Nuclear reactor from the inside. How a nuclear reactor is arranged and works

Nuclear reactor works smoothly and accurately. Otherwise, as you know, there will be trouble. But what's going on inside? Let's try to formulate the principle of operation of a nuclear (atomic) reactor briefly, clearly, with stops.

In fact, the same process is going on there as in a nuclear explosion. Only now the explosion occurs very quickly, and in the reactor all this stretches for long time. In the end, everything remains safe and sound, and we get energy. Not so much that everything around immediately smashed, but quite enough to provide electricity to the city.

how a reactor worksNPP cooling towers
Before you understand how a controlled nuclear reaction works, you need to know what a nuclear reaction is in general.

A nuclear reaction is the process of transformation (fission) of atomic nuclei when they interact with elementary particles and gamma rays.

Nuclear reactions can take place both with absorption and with the release of energy. Second reactions are used in the reactor.

A nuclear reactor is a device whose purpose is to maintain a controlled nuclear reaction with the release of energy.

Often a nuclear reactor is also called a nuclear reactor. Note that there is no fundamental difference here, but from the point of view of science, it is more correct to use the word "nuclear". There are now many types of nuclear reactors. These are huge industrial reactors designed to generate energy at power plants, nuclear submarine reactors, small experimental reactors used in scientific experiments. There are even reactors used to desalinate seawater.

The history of the creation of a nuclear reactor

The first nuclear reactor was launched in the not so distant 1942. It happened in the USA under the leadership of Fermi. This reactor was called the "Chicago woodpile".

In 1946, the first Soviet reactor started up under the leadership of Kurchatov. The body of this reactor was a ball seven meters in diameter. The first reactors did not have a cooling system, and their power was minimal. By the way, the Soviet reactor had an average power of 20 watts, while the American one had only 1 watt. For comparison: the average power of modern power reactors is 5 Gigawatts. Less than ten years after the launch of the first reactor, the world's first industrial nuclear power plant was opened in the city of Obninsk.

The principle of operation of a nuclear (atomic) reactor

Any nuclear reactor has several parts: core with fuel and moderator, neutron reflector, coolant, control and protection system. The isotopes of uranium (235, 238, 233), plutonium (239) and thorium (232) are most often used as fuel in reactors. The active zone is a boiler through which ordinary water (coolant) flows. Among other coolants, “heavy water” and liquid graphite are less commonly used. If we talk about the operation of a nuclear power plant, then a nuclear reactor is used to generate heat. Electricity itself is generated in the same way as in other types of power plants - steam rotates a turbine, and the energy of movement is converted into electrical energy.

Below is a diagram of the operation of a nuclear reactor.

scheme of operation of a nuclear reactorScheme of a nuclear reactor at a nuclear power plant

As we have already said, the decay of a heavy uranium nucleus produces lighter elements and a few neutrons. The resulting neutrons collide with other nuclei, also causing them to fission. In this case, the number of neutrons grows like an avalanche.

Here it is necessary to mention the neutron multiplication factor. So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction dies out. But if you maintain the value of the coefficient equal to one, the reaction will proceed for a long time and stably.

The question is how to do it? In the reactor, the fuel is in the so-called fuel elements (TVELs). These are rods that contain nuclear fuel in the form of small pellets. The fuel rods are connected into hexagonal cassettes, of which there can be hundreds in the reactor. Cassettes with fuel rods are located vertically, while each fuel rod has a system that allows you to adjust the depth of its immersion in the core. In addition to the cassettes themselves, there are control rods and emergency protection rods among them. The rods are made of a material that absorbs neutrons well. Thus, the control rods can be lowered to different depths in the core, thereby adjusting the neutron multiplication factor. The emergency rods are designed to shut down the reactor in the event of an emergency.

How is a nuclear reactor started?

We figured out the very principle of operation, but how to start and make the reactor function? Roughly speaking, here it is - a piece of uranium, but after all, a chain reaction does not start in it by itself. The fact is that in nuclear physics there is the concept of critical mass.

Nuclear fuelNuclear fuel

Critical mass is the mass of fissile material necessary to start a nuclear chain reaction.

With the help of fuel elements and control rods, a critical mass of nuclear fuel is first created in the reactor, and then the reactor is brought to the optimal power level in several stages.

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In this article, we have tried to give you a general idea of ​​the structure and principle of operation of a nuclear (atomic) reactor. If you still have questions on the topic or the university asked a problem in nuclear physics - please contact the specialists of our company. We, as usual, are ready to help you solve any pressing issue of your studies. In the meantime, we are doing this, your attention is another educational video!

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Nuclear reactor, principle of operation, operation of a nuclear reactor.

Every day we use electricity and do not think about how it is produced and how it came to us. However, it is one of the most important parts 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. That is what we will be discussing 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 as a drive. big ships. In order to imagine the power and efficiency of nuclear reactors, one can give an example. Where an average nuclear reactor would need 30 kilograms of uranium, an average thermal power plant would need 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 (subsequently the word"Pile" along with other meanings began to denote a nuclear reactor). This name was given to him due to the fact that he resembled a large stack of graphite blocks laid one on top of the other.

Between the blocks was placed spherical "working bodies" of natural uranium and its dioxide.

In the USSR, the first reactor was built under the leadership of Academician IV Kurchatov. The F-1 reactor was put into operation on December 25, 1946. The reactor was in the form of a ball and had a diameter of about 7.5 meters. It did not have a cooling system, so it worked 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 was put into operation in the city of Obninsk.

The principle of operation 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 U 235 . In 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 uncontrolled chain reaction.

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

The device of a nuclear reactor.

At present, there are two types of nuclear reactors VVER (pressure water 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 - CPS drive; 2 - reactor cover; 3 - reactor vessel; 4 - block of protective pipes (BZT); 5 - mine; 6 - core baffle; 7 - fuel assemblies (FA) and control rods;

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

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

TVEL raktor RBMK. The device of the fuel element of the RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.

TVEL also includes a spring system for holding fuel pellets at the same level, which makes it possible to more accurately control the depth of immersion/removal of fuel into the core. They are assembled into hexagonal cassettes, each of which includes several dozen fuel rods. The coolant flows through the channels in each cassette.

The fuel elements 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 - the body, which also plays the role of a neutron reflector. Among the cassettes, control rods and emergency protection rods of the reactor are inserted at regular intervals, which, in case of overheating, are designed to shut down the reactor.

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

The controllers can move up and down by sinking, or vice versa, leaving the core, where the reaction is most intense. This is provided by powerful electric motors, together with the control system. Emergency protection rods are designed to shut down the reactor in case of an emergency, falling into the core and absorbing more free neutrons.

Each reactor has a lid through which the 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 not to release 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 not sufficiently enriched, and is divided into TVELs. Even if the core melts, the fuel will not be able to react so actively. The maximum that can happen is a thermal explosion, like at Chernobyl, when the pressure in the reactor reached such values ​​that the metal case was simply torn apart, and the reactor lid, weighing 5000 tons, made a flip jump, breaking through the roof of the reactor compartment and releasing steam out. If Chernobyl nuclear power plant was equipped with the correct biological protection, like today's sarcophagus, the catastrophe cost humanity much less.

The work of a nuclear power plant.

In a nutshell, the raboboa looks like this.

Nuclear power plant. (clickable)

After entering the reactor core with the help of pumps, the water is heated from 250 to 300 degrees and exits from the “other side” of the reactor. This is called the first loop. Then it goes to the heat exchanger, where it meets with the second circuit. After that, the steam under pressure enters the turbine blades. Turbines generate electricity.

The chain reaction of fission is always accompanied by the release of energy of enormous magnitude. The practical use of this energy is the main task of a nuclear reactor.

A nuclear reactor is a device in which a controlled, or controlled, nuclear fission reaction takes place.

According to the principle of operation, nuclear reactors are divided into two groups: thermal neutron reactors and fast neutron reactors.

How does a thermal neutron nuclear reactor work?

A typical nuclear reactor has:

• Core and moderator;
• Neutron reflector;
• Coolant;
• Chain reaction control system, emergency protection;
• System of control and radiation protection;
• Remote control system.

