elements to spontaneous radiation began to be called radioactivity (from Latin radio - I radiate and activus - effective).

Becquerel came up with the idea: is not any luminescence accompanied by x-rays? To test his guess, he took several compounds, including one of the uranium salts, which phosphorescent yellow-green light. After illuminating it with sunlight, he wrapped the salt in black paper and placed it in a dark closet on a photographic plate, also wrapped in black paper. Some time later, having shown the plate, Becquerel really saw the image of a piece of salt. But luminescent radiation could not pass through the black paper, and only X-rays could illuminate the plate under these conditions. Becquerel repeated the experiment several times with equal success. At the end of February 1896, at a meeting of the French Academy of Sciences, he made a report on the X-ray emission of phosphorescent substances. After some time, a plate was accidentally developed in Becquerel's laboratory, on which lay uranium salt, not irradiated by sunlight. She, of course, did not phosphoresce, but the imprint on the plate turned out. Then Becquerel began to test various compounds and minerals of uranium (including those that do not show phosphorescence), as well as metallic uranium. The plate was constantly lit up. By placing a metal cross between the salt and the plate, Becquerel obtained the weak contours of the cross on the plate. Then it became clear that new rays were discovered that pass through opaque objects, but are not X-rays.

Becquerel shares his discovery with the scientists with whom he collaborated. In 1898, Marie Curie and Pierre Curie discovered the radioactivity of thorium, and later they discovered the radioactive elements polonium and radium. They found that all uranium compounds and, to the greatest extent, uranium itself have the property of natural radioactivity. Becquerel returned to the luminophores that interested him. True, he made another major discovery related to radioactivity. Once, for a public lecture, Becquerel needed a radioactive substance, he took it from the Curies and put the test tube in his vest pocket. After giving a lecture, he returned the radioactive preparation to the owners, and the next day he found redness of the skin in the form of a test tube on the body under the vest pocket. Becquerel told Pierre Curie about this, and he set up an experiment: for ten hours he wore a test tube with radium tied to his forearm. A few days later he also developed redness, which then turned into a severe ulcer, from which he suffered for two months. Thus, the biological effect of radioactivity was discovered for the first time.

In 1899, as a result of an experiment conducted under the guidance of the English physicist Ernest Rutherford, it was found that the radioactive radiation of radium is inhomogeneous, i.e. has a complex composition. In the middle there is a stream (radiation) that does not have an electric charge, and 2 streams of charged particles lined up on the sides. Positively charged particles are called alpha particles, which are fully ionized helium atoms, and negatively charged particles, beta particles, which are electrons. Neutral are called gamma particles or gamma quanta. Gamma radiation, as it turned out later, is one of the ranges of electromagnetic radiation.

Because it was known that the atom as a whole is neutral, the phenomenon of radioactivity allowed scientists to create a rough model of the atom. The first person to do this was the English physicist Joseph John Thomson, who created one of the first models of the atom in 1903. The model was a sphere, throughout the entire volume of which a positive charge was evenly distributed. Inside the ball were electrons, each of which could oscillate around its equilibrium position. The model resembled a cake with raisins in shape and structure. The positive charge is equal in absolute value to the total negative charge of the electrons, so the charge of the atom as a whole is zero.

Thomson's model of the structure of the atom needed experimental verification, which was taken up in 1911 by Rutherford. He conducted experiments and came to the conclusion that the model of the atom is a ball, in the center of which there is a positively charged nucleus, which occupies a small volume of the entire atom. Electrons move around the nucleus, the mass of which is much less. An atom is electrically neutral because the charge of the nucleus is equal to the modulus of the total charge of the electrons. Rutherford also found that the nucleus of an atom has a diameter of about 10-14 – 10 -15 m, i.e. it is hundreds of thousands of times smaller than an atom. It is the nucleus that undergoes a change during radioactive transformations, i.e. radioactivity is the ability of some atomic nuclei to spontaneously transform into other nuclei with the emission of particles. In order to register (see) particles, in 1908 the German physicist Hans Geiger invented the so-called Geiger counter.

Later, positively charged particles in an atom were called protons, and negative ones - neutrons. Protons and neutrons are collectively known as nucleons.

fission of uranium. Chain reaction.

The fission of uranium nuclei during its bombardment with neutrons was discovered in 1939 by the German scientists Otto Hahn and Fritz Strassmann.

Let's consider the mechanism of this phenomenon. Having absorbed an extra neutron, the nucleus comes into action and deforms, acquiring an elongated shape.

There are 2 types of forces in the nucleus: electrostatic repulsive forces between protons, tending to break the nucleus, and nuclear forces of attraction between all nucleons, due to which the nucleus does not decay. But the nuclear forces are short-range, so in an elongated nucleus they can no longer hold the parts of the nucleus that are very distant from each other. Under the action of electrostatic forces, the nucleus is torn into two parts, which scatter in different directions at great speed and emit 2-3 neutrons. Part of the internal energy is converted into kinetic energy. Fragments of the nucleus quickly slow down in the environment, as a result of which their kinetic energy is converted into the internal energy of the environment. With the simultaneous fission of a large number of uranium nuclei, the internal energy of the medium surrounding uranium and, accordingly, its temperature increase. Thus, the reaction of fission of uranium nuclei goes with the release of energy into the environment. The energy is colossal. With the complete fission of all the nuclei present in 1 g of uranium, as much energy is released as is released during the combustion of 2.5 tons of oil. To convert the internal energy of atomic nuclei into electrical energy, nuclear fission chain reactions are used, based on the fact that 2-3 neutrons released during the fission of the first nucleus can take part in the fission of other nuclei that capture them. To maintain the continuity of the chain reaction, it is important to take into account the mass of uranium. If the mass of uranium is too small, then the neutrons fly out of it without meeting the nucleus on their way. The chain reaction stops. The larger the mass of a piece of uranium, the larger its dimensions and the longer the path that neutrons travel in it. The probability of neutrons meeting with atomic nuclei increases. Accordingly, the number of nuclear fissions and the number of emitted neutrons increase. The number of neutrons that appeared after the fission of nuclei is equal to the number of neutrons lost, so the reaction can continue for a long time. In order for the reaction not to stop, it is necessary to take a mass of uranium of a certain value - critical. If the mass of uranium is more than critical, then as a result of a sharp increase in free neutrons, the chain reaction leads to an explosion.

Nuclear reactor. Nuclear reaction. Converting the internal energy of atomic nuclei into electrical energy.

Nuclear reactor - This is a device in which a controlled nuclear chain reaction is carried out, accompanied by the release of energy. The first nuclear reactor, called SR-1, was built in December 1942 in the USA under the leadership of E. Fermi. At present, according to the IAEA, there are 441 reactors in the world in 30 countries. Another 44 reactors are under construction.

In a nuclear reactor, uranium-235 is mainly used as a fissile material. Such a reactor is called a slow neutron reactor. moderator Neutrons can be different substances:

1. Water . The advantages of ordinary water as a moderator are its availability and low cost. The disadvantages of water are the low boiling point (100 °C at a pressure of 1 atm) and the absorption of thermal neutrons. The first drawback is eliminated by increasing the pressure in the primary circuit. The absorption of thermal neutrons by water is compensated by the use of nuclear fuel based on enriched uranium.
2. Heavy water . Heavy water differs little from ordinary water in its chemical and thermophysical properties. It practically does not absorb neutrons, which makes it possible to use natural uranium as a nuclear fuel in reactors with a heavy water moderator. The disadvantage of heavy water is its high cost.
3. Graphite . Reactor graphite is obtained artificially from a mixture of petroleum coke and coal tar. First, blocks are pressed from the mixture, and then these blocks are thermally treated at a high temperature. Graphite has a density of 1.6-1.8 g/cm3. It sublimates at a temperature of 3800-3900 °C. Graphite heated in air to 400 °C ignites. Therefore, in power reactors, it is contained in an atmosphere of inert gas (helium, nitrogen).
4. Beryllium . One of the best retarders. It has a high melting point (1282°C) and thermal conductivity, and is compatible with carbon dioxide, water, air, and some liquid metals. However, helium appears in the threshold reaction, therefore, under intense irradiation with fast neutrons, gas accumulates inside beryllium, under the pressure of which beryllium swells. The use of beryllium is also limited by its high cost. In addition, beryllium and its compounds are highly toxic. Beryllium is used to make reflectors and water displacers in the core of research reactors.

