Atomic and nuclear reactor difference. NPP operation safety. What happened at the Chernobyl nuclear power plant

The immense energy of a tiny atom

“Good science is physics! Only life is short." These words belong to a scientist who has done amazingly much in physics. They were once pronounced by an academician Igor Vasilievich Kurchatov, creator of the world's first nuclear power plant.

On June 27, 1954, this unique power plant went into operation. Humanity has another powerful source of electricity.

The path to mastering the energy of the atom was long and difficult. It began in the first decades of the 20th century with the discovery of natural radioactivity by the Curies, with Bohr's postulates, Rutherford's planetary model of the atom, and the proof of such, as it seems now, an obvious fact - the nucleus of any atom consists of positively charged protons and neutral neutrons.

In 1934, Frédéric and Irene Joliot-Curie (daughter of Marie Sklodowska-Curie and Pierre Curie) discovered that by bombarding them with alpha particles (the nuclei of helium atoms), ordinary chemical elements could be turned into radioactive ones. The new phenomenon is called artificial radioactivity.

I. V. Kurchatov (right) and A. I. Alikhanov (center) with their teacher A. F. Ioffe. (Early 30s.)

If such a bombardment is carried out with very fast and heavy particles, then a cascade of chemical transformations begins. Elements with artificial radioactivity will gradually give way to stable elements that will no longer decay.

With the help of irradiation or bombardment, it is easy to make the dream of alchemists come true - to make gold from other chemical elements. Only the cost of such a transformation will significantly exceed the price of the received gold ...

Fission of uranium nuclei

More benefit (and, unfortunately, anxiety) was brought to mankind by the discovery in 1938-1939 by a group of German physicists and chemists fission of uranium nuclei. When irradiated with neutrons, heavy uranium nuclei decay into lighter chemical elements belonging to the middle part periodic system Mendeleev, and emit several neutrons. For the nuclei of light elements, these neutrons turn out to be superfluous ... When the nuclei of uranium are “splitting”, a chain reaction can begin: each of the two or three resulting neutrons is capable, in turn, of producing several neutrons, hitting the nucleus of a neighboring atom.

The total mass of the products of such a nuclear reaction turned out, as scientists calculated, to be less than the mass of the nuclei of the original substance - uranium.

According to Einstein's equation, which relates mass to energy, one can easily determine that a huge amount of energy must be released in this case! And it will happen in a very short time. Unless, of course, the chain reaction becomes uncontrollable and goes to the end ...

Walking after the conference E. Fermi (right) with his student B. Pontecorvo. (Basel, 1949)

huge physical and technical capabilities, hidden in the fission process of uranium, was one of the first to estimate Enrico Fermi, in those distant thirties of our century, still a very young, but already recognized head of the Italian school of physicists. Long before the Second World War, he and a group of talented employees investigated the behavior various substances with neutron irradiation and determined that the efficiency of the uranium fission process can be significantly increased ... by slowing down the movement of neutrons. Strange as it may seem at first glance, with a decrease in the speed of neutrons, the probability of their capture by uranium nuclei increases. Quite accessible substances serve as effective "moderators" of neutrons: paraffin, carbon, water ...

Moving to the US, Fermi continued to be the brain and heart of the nuclear research there. Two talents, usually mutually exclusive, were combined in Fermi: an outstanding theorist and a brilliant experimenter. “It will be a long time before we can see a person equal to him,” wrote the prominent scientist W. Zinn after Fermi’s untimely death from a malignant tumor in 1954 at the age of 53.

A team of scientists who rallied around Fermi during the Second World War decided to create a weapon of unprecedented destructive power based on a chain reaction of uranium fission - atomic bomb. Scientists were in a hurry: what if Nazi Germany will be the first to make a new weapon and use it in its inhuman desire to enslave other peoples?