1 - active zone; 2 - reflector; 3 - protection; 4 - control rods; 5 - coolant; 6 - pumps; 7 - heat exchanger; 8 - turbine; 9 - generator; 10 - capacitor.

Core and moderator

It is in the core that the controlled fission chain reaction takes place.

Most nuclear reactors run on heavy isotopes of uranium-235. But in natural samples of uranium ore, its content is only 0.72%. This concentration is not enough for a chain reaction to develop. Therefore, the ore is artificially enriched, bringing the content of this isotope to 3%.

Fissile material, or nuclear fuel, in the form of pellets is placed in hermetically sealed rods called TVELs (fuel elements). They permeate the entire active zone filled with moderator neutrons.

Why is a neutron moderator needed in a nuclear reactor?

The fact is that neutrons born after the decay of uranium-235 nuclei have a very high speed. The probability of their capture by other uranium nuclei is hundreds of times less than the probability of capture of slow neutrons. And if you do not reduce their speed, the nuclear reaction may fade over time. The moderator solves the problem of reducing the speed of neutrons. If water or graphite is placed in the path of fast neutrons, their speed can be artificially reduced and thus the number of particles captured by atoms can be increased. At the same time, a smaller amount of nuclear fuel is needed for a chain reaction in a reactor.

As a result of the deceleration process, thermal neutrons, whose speed is almost equal to the speed thermal motion gas molecules at room temperature.

As a moderator in nuclear reactors, water, heavy water (deuterium oxide D 2 O), beryllium, and graphite are used. But the best moderator is heavy water D 2 O.

Neutron reflector

To avoid leakage of neutrons into the environment, the core of a nuclear reactor is surrounded by neutron reflector. As a material for reflectors, the same substances are often used as in moderators.

coolant

The heat released during a nuclear reaction is removed using a coolant. As a coolant in nuclear reactors, ordinary natural water is often used, previously purified from various impurities and gases. But since water boils already at a temperature of 100 0 C and a pressure of 1 atm, in order to increase the boiling point, the pressure in the primary coolant circuit is increased. The water of the primary circuit, circulating through the reactor core, washes the fuel elements, while heating up to a temperature of 320 0 C. Further inside the heat exchanger, it gives off heat to the water of the second circuit. The exchange passes through the heat exchange tubes, so there is no contact with the water of the secondary circuit. This excludes the ingress of radioactive substances into the second circuit of the heat exchanger.

And then everything happens as in a thermal power plant. Water in the second circuit turns into steam. The steam turns a turbine, which drives an electric generator, which produces electricity.

In heavy water reactors, the coolant is heavy water D 2 O, and in reactors with liquid metal coolants, it is molten metal.

Chain reaction control system

The current state of the reactor is characterized by a quantity called reactivity.

ρ = ( k-1)/ k ,

k = n i / n i -1 ,

where k is the neutron multiplication factor,

n i is the number of neutrons of the next generation in a nuclear fission reaction,

n i -1 , - number of neutrons previous generation in the same reaction.

If a k ˃ 1 , the chain reaction builds up, the system is called supercritical th. If a k< 1 , the chain reaction decays, and the system is called subcritical. At k = 1 the reactor is in stable critical condition, since the number of fissile nuclei does not change. In this state, reactivity ρ = 0 .

The critical state of the reactor (the required neutron multiplication factor in a nuclear reactor) is maintained by moving control rods. The material from which they are made includes substances that absorb neutrons. Pushing or pushing these rods into the core controls the rate of the nuclear fission reaction.

The control system provides control of the reactor during its start-up, planned shutdown, operation at power, as well as emergency protection of the nuclear reactor. This is achieved by changing the position of the control rods.

If any of the reactor parameters (temperature, pressure, power slew rate, fuel consumption, etc.) deviates from the norm, and this can lead to an accident, in central part core are dropped special emergency rods and there is a rapid cessation of the nuclear reaction.

To ensure that the parameters of the reactor comply with the standards, monitor monitoring and radiation protection systems.

For guard environment from radioactive radiation the reactor is placed in a thick concrete case.

Remote control systems

All signals about the state of a nuclear reactor (coolant temperature, radiation level in different parts reactor, etc.) are sent to the reactor control panel and processed in computer systems. The operator receives all the necessary information and recommendations to eliminate certain deviations.

Fast neutron reactors

The difference between this type of reactors and thermal neutron reactors is that fast neutrons that arise after the decay of uranium-235 are not slowed down, but are absorbed by uranium-238 with its subsequent transformation into plutonium-239. Therefore, fast neutron reactors are used to produce weapons-grade plutonium-239 and thermal energy, which is converted into electrical energy by nuclear power plant generators.

The nuclear fuel in such reactors is uranium-238, and the raw material is uranium-235.

In natural uranium ore, 99.2745% is uranium-238. When a thermal neutron is absorbed, it does not fission, but becomes an isotope of uranium-239.

Some time after the β-decay, uranium-239 turns into the nucleus of neptunium-239:

239 92 U → 239 93 Np + 0 -1 e

After the second β-decay, fissile plutonium-239 is formed:

239 9 3 Np → 239 94 Pu + 0 -1 e

And finally, after the alpha decay of the plutonium-239 nucleus, uranium-235 is obtained:

239 94 Pu → 235 92 U + 4 2 He

Fuel elements with raw materials (enriched uranium-235) are located in the reactor core. This zone is surrounded by a breeding zone, which is fuel rods with fuel (depleted uranium-238). Fast neutrons emitted from the core after the decay of uranium-235 are captured by uranium-238 nuclei. The result is plutonium-239. Thus, new nuclear fuel is produced in fast neutron reactors.

Liquid metals or their mixtures are used as coolants in fast neutron nuclear reactors.

Classification and application of nuclear reactors

Nuclear reactors are mainly used in nuclear power plants. With their help, electrical and thermal energy is obtained on an industrial scale. Such reactors are called energy .

Nuclear reactors are widely used in the propulsion systems of modern nuclear submarines, surface ships, and in space technology. They supply electrical energy to the engines and are called transport reactors .

For scientific research in the field of nuclear physics and radiation chemistry, they use fluxes of neutrons, gamma quanta, which are obtained in the core research reactors. The energy generated by them does not exceed 100 MW and is not used for industrial purposes.

Power experimental reactors even less. It reaches a value of only a few kW. These reactors are used to study various physical quantities, whose significance is important in the design of nuclear reactions.

To industrial reactors include reactors for the production of radioactive isotopes used for medical purposes, as well as in various fields of industry and technology. Seawater desalination reactors are also industrial reactors.

Nuclear power is a modern and rapidly developing way of generating electricity. Do you know how nuclear power plants are arranged? What is the principle of operation of a nuclear power plant? What types of nuclear reactors exist today? We will try to consider in detail the scheme of operation of a nuclear power plant, delve into the structure of a nuclear reactor and find out how safe the atomic method of generating electricity is.

Any station is a closed area far from the residential area. There are several buildings on its territory. The most important building is the reactor building, next to it is the turbine hall from which the reactor is controlled, and the safety building.

The scheme is impossible without a nuclear reactor. An atomic (nuclear) reactor is a device of a nuclear power plant, which is designed to organize a chain reaction of neutron fission with the obligatory release of energy in this process. But what is the principle of operation of a nuclear power plant?

The entire reactor plant is placed in the reactor building, a large concrete tower that hides the reactor and, in the event of an accident, will contain all the products of a nuclear reaction. This large tower is called containment, hermetic shell or containment.

The containment zone in the new reactors has 2 thick concrete walls - shells.
An 80 cm thick outer shell protects the containment area from external influences.

The inner shell with a thickness of 1 meter 20 cm has special steel cables in its device, which increase the strength of concrete by almost three times and will not allow the structure to crumble. On the inside, it is lined with a thin sheet of special steel, which is designed to serve as additional protection for the containment and, in the event of an accident, prevent the contents of the reactor from being released outside the containment area.

Such a device of a nuclear power plant can withstand the fall of an aircraft weighing up to 200 tons, an 8-magnitude earthquake, tornado and tsunami.

The first pressurized enclosure was built at the American nuclear power plant Connecticut Yankee in 1968.

The total height of the containment area is 50-60 meters.

What is a nuclear reactor made of?

To understand the principle of operation of a nuclear reactor, and hence the principle of operation of a nuclear power plant, you need to understand the components of the reactor.

