Regulation of the fission rate of heavy nuclei. The operating principle of a nuclear reactor. Anatomy of the Chernobyl accident in a nutshell

The schematic diagram of a nuclear reactor using thermal (slow) neutrons is shown in Fig. 5.1, here 1 - control rods, 2 - biological protection, 3 - thermal protection, 4 - moderator, 5 - nuclear fuel (fuel rods).

When a neutron hits the nucleus of the uranium 235 isotope, it splits into two parts and several (2.5-3) new secondary neutrons are emitted. In order for a chain reaction to be maintained in a nuclear reactor, it is necessary that the mass of nuclear fuel in the reactor core be no less than critical. The reactor must contain this amount 235 U so that, on average, at least one of the resulting neutrons in each fission event can cause the next fission event before it leaves the reactor core.

Figure 5.1. Schematic diagram of a thermal neutron nuclear reactor

If the number of neutrons is kept constant, then the fission reaction will have a stationary character. The higher the steady-state level of the number of existing neutrons, the greater the power of the reactor. A power of 1 MW corresponds to a chain reaction in which 3 10 16 divisions occur in 1 second.

If the number of neutrons increases, a thermal explosion will occur; if it decreases, the reaction will stop. The reaction rate is controlled using control rods 1.

The current state of a nuclear reactor can be characterized as efficient neutron multiplication factor or reactivity, which are interconnected by the relationship:

The following values ​​are typical for these quantities:

· - the chain reaction increases over time, the reactor is in a supercritical state, its reactivity;

· , - the number of nuclear fissions is constant, the reactor is in a stable critical state.

A nuclear reactor can operate at a given power for a long time only if it has a reactivity reserve at the beginning of operation. During the operation of a nuclear reactor, due to the accumulation of fission fragments in the fuel, its isotopic and chemical composition changes, and transuranic elements, mainly Pu, are formed. The processes occurring in the reactor reduce the possibility of a chain reaction of fission of atomic nuclei.

To maintain and implement a chain reaction, it is necessary to limit the absorption of neutrons by the materials surrounding the reactor core. This is achieved by using materials (for biological 2 and thermal 3 protection) that at least partially (ideally 50%) reflect neutrons, i.e. didn't absorb them. Of particular importance is the choice of coolant used to transfer heat from the core to the turbine.

The neutrons produced as a result of fission can be fast (high speed) or slow (thermal). Probability of capture of a slow neutron by a nucleus 235 U and its subsequent splitting is greater than that of a fast neutron. Therefore, fuel rods 5 are surrounded by special moderators 4, which slow down neutrons, weakly absorbing them. To reduce neutron leakage from the reactor, it is equipped with a reflector. The most commonly used moderators and reflectors are graphite, heavy ( D2O), ordinary water, etc.

The number of stationary existing neutrons determines the number of nuclear fission fragments formed, which fly away in different directions at enormous speed. The braking of fragments leads to heating of the fuel and the walls of the fuel rods. To remove this heat, the reactor is fed coolant, heating of which is the purpose of the reactor. Often the same substance, for example ordinary water, performs the functions coolant, moderator and reflector. Water is supplied to the reactor using main circulation pumps(MCP).

The neutron nuclear reaction of fission of heavy nuclei, as already noted, is the main and central reaction in nuclear reactors. Therefore, it makes sense from the very beginning to get acquainted with the physical concepts of the fission reaction and those of its features that in one way or another leave their mark on all aspects of life and everyday life of the most complex technical complex, which is called a Nuclear Power Plant.

An idea of ​​the fission of the uranium-235 nucleus in visual images is given in Fig. 2.6.

Neutron Nucleus of mass A Excited compound nucleus Fission fragments

Fission neutrons

Fig.2.6. Schematic representation of 235 U nuclear fission.

Based on this diagram, the generalized fission reaction "equation" (which is logical rather than strictly mathematical) can be written as:

235 U + 1 n  (236 U) *  (F 1)* + (F 2)* +  5. 1 n + a + b + c + E

- (F 1)* and (F 2)* - symbolic designations excited fission fragments (the index (*) hereinafter denotes unstable, excited or radioactive elements); fragment (F 1)* has mass A 1 and charge Z 1, fragment (F 2)* has mass A 2 and charge Z 2;

-  5 . 1 n are designated  5 fission neutrons released on average in each fission event of the uranium-235 nucleus;

- ,  and  - -particles, -particles and -quanta, the average numbers of which per act of fission of the uranium-235 nucleus are equal to a, b and c, respectively;

E is the average amount of energy released in the fission act.

