Chapter VIII Principle of operation and capabilities of a nuclear reactor I. Design of a nuclear reactor A nuclear reactor consists of the following five main elements: 1) nuclear fuel; 2) neutron moderator; 3) control system; 4) cooling system; 5) protective

Chapter 11 INTERNAL STRUCTURE OF DIELECTRICS §1. Molecular dipoles§2. Electronic polarization §3. Polar molecules; orientation polarization§4. Electric fields in dielectric voids§5. Dielectric constant of liquids; Clausius-Mossotti formula§6.

This nondescript gray cylinder is the key link in the Russian nuclear industry. It doesn’t look very presentable, of course, but once you understand its purpose and look at the technical characteristics, you begin to understand why the secret of its creation and structure is protected by the state like the apple of its eye.

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

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

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

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

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

A little history

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

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

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

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

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

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

How does a centrifuge work?

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

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

Technical features.

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

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

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

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

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

Who invented it?

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

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

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

So what did Sergeev come up with?

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

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

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

Our days

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

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

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

What is a nuclear reactor?

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

Operating principle of a nuclear reactor

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

The process of nuclear fission

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

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

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

Heat release in a nuclear reactor

The reactor work area generates heat in several ways:

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

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

Nuclear reactor cooling system

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

Monitoring the neutron flux in the reactor

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

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

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

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

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

Most types of reactors are sensitive to a process known as the "xenon pit" or "iodine pit". The widespread decay product xenon-135, resulting from the fission reaction, plays the role of a neutron absorber that tends to shut down the reactor. The accumulation of xenon-135 can be controlled by maintaining a power level high enough to destroy it by absorbing neutrons as quickly as it is produced. Fission also results in the formation of iodine-135, which in turn decays (with a half-life of 6.57 hours) to form xenon-135. When the reactor is shut down, iodine-135 continues to decay to form xenon-135, which makes restarting the reactor more difficult within a day or two as xenon-135 decays to form cesium-135, which is not a neutron absorber like xenon-135. 135, with a half-life of 9.2 hours. This temporary state is an “iodine hole.” If the reactor has sufficient additional power, it can be restarted. The more xenon-135 turns into xenon-136, which is less of a neutron absorber, and within a few hours the reactor experiences what is called a "xenon burnup stage." Additionally, control rods must be inserted into the reactor to compensate for the absorption of neutrons to replace the lost xenon-135. The failure to correctly follow such a procedure was a key cause of the Chernobyl accident.

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

How is electricity produced?

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

The history of the first reactors

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

The impetus for creating a new type of reactor using uranium was the discovery by Lise Meitner, Fritz Strassmann and Otto Hahn in 1938, who “bombarded” uranium with neutrons (using the alpha decay reaction of beryllium, a “neutron gun”) to produce barium, which they believed it arose from the decay of uranium nuclei. Subsequent research in early 1939 (Szilard and Fermi) showed that some neutrons were also produced by atomic fission, making possible the nuclear chain reaction that Szilard had envisioned six years earlier.

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

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

The following year, the US government received the Frisch-Peierls Memorandum from Great Britain, which stated that the amount of uranium required to carry out the chain reaction was much less than previously thought. The memorandum was created with the participation of Maud Committee, which worked on the atomic bomb project in Great Britain, later known under the code name "Tube Alloys" and later included in the Manhattan Project.

Ultimately, the first man-made nuclear reactor, called Chicago Woodpile 1, was built at the University of Chicago by a team led by Enrico Fermi in late 1942. By this time, the US atomic program had already been accelerated due to the country's entry into the war. The Chicago Woodpile reached its critical point on December 2, 1942, at 3:25 p.m. The reactor frame was made of wood, holding together a stack of graphite blocks (hence the name) with nested "briquettes" or "pseudo-spheres" of natural uranium oxide.