• active zone. This is the area where the nuclear fuel (heat releaser) and the moderator are placed. Atoms of fuel (most often uranium is the fuel) perform a fission chain reaction. The moderator is designed to control the fission process, and allows you to carry out the reaction required in terms of speed and strength.
• Neutron reflector. The reflector surrounds the active zone. It consists of the same material as the moderator. In fact, this is a box, the main purpose of which is to prevent neutrons from leaving the core and getting into the environment.
• Coolant. The coolant must absorb the heat that was released during the fission of fuel atoms and transfer it to other substances. The coolant largely determines how a nuclear power plant is designed. The most popular coolant today is water.
  Reactor control system. Sensors and mechanisms that bring the nuclear power plant reactor into action.

Fuel for nuclear power plants

What does a nuclear power plant do? Fuel for nuclear power plants are chemical elements that have radioactive properties. At all nuclear power plants, uranium is such an element.

The design of the stations implies that nuclear power plants operate on complex composite fuel, and not on pure chemical element. And in order to extract uranium fuel from natural uranium, which is loaded into a nuclear reactor, you need to carry out a lot of manipulations.

Enriched uranium

Uranium consists of two isotopes, that is, it contains nuclei with different masses. They were named by the number of protons and neutrons isotope -235 and isotope-238. Researchers of the 20th century began to extract uranium 235 from the ore, because. it was easier to decompose and transform. It turned out that there is only 0.7% of such uranium in nature (the remaining percentages went to the 238th isotope).

What to do in this case? They decided to enrich uranium. Enrichment of uranium is a process when there are many necessary 235x isotopes and few unnecessary 238x isotopes left in it. The task of uranium enrichers is to make almost 100% uranium-235 from 0.7%.

Uranium can be enriched using two technologies - gas diffusion or gas centrifuge. For their use, uranium extracted from ore is converted into a gaseous state. In the form of gas, it is enriched.

uranium powder

Enriched uranium gas is converted into a solid state - uranium dioxide. This pure solid uranium 235 looks like large white crystals that are later crushed into uranium powder.

Uranium tablets

Uranium pellets are solid metal washers, a couple of centimeters long. In order to mold such tablets from uranium powder, it is mixed with a substance - a plasticizer, it improves the quality of tablet pressing.

Pressed washers are baked at a temperature of 1200 degrees Celsius for more than a day to give the tablets special strength and resistance to high temperatures. The way a nuclear power plant works directly depends on how well the uranium fuel is compressed and baked.

Tablets are baked in molybdenum boxes, because. only this metal is able not to melt at "hellish" temperatures over one and a half thousand degrees. After that, uranium fuel for nuclear power plants is considered ready.

What is TVEL and TVS?

The reactor core looks like a huge disk or pipe with holes in the walls (depending on the type of reactor), 5 times larger than a human body. These holes contain uranium fuel, the atoms of which carry out the desired reaction.

It’s impossible to simply throw fuel into a reactor, well, if you don’t want to get an explosion of the entire station and an accident with consequences for a couple of nearby states. Therefore, uranium fuel is placed in fuel rods, and then collected in fuel assemblies. What do these abbreviations mean?

• TVEL - fuel element (not to be confused with the same name of the Russian company that produces them). In fact, this is a thin and long zirconium tube made of zirconium alloys, into which uranium pellets are placed. It is in fuel rods that uranium atoms begin to interact with each other, releasing heat during the reaction.

Zirconium was chosen as a material for the production of fuel rods due to its refractoriness and anti-corrosion properties.

The type of fuel elements depends on the type and structure of the reactor. As a rule, the structure and purpose of fuel rods does not change; the length and width of the tube can be different.

The machine loads more than 200 uranium pellets into one zirconium tube. In total, about 10 million uranium pellets work simultaneously in the reactor.
FA - fuel assembly. NPP workers call fuel assemblies bundles.

In fact, these are several TVELs fastened together. Fuel assemblies are ready-made nuclear fuel, what a nuclear power plant runs on. It is fuel assemblies that are loaded into a nuclear reactor. About 150 - 400 fuel assemblies are placed in one reactor.
Depending on which reactor the fuel assembly will operate in, they come in different shapes. Sometimes the bundles are folded into a cubic, sometimes into a cylindrical, sometimes into a hexagonal shape.

One fuel assembly for 4 years of operation generates the same amount of energy as when burning 670 wagons of coal, 730 tanks with natural gas or 900 tanks loaded with oil.
Today, fuel assemblies are produced mainly at factories in Russia, France, the USA and Japan.

To deliver fuel for nuclear power plants to other countries, fuel assemblies are sealed in long and wide metal pipes, air is pumped out of the pipes and special machines delivered on board cargo aircraft.

Nuclear fuel for nuclear power plants weighs prohibitively much, tk. uranium is one of the most heavy metals on the planet. His specific gravity 2.5 times more than steel.

Nuclear power plant: principle of operation

What is the principle of operation of a nuclear power plant? The principle of operation of nuclear power plants is based on a chain reaction of fission of atoms of a radioactive substance - uranium. This reaction takes place in the core of a nuclear reactor.

IT'S IMPORTANT TO KNOW:

If you do not go into the intricacies of nuclear physics, the principle of operation of a nuclear power plant looks like this:
After the nuclear reactor is started, absorbing rods are removed from the fuel rods, which prevent the uranium from reacting.

As soon as the rods are removed, the uranium neutrons begin to interact with each other.

When neutrons collide, a mini-explosion occurs at the atomic level, energy is released and new neutrons are born, a chain reaction begins to occur. This process releases heat.

The heat is transferred to the coolant. Depending on the type of coolant, it turns into steam or gas, which rotates the turbine.

The turbine drives an electric generator. It is he who, in fact, generates electricity.

If you do not follow the process, uranium neutrons can collide with each other until the reactor is blown up and the entire nuclear power plant is blown to smithereens. Computer sensors control the process. They detect an increase in temperature or a change in pressure in the reactor and can automatically stop the reactions.

What is the difference between the principle of operation of nuclear power plants and thermal power plants (thermal power plants)?

Differences in work are only at the first stages. In nuclear power plants, the coolant receives heat from the fission of atoms of uranium fuel, in thermal power plants, the coolant receives heat from the combustion of organic fuel (coal, gas or oil). After either the atoms of uranium or the gas with coal have released heat, the schemes of operation of nuclear power plants and thermal power plants are the same.

Types of nuclear reactors

How a nuclear power plant works depends on how it works. atomic reactor. Today there are two main types of reactors, which are classified according to the spectrum of neurons:
A slow neutron reactor, also called a thermal reactor.

For its operation, 235 uranium is used, which goes through the stages of enrichment, the creation of uranium tablets, etc. Today, slow neutron reactors are in the vast majority.
Fast neutron reactor.

These reactors are the future, because they work on uranium-238, which is a dime a dozen in nature and it is not necessary to enrich this element. The disadvantage of such reactors is only in very high costs for design, construction and launch. Today, fast neutron reactors operate only in Russia.

The coolant in fast neutron reactors is mercury, gas, sodium or lead.

Slow neutron reactors, which are used today by all nuclear power plants in the world, also come in several types.

The IAEA organization (International Atomic Energy Agency) has created its own classification, which is used most often in the world nuclear industry. Since the principle of operation of a nuclear power plant largely depends on the choice of coolant and moderator, the IAEA has based its classification on these differences.

From a chemical point of view, deuterium oxide is an ideal moderator and coolant, because its atoms most effectively interact with the neutrons of uranium compared to other substances. Simply put, heavy water performs its task with minimal losses and maximum results. However, its production costs money, while it is much easier to use the usual “light” and familiar water for us.

A few facts about nuclear reactors...

It is interesting that one nuclear power plant reactor is built for at least 3 years!
To build a reactor, you need equipment that runs on an electric current of 210 kilo amperes, which is a million times the current that can kill a person.

One shell (structural element) of a nuclear reactor weighs 150 tons. There are 6 such elements in one reactor.

Pressurized water reactor

We have already found out how the nuclear power plant works in general, in order to “sort it out” let's see how the most popular pressurized nuclear reactor works.
All over the world today, generation 3+ pressurized water reactors are used. They are considered the most reliable and safe.

