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How does a nuclear reaction occur in a reactor? Chain reaction and criticality

Design and principle of operation

Energy release mechanism

The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energy. The latter means that microparticles of a substance are in a state with a rest energy greater than in another possible state to which a transition exists. A spontaneous transition is always prevented by an energy barrier, to overcome which the microparticle must receive a certain amount of energy from the outside - excitation energy. The exoenergetic reaction consists in the fact that in the transformation following excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of colliding particles, or due to the binding energy of the joining particle.

If we keep in mind the macroscopic scale of energy release, then all or initially at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is chemical reactions, such an increase is usually hundreds of kelvins, but in the case of nuclear reactions it is at least 10 7 due to the very high altitude Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by joining particles does not require large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the attractive forces of particles. But to excite reactions, the particles themselves are necessary. And if we again mean not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter occurs when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Design

Any nuclear reactor consists of the following parts:

Core with nuclear fuel and moderator;
Neutron reflector surrounding the core;
Chain reaction control system, including emergency protection;
Radiation protection;
Remote control system.

Physical principles of operation

Iodine pit

Main article: Iodine pit

Iodine pit - a state of a nuclear reactor after it is turned off, characterized by the accumulation of the short-lived isotope xenon. This process leads to the temporary appearance of significant negative reactivity, which, in turn, makes it impossible to bring the reactor to its design capacity within a certain period (about 1-2 days).

Classification

By purpose

According to the nature of their use, nuclear reactors are divided into:

Power reactors designed to produce electrical and thermal energy used in the energy sector, as well as for desalination of sea water (desalination reactors are also classified as industrial). Such reactors are mainly used in nuclear power plants. The thermal power of modern power reactors reaches 5 GW. A separate group includes:
- Transport reactors, designed to supply energy to vehicle engines. The widest groups of applications are marine transport reactors used on submarines and various surface vessels, as well as reactors used in space technology.
Experimental reactors, intended for the study of various physical quantities, the value of which is necessary for the design and operation of nuclear reactors; The power of such reactors does not exceed several kW.
Research reactors, in which fluxes of neutrons and gamma quanta created in the core are used for research in the field of nuclear physics, solid state physics, radiation chemistry, biology, for testing materials intended to operate in intense neutron fluxes (including parts nuclear reactors) for the production of isotopes. The power of research reactors does not exceed 100 MW. The released energy is usually not used.
Industrial (weapons, isotope) reactors, used to produce isotopes used in various fields. Most widely used to produce nuclear weapons materials, such as 239 Pu. Also classified as industrial are reactors used for desalination of sea water.

Often reactors are used to solve two or more different problems, in which case they are called multi-purpose. For example, some power reactors, especially in the early days of nuclear power, were designed primarily for experimentation. Fast neutron reactors can simultaneously produce energy and produce isotopes. Industrial reactors, in addition to their main task, often generate electrical and thermal energy.

According to the neutron spectrum

Thermal (slow) neutron reactor (“thermal reactor”)
Fast neutron reactor ("fast reactor")

By fuel placement

Heterogeneous reactors, where fuel is placed discretely in the core in the form of blocks, between which there is a moderator;
Homogeneous reactors, where the fuel and moderator are a homogeneous mixture (homogeneous system).

In a heterogeneous reactor, the fuel and moderator can be spatially separated, in particular, in a cavity reactor, the moderator-reflector surrounds a cavity with fuel that does not contain a moderator. From a nuclear physical point of view, the criterion for homogeneity/heterogeneity is not the design, but the placement of fuel blocks at a distance exceeding the neutron moderation length in a given moderator. Thus, reactors with the so-called “close lattice” are designed as homogeneous, although in them the fuel is usually separated from the moderator.

Nuclear fuel blocks in a heterogeneous reactor are called fuel assemblies (FA), which are located in the core at the nodes of a regular lattice, forming cells.

By fuel type

uranium isotopes 235, 238, 233 (235 U, 238 U, 233 U)
plutonium isotope 239 (239 Pu), also isotopes 239-242 Pu in the form of a mixture with 238 U (MOX fuel)
thorium isotope 232 (232 Th) (via conversion to 233 U)

By degree of enrichment:

natural uranium
weakly enriched uranium
highly enriched uranium

By chemical composition:

metal U
UC (uranium carbide), etc.

By type of coolant

Gas, (see Graphite-gas reactor)
D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)

By type of moderator

C (graphite, see Graphite-gas reactor, Graphite-water reactor)
H2O (water, see Light water reactor, Water-cooled reactor, VVER)
D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)
Metal hydrides
Without moderator (see Fast reactor)

By design

By steam generation method

Reactor with external steam generator (See Water-water reactor, VVER)

IAEA classification

PWR (pressurized water reactors) - water-water reactor (pressurized water reactor);
BWR (boiling water reactor) - boiling water reactor;
FBR (fast breeder reactor) - fast breeder reactor;
GCR (gas-cooled reactor) - gas-cooled reactor;
LWGR (light water graphite reactor) - graphite-water reactor
PHWR (pressurized heavy water reactor) - heavy water reactor

The most common in the world are pressurized water (about 62%) and boiling water (20%) reactors.