Parts of a slow neutron reactor: in the core there is nuclear fuel in the form of uranium rods and a neutron moderator (for example, water), a reflector (a layer of matter that surrounds the core) and a protective shell made of concrete. The reaction is controlled by control rods that effectively absorb neutrons. To start the reactor, they are gradually removed from the core. The neutrons and fragments of nuclei formed during this reaction, flying apart at high speed, fall into the water, collide with the nuclei of hydrogen and oxygen atoms, and give them part of their kinetic energy. At the same time, the water heats up, and after some time the slowed down neutrons again fall into the uranium rods and participate in nuclear fission. The active zone is connected to the heat exchanger by means of pipes, forming the first closed circuit. Pumps provide water circulation in it. The heated water passes through the heat exchanger, heats the water in the secondary coil and turns it into steam. Thus, the water in the core serves not only as a neutron moderator, but also as a coolant that removes heat. After the steam energy in the coil is converted into electrical energy. The steam turns the turbine, which drives the rotor of the electric current generator. The exhaust steam enters the condenser and turns into water. Then the whole cycle is repeated.

nuclear engineuses the energy of nuclear fission or fusion to create jet thrust. The traditional nuclear engine as a whole is a design of a nuclear reactor and the engine itself. The working fluid (more often - ammonia or hydrogen) is supplied from the tank to the reactor core, where, passing through the channels heated by the nuclear decay reaction, it is heated to high temperatures and then ejected through the nozzle, creating jet thrust.

Nuclear power.

Nuclear power- a field of technology based on the use of the fission reaction of atomic nuclei to generate heat and generate electricity. The nuclear energy sector is most significant in France, Belgium, Finland, Sweden, Bulgaria and Switzerland, i.e. in those industrialized countries where there is not enough natural energy resources. These countries generate between a quarter and a half of their electricity from nuclear power plants.

The first European reactor was created in 1946 in the Soviet Union under the leadership of Igor Vasilyevich Kurchatov. In 1954, the first nuclear power plant was put into operation in Obninsk. NPP advantages:

1. The main advantage is the practical independence from fuel sources due to the small amount of fuel used. In Russia, this is especially important in the European part, since the delivery of coal from Siberia is too expensive. The operation of a nuclear power plant is much cheaper than a thermal power plant. True, the construction of a thermal power plant is cheaper than the construction of a nuclear power plant.
2. A huge advantage of a nuclear power plant is its relative environmental cleanliness. At TPPs, the total annual emissions of harmful substances are approximately 13,000 tons per year for gas and 165,000 tons for pulverized coal TPPs. There are no such emissions at nuclear power plants. Thermal power plants consume 8 million tons of oxygen per year for fuel oxidation, while nuclear power plants do not consume oxygen at all. In addition, a coal plant gives a higher specific release of radioactive substances. Coal always contains natural radioactive substances; when coal is burned, they almost completely enter the external environment. Most radionuclides from thermal power plants are long-lived. Most of the radionuclides from nuclear power plants quickly decay, turning into non-radioactive.
3. For most countries, including Russia, the production of electricity at nuclear power plants is not more expensive than at pulverized-coal and, even more so, gas-oil thermal power plants. The advantage of nuclear power plants in the cost of electricity produced is especially noticeable during the so-called energy crises that began in the early 1970s. Falling oil prices automatically reduce the competitiveness of nuclear power plants.

The use of nuclear engines in modern times.

With the development of nuclear physics, the prospect of creating atomic power plants became more and more clear. The first practical step in this direction was taken by the Soviet Union, where in 1954. a nuclear power plant was built.

In 1959 The first nuclear-powered vessel in the world, the Lenin icebreaker, was put into operation under the flag of the USSR.

In the last years of the 19th century, the powerful Soviet nuclear-powered icebreakers Arktika and Sibir entered the Arctic watch...

Nuclear power has opened up especially great opportunities for submarines, making it possible to solve the two most urgent problems - increasing underwater speed and increasing the duration of swimming under water without surfacing. After all, the most advanced diesel-electric submarines cannot develop more than 18-20 knots under water, and even this speed is maintained only for about an hour, after which they are forced to surface to charge the batteries.

Under such conditions, at the direction of the Central Committee of the CPSU and the Soviet government, an atomic submarine fleet was created in our country in the shortest possible time. Soviet nuclear-powered submarines repeatedly crossed the Arctic Ocean under the ice, surfaced in the region of the North Pole. On the eve of the XXIII Congress of the CPSU, a group of nuclear submarines circumnavigated the world, passing about 22 thousand miles under water without surfacing ...

The main difference between a nuclear submarine and a steam-powered one is the replacement of a steam boiler with a reactor in which a controlled chain reaction of fission of nuclear fuel atoms is carried out with the release of heat used to produce steam in a steam generator.

The nuclear installation created a real prospect for submarines not only to equal speed with surface ships, but also to surpass them. As we know, in a submerged state, a submarine does not experience wave resistance, to overcome which high-speed surface displacement ships spend most of the power of the power plant.

The biological effect of radiation.

Radiation, by its very nature, is harmful to life. Small doses of radiation can “start” a not yet fully understood chain of events leading to cancer or genetic damage. At high doses, radiation can destroy cells, damage organ tissues and cause the death of an organism. Damage caused by high doses of radiation usually shows up within hours or days. Cancers, however, appear many years after exposure, usually not earlier than one to two decades. And congenital malformations and other hereditary diseases caused by damage to the genetic apparatus, by definition, appear only in the next or subsequent generations: these are children, grandchildren and more distant descendants of an individual who has been exposed to radiation.

Depending on the type of radiation, radiation dose and its conditions, various types of radiation injury are possible. These are acute radiation sickness (ARS) - from external exposure, ARS - from internal exposure, chronic radiation sickness, various clinical forms with predominantly local lesions of individual organs, which can be characterized by acute, subacute or chronic course; these are long-term consequences, among which the most significant is the occurrence of malignant tumors; degenerative and dystrophic processes (cataract, sterility, sclerotic changes). This also includes the genetic consequences observed in the offspring of exposed parents. The ionizing radiations that cause their development, due to their high penetrating ability, affect tissues, cells, intracellular structures, molecules and atoms anywhere in the body.

Living beings react differently to the effects of radiation, and the development of radiation reactions largely depends on the dose of radiation. Therefore, it is advisable to distinguish between: 1) the impact of small doses, up to about 10 rad; 2) exposure to medium doses, usually used for therapeutic purposes, which border on its upper limit with exposure to high doses. When exposed to radiation, there are reactions that occur immediately, early reactions, as well as late (remote) manifestations. The final result of irradiation often depends largely on the dose rate, various irradiation conditions, and especially on the nature of the radiations. This also applies to the field of application of radiation in clinical practice for therapeutic purposes.

Radiation affects people differently depending on gender and age, the state of the body, its immune system, etc., but it is especially strong on infants, children and adolescents.

Cancer is the most serious of all the consequences of human exposure to low doses. Extensive surveys covering 100,000 survivors of the atomic bombings of Hiroshima and Nagasaki have shown that so far cancer is the only cause of increased mortality in this population group.

Conclusion.

After conducting research, we found out that nuclear fuel and nuclear engines bring great benefits to humans. Thanks to them, a person found cheap sources of heat and energy (one nuclear power plant replaces several tens or even hundreds of conventional thermal power plants for a person), was able to get through the ice to the North Pole and sink to the bottom of the ocean. But all this works only when it is applied correctly, i.e. in the right amount and only for peaceful purposes. There have been many cases of explosions of nuclear power plants (Chernobyl, Fukushima) and explosions of atomic bombs (Hiroshima and Nagasaki).

But no one is protected from the consequences of radioactive waste. Many people suffer from radiation sickness and cancer caused by radiation. But we think that in a few years, scientists will come up with methods for disposing of radioactive waste without harm to health and invent cures for all these diseases.

Bibliography.