Let us emphasize once again: the expression written above is not an equation in the strict sense of the word; rather, it is simply an easy-to-remember form of notation that reflects the main features of the neutron fission reaction:

a) formation of fission fragments;

b) the formation of new free neutrons during fission, which we will henceforth briefly call fission neutrons;

c) radioactivity of fission fragments, which causes their further transformation into more stable formations, which results in a number of side effects - both positive, useful, and negative, which must be taken into account when designing, constructing and operating nuclear reactors;

d) the release of energy during fission is the main property of the fission reaction, which makes it possible to create energetic nuclear reactor.

Each of the physical processes listed above that accompany the fission reaction plays a certain role in the reactor and has its own practical meaning. Therefore, let's get to know them in more detail.

2.2.1. Formation of fission fragments. A single act of nuclear fission can be spoken of as a phenomenon to a certain extent random, bearing in mind that the heavy uranium nucleus, consisting of 92 protons and 143 neutrons, is fundamentally capable of splitting into a different number of fragments with different atomic masses. In this case, assessing the possibility of dividing a nucleus into 2, 3 or more fragments can be approached with probabilistic measures. According to the data given in, the probability of a nucleus dividing into two fragments is more than 98%, therefore, the vast majority of fissions end in the formation of exactly two fragments.

Spectroscopic studies of fission products have identified more than 600 qualitatively different fission fragments with different atomic masses. And here, in an apparent accident, with a large number of divisions, one immediately emerged general pattern which can be briefly expressed as follows:

The probability of the appearance of a fragment of a certain atomic mass during the mass fission of a particular nuclide is a strictly defined value characteristic of this fissile nuclide.

This quantity is usually called specific fragment yield , denoted by a small Greek letter  i(gamma) with a subscript - a symbol of the chemical element of which this fragment is the nucleus, or a symbol of an isotope.

For example, in physical experiments it has been recorded that a fragment of xenon-135 (135 Xe) appears on average in three cases every thousand fissions of 235 U nuclei. This means that the specific yield of 135 Xe fragments is

 Xe= 3/1000 = 0.003 of all divisions,

and in relation to a single fission event of the 235 U nucleus, the value  Xe = 0.003 = 0.3% - is the probability that fission will result in the formation of a fragment 135 Heh.

A clear assessment of the pattern of formation of fission fragments of different atomic masses is given by the curves of the specific yield of fragments (Fig. 2.7).

10

70 80 90 100 110 120 130 140 150 A, a.m.u.

Rice. 2.7. Specific yields of fission fragments of various atomic masses

during the fission of 235 U (solid line) and 239 Pu (dashed line) nuclei.

The nature of these curves allows us to conclude the following:

a) The atomic masses of fragments formed during fission, in the vast majority of cases, lie within the range of 70  165 amu. The specific yield of lighter and heavier fragments is very small (does not exceed 10 -4%).

b) Symmetrical fission of nuclei (that is, fission into two fragments of equal masses) are extremely rare: their specific yield does not exceed 0.01% for uranium-235 nuclei and 0.04% for plutonium-239 nuclei.

c) Most often formed lungs fragments with mass numbers within 83  104 amu. And heavy fragments with A = 128  149 a.m.u. (their specific yield is 1% or more).

d) The fission of 239 Pu under the influence of thermal neutrons leads to the formation of several more severe fragments compared to 235 U fission fragments.

*) In the future, when studying the kinetics of the reactor and the processes of its poisoning and slagging, we will more than once have to refer to the values ​​of the specific yields of many fission fragments when drawing up differential equations that describe the physical processes in the reactor core.