Reactor materials

The materials from which reactors are built operate at high temperatures in a field of neutrons, γ quanta and fission fragments. Therefore, not all materials used in other branches of technology are suitable for reactor construction. When choosing reactor materials, their radiation resistance, chemical inertness, absorption cross section and other properties are taken into account.

The radiation instability of materials has less effect at high temperatures. The mobility of atoms becomes so great that the probability of the return of atoms knocked out of the crystal lattice to their place or the recombination of hydrogen and oxygen into a water molecule increases markedly. Thus, the radiolysis of water is insignificant in energy non-boiling reactors (for example, VVER), while in powerful research reactors a significant amount of explosive mixture is released. Reactors have special systems for burning it.

Reactor materials are in contact with each other (fuel shell with coolant and nuclear fuel, fuel cassettes with coolant and moderator, etc.). Naturally, the contacting materials must be chemically inert (compatible). An example of incompatibility is uranium and hot water entering into a chemical reaction.

For most materials, the strength properties deteriorate sharply with increasing temperature. In power reactors, structural materials operate at high temperatures. This limits the choice of construction materials, especially for those parts of the power reactor that must withstand high pressure.

Burnout and reproduction of nuclear fuel

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 isotopes, are formed. The effect of fission fragments on the reactivity of a nuclear reactor is called poisoning(for radioactive fragments) and slagging(for stable isotopes).

The main reason for reactor poisoning is , which has the largest neutron absorption cross section (2.6·10 6 barn). Half-life of 135 Xe T 1/2 = 9.2 hours; The yield during division is 6-7%. The bulk of 135 Xe is formed as a result of the decay ( T 1/2 = 6.8 hours). In case of poisoning, Keff changes by 1-3%. The large absorption cross section of 135 Xe and the presence of the intermediate isotope 135 I lead to two important phenomena:

To an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of the reactor after it is stopped or the power is reduced (“iodine pit”), which makes short-term stops and fluctuations in output power impossible. This effect is overcome by introducing a reactivity reserve in regulatory bodies. The depth and duration of the iodine well depend on the neutron flux Ф: at Ф = 5·10 18 neutron/(cm²·sec) the duration of the iodine well is ˜ 30 hours, and the depth is 2 times greater than the stationary change in Keff caused by 135 Xe poisoning.
Due to poisoning, spatiotemporal fluctuations in the neutron flux F, and, consequently, in the reactor power, can occur. These oscillations occur at Ф > 10 18 neutrons/(cm² sec) and large sizes reactor. Oscillation periods ˜ 10 hours.

Nuclear fission produces a large number of stable fragments, which differ in absorption cross sections compared to the absorption cross section of the fissile isotope. The concentration of fragments with a large absorption cross section reaches saturation within the first few days of reactor operation. These are mainly fuel rods of different “ages”.

In the case of a complete fuel change, the reactor has excess reactivity that needs to be compensated, while in the second case compensation is required only when the reactor is first started. Continuous overloading makes it possible to increase the burnup depth, since the reactivity of the reactor is determined by the average concentrations of fissile isotopes.

The mass of loaded fuel exceeds the mass of unloaded fuel due to the “weight” of the released energy. After the reactor is shut down, first mainly due to fission by delayed neutrons, and then, after 1-2 minutes, due to β- and γ-radiation of fission fragments and transuranium elements, the release of energy in the fuel continues. If the reactor worked long enough before stopping, then 2 minutes after stopping, the energy release is about 3%, after 1 hour - 1%, after a day - 0.4%, after a year - 0.05% of the initial power.

The ratio of the number of fissile Pu isotopes formed in a nuclear reactor to the amount of burnt 235 U is called conversion rate K K . The value of K K increases with decreasing enrichment and burnup. For a heavy water reactor using natural uranium, with a burnup of 10 GW day/t K K = 0.55, and with small burnups (in this case K K is called initial plutonium coefficient) K K = 0.8. If a nuclear reactor burns and produces the same isotopes (breeder reactor), then the ratio of the reproduction rate to the burnup rate is called reproduction rate K V. In nuclear reactors using thermal neutrons K V< 1, а для реакторов на быстрых нейтронах К В может достигать 1,4-1,5. Рост К В для реакторов на быстрых нейтронах объясняется главным образом тем, что, особенно в случае 239 Pu, для быстрых нейтронов g grows and A falls.

Nuclear reactor control

Control of a nuclear reactor is possible only due to the fact that during fission, some of the neutrons fly out of the fragments with a delay, which can range from several milliseconds to several minutes.

To control the reactor, absorber rods are used, introduced into the core, made of materials that strongly absorb neutrons (mainly, and some others) and/or a solution of boric acid, added to the coolant in a certain concentration (boron control). The movement of the rods is controlled by special mechanisms, drives, operating according to signals from the operator or equipment for automatic control of the neutron flux.

In case of different emergency situations In each reactor, an emergency termination of the chain reaction is provided, carried out by dropping all absorbing rods into the core - an emergency protection system.

Residual Heat

An important issue directly related to nuclear safety is decay heat. This is a specific feature of nuclear fuel, which consists in the fact that, after the cessation of the fission chain reaction and the thermal inertia usual for any energy source, the release of heat in the reactor continues for a long time, which creates a number of technically complex problems.

Residual heat is a consequence of the β- and γ-decay of fission products that accumulated in the fuel during the operation of the reactor. Fission product nuclei, due to decay, transform into a more stable or completely stable state with the release of significant energy.

