Nuclear fission. Fission of heavy nuclei

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 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 nuclear fission, enormous energy is released, the vast majority of it is released in the form of kinetic energy of fission fragments.

Distribution of fission products by mass. The uranium nucleus in most cases divides asymmetrically. Two nuclear fragments have, respectively, different speeds and different masses.

The fragments fall into two groups based on their masses; one near krypton and the other near xenon. The masses of the fragments relate 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 relative to the vertical straight line passing through the point. The significant width of the maxima indicates the variety of fission paths.

Rice. 82. Distribution of uranium fission products by mass

The listed characteristics relate mainly to fission under the influence of thermal neutrons; In the case of fission under the influence of neutrons with energies of several or more, the nucleus disintegrates into two more symmetrical fragments in mass.

Properties of fission products. During the fission of a uranium atom, very many shell electrons are stripped, and the fission fragments are approximately multiply ionized positive ions, which, when passing through the substance, strongly ionize the atoms. Therefore, the ranges of 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 possible options fission produces radioactive bromine. In Fig. 83 shows a diagram of its decay, at the end of which there are stable isotopes

An interesting feature of this chain: 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 decays exponentially, as with 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 during fission, delayed neutrons play an important role in the chain reaction.

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

When studying the fission process, a 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 fission subsequent nuclei, i.e., the possibility of creating a chain reaction arises. To resolve this issue in 1939-1940. worked in almost all the largest nuclear laboratories in the world.

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

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

Fission of atomic nuclei is the process of splitting a nucleus into two approximately equal parts. Typically, this process occurs when some particle - a neutron, proton, alpha particle, etc. - enters a heavy nucleus. In such cases, fission is called forced. But sometimes division occurs spontaneously; such division is called spontaneous.

The mechanism of forced division. When a particle (for example, a neutron) enters the nucleus, its binding energy is released inside the nucleus E St.. A significant part of the kinetic energy of the particle is added to it E, as a result of which the nucleus comes into an excited state, and its total excitation energy turns out to be equal to E * = E St. + E·A/(A+1). This excitation manifests itself in the form of accelerated movement of all the nucleons of the nucleus, the nucleus “boils,” waves run along its surface, etc. Then one of two things happens. Either the excess energy will leave the nucleus with the emission of one or more gamma quanta (i.e., radiative capture of the incoming particle will occur). Either, as a result of oscillations of the nuclear “liquid”, a constriction is formed in the core, the core will take the shape of a dumbbell, and under the influence of the Coulomb repulsion of the charges of the two halves of this “dumbbell”, the constriction will burst, and the two parts of the former nucleus will fly apart in opposite directions with great energy received from those the same forces of Coulomb repulsion of like electric charges. The resulting halves of the original nucleus are called fission fragments. Under the influence of surface tension forces, they will acquire a spherical shape and become the nuclei of new atoms with masses equal to approximately half the mass of the uranium nucleus, i.e. atoms of elements lying in the middle of the periodic table.

Potential fission barrier. In order for a nucleus to split, it must first be given a sufficiently large deformation, which arises as a result of the excitation energy imparted to the nucleus - otherwise the nucleus will contract into a sphere and division will not occur. The minimum excitation energy at which fission becomes possible is called potential fission barrier and is indicated by the symbol U f. Fission is possible if the excitation energy of the nucleus E * > U f. If E * < U f, then division is impossible. All heavy nuclei (thorium, uranium, plutonium, etc.) have values U f approximately the same and equal to 5.1 – 5.4 MeV. Under such conditions, all heavy nuclei would have to exhibit the same ability to fission. However, this is not true.

It is known that with respect to fission by neutrons, nuclei are divided into two different groups:

odd kernels such as 233 U, 235 U, 239 Pu, 241 Pu. They are easily fissile by any, even thermal neutrons, which is why they are often called “fuel” nuclei;

even-even kernels 232 Th, 234 U, 238 U, 240 Pu, 242 Pu thermal neutrons do not fission, so they are often called “raw neutrons”.

This happens because when a neutron hits an odd nucleus, an even-even nucleus is formed (for example, 235 U + n → 236 U), the binding energy of the neutron in which is especially high, so that even at zero kinetic energy of the neutron, the excitation energy turns out to be greater than the height of the fission barrier, and the nucleus easily divides.

When a neutron hits an even-even nucleus (for example, 238 U + n → 239 U), an odd nucleus is formed, the binding energy of the neutron in which is much lower, and it is not enough to overcome the fission barrier. But if in the latter case, not a thermal, but a fast neutron with a sufficiently large kinetic energy enters the nucleus, then it may turn out that the total excitation energy E * > U f, and division will occur. The minimum kinetic energy of a neutron at which fission of an even-even nucleus becomes possible is called threshold fission energy E since then. For the kernel 238 U this energy E since then≈ 1 MeV. Threshold energies for other even-even nuclei have approximately the same values. So all such nuclei also fission, but only by fast neutrons.

Spontaneous division. Due to the large overload of protons, which repel each other by electrostatic forces and thereby try to tear the nucleus apart, heavy nuclei turn out to be extremely unstable and therefore are able to divide on their own, without any external influence. This spontaneous division is called spontaneous fission. Spontaneous fission occurs, similar to alpha decay, through the tunneling effect of fragments passing through the fission barrier. But due to the large charge of the fragments, their probability of passing through the potential barrier during the fission of uranium nuclei turns out to be much less than for alpha particles, and the half-life in relation to spontaneous fission is, accordingly, much longer. So for the alpha decay of uranium-238 nuclei the period T α = 4.5 10 9 years, whereas for spontaneous fission T f= 10 16 years, i.e. 2.5 million times more. As the nuclear charge increases, the values T f are rapidly decreasing. So for nuclei of artificial transuranium elements (see below) with Z>100 the value T f measured in minutes and even seconds, and for some nuclides spontaneous fission becomes an even more preferable type of decay. This allows us to count spontaneous fission fourth type of radioactive decay in addition to alpha, beta and gamma decays.