All pressurized water reactors in the world over all the years of their operation in total have already managed to gain more than 1000 years of trouble-free operation and have never given serious deviations.

The structure of nuclear power plants based on pressurized water reactors implies that distilled water circulates between the fuel rods, heated to 320 degrees. To prevent it from going into a vapor state, it is kept under a pressure of 160 atmospheres. The NPP scheme calls it primary water.

The heated water enters the steam generator and gives off its heat to the water of the secondary circuit, after which it “returns” to the reactor again. Outwardly, it looks like the pipes of the primary water circuit are in contact with other pipes - the water of the second circuit, they transfer heat to each other, but the waters do not contact. Tubes are in contact.

Thus, the possibility of radiation getting into the water of the secondary circuit, which will further participate in the process of generating electricity, is excluded.

Nuclear power plant safety

Having learned the principle of operation of nuclear power plants, we must understand how safety is arranged. The design of nuclear power plants today requires increased attention to safety rules.
The cost of nuclear power plant safety is approximately 40% of the total cost of the plant itself.

The NPP scheme includes 4 physical barriers that prevent the release of radioactive substances. What are these barriers supposed to do? At the right time, be able to stop the nuclear reaction, ensure constant heat removal from the core and the reactor itself, and prevent the release of radionuclides from the containment (containment zone).

• The first barrier is the strength of uranium pellets. It is important that they do not collapse under the influence of high temperatures in a nuclear reactor. In many ways, how a nuclear power plant works depends on how the uranium pellets were "baked" at the initial stage of production. If the uranium fuel pellets are baked incorrectly, the reactions of the uranium atoms in the reactor will be unpredictable.
• The second barrier is the tightness of fuel rods. Zirconium tubes must be tightly sealed, if the tightness is broken, then at best the reactor will be damaged and work stopped, at worst everything will fly into the air.
• The third barrier is a strong steel reactor vessel a, (that same large tower - a containment area) which "holds" all radioactive processes in itself. The hull is damaged - radiation will be released into the atmosphere.
• The fourth barrier is emergency protection rods. Above the active zone, rods with moderators are suspended on magnets, which can absorb all neutrons in 2 seconds and stop the chain reaction.

If, despite the construction of a nuclear power plant with many degrees of protection, it is not possible to cool the reactor core at the right time, and the fuel temperature rises to 2600 degrees, then the last hope of the safety system comes into play - the so-called melt trap.

The fact is that at such a temperature the bottom of the reactor vessel will melt, and all the remnants of nuclear fuel and molten structures will flow into a special “glass” suspended above the reactor core.

The melt trap is refrigerated and refractory. It is filled with the so-called "sacrificial material", which gradually stops the fission chain reaction.

Thus, the NPP scheme implies several degrees of protection, which almost completely exclude any possibility of an accident.

What is a nuclear reactor?

A nuclear reactor, formerly known as a "nuclear boiler" is a device used to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used in nuclear power plants to generate electricity and for ship engines. The heat from nuclear fission is transferred to the working fluid (water or gas) which is passed through the steam turbines. Water or gas drives the ship's blades or rotates electric generators. The steam resulting from a nuclear reaction can, in principle, be used for the thermal industry or for district heating. Some reactors are used to produce isotopes for medical and industrial applications or to produce weapons-grade plutonium. Some of them are for research purposes only. Today, there are about 450 nuclear power reactors that are used to generate electricity in about 30 countries around the world.

The principle of operation of a nuclear reactor

Just as conventional power plants generate electricity by using the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion into mechanical or electrical forms.

Nuclear fission process

When a significant number of decaying atomic nuclei (such as uranium-235 or plutonium-239) absorb a neutron, the process of nuclear decay can occur. A heavy nucleus decays into two or more light nuclei, (fission products), releasing kinetic energy, gamma rays and free neutrons. Some of these neutrons can later be absorbed by other fissile atoms and cause further fission, which releases even more neutrons, and so on. This process is known as a nuclear chain reaction.

To control such a nuclear chain reaction, neutron absorbers and moderators can change the proportion of neutrons that go into fission of more nuclei. Nuclear reactors are controlled manually or automatically to be able to stop the decay reaction when dangerous situations are identified.

Commonly used neutron flux regulators are ordinary ("light") water (74.8% of reactors in the world), solid graphite (20% of reactors) and "heavy" water (5% of reactors). In some experimental types of reactors, it is proposed to use beryllium and hydrocarbons.

Heat generation in a nuclear reactor

The working zone of the reactor generates heat in several ways:

• Kinetic energy fission products are converted into thermal energy when nuclei collide with neighboring atoms.
• The reactor absorbs some of the gamma radiation produced during fission and converts its energy into heat.
• Heat is generated from the radioactive decay of fission products and those materials that have been affected by neutron absorption. This heat source will remain unchanged for some time, even after the reactor is shut down.

During nuclear reactions, a kilogram of uranium-235 (U-235) releases about three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 compared to 2.4 × 107 joules per kilogram coal) ,

Nuclear reactor cooling system

The coolant of a nuclear reactor - usually water, but sometimes gas, liquid metal (such as liquid sodium), or molten salt - is circulated around the reactor core to absorb the heat generated. Heat is removed from the reactor and then used to generate steam. Most reactors use a cooling system that is physically isolated from the water that boils and generates steam used for turbines, much like a pressurized water reactor. However, in some reactors, water for steam turbines is boiled directly in the reactor core; for example, in a pressurized water reactor.

Neutron flux control in the reactor

The reactor power output is controlled by controlling the number of neutrons capable of causing more fissions.

Control rods that are made from "neutron poison" are used to absorb neutrons. The more neutrons absorbed by the control rod, the fewer neutrons can cause further fission. Thus, immersing the absorption rods deep into the reactor reduces its output power and, conversely, removing the control rod will increase it.

At the first level of control in all nuclear reactors, the delayed emission of neutrons from a number of neutron-enriched fission isotopes is an important physical process. These delayed neutrons are about 0.65% of total number neutrons produced during fission, and the rest (the so-called "fast neutrons") are formed immediately during fission. The fission products that form the delayed neutrons have half-lives ranging from milliseconds to several minutes, and therefore it takes a considerable amount of time to determine exactly when the reactor reaches its critical point. Maintaining the reactor in a chain reactivity mode, where delayed neutrons are needed to reach critical mass, is achieved by mechanical devices or human control to control the chain reaction in "real time"; otherwise, the time between reaching criticality and melting the core of a nuclear reactor as a result of the exponential power surge in a normal nuclear chain reaction would be too short to intervene. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as prompt criticality. There is a scale for describing criticality in numerical form, in which the initial criticality is indicated by the term "zero dollars", the fast critical point as "one dollar", other points in the process are interpolated in "cents".

In some reactors, the coolant also acts as a neutron moderator. The moderator increases the power of the reactor by causing the fast neutrons that are released during fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. If the coolant is also a neutron moderator, then changes in temperature can affect the density of the coolant/moderator and hence the change in reactor power output. The higher the temperature of the coolant, the less dense it will be, and therefore the less effective moderator.

In other types of reactors, the coolant acts as a "neutron poison", absorbing neutrons in the same way as control rods. In these reactors, power output can be increased by heating the coolant, making it less dense. Nuclear reactors typically have automatic and manual systems for shutting down the reactor for emergency shutdown. These systems put a large number"neutron poison" (often boron in the form of boric acid) into the reactor to stop the fission process if dangerous conditions are detected or suspected.

Most types of reactors are sensitive to a process known as "xenon pit" or "iodine pit". A common fission product, xenon-135, acts as a neutron absorber that seeks to shut down the reactor. The accumulation of xenon-135 can be controlled by maintaining enough high level power to destroy it by absorbing neutrons as fast as it is produced. Fission also results in the formation of iodine-135, which in turn decays (with a half-life of 6.57 hours) to form xenon-135. When the reactor is shut down, iodine-135 continues to decay to form xenon-135, which makes restarting the reactor more difficult within a day or two, as xenon-135 decays to form cesium-135, which is not a neutron absorber like xenon-135. 135, with a half-life of 9.2 hours. This temporary state is the "iodine pit". If the reactor has sufficient additional power, then it can be restarted. The more xenon-135 will turn into xenon-136, which is less than the neutron absorber, and within a few hours the reactor experiences the so-called "xenon burn-up stage". Additionally, control rods must be inserted into the reactor to compensate for the absorption of neutrons to replace the lost xenon-135. Failure to properly follow this procedure was a key reason for the accident at the Chernobyl nuclear power plant.

