Nuclear reactor operating principle. The world's first nuclear reactor

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. There are now 442 nuclear reactors built worldwide, 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 about radioactive waste from the reactors. As a result, construction of new nuclear power plants in the United States has stalled. When a more serious accident occurred at the 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've forgotten high school physics, we'll 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 a 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 several different types of nuclear reactors, but they all have some General 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 ordinary 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 various designs reactors 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, such as from a 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 man-made sources ranging from chest X-rays, 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, Physicians 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— disposal of nuclear waste. Some of the spent fuel from the reactors still releases radioactivity. Other unnecessary material that should be saved is radioactive waste high level , 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 of reactors are dangerous to humans and other life forms because they can release huge, lethal doses of radiation even from a 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 fall into food chain and put a large number of people at risk.

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. Heavy nuclear accident happened in April 1986 at the 4th power unit at the Chernobyl nuclear power plant. This was caused by a combination of system failures, 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 amount of radiation spread over an area of ​​323,749 square kilometers. The number of deaths caused by radiation is still unclear, but the World Health Organization says 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. A case in point is the March 2011 accident at the Fukushima 1 nuclear power plant in Japan. The station was reportedly designed to withstand strong earthquake, but not as catastrophic as the 9.0 magnitude earthquake that raised a 14-meter tsunami wave over 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 trucks and barges, the Japanese were able to pump fresh water into the 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.

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

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

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

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

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

The inner shell, 1 meter 20 cm thick, has special steel cables that increase the strength of concrete almost three times and will prevent the structure from crumbling. WITH 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, not to release the contents of the reactor outside the containment zone.

This design of the nuclear power plant allows it to withstand an airplane crash weighing up to 200 tons, a magnitude 8 earthquake, a tornado and a tsunami.

The first sealed shell was built at the American Connecticut Yankee nuclear power plant in 1968.

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

What does a nuclear reactor consist of?

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

• Active zone. This is the area where the nuclear fuel (fuel generator) and moderator are placed. Fuel atoms (most often uranium is the fuel) undergo a chain fission reaction. The moderator is designed to control the fission process and allows for the required reaction in terms of speed and strength.
• Neutron reflector. A reflector surrounds the core. It consists of the same material as the moderator. In essence, this is a box, the main purpose of which is to prevent neutrons from leaving the core and entering the environment.
• Coolant. The coolant must absorb the heat 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 power a nuclear power plant reactor.

Fuel for nuclear power plants

What does a nuclear power plant operate on? Fuel for nuclear power plants are chemical elements with radioactive properties. At all nuclear power plants, this element is uranium.

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 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 ore, because... it was easier to decompose and transform. It turned out that such uranium in nature is only 0.7% (the remaining percentage goes to the 238th isotope).

What to do in this case? They decided to enrich uranium. Uranium enrichment is a process in which a lot of the necessary 235x isotopes remain in it and few unnecessary 238x isotopes. The task of uranium enrichers is to turn 0.7% into almost 100% uranium-235.

Uranium can be enriched using two technologies: gas diffusion or gas centrifuge. To use them, uranium extracted from ore is converted into a gaseous state. It is enriched in the form of gas.

Uranium powder

Enriched uranium gas is converted into a solid state - uranium dioxide. This pure solid uranium 235 appears as large white crystals, which are later crushed into uranium powder.

Uranium tablets

Uranium tablets are solid metal discs, a couple of centimeters long. To form such tablets from uranium powder, it is mixed with a substance - a plasticizer; it improves the quality of pressing the tablets.

The pressed pucks 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. How a nuclear power plant operates directly depends on how well the uranium fuel is compressed and baked.

The tablets are baked in molybdenum boxes, because only this metal is capable of not melting at “hellish” temperatures of over one and a half thousand degrees. After this, uranium fuel for nuclear power plants is considered ready.

What are TVEL and FA?

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

It’s impossible to just throw fuel into the reactor, well, unless you want to cause 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 Russian company, which produces them). It is essentially a thin and long zirconium tube made from zirconium alloys into which uranium tablets 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 rods 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 are working simultaneously in the reactor.
FA – fuel assembly. NPP workers call fuel assemblies bundles.