Energy release during nuclear fission. The graph in Fig. 1.1. shows that the specific binding energy of nucleons in uranium nuclei (≈ 7.5 MeV/nucleon) is significantly less than that of nuclei with half the mass (≈ 8.4 MeV/nucleon), which are obtained during fission in the form of fragments. This means that the fragments are bound much more strongly than uranium nuclei, and during their formation, due to the rearrangement of nucleons, excess binding energy is released in an amount of approximately 0.9 MeV per nucleon. And since 236 nucleons are involved in the fission of one nucleus, the total energy release during the fission of one nucleus is 236·0.9 ≈ 212 MeV. The bulk of this energy goes to the fragments in the form of their kinetic energy. But when nuclei fission, in addition to fragments, several different particles are released, which carry away the rest of the energy. The approximate distribution of energy between various particles during the fission of uranium nuclei by thermal neutrons is given in Table 1.3. The total amount of energy (215 MeV) agrees well with the estimate made above (212 MeV). Of this amount of energy, 10 MeV are carried away by antineutrinos into outer space and are thus "irretrievable losses". The rest of the energy is absorbed in various reactor materials and is ultimately converted into thermal energy, which is used either directly (in nuclear power plants and nuclear power plants) or to generate electric current (in nuclear power plants and nuclear power plants).

Table 1.3. Energy distribution during fission of heavy nuclei

Selection form	Energy (MeV)
Kinetic energy of fission fragments
Kinetic energy of secondary fission neutrons
Prompt gamma radiation energy during fission
Energy carried away by electrons during beta decay of fragments
Energy carried away by antineutrinos during beta decay of fragments
Energy of gamma radiation accompanying beta decay of fragments
Energy released when neutrons are captured by nuclei of a medium

Effective fission cross sections. Nuclei fissioning with thermal neutrons are also capable of fissioning with intermediate and fast neutrons; therefore, for them, as with radiative capture (see above), it is necessary to consider the features of the behavior of fission cross sections in all three regions.

In the region of thermal neutrons, fission cross sections also change with increasing energy according to the “1/v” law. The values ​​of fission cross sections σ f averaged over this region are given in Table 1.4.

Table 1.4. Cross sections for fission of some nuclei by thermal neutrons

Parameter	Unit of measurement	Fissile nuclides
α = σ n,γ /σ f

Unfortunately, when a neutron enters a uranium or plutonium nucleus, not only fission can occur, but also radiation capture of a neutron without fission, for example 235 U(n,γ) 236 U. This process is harmful to the operation of the reactor, and doubly so:

a neutron is lost, which cannot participate in the fission chain reaction;

the nuclear fuel core 235 U is lost, turning into the even-even nucleus 236 U, which, as noted above, is not fissioned by thermal neutrons.

But as can be seen from Table 5, the fission cross sections in all cases turn out to be larger than the radiation capture cross sections, so the useful fission process occurs with a greater probability than the undesirable radiation capture process. This is especially clearly demonstrated by the ratios of the cross sections of these two processes (the last line in Table 1.4).

In the region of intermediate neutrons, the dependence of fission cross sections on energy, as well as during radiative capture, reveals resonance peaks. On average, in this region the values ​​of the “alpha” parameter turn out to be even slightly higher than in the region of thermal neutrons, therefore, although intermediate neutron reactors are being built, they are not widely used.

In the region of fast neutrons, the dependence of fission cross sections on neutron energy becomes smooth, but unlike radiative capture cross sections, fission cross sections not only do not decrease with increasing neutron energy, but even increase somewhat. This leads to a significant improvement in the ratio of the probabilities of radiative neutron capture and fission, especially for plutonium, for which the ratio α = 0.029 for fast neutrons, i.e. more than 12 times better than for thermal neutrons. This circumstance is associated with one of the main advantages of nuclear reactors operating on fast neutrons compared to thermal reactors.

The fission cross sections of even-even nuclides up to the fission threshold are naturally equal to zero, and above the threshold, although they differ from zero, they never acquire large values. So the fission cross section is 238 U at energies above 1 MeV it turns out to be about 0.5 barn.

Fission fragments. Despite the high energy (approximately 82 MeV for each fragment), the ranges of fragments in the air turn out to be no more, and even somewhat less than the ranges of alpha particles (about 2 cm). And this is despite the fact that alpha particles have significantly lower energies (4 – 9 MeV). This happens because the electrical charge of the fragment is much greater than the charge of the alpha particle, and therefore it loses energy much more intensely to ionize and excite the atoms of the medium.

More accurate measurements showed that the paths of the fragments, as a rule, are not the same, and are grouped around the values ​​of 1.8 and 2.2 cm.

In general, fission can produce fragments with a wide variety of mass numbers ranging from 70 to 160 (i.e., about 90 different values), but fragments with such masses are formed with different probabilities. These probabilities are usually expressed by the so-called fragment exits Y A with a given mass number A: Y A = N A / N f, Where N A– number of fragments with mass number A that arose during N f, nuclear fission. Usually the size Y A expressed as a percentage.

The distribution curve of fission fragment yields by mass numbers has two maxima (or “humps”), with one maximum located in the region A = 90, and the second in the region A = 140. Note that it is nuclei of approximately these masses that are most often found in traces - fallout after nuclear tests or nuclear accidents. It is enough to recall traces of such nuclides as 131 I, 133 I, 90 Sr, 137 Cs.