The convenience of this value is that, knowing the rate of the fission reaction (the number of fissions per unit volume of the fuel composition per unit time), it is easy to calculate the rate of formation of any fission fragments, the accumulation of which in the reactor in one way or another affects its operation:

Generation rate of i-th fragment =  i  (rate of fission reaction)

And one more note related to the formation of fission fragments. The fission fragments generated during fission have high kinetic energies. By transferring their kinetic energy during collisions with atoms of the fuel composition medium, fission fragments thereby increase the average level of kinetic energy of atoms and molecules, which, in accordance with the ideas of kinetic theory, is perceived by us as temperature increase fuel composition or how heat generation in it.

Most of the heat in the reactor is generated in this way.

This is a certain positive role of the formation of fragments in the operating process of a nuclear power reactor.

2.2.2. Production of fission neutrons. The key physical phenomenon accompanying the process of fission of heavy nuclei is emission of secondary fast neutrons by excited fission fragments, otherwise called prompt neutrons or fission neutrons.

The significance of this phenomenon (discovered by F. Joliot-Curie and his colleagues - Albano and Kowarski - in 1939) is undeniable: it is thanks to it that during the fission of heavy nuclei, new free neutrons appear to replace those that caused the fission; these new neutrons can interact with other fissile nuclei in the fuel and cause them to fission, followed by the emission of new fission neutrons, etc. That is, due to the formation of fission neutrons, it becomes possible organize a process of fissions uniformly following each other in time without the supply of free neutrons to the fuel-containing medium from an external source. In such a delivery, simply put, not necessary, as long as the “tools” with the help of which nuclear fission is carried out are found here, in this very environment, in a bound state in fissile nuclei; in order to “put into action” bound neutrons, they only need to be made free, that is, the nucleus must be divided into fragments, and then the fragments themselves will complete everything: due to their excited state, they will emit “extra” neutrons from their composition, interfering with them stability, and this will happen in a time of the order of 10 -15 - 10 -13 s, coinciding in order of magnitude with the time the compound nucleus remains in an excited state. This coincidence gave rise to the idea that fission neutrons appear not from excited fission fragments supersaturated with neutrons after the end of fission, but directly in that short period of time during which nuclear fission occurs. That is, not after act of division, and during this act, as if simultaneously with the destruction of the core. For the same reason, these neutrons are often called prompt neutrons.

An analysis of possible combinations of protons and neutrons in stable nuclei of various atomic masses (remember the diagram of stable nuclei) and their comparison with the qualitative composition of fission products showed that probability of formationsustainable There are very few fragments during fission. This means that the vast majority of fragments are born unstable and can emit one, two, three or even more “extra” fission neutrons for their stability, and, it is clear that each specific excited fragment must emit your own, strictly defined, the number of fission neutrons “extra” for its stability.

But since each fragment with a large number of fissions has a strictly defined specific yield, then with a certain large number of fissions the number of fission fragments of each type formed will also be certain, and, consequently, the number of fission neutrons emitted by fragments of each type will also be certain, and, This means that their total number will also be certain. Dividing the total number of neutrons produced in fissions by the number of fissions in which they were produced, we should get average number of fission neutrons emitted in one fission event, which, based on the above reasoning, should also be strictly defined and constant for each type of fissile nuclide. This physical constant of a fissile nuclide is designated  .

According to 1998 data (the value of this constant is periodically updated based on the results of an analysis of physical experiments around the world) during fission under the influence of thermal neutrons

For uranium-235  5 = 2.416,

For plutonium-239  9 = 2.862,

For plutonium-241  1 = 2.938, etc.

The last remark is useful: the value of the constant  depends significantly on the magnitude of the kinetic energy of the neutrons causing fission and, as the latter increases, it increases approximately in direct proportion to E.

For the two most important fissile nuclides, the approximate dependences (E) are described by empirical expressions:

For uranium-235  5 (E) = 2.416 + 0.1337 E;

For plutonium-239  9 (E) = 2.862 + 0.1357 E.

*) Neutron energy E is substituted in [MeV].

Thus, the value of the constant , calculated using these empirical formulas, at different neutron energies can reach the following values:

So, the first characteristic of fission neutrons emitted during the fission of specific fissile nuclides is the inherent average number of fission neutrons produced in a fission event .