Reactors used in marine nuclear installations (especially nuclear submarines) often cannot be started in a continuous power mode in the same way as land-based power reactors. In addition, such power plants must have a long period of operation without changing the fuel. For this reason, many designs use highly enriched uranium but contain a burnable neutron absorber in the fuel rods. This makes it possible to design a reactor with an excess of fissile material, which is relatively safe at the beginning of the burnup of the reactor fuel cycle due to the presence of neutron absorbing material, which is subsequently replaced by conventional long-lived neutron absorbers (more durable than xenon-135), which gradually accumulate over the life of the reactor. fuel.

How is electricity produced?

The energy generated during fission generates heat, some of which can be converted into useful energy. A common method of harnessing this thermal energy is to use it to boil water and produce pressurized steam, which in turn drives a steam turbine that drives an alternator and generates electricity.

The history of the appearance of the first reactors

Neutrons were discovered in 1932. The scheme of a chain reaction provoked by nuclear reactions as a result of exposure to neutrons was first carried out by the Hungarian scientist Leo Sillard in 1933. He applied for a patent for his simple reactor idea during the next year at the Admiralty in London. However, Szilard's idea did not include the theory of nuclear fission as a source of neutrons, since this process had not yet been discovered. Szilard's ideas for nuclear reactors using a neutron-mediated nuclear chain reaction in light elements proved unworkable.

The impetus for the creation of a new type of reactor using uranium was the discovery of Lise Meitner, Fritz Strassmann and Otto Hahn in 1938, who "bombarded" uranium with neutrons (using the alpha decay reaction of beryllium, the "neutron gun") to form barium, which, as they believed it originated from the decay of uranium nuclei. Subsequent studies in early 1939 (Szilard and Fermi) showed that some neutrons were also produced during the fission of the atom, and this made it possible to carry out a nuclear chain reaction, as Szilard had foreseen six years earlier.

On August 2, 1939, Albert Einstein signed a letter written by Szilard to President Franklin D. Roosevelt stating that the discovery of uranium fission could lead to the creation of "extremely powerful bombs new type. "This gave impetus to the study of reactors and radioactive decay. Szilard and Einstein knew each other well and worked together for many years, but Einstein never thought about such a possibility for nuclear power until Szilard told him, in fact beginning his quest to and write a letter to Einstein-Szilard to warn the US government

Shortly thereafter, in 1939 Nazi Germany attacked Poland, starting World War II in Europe. Officially, the US was not yet at war, but in October, when the Einstein-Szilard letter was delivered, Roosevelt noted that the purpose of the study was to make sure "the Nazis don't blow us up." nuclear project The US began, albeit with some delay, as skepticism remained (particularly from Fermi), and also because of the small number of government officials who initially oversaw the project.

The following year, the US government received a Frisch-Peierls memorandum from Britain stating that the amount of uranium needed to carry out a chain reaction was much less than previously thought. The memorandum was created with the participation of Maud Commity, who worked on the atomic bomb project in the UK, later known under the code name "Tube Alloys" (Tubular Alloys) and later included in the Manhattan Project.

Ultimately, the first man-made nuclear reactor, called Chicago Woodpile 1, was built at the University of Chicago by a team led by Enrico Fermi in late 1942. By this time, the US nuclear program had already been accelerated by the country's entry into the war. "Chicago Woodpile" reached a critical point on December 2, 1942 at 15 hours 25 minutes. The frame of the reactor was wooden, holding together a stack of graphite blocks (hence the name) with nested "briquettes" or "pseudospheres" of natural uranium oxide.

Beginning in 1943, shortly after the creation of the Chicago Woodpile, the US military developed a whole series of nuclear reactors for the Manhattan Project. The main purpose of the largest reactors (located in the Hanford complex in Washington state) was the mass production of plutonium for nuclear weapons. Fermi and Szilard filed a patent application for the reactors on December 19, 1944. Its issuance was delayed by 10 years due to wartime secrecy.

"World's First" - this inscription was made at the site of the EBR-I reactor, which is now a museum near the city of Arco, Idaho. Originally named "Chicago Woodpile-4", this reactor was built under the direction of Walter Zinn for the Aregonne National Laboratory. This experimental fast breeder reactor was at the disposal of the US Atomic Energy Commission. The reactor produced 0.8 kW of power in testing on December 20, 1951, and 100 kW of power (electrical) the next day, with a design capacity of 200 kW (electrical power).

In addition to the military use of nuclear reactors, there were political reasons to continue research into atomic energy for peaceful purposes. US President Dwight Eisenhower delivered his famous "Atoms for Peace" speech at General Assembly UN December 8, 1953 This diplomatic move led to the spread of reactor technology both in the US and around the world.

The first nuclear power plant built for civilian purposes was the AM-1 nuclear power plant in Obninsk, launched on June 27, 1954 in the Soviet Union. It produced about 5 MW of electrical energy.

After World War II, the US military looked for other applications for nuclear reactor technology. Studies conducted in the Army and Air Force were not implemented; However, the US Navy was successful with the launch of the nuclear submarine USS Nautilus (SSN-571) on January 17, 1955.

The first commercial nuclear power plant (Calder Hall in Sellafield, England) opened in 1956 with an initial capacity of 50 MW (later 200 MW).

The first portable nuclear reactor "Alco PM-2A" was used to generate electricity (2 MW) for the American military base"Camp Century" since 1960.

Main components of a nuclear power plant

The main components of most types of nuclear power plants are:

Elements of a nuclear reactor

• Nuclear fuel (nuclear reactor core; neutron moderator)
• Initial source of neutrons
• Neutron absorber
• Neutron gun (provides a constant source of neutrons to re-initiate the reaction after being turned off)
• Cooling system (often neutron moderator and coolant are the same, usually purified water)
• control rods
• Nuclear reactor vessel (NRC)

Boiler water pump

• Steam generators (not in boiling water reactors)
• Steam turbine
• Electricity generator
• Capacitor
• Cooling tower (not always required)
• Processing system radioactive waste(part of the station for the disposal of radioactive waste)
• Nuclear fuel reloading site
• Spent fuel pool

Radiation safety system

• Rector protection system (SZR)
• Emergency diesel generators
• Reactor Core Emergency Cooling System (ECCS)
• Emergency fluid control system (boron emergency injection, in boiling water reactors only)
• Service water supply system for responsible consumers (SOTVOP)

Protective shell

• Remote Control
• Emergency installation
• Nuclear training complex (as a rule, there is a simulation of the control panel)

Classifications of nuclear reactors

Types of nuclear reactors

Nuclear reactors are classified in several ways; summary these classification methods are presented below.

Classification of nuclear reactors by type of moderator

Used thermal reactors:

• Graphite reactors
  
• Pressurized water reactors
• Heavy water reactors(used in Canada, India, Argentina, China, Pakistan, Romania and South Korea).
• Light water reactors(LVR). Light water reactors (the most common type of thermal reactor) use ordinary water to control and cool the reactors. If the temperature of the water rises, then its density decreases, slowing down the neutron flux enough to cause further chain reactions. This negative feedback stabilizes the rate of the nuclear reaction. Graphite and heavy water reactors tend to heat up more intensely than light water reactors. Due to the extra heat, such reactors can use natural uranium/unenriched fuel.
• Reactors based on light element moderators.
• Molten salt moderated reactors(MSR) are controlled by the presence of light elements, such as lithium or beryllium, which are part of the LiF and BEF2 coolant/fuel matrix salts.
• Reactors with liquid metal coolers, where the coolant is a mixture of lead and bismuth, can use BeO oxide in the neutron absorber.
• Reactors based on organic moderator(OMR) use diphenyl and terphenyl as moderator and coolant components.