The ratio of the number of neutrons to the number of protons in the fragments at the first moment turns out to be approximately the same as it was in the uranium nucleus, i.e. 143:92 = 1.55. But for stable nuclei with average masses, to which fragments belong, this ratio is much closer to unity: for example, for the stable nucleus 118 Sn this ratio is 1.36. This means that the fragment nuclei are heavily overloaded with neutrons, and they will try to get rid of this overload through beta decay, in which neutrons turn into protons. At the same time, in order for the primary fragment to turn into a stable nuclide, several successive beta decays may be required, forming a whole chain, for example:

(stable).

Here, under the arrows, are the half-lives of nuclides: s-seconds, h-watch, y-years. Note that a fission fragment is usually called only the very first nucleus that directly arises during the fission of a uranium nucleus (in this case - 135 Sb). All other nuclides resulting from beta decays, together with fragments and stable final nuclides, are called fission products. Since the mass number does not change along the chain, the total number of such chains that can be formed during the fission of uranium nuclei is equal to the number of mass numbers that can arise, i.e. approximately 90. And since each chain contains an average of 5 radioactive nuclides, in total among the fission products one can count about 450 radionuclides with a wide variety of half-lives from fractions of a second to millions of years. In a nuclear reactor, the accumulation of fission products creates certain problems, because firstly, they absorb neutrons and thereby complicate the fission chain reaction, and secondly, due to their beta decay, residual heat generation occurs, which can continue for a very long time after the reactor is shut down (heat generation continues in the remains of the Chernobyl reactor to this day) . The radioactivity of fission products also poses a significant danger to humans.

Secondary fission neutrons. Neutrons that cause nuclear fission are called primary, and neutrons produced during nuclear fission are called secondary. Secondary fission neutrons are emitted by fragments at the very beginning of their movement. As already noted, the fragments immediately after fission are heavily overloaded with neutrons; in this case, the excitation energy of the fragments exceeds the binding energy of the neutrons in them, which predetermines the possibility of the escape of neutrons. Leaving the fragment core, the neutron carries away some of the energy with it, as a result of which the excitation energy of the fragment core decreases. After the excitation energy of the fragment nucleus becomes less than the binding energy of the neutron in it, the emission of neutrons stops.

When different nuclei fission, different numbers of secondary neutrons are produced, usually from 0 to 5 (most often 2-3). For reactor calculations, the average number of secondary neutrons emitted per fission event is of particular importance. This number is usually denoted by the Greek letter ν (nu) or, more often, ν f. The values ​​of ν f depend on the type of fissile nucleus and on the energy of the primary neutrons. Some examples are given in Table 1.5. The data presented in this table show that the values ​​of ν f increase both with increasing charge and mass of the fissile nucleus, and with increasing energy of the primary neutrons.

Table 1.5. Average amounts of secondary neutrons produced during nuclear fission by thermal and fast neutrons

Original	Values ​​of ν f at different energies of primary neutrons
	Thermal neutrons	Fast neutrons

Another advantage of fast neutron reactors is associated with the latter circumstance - a larger number of secondary neutrons allows them to carry out the process of expanded reproduction of nuclear fuel (see below). Secondary neutrons also arise during spontaneous fission of nuclei. So ν f (U-238) = 1.98, and ν f (Cf-252) = 3.767.

The process of emission of secondary neutrons by highly excited nuclei of fragments resembles the process of evaporation of molecules from the surface of a highly heated drop of liquid. Therefore, the energy spectrum of secondary neutrons is similar to the Maxwell distribution of molecules during thermal motion. The maximum of this spectrum lies at an energy of 0.8 MeV, and the average energy of secondary fission neutrons is about 2 MeV.

The main part of secondary neutrons escapes from the fragment nuclei on average within 10 -14 s after nuclear fission, i.e. almost instantly. Therefore, this part of secondary neutrons is called prompt neutrons. But there are also so-called delayed neutrons, which play an important and very special role in reactors .

Delayed neutrons during nuclear fission. Experience shows that a small fraction of secondary neutrons (usually< 1 %) испускается облученным нейтронами образцом делящегося материала спустя долгое время после прекращения облучения, когда деления ядер в образце тоже, естественно, уже не происходят. Происхождение запаздывающих нейтронов связано с бета-распадом некоторых осколков деления. Если бета-распад происходит на уровень конечного ядра, энергия возбуждения которого превышает энергию связи нейтрона, то распад ядра из этого состояния может произойти не путем испускания гамма-кванта, как обычно, а путем испускания нейтрона. Вылет нейтрона происходит практически в то же мгновение, как только образуется возбужденное ядро, но относительно процесса деления исходного ядра этот момент оказывается отодвинутым на время, которое потребовалось для бета-распада осколка. Поэтому запаздывающие нейтроны вылетают практически одновременно с бета-частицами, и их выход во времени описывается таким же экспоненциальным законом и с тем же периодом полураспада, что и бета-распад осколка.

The fraction of delayed neutrons is defined as the ratio of the number of delayed neutrons to the number of all secondary fission neutrons: β = N zap. n / N n. The values ​​of β for some nuclei when fissioned by neutrons of different energies are given in Table 1.6.

Table 1.6. Fractions of delayed neutrons during nuclear fission

Original	B (%) during nuclear fission
	Thermal neutrons	Neutrons with energy 2 MeV
233 U
235 U
238 U
239 Pu

Since delayed neutrons can arise from the decay of various fragment nuclei (called delayed neutron precursor nuclei), each of which decays with its own half-life, delayed neutrons form several groups, each of which has its own half-life. The main parameters of these groups are given in table. 1.7. In this table, the relative yields of delayed neutrons are normalized to unity. The energies of delayed neutrons are somewhat lower than the average energy of prompt neutrons (2 MeV), since they are emitted from less excited fragments. The half-lives of groups of delayed neutrons do not exactly coincide with the half-lives of the isolated precursors, since in fact there are much more precursors of delayed neutrons - some researchers have found up to several dozen of them. Neutrons from predecessors with similar periods merge into one group with a certain average period, which is entered into the tables. For the same reason, the yields of groups and their periods depend on the type of fissile nucleus and the energy of the primary neutrons, since when these two parameters change, the yields of fission fragments change, and, consequently, the composition of the groups also changes.