It is a fact that for all fissile nuclides  > 1, creates a prerequisite for feasibility chain neutron fission reaction. It is clear that to implement self-sustaining fission chain reaction it is necessary to create conditions so that one from  neutrons obtained in the fission act definitely called the next division of another nucleus, and rest ( - 1) neutrons somehow excluded from the process of nuclear fission. Otherwise, the intensity of divisions will increase in time like an avalanche (which is what happens in atomic bomb).

Since it is now known that the value of the constant  increases with increasing energy of fission-causing neutrons, a logical question arises: with what kinetic energy born fission neutrons?

The answer to this question is given by the second characteristic of fission neutrons, called energy spectrum of fission neutrons and representing the distribution function of fission neutrons over their kinetic energies.

If in a unit (1 cm3) volume of the medium at some considered moment in time appear n fission neutrons of all possible energies, then normalized energy spectrum is a function of the amount of energy E, the value of which at any particular value of E shows what part (proportion) of all these neutrons are neutrons with energies of the elementary interval dE near the energy E. In other words, we are talking about the expression

The energy distribution of fission neutrons is described quite accurately Watt's spectral function(Watt):

n(E) = 0.4839
, (2.2.2)

a graphic illustration of which is Fig. 2.8. on the next page.

Watt's spectrum shows that, although fission neutrons are produced with very different energies, lying in a very wide range, most neutrons have initial energy,equal to E nv = 0.7104 MeV, corresponding to the maximum of Watt's spectral function. In meaning, this value is the most probable energy of fission neutrons.

Another quantity characterizing the energy spectrum of fission neutrons is average energy of fission neutrons , that is, the amount of energy that each fission neutron would have if the total real energy of all fission neutrons were equally divided between them:

E av =  E n(E) dE /  n(E) dE (2.2.3)

Substituting expression (2.2.2) into (2.2.3) gives the value of the average energy of fission neutrons

E Wed = 2.0 MeV

And this means that almost everything fission neutrons are born fast(that is, with energies E > 0.1 MeV). But few fast neutrons with relatively high kinetic energies are produced (less than 1%), although a noticeable number of fission neutrons appear with energies up to 18 - 20 MeV.

0 1 2 3 4 5 E, MeV

Fig.2.8. The energy spectrum of fission neutrons is the Watt spectrum.

Fission neutron spectra for different fissile nuclides differ from each other slightly. Let's say, for the nuclides 235 U and 239 Pu that we are primarily interested in, the values ​​of the average energies of fission neutrons (corrected based on the results of physical experiments):

E av = 1.935 MeV - for 235 U and E av = 2.00 MeV - for 239 Pu

The value of the average energy of the spectrum of fission neutrons increases with increasing energy of neutrons causing fission, but this increase is insignificant(at least within the range of 10 - 12 MeV). This allows us to ignore it and approximately calculate the energy spectrum of fission neutrons uniform for various nuclear fuels and for different spectrum (fast, intermediate and thermal) reactors.

For uranium-238, despite the threshold nature of its fission, the spectrum of fission neutrons also practically coincides with the expression(2.2.2), and the dependence of the average number of fission neutrons  8 from the energy of fission-causing neutrons - also practically linear at energies above the threshold ( E P = 1.1 MeV):

 8 (E) = 2.409 + 0.1389E. (2.2.4)

2.2.3. Radioactivity of fission fragments. It has already been said that about 600 types of fission fragments have been identified, differing in mass and proton charge, and that practically All they are bornvery excited .

The matter is further complicated by the fact that they carry significant excitement and after emission of fission neutrons. Therefore, in a natural desire for stability, they continue to “dump” excess energy above the level of the ground state until this level is reached.

This discharge is carried out by the sequential emission of fragments of all types of radioactive radiation (alpha, beta and gamma radiation), and for different fragments, different types of radioactive decay occur in different sequences and (due to differences in the values ​​of decay constants ) are stretched to varying degrees in time.

Thus, in an operating nuclear reactor, not only the process savings radioactive fragments, but also the process of their continuous transformation: a large number is known chains successive transformations, ultimately leading to the formation of stable nuclei, but all these processes require different times, for some chains - very short, and for others - quite long.

Therefore, radioactive radiation not only accompanies the fission reaction in working reactor, but are also emitted by the fuel for a long time after it is shut down.