1. A. V. Pyoryshkin, E. M. Gutnik. "Textbook on physics for grade 9".
2. G. Kessler. "Nuclear energy".
3. R. G. Perelman. "Nuclear Engines".
4. E. Rutherford. Selected Scientific Works. The structure of the atom and artificial transformation.
5. https://en.wikipedia.org

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NUCLEAR REACTIONS IN NATURE - are divided into 2 classes: thermonuclear reactions and reactions under the action of nuclear particles and nuclear fission. The former require a temperature of ~ several million degrees for their implementation and occur only in the interiors of stars or during explosions of H-bombs. The latter occur in the atmosphere and lithosphere due to cosmic radiation and due to nuclear-active particles in the upper shells of the Earth. Fast cosmic particles (mean energy ~2 10 9 eV), entering the Earth's atmosphere, often cause complete splitting of atmospheric atoms (N, O) into lighter nuclear fragments, including neutrons. The formation rate of the latter reaches 2.6 neutrons (cm -2 sec -1). Neutrons interact predominantly with atmospheric N, providing a constant production of radioactive isotopes carbon C 14 (T 1/2 = 5568 years) and tritium H 3 (T 1/2 = 12.26 years) according to the following reactions N 14 + P\u003d C 14 + H 1; N 14+ n\u003d C 12 + H 3. The annual formation of radiocarbon in the earth's atmosphere is about 10 kg. The formation of radioactive Be 7 and Cl 39 in the atmosphere was also noted. Nuclear reactions in the lithosphere occur mainly due to α-particles and neutrons arising from the decay of long-lived radioactive elements (mainly U and Th). It should be noted the accumulation of He 3 in some mls containing Li (see. Helium isotopes in geology), the formation of individual isotopes of neon in euxenite, monazite, and other m-lahs according to the reactions: O 18 + He 4 \u003d Ne 21 + P; Fe 19 + He \u003d Na 22 + P; Na 22 → Ne 22 . The formation of argon isotopes in radioactive substances according to the reactions: Cl 35 + Not = Ar 38 + n; Cl 35 + He \u003d K 38 + H 1; K 38 → Ar 38. During spontaneous and neutron-induced fission of uranium, the formation of heavy isotopes of krypton and xenon is observed (see Xenon absolute age determination method). In the m-lakh of the lithosphere, the artificial fission of atomic nuclei causes the accumulation of certain isotopes in the amount of 10 -9 -10 -12% of the mass of the m-la.

Geological dictionary: in 2 volumes. - M.: Nedra. Edited by K. N. Paffengolts et al.. 1978 .

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Chem. transformations and nuclear processes, in which the appearance of an intermediate active particle (a free radical, an atom, an excited molecule in chemical transformations, a neutron in nuclear processes) causes a chain of transformations of the initial ones into c. Examples of chem. C. r ... Chemical Encyclopedia

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• Obtaining nuclear energy and rare and precious metals as a result of nuclear transformations. Binding Energy and Potential Energy of Electric Interaction of Electric Charges in Neutron, Deuter, Larin V.I. The first part of this book deals with various nuclear reactions to obtain energy and precious metals as a result of forced nuclear transformations of stable isotopes.…

Like chemical reactions, nuclear reactions can be endothermic and exothermic.

Nuclear reactions are divided into decay reactions and fusion reactions. A special type of nuclear reaction is nuclear fission. The timing of nuclear decay and nuclear fission means completely different types of reactions [ ].

1. History

The first artificially induced nuclear reaction was observed in the year by Ernest Rutherford, irradiating nitrogen with alpha particles. The reaction proceeded according to the scheme

.

2. Laws of conservation in nuclear reactions

During nuclear reactions, the general laws of conservation of energy, momentum, angular momentum, and electric charge are fulfilled.

In addition, there are a number of special conservation laws inherent in the nuclear interaction, for example, the law of conservation of baryon charge.

3. Energy yield of a nuclear reaction

If the sum of the rest masses of the particles in the reaction is greater than the sum of the rest masses of the particles after the reaction, then such a reaction occurs with the release of energy. This energy is called the energy yield of a nuclear reaction. The energy yield of a nuclear reaction is calculated by the formula ΔE = Δmc 2, where Δm is the mass defect, c is the speed of light.

4. Types of nuclear reactions

4.1. Nuclear fusion reactions

During nuclear fusion reactions, new, heavier nuclei are formed from light nuclei of elements.

Usually, fusion reactions are possible only under conditions where the nuclei have a large kinetic energy, since the forces of electrostatic repulsion prevent the approach of equally charged nuclei, creating the so-called Coulomb barrier.

This can be achieved artificially with the help of charged particle accelerators, in which ions, protons or α-particles are accelerated by an electric field, or thermonuclear reactors, where ions of a substance acquire kinetic energy due to thermal motion. In the latter case, we are talking about a thermonuclear fusion reaction.

4.1.1. Nuclear fusion in nature

In nature, fusion reactions began in the first minutes after the Big Bang. During the primary nucleosynthesis, only some light nuclei (deuterium, helium, lithium) were formed from protons.
Now nuclear reactions take place in the cores of stars, for example, in the Sun. The main process is the formation of a helium nucleus from four protons, which can occur either in a proton-proton chain or in the Bethe-Weizsäcker cycle.

In stars whose mass exceeds half M ☉ , other, heavier elements can be formed. This process begins with the formation of carbon nuclei in a triple α reaction. The resulting nuclei interact with protons and α-particles and thus form chemical elements up to the iron peak.

The formation of heavy nuclei (from iron to Bismuth) occurs in the shells of fairly massive stars at the red giant stage, mainly due to the s-process and, in part, due to the p-process. Navagchi (unstable) nuclei are formed during supernova explosions.

4.2. Nuclear decay reactions

Decay reactions are due to alpha and beta radioactivity. During alpha decay, an alpha particle 4 He flies out of the nucleus, and the mass number and charge number of the nucleus change to 4 and 2, respectively. During beta decay, an electron or a positron flies out of the nucleus, the mass number of the nucleus does not change, and the charge number increases or decreases by 1. Both types of decay occur spontaneously.

4.3. Nuclear fission

A small number of isotopes are capable of fission - a reaction in which the nucleus is divided into two large parts. Nuclear fission can take place spontaneously, so forced- under the influence of other particles, mainly - neutrons.

years, it was revealed that uranium-235 nuclei are capable of not only spontaneous fission(on two light nuclei) with the release of ~ 200 MeV energy and the emission of two or three neutrons, but also to forced division, triggered by neutrons. Considering that as a result of such a separation, neutrons are also emitted, which can cause new reactions of forced fission of neighboring uranium nuclei, the possibility of a nuclear chain reaction has become obvious. Such a reaction does not occur in nature only because natural uranium consists of 99.3% of the uranium-238 isotope, and only uranium-235, which only 0.7% is contained in natural uranium, is capable of fission reactions.

The mechanism of a nuclear fission reaction is as follows. Nuclear forces through the interaction of exchange virtual particles (in most cases pion-nucleon interaction occurs) have a non-central character. This means that nucleons cannot interact simultaneously with all nucleons in the nucleus, especially in multi-nucleon nuclei. With a large number of nucleons in the nucleus, this causes an asymmetry in the density of nuclear forces and a further asymmetry in the nucleon bond, and, consequently, an asymmetry in energy over the volume of the nucleus. The nucleus acquires a shape that differs significantly from spherical. In this case, the electrostatic interaction between protons can approach the strong interaction in terms of energy.

Thus, due to asymmetry, the fission energy barrier is overcome and the nucleus decays into lighter nuclei that are asymmetric in mass.

Sometimes the nucleus can tunnel into a state of lower energy.

5. Nuclear reactions in human life

5.1. Atomic bomb

The chain reaction of fission of atomic nuclei in the twentieth century began to be used in atomic bombs. Due to the fact that for an intense nuclear reaction it is necessary to have a critical mass (the mass necessary for the development of a chain reaction), then in order to carry out an atomic explosion, several parts with masses less than critical are combined, a supercritical mass is formed and a fission chain reaction occurs in it, accompanied by the release of a large amount of energy - an atomic explosion occurs.

5.2. Nuclear reactor

A nuclear reactor is used to convert the thermal energy of nuclear decay into electrical energy. As fuel in the reactor, a mixture of isotopes of uranium-235 and uranium-238 or plutonium-239 is used. When fast neutrons hit the nucleus of an atom of uranium-238, it is converted into plutonium-239 and its subsequent decay with the release of energy. The process can be cyclic, but this requires reactors operating on fast neutrons. Now, uranium-235 nuclide is used as the main component in reactors. For its interaction with fast neutrons, they must be slowed down. How to use a retarder:

According to the type of water used in the reactors, D 2 O or H 2 O, the reactors are divided into heavy water and light water respectively. In heavy water reactors, uranium-238 nuclide is used as fuel, in light water reactors - Uranium-235. Control rods containing isotopes of boron or cadmium are used to control the decay reaction and stop it. The energy that is released during the fission chain reaction is removed by the coolant. Therefore, it heats up, and when it enters the water, it heats it, turning it into steam (often the water itself is the coolant). The steam inverts a steam turbine which turns the rotor of an alternator.