Construction of a nuclear reactor in our country

Already in 1942, scientists managed to assemble and launch on the territory of the stadium of the University of Chicago first nuclear reactor. The uranium rods in the reactor were interspersed with carbon "bricks" - moderators, and if the chain reaction nevertheless became too violent, it could be quickly stopped by introducing cadmium plates into the reactor, which separated the uranium rods and completely absorbed the neutrons.

The researchers were very proud of their inventions. simple fixtures to the reactor, which now make us smile. One of Fermi's employees in Chicago, the famous physicist G. Anderson, recalls that cadmium tin was nailed to a wooden bar, which, if necessary, instantly lowered into the boiler under the influence of its own gravity, which was the reason to give it the name "instant". G. Anderson writes: “Before starting the boiler, this rod should have been pulled up and secured with a rope. In the event of an accident, the rope could be cut and the "moment" would take its place inside the boiler.

A controlled chain reaction was obtained at an atomic reactor, theoretical calculations and predictions were verified. A chain of chemical transformations took place in the reactor, as a result of which a new chemical element- plutonium. It, like uranium, can be used to create an atomic bomb.

Scientists have determined that there is a "critical mass" of uranium or plutonium. If there is enough atomic matter, the chain reaction leads to an explosion, if it is small, less than the “critical mass”, then heat is simply released.

Construction of a nuclear power plant

In the atomic bomb the simplest design two pieces of uranium or plutonium are stacked side by side, and the mass of each is a little less than the critical one. At the right moment, a fuse from an ordinary explosive connects the pieces, the mass of atomic fuel exceeds a critical value - and the release of destructive energy of monstrous force occurs instantly ...

Blinding light radiation, a shock wave that sweeps away everything in its path, and penetrating radioactive radiation fell upon the inhabitants of two Japanese cities - Hiroshima and Nagasaki - after the explosion of American atomic bombs in 1945, and since then, people have been alarmed by the terrible consequences of the use of atomic bombs. weapons.

Under the unifying scientific leadership of IV Kurchatov, Soviet physicists developed atomic weapons.

But the leader of these works did not stop thinking about the peaceful use of atomic energy. After all, a nuclear reactor has to be intensively cooled, why is this heat not “given away” to a steam or gas turbine, not used to heat houses?

Pipes with liquid low-melting metal were passed through the nuclear reactor. The heated metal entered the heat exchanger, where it transferred its heat to the water. The water turned into superheated steam, the turbine began to work. The reactor was surrounded by a protective shell of concrete with metal filler: radioactive radiation should not escape.

The nuclear reactor has turned into a nuclear power plant, bringing calm light to people, cozy warmth the desired world...

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 velocity is practically equal to the velocity of thermal motion of 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 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, conventional natural water, 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 rods, 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 , is the number of neutrons of the 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, 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.

To protect the 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.) arrive at the reactor control panel and are 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, receive electrical and thermal energy 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. Desalination reactors sea ​​water also apply to industrial reactors.

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 have been seen as both a miracle and a threat.

When the first US commercial reactor went online at Shippingport, Pennsylvania in 1956, the technology was hailed as the powerhouse of the future, with some believing that reactors would make electricity generation too cheap. Now 442 nuclear reactors have been built around the world, about a quarter of these reactors are in the United States. The world has become dependent on nuclear reactors, which generate 14 percent of the electricity. Futurists even fantasized about atomic cars.

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

But in the early 2000s, nuclear reactors began to make a comeback, thanks to a growing demand for energy and a declining supply of fossil fuels, as well as growing concerns about climate change from carbon dioxide emissions.

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

Use of nuclear reaction

Simply put, in a nuclear reactor, atoms split and release the energy that holds their parts together.