Although the decay heat release rate quickly decreases to values small compared to steady-state values, in high-power power reactors it is significant in absolute terms. For this reason, residual heat generation necessitates long time ensure heat removal from the reactor core after shutdown. This task requires the design of the reactor installation to include cooling systems with a reliable power supply, and also necessitates long-term (3-4 years) storage of spent nuclear fuel in storage facilities with special temperature conditions- cooling pools, which are usually located in close proximity to the reactor.

Literature

Levin V. E. Nuclear physics and nuclear reactors. 4th ed. - M.: Atomizdat, 1979.
Shukolyukov A. Yu. “Uranium. Natural nuclear reactor." “Chemistry and Life” No. 6, 1980, p. 20-24

Notes

"ZEEP - Canada's First Nuclear Reactor", Canada Science and Technology Museum.
Greshilov A. A., Egupov N. D., Matushchenko A. M. Nuclear shield. - M.: Logos, 2008. - 438 p. -

We are so accustomed to electricity that we don’t think about where it comes from. Basically, it is produced at power plants, which use various sources for this. Power plants can be thermal, wind, geothermal, solar, hydroelectric, and nuclear. It is the latter that causes the most controversy. They argue about their necessity and reliability.

In terms of productivity, nuclear energy today is one of the most efficient and its share in global production electrical energy quite significant, more than a quarter.

How does a nuclear power plant work and how does it generate energy? The main element of a nuclear power plant is a nuclear reactor. A nuclear chain reaction occurs in it, resulting in the release of heat. This reaction is controlled, which is why we can use energy gradually, rather than getting a nuclear explosion.

Basic elements of a nuclear reactor

Nuclear fuel: enriched uranium, isotopes of uranium and plutonium. The most commonly used is uranium 235;
Coolant for removing the energy generated during reactor operation: water, liquid sodium, etc.;
Control rods;
Neutron moderator;
Radiation protection sheath.

Video of a nuclear reactor in operation

How does a nuclear reactor work?

In the reactor core there are fuel elements (fuel elements) - nuclear fuel. They are assembled into cassettes containing several dozen fuel rods. The coolant flows through the channels through each cassette. Fuel rods regulate the power of the reactor. A nuclear reaction is possible only at a certain (critical) mass of the fuel rod. The mass of each rod individually is below critical. The reaction begins when all the rods are in the active zone. By inserting and removing fuel rods, the reaction can be controlled.

So, when the critical mass is exceeded, radioactive fuel elements emit neutrons that collide with atoms. The result is an unstable isotope that immediately decays, releasing energy in the form of gamma radiation and heat. Particles colliding impart kinetic energy to each other, and the number of decays increases exponentially. This is a chain reaction - the principle of operation of a nuclear reactor. Without control, it occurs at lightning speed, which leads to an explosion. But in a nuclear reactor the process is under control.

Thus, thermal energy is released in the core, which is transferred to the water washing this zone (primary circuit). Here the water temperature is 250-300 degrees. Next, the water transfers heat to the second circuit, and then to the turbine blades that generate energy. The conversion of nuclear energy into electrical energy can be represented schematically:

Internal energy of the uranium nucleus,
Kinetic energy of fragments of decayed nuclei and released neutrons,
Internal energy of water and steam,
Kinetic energy of water and steam,
Kinetic energy of turbine and generator rotors,
Electric Energy.

The reactor core consists of hundreds of cassettes united by a metal shell. This shell also plays the role of a neutron reflector. Control rods for adjusting the reaction speed and reactor emergency protection rods are inserted among the cassettes. Next, thermal insulation is installed around the reflector. On top of the thermal insulation there is a protective shell made of concrete, which traps radioactive substances and does not allow them to pass into the surrounding space.

Where are nuclear reactors used?

Energy nuclear reactors are used in nuclear power plants, in ships electrical installations, at nuclear heat supply stations.
Convector and breeder reactors are used to produce secondary nuclear fuel.
Research reactors are needed for radiochemical and biological research and the production of isotopes.

Despite all the controversy and controversy regarding nuclear energy, nuclear power plants continue to be built and operated. One of the reasons is cost efficiency. A simple example: 40 tanks of fuel oil or 60 wagons of coal produce the same amount of energy as 30 kilograms of uranium.

This nondescript gray cylinder is the key link in the Russian nuclear industry. It doesn’t look very presentable, of course, but it’s worth understanding its purpose and taking a look at specifications, as you begin to realize why the secret of its creation and structure is protected by the state like the apple of its eye.

Yes, I forgot to introduce: here is a gas centrifuge for separating uranium isotopes VT-3F (nth generation). The principle of operation is elementary, like a milk separator; the heavy is separated from the light by the influence of centrifugal force. So what is the significance and uniqueness?

First, let's answer another question - in general, why separate uranium?

Natural uranium, which lies right in the ground, is a cocktail of two isotopes: uranium-238 And uranium-235(and 0.0054% U-234).
Uran-238, it's just heavy, gray metal. You can use it to make an artillery shell, or... a keychain. Here's what you can do from uranium-235? Well, firstly, an atomic bomb, and secondly, fuel for nuclear power plants. And here we come to the key question - how to separate these two, almost identical atoms, from each other? No, really HOW?!