Table 1.7. Parameters of groups of delayed neutrons during fission of 235 U by thermal neutrons

Group number	Half-life (sec)	Relative output	Average energy (keV)	Main predecessor
				I-137
				I-138

Delayed neutrons play a decisive role in controlling the fission chain reaction and the operation of the entire nuclear reactor as a whole.

Instantaneous gamma radiation during fission. When, after the last neutron leaves the fragment, the excitation energy of the fragment's nucleus is lower than the binding energy of the neutron in it, further escape of prompt neutrons becomes impossible. But some extra energy still remains in the fragment. This excess energy is carried away from the nucleus in a series of emitted gamma rays. As noted above, the total energy of instantaneous gamma rays is about 8 MeV, their average number per fission is approximately 10, therefore, the average energy of one gamma ray during the fission of heavy nuclei is approximately 0.8 MeV.

Thus, a nuclear reactor is a powerful source of not only neutrons, but also gamma radiation, and it is necessary to protect against both of these types of radiation.

URANIUM NUCLEI FISSION

Only nuclei can divide some severe elements such as uranium.

The nucleus of uranium - 235 has the shape of a ball. Having absorbed a neutron, the nucleus becomes excited and begins to deform.
It stretches from side to side until the Coulomb repulsive forces between protons begin to prevail over the nuclear forces of attraction. After this, the core breaks into two parts and the fragments fly away at a speed of 1/30 the speed of light. When a nucleus fissions, 2 or 3 more neutrons are produced.
The appearance of neutrons is explained by the fact that the number of neutrons in the fragments is greater than acceptable.

The flying fragments, which have enormous speed, are slowed down by the environment.
The kinetic energy of the fragments is converted into internal energy of the environment, which heats up.
Thus, the fission of uranium nuclei is accompanied highlighting large quantity energy.

NUCLEAR CHAIN ​​REACTION

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

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

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

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

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

To carry out a chain reaction, it is impossible to use any nuclei that fission under the influence of neutrons.

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

In nature, uranium-235 isotopes make up only 0.7% of the total uranium reserve, but they are the ones that are suitable for carrying out a chain reaction, because share under the influence slow neutrons.

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

Conditions for a chain reaction to occur in uranium-235:

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

Critical mass:

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

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

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

Remember the theme " Atomic physics"for 9th grade:

Radioactivity.
Radioactive transformations.
Composition of the atomic nucleus. Nuclear forces.
Energy of communication. Mass defect
Fission of uranium nuclei.
Nuclear chain reaction.
Nuclear reactor.
Thermonuclear reaction.

Other pages on the topic "Atomic Physics" for grades 10-11:

A BIT OF HISTORY

In 1930, in Cambridge, J. Cockcroft and E. Walson split the atom. The head of the Cavendish Laboratory, Lord E. Rutherford, publicly spoke about this experiment: “The splitting of the atom is just the most elegant experiment and its elegance lies in the fact that it has no practical application."
___

When did work begin in France? on creation atomic weapons and, accordingly, upon purification of uranium isotopes, it was suddenly discovered that uranium from the vicinity of the West African village of Oklo, instead of 0.71% for uranium-235, suitable for ammunition, contains only 0.68%. The ensuing investigation led to the discovery of a unique, truly one-of-a-kind object - natural nuclear reactor! At the same time, during the operation of this reactor, part of the uranium-235 was consumed.
___

Recently humanity celebrated 50th anniversary atomic bombings Hiroshima and Nagasaki. The path to these tragic events also passed under the main grandstand of the Chicago Stadium, where on December 2, 1942, first nuclear chain reaction.
___

From an anecdote about what is a chain reaction: “If someone walks near a dog sitting on a chain, it begins to bark, and other dogs follow.”

Contents of the article

NUCLEUS FISSION, nuclear reaction in which atomic nucleus When bombarded by neutrons, it splits into two or more fragments. The total mass of the fragments is usually less than the sum of the masses of the original nucleus and the bombarding neutron. "Missing Mass" m turns into energy E according to Einstein's formula E = mc 2 where c– speed of light. Since the speed of light is very high (299,792,458 m/s), a small mass corresponds to enormous energy. This energy can be converted into electricity.

The energy released during nuclear fission is converted into heat when the fission fragments are decelerated. The rate of heat release depends on the number of nuclei dividing per unit time. When not large volume In a short time, a large number of nuclei fission occurs, then the reaction has the character of an explosion. This is the operating principle atomic bomb. If a relatively small number of nuclei are divided in a large volume over a longer period of time, the result will be the release of heat that can be used. This is what nuclear power plants are based on. In nuclear power plants, the heat released in nuclear reactors as a result of nuclear fission is used to produce steam, which is supplied to turbines that turn electric generators.

For practical use of fission processes, uranium and plutonium are most suitable. They have isotopes (atoms of this element with different mass numbers), which fission when absorbing neutrons even with very low energies.

The key to the practical use of fission energy was the fact that some elements emit neutrons during the fission process. Although one neutron is absorbed during nuclear fission, this loss is made up by the creation of new neutrons during the fission process. If the device in which fission occurs has a sufficiently large (“critical”) mass, then a “chain reaction” can be maintained due to new neutrons. The chain reaction can be controlled by adjusting the number of neutrons capable of causing fission. If it is greater than one, then the fission intensity increases, and if it is less than one, it decreases.