If you forgot physics high school we will remind you how nuclear fission works. Atoms are 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 that are bound together. The force that binds the elements of the nucleus is hard to even imagine. It is many billion times stronger than the force of gravity. Despite this enormous force, it is possible to split the nucleus by firing neutrons at it. When this is done, a lot of energy will be released. When atoms break up, their particles crash into nearby atoms, splitting them, and those, in turn, next, next, 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 core is relatively weak compared to other elements. Nuclear reactors use a specific isotope called Atran-235 . Uranium-235 is rare in nature, with ore from uranium mines containing only about 0.7% U-235. That's why reactors use enrichedAtrun, which is created by isolating and concentrating Uranium-235 through a gas diffusion process.

A chain reaction process can be created in an 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, which absorb some of the neutrons. This still allows the fission process to release enough energy to heat water to about 270 degrees Celsius and turn it into steam, which is used to turn the power plant's turbines and generate electricity. In principle, in this case, a controlled nuclear bomb works instead of coal, creating electricity, except that the energy to boil 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. All of them have a stock of radioactive fuel pellets- usually uranium oxide, which are arranged in tubes to form fuel rods in coreereactor.

The reactor also has the previously mentioned managerserodand— of a neutron-absorbing material such as cadmium, hafnium or boron, which is inserted to control or stop the 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 plain water, but reactors in other countries sometimes use graphite, or heavywowwatersat, 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 shellat, a large, heavy structure, usually several meters thick, of steel and concrete that keeps radioactive gases and liquids inside where they can't harm anyone.

There are a number of 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 in the steam generator comes into contact with water supplied without pressure, which turns into steam that rotates the turbines. There is also a design reactor high power channel type (RBMK) with one water circuit and fast neutron reactor with two sodium and one water circuit.

How safe is a nuclear reactor?

The answer to this question is quite difficult and it depends on who you ask and what you mean by "safe". Are you worried 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 valid 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 energy, however, point out that we are all regularly exposed to radiation from various sources, including cosmic rays and natural radiation emitted by the Earth. The average annual radiation dose is about 6.2 millisieverts (mSv), half of it from natural sources and half from artificial sources ranging from chest x-rays, smoke detectors and luminous watch faces. How much radiation do we get from nuclear reactors? Only a tiny fraction of a percent of our typical annual exposure, 0.0001 mSv.

While all nuclear plants inevitably leak small amounts of radiation, regulatory commissions keep nuclear plant operators under stringent regulations. 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 health risks to humans.

But it is important to note that not everyone agrees with such a complacent assessment of radiation risks. For example, Physicians for Social Responsibility, a longtime critic of the nuclear industry, has studied children living around German nuclear power plants. The study showed that people living within 5 km of the plants had double the risk of contracting leukemia compared to those living farther from the nuclear power plant.

nuclear waste reactor

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 another environmental problem: nuclear waste disposal. Some of the spent fuel waste from reactors still releases radioactivity. Other unnecessary stuff that should be saved is radioactive waste high level , the liquid residue from the processing of spent fuel, in which part of the uranium remains. Right now, most of this waste is stored locally at nuclear power plants in ponds of water that 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 during fission. When large uranium atoms are fissured, they create by-products - 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 Pperiodohmhalf-life. For other radioactive elements, the half-life will be different. In addition, some uranium atoms also capture neutrons, forming heavier elements such as plutonium. These transuranium elements do not generate 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 radioactiveedepartures high level from reactors are dangerous to humans and other life forms because they can emit a huge, lethal dose radiation even from a short exposure. Ten years after removing fuel from a reactor, for example, they emit 200 times more radioactivity per hour than it takes to kill a person. And if the waste ends up in groundwater or rivers, they can enter the food chain and endanger large numbers of people.

Because waste is so dangerous, many people are in a difficult position. 60,000 tons of waste is located at nuclear plants close to big cities. But finding a safe place to store waste is very difficult.

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 optimum safety. It's just that they don't break, work properly, and have backups if things don't go according to plan. As a result, year after year, nuclear plants appear to be fairly safe compared to, say, air travel, which routinely kills between 500 and 1,100 people a year worldwide.

