Regulation of the rate of nuclear fission of heavy atoms. "peaceful" atom. Environmental disasters at nuclear power plants

After an uncontrolled chain reaction was carried out, which made it possible to obtain a gigantic amount of energy, scientists set the task of carrying out a controlled chain reaction. The essence of a controlled chain reaction is 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 pressurized water reactor.

The heat carrier 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 - it slows down fast neutrons so that they hit the uranium nuclei and cause a chain reaction.

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

Released upon division a large number of heat. The heated water leaves the core under pressure at 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 sent to the blades of the turbine and rotates it. The rotating shaft transfers energy to the generator. In the generator, the mechanical energy of rotation is converted into electrical energy. The steam is cooled and the water is returned back to the reactor.

As a result of these complex processes, a nuclear power plant generates electricity.

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 by control rods made of boron or cadmium. Control rods, like fuel rods, are located in the reactor core and, like a sponge that absorbs 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: slows it down by lowering the control rods into the reactor core; or accelerates - raising the rods.

It would seem that everything is fine - nuclear energy is an inexhaustible high-tech source of electricity and the future belongs to it. So people thought before August 26, 1986. The accident at the fourth block of the Chernobyl nuclear power plant turned everything upside down - the "peaceful" atom turned out to be not so peaceful, if treated with disdain.

A lot of material has been written about this. Here the quintessence (compressed essence) of the catastrophe will be given.

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

1. Insufficiently well-thought-out program of the technological experiment on the run-out of the turbogenerator;
2. The miscalculations of the developers of the nuclear RBMK reactor, where a significant role was played by the lack of operational information on the reactivity margin in the core in the control system;
3. The "liberties" of the NPP personnel who conducted the experiment and allowed deviations from the regulations of 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 block, and the reactor allowed them to do it". Part of the Chernobyl fault lies with almost everyone - and on physicists who carry out calculations using simplified models, and on assemblers who carelessly weld seams, and on operators who allow themselves to disregard the work schedule.

Anatomy of the Chernobyl accident in a nutshell

1. It was allowed to reduce the reactor power 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" - it was necessary to shut down the reactor, but in this case, the experiment on the run-out of the turbine would not have been carried out, with all the administrative consequences that follow from this. As a result, the Chernobyl personnel decided to increase the reactor power and continue the experiment.

2. It can be seen from the material presented above that the NPP operator can control the nuclear reaction rate (reactor power) by moving the 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 a reader who is not familiar with the "nuclear subtleties", the following analogy can be made 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 (the power of the reactor).
• In the normal position, the weight and the spring are in equilibrium - the weight is at a certain height, and the spring is stretched by a certain amount.
• With the failure of the reactor power ("iodine pit") - the cargo 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 just the opposite - to insert all the rods and shut off 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 down. And the operator continues to pull up.
• Finally, the load reaches its lowest point, and under the influence of (already decent) spring forces, it begins to move upward - the reactor power begins to increase sharply. The load is flying upwards 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 operators of the Chernobyl nuclear power plant, who allowed the explosion of the power unit, paid the highest price for their mistake - their lives.

Why did the personnel of the Chernobyl nuclear power plant 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 was":

On April 26, 1986, at one hour twenty-three minutes forty seconds, Alexander Akimov, the shift supervisor of Chernobyl Unit 4, ordered the reactor to be shut down at the end of the work carried out before the shutdown of the power unit for the planned repairs. The command was given in a calm working environment, the centralized control system does not record a single emergency or warning signal about the deviation of the parameters of the reactor or service systems. The reactor operator Leonid Toptunov removed the cap from the AZ button, which prevents accidental erroneous pressing, and pressed the button. At this signal, 187 control rods of the reactor began to move down into the core. The backlight lamps on the mnemonic panel lit up, and the arrows of the rod position indicators began to move. Alexander Akimov, standing half-turned to the reactor control panel, watched 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 was observed in the experiment.
But then something happened that even the most unbridled fantasy could not predict. After a slight decrease, the reactor power suddenly began to increase at an ever-increasing rate, alarms appeared. L. Toptunov shouted about an emergency increase in power. But there was nothing he could do. He did everything he could - he held the AZ button, the CPS rods went into the core. There are no other resources at his disposal. Yes, and everyone else too. A. Akimov sharply shouted: "Turn off the reactor!" He jumped to the console and de-energized the electromagnetic clutches of the CPS 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 before going half way. They had nowhere else to go.
In one hour, twenty-three minutes, forty-seven seconds, the reactor was destroyed by a power boost on prompt neutrons. This is a collapse, the ultimate catastrophe that can happen in a power reactor. They didn’t comprehend it, they didn’t prepare for it, no technical measures for localization at the block and station are provided ...