HISTORICAL BACKGROUND

The history of the discovery of nuclear fission begins with the work of A. Becquerel (1852–1908). Exploring phosphorescence in 1896 various materials, he discovered that minerals containing uranium spontaneously emit radiation that causes a photographic plate to turn black even if an opaque material is placed between the mineral and the plate. solid. Various experimenters have found that this radiation consists of alpha particles (helium nuclei), beta particles (electrons) and gamma quanta (hard electromagnetic radiation).

The first nuclear transformation artificially caused by man was carried out in 1919 by E. Rutherford, who converted nitrogen into oxygen by irradiating nitrogen with alpha particles of uranium. This reaction was accompanied by the absorption of energy, since the mass of its products - oxygen and hydrogen - exceeds the mass of the particles entering the reaction - nitrogen and alpha particles. The release of nuclear energy was first achieved in 1932 by J. Cockcroft and E. Walton, who bombarded lithium with protons. In this reaction, the mass of the nuclei entering the reaction was slightly greater than the mass of the products, as a result of which energy was released.

In 1932, J. Chadwick discovered the neutron, a neutral particle with a mass approximately equal to the mass of the nucleus of a hydrogen atom. Physicists around the world began studying the properties of this particle. It was assumed that a neutron, deprived of an electric charge and not repelled by a positively charged nucleus, would be more likely to cause nuclear reactions. Later results confirmed this guess. In Rome, E. Fermi and his colleagues irradiated almost all the elements of the periodic table with neutrons and observed nuclear reactions with the formation of new isotopes. Evidence of the formation of new isotopes was “artificial” radioactivity in the form of gamma and beta radiation.

The first indications of the possibility of nuclear fission.

Fermi is responsible for the discovery of many neutron reactions known today. In particular, he tried to obtain element with serial number 93 (neptunium) by bombarding uranium (element with serial number 92) with neutrons. At the same time, he recorded electrons emitted as a result of the capture of neutrons in the proposed reaction

238 U + 1 n ® 239 Np + b–,

where 238 U is the isotope of uranium-238, 1 n is a neutron, 239 Np is neptunium and b- – electron. However, the results were mixed. To exclude the possibility that the detected radioactivity belongs to isotopes of uranium or other elements located in periodic table before uranium, had to be carried out chemical analysis radioactive elements.

The results of the analysis showed that the unknown elements correspond to serial numbers 93, 94, 95 and 96. Therefore, Fermi concluded that he had obtained transuranium elements. However, O. Hahn and F. Strassman in Germany, after conducting a thorough chemical analysis, found that among the elements resulting from irradiation of uranium with neutrons, radioactive barium was present. This meant that some of the uranium nuclei were probably splitting into two large fragments.

Confirmation of the possibility of division.

After this, Fermi, J. Dunning and J. Pegram from Columbia University conducted experiments that showed that nuclear fission actually takes place. The fission of uranium by neutrons was confirmed by methods of proportional counters, a cloud chamber, and the accumulation of fission fragments. The first method showed that when a neutron source approaches a uranium sample, high-energy pulses are emitted. In the cloud chamber it was seen that a uranium nucleus, bombarded by neutrons, splits into two fragments. The latter method made it possible to establish that, as theory predicted, the fragments were radioactive. All this taken together convincingly proved that fission actually occurs, and made it possible to confidently judge the energy released during fission.

Since the permissible ratio of the number of neutrons to the number of protons in stable nuclei decreases with decreasing nuclear size, the fraction of neutrons in fragments should be less than in the original uranium nucleus. Thus, there was every reason to assume that the fission process is accompanied by the emission of neutrons. This was soon experimentally confirmed by F. Joliot-Curie and his colleagues: the number of neutrons emitted during the fission process was greater than the number of absorbed neutrons. It turned out that for every absorbed neutron there are approximately two and a half new neutrons. The possibility of a chain reaction and the prospects for creating an exceptionally powerful source of energy and its use for military purposes immediately became obvious. After this, in a number of countries (especially Germany and the USA), work began on the creation of an atomic bomb in conditions of deep secrecy.

Developments during the Second World War.

From 1940 to 1945, the direction of development was determined by military considerations. In 1941, small amounts of plutonium were obtained and a number of nuclear parameters of uranium and plutonium were established. In the USA, the most important production and research enterprises necessary for this were under the jurisdiction of the Manhattan Military Engineering District, to which the Uranium Project was transferred on August 13, 1942. At Columbia University (New York), a group of employees led by E. Fermi and W. Zinn carried out the first experiments in which they studied the multiplication of neutrons in a lattice of blocks of uranium dioxide and graphite - an atomic “boiler”. In January 1942, this work was transferred to the University of Chicago, where in July 1942 results were obtained showing the possibility of a self-sustaining chain reaction. Initially, the reactor operated at a power of 0.5 W, but after 10 days the power was increased to 200 W. Possibility of obtaining large quantities nuclear energy was first demonstrated on July 16, 1945 with the explosion of the first atomic bomb at the Alamogordo test site (New Mexico).

NUCLEAR REACTORS

A nuclear reactor is a facility in which a controlled, self-sustaining chain reaction of nuclear fission is possible. Reactors can be classified by the fuel used (fissile and raw isotopes), by the type of moderator, by the type of fuel elements and by the type of coolant.

Fissile isotopes.

There are three fissile isotopes - uranium-235, plutonium-239 and uranium-233. Uranium-235 is obtained by isotope separation; plutonium-239 - in reactors in which uranium-238 is converted into plutonium, 238 U ® 239 U ® 239 Np ® 239 Pu; uranium-233 - in reactors in which thorium-232 is processed into uranium. Nuclear fuel for a power reactor is selected taking into account its nuclear and chemical properties, as well as cost.