By the way: The radius of the nucleus of a uranium atom is 1.5 10 -8 cm.

In order for uranium atoms to be driven into the technological chain, it (uranium) needs to be converted into gaseous state. There is no point in boiling, it is enough to combine uranium with fluorine and get uranium hexafluoride HFC. The technology for its production is not very complicated and expensive, and therefore HFC they get it right where this uranium is mined. UF6 is the only highly volatile uranium compound (when heated to 53°C, the hexafluoride (pictured) directly transforms from a solid to a gaseous state). Then it is pumped into special containers and sent for enrichment.

A little history

At the beginning nuclear race, the greatest scientific minds of both the USSR and the USA mastered the idea of diffusion separation - passing uranium through a sieve. Small 235th the isotope will slip through, and the “fat” 238th will get stuck. Moreover, making a sieve with nano-holes for Soviet industry in 1946 was not the most difficult task.

From the report of Isaac Konstantinovich Kikoin at the scientific and technical council under the Council of People's Commissars (presented in a collection of declassified materials on the USSR atomic project (Ed. Ryabev)): Currently, we have learned to make meshes with holes of about 5/1,000 mm, i.e. 50 times greater than the free path of molecules at atmospheric pressure. Consequently, the gas pressure at which isotope separation on such grids will occur must be less than 1/50 atmospheric pressure. In practice, we assume to work at a pressure of about 0.01 atmospheres, i.e. under good vacuum conditions. Calculations show that to obtain a product enriched to a concentration of 90% with a light isotope (this concentration is sufficient to produce an explosive), it is necessary to combine about 2,000 such stages in a cascade. In the machine we are designing and partially manufacturing, it is expected to produce 75-100 g of uranium-235 per day. The installation will consist of approximately 80-100 “columns”, each of which will have 20-25 stages installed.”

Below is a document - Beria’s report to Stalin on the preparation of the first atomic bomb explosion. Below is a short information about the nuclear materials produced by the beginning of the summer of 1949.

And now imagine for yourself - 2000 hefty installations, for the sake of just 100 grams! Well, what to do with it, we need bombs. And they began to build factories, and not just factories, but entire cities. And okay, only the cities, these diffusion plants required so much electricity that they had to build separate power plants nearby.

In the USSR, the first stage D-1 of plant No. 813 was designed for a total output of 140 grams of 92-93% uranium-235 per day at 2 cascades of 3100 separation stages identical in power. An unfinished aircraft plant in the village of Verkh-Neyvinsk, 60 km from Sverdlovsk, was allocated for production. Later it turned into Sverdlovsk-44, and plant 813 (pictured) into the Ural Electrochemical Plant - the world's largest separation plant.

And although the technology of diffusion separation, albeit with great technological difficulties, was debugged, the idea of developing a more economical centrifuge process did not leave the agenda. After all, if we manage to create a centrifuge, then energy consumption will be reduced from 20 to 50 times!

How does a centrifuge work?

Its structure is more than elementary and similar to the old one washing machine operating in the “spin/dry” mode. The rotating rotor is located in a sealed casing. Gas is supplied to this rotor (UF6). Due to the centrifugal force, hundreds of thousands of times greater than the Earth’s gravitational field, the gas begins to separate into “heavy” and “light” fractions. Light and heavy molecules begin to group into different zones rotor, but not in the center and around the perimeter, but at the top and bottom.

This occurs due to convection currents - the rotor cover is heated and a counterflow of gas occurs. There are two small intake tubes installed at the top and bottom of the cylinder. A lean mixture enters the lower tube, and a mixture with a higher concentration of atoms enters the upper tube. 235U. This mixture goes into the next centrifuge, and so on, until the concentration 235th will not reach uranium desired value. A chain of centrifuges is called a cascade.

Technical features.

Well, firstly, the rotation speed - in the modern generation of centrifuges it reaches 2000 rps (I don’t even know what to compare it with... 10 times faster than the turbine in an aircraft engine)! And it has been working non-stop for THREE DECADES! Those. Now centrifuges, turned on under Brezhnev, are rotating in cascades! The USSR no longer exists, but they keep spinning and spinning. It is not difficult to calculate that during its working cycle the rotor makes 2,000,000,000,000 (two trillion) revolutions. And what bearing will withstand this? Yes, none! There are no bearings there.

The rotor itself is an ordinary top, at the bottom it has a strong needle resting on a corundum bearing, and the upper end hangs in a vacuum, being held electromagnetic field. The needle is also not simple, made from ordinary wire for piano strings, it is very hardened in a cunning way(which is GT). It is not difficult to imagine that with such a frantic rotation speed, the centrifuge itself must be not just durable, but extremely durable.

Academician Joseph Friedlander recalls: “They could have shot me three times. Once, when we had already received the Lenin Prize, there was a major accident, the lid of the centrifuge flew off. The pieces scattered and destroyed other centrifuges. A radioactive cloud rose. We had to stop the entire line - a kilometer of installations! At Sredmash, General Zverev commanded the centrifuges; before the atomic project, he worked in Beria’s department. The general at the meeting said: “The situation is critical. The country's defense is at risk. If we don’t quickly rectify the situation, ’37 will repeat for you.” And immediately closed the meeting. Then we completely came up with new technology with a completely isotropic uniform cover structure, but very complex installations were required. Since then, these types of lids have been produced. There were no more troubles. In Russia there are 3 enrichment plants, many hundreds of thousands of centrifuges.”
In the photo: tests of the first generation of centrifuges

The rotor housings were also initially made of metal, until they were replaced by... carbon fiber. Lightweight and highly tensile, it is an ideal material for a rotating cylinder.