Essentially, these are several fuel rods fastened together. FA is finished nuclear fuel, what a nuclear power plant operates on. It is the fuel assemblies that are loaded into the nuclear reactor. About 150 – 400 fuel assemblies are placed in one reactor.
Depending on the reactor in which the fuel assemblies will operate, 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 over 4 years of operation produces the same amount of energy as when burning 670 cars 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 aboard cargo planes.

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

Nuclear power plant: operating principle

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

IT IS IMPORTANT TO KNOW:

Without going into the intricacies of nuclear physics, the operating principle of a nuclear power plant looks like this:
After the start-up of a nuclear reactor, absorber rods are removed from the fuel rods, which prevent the uranium from reacting.

Once 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 generates heat.

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 actually generates the electric current.

If you do not monitor the process, uranium neutrons can collide with each other until they explode the reactor and smash the entire nuclear power plant to smithereens. The process is controlled by computer sensors. They detect an increase in temperature or change in pressure in the reactor and can automatically stop reactions.

How does the operating principle of nuclear power plants differ from thermal power plants (thermal power plants)?

There are differences in work only in the first stages. In a nuclear power plant, the coolant receives heat from the fission of atoms of uranium fuel; in a thermal power plant, the coolant receives heat from the combustion of organic fuel (coal, gas or oil). After either uranium atoms or gas and coal have released heat, the operation schemes 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, uranium 235 is used, which goes through the stages of enrichment, creation of uranium pellets, etc. Today, the vast majority of reactors use slow neutrons.
Fast neutron reactor.

These reactors are the future, because... They work on uranium-238, which is a dime a dozen in nature and there is no need to enrich this element. The only downside of such reactors is the very high costs of design, construction and startup. Today, fast neutron reactors operate only in Russia.

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

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

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

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

A few facts about nuclear reactors...

It’s interesting that one nuclear power plant reactor takes at least 3 years to build!
To build a reactor you need equipment that runs on electric current at 210 kilo Amperes, which is a million times greater than 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 a nuclear power plant works in general; to put everything into perspective, let’s look at how the most popular pressurized water 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, have already accumulated more than 1000 years of trouble-free operation and have never given serious deviations.

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

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

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

NPP operational safety

Having learned the principle of operation of nuclear power plants, we must understand how safety works. The construction of nuclear power plants today requires increased attention to safety rules.
NPP safety costs account for approximately 40% of the total cost of the plant itself.

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

• The first barrier is the strength of uranium pellets. It is important that they are not destroyed by high temperatures in a nuclear reactor. In many ways, how a nuclear power plant operates depends on how the uranium pellets are “baked” at initial stage manufacturing. If the uranium fuel pellets are not baked correctly, 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 seal is broken, then best case scenario the reactor will be damaged and work will be stopped, in the worst case, everything will blow up.
• The third barrier is a durable steel reactor vessel a, (that same large tower - hermetic zone) which “contains” all radioactive processes. If the housing is damaged, radiation will escape into the atmosphere.
• The fourth barrier is emergency protection rods. Rods with moderators are suspended above the core by magnets, which can absorb all neutrons in 2 seconds and stop the chain reaction.

If, despite the design 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 this temperature the bottom of the reactor vessel will melt, and all the remains of nuclear fuel and molten structures will flow into a special “glass” suspended above the reactor core.

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

Thus, the nuclear power plant design implies several degrees of protection, which almost completely eliminate any possibility of an accident.

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 electrical energy production is 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 in geometric progression increases. 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:

1. Internal energy of the uranium nucleus,
2. Kinetic energy of fragments of decayed nuclei and released neutrons,
3. Internal energy of water and steam,
4. Kinetic energy of water and steam,
5. Kinetic energy of turbine and generator rotors,
6. 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.

For ordinary person Modern high-tech devices are so mysterious and enigmatic that they can be worshiped like the ancients worshiped lightning. School physics lessons, replete with mathematical calculations, do not solve the problem. But you can even tell an interesting story about a nuclear reactor, the principle of operation of which is clear even to a teenager.