That is, a few seconds before the disaster, the staff did not even suspect about the approaching danger! The end of this whole absurd situation was the pressing of the emergency button, after which an explosion occurred - you rush in a car and press the brakes in front of an obstacle, but the car accelerates even more and crashes into an obstacle. In fairness, it should be said that pressing the emergency button could no longer affect the situation - it only accelerated the inevitable explosion of the reactor for a few moments, but the fact remains - emergency protection blew up the reactor !

The impact of radiation on humans

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

In addition to releasing an enormous amount of energy, which leads to great destruction, nuclear reactions are accompanied by radiation and, as a result, radiation contamination of the area.

Why is radiation so harmful to a living organism? If it did not bring such harm to all living things, then everyone would have long forgotten about the Chernobyl accident, and atomic bombs would be 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, by analogy with a sunburn, causes a radiation burn, destroying the tissues of the body.

The neutron nuclear fission reaction 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 life of the most complex technical complex, which is called the Nuclear Power Plant.

Fig. 2.6 gives an idea of ​​the fission of the uranium-235 nucleus in visual images.

Neutron Nucleus of mass A Excited compound nucleus Fission fragments

fission neutrons

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

Based on this diagram, the generalized "equation" for the fission reaction (which is more logical 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)* - symbols excited fission fragments (hereinafter, the index (*) denotes unstable, excited, or radioactive elements); a fragment (F 1)* has a mass A 1 and a charge Z 1 , a fragment (F 2)* has a mass A 2 and a charge Z 2 ;

-  5 . 1 n denotes  5 fission neutrons released on average in each act of fission 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 respectively a, b and c;

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

We emphasize again: the expression written above is not an equation in the strict sense of the word; it is rather just a form of notation that is easy to remember and reflects the main features of the neutron fission reaction:

a) the formation of fission fragments;

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

c) the radioactivity of fission fragments, which causes their further transformations to more stable formations, due to which a series of side effects- both positive, useful, and negative, which must be taken into account when designing, building and operating nuclear reactors;

d) the release of energy during fission - the main property of the fission reaction, which allows you to create energy nuclear reactor.

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

2.2.1. The formation of fission fragments. One can speak of a single act of nuclear fission as a phenomenon to a certain extent random, bearing in mind that the heavy nucleus of uranium, 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, the assessment of the possibility of nuclear fission into 2, 3, or more fragments can be approached with probabilistic measures. According to the data given in, the probability of nuclear fission into two fragments is more than 98%, therefore, the vast majority of fission ends with the formation of exactly two fragments.

Spectroscopic studies of fission products have established more than 600 qualitatively different fission fragments with different atomic masses. And here, in seeming chance, with a large number of divisions, one general rule, 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 called fragment yield , denoted by a small Greek letter  i(gamma) with a subscript - the symbol of the chemical element, the nucleus of which is this fragment, or the symbol of the isotope.

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

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

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

A clear assessment of the patterns of formation of fission fragments of various 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, amu

Rice. 2.7. Specific Yields of Fission Fragments of Different Atomic Masses

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

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

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

b) Symmetrical nuclear fission (that is, fission into two fragments of equal masses) is 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 a.m.u. 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 action of thermal neutrons leads to the formation of several heavier 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 turn to the values ​​of the specific yields of many fission fragments when compiling differential equations describing the physical processes in the reactor core.

The convenience of this value is that, knowing the fission reaction rate (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 somehow affects its operation:

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

And one more remark related to the formation of fission fragments. Fission fragments generated during fission have high kinetic energies. By transferring their kinetic energy during collisions with the atoms of the medium of the fuel composition, the fission fragments thereby increase the average level of kinetic energy of atoms and molecules, which, in accordance with the ideas of the kinetic theory, is perceived by us as temperature increase fuel composition or how heat dissipation 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 working process of a nuclear power reactor.