UEIP General Director (2009-2012) Alexander Kurkin recalls: “It was getting ridiculous. When they were testing and checking a new, more “resourceful” generation of centrifuges, one of the employees did not wait for the rotor to stop completely, disconnected it from the cascade and decided to carry it by hand to the stand. But instead of moving forward, no matter how he resisted, he embraced this cylinder and began to move backward. So we saw with our own eyes that the earth rotates, and the gyroscope is a great force.”

Who invented it?

Oh, it's a mystery, wrapped in mystery and shrouded in suspense. Here you will find captured German physicists, the CIA, SMERSH officers and even the downed spy pilot Powers. In general, the principle of a gas centrifuge was described at the end of the 19th century.

Even at the dawn of the Atomic Project, Viktor Sergeev, an engineer at the Special Design Bureau of the Kirov Plant, proposed a centrifuge separation method, but at first his colleagues did not approve of his idea. In parallel, scientists from defeated Germany struggled to create a separation centrifuge at a special research institute-5 in Sukhumi: Dr. Max Steenbeck, who worked as a leading Siemens engineer under Hitler, and former Luftwaffe mechanic, graduate of the University of Vienna, Gernot Zippe. In total, the group included about 300 “exported” physicists.

Remembers CEO CJSC Centrotech-SPb State Corporation Rosatom Alexey Kaliteevsky: “Our experts came to the conclusion that the German centrifuge is absolutely unsuitable for industrial production. Steenbeck's apparatus did not have a system for transferring the partially enriched product to the next stage. It was proposed to cool the ends of the lid and freeze the gas, and then defrost it, collect it and put it into the next centrifuge. That is, the scheme is inoperative. However, the project had several very interesting and unusual technical solutions. These “interesting and unusual solutions” were combined with the results obtained by Soviet scientists, in particular with the proposals of Viktor Sergeev. Relatively speaking, our compact centrifuge is one-third the fruit of German thought, and two-thirds Soviet.” By the way, when Sergeev came to Abkhazia and expressed his thoughts about the selection of uranium to the same Steenbeck and Zippe, Steenbeck and Zippe dismissed them as unrealizable.

So what did Sergeev come up with?

And Sergeev’s proposal was to create gas selectors in the form of pitot tubes. But Dr. Steenbeck, who, as he believed, had eaten his teeth on this topic, was categorical: “They will slow down the flow, cause turbulence, and there will be no separation!” Years later, while working on his memoirs, he would regret it: “An idea worthy of coming from us! But it never occurred to me...”

Later, once outside the USSR, Steenbeck no longer worked with centrifuges. But before leaving for Germany, Geront Zippe had the opportunity to get acquainted with a prototype of Sergeev’s centrifuge and the ingeniously simple principle of its operation. Once in the West, “the cunning Zippe,” as he was often called, patented the centrifuge design under his own name (patent No. 1071597 of 1957, declared in 13 countries). In 1957, having moved to the USA, Zippe built a working installation there, reproducing Sergeev’s prototype from memory. And he called it, let’s pay tribute, “Russian centrifuge” (pictured).

By the way, Russian engineering has shown itself in many other cases. An example is a simple emergency shut-off valve. There are no sensors, detectors or electronic circuits. There is only a samovar faucet, which touches the cascade frame with its petal. If something goes wrong and the centrifuge changes its position in space, it simply turns and closes the inlet line. It's like the joke about an American pen and a Russian pencil in space.

Our days

This week the author of these lines attended significant event– closure of the Russian office of US Department of Energy observers under contract HEU-LEU. This deal (highly enriched uranium - low enriched uranium) was, and remains, the largest agreement in the field of nuclear energy between Russia and America. Under the terms of the contract, Russian nuclear scientists processed 500 tons of our weapons-grade (90%) uranium into fuel (4%) HFCs for American nuclear power plants. Revenues for 1993-2009 amounted to 8.8 billion US dollars. This was the logical outcome of the technological breakthrough of our nuclear scientists in the field of isotope separation made in the post-war years.
In the photo: cascades of gas centrifuges in one of the UEIP workshops. There are about 100,000 of them here.

Thanks to centrifuges, we have obtained thousands of tons of relatively cheap, both military and commercial product. The nuclear industry, one of the few remaining ( military aviation, space), where Russia holds undisputed primacy. Foreign orders alone for ten years in advance (from 2013 to 2022), Rosatom’s portfolio excluding the contract HEU-LEU is 69.3 billion dollars. In 2011 it exceeded 50 billion...
The photo shows a warehouse of containers with HFCs at the UEIP.

On September 28, 1942, Resolution of the State Defense Committee No. 2352ss “On the organization of work on uranium” was adopted. This date is considered the official beginning of the history of the Russian nuclear industry.

Every day we use electricity and do not think about how it is produced and how it got to us. Nevertheless, it is one of the most important parts of modern civilization. Without electricity there would be nothing - no light, no heat, no movement.