How does a nuclear reactor work?

The operating principle of this high-tech device is as follows:

1. When a neutron is absorbed, nuclear fuel (most often this uranium-235 or plutonium-239) fission of the atomic nucleus occurs;
2. Kinetic energy, gamma radiation and free neutrons are released;
3. Kinetic energy is converted into thermal energy (when nuclei collide with surrounding atoms), gamma radiation is absorbed by the reactor itself and also turns into heat;
4. Some of the neutrons produced are absorbed by fuel atoms, which causes a chain reaction. To control it, neutron absorbers and moderators are used;
5. With the help of a coolant (water, gas or liquid sodium), heat is removed from the reaction site;
6. Pressurized steam from heated water is used to drive steam turbines;
7. With the help of a generator, the mechanical energy of turbine rotation is converted into alternating electric current.

Approaches to classification

There can be many reasons for the typology of reactors:

• By type of nuclear reaction. Fission (all commercial installations) or fusion (thermonuclear energy, widespread only in some research institutes);
• By coolant. In the vast majority of cases, water (boiling or heavy) is used for this purpose. Alternative solutions are sometimes used: liquid metal (sodium, lead-bismuth, mercury), gas (helium, carbon dioxide or nitrogen), molten salt (fluoride salts);
• By generation. The first was early prototypes that made no commercial sense. Second, most of the nuclear power plants currently in use were built before 1996. The third generation differs from the previous one only in minor improvements. Work on the fourth generation is still underway;
• By state of aggregation fuel (gas fuel currently exists only on paper);
• By purpose of use(for electricity production, engine starting, hydrogen production, desalination, elemental transmutation, obtaining neural radiation, theoretical and investigative purposes).

Nuclear reactor design

The main components of reactors in most power plants are:

1. Nuclear fuel is a substance needed to produce heat for power turbines (usually low-enriched uranium);
2. The nuclear reactor core is where the nuclear reaction takes place;
3. Neutron moderator - reduces the speed of fast neutrons, turning them into thermal neutrons;
4. Starting neutron source - used for reliable and stable starting of a nuclear reaction;
5. Neutron absorber - available in some power plants to reduce the high reactivity of fresh fuel;
6. Neutron howitzer - used to re-initiate a reaction after shutdown;
7. Coolant (purified water);
8. Control rods - to regulate the rate of fission of uranium or plutonium nuclei;
9. Water pump - pumps water into the steam boiler;
10. Steam turbine - turns thermal energy pair into rotary mechanical;
11. Cooling tower - a device for removing excess heat into the atmosphere;
12. Radioactive waste reception and storage system;
13. Safety systems (emergency diesel generators, devices for emergency core cooling).

How the latest models work

The latest 4th generation of reactors will be available for commercial operation no earlier than 2030. Currently, the principle and structure of their operation are at the development stage. According to modern data, these modifications will differ from existing models in such advantages:

• Rapid gas cooling system. It is assumed that helium will be used as a coolant. According to project documentation, in this way it is possible to cool reactors with a temperature of 850 °C. To work with such high temperatures Specific raw materials will also be required: composite ceramic materials and actinide compounds;
• It is possible to use lead or a lead-bismuth alloy as the primary coolant. These materials have a low neutron absorption rate and a relatively low melting point;
• Also, a mixture of molten salts can be used as the main coolant. This will make it possible to operate at higher temperatures than modern water-cooled counterparts.

Natural analogues in nature

A nuclear reactor is perceived in the public consciousness exclusively as a product of high technology. However, in fact, the first such the device is of natural origin. It was discovered in the Oklo region of the Central African state of Gabon:

• The reactor was formed due to the flooding of uranium rocks by groundwater. They acted as neutron moderators;
• The thermal energy released during the decay of uranium turns water into steam, and the chain reaction stops;
• After the coolant temperature drops, everything repeats again;
• If the liquid had not boiled away and stopped the reaction, humanity would have faced a new natural disaster;
• Self-sustaining nuclear fission began in this reactor about one and a half billion years ago. During this time, approximately 0.1 million watts of power output was provided;
• Such a wonder of the world on Earth is the only one known. The emergence of new ones is impossible: the share of uranium-235 in natural raw materials is much lower than the level necessary to maintain a chain reaction.