2.2.2. Formation of fission neutrons. The key physical phenomenon that accompanies 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 with collaborators - Albano and Kovarsky - in 1939) is indisputable: it is thanks to him that during the fission of heavy nuclei, new free neutrons appear to replace those that caused fission; these new neutrons can interact with other fissile nuclei in the fuel and cause them to fission, accompanied by the emission of new fission neutrons, and so on. That is, due to the formation of fission neutrons, it becomes possible organize the process of fissions uniformly following each other in time without supplying free neutrons to the fuel-containing medium from an external source. In such a delivery, simply put, not necessary, as soon as the "tools" with which nuclear fission is carried out are found here, in this very environment, in a bound state in fissile nuclei; in order to "use" the bound neutrons, they only need to be made free, that is, the nucleus is divided into fragments, and then the fragments themselves will finish everything: due to their excited state, they will emit "extra" neutrons from their composition, interfering with their stability, moreover, this will happen in a time of the order of 10 -15 - 10 -13 s, coinciding in order of magnitude with the time spent by the compound nucleus in an excited state. This coincidence gave rise to the notion 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 the act of division, and during this act, as if simultaneously with the destruction of the nucleus. For the same reason, these neutrons are often referred to as 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 fragments during fission is very small. And this means that the vast majority of fragments are born unstable and can emit one, two, three or even more fission neutrons "superfluous" for their stability, moreover, it is clear that each specific excited fragment must emit own, strictly defined the number of fission neutrons "superfluous" 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 formed fission fragments of each type will also be certain, and, consequently, the number of fission neutrons emitted by fragments of each type will also be certain, but, This means that their total number will also be certain. By dividing the total number of neutrons received in fissions by the number of fissions in which they are received, we must obtain average number of fission neutrons emitted in one fission event, which, based on the above reasoning, must also be strictly defined and constant for each kind of fissile nuclides. This physical constant of the fissile nuclide is denoted  .

According to 1998 data (the value of this constant is periodically updated based on the results of the analysis of physical experiments around the world) in fission under the action 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  essentially depends on the value of the kinetic energy of the neutrons that cause fission, and with the growth of the latter 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.

*) The neutron energy E is substituted in [MeV].

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

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

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

Since it is now known that the value of the constant  increases with an increase in the energy of the 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 fission neutron energy spectrum and representing the distribution function of fission neutrons over their kinetic energies.

If in a unit (1 cm 3) volume of the medium at some given point in time, n fission neutrons of all possible energies, then normalized energy spectrum is a function of the energy value E, the value of which, for any particular value of E, shows what part (fraction) 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)

graphic illustration of which is fig.2.8. on the next page.

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

Another quantity characterizing the energy spectrum of fission neutrons is average fission neutron energy , 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)

Substitution in (2.2.3) of expression (2.2.2) gives the value of the average energy of fission neutrons

E Wed = 2.0 MeV

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

0 1 2 3 4 5 Е, 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 235U and 239Pu that are primarily of interest to us, the average energies of fission neutrons (corrected according to 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 fission neutron spectrum increases with the energy of the neutrons that cause fission, but this increase is negligible(at least within the range of 10 - 12 MeV). This makes it possible not to take it into account and to approximately calculate the energy spectrum of fission neutrons common for different nuclear fuels and for different spectrum (fast, intermediate and thermal) reactors.

For uranium-238, despite the threshold nature of its fission, the fission neutron spectrum 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 almost 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 established, 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 considerable excitement and after fission neutron emission. Therefore, in their natural striving for stability, they continue to "discharge" excess energy above the level of the ground state until this level is reached.

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

Thus, in a working nuclear reactor, not only the process accumulation 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 small, and for others - quite long.

Therefore, radioactive radiation not only accompanies the fission reaction in working reactor, but also emitted by the fuel for a long time after its shutdown.

This factor, firstly, gives rise to a special kind of physical danger - the danger personnel exposure, servicing the reactor plant, abbreviated as radiation hazard. This forces the designers of the reactor plant to provide for its environment. biological defense, 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 environment.