Everyone knows that electricity is generated at power plants, including nuclear ones. The heart of every nuclear power plant is nuclear reactor. This is what we will be looking at in this article.

Nuclear reactor, a device in which a controlled nuclear chain reaction occurs with the release of heat. These devices are mainly used to generate electricity and to drive large ships. In order to imagine the power and efficiency of nuclear reactors, we can give an example. Where an average nuclear reactor will require 30 kilograms of uranium, an average thermal power plant will require 60 wagons of coal or 40 tanks of fuel oil.

Prototype nuclear reactor was built in December 1942 in the USA under the direction of E. Fermi. It was the so-called “Chicago stack”. Chicago Pile (later the word“Pile”, along with other meanings, has come to mean a nuclear reactor). It was given this name because it resembled a large stack of graphite blocks placed one on top of the other.

Between the blocks were placed spherical “working fluids” made of natural uranium and its dioxide.

In the USSR, the first reactor was built under the leadership of Academician I.V. Kurchatov. The F-1 reactor was operational on December 25, 1946. The reactor was spherical in shape and had a diameter of about 7.5 meters. It had no cooling system, so it operated at very low power levels.

Research continued and on June 27, 1954, the world's first nuclear power plant with a capacity of 5 MW came into operation in Obninsk.

The operating principle of a nuclear reactor.

During the decay of uranium U 235, heat is released, accompanied by the release of two or three neutrons. According to statistics - 2.5. These neutrons collide with other uranium atoms U235. During a collision, uranium U 235 turns into an unstable isotope U 236, which almost immediately decays into Kr 92 and Ba 141 + these same 2-3 neutrons. The decay is accompanied by the release of energy in the form of gamma radiation and heat.

This is called a chain reaction. Atoms divide, the number of decays increases exponentially, which ultimately leads to a lightning-fast, by our standards, release of a huge amount of energy - an atomic explosion occurs as a consequence of an uncontrollable chain reaction.

However, in nuclear reactor we are dealing with controlled nuclear reaction. How this becomes possible is described further.

The structure of a nuclear reactor.

Currently, there are two types of nuclear reactors: VVER (water-cooled power reactor) and RBMK (high-power channel reactor). The difference is that RBMK is a boiling water reactor, while VVER uses water under pressure of 120 atmospheres.

VVER 1000 reactor. 1 - control system drive; 2 - reactor cover; 3 - reactor body; 4 - block of protective pipes (BZT); 5 - shaft; 6 - core enclosure; 7 - fuel assemblies (FA) and control rods;

Each industrial nuclear reactor is a boiler through which coolant flows. As a rule, this is ordinary water (about 75% in the world), liquid graphite (20%) and heavy water (5%). For experimental purposes, beryllium was used and was assumed to be a hydrocarbon.

TVEL- (fuel element). These are rods in a zirconium shell with niobium alloy, inside of which uranium dioxide tablets are located.

The fuel rods in the cassette are highlighted in green.

Fuel cassette assembly.

The reactor core consists of hundreds of cassettes placed vertically and united together by a metal shell - a body, which also plays the role of a neutron reflector. Among the cassettes, control rods and reactor emergency protection rods are inserted at regular intervals, which are designed to shut down the reactor in case of overheating.

Let us give as an example data on the VVER-440 reactor:

The controllers can move up and down, plunging, or vice versa, leaving the active zone, where the reaction is most intense. This is ensured by powerful electric motors, in conjunction with a control system. The emergency protection rods are designed to shut down the reactor in the event of an emergency, falling into the core and absorbing more free neutrons.

Each reactor has a lid through which used and new cassettes are loaded and unloaded.

Thermal insulation is usually installed on top of the reactor vessel. The next barrier is biological protection. This is usually a reinforced concrete bunker, the entrance to which is closed by an airlock with sealed doors. Biological protection is designed to prevent the release of radioactive steam and pieces of the reactor into the atmosphere if an explosion does occur.

A nuclear explosion in modern reactors is extremely unlikely. Because the fuel is quite slightly enriched and divided into fuel elements. Even if the core melts, the fuel will not be able to react as actively. The worst that can happen is a thermal explosion like at Chernobyl, when the pressure in the reactor reached such values that the metal casing simply burst, and the reactor cover, weighing 5,000 tons, made an inverted jump, breaking through the roof of the reactor compartment and releasing steam outside. If the Chernobyl nuclear power plant had been equipped with proper biological protection, like today’s sarcophagus, then the disaster would have cost humanity much less.

Operation of a nuclear power plant.

In a nutshell, this is what raboboa looks like.

Nuclear power plant. (Clickable)

After entering the reactor core using pumps, the water is heated from 250 to 300 degrees and exits from the “other side” of the reactor. This is called the first circuit. After which it is sent to the heat exchanger, where it meets the second circuit. After which the steam under pressure flows onto the turbine blades. Turbines generate electricity.

Nuclear reactors have one job: to split atoms in a controlled reaction and use the released energy to generate electrical power. For many years, reactors were seen as both a miracle and a threat.