Classification of nuclear reactors by type of coolant

• Water cooled reactor. There are 104 operating reactors in the United States. Of these, 69 are pressurized water reactors (PWRs) and 35 are boiling water reactors (BWRs). Pressurized water nuclear reactors (PWRs) make up the vast majority of all Western nuclear power plants. The main characteristic of the RVD type is the presence of a supercharger, a special high-pressure vessel. Most commercial high pressure reactors and naval reactor plants use superchargers. During normal operation, the blower is partially filled with water and a steam bubble is maintained above it, which is created by heating the water with immersion heaters. In the normal mode, the supercharger is connected to the pressure vessel of the reactor (HRV) and the pressure compensator provides a cavity in case of a change in the volume of water in the reactor. Such a scheme also provides control of the pressure in the reactor by increasing or decreasing the steam pressure in the compensator using heaters.

• High pressure heavy water reactors belong to a variety of pressurized water reactors (PWR), combining the principles of using pressure, an isolated thermal cycle, assuming the use of heavy water as a coolant and moderator, which is economically beneficial.
• boiling water reactor(BWR). Models of boiling water reactors are characterized by the presence of boiling water around the fuel rods at the bottom of the main reactor vessel. The boiling water reactor uses enriched 235U as fuel, in the form of uranium dioxide. The fuel is arranged in rods placed in a steel vessel, which, in turn, is immersed in water. The nuclear fission process causes water to boil and steam to form. This steam passes through pipelines in the turbines. The turbines are powered by steam, and this process generates electricity. During normal operation, the pressure is controlled by the amount of steam flowing from the reactor pressure vessel into the turbine.
• Pool type reactor
• Reactor with liquid metal coolant. Since water is a neutron moderator, it cannot be used as a coolant in a fast neutron reactor. Liquid metal coolants include sodium, NaK, lead, lead-bismuth eutectic, and for early generation reactors, mercury.
• Fast neutron reactor with sodium coolant.
• Reactor on fast neutrons with lead coolant.
• Gas cooled reactors are cooled by circulating inert gas, conceived with helium in high-temperature structures. At the same time, carbon dioxide was used earlier at British and French nuclear power plants. Nitrogen has also been used. The use of heat depends on the type of reactor. Some reactors are so hot that the gas can directly drive a gas turbine. Older reactor designs typically involved passing gas through a heat exchanger to generate steam for a steam turbine.
• Molten salt reactors(MSR) are cooled by circulating molten salt (usually eutectic mixtures of fluoride salts such as FLiBe). In a typical MSR, the coolant is also used as a matrix in which the fissile material is dissolved.

Generations of nuclear reactors

• First generation reactor(early prototypes, research reactors, non-commercial power reactors)
• Second generation reactor(most modern nuclear power plants 1965-1996)
• Third generation reactor(evolutionary improvements to existing designs 1996-present)
• Reactor fourth generation (technologies still under development, unknown start date, possibly 2030)

In 2003, the French Commissariat for Atomic Energy (CEA) introduced the designation "Gen II" for the first time during its Nucleonics Week.

The first mention of "Gen III" in 2000 was made in connection with the start of the Generation IV International Forum (GIF).

"Gen IV" was mentioned in 2000 by the United States Department of Energy (DOE) for the development of new types of power plants.

Classification of nuclear reactors by type of fuel

• Solid fuel reactor
• liquid fuel reactor
• Homogeneous Water Cooled Reactor
• Molten salt reactor
• Gas-fired reactors (theoretically)

Classification of nuclear reactors by purpose

• Electricity generation
• Nuclear power plants, including small cluster reactors
• Self-propelled devices (see nuclear power plants)
• Nuclear offshore installations
• Various proposed types of rocket engines
• Other uses of heat
• Desalination
• Heat generation for domestic and industrial heating
• Hydrogen production for use in hydrogen energy
• Production reactors for element conversion
• Breeder reactors capable of producing more fissile material than they consume during the chain reaction (by converting the parent isotopes U-238 to Pu-239, or Th-232 to U-233). Thus, having worked out one cycle, the uranium breeder reactor can be repeatedly refueled with natural or even depleted uranium. In turn, the thorium breeder reactor can be refilled with thorium. However, an initial supply of fissile material is needed.
• Creation of various radioactive isotopes, such as americium for use in smoke detectors and cobalt-60, molybdenum-99 and others, used as tracers and for treatment.
• Production of materials for nuclear weapons, such as weapons-grade plutonium
• Creation of a source of neutron radiation (for example, the Lady Godiva pulsed reactor) and positron radiation (for example, neutron activation analysis and potassium-argon dating)
• Research Reactor: Typically, reactors are used for scientific research and teaching, material testing, or the production of radioisotopes for medicine and industry. They are much smaller than power reactors or ship reactors. Many of these reactors are located on university campuses. There are about 280 such reactors operating in 56 countries. Some operate with highly enriched uranium fuel. International efforts are underway to replace low enriched fuels.

Modern nuclear reactors

These reactors use a pressure vessel to contain the nuclear fuel, control rods, moderator, and coolant. Reactors are cooled and neutrons are moderated by liquid water under high pressure. The hot radioactive water that exits the pressure vessel passes through the steam generator circuit, which in turn heats the secondary (non-radioactive) circuit. These reactors are most modern reactors. This is the neutron reactor heating design device, the latest of which are the VVER-1200, the advanced pressurized water reactor and the European pressurized water reactor. The US Navy reactors are of this type.

Boiling water reactors are similar to pressurized water reactors without a steam generator. Boiling water reactors also use water as the coolant and neutron moderator as pressurized water reactors, but at a lower pressure, which allows the water to boil inside the boiler, creating steam that turns turbines. Unlike a pressurized water reactor, there is no primary and secondary circuit. The heating capacity of these reactors can be higher, and they can be simpler in design, and even more stable and safer. This is a thermal neutron reactor device, the latest of which are the advanced boiling water reactor and the economical simplified boiling water nuclear reactor.

A Canadian design (known as CANDU), these are pressurized heavy water moderated reactors. Instead of using a single pressure vessel, as in pressurized water reactors, the fuel is in hundreds of high pressure channels. These reactors run on natural uranium and are thermal neutron reactors. Heavy water reactors can be refueled while operating at full power, making them very efficient when using uranium (this allows precise control of the flow in the core). Heavy water CANDU reactors have been built in Canada, Argentina, China, India, Pakistan, Romania and South Korea. India also operates a number of heavy water reactors, often referred to as "CANDU-derivatives", built after the Canadian government ended nuclear relations with India following the "Smiling Buddha" nuclear weapons test in 1974.

Soviet development, designed to produce plutonium, as well as electricity. RBMKs use water as a coolant and graphite as a neutron moderator. RBMKs are similar in some respects to CANDUs, as they can be recharged while in service and use pressure tubes instead of a pressure vessel (as they do in pressurized water reactors). However, unlike CANDU, they are very unstable and bulky, making the reactor cap expensive. A number of critical safety deficiencies have also been identified in RBMK designs, although some of these deficiencies were corrected after the Chernobyl disaster. Them main feature is the use of light water and unenriched uranium. As of 2010, 11 reactors remain open, largely due to improved safety and support from international safety organizations such as the US Department of Energy. Despite these improvements, RBMK reactors are still considered one of the most dangerous reactor designs to use. RBMK reactors were only used in the former Soviet Union.

They typically use a graphite neutron moderator and a CO2 cooler. Due to the high operating temperatures, they can have higher efficiency for heat generation than pressurized water reactors. There are a number of operational reactors of this design, mainly in the United Kingdom, where the concept was developed. Older developments (i.e. Magnox stations) are either closed or will be closed in the near future. However, improved gas-cooled reactors have an estimated operating life of another 10 to 20 years. Reactors of this type are thermal neutron reactors. The monetary costs of decommissioning such reactors can be high due to the large volume of the core.