Secondly, after the shutdown of the reactor, 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 by this moment, and the rate of its decrease in the future is determined by the half-lives of these fragments. From what has been said follows another negative factor due to the radioactivity of fission fragments - necessitylongdampening reactor core after shutdown in order to remove residual heat, and this is associated with a significant consumption of electricity and motor resources of the circulation equipment.

Thus, the formation of radioactive fragments during fission in a reactor is a phenomenon mainly negative, but ... there is no silver lining!

In the radioactive transformations of fission fragments, one can also see positive aspect to which nuclear reactors are literally owe their existence . The fact is that out of a large number of fission fragments, there are about 60 types of such that, after the first -decay, become neutron active 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 precisely due to their existence that it is possible safe management nuclear reactor; We will verify this when studying the kinetics of a nuclear reactor.

2.2.4. The 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 mass defect, and the proportionality factor in this relationship is the square of the speed of light:

E=  mc 2

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

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

mc 2 = 200 MeV

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

The fission energy is dissipated from the volume where the fission of the nucleus 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 prompt 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 less than 0.1 s (counting from the moment of fission), and therefore are called instant heat sources.

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

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

The second practical question for the 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 exit of the resulting fragments from the fuel composition of fuel elements (fuel rods) into the coolant cooling them. And, if the fission fragments in a working reactor do not leave the fuel rods, 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 power of neutrons and  radiation generates carryover 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 you to calculate another practically important characteristic, called specific volumetric heat release in fuel rods (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, 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 obtained as 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 equal to 8.3%, and for RBMK-1000 - about 7%).

Thus, the share of total heat release in the core volume from 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 power 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 in 1 MeV in the SI system 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 energy density averaged over the volume of the active zone is known, then thermal power of the reactor will obviously be:

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

The thermal power of the reactor is directly proportional to average speed

fission reactions in its active zone.

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

The considered ratios 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 the reactor with multicomponent fuel composition, it is easy to verify the proportionality of the average rate of the fission reaction and the thermal power of the reactor in the most general case.

Thus, the thermal power of the reactor and distribution of heat release 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 the fission reaction should be related to the number of free neutrons in the core medium, 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 be clearly related to neutron field characteristics in its scope.

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Lesson Objectives:

• Educational: updating existing knowledge; continue the formation of concepts: fission of uranium nuclei, nuclear chain reaction, conditions for its occurrence, critical mass; introduce new concepts: a nuclear reactor, the main elements of a nuclear reactor, the design of a nuclear reactor and the principle of its operation, the control of a nuclear reaction, the classification of nuclear reactors and their use;
• Developing: continue the formation of the ability to observe and draw conclusions, as well as develop the intellectual abilities and curiosity of students;
• Educational: to continue the education of attitude towards physics as an experimental science; to cultivate a conscientious attitude to work, discipline, a positive attitude to knowledge.

Lesson type: learning new material.

Equipment: multimedia installation.

During the classes

1. Organizational moment.

Guys! Today in the lesson we will repeat the fission of uranium nuclei, a nuclear chain reaction, the conditions for its occurrence, the critical mass, we will learn what a nuclear reactor is, the main elements of a nuclear reactor, the design of a nuclear reactor and the principle of its operation, control of a nuclear reaction, the classification of nuclear reactors and their use.

2. Checking the studied material.

1. Mechanism of fission of uranium nuclei.
2. Describe the mechanism of a nuclear chain reaction.
3. Give an example of a nuclear fission reaction of the uranium nucleus.
4. What is called critical mass?
5. How does a chain reaction proceed in uranium if its mass is less than critical, more than critical?
6. What is the critical mass of uranium 295, is it possible to reduce the critical mass?
7. How can you change the course of a nuclear chain reaction?
8. What is the purpose of slowing down fast neutrons?
9. What substances are used as moderators?
10. Due to what factors can the number of free neutrons in a piece of uranium be increased, thereby ensuring the possibility of a reaction occurring in it?

3. Explanation of new material.

Guys, answer this question: What is the main part of any nuclear power plant? ( nuclear reactor)

Well done. So, guys, now let's dwell on this issue in more detail.

Historical reference.