The table below presents the main parameters of fissile isotopes. The total cross section characterizes the probability of interaction of any type between a neutron and a given nucleus. The fission cross section characterizes the probability of a nucleus fission by a neutron. The energy output per absorbed neutron depends on what fraction of nuclei does not participate in the fission process. The number of neutrons emitted in one fission event is important from the point of view of maintaining a chain reaction. The number of new neutrons per absorbed neutron is important because it characterizes the intensity of fission. The fraction of delayed neutrons emitted after fission has occurred is related to the energy stored in the material.

CHARACTERISTICS OF FISSILE ISOTOPES

CHARACTERISTICS OF FISSILE ISOTOPES
Isotope	Uran-235		Uran-233		Plutonium-239
Neutron energy	1 MeV	0.025 eV	1 MeV	0.025 eV	1 MeV	0.025 eV
Full section	6.6 ± 0.1	695 ± 10	6.2 ± 0.3	600±10	7.3 ± 0.2	1005 ± 5
Fission section	1.25 ± 0.05	581 ± 6	1.85 ± 0.10	526 ± 4	1.8 ± 0.1	751 ± 10
Proportion of nuclei not involved in fission	0.077 ± 0.002	0.174 ± 0.01	0.057 ± 0.003	0.098 ± 0.004	0.08 ± 0.1	0.37 ± 0.03
Number of neutrons emitted in one fission event	2.6 ± 0.1	2.43 ± 0.03	2.65 ± 0.1	2.50 ± 0.03	3.03 ± 0.1	2.84 ± 0.06
Number of neutrons per absorbed neutron	2.41 ± 0.1	2.07 ± 0.02	2.51 ± 0.1	2.28 ± 0.02		2.07 ± 0.04
Fraction of delayed neutrons, %	(0.64 ± 0.03)	(0.65 ± 0.02)	(0.26 ± 0.02)	(0.26 ± 0.01)	(0.21 ± 0.01)	(0.22 ± 0.01)
Fission energy, MeV
All sections are given in barns (10 -28 m2).

The table data shows that each fissile isotope has its own advantages. For example, in the case of the isotope with the largest cross section for thermal neutrons (with an energy of 0.025 eV), less fuel is needed to achieve critical mass when using a neutron moderator. Since greatest number neutrons per absorbed neutron occurs in a plutonium fast reactor (1 MeV), in breeding mode it is better to use plutonium in a fast reactor or uranium-233 in a thermal reactor than uranium-235 in a thermal reactor. Uranium-235 is more preferable from the point of view of ease of control, since it has more share delayed neutrons.

Raw material isotopes.

There are two raw material isotopes: thorium-232 and uranium-238, from which the fissile isotopes uranium-233 and plutonium-239 are obtained. The technology for using raw material isotopes depends on various factors, such as the need for enrichment. Uranium ore contains 0.7% uranium-235, and thorium ore contains no fissile isotopes. Therefore, an enriched fissile isotope must be added to thorium. The number of new neutrons per absorbed neutron is also important. Taking this factor into account, we have to give preference to uranium-233 in the case of thermal neutrons (slowed down to an energy of 0.025 eV), since under such conditions the number of emitted neutrons is greater, and therefore the conversion factor is the number of new fissile nuclei per one “spent” fissile nucleus.

Retarders.

The moderator serves to reduce the energy of the neutrons emitted during the fission process from about 1 MeV to thermal energies of about 0.025 eV. Since moderation occurs mainly as a result of elastic scattering on the nuclei of non-fissile atoms, the mass of the moderator atoms must be as small as possible so that the neutron can transfer maximum energy to them. In addition, the moderator atoms must have a small (compared to the scattering cross section) capture cross section, since the neutron has to collide with the moderator atoms many times before it is slowed down to thermal energy.

The best moderator is hydrogen, since its mass is almost equal to the mass of the neutron and, therefore, the neutron loses the greatest amount of energy upon collision with hydrogen. But ordinary (light) hydrogen absorbs neutrons too strongly, and therefore more suitable moderators, despite their slightly larger mass, are deuterium (heavy hydrogen) and heavy water, since they absorb less neutrons. Beryllium can be considered a good moderator. Carbon has such a small neutron absorption cross section that it effectively slows down neutrons, although it requires many more collisions to slow down than hydrogen.

Average N elastic collisions required to slow a neutron from 1 MeV to 0.025 eV using hydrogen, deuterium, beryllium and carbon are approximately 18, 27, 36 and 135, respectively. The approximate nature of these values ​​is due to the fact that, due to the presence of chemical energy of bonds in the moderator, collisions at energies below 0.3 eV are unlikely to be elastic. At low energies, the atomic lattice can transfer energy to neutrons or change the effective mass in a collision, thereby disrupting the moderation process.

Coolants.

The coolants used in nuclear reactors are water, heavy water, liquid sodium, liquid sodium potassium alloy (NaK), helium, carbon dioxide and organic liquids such as terphenyl. These substances are good coolants and have small neutron absorption cross sections.

Water is an excellent moderator and coolant, but it absorbs neutrons too much and has too high blood pressure vapors (14 MPa) at operating temperature 336° C. The best known moderator is heavy water. Its characteristics are close to those of ordinary water, and the neutron absorption cross section is smaller. Sodium is an excellent coolant, but is not effective as a neutron moderator. This is why it is used in fast neutron reactors, where fission produces more neutrons. True, sodium has a number of disadvantages: it induces radioactivity, it has a low heat capacity, it is chemically active and solidifies at room temperature. A sodium-potassium alloy has properties similar to sodium, but remains liquid at room temperature. Helium is an excellent coolant, but it has little specific heat. Carbon dioxide is a good coolant and has been widely used in graphite-moderated reactors. Terphenyl has the advantage over water that it has a low vapor pressure at operating temperature, but it decomposes and polymerizes when exposed to the high temperatures and radiation fluxes found in reactors.