The design of this reactor is cooled by liquid metal, without a moderator and produces more fuel than it consumes. They are said to "breed" fuel as they produce fissile fuel in the course of neutron capture. Such reactors can function in the same way as pressurized water reactors in terms of efficiency, they need to compensate for increased pressure, because liquid metal is used that does not create excess pressure even at very high temperatures. The BN-350 and BN-600 in the USSR and the Superphoenix in France were reactors of this type, as was Fermi I in the United States. The Monju reactor in Japan, damaged by a sodium leak in 1995, resumed operations in May 2010. All of these reactors use/used liquid sodium. These reactors are fast neutron reactors and do not belong to thermal neutron reactors. These reactors are of two types:

The use of lead as the liquid metal provides excellent radiation shielding and allows operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost to the coolant and the coolant doesn't become radioactive. Unlike sodium, lead is generally inert, so there is less risk of an explosion or accident, but such large amounts of lead can cause toxicity and waste disposal problems. Often lead-bismuth eutectic mixtures can be used in reactors of this type. In this case, bismuth will pose a small interference to the radiation, since it is not completely transparent to neutrons, and can change into another isotope more easily than lead. The Russian Alpha-class submarine uses a lead-bismuth-cooled fast neutron reactor as its main power generation system.

Most liquid metal breeding reactors (LMFBRs) are of this type. Sodium is relatively easy to obtain and work with, and it also helps prevent corrosion. various parts reactor immersed in it. However, sodium reacts violently on contact with water, so care must be taken, although such explosions will not be much more powerful than, for example, superheated liquid leaks from SCWRs or RWDs. EBR-I is the first reactor of this type, where the core consists of a melt.

They use fuel pressed into ceramic balls in which gas is circulated through the balls. As a result, they are efficient, unpretentious, very safe reactors with inexpensive, standardized fuel. The prototype was the AVR reactor.

In them, the fuel is dissolved in fluoride salts, or fluorides are used as a coolant. Their diverse security systems, high efficiency and high energy density are suitable for Vehicle. Remarkably, they have no parts subjected to high pressures or combustible components in the core. The prototype was the MSRE reactor, which also used a thorium fuel cycle. As a breeder reactor, it reprocesses spent fuel, recovering both uranium and transuranium elements, leaving only 0.1% of transuranium waste compared to conventional once-through uranium light water reactors currently in operation. A separate issue is radioactive fission products, which are not recycled and must be disposed of in conventional reactors.

These reactors use fuel in the form of soluble salts that are dissolved in water and mixed with a coolant and neutron moderator.

Innovative nuclear systems and projects

advanced reactors

More than a dozen advanced reactor projects are at various stages of development. Some of these have evolved from RWD, BWR and PHWR designs, some differ more significantly. The former include the Advanced Boiling Water Reactor (ABWR) (two of which are currently operational and others under construction), as well as the planned Economic Simplified Passive Safety Boiling Water Reactor (ESBWR) and AP1000 installations (see below). Nuclear Power Program 2010).

Integral fast neutron nuclear reactor(IFR) was built, tested, and tested throughout the 1980s, then decommissioned after the resignation of the Clinton administration in the 1990s due to nuclear non-proliferation policies. The reprocessing of spent nuclear fuel is at the heart of its design and hence it produces only a fraction of the waste from operating reactors.

Modular high-temperature gas-cooled reactor reactor (HTGCR), designed in such a way that high temperatures reduce the output power due to the Doppler broadening of the cross section of the neutron beam. The reactor uses a ceramic type of fuel, so its safe operating temperatures exceed the derating temperature range. Most structures are cooled with inert helium. Helium cannot cause an explosion due to vapor expansion, does not absorb neutrons, which would lead to radioactivity, and does not dissolve contaminants that could be radioactive. Typical designs consist of more layers of passive protection (up to 7) than in light water reactors (typically 3). A unique feature that can provide safety is that the fuel balls actually form the core and are replaced one by one over time. The design features of fuel cells make them expensive to recycle.

Small, closed, mobile, autonomous reactor (SSTAR) was originally tested and developed in the USA. The reactor was conceived as a fast neutron reactor, with a passive protection system that could be shut down remotely in case a malfunction was suspected.

Clean and environmentally friendly advanced reactor (CAESAR) is a concept for a nuclear reactor that uses steam as a neutron moderator - this design is still in development.

The Reduced Water Moderated Reactor is based on the Advanced Boiling Water Reactor (ABWR) currently in operation. This is not a full fast neutron reactor, but uses mainly epithermal neutrons, which have intermediate velocities between thermal and fast.

Self-Regulating Nuclear Power Module with Hydrogen Moderator (HPM) is a design type of reactor released by Los Alamos National Laboratory that uses uranium hydride as fuel.

Subcritical nuclear reactors designed as safer and more stable-working, but are difficult in engineering and economic terms. One example is the "Energy Amplifier".

Thorium based reactors. It is possible to convert thorium-232 to U-233 in reactors designed specifically for this purpose. In this way, thorium, which is four times more common than uranium, can be used to make nuclear fuel based on U-233. U-233 is believed to have favorable nuclear properties over conventional U-235, in particular better neutron efficiency and reduced long lived transuranium waste production.

Advanced Heavy Water Reactor (AHWR)- the proposed heavy water reactor, which will represent the development of the next generation of the PHWR type. Under development at Bhabha Nuclear Research Center (BARC), India.

KAMINI- a unique reactor using the uranium-233 isotope as fuel. Built in India at the BARC Research Center and the Indira Gandhi Nuclear Research Center (IGCAR).

India also plans to build fast neutron reactors using the thorium-uranium-233 fuel cycle. FBTR (fast neutron reactor) (Kalpakkam, India) uses plutonium as fuel and liquid sodium as coolant during operation.

What are fourth generation reactors

The fourth generation of reactors is a set of different theoretical projects that are currently being considered. These projects are not likely to be implemented by 2030. Modern reactors in operation are generally considered to be second or third generation systems. First generation systems have not been used for some time. The development of this fourth generation of reactors was officially launched on International Forum Generation IV (GIF) based on eight technology goals. The main objectives were to improve nuclear safety, increase security against proliferation, minimize waste and use natural resources, as well as to reduce the cost of building and running such stations.

• Gas-cooled fast neutron reactor
• Fast neutron reactor with lead cooler
• Liquid salt reactor
• Sodium-cooled fast neutron reactor
• Supercritical water-cooled nuclear reactor
• Ultra high temperature nuclear reactor

What are fifth generation reactors?

The fifth generation of reactors are projects, the implementation of which is possible from a theoretical point of view, but which are not currently the subject of active consideration and research. Although such reactors can be built in the current or short term, they are of little interest for reasons of economic feasibility, practicality or safety.

• liquid phase reactor. A closed loop with liquid in the core of a nuclear reactor, where the fissile material is in the form of molten uranium or a uranium solution cooled with the help of a working gas injected into through holes in the base of the containment vessel.
• Reactor with a gas phase in the core. Closed-loop variant for rocket with nuclear engine, where the fissile material is gaseous uranium hexafluoride located in a quartz container. A working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation resulting from a nuclear reaction. This design could be used as rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. Theoretically, the use of uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would lead to lower energy generation costs, as well as significantly reduce the size of the reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled neutron flux, weakening the strength properties of most of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of materials similar to those used by the International Project for the Implementation of a Fusion Irradiation Facility.
• Gas phase electromagnetic reactor. Similar to a gas phase reactor but with photovoltaic cells converting ultraviolet light directly into electricity.
• Fragmentation based reactor
• Hybrid nuclear fusion. The neutrons emitted during the fusion and decay of the original or "substance in the reproduction zone" are used. For example, transmutation of U-238, Th-232, or spent fuel/radioactive waste from another reactor into relatively more benign isotopes.

Reactor with a gas phase in the active zone. A closed-loop variant for a nuclear-powered rocket, where the fissile material is gaseous uranium hexafluoride located in a quartz vessel. The working gas (such as hydrogen) will flow around this vessel and absorb the ultraviolet radiation resulting from the nuclear reaction. Such a design could be used as a rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. Theoretically, the use of uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would lead to lower energy generation costs, as well as significantly reduce the size of the reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled neutron flux, weakening the strength properties of most of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of materials similar to those used by the International Project for the Implementation of a Fusion Irradiation Facility.

Gas-phase electromagnetic reactor. Similar to a gas phase reactor but with photovoltaic cells converting ultraviolet light directly into electricity.

Fragmentation based reactor

Hybrid nuclear fusion. The neutrons emitted during the fusion and decay of the original or "substance in the reproduction zone" are used. For example, transmutation of U-238, Th-232, or spent fuel/radioactive waste from another reactor into relatively more benign isotopes.