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

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

The USSR was less fortunate. A severe nuclear accident occurred in April 1986 at the 4th power unit at the Chernobyl nuclear power plant. This was caused by a combination of system breakdowns, design flaws, and poorly trained personnel. During a routine test, the reaction suddenly increased and the control rods jammed, preventing the 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 destroy, as a result of which the fuel took on a liquid form. Many workers of the station and liquidators of the accident died. A large number of radiation spread over an area of ​​323,749 square kilometers. The number of deaths caused by radiation is still unclear, but the World Health Organization says it may have caused 9,000 cancer deaths.

The builders of nuclear reactors give guarantees based on probabilistic estimatee in which they try to balance the potential harm of an event with the likelihood that it actually occurs. But some critics say they should prepare, instead, for the rare, most unexpected, but very dangerous events. An illustrative example is the accident in March 2011 at the Fukushima 1 nuclear power plant in Japan. The station was reportedly designed to withstand a large quake, but not as catastrophic as the 9.0 quake that kicked up 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 meant to power the cooling system of the six nuclear power plant reactors in the event of a power outage. Thus, even after the control rods of the Fukushima reactors stopped the fission reaction, the still hot fuel allowed the temperature inside the destroyed reactors.

Japanese officials have resorted to the last resort - flooding the reactors with huge amounts of sea water laced with boric acid, which could prevent a catastrophe, 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 this nuclear power plant, the radioactive element Cesium-137 turned out to be at levels much higher than after the Chernobyl disaster, which raised doubts about the possibility of human habitation in this zone.

For an ordinary person, modern high-tech devices are so mysterious and mysterious that it is just right to worship them, as the ancients worshiped lightning. School physics lessons, replete with mathematical calculations, do not solve the problem. But it’s interesting to tell even about a nuclear reactor, the principle of operation of which is clear even to a teenager.

How does a nuclear reactor work?

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

1. When a neutron is absorbed, nuclear fuel (most often this uranium-235 or plutonium-239) the division of the atomic nucleus occurs;
2. Kinetic energy, gamma radiation and free neutrons are released;
3. Kinetic energy is converted into thermal energy (when the nuclei collide with surrounding atoms), gamma radiation is absorbed by the reactor itself and is also converted into heat;
4. Some of the generated neutrons are absorbed by the 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 a generator mechanical energy 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 power, is 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 alloy, mercury), gas (helium, carbon dioxide or nitrogen), molten salt (fluoride salts);
• By generation. The first is the early prototypes, which didn't make any commercial sense. The second is the majority of currently used nuclear power plants that were built before 1996. The third generation differs from the previous one only in minor improvements. Work on the fourth generation is still underway;
• According to aggregate state fuel (gas still exists only on paper);
• By purpose of use(for the production of electricity, engine start, hydrogen production, desalination, transmutation of elements, obtaining neural radiation, theoretical and investigative purposes).

Nuclear reactor device

The main components of reactors in most power plants are:

1. Nuclear fuel - a substance that is necessary for the production of heat for power turbines (usually low enriched uranium);
2. The active zone of the nuclear reactor - this 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 launch 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 being turned off;
7. Coolant (purified water);
8. Control rods - to control the rate of fission of uranium or plutonium nuclei;
9. Water pump - pumps water to the steam boiler;
10. Steam turbine - converts the thermal energy of steam into rotational mechanical energy;
11. Cooling tower - a device for removing excess heat into the atmosphere;
12. System for receiving and storing radioactive waste;
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 arrangement of their work are at the development stage. According to current data, these modifications will differ from existing models in such benefits:

• Rapid gas cooling system. It is assumed that helium will be used as a coolant. According to project documentation, thus it is possible to cool reactors with a temperature of 850 °C. To work with such high temperatures ah, 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 a primary coolant. These materials have a low neutron absorption and are relatively low temperature melting;
• Also, a mixture of molten salts can be used as the main coolant. Thus, it will be possible to work at higher temperatures than modern water-cooled counterparts.