How many nuclear reactors are there in South Korea?

Poor na Natural resources, but the industrialized and overpopulated Republic of Korea has an extraordinary need for energy. Against the backdrop of Germany's refusal to use the peaceful atom, this country has high hopes for curbing nuclear technology:

• It is planned that by 2035 the share of electricity generated by nuclear power plants will reach 60%, and total production will be more than 40 gigawatts;
• The country does not have atomic weapons, but research in nuclear physics is ongoing. Korean scientists have developed designs for modern reactors: modular, hydrogen, with liquid metal, etc.;
• The successes of local researchers make it possible to sell technologies abroad. The country is expected to export 80 such units over the next 15-20 years;
• But as of today, most nuclear power plants were built with the assistance of American or French scientists;
• The number of operating plants is relatively small (only four), but each of them has a significant number of reactors - a total of 40, and this figure will grow.

When bombarded by neutrons, nuclear fuel goes into a chain reaction, resulting in a huge amount of heat. The water in the system takes this heat and turns into steam, which turns turbines that produce electricity. Here simple circuit operation of a nuclear reactor, the most powerful source of energy on Earth.

Video: how nuclear reactors work

In this video, nuclear physicist Vladimir Chaikin will tell you how electricity is generated in nuclear reactors and their detailed structure:

A fission chain reaction is always accompanied by the release of enormous energy. 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 occurs.

Based on 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;
• Control and radiation protection system;
• 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 a controlled fission chain reaction occurs.

Most nuclear reactors operate 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 tablets is placed in hermetically sealed rods, which are called fuel rods (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 the 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 their speed is not reduced, the nuclear reaction may die out 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 chain reaction in the reactor will require less nuclear fuel.

As a result of the slowdown process, thermal neutrons, the speed of which is almost equal to the speed of thermal movement of gas molecules at room temperature.

Water, heavy water (deuterium oxide D 2 O), beryllium, and graphite are used as a moderator in nuclear reactors. But the best moderator is heavy water D2O.

Neutron reflector

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

Coolant

The heat released during a nuclear reaction is removed using a coolant. Ordinary natural water, previously purified from various impurities and gases, is often used as a coolant in nuclear reactors. 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 primary circuit water circulating through the reactor core washes the fuel rods, heating up to a temperature of 320 0 C. Then, inside the heat exchanger, it gives off heat to the secondary circuit water. The exchange takes place through heat exchange tubes, so there is no contact with the secondary circuit water. This prevents radioactive substances from entering the second circuit of the heat exchanger.

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

In heavy water reactors, the coolant is heavy water D2O, 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 – neutron multiplication factor,

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

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

If k ˃ 1 , the chain reaction grows, the system is called supercritical y. If k< 1 , the chain reaction dies out, 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 neutron absorbent substances. By extending or pushing these rods into the core, the rate of the nuclear fission reaction is controlled.

The control system provides control of the reactor during its start-up, scheduled 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, rate of power rise, fuel consumption, etc.) deviates from the norm, and this can lead to an accident, special emergency rods and the nuclear reaction quickly stops.

Ensure that the reactor parameters comply with the standards control and radiation protection systems.

For guard environment to protect against radioactive radiation, the reactor is placed in a thick concrete casing.

Remote control systems

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

Fast reactors

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

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 β-decay, uranium-239 turns into a neptunium-239 nucleus:

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 rods with raw materials (enriched uranium-235) are located in the reactor core. This zone is surrounded by a breeding zone, which consists of 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. As a result, plutonium-239 is formed. Thus, new nuclear fuel is produced in fast neutron reactors.

Liquid metals or mixtures thereof 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 produced 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 motors with electrical energy and are called transport reactors .

For scientific research in the field of nuclear physics and radiation chemistry, fluxes of neutrons and gamma quanta are used, 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 study various physical quantities, the meaning of which 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 classified as industrial reactors.