When the first commercial U.S. reactor came online at Shippingport, Pennsylvania, in 1956, the technology was hailed as the energy source of the future, and some believed the reactors would make generating electricity too cheap. Currently, 442 have been built worldwide. nuclear reactor, about a quarter of these reactors are in the United States. The world has become dependent on nuclear reactors, producing 14 percent of its electricity. Futurists even fantasized about nuclear cars.

When the Unit 2 reactor at the Three Mile Island Power Plant in Pennsylvania experienced a cooling system failure and partial meltdown of its radioactive fuel in 1979, the warm feelings about reactors changed radically. Even though the destroyed reactor was contained and no serious radiation emitted, many people began to view the reactors as too complex and vulnerable, with potentially catastrophic consequences. People were also concerned radioactive waste from reactors. As a result, construction of new nuclear power plants in the United States has stalled. When a more serious accident occurred on Chernobyl nuclear power plant in the Soviet Union in 1986, nuclear power seemed doomed.

But in the early 2000s, nuclear reactors began to make a comeback, thanks to rising energy demands and dwindling supplies of fossil fuels, as well as growing concerns about climate change resulting from carbon dioxide emissions.

But in March 2011, another crisis occurred - this time the Fukushima 1 nuclear power plant in Japan was badly damaged by an earthquake.

Use of nuclear reaction

Simply put, a nuclear reactor splits atoms and releases the energy that holds their parts together.

If you have forgotten physics high school, we will remind you how nuclear fission works. Atoms are like tiny solar systems, with a core like the Sun and electrons like planets in orbit around it. The nucleus is made up of particles called protons and neutrons, which are bound together. The force that binds the elements of the core is difficult to even imagine. It is many billions of times stronger than the force of gravity. Despite this enormous power, you can split the nucleus by shooting neutrons at it. When this is done, a lot of energy will be released. When atoms decay, their particles crash into nearby atoms, splitting them, and those, in turn, are next, and next, and next. There is a so-called chain reaction.

Uranium, an element with large atoms, is ideal for the fission process because the force that binds the particles of its nucleus is relatively weak compared to other elements. Nuclear reactors use a specific isotope called Uran-235 . Uranium-235 is rare in nature, with ore from uranium mines containing only about 0.7% Uranium-235. This is why reactors are used enrichedUwounds, which is created by separating and concentrating Uranium-235 through a gas diffusion process.

A chain reaction process can be created in atomic bomb, similar to those dropped on the Japanese cities of Hiroshima and Nagasaki during World War II. But in a nuclear reactor, the chain reaction is controlled by inserting control rods made of materials such as cadmium, hafnium or boron that absorb some of the neutrons. This still allows the fission process to release enough energy to heat the water to about 270 degrees Celsius and turn it into steam, which is used to spin the power plant's turbines and generate electricity. Basically, in this case, a controlled nuclear bomb works instead of coal to create electricity, except that the energy to boil the water comes from splitting atoms instead of burning carbon.

Nuclear Reactor Components

There are a few various types nuclear reactors, but they all have some common characteristics. They all have a supply of radioactive fuel pellets - usually uranium oxide - which are arranged in tubes to form fuel rods in active zonesereactor.

The reactor also has the previously mentioned managerserodAnd- made of a neutron-absorbing material such as cadmium, hafnium or boron, which is inserted to control or stop a reaction.

The reactor also has moderator, a substance that slows down neutrons and helps control the fission process. Most reactors in the United States use ordinary water, but reactors in other countries sometimes use graphite, or heavywowwaterat, in which hydrogen is replaced by deuterium, an isotope of hydrogen with one proton and one neutron. Another important part of the system is coolingand Iliquidb, usually, plain water, which absorbs and transfers heat from the reactor to create steam to spin the turbine and cools the reactor area so that it does not reach the temperature at which the uranium will melt (about 3815 degrees Celsius).

Finally, the reactor is enclosed in shellsat, a large, heavy structure, usually several meters thick, made of steel and concrete that keeps radioactive gases and liquids inside where they can't harm anyone.

There are a number of different reactor designs in use, but one of the most common is pressurized water power reactor (VVER). In such a reactor, water is forced into contact with the core and then remains there under such pressure that it cannot turn into steam. This water then comes into contact with unpressurized water in the steam generator, which turns into steam, which rotates the turbines. There is also a design high-power channel-type reactor (RBMK) with one water circuit and fast neutron reactor with two sodium and one water circuits.

How safe is a nuclear reactor?

Answering this question is quite difficult and depends on who you ask and how you define “safe”. Are you concerned about radiation or radioactive waste generated in reactors? Or are you more worried about the possibility of a catastrophic accident? What degree of risk do you consider an acceptable trade-off for the benefits of nuclear power? And to what extent do you trust the government and nuclear energy?

"Radiation" is a strong argument, mainly because we all know that large doses of radiation, for example from an explosion nuclear bomb, can kill many thousands of people.

Proponents of nuclear power, however, point out that we are all regularly exposed to radiation from a variety of sources, including cosmic rays and natural radiation emitted by the Earth. The average annual radiation dose is about 6.2 millisieverts (mSv), half of it from natural sources, and half from artificial sources, ranging from x-rays chest, smoke detectors and luminous watch dials. How much radiation do we get from nuclear reactors? Only a tiny fraction of a percent of our typical annual exposure is 0.0001 mSv.