Fuel elements.

The fuel element (fuel element) is a fuel core with a sealed shell. The shell prevents the leakage of fission products and the interaction of fuel with the coolant. The shell material must weakly absorb neutrons and have acceptable mechanical, hydraulic and thermal conductivity characteristics. Fuel elements are usually pellets of sintered uranium oxide in tubes made of aluminum, zirconium or stainless steel; tablets of uranium alloys with zirconium, molybdenum and aluminum, coated with zirconium or aluminum (in the case of an aluminum alloy); graphite tablets with dispersed uranium carbide, coated with impenetrable graphite.

All these fuel elements have their uses, but for pressurized water reactors, uranium oxide pellets in stainless steel tubes are most preferred. Uranium dioxide does not react with water, has high radiation resistance and is characterized by a high melting point.

For high-temperature gas-cooled reactors, graphite fuel cells appear to be quite suitable, but they have the serious disadvantage that gaseous fission products can penetrate through their cladding due to diffusion or due to defects in the graphite.

Organic coolants are incompatible with zirconium fuel elements and therefore require the use of aluminum alloys. The prospects for organic-cooled reactors depend on whether aluminum alloys or powder metallurgy products that would have the strength (at operating temperatures) and thermal conductivity necessary for the use of fins that increase heat transfer to the coolant. Since the heat exchange between the fuel and the organic coolant due to thermal conductivity is small, it is desirable to use surface boiling to increase heat transfer. There will be new problems associated with surface boiling, but these must be resolved if the use of organic fluids is to be beneficial.

REACTOR TYPES

Theoretically, more than 100 are possible different types reactors differing in fuel, moderator and coolants. Most conventional reactors use water, either under pressure or boiling, as the coolant.

Pressurized water reactor.

In such reactors, water serves as a moderator and coolant. The heated water is pumped under pressure into a heat exchanger, where the heat is transferred to the water in the secondary circuit, which produces steam that rotates the turbine.

Boiling reactor.

In such a reactor, water boils directly in the reactor core and the resulting steam enters the turbine. Most boiling water reactors also use water as a moderator, but sometimes a graphite moderator is used.

Liquid metal cooled reactor.

In such a reactor, liquid metal circulating through pipes is used to transfer the heat released during the fission process in the reactor. Almost all reactors of this type use sodium as the coolant. The steam generated on the other side of the primary circuit pipes is fed to a conventional turbine. A liquid metal cooled reactor can use relatively high energy neutrons (fast neutron reactor) or neutrons moderated in graphite or beryllium oxide. Liquid-metal-cooled fast neutron reactors are more preferable as breeder reactors, since in this case there are no neutron losses associated with moderation.

Gas-cooled reactor.

In such a reactor, the heat released during the fission process is transferred to a steam generator by gas - carbon dioxide or helium. The neutron moderator is usually graphite. A gas-cooled reactor can operate at much higher high temperatures, rather than a liquid-cooled reactor, and is therefore suitable for industrial heat supply systems and for power plants with high efficiency. Small gas-cooled reactors are characterized by increased operational safety, in particular, there is no risk of reactor meltdown.

Homogeneous reactors.

The core of homogeneous reactors uses a homogeneous liquid containing a fissile isotope of uranium. The liquid is usually a molten compound of uranium. It is pumped into a large spherical pressure vessel, where a fission chain reaction occurs at a critical mass. The liquid is then fed into the steam generator. Homogeneous reactors have not become widespread due to design and technological difficulties.

REACTIVITY AND CONTROL

The possibility of a self-sustaining chain reaction in a nuclear reactor depends on how much neutron leakage is from the reactor. Neutrons produced during fission disappear as a result of absorption. In addition, neutron leakage is possible due to diffusion through a substance, similar to the diffusion of one gas through another.

To control a nuclear reactor, you need to be able to regulate the neutron multiplication factor k, defined as the ratio of the number of neutrons in one generation to the number of neutrons in the previous generation. At k= 1 (critical reactor) a stationary chain reaction with constant intensity takes place. At k> 1 (supercritical reactor), the intensity of the process increases, and at k r = 1 – (1/ k) is called reactivity.)

Due to the phenomenon of delayed neutrons, the time of “birth” of neutrons increases from 0.001 s to 0.1 s. This characteristic reaction time allows it to be controlled using mechanical actuators - control rods made of a material that absorbs neutrons (B, Cd, Hf, In, Eu, Gd, etc.). The control time constant should be on the order of 0.1 s or more. To ensure safety, a reactor operating mode is chosen in which delayed neutrons are needed in each generation to maintain a stationary chain reaction.

Control rods and neutron reflectors are used to ensure a given power level, but the control task can be greatly simplified correct calculation reactor. For example, if a reactor is designed so that reactivity decreases as power or temperature increases, then it will be more stable. For example, if the slowdown is insufficient due to an increase in temperature, the water in the reactor expands, i.e. the density of the moderator decreases. As a result, the absorption of neutrons in uranium-238 increases, since they do not have time to effectively slow down. Some reactors take advantage of the factor of increasing neutron leakage from the reactor due to the decrease in water density. Another way to stabilize a reactor is by heating a “resonant neutron absorber,” such as uranium-238, which then absorbs neutrons more strongly.

Security systems.

The safety of the reactor is ensured by one or another mechanism for stopping it in the event of a sharp increase in power. This may be the mechanism of a physical process or the operation of a control and protection system, or both. When designing pressurized water reactors, emergency situations associated with the flow of cold water into the reactor, a drop in coolant flow and too much reactivity at start-up. Since the intensity of the reaction increases with decreasing temperature, when cold water suddenly enters the reactor, reactivity and power increase. The protection system usually includes an automatic lock to prevent the flow of cold water. When coolant flow decreases, the reactor overheats, even if its power does not increase. In such cases, automatic shutdown is necessary. In addition, coolant pumps must be designed to supply the coolant required to shut down the reactor. An emergency situation may arise when starting up a reactor with too high reactivity. Because of low level power, the reactor does not have time to heat up enough for the temperature protection to operate until it is too late. The only reliable measure in such cases is careful startup of the reactor.