Igor Vasilyevich Kurchatov - an outstanding Soviet physicist, academician, founder and first director of the Institute atomic energy from 1943 to 1960, chief scientific leader of the atomic problem in the USSR, one of the founders of the use of nuclear energy for peaceful purposes. Academician of the Academy of Sciences of the USSR (1943). The first Soviet atomic bomb was tested in 1949. Four years later, the world's first hydrogen bomb was successfully tested. And in 1949, Igor Vasilievich Kurchatov began work on the project of a nuclear power plant. The nuclear power plant is a messenger of the peaceful use of atomic energy. The project was successfully completed: on July 27, 1954, our nuclear power plant became the first in the world! Kurchatov rejoiced and had fun like a child!

Definition of a nuclear reactor.

A nuclear reactor is a device in which a controlled chain reaction of fission of some heavy nuclei is carried out and maintained.

The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of IV Kurchatov.

The main elements of a nuclear reactor are:

• nuclear fuel (uranium 235, uranium 238, plutonium 239);
• neutron moderator (heavy water, graphite, etc.);
• coolant for the output of energy generated during the operation of the reactor (water, liquid sodium, etc.);
• Control rods (boron, cadmium) - strongly absorbing neutrons
• Protective shell that delays radiation (concrete with iron filler).

Operating principle nuclear reactor

Nuclear fuel is located in the active zone in the form of vertical rods called fuel elements (TVEL). Fuel rods are designed to control the power of the reactor.

The mass of each fuel rod is much less than the critical mass, so a chain reaction cannot occur in one rod. It begins after immersion in the active zone of all uranium rods.

The active zone is surrounded by a layer of a substance that reflects neutrons (reflector) and a protective shell of concrete that traps neutrons and other particles.

Heat removal from fuel cells. The coolant - water washes the rod, heated to 300 ° C at high pressure enters the heat exchangers.

The role of the heat exchanger - water heated to 300 ° C, gives off heat to ordinary water, turns into steam.

Nuclear reaction control

The reactor is controlled by rods containing cadmium or boron. With the rods extended from the reactor core, K > 1, and with the rods fully retracted, K< 1. Вдвигая стержни внутрь активной зоны, можно в любой момент времени приостановить развитие цепной реакции. Управление ядерными реакторами осуществляется дистанционно с помощью ЭВМ.

Reactor on slow neutrons.

The most efficient fission of uranium-235 nuclei occurs under the action of slow neutrons. Such reactors are called slow neutron reactors. The secondary neutrons produced in the fission reaction are fast. In order for their subsequent interaction with uranium-235 nuclei in a chain reaction to be most effective, they are slowed down by introducing a moderator into the core - a substance that reduces the kinetic energy of neutrons.

Fast neutron reactor.

Fast neutron reactors cannot operate on natural uranium. The reaction can only be maintained in an enriched mixture containing at least 15% of the uranium isotope. The advantage of fast neutron reactors is that their operation produces a significant amount of plutonium, which can then be used as nuclear fuel.

Homogeneous and heterogeneous reactors.

Nuclear reactors, depending on the mutual arrangement of fuel and moderator, are divided into homogeneous and heterogeneous. In a homogeneous reactor, the core is a homogeneous mass of fuel, moderator and coolant in the form of a solution, mixture or melt. A reactor is called heterogeneous, in which fuel in the form of blocks or fuel assemblies is placed in the moderator, forming a regular geometric lattice in it.

Converting the internal energy of atomic nuclei into electrical energy.

A nuclear reactor is the main element of a nuclear power plant (NPP), which converts thermal nuclear energy into electrical energy. Energy conversion occurs according to the following scheme:

• internal energy of uranium nuclei -
• kinetic energy of neutrons and fragments of nuclei -
• internal energy of water -
• steam internal energy -
• steam kinetic energy -
• kinetic energy of the turbine rotor and generator rotor -
• Electric Energy.

Use of nuclear reactors.

Depending on the purpose, nuclear reactors are power, converters and breeders, research and multi-purpose, transport and industrial.

Nuclear power reactors are used to generate electricity at nuclear power plants, in ship power plants, nuclear combined heat and power plants, as well as at nuclear heat supply stations.

Reactors designed to produce secondary nuclear fuel from natural uranium and thorium are called converters or breeders. In the reactor-converter secondary nuclear fuel is formed less than originally consumed.

In the breeder reactor, expanded reproduction of nuclear fuel is carried out, i.e. it turns out more than was spent.