While all nuclear plants inevitably leak small amounts of radiation, regulatory commissions hold plant operators to stringent requirements. They cannot expose people living around the plant to more than 1 mSv of radiation per year, and workers at the plant have a threshold of 50 mSv per year. That may seem like a lot, but according to the Nuclear Regulatory Commission, there is no medical evidence that annual radiation doses below 100 mSv pose any risks to human health.

But it's important to note that not everyone agrees with this complacent assessment of radiation risks. For example, the organization “Doctors for social responsibility", a longtime critic of the nuclear industry, studied children living around German nuclear power plants. The study found that people living within 5 km of plants had double the risk of contracting leukemia compared to those living further from nuclear power plants.

Nuclear reactor waste

Nuclear power is touted by its proponents as "clean" energy because the reactor does not emit large amounts of greenhouse gases into the atmosphere compared to coal-fired power plants. But critics point to something else environmental problem— recycling nuclear waste. Some of the spent fuel from the reactors still releases radioactivity. Other unnecessary material that should be saved is high level radioactive waste, a liquid residue from the reprocessing of spent fuel, in which some of the uranium remains. Right now, most of this waste is stored locally at nuclear power plants in ponds of water, which absorb some of the remaining heat produced by the spent fuel and help shield workers from radiation exposure.

One of the problems with spent nuclear fuel is that it has been altered by the fission process. When large uranium atoms are split, they create byproducts—radioactive isotopes of several light elements such as Cesium-137 and Strontium-90, called fission products. They are hot and highly radioactive, but eventually, over a period of 30 years, they decay into less dangerous forms. This period is called for them Pperiodohmhalf-life. Other radioactive elements will have different half-lives. In addition, some uranium atoms also capture neutrons, forming heavier elements such as Plutonium. These transuranium elements do not create as much heat or penetrating radiation as fission products, but they take much longer to decay. Plutonium-239, for example, has a half-life of 24,000 years.

These radioactiveewastes high level from reactors are dangerous to humans and other life forms because they can release huge, lethal dose radiation even from short exposure. Ten years after removing the remaining fuel from a reactor, for example, they are emitting 200 times more radioactivity per hour than it would take to kill a person. And if the waste ends up in groundwater or rivers, they can enter the food chain and endanger large numbers of people.

Because waste is so dangerous, many people are in a difficult situation. 60,000 tons of waste are located at nuclear power plants close to major cities. But finding a safe place to store waste is not easy.

What can go wrong with a nuclear reactor?

With government regulators looking back on their experience, engineers have spent a lot of time over the years designing reactors for optimal safety. It's just that they don't break down, work properly, and have backup safety measures if something doesn't go according to plan. As a result, year after year, nuclear power plants appear to be fairly safe compared to, say, air travel, which regularly kills between 500 and 1,100 people a year worldwide.

However, nuclear reactors suffer major breakdowns. On the International Nuclear Event Scale, which rates reactor accidents from 1 to 7, there have been five accidents since 1957 that rate from 5 to 7.

The worst nightmare is a cooling system failure, which leads to overheating of the fuel. The fuel turns to liquid and then burns through the containment, releasing radioactive radiation. In 1979, Unit 2 at the Three Mile Island nuclear power plant (USA) was on the verge of this scenario. Fortunately, a well-designed containment system was strong enough to stop the radiation from escaping.

The USSR was less fortunate. A severe nuclear accident occurred in April 1986 at the 4th power unit at the Chernobyl nuclear power plant. This was caused by a combination system breakdowns, design flaws and poorly trained personnel. During a routine test, the reaction suddenly intensified and the control rods jammed, preventing an emergency shutdown. The sudden buildup of steam caused two thermal explosions, throwing the reactor's graphite moderator into the air. In the absence of anything to cool the reactor fuel rods, they began to overheat and completely collapse, as a result of which the fuel took on a liquid form. Many station workers and accident liquidators died. A large number of radiation spread over an area of 323,749 square kilometers. The number of deaths caused by radiation is still unclear, but World organization health officials say it may have caused 9,000 cancer deaths.

Nuclear reactor manufacturers provide guarantees based on probabilistic assessmente, in which they try to balance the potential harm of an event with the likelihood with which it actually occurs. But some critics say they should prepare instead for rare, unexpected but highly dangerous events. Case in point- an accident in March 2011 at the Fukushima 1 nuclear power plant in Japan. The station was reportedly designed to withstand a strong earthquake, but not one as catastrophic as the 9.0 magnitude quake that sent a 14-meter tsunami wave above dikes designed to withstand a 5.4-meter wave. The onslaught of the tsunami destroyed the backup diesel generators that were intended to power the cooling system of the plant's six reactors in the event of a power outage. So even after the Fukushima reactors' control rods stopped fission, the still-hot fuel allowed temperatures to rise dangerously inside the destroyed ones. reactors.

Japanese officials resorted to a last resort - flooding the reactors with huge amounts of sea water with additive boric acid, which was able to prevent a disaster, but destroyed the reactor equipment. Eventually, with the help of fire engines and barges, the Japanese were able to pump fresh water into reactors. But by then, monitoring had already shown alarming levels of radiation in the surrounding land and water. In one village 40 km from the plant, the radioactive element Cesium-137 was found at levels much higher than after the Chernobyl disaster, raising doubts about the possibility of human habitation in the area.