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NUCLEAR REACTIONS
Nuclear reactions - transformations atomic nuclei when interacting with other nuclei,elementary particlesor quants. This definition delimits the nuclear reactions and processes of spontaneous transformation of nuclei during radioactive decay (see.Radioactivity), although in both cases we are talking about the formation of new nuclei.
Nuclear reactions carried out under the action of incident, or bombarding, particles ( neutrons p, protons p, deuterons d, electrons e, atomic nuclei various. elements) or quanta, which are irradiated with heavier nuclei contained in the target. According to the energies of the bombarding particles, nuclear reactions at low (< 1 МэВ), средних (1-100 МэВ) и высоких (>100 MeV) energies. Distinguish districts on light nuclei ( mass number target nuclei A< 50), ядрах ср. массы (50 < А < 100) и тяжелых ядрах (А > 100).
Nuclear reaction can occur if the two particles involved in it approach at a distance less than the diameter of the nucleus (about 10 -13 cm), i.e., at a distance at which intranuclear forces act and interact. between the constituent nucleons. If both involved in nuclear reactions particles - both the bombarding and the target core - are positively charged, then the approach of the particles is prevented by the repulsive force of the two put. charges, and the bombarding particle must overcome the so-called. Coulomb potential barrier. The height of this barrier depends on the charge of the bombarding particle and the charge of the target nucleus. For nuclei with atoms from cf. values atomic number , and bombarding particles with charge +1, the barrier height is about 10 MeV. In the event that in a nuclear reactions particles that do not have a charge are involved ( neutrons ), there is no Coulomb potential barrier, and nuclear reactions can flow with the participation of particles having thermal energy (i.e., energy corresponding to thermal vibrations atoms ).
The possibility of the occurrence of nuclear reactions not as a result of the bombardment of the target nuclei by incident particles, but due to the super-strong approach of the nuclei (i.e., the approach at distances comparable to the diameter of the nucleus) located in the solid matrix or on the surface solid body (for example, with the participation of nuclei deuterium gas atoms , dissolved in palladium ); so far (1995) reliable data on the implementation of such nuclear reactions ("cold fusion") no.
Nuclear reactions obey the same general laws of nature as ordinary chem. reactions (law of conservation of massand energy, conservation of charge, momentum). In addition, during the course of nuclear reactions there are also some specific laws that are not manifested in chem. reactions, for example, the law of conservation of baryon charge (baryons are heavyelementary particles).
Record nuclear reactions possible, as shown by the example of the transformation of Pu nuclei into Ku nuclei during irradiation of a plutonium target with nuclei not she :

From this entry it can be seen that the sums of charges on the left and right (94 + 10 = 104) and the sums mass numbers (242 + 22 = 259 + 5) are equal to each other. Since the symbol of chem. element uniquely indicates its atomic number (nuclear charge), then when writing nuclear reactions particle charge values ​​are usually not indicated. More often nuclear reactions write shorter. Yes, nuclear radionuclide formation reaction 14 C during irradiation of nuclei 14 N neutrons write down as follows: 14 N (n, p) 14 C.
In parentheses indicate first the bombarding particle or quantum, then, separated by a comma, the resulting light particles or quantum. In accordance with this method of recording, (n, p), (d, p), (n, 2n) and other nuclear reactions .
In the collision of the same particles, nuclear reactions can go in different ways. For example, when an aluminum target is irradiated neutrons may follow. nuclear reactions : 27 A1(n,) 28 A1, 27 A1(n, n) 27 A1, 27 A1(n, 2n) 26 A1, 27 A1(n, p) 27 Mg, 27 Al(n,) 24 Na, etc. The set of colliding particles is called the inlet channel of the nuclear reactions , and particles produced as a result of nuclear reactions , form the output channel.
Nuclear reactions can proceed with the release and absorption of energy Q. If we write the nuclear reaction as A(a, b)B, then for such a nuclear reactions the energy is: Q \u003d [(M A + M a) - (M in + M b)] x c 2, where M is the mass involved in the nuclear reactions particles; c is the speed of light. In practice, it is more convenient to use the values mass defects delta M (see. Nucleus atomic ), then the expression for calculating Q has the form: moreover, for reasons of convenience, it is usually expressed in kiloelectronvolts (keV, 1 amu = 931501.59 keV = 1.492443 x 10 -7 kJ).
The change in energy that accompanies nuclear reaction , can be 10 6 times or more higher than the energy released or absorbed during chemical. reactions. Therefore, with nuclear reactions the change in the masses of the interacting nuclei becomes noticeable: the energy released or absorbed is equal to the difference between the sums of the masses of the particles before and after the nuclear reactions . The ability to release huge amounts of energy in the implementation of nuclear reactions lies at the heart of nuclear energy . Investigation of the relationships between the energies of particles involved in nuclear reactions , as well as the relationship between the angles at which the resulting particles expand, constitutes a section of nuclear physics - the kinematics of nuclear reactions.
Nuclear reactions .
The nature of the interaction of an incident particle with a target nucleus depends on the individual properties of the interacting particles and the energy of the incident particle. An incident particle can enter the target core and fly out of it only by changing its trajectory. This phenomenon is called elastic interaction (or elastic scattering). In the above example with the participation of nuclei 27 A1, it corresponds to the nuclear reaction 27 A1(r, r) 27 A1. The nucleon of the bombarding particle, hitting the nucleus, can collide with the nucleon of the nucleus. If, in this case, the energy of one or both nucleons turns out to be greater than the energy needed to escape from the nucleus, then both of them (or at least one of them) will leave the nucleus. This is the so-called direct process. The time during which it flows corresponds to the time during which the bombarding particle passes through the space occupied by the target nucleus. It is estimated to be about 10 -22 s. The direct process is possible at high energies of the bombarding particle.
At medium and low energies of the bombarding particle, its excess energy is redistributed among many nucleons of the nucleus. This happens in a time of 10 -15 -10 -16 s. This time corresponds to the lifetime of the so-called compound nucleus of a nuclear system, which is formed in the course of a nuclear explosion. reactions as a result of the merger of the incident particle with the target nucleus. During this period, the excess energy received by the compound nucleus from the incident particle is redistributed. It can concentrate on one or more nucleons that make up the compound nucleus. As a result, the compound nucleus emits, for example, a deuteron d, a triton t, or a particle.
If the energy introduced into the compound nucleus by the incident particle turned out to be less than the height of the potential barrier, which must be overcome by the light particle emitted from the compound nucleus, then in this case the compound nucleus emits a quantum (radiative capture). As a result of the decay of the compound nucleus, a relatively heavy new nucleus is formed, which can be both in the main and inexcited state. In the latter case, there will be a gradual transition of the excited nucleus to the ground state.

Effective cross section of nuclear reactions .

Unlike most chemical reactions, in which the starting substances, taken in stoichiometric quantities, react with each other completely, nuclear reaction causes only a small fraction of all the bombarding particles that fell on the target. This is due to the fact that the nucleus occupies a negligible part of the volume atom , so that the probability of an incident particle passing through the target meeting the nucleus atom very small. The Coulomb potential barrier between the incident particle and the nucleus (with the same charge) also prevents nuclear reactions . For quantities. characteristics of the probability of a nuclear reactions use the concept of effective section a. It characterizes the probability of the transition of two colliding particles to a certain final state and is equal to the ratio of the number of such transitions per unit time to the number of bombarding particles passing per unit time through a unit area perpendicular to the direction of their motion. The effective section has the dimension of area and is comparable in order of magnitude to the cross-sectional area atomic nuclei (about 10 -28 m 2). Previously, an off-system unit of the effective section was used - barn (1 barn \u003d 10 -28 m 2).
Real values ​​for various nuclear reactions vary widely (from 10 -49 to 10 -22 m 2). The value depends on the nature of the bombarding particle, its energy, and, to a particularly large extent, on the properties of the irradiated nucleus. In the case of nuclear irradiation neutrons when varying energy neutrons you can observe the so-called. resonant capture neutrons , which is characterized by a resonant cross section. Resonance capture is observed when the kinetic energy neutron is close to the energy of one of the stationary states of the compound nucleus. The cross section corresponding to the resonant capture of a bombarding particle can exceed the nonresonant cross section by several orders of magnitude.
If a bombarding particle is capable of causing a nuclear reactions over several channels, then the sum of the effective cross sections of various processes occurring with a given irradiated nucleus is often called the total cross section.
Effective cross sections of nuclear reactions for various nuclei isotopes c.-l. elements are often very different from each other. Therefore, when using a mixture isotopes for nuclear reactions effective cross sections must be taken into account for each nuclide taking into account its prevalence in the mixture isotopes.
Nuclear exits reactions
Yields of nuclear reactions -number ratio acts of nuclear reactions the number of particles falling per unit area (1 cm 2 ) of the target usually does not exceed 10 -6 -10 -3 . For thin targets (simplistically, a target can be called thin, when passing through which the flow of bombarding particles does not noticeably weaken), the yield of nuclear reactions is proportional to the number of particles falling on 1 cm 2 of the target surface, the number of nuclei contained in 1 cm 2 of the target, and also to the value of the effective cross section of the nuclear reactions . Even when using such a powerful source of projectile particles as a nuclear reactor, within 1 hour it is possible, as a rule, to obtain in the implementation of nuclear reactions under the influence of neutrons no more than a few mg atoms containing new nuclei. Usually, the mass of a substance obtained in one or another nuclear reactions , significantly less.