Natural analogues in nature

The nuclear reactor is perceived in the public mind solely as a product high technology. However, in fact the first the device is of natural origin. It was discovered in the Oklo region, in 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 off and stopped the course of 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, about 0.1 million watts of output power was allocated;
• Such a wonder of the world on Earth is the only one known. The appearance of new ones is impossible: the proportion of uranium-235 in natural raw materials is much lower than the level necessary to maintain a chain reaction.

How many nuclear reactors are in South Korea?

Poor in natural resources, but industrialized and overpopulated, the Republic of Korea is in dire need of energy. Against the backdrop of Germany's rejection of 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 the total production - 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 success of local researchers allows you to sell technology abroad. It is expected that in the next 15-20 years the country will export 80 such units;
• But as of today, most of the nuclear power plants have been built with the assistance of American or French scientists;
• The number of operating stations is relatively small (only four), but each of them has a significant number of reactors - 40 in total, and this figure will grow.

When bombarded with neutrons, nuclear fuel enters into a chain reaction, as a result of which a huge amount of heat is generated. The water in the system takes this heat and turns it 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, their detailed structure:

Today we will make a short journey into the world of nuclear physics. The theme of our tour will be nuclear reactor. You will learn how it works, what physical principles underlie its operation and where this device is used.

The birth of nuclear energy

The world's first nuclear reactor was built in 1942 in the USA. experimental group of physicists led by the laureate nobel prize Enrico Fermi. At the same time, they carried out a self-sustaining uranium fission reaction. The atomic genie has been released.

The first Soviet nuclear reactor was launched in 1946, and 8 years later, the world's first nuclear power plant in the city of Obninsk gave current. The chief scientific supervisor of work in the nuclear power industry of the USSR was an outstanding physicist Igor Vasilievich Kurchatov.

Since then, several generations of nuclear reactors have changed, but the main elements of its design have remained unchanged.

Anatomy of a nuclear reactor

This nuclear facility is a thick-walled steel tank with a cylindrical capacity ranging from a few cubic centimeters to many cubic meters.

Inside this cylinder is the holy of holies - reactor core. It is here that the chain reaction of fission of nuclear fuel takes place.

Let's see how this process takes place.

The nuclei of heavy elements, in particular Uranium-235 (U-235), under the influence of a small energy push, they are able to fall apart into 2 fragments of approximately equal mass. The causative agent of this process is the neutron.

Fragments are most often barium and krypton nuclei. Each of them carries positive charge, so the forces of Coulomb repulsion force them to scatter in different sides at about 1/30 of the speed of light. These fragments are carriers of colossal kinetic energy.

For practical use energy, it is necessary that its release be self-sustaining. Chain reaction, which is in question is all the more interesting because each fission event is accompanied by the emission of new neutrons. For one initial neutron, on average, 2-3 new neutrons arise. The number of fissile uranium nuclei is growing like an avalanche, causing the release of enormous energy. If this process is not controlled, it will nuclear explosion. It takes place in .

To control the number of neutrons materials that absorb neutrons are introduced into the system, providing a smooth release of energy. Cadmium or boron are used as neutron absorbers.

How to curb and use the huge kinetic energy of the fragments? For these purposes, a coolant is used, i.e. a special medium, moving in which the fragments are decelerated and heated to extremely high temperatures. Such a medium can be ordinary or heavy water, liquid metals (sodium), as well as some gases. In order not to cause the transition of the coolant into a vapor state, supported in the core high pressure(up to 160 atm). For this reason, the walls of the reactor are made of ten-centimeter steel of special grades.

If the neutrons fly out of the nuclear fuel, then the chain reaction can be interrupted. Therefore, there is a critical mass of fissile material, i.e. its minimum mass at which a chain reaction will be maintained. It depends on various parameters, including the presence of a reflector surrounding the reactor core. It serves to prevent leakage of neutrons into the environment. The most common material for this structural element is graphite.