This factor, firstly, gives rise to a special type of physical danger - danger personnel exposure, servicing the reactor installation, briefly referred to as radiation hazard. This forces reactor plant designers to provide for its environment. biological protection, place it in rooms isolated from the environment and take a number of other measures to eliminate the possibility of dangerous exposure of people and radioactive contamination of the environment.

Secondly, after the reactor is shut down, all types of radioactive radiation, although decreasing in intensity, continue to interact with the materials of the core and, like the fission fragments themselves in the initial period of their free existence, transfer their kinetic energy to the atoms of the core medium, increasing their average kinetic energy. That is in the reactor after its shutdown takes place decay heat .

It is easy to understand that the power of residual heat release in the reactor at the moment of shutdown is directly proportional to the number of fragments accumulated during the operation of the reactor at that moment, and the rate of its decline is subsequently determined by the half-lives of these fragments. From what has been said another follows negative factor due to the radioactivity of fission fragments - necessitylong-termcooling down reactor core after shutdown in order to remove residual heat, and this is associated with a significant consumption of electricity and the motor life of the circulation equipment.

Thus, the formation of radioactive fragments during fission in a reactor is a phenomenon mainly negative, but... every cloud has a silver lining!

In the radioactive transformations of fission fragments one can also see positive aspect that nuclear reactors literally owe their existence . The fact is that out of a large variety of fission fragments, there are about 60 types that, after the first -decay, become neutronactive , capable of emitting so-called lagging neutrons. Relatively few delayed neutrons are emitted in the reactor (approximately 0.6% of the total number of generated neutrons), but it is thanks to their existence that it is possible safe management nuclear reactor; We will be convinced of this when studying the kinetics of a nuclear reactor.

2.2.4. Release of energy during fission. The nuclear fission reaction in physics is one of the clear confirmations of A. Einstein’s hypothesis about the relationship between mass and energy, which in relation to nuclear fission is formulated as follows:

The amount of energy released during nuclear fission is directly proportional to the size of the mass defect, and the coefficient of proportionality in this relationship is the square of the speed of light:

E=  mс 2

During nuclear fission, the excess (defect) of mass is defined as the difference in the sum of the rest masses of the initial products of the fission reaction (i.e., nucleus and neutron) and the resulting products of nuclear fission (fission fragments, fission neutrons and other microparticles emitted both during the fission process and after him).

Spectroscopic analysis made it possible to determine the majority of fission products and their specific yields. On this basis it turned out to be not so difficult to calculate private the magnitude of mass defects for various results of fission of uranium-235 nuclei, and from them - calculate the average amount of energy released in a single fission, which turned out to be close to

mc 2 = 200 MeV

It is enough to compare this value with the energy released in the act of one of the most endothermic chemical reactions - oxidation reactions of rocket fuel (value less than 10 eV) - to understand that at the level of microscopic objects (atoms, nuclei) 200 MeV - very high energy: it is at least eight orders of magnitude (100 million times) greater than the energy obtained from chemical reactions.

Fission energy is dissipated from the volume where nuclear fission occurred through various material carriers: fission fragments, fission neutrons, - and -particles, -quanta and even neutrinos and antineutrinos.

The distribution of fission energy between material carriers during the fission of 235 U and 239 Pu nuclei is given in Table 2.1.

Table 2.1. Distribution of fission energy of uranium-235 and plutonium-239 nuclei between fission products.

Fission energy carriers		Plutonium-239
1. Kinetic energy of fission fragments
2. Kinetic energy of fission neutrons
3. Energy of instantaneous gamma quanta
4. Energy of -quanta from fission products
5. Kinetic energy of -radiation of fragments
6. Antineutrino energy

Various components of fission energy are transformed into heat not at the same time.

The first three components turn into heat in a time of less than 0.1 s (counting from the moment of division), and therefore are called instant sources of heat release.

- and -radiations from fission products are emitted by excited fragments with the most varied half-lives(from a few fractions of a second to several tens of days, if we take into account only fragments with noticeable specific yield), and therefore the process mentioned above decay heat, which is precisely caused by radioactive emissions from fission products, can last tens of days after the reactor is shut down.