bombarding particles.
For the implementation of nuclear reactions use neutrons n, protons p, deuterons d, tritons t, particles, heavy ions (12 C, 22 Ne, 40 Ar, etc.), electrons e and quanta. Sources neutrons (see neutron sources) when conducting nuclear reactions serve as: mixtures of metal Be and a suitable emitter, e.g. 226 Ra (the so-called ampoule sources), neutron generators, nuclear reactors. Because in most cases of nuclear reactions higher for neutrons with low energies (thermal neutrons ), then before directing the flow neutrons on the target, they are usually slowed down using paraffin, graphite and other materials. In case of slow neutrons basic. process for almost all nuclei - radiation capture - nuclear reaction type because the Coulomb barrier of the nucleus prevents the flight protons and particles. Under the influence neutron fission chain reactions .
In case of use as bombarding particles protons , deuterons, etc., flow carrying a positive charge, the bombarding particle is accelerated to high energies (from tens of MeV to hundreds of GeV) using various accelerators. This is necessary so that the charged particle can overcome the Coulomb potential barrier and enter the irradiated nucleus. When targets are irradiated with positively charged particles, max. nuclear outputs reactions achieved using deuterons. This is due to the fact that the binding energy proton and neutron in the deuteron is relatively small, and, accordingly, the distance between proton and neutron .
When deuterons are used as bombarding particles, only one nucleon often penetrates into the irradiated nucleus - proton or neutron , the second nucleon of the deuteron nucleus flies further, usually in the same direction as the incident deuteron. High effective cross sections can be achieved in nuclear reactions between deuterons and light nuclei at relatively low energies of incident particles (1-10 MeV). Therefore, nuclear reactions with the participation of deuterons can be carried out not only with the use of deuterons accelerated at the accelerator, but also by heating the mixture of interacting nuclei to a temperature of about 10 7 K. Such nuclear reactions called thermonuclear. Under natural conditions, they occur only in the depths of stars. On Earth, thermonuclear reactions involving deuterium, deuterium and tritium, deuterium and lithium and others were carried out with explosions thermonuclear (hydrogen) bombs.
For particles, the Coulomb barrier for heavy nuclei reaches ~25 MeV. Equally probable nuclear reactions and Nuclear Products reactions usually radioactive, for nuclear reactions - usually stable kernels.
For the synthesis of new superheavy chemical. elements, nuclear reactions , flowing with the participation of accelerator-accelerated heavy ions (22 Ne, 40 Ar, etc.). For example, for nuclear reactions m. b. synthesis fermium. For nuclear reactions with heavy ions characterized by a large number of output channels. For example, when bombarding nuclei with 232 Th ions 40 Ar, Ca, Ar, S, Si, Mg, Ne nuclei are formed.
For the implementation of nuclear reactions under the action of quanta, high-energy quanta (tens of MeV) are suitable. Quanta with lower energies experience only elastic scattering on nuclei. Nuclear processes occurring under the action of incident quanta reactions called photonuclear, these reactions reach 10 30 m 2.
Although electrons have a charge opposite to that of the nuclei, penetration electrons into the nucleus is possible only in those cases when nuclear radiation is used electrons , whose energy exceeds tens of MeV. To receive such electrons betatrons and other accelerators are used.
Nuclear research reactions give a variety of information about the internal structure of nuclei. Nuclear reactions involving neutrons allow you to get a huge amount of energy in nuclear reactors. As a result of nuclear fission reactions under the action of neutrons a large number of different radionuclides , which can be used, in particular, in chemistry like isotope tracers. In some cases, nuclear reactions allow you to receivelabeled compounds. Nuclear reactions are the basis activation analysis. With the help of nuclear reactions synthesized artificial chem. elements ( technetium, promethium, transuranic elements, transactinoid).

History of the discovery of uranium fission

Fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn and F. Strassmann. They managed to establish that when bombarding uranium nuclei with neutrons, elements of the middle part of the periodic system are formed: barium, krypton, etc. The Austrian physicist L. Meitner and the English physicist O. Frisch gave the correct interpretation of this fact. They explained the appearance of these elements by the decay of uranium nuclei, which captured a neutron, into two approximately equal parts. This phenomenon is called nuclear fission, and the resulting nuclei are called fission fragments.

Drop model of the nucleus

This fission reaction can be explained based on the drop model of the nucleus. In this model, the nucleus is considered as a drop of an electrically charged incompressible liquid. In addition to the nuclear forces acting between all the nucleons of the nucleus, protons experience an additional electrostatic repulsion, due to which they are located on the periphery of the nucleus. In the unexcited state, the electrostatic repulsion forces are compensated, so the nucleus has a spherical shape (Fig. 1).

Rice. one

After the capture of a neutron by a nucleus, an intermediate nucleus is formed, which is in an excited state. In this case, the neutron energy is evenly distributed among all nucleons, and the intermediate nucleus itself is deformed and begins to oscillate. If the excitation is small, then the nucleus (Fig. 1, b), freeing itself from excess energy by emitting ? -quantum or neutron, returns to a stable state. If the excitation energy is sufficiently high, then the deformation of the core during vibrations can be so large that a constriction is formed in it (Fig. 1c), similar to the constriction between two parts of a splitting liquid drop. Nuclear forces acting in a narrow waist can no longer resist the significant Coulomb force of repulsion of parts of the nucleus. The constriction breaks, and the nucleus breaks up into two "fragments" (Fig. 1d), which scatter in opposite directions.
Currently, about 100 different isotopes with mass numbers from about 90 to 145 are known, arising from the fission of this nucleus. Two typical fission reactions of this nucleus have the form:
.
Note that as a result of nuclear fission initiated by a neutron, new neutrons are produced that can cause fission reactions in other nuclei. The fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.
During the fission of the nuclei of heavy atoms (), a very large energy is released - about 200 MeV during the fission of each nucleus. About 80% of this energy is released in the form of fragment kinetic energy; the remaining 20% ​​is accounted for by the energy of the radioactive radiation of fragments and the kinetic energy of prompt neutrons.
The energy released during nuclear fission can be estimated using the specific binding energy of nucleons in the nucleus. The specific binding energy of nucleons in nuclei with a mass number A? 240 is about 7.6 MeV/nucleon, while in nuclei with mass numbers A= 90 – 145 specific energy is approximately equal to 8.5 MeV/nucleon. Therefore, the fission of a uranium nucleus releases an energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. With the complete fission of all the nuclei contained in 1 g of uranium, the same energy is released as during the combustion of 3 tons of coal or 2.5 tons of oil.

Nuclear chain reaction

Nuclear chain reaction - a sequence of singlenuclear reactions , each of which is caused by a particle that appeared as a product of the reaction at the previous step of the sequence. An example of a nuclear chain reaction is the chain reactionnuclear fission heavy elements, at which the main number of fission events is initiatedneutrons obtained by nuclear fission in the previous generation.

In the fission of a uranium-235 nucleus, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will already appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. The scheme for the development of a chain reaction of fission of uranium nuclei is shown in fig. 3.

Rice. 3

Uranium occurs in nature in the form of two isotopes: (99.3%) and (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction proceeds most intensively with slow (thermal) neutrons, while nuclei enter into a fission reaction only with fast neutrons with an energy of the order of 1 MeV. Otherwise, the excitation energy of the formed nuclei
is insufficient for fission, and then instead of fission, nuclear reactions occur:
.
Uranium isotope ? -radioactive, half-life 23 min. The isotope of neptunium is also radioactive, with a half-life of about 2 days.
.

The plutonium isotope is relatively stable, with a half-life of 24,000 years. The most important property of plutonium is that it is fissile under the influence of neutrons in the same way as. Therefore, with help, a chain reaction can be carried out.
The chain reaction scheme discussed above is an ideal case. In real conditions, not all neutrons produced during fission participate in the fission of other nuclei. Some of them are captured by non-fissile nuclei of foreign atoms, others fly out of uranium (neutron leakage).
Therefore, the chain reaction of fission of heavy nuclei does not always occur and not for any mass of uranium.