Research reactors are used to study the processes of interaction of neutrons with matter, study the behavior of reactor materials in intense fields of neutron and gamma radiation, radiochemical and biological research, production of isotopes, experimental research in the physics of nuclear reactors.

Reactors have different power, stationary or pulsed mode of operation. Multi-purpose reactors are reactors that serve multiple purposes, such as power generation and nuclear fuel production.

Environmental disasters at nuclear power plants

• 1957 - accident in the UK
• 1966 - Partial core meltdown after reactor cooling failure near Detroit.
• 1971 - A lot of polluted water went into the US river
• 1979 - biggest accident in USA
• 1982 - release of radioactive steam into the atmosphere
• 1983 - a terrible accident in Canada (radioactive water flowed out for 20 minutes - a ton per minute)
• 1986 - accident in the UK
• 1986 - accident in Germany
• 1986 - Chernobyl nuclear power plant
• 1988 - fire at a nuclear power plant in Japan

Modern nuclear power plants are equipped with a PC, and earlier, even after an accident, the reactors continued to operate, since there was no automatic shutdown system.

4. Fixing the material.

1. What is a nuclear reactor?
2. What is nuclear fuel in a reactor?
3. What substance serves as a neutron moderator in a nuclear reactor?
4. What is the purpose of a neutron moderator?
5. What are control rods for? How are they used?
6. What is used as a coolant in nuclear reactors?
7. Why is it necessary that the mass of each uranium rod be less than the critical mass?

5. Test execution.

1. What particles are involved in the fission of uranium nuclei?
   A. protons;
   B. neutrons;
   B. electrons;
   G. helium nuclei.
2. What mass of uranium is critical?
   A. the largest at which a chain reaction is possible;
   B. any mass;
   V. the smallest at which a chain reaction is possible;
   D. the mass at which the reaction will stop.
3. What is the approximate critical mass of uranium 235?
   A. 9 kg;
   B. 20 kg;
   B. 50 kg;
   G. 90 kg.
4. Which of the following substances can be used in nuclear reactors as neutron moderators?
   A. graphite;
   B. cadmium;
   B. heavy water;
   G. bor.
5. For a nuclear chain reaction to occur at a nuclear power plant, it is necessary that the neutron multiplication factor be:
   A. is equal to 1;
   B. more than 1;
   V. less than 1.
6. Regulation of the fission rate of nuclei of heavy atoms in nuclear reactors is carried out:
   A. due to the absorption of neutrons when lowering the rods with an absorber;
   B. due to an increase in heat removal with an increase in the speed of the coolant;
   B. by increasing the supply of electricity to consumers;
   G. by reducing the mass of nuclear fuel in the core when removing the fuel rods.
7. What energy transformations take place in a nuclear reactor?
   A. the internal energy of atomic nuclei is converted into light energy;
   B. the internal energy of atomic nuclei is converted into mechanical energy;
   B. the internal energy of atomic nuclei is converted into electrical energy;
   G. there is no correct answer among the answers.
8. In 1946, the first nuclear reactor was built in the Soviet Union. Who was the leader of this project?
   A. S. Korolev;
   B. I. Kurchatov;
   V. D. Sakharov;
   G. A. Prokhorov.
9. What way do you consider the most appropriate for increasing the reliability of nuclear power plants and preventing contamination of the external environment?
   A. development of reactors capable of automatically cooling the reactor core, regardless of the will of the operator;
   B. increasing the literacy of NPP operation, the level of professional training of NPP operators;
   B. development of highly efficient technologies for dismantling nuclear power plants and processing radioactive waste;
   D. the location of the reactors deep underground;
   E. refusal to build and operate nuclear power plants.
10. What sources of environmental pollution are associated with the operation of nuclear power plants?
    A. uranium industry;
    B. nuclear reactors different types;
    B. radiochemical industry;
    D. places of processing and disposal of radioactive waste;
    E. use of radionuclides in the national economy;
    E. nuclear explosions.

Answers: 1 B; 2 V; 3 V; 4 A, B; 5 A; 6 A; 7 V;. 8 B; 9 B. V; 10 A, B, C, D, F.

6. The results of the lesson.

What new did you learn at the lesson today?

What did you like about the lesson?

What are the questions?

THANK YOU FOR YOUR WORK IN THE LESSON!