*) According to very rough estimates, the power of residual heat release in the reactor after its shutdown decreases in the first minute - by 30-35%; after the first hour of shutdown of the reactor, it is approximately 30% of the power at which the reactor operated before shutdown, and after the first day parking - approximately 25 percent. It is clear that stopping the forced cooling of the reactor under such conditions is out of the question, because Even a short-term cessation of coolant circulation in the core is fraught with the danger of thermal destruction of fuel elements. Only after several days of forced cooling of the reactor, when the power of residual heat release is reduced to the level of the coolant removed due to natural convection, can the circulation means of the primary circuit be stopped.

The second practical question for an engineer: where and what part of the fission energy is transformed into heat in the reactor? - since this is due to the need to organize a balanced heat removal from its various internal parts, designed in various technological designs.

Fuel composition, which contains fissile nuclides, is contained in sealed shells that prevent the release of formed fragments from the fuel composition of fuel elements (fuel elements) into the coolant that cools them. And, if fission fragments in a working reactor do not leave the fuel elements, it is clear that the kinetic energies of the fragments and weakly penetrating -particles are converted into heat inside fuel rods.

The energies of fission neutrons and -radiation are transformed into heat inside the fuel elements only partially: the penetrating ability of neutrons and -radiation generates entrainment most of their initial kinetic energy from their birthplaces.

Knowing the exact value of the fission energy and its share of the resulting heat inside the fuel elements is of great practical importance, allowing one to calculate another practically important characteristic called specific volumetric heat release in fuel rod fuel (q v).

For example, if it is known that in 1 cm 3 of the fuel composition of a fuel element, in 1 s R f fissions of uranium-235 nuclei, then it is obvious: the amount of thermal energy generated every second in this unit volume (= thermal power of 1 cm 3 of fuel) is the specific volumetric heat release (or energy intensity) fuel, and this value will be equal to:

q v = 0.9 .  E . R f (2.2.5)

The share of fission energy received in the form of heat outside the fuel elements in the reactor core depends on its type and design and lies within (6  9)% of the total fission energy. (For example, for VVER-1000 this value is approximately 8.3%, and for RBMK-1000 it is about 7%).

Thus, the share of the total heat release in the core volume of the total fission energy is 0.96  0.99, i.e. with technical precision coincides with the total fission energy.

Hence another technical characteristic of the reactor core:

- average energy intensity of the core(q v) az - thermal power received per unit volume of the core:

(q v) az = (0.96-0.99)  E . R f   E . R f (2.2.6)

Since the energy is 1 MeV in the SI system it corresponds to 1.602. 10 -13 J, then the value of the energy intensity of the reactor core:

(q v) az  3.204 . 10 -11 R f .

Therefore, if the value of the average energy intensity over the core volume is known, then reactor thermal power will obviously be:

Q p= (q v) az. V az 3.204. 10–11 . R f . V az [W] (2.2.7)

The thermal power of the reactor is directly proportional average speed

fission reactions in its core.

Practical consequence : Do you want the reactor to work atconstant power level? - Create conditions in it such that the fission reaction in its active zone occurs with a constant average speed over time. Do you need to increase (decrease) the reactor power? - Find ways to increase (or decrease) the reaction rate accordingly de leniya. This is the primary meaning of controlling the power of a nuclear reactor.

The considered relationships and conclusions seem obvious only in the simplest case, when the fuel component in the reactor is one uranium-235. However, repeating the reasoning for a reactor with multicomponent fuel composition, it is easy to verify the proportionality of the average fission reaction rate and the thermal power of the reactor in the most general case.

Thus, the thermal power of the reactor and heat distribution in its core are directly proportional to the distribution of the fission reaction rate over the volume of the fuel composition of the reactor core.

But from what has been said it is also clear that the rate of fission reaction must be related to the number of free neutrons in the core environment, since it is they (free neutrons) that cause fission reactions, radiative capture, scattering and other neutron reactions. In other words, the rate of the fission reaction, the energy release in the core and the thermal power of the reactor must clearly be related to characteristics of the neutron field in its volume.