The processes occurring in the reactor are accompanied by the release of the dangerous kind radiation - gamma radiation. To minimize this danger, it provides anti-radiation protection.

How a nuclear reactor works

Nuclear fuel, called fuel elements, is placed in the reactor core. They are tablets formed from a fissile material and packed into thin tubes about 3.5 m long and 10 mm in diameter.

Hundreds of fuel assemblies of the same type are placed in the core, and they become sources of thermal energy released during the chain reaction. The coolant washing the fuel rods forms the first circuit of the reactor.

heated up high parameters, it is pumped by the pump to the steam generator, where it transfers its energy to the water of the secondary circuit, turning it into steam. The resulting steam rotates the turbine generator. The electricity generated by this unit is transferred to the consumer. And the exhaust steam, cooled by water from the cooling pond, in the form of condensate, is returned to the steam generator. The cycle closes.

Such double circuit the operation of a nuclear installation excludes the penetration of radiation accompanying the processes occurring in the core beyond its limits.

So, a chain of energy transformations takes place in the reactor: the nuclear energy of the fissile material → into the kinetic energy of fragments → the thermal energy of the coolant → the kinetic energy of the turbine → and into electrical energy in the generator.

The inevitable loss of energy leads to the fact that The efficiency of nuclear power plants is relatively low, 33-34%.

In addition to generating electrical energy at nuclear power plants, nuclear reactors are used to produce various radioactive isotopes, for research in many areas of industry, and to study the permissible parameters of industrial reactors. Transport reactors, which provide energy to vehicle engines, are becoming more and more widespread.

Types of nuclear reactors

Typically, nuclear reactors run on uranium U-235. However, its content natural material extremely small, only 0.7%. The main mass of natural uranium is the U-238 isotope. A chain reaction in U-235 can only be caused by slow neutrons, and the U-238 isotope is only fissioned by fast neutrons. As a result of nuclear fission, both slow and fast neutrons are born. Fast neutrons, experiencing deceleration in the coolant (water), become slow. But the amount of the U-235 isotope in natural uranium is so small that it is necessary to resort to its enrichment, bringing its concentration to 3-5%. This process is very expensive and economically disadvantageous. In addition, exhaustion time natural resources this isotope is estimated to be only 100-120 years old.

Therefore, in the nuclear industry there is a gradual transition to reactors operating on fast neutrons.

Their main difference is that liquid metals are used as a coolant, which do not slow down neutrons, and U-238 is used as nuclear fuel. The nuclei of this isotope pass through a chain of nuclear transformations into Plutonium-239, which is subject to a chain reaction in the same way as U-235. That is, there is a reproduction of nuclear fuel, and in an amount exceeding its consumption.

According to experts Uranium-238 isotope reserves should last for 3,000 years. This time is quite enough for humanity to have enough time to develop other technologies.

Problems in the use of nuclear energy

As well as obvious benefits nuclear energy, the scale of the problems associated with the operation of nuclear facilities cannot be underestimated.

The first of these is disposal of radioactive waste and dismantled equipment nuclear energy. These elements have an active radiation background, which persists for a long period. For the disposal of these wastes, special lead containers are used. They are supposed to be buried in the areas permafrost at depths up to 600 meters. Therefore, work is constantly underway to find a way to process radioactive waste, which should solve the problem of disposal and help preserve the ecology of our planet.

The second major problem is ensuring safety during NPP operation. Major accidents like Chernobyl can take many human lives and put vast territories out of use.

accident on Japanese nuclear power plant Fukushima-1 only confirmed the potential danger that manifests itself in the event of an emergency situation at nuclear facilities.

However, the possibilities of nuclear energy are so great that ecological problems fade into the background.

Today, humanity has no other way to satisfy the ever-increasing energy hunger. The basis of the nuclear power industry of the future will probably be "fast" reactors with the function of breeding nuclear fuel.

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