Neutron multiplication factor

The development of a chain reaction is characterized by the so-called neutron multiplication factor To, which is measured by the ratio of the number N i neutrons that cause nuclear fission of matter at one of the stages of the reaction, to the number N i-1 neutrons that caused fission at the previous stage of the reaction:
.
The multiplication factor depends on a number of factors, in particular, on the nature and amount of the fissile material, and on the geometric shape of the volume it occupies. The same amount of a given substance has a different value To. To maximum if the substance has a spherical shape, since in this case the loss of prompt neutrons through the surface will be the smallest.
The mass of fissile material in which the chain reaction proceeds with the multiplication factor To= 1 is called the critical mass. In small pieces of uranium, most of the neutrons, without hitting any nucleus, fly out.
The value of the critical mass is determined by the geometry of the physical system, its structure and the external environment. So, for a ball of pure uranium, the critical mass is 47 kg (a ball with a diameter of 17 cm). The critical mass of uranium can be reduced many times over by using so-called neutron moderators. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capture of slow neutrons by uranium-235 nuclei is hundreds of times greater than that of fast ones. The best moderator of neutrons is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
A good moderator is also graphite, whose nuclei do not absorb neutrons. Upon elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal velocities.
The use of neutron moderators and a special beryllium shell that reflects neutrons makes it possible to reduce the critical mass to 250 g.
With a multiplication factor To= 1 the number of fissile nuclei is maintained at a constant level. This mode is provided in nuclear reactors.
If the mass of nuclear fuel is less than the critical mass, then the multiplication factor To < 1; каждое новое поколение вызывает все меньшее и меньшее число делений, и реакция без внешнего источника нейтронов быстро затухает.
If the mass of nuclear fuel is greater than the critical one, then the multiplication factor To> 1 and each new generation of neutrons causes an increasing number of fissions. The chain reaction grows like an avalanche and has the character of an explosion, accompanied by a huge release of energy and an increase in the ambient temperature up to several million degrees. A chain reaction of this kind occurs when an atomic bomb explodes.
Nuclear reactor

A nuclear reactor is a device in which controllednuclear chain reaction accompanied by the release of energy. The first nuclear reactor was built in December 1942 in the USA under the direction of E.Fermi . In Europe, the first nuclear reactor was launched in December 1946 in Moscow under the leadership of I.V.Kurchatov . By 1978, there were already about a thousand nuclear reactors of various types operating in the world. The components of any nuclear reactor are:core with nuclear fuel , usually surrounded by a neutron reflector,coolant , chain reaction control system, radiation protection, remote control system. The main characteristic of a nuclear reactor is its power. Power in 1 Met corresponds to a chain reaction in which 3 10 16 fission events occur in 1 sec.

The core of a nuclear reactor contains nuclear fuel, a chain reaction of nuclear fission proceeds, and energy is released. The state of a nuclear reactor is characterized by an effective coefficient Kef neutron multiplication or reactivity r:

R \u003d (K? - 1) / K eff. (one)

If a To ef > 1, then the chain reaction grows with time, the nuclear reactor is in a supercritical state and its reactivity is r > 0; if To ef < 1 , then the reaction decays, the reactor is subcritical, r< 0; при To ? = 1, r = 0, the reactor is in a critical state, a stationary process is in progress, and the number of fissions is constant in time. To initiate a chain reaction during the start-up of a nuclear reactor, a neutron source is usually introduced into the core (a mixture of Ra and Be, 252 Cf etc.), although this is not necessary, since spontaneous nuclear fission uranium and cosmic rays give a sufficient number of initial neutrons for the development of a chain reaction at To ef > 1.

235 U is used as the fissile material in most nuclear reactors. . If the core, in addition to nuclear fuel (natural or enriched Uranus), contains a neutron moderator (graphite, water and other substances containing light nuclei, see below).Neutron moderation ), then the main part of the divisions occurs under the actionthermal neutrons (thermal reactor ). In a thermal neutron nuclear reactor, natural Uranus , not enriched 235 U (these were the first nuclear reactors). If there is no moderator in the core, then the main part of fissions is caused by fast neutrons with energy x n > 10 kev(fast reactor ). Intermediate neutron reactors with an energy of 1-1000 ev.

By design, nuclear reactors are divided into heterogeneous reactors , in which nuclear fuel is distributed discretely in the core in the form of blocks, between which there is a neutron moderator, andhomogeneous reactors , in which nuclear fuel and moderator are a homogeneous mixture (solution or suspension). Blocks with nuclear fuel in a heterogeneous in a nuclear reactor are calledfuel elements (TVEL "ami), form a regular lattice; the volume per one fuel element is called a cell. According to the nature of the use, a nuclear reactor is divided into power reactors andresearch reactors . Often one nuclear reactor performs several functions .

Under criticality conditions, a nuclear reactor has the form:

To ef = K ? ? P = 1, (1)

Where 1 - P is the probability of exit (leakage) of neutrons from the active zone of a nuclear reactor, To ? - the neutron multiplication factor in the core of infinitely large dimensions, determined for thermal nuclear reactors by the so-called "formula of 4 factors":

To? = neju. (2)

Here n is the average number of secondary (fast) neutrons arising from the fission of the 235 U nucleus thermal neutrons, e is the multiplication factor for fast neutrons (an increase in the number of neutrons due to fission of nuclei, mainly nuclei 238 U , fast neutrons); j is the probability that the neutron is not captured by the nucleus 238 U during the deceleration process, u is the probability that a thermal neutron will cause fission. The value h \u003d n / (l + a) is often used, where a is the ratio of the radiation capture cross section s p to the fission cross section s d.

Condition (1) determines the dimensions of the Nuclear Reactor. For example, for a nuclear reactor from natural uranium and graphite n = 2.4. e » 1.03, eju » 0.44, whence To? =1.08. This means that for To ? > 1 required R<0,93, что соответствует (как показывает теория Ядерного реактора) размерам активной зоны Ядерный реактор ~ 5-10 m. The volume of a modern nuclear power reactor reaches hundreds m 3 and is determined mainly by the possibilities of heat removal, and not by the conditions of criticality. The volume of the active zone of a nuclear reactor in a critical state is called the critical volume of the nuclear reactor, and the mass of fissile material is called the critical mass. The nuclear reactor with fuel in the form of solutions of salts of pure fissile isotopes in water and with a water neutron reflector have the smallest critical mass. For 235 U this mass is 0.8 kg, for 239 Pu - 0,5 kg. 251 has the smallest critical mass cf (theoretically 10 g). Critical parameters of a graphite nuclear reactor with natural uranium: mass of uranium 45 t, volume of graphite 450 m 3 . To reduce the leakage of neutrons, the core is given a spherical or close to spherical shape, for example, a cylinder with a height of the order of the diameter or a cube (the smallest ratio of surface to volume).

The value of n is known for thermal neutrons with an accuracy of 0.3% (Table 1). With an increase in the energy x n of the neutron that caused fission, n grows according to the law: n \u003d n t + 0.15x n (x n in mev), where n t corresponds to fission by thermal neutrons.

Tab. 1. - Values ​​n and h) for thermal neutrons (according to data for 1977)

233 U	235 U	239 Pu	241 Pu
n 2.479	2,416	2,862	2,924
h 2.283	2,071	2,106	2,155

The value of (e-1) is usually only a few %; nevertheless, the role of fast neutron multiplication is significant, since for large nuclear reactors ( To ? - 1) << 1 (графитовые Ядерный реактор с естественным uranium in which a chain reaction was first carried out, it would not have been possible to create if fission on fast neutrons did not exist).

The maximum possible value of J is achieved in a nuclear reactor that contains only fissile nuclei. Power nuclear reactors use lightly enriched

Uranus (concentration 235 U ~ 3-5%), and cores 238 U absorb an appreciable fraction of the neutrons. So, for a natural mixture of isotopes uranium maximum value nJ = 1.32. The absorption of neutrons in the moderator and structural materials usually does not exceed 5-20% of the absorption by all isotopes of nuclear fuel. Of the moderators, heavy water has the lowest absorption of neutrons, and of structural materials - Al and Zr .