After an uncontrolled chain reaction was carried out, which made it possible to obtain a gigantic amount of energy, scientists set the task of implementing a controlled chain reaction. The essence of a controlled chain reaction lies in the ability to control neutrons. This principle has been successfully applied in nuclear power plants (NPPs).

The fission energy of uranium nuclei is used in nuclear power plants (NPPs). The fission process of uranium is very dangerous. Therefore, nuclear reactors are surrounded by dense protective shells. A common type of reactor is pressurized water.

The coolant is water. Cold water enters the reactor under very high pressure, which prevents it from boiling.

Cold water passing through the reactor core also acts as a moderator - slowing down fast neutrons so that they hit the uranium nuclei and cause a chain reaction.

Nuclear fuel (uranium) is located in the core in the form of fuel assembly rods. The fuel rods in the assembly alternate with control rods, which regulate the rate of nuclear fission by absorbing fast neutrons.

Fission releases a large amount of heat. The heated water leaves the core under pressure with a temperature of 300? C and enters the power plant, which houses generators and turbines.

Hot water from the reactor heats the secondary circuit water to a boil. The steam is directed to the turbine blades and rotates it. The rotating shaft transfers energy to the generator. In the generator, mechanical rotational energy is converted into electrical energy. The steam cools and the water returns back to the reactor.

As a result of these complex processes, a nuclear power plant produces electric current.

As you can see, the fissile isotope is located in the fuel rods located in the reactor core, forming a critical mass. The nuclear reaction is controlled using control rods made of boron or cadmium. Control rods, like fuel rods, are located in the reactor core and, like a sponge absorbing water, act on neutrons, absorbing them. The NPP operator, by adjusting the number of control rods in the reactor core, controls the speed of the nuclear process: he slows it down by lowering the control rods into the reactor core; or speeds it up by raising the rods.

It would seem that everything is wonderful - nuclear energy is an inexhaustible high-tech source of electricity and it is the future. That's what people thought until August 26, 1986. The accident at the fourth unit of the Chernobyl nuclear power plant turned everything upside down - the “peaceful” atom turned out to be not so peaceful if treated with disdain.

Quite a lot of material has been written about this. Here the quintessence (condensed essence) of the disaster will be given.

The main causes of the accident of the 4th power unit of the Chernobyl nuclear power plant:

1. An insufficiently well-thought-out program for a technological experiment on the run-down of a turbogenerator;
2. Miscalculations by the developers of the RBMK nuclear reactor, where a significant role was played by the lack of operational information in the control system about the reactivity reserve in the core;
3. The “liberties” of the nuclear power plant personnel who conducted the experiment and allowed deviations from the regulations for the work being carried out.

All this together led to disaster. Among the specialists investigating the events in Chernobyl, there was something like this formula: "the operators managed to blow up the unit, and the reactor allowed them to do it". Part of the Chernobyl guilt lies with almost everyone - and on physicists who carry out calculations using simplified models, and on installers who carelessly weld seams, and on operators who allow themselves to ignore work regulations.

Anatomy of the Chernobyl accident in a nutshell

1. The reactor power was allowed to decrease to a very small value (approximately 1% of the nominal value). This is “bad” for the reactor, because it falls into the “iodine pit” and xenon poisoning of the reactor begins. According to the “normal” approach, it was necessary to shut down the reactor, but in this case the turbine run-down experiment would not have been carried out, with all the ensuing administrative consequences. As a result, the Chernobyl NPP personnel decided to increase the power of the reactor and continue the experiment.

2. From the above material it is clear that the operator of a nuclear power plant can control the rate of nuclear reaction (reactor power) by moving control rods into the reactor core. To increase the power of the reactor (to complete the experiment), almost all control rods were removed from the reactor core.