Fusion reactors

Controlled fusion can be used in fusion power plants to produce electricity without the complexities of working with actinides. However, serious scientific and technological hurdles remain. Several fusion reactors have been built, but only recently have the reactors been able to release more energy than they consume. Despite the fact that research began in the 1950s, it is assumed that a commercial fusion reactor will not be operational until 2050. The ITER project is currently making efforts to use fusion energy.

Nuclear fuel cycle

Thermal reactors generally depend on the degree of purification and enrichment of uranium. Some nuclear reactors can run on a mixture of plutonium and uranium (see MOX fuel). The process by which uranium ore is mined, processed, enriched, used, possibly recycled and disposed of is known as the nuclear fuel cycle.

Up to 1% of uranium in nature is the easily fissile isotope U-235. Thus, the design of most reactors involves the use of enriched fuel. Enrichment involves increasing the proportion of U-235 and is usually carried out using gaseous diffusion or in a gas centrifuge. The enriched product is further converted into uranium dioxide powder, which is compressed and fired into pellets. These granules are placed in tubes, which are then sealed. Such tubes are called fuel rods. Each nuclear reactor uses many of these fuel rods.

Most commercial BWRs and PWRs use uranium enriched to 4% U-235, approximately. In addition, some industrial reactors with high neutron economy do not require enriched fuel at all (that is, they can use natural uranium). According to the International Atomic Energy Agency, there are at least 100 research reactors in the world using highly enriched fuel (weapons grade / 90% enriched uranium). The risk of theft of this type of fuel (possible for use in the manufacture of nuclear weapons) has led to a campaign calling for a switch to the use of reactors with low enriched uranium (which poses less of a proliferation threat).

Fissile U-235 and non-fissile, fissionable U-238 are used in the nuclear transformation process. U-235 is fissioned by thermal (i.e. slow moving) neutrons. A thermal neutron is one that moves at about the same speed as the atoms around it. Since the vibrational frequency of atoms is proportional to their absolute temperature, the thermal neutron has a greater ability to split U-235 when it is moving at the same vibrational speed. On the other hand, U-238 is more likely to capture a neutron if the neutron is moving very fast. The U-239 atom decays as quickly as possible to form plutonium-239, which is itself a fuel. Pu-239 is a complete fuel and should be considered even when using highly enriched uranium fuel. Plutonium fission processes will take precedence over U-235 fission processes in some reactors. Especially after the original loaded U-235 is depleted. Plutonium fissions in both fast and thermal reactors, making it ideal for both nuclear reactors and nuclear bombs.

Most existing reactors are thermal reactors, which typically use water as a neutron moderator (moderator means that it slows down a neutron to thermal speed) and also as a coolant. However, in a fast neutron reactor, a slightly different kind of coolant is used, which will not slow down the neutron flux too much. This allows fast neutrons to predominate, which can be effectively used to constantly replenish the fuel supply. By simply placing cheap, unenriched uranium in the core, spontaneously non-fissile U-238 will convert into Pu-239, "reproducing" the fuel.

In a thorium-based fuel cycle, thorium-232 absorbs a neutron in both fast and thermal reactors. The beta decay of thorium produces protactinium-233 and then uranium-233, which in turn is used as fuel. Therefore, like uranium-238, thorium-232 is a fertile material.

Maintenance of nuclear reactors

The amount of energy in a nuclear fuel tank is often expressed in terms of "full power days", which is the number of 24-hour periods (days) the reactor is operated at full power to generate thermal energy. The days of full power operation in a reactor operating cycle (between the intervals required for refueling) are related to the amount of decaying uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. The higher the percentage of U-235 in the core at the beginning of the cycle, the more days of full power operation will allow the reactor to operate.

At the end of the operating cycle, the fuel in some assemblies is "used out", unloaded and replaced in the form of new (fresh) fuel assemblies. Also, such a reaction of accumulation of decay products in nuclear fuel determines the service life of nuclear fuel in the reactor. Even long before the final fission process occurs, long-lived neutron-absorbing decay by-products have time to accumulate in the reactor, preventing the chain reaction from proceeding. The proportion of the reactor core that is replaced during refueling is typically one quarter for a boiling water reactor and one third for a pressurized water reactor. Disposal and storage of this spent fuel is one of the most difficult tasks in the organization of the work of an industrial nuclear power plant. Such nuclear waste is extremely radioactive and its toxicity has been a danger for thousands of years.

Not all reactors need to be taken out of service for refueling; for example, spherical bed nuclear reactors, RBMK (high power ducted reactor), molten salt reactors, Magnox, AGR and CANDU reactors allow fuel elements to be moved during plant operation. In the CANDU reactor, it is possible to place individual fuel elements in the core in such a way as to adjust the content of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its burnup, which is expressed in terms of thermal energy generated by the initial unit weight of the fuel. Burnup is usually expressed as thermal megawatt days per tonne of the original heavy metal.

Nuclear power safety

Nuclear safety is actions aimed at preventing nuclear and radiation accidents or localizing their consequences. The nuclear power industry has improved the safety and performance of reactors, and has also come up with new, safer reactor designs (which have generally not been tested). However, there is no guarantee that such reactors will be designed, built and can operate reliably. Mistakes occur when the reactor designers at the Fukushima nuclear power plant in Japan did not expect that the tsunami generated by the earthquake would disable the back-up system that was supposed to stabilize the reactor after the earthquake, despite numerous warnings from the NRG (national research group) and the Japanese Nuclear Safety Administration. According to UBS AG, the Fukushima I nuclear accidents cast doubt on whether even advanced economies like Japan can provide nuclear safety. Catastrophic scenarios, including terrorist attacks, are also possible. An interdisciplinary team from MIT (Massachusetts Institute of Technology) has calculated that, given the expected growth in nuclear power, at least four serious nuclear accidents should be expected in the period 2005-2055.

Nuclear and radiation accidents

Some of the serious nuclear and radiation accidents that have occurred. Nuclear power plant accidents include the SL-1 incident (1961), the Three Mile Island accident (1979), Chernobyl disaster(1986) and also nuclear disaster Fukushima Daichi (2011). Nuclear-powered accidents include the reactor accidents on K-19 (1961), K-27 (1968), and K-431 (1985).

Nuclear reactors have been launched into orbit around the Earth at least 34 times. A series of incidents involving the Soviet nuclear-powered unmanned satellite RORSAT led to the penetration of spent nuclear fuel into the Earth's atmosphere from orbit.

natural nuclear reactors

Although it is often believed that nuclear fission reactors are the product of modern technology, the first nuclear reactors are available in natural conditions. A natural nuclear reactor can be formed under certain conditions that mimic conditions in a designed reactor. So far, up to fifteen natural nuclear reactors have been discovered within three separate ore deposits of the Oklo uranium mine in Gabon (West Africa). The well-known "dead" Ocllo reactors were first discovered in 1972 by the French physicist Francis Perrin. A self-sustaining nuclear fission reaction took place in these reactors approximately 1.5 billion years ago, and was maintained for several hundred thousand years, generating an average of 100 kW of power output during this period. The concept of a natural nuclear reactor was explained in terms of theory as early as 1956 by Paul Kuroda at the University of Arkansas.

Such reactors can no longer be formed on Earth: radioactive decay during this vast period of time, reduced the proportion of U-235 in natural uranium below the level required to sustain the chain reaction.

Natural nuclear reactors formed when the rich uranium mineral deposits began to fill up groundwater, which acted as a neutron moderator and the onset of a significant chain reaction. The neutron moderator in the form of water evaporated, causing the reaction to accelerate, and then condensed back, causing the nuclear reaction to slow down and prevent melting. The fission reaction persisted for hundreds of thousands of years.

Such natural reactors have been extensively studied by scientists interested in the disposal of radioactive waste in a geological setting. They propose a case study on how radioactive isotopes would migrate through the earth's crust. it key moment for critics of waste disposal in a geological setting who fear that isotopes contained in the waste may end up in water supplies or migrate into the environment.

Environmental problems of nuclear power

A nuclear reactor releases small amounts of tritium, Sr-90, into the air and into groundwater. Water contaminated with tritium is colorless and odorless. Large doses of Sr-90 increase the risk of bone cancer and leukemia in animals, and presumably in humans.