Probability of resonant capture of neutrons by nuclei 238

U in the process of deceleration (1-j) decreases significantly in heterogeneous nuclear reactors. The decrease in (1 - j) is due to the fact that the number of neutrons with energies close to the resonant one sharply decreases inside the fuel block and only the outer layer of the block participates in resonant absorption. The heterogeneous structure of the nuclear reactor makes it possible to carry out a chain process on a natural uranium . It reduces the value of O, but this loss in reactivity is much smaller than the gain due to the decrease in resonant absorption.

To calculate thermal nuclear reactors, it is necessary to determine the spectrum of thermal neutrons. If the absorption of neutrons is very weak and the neutron has time to collide many times with the nuclei of the moderator before absorption, then thermodynamic equilibrium (thermalization of neutrons) is established between the moderating medium and the neutron gas, and the spectrum of thermal neutrons is described

Maxwell distribution . In reality, the absorption of neutrons in the active zone of a nuclear reactor is quite large. This leads to a deviation from the Maxwell distribution - the average energy of neutrons is greater than the average energy of the molecules of the medium. The thermalization process is influenced by the movements of the nuclei, chemical bonds of atoms and etc.

Burnup and reproduction of nuclear fuel.

During the operation of a nuclear reactor, a change in the composition of the fuel occurs, associated with the accumulation of fission fragments in it and with the formationtransuranic elements , mainly isotopes Pu . The influence of fission fragments on the reactivity of a nuclear reactor is called poisoning (for radioactive fragments) and slagging (for stable ones). Poisoning is mainly due to 135 Xe which has the largest neutron absorption cross section (2.6 10 6 barn). Its half-life T 1/2 = 9.2 h, the fission yield is 6-7%. Main body 135 Xe formed as a result of decay 135 ] (Tts = 6,8 h). When poisoned, Kef changes by 1-3%. Large absorption cross section 135 Xe and the presence of an intermediate isotope 135 I lead to two important phenomena: 1) to an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of the Nuclear Reactor after its shutdown or power reduction (“iodine pit”). This makes it necessary to have an additional reactivity margin in the regulatory bodies or makes short-term stops and power fluctuations impossible. Depth and Duration iodine wells depend on the neutron flux Ф: at Ф = 5 10 13 neutron/cm2? sec duration iodine holes ~ 30 h, and the depth is 2 times greater than the stationary change To ef caused by poisoning 135 Xe . 2) Due to poisoning, spatio-temporal fluctuations of the neutron flux Ф, and hence the power of the nuclear reactor, can occur. These fluctuations occur at Ф> 10 13 neutrons / cm 2? sec and large dimensions of the nuclear reactor. Oscillation periods ~ 10 h.

The number of different stable fragments arising from nuclear fission is large. There are fragments with large and small absorption cross sections compared to the absorption cross section of a fissile isotope. The concentration of the former reaches saturation during the first few days of operation of the Nuclear Reactor (mainly 149 Sm , changing K eff by 1%). The concentration of the latter and the negative reactivity introduced by them increase linearly with time.

The formation of transuranium elements in a nuclear reactor occurs according to the following schemes:

Here 3 means neutron capture, the number under the arrow is the half-life.

Accumulation of 239 Pu (nuclear fuel) at the beginning of the operation of a nuclear reactor occurs linearly in time, and the faster (with a fixed burnup of 235 U ), the less enrichment uranium. Then concentration 239 Pu tends to a constant value, which does not depend on the degree of enrichment, but is determined by the ratio of the neutron capture cross sections 238 U and 239 Pu . Characteristic time of establishment of the equilibrium concentration 239 Pu ~ 3/ F years (F in units 10 13 neutrons/ cm 2 ?sec). Isotopes 240 Pu, 241 Pu reach an equilibrium concentration only when the fuel is re-burned in a nuclear reactor after the regeneration of nuclear fuel.

Burnup of nuclear fuel is characterized by the total energy released in a nuclear reactor per 1 t fuel. For nuclear reactors fueled by natural uranium, maximum burnout ~ 10 gwt?day/t(heavy water nuclear reactors). In a nuclear reactor with weakly enriched uranium (2-3% 235 U ) burnout ~ 20-30 GW-day/t. In a fast neutron nuclear reactor - up to 100 GW-day/t. Burnout 1 GW-day/t corresponds to the combustion of 0.1% of nuclear fuel.

When the nuclear fuel burns out, the reactivity of the nuclear reactor decreases (in a nuclear reactor running on natural uranium at low burnups, some increase in reactivity occurs). The replacement of burnt-out fuel can be carried out immediately from the entire core or gradually along the fuel rods so that there are fuel rods of all ages in the core - continuous refueling mode (intermediate options are possible). In the first case, a nuclear reactor with fresh fuel has an excess reactivity that needs to be compensated. In the second case, such compensation is needed only at the initial start-up, before entering the continuous overload mode. Continuous refueling makes it possible to increase the burnup depth, since the reactivity of a nuclear reactor is determined by the average concentrations of fissile nuclides (TVELs with a minimum concentration of fissile nuclides are unloaded). Table 2 shows the composition of the extracted nuclear fuel (in kg) inpressurized water reactor power 3 Gwt. The entire core is unloaded simultaneously after the operation of the nuclear reactor for 3 years and "excerpts" 3 years(Ф = 3?1013 neutron/cm2?sec). Starting Lineup: 238 U - 77350, 235 U - 2630, 234 U - 20.