Avoid those listed emergency situations quite simple, if you follow the following rule: all actions that can increase the reactivity of the system must be performed carefully and slowly. The most important thing in the issue of reactor safety is the absolute necessity of long-term cooling of the reactor core after the fission reaction in it ceases. The fact is that the radioactive fission products remaining in the fuel cassettes generate heat. It is much less than the heat generated in the mode full power, but it is enough to melt the fuel elements in the absence of the necessary cooling. A brief interruption in the supply of cooling water led to significant damage to the core and a reactor accident at Three Mile Island (USA). Destruction of the reactor core is minimal damage in the event of such an accident. It would be worse if dangerous radioactive isotopes leaked. Most industrial reactors are equipped with hermetically sealed safety vessels, which should prevent the release of isotopes into the environment in the event of an accident.

In conclusion, we note that the possibility of destruction of a reactor largely depends on its design and design. Reactors can be designed in such a way that reducing coolant flow will not lead to major problems. These are various types gas-cooled reactors.

All this confusion is now quite clear. It turned out that under the influence of neutrons a new type of nuclear transformation can occur in uranium. This transformation, discovered in 1938 by Hahn and Strassmann and became known in early 1939, is that, having captured a neutron, the uranium nucleus can split into two halves.

In all other nuclear reactions, at most an alpha particle is ejected from the nucleus. Here, two nuclei of average atomic weight are obtained from uranium, for example, krypton and barium:

(uranium) 2|| + neutron ->. (uranium) Ш (krypton) ^ -[- (barium)’|?.

The binding energy of the fragments, i.e., the nuclei of krypton and barium, is significantly greater than that of uranium. Therefore, the fission of uranium releases a huge energy of 170 million volts, i.e. 10 times more than when ligium is destroyed by protons. The energy released during fission transforms into the kinetic energy of uranium fragments, that is, these fragments acquire enormous speed.

The fission of uranium, by the way, is similar to the fission of LITHIUM:

(lithium) -(- proton) (beryllium) ® -".(helium) 2+ (helium) *.

In both cases, the nucleus is divided into two halves, and the reasons for the release of energy are also the same. However, nuclei heavier than lithium always emit, at most, an alpha particle; When lithium is destroyed, only alpha particles are also obtained. Therefore, the fission of uranium is a very special phenomenon.

Let's see how this fission of uranium occurs. The uranium nucleus, consisting of more than two hundred particles, is like a small round charged droplet and has a spherical shape (Fig. 16, a). If we begin to change the shape of the nucleus, then exactly the same thing will happen as with the droplet. With little

When the core is stretched, it tends to return to its original spherical shape, since in this case the surface of the core is the smallest; Increasing the surface area is not beneficial; it requires energy expenditure.

But if we greatly change the shape of the core, as shown in Fig. 16,v, - then you will be the core

It is better to fall apart into two halves, because both parts of the nucleus repel each other by electrical forces, and this repulsion becomes significant.

No, than the loss of energy associated with an increase in surface area.

Thus, in order for the fission of the uranium nucleus to occur, it is necessary to cause strong movements in the nucleus, which would lead to the desired change in its shape.

4 V. L. Ginzburg 49

A neutron entering a uranium nucleus can excite strong movements and thereby lead to the fission of this nucleus. When fission occurs, various fragments are obtained, for example, krypton and barium, or rubidium and cesium (from case to case, either one pair of nuclei or another can be obtained).

The fragments can be observed in the cloud chamber (Fig. 17).

All fragments resulting from the fission of uranium are, however, characterized by one feature - they turn out to be very overloaded with neutrons. The point is

The fact is that in heavier elements the ratio of the number of neutrons to the number of protons is greater than in light elements.

For example, in uranium2!! There are 146 neutrons and 92 protons, and in oxygen the number of neutrons and protons is the same.

The naturally occurring isotopes of krypton and barium have at most 50 and 82 neutrons, respectively, or a total of 132 neutrons. Meanwhile, the 239-weight uranium nucleus, which decays into krypton and barium, has 147 neutrons; therefore, the krypton and barium nuclei formed during the fission of uranium will together have 50

15 extra neutrons. This circumstance leads to the fact that in the fragments resulting from the fission of uranium, excess neutrons turn into protons, i.e. these fragments turn out to be radioactive and emit beta particles. Krypton, for example, decays like this:

(krypton) 3(R> (rubidium) 37-- (electron) (strontium) 38-)- (electron).

Thus, when uranium fissions, many elements appear, most of which are radioactive.

But the overload of fragments with neutrons is so great that the matter is not limited to radioactivity, and several neutrons simply fly out freely.

Consequently, during the fission of uranium caused by neutrons, new neutrons are released, the number of which is equal to two or three per decaying nucleus (Fig. 18).

This fact plays a decisive role in the use of nuclear energy.

The fission of uranium turns out to be just the type of nuclear transformation in which one neutron leads to the emission of several new neutrons. At the same time, a lot of energy is released. If the neutrons produced during fission can successfully cause new fission of nuclei, then the number of neutrons and broken nuclei will increase all the time, and the reaction will not stop.

Moreover, if special measures are not taken, this reaction will grow so violently that an explosion will result. Such a reaction, growing without any external sources, as we have already said, is called a chain reaction.

It turned out that in uranium such a chain reaction occurs when certain conditions can be implemented.

This is how nuclear energy was first released.