To make it clearer for the reader who is not familiar with the “nuclear subtleties”, we can give the following analogy with a load suspended on a spring:

• The load (or rather its position) is the power of the reactor;
• The spring is a means of controlling the load (reactor power).
• In the normal position, the load and the spring are in equilibrium - the load is at a certain height, and the spring is stretched by a certain amount.
• When the reactor power failed ("iodine pit"), the load went down to the ground (and went very strongly).
• To “pull out” the reactor, the operator “pulled the spring” (pulled out the control rods; but it was necessary to do just the opposite - insert all the rods and shut down the reactor, i.e., release the spring so that the load falls to the ground). But the load-spring system has some inertia, and for some time after the operator began to pull the spring up, the load still moves downwards. And the operator continues to pull up.
• Finally, the load reaches the lowest point, and under the influence of (already decent) spring forces it begins to move upward - the power of the reactor begins to increase sharply. The load flies upward faster and faster (an uncontrolled chain reaction with the release of a huge amount of heat), and the operator can no longer do anything to extinguish the inertia of the upward movement of the load. As a result, the load hits the operator in the forehead.

Yes, the Chernobyl nuclear power plant operators, who allowed the power unit to explode, paid the highest price for their mistake - their lives.

Why did the Chernobyl NPP personnel act in this way? One of the reasons was the fact that the nuclear reactor control system did not provide the operator with operational information about the dangerous processes occurring in the reactor.

This is how A.S. Dyatlov begins his book "Chernobyl. How it happened":

On April 26, 1986, at one hour, twenty-three minutes, forty seconds, the shift supervisor of Unit No. 4 of the Chernobyl Nuclear Power Plant, Alexander Akimov, ordered the reactor to be shut down upon completion of the work carried out before shutting down the power unit for planned repairs. The command was issued in a calm working environment; the centralized control system does not record a single emergency or warning signal about deviations in the parameters of the reactor or service systems. Reactor operator Leonid Toptunov removed the cap from the AZ button, which protects against accidental mistaken pressing, and pressed the button. At this signal, 187 reactor control rods began moving down into the core. The backlight lights on the mnemonic board lit up, and the arrows of the rod position indicators began to move. Alexander Akimov, standing half-turned to the reactor control panel, observed this, also saw that the “bunnies” of the AR imbalance indicators “darted to the left” (his expression), as it should be, which meant a decrease in the reactor power, turned to the safety panel, behind which I observed in the experiment.
But then something happened that even the wildest imagination could not predict. After a slight decrease, the reactor power suddenly began to increase at an ever-increasing speed, and alarm signals appeared. L. Toptunov shouted about an emergency increase in power. But he was unable to do anything. All he could do was hold down the AZ button, the control rods went into the active zone. He has no other means at his disposal. And everyone else too. A. Akimov sharply shouted: “Shut down the reactor!” He jumped to the control panel and de-energized the electromagnetic clutches of the control rod drives. The action is correct, but useless. After all, the CPS logic, that is, all its elements of logical circuits, worked correctly, the rods went into the zone. Now it is clear - after pressing the AZ button there were no correct actions, there were no means of salvation. Other logic failed!
Two powerful explosions followed with a short interval. The AZ rods stopped moving without going even half way. They had nowhere else to go.
At one hour, twenty-three minutes, forty-seven seconds, the reactor was destroyed by a power run-up using prompt neutrons. This is a collapse, the ultimate disaster that can happen at a power reactor. They didn’t comprehend it, they didn’t prepare for it, no technical measures for localization at the block and station were provided...

That is, a few seconds before the disaster, the personnel did not even suspect the approaching danger! The end of this whole absurd situation was pressing the emergency button, after which an explosion occurred - you are racing in a car and in front of an obstacle you press the brake, but the car accelerates even more and crashes into the obstacle. To be fair, it should be said that pressing the emergency button could not influence the situation in any way - it only accelerated the inevitable explosion of the reactor by a few moments, but the fact remains - emergency protection blew up the reactor !

Impact of radiation on humans

Why are man-made nuclear disasters (not to mention nuclear weapons) so dangerous?

In addition to the release of colossal amounts of energy, which leads to great destruction, nuclear reactions are accompanied by radiation and, as a consequence, radiation contamination of the area.

Why is radiation so harmful to a living organism? If it had not brought such harm to all living things, then everyone would have forgotten about the Chernobyl accident long ago, and atomic bombs would have been thrown left and right.

Radiation destroys the cells of a living organism in two ways:

1. due to heating (radiation burn);
2. due to ionization of cells (radiation sickness).

Radioactive particles and radiation itself have high kinetic energy. Radiation generates heat. This heat, similar to a sunburn, causes a radiation burn, destroying body tissue.