Tab. 2. - The composition of the unloaded fuel, kg

238 etc.................
Nuclear reaction (NR) - a process in which the nucleus of an atom changes by crushing or combining with the nucleus of another atom. Thus, it must lead to the transformation of at least one nuclide into another. Sometimes, if a nucleus interacts with another nucleus or particle without changing the nature of any nuclide, the process is referred to as nuclear scattering. Perhaps most notable are the reactions of light elements that affect the energy production of stars and the sun. Natural reactions also occur in the interaction of cosmic rays with matter. natural nuclear reactor The most notable human-controlled reaction is the fission reaction, which takes place in these devices for initiating and controlling a nuclear chain reaction. But there are not only artificial reactors. The world's first natural nuclear reactor was discovered in 1972 at Oklo in Gabon by French physicist Francis Perrin. The conditions under which the natural energy of a nuclear reaction could be generated were predicted in 1956 by Paul Kazuo Kuroda. The only known site in the world consists of 16 sites where self-sustaining reactions of this type have occurred. This is believed to have been about 1.7 billion years ago and continued for several hundred thousand years, as evidenced by xenon isotopes (a fission product gas) and varying U-235/U-238 ratios (natural uranium enrichment). Nuclear fission The binding energy plot suggests that nuclides with a mass greater than 130 a.m.u. should spontaneously separate from each other to form lighter and more stable nuclides. Experimentally, scientists have found that spontaneous fission reactions of the elements of a nuclear reaction occur only for the heaviest nuclides with a mass number of 230 or more. Even if this is done, it is very slow. The half-life for spontaneous fission of 238 U, for example, is 10-16 years, or about two million times longer than the age of our planet! Fission reactions can be induced by irradiating samples of heavy nuclides with slow thermal neutrons. For example, when 235 U absorbs a thermal neutron, it breaks into two particles of uneven mass and releases an average of 2.5 neutrons. The absorption of the 238 U neutron induces oscillations in the nucleus, which deform it until it breaks into fragments, just as a drop of liquid can shatter into smaller droplets. More than 370 daughter nuclides with atomic masses between 72 and 161 a.m.u. produced by fission with the thermal neutron 235U, including the two products shown below. Nuclear reaction isotopes, such as uranium, undergo induced fission. But the only natural isotope 235 U is present in abundance at only 0.72%. The induced fission of this isotope releases on average 200 MeV per atom, or 80 million kilojoules per gram of 235 U. The attraction of nuclear fission as an energy source can be understood by comparing this value with the 50 kJ/g released when natural gas is burned. First nuclear reactor The first artificial nuclear reactor was built by Enrico Fermi and put into operation by employees under the football stadium on December 2, 1942. This reactor, which produced several kilowatts of power, consisted of a pile of 385 tons of graphite blocks stacked in layers around a cubic lattice of 40 tons of uranium and uranium oxide. Spontaneous fission of 238 U or 235 U in this reactor produced very few neutrons. But there was enough uranium, so that one of these neutrons induced 235 U, thereby releasing an average of 2.5 neutrons, which catalyzed the fission of additional 235 U nuclei in a chain reaction (nuclear reactions). The amount of fissile material needed to sustain the chain reaction is called The green arrows show the splitting of the uranium nucleus in two fission fragments emitting new neutrons. Some of these neutrons can trigger new fission reactions (black arrows). Some of the neutrons may be lost in other processes (blue arrows). The red arrows show delayed neutrons that come later from radioactive fission fragments and can cause new fission reactions. Designation of nuclear reactions Consider the basic properties of atoms, including atomic number and atomic mass. The atomic number is the number of protons in the nucleus of an atom, and isotopes have the same atomic number but differ in the number of neutrons. If the initial nuclei are denoted a and b, and the kernels of the product are denoted with and d, then the reaction can be represented by the equation you can see below. Which nuclear reactions cancel out for light particles instead of using full equations? In many situations, the compact form is used to describe such processes: a (b, c) d equivalent to a+b producing c + d. Light particles often shrink: usually p means proton, n- neutron, d- deuteron, α - alpha particle, or helium-4, β beta particle or electron γ - gamma photon, etc. Types of nuclear reactions Although the number of possible such reactions is enormous, they can be sorted by type. Most of these reactions are accompanied by gamma radiation. Here are some examples: Elastic scattering. Occurs when no energy is transferred between the target nucleus and the incident particle. Inelastic scattering. Occurs when energy is transferred. The difference in kinetic energies is conserved in the excited nuclide. capture reactions. Both charged and neutral particles can be captured by nuclei. This is accompanied by the emission of ɣ-rays. The particles of nuclear reactions in the reaction of neutron capture are called radioactive nuclides (induced radioactivity). Transfer reactions. The absorption of a particle accompanied by the emission of one or more particles is called a transfer reaction. Fission reactions. Nuclear fission is a reaction in which the nucleus of an atom is split into smaller pieces (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays) and releases large amounts of energy. Fusion reactions. Occur when two or more atomic nuclei collide at a very high speed and combine to form a new type of atomic nucleus. Particles from deuterium-tritium nuclear fusion reactions are of particular interest because of their potential to provide energy in the future. splitting reactions. Occurs when a nucleus is hit by a particle with enough energy and momentum to knock out a few small fragments or break it into many fragments. rearrangement reactions. This is the absorption of a particle, accompanied by the emission of one or more particles: 197Au (p, d) 196mAu 4He (a, p) 7Li 27Al (a, n) 30P 54Fe (a, d) 58Co 54Fe(a, 2n) 56Ni 54Fe (32S, 28Si) 58Ni Various rearrangement reactions change the number of neutrons and the number of protons. nuclear decay Nuclear reactions occur when an unstable atom loses energy through radiation. It is a random process at the level of single atoms, since according to quantum theory it is impossible to predict when an individual atom will decay. There are many types of radioactive decay: Alpha radioactivity. Alpha particles are made up of two protons and two neutrons bound together with a particle identical to a helium nucleus. Due to its very large mass and its charge, it strongly ionizes the material and has a very short range. Beta radioactivity. It is high-energy, high-speed positrons, or electrons, emitted from certain types of radioactive nuclei, such as potassium-40. Beta particles have a greater penetration range than alpha particles, but still much less than gamma rays. Ejected beta particles are a form of ionizing radiation, also known as nuclear chain reaction beta rays. The production of beta particles is called beta decay. Gamma radioactivity. Gamma rays are electromagnetic radiation of very high frequency and are therefore high energy photons. They are formed when nuclei decay as they go from a high-energy state to a lower state known as gamma decay. Most nuclear reactions are accompanied by gamma radiation. Neutron emission. Neutron emission is a type of radioactive decay of nuclei containing excess neutrons (especially fission products), in which the neutron is simply ejected from the nucleus. This type of radiation plays a key role in the control of nuclear reactors because these neutrons are delayed. Energy The Q-value of the energy of a nuclear reaction is the amount of energy released or absorbed during the reaction. It is called or Q-value of the reaction. This energy is expressed as the difference between the kinetic energy of the product and the amount of the reactant. General view of the reaction: x + X ⟶ Y + y + Q……(i) x + X ⟶ Y + y + Q……(i), where x and X are reagents, and y and Y- reaction product, which can determine the energy of a nuclear reaction, Q - energy balance. Q-value NR means the energy released or absorbed in the reaction. It is also called the NR energy balance, which can be positive or negative depending on the character. If the Q-value is positive, the reaction will be exothermic, also called exoergic. She releases energy. If the Q-value is negative, the reaction is endoergic, or endothermic. Such reactions are carried out by absorbing energy. In nuclear physics, such reactions are defined by the Q-value, as the difference between the sum of the masses of the initial reactants and the final products. It is measured in energy units MeV. Consider a typical reaction in which the projectile a and purpose A inferior to two products B and b. This can be expressed as follows: a + A → B + B, or even in a more compact notation - A (a, b) B. The types of energies in a nuclear reaction and the value of this reaction are determined by the formula: Q = c2, which coincides with the excess kinetic energy of the final products: Q = T final - T initial For reactions in which an increase in the kinetic energy of the products is observed, Q is positive. Positive Q-reactions are called exothermic (or exogenous). There is a net release of energy, since the kinetic energy of the final state is greater than that of the initial state. For reactions in which a decrease in the kinetic energy of the products is observed, Q is negative. The half-life of a radioactive substance is a characteristic constant. It measures the time required for a given amount of matter to be reduced by half through decay and hence radiation. Archaeologists and geologists use the half-life to date on organic objects in a process known as carbon dating. During beta decay, carbon 14 is converted to nitrogen 14. At the time of death, organisms stop producing carbon 14. Because the half-life is constant, the ratio of carbon 14 to nitrogen 14 provides a measure of the age of the sample. In the medical field, the energy sources for nuclear reactions are radioactive isotopes of Cobalt 60, which has been used for radiation therapy to shrink tumors that will later be removed surgically, or to kill cancer cells in inoperable tumors. When it decays into stable nickel, it emits two relatively high energies - gamma rays. Today it is being replaced by electron beam radiotherapy systems. Half-life of isotopes from some samples: oxygen 16 - infinite; uranium 238 - 4,460,000,000 years; uranium 235 - 713,000,000 years; carbon 14 - 5,730 years; cobalt 60 - 5.27 years; silver 94 - 0.42 seconds. radiocarbon dating At a very steady rate, unstable carbon 14 gradually decays into carbon 12. The ratio of these carbon isotopes reveals the age of some of the oldest inhabitants of the earth. Radiocarbon dating is a method that provides objective estimates of the age of carbon-based materials. Age can be estimated by measuring the amount of carbon 14 present in a sample and comparing it to an international standard reference. The impact of radiocarbon dating on the modern world has made it one of the most significant discoveries of the 20th century. Plants and animals assimilate carbon 14 from carbon dioxide throughout their lives. When they die, they cease to exchange carbon with the biosphere, and the carbon 14 content in them begins to decrease at a rate determined by the law of radioactive decay. Radiocarbon dating is essentially a method for measuring residual radioactivity. Knowing how much carbon 14 is left in the sample, you can find out the age of the organism when it died. It should be noted that the results of radiocarbon dating show when the organism was alive. Basic methods for measuring radiocarbon There are three main methods used to measure carbon 14 in any given sampler proportional count, liquid scintillation counter, and accelerator mass spectrometry. Proportional gas counting is a common radiometric dating technique that takes into account beta particles emitted from a given sample. Beta particles are decay products of radiocarbon. In this method, the carbon sample is first converted to carbon dioxide gas before being measured in gas proportional meters. Scintillation fluid counting is another method of radiocarbon dating that was popular in the 1960s. In this method, the sample is in liquid form and a scintillator is added. This scintillator creates a flash of light when it interacts with a beta particle. The sample tube is passed between two photomultipliers, and when both devices register a flash of light, a count is made. Benefits of Nuclear Science The laws of nuclear reactions are used in a wide range of branches of science and technology, such as medicine, energy, geology, space and environmental protection. Nuclear medicine and radiology are medical practices that involve the use of radiation or radioactivity to diagnose, treat, and prevent disease. While radiology has been in use for almost a century, the term "nuclear medicine" began to be used about 50 years ago. Nuclear power has been in use for decades and is one of the fastest growing energy options for countries seeking energy security and low emission energy saving solutions. Archaeologists use a wide range of nuclear methods to determine the age of objects. Artifacts such as the Shroud of Turin, the Dead Sea Scrolls and the Crown of Charlemagne can be dated and authenticated using nuclear techniques. Nuclear techniques are being used in agricultural communities to fight disease. Radioactive sources are widely used in the mining industry. For example, they are used in non-destructive testing of blockages in pipelines and welds, in measuring the density of the punched material. Nuclear science plays a valuable role in helping us understand the history of our environment. Did not find an answer to your question? 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