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

"World's First" is the inscription on the site of the EBR-I reactor, which is now a museum near Arco, Idaho. Originally called Chicago Woodpile 4, this reactor was created under the direction of Walter Sinn for the Aregon National Laboratory. This experimental fast breeder reactor was operated by the US Atomic Energy Commission. The reactor produced 0.8 kW of power when tested on December 20, 1951, and 100 kW of power (electrical) the next day, having a design capacity of 200 kW (electrical power).

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

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

After World War II, the US military sought other applications for nuclear reactor technology. Research conducted by the Army and Air Force was not implemented; However, the US Navy achieved success by launching the nuclear submarine USS Nautilus (SSN-571) on January 17, 1955.

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

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

Main components of a nuclear power plant

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

Nuclear reactor elements

• Nuclear fuel (nuclear reactor core; neutron moderator)
• Original neutron source
• Neutron absorber
• Neutron gun (provides a constant source of neutrons to re-initiate the reaction after shutdown)
• Cooling system (often the neutron moderator and coolant are the same thing, usually purified water)
• Control rods
• Nuclear reactor vessel (NRP)

Boiler water supply pump

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

Radiation safety system

• Rector protection system (RPS)
• Emergency diesel generators
• Emergency reactor core cooling system (ECCS)
• Emergency liquid control system (emergency boron injection, only in boiling-water nuclear reactors)
• System for supplying process water to responsible consumers (SOTVOP)

Protective shell

• Remote Control
• Emergency installation
• Nuclear training complex (as a rule, there is an imitation control panel)

Classifications of nuclear reactors

Types of nuclear reactors

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

Classification of nuclear reactors by moderator type

Thermal reactors used:

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

Classification of nuclear reactors by type of coolant

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

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

Generations of nuclear reactors

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

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

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

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

Classification of nuclear reactors by type of fuel

• Solid fuel reactor
• Liquid fuel reactor
• Homogeneous water cooled reactor
• Molten salt reactor
• Gas-fueled reactors (theoretically)

Classification of nuclear reactors by purpose

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

Modern nuclear reactors

These reactors use a high-pressure vessel to hold nuclear fuel, control rods, moderator, and coolant. Cooling of reactors and moderation of neutrons occurs with liquid water under high pressure. The hot radioactive water that leaves the high pressure vessel passes through a steam generator circuit, which in turn heats the secondary (non-radioactive) circuit. These reactors make up the majority of modern reactors. This is a neutron reactor heating structure device, the newest of which are the VVER-1200, the Advanced Pressurized Water Reactor and the European Pressurized Water Reactor. US Navy reactors are of this type.

Boiling water reactors are similar to pressurized water reactors without a steam generator. Boiling water reactors also use water as a coolant and neutron moderator as pressurized water reactors, but at a lower pressure, allowing the water to boil inside a boiler, creating steam that turns turbines. Unlike a pressurized water reactor, there is no primary or secondary circuit. The heating capacity of these reactors may be higher, and they may be simpler in design, and even more stable and safe. This is a thermal neutron reactor device, the newest of which are the Advanced Boiling Water Reactor and the Economical Simplified Boiling Water Nuclear Reactor.

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

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

They typically use a graphite neutron moderator and CO2 coolant. Because of their high operating temperatures, they can be more efficient at producing heat than pressurized water reactors. There are a number of operating reactors of this design, mainly in the United Kingdom where the concept was developed. The older developments (i.e. Magnox Station) are either closed or will be closed in the near future. However, improved gas-cooled reactors have an expected operating life of another 10 to 20 years. Reactors of this type are thermal neutron reactors. The monetary costs of decommissioning such reactors can be high due to the large volume of the core.

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

The use of lead as a liquid metal provides excellent protection against radioactive radiation, and allows operation at very high temperatures. Additionally, lead is (mostly) transparent to neutrons, so fewer neutrons are lost to the coolant and the coolant does not become radioactive. Unlike sodium, lead is generally inert, so there is less risk of explosion or accident, but such large quantities of lead can cause problems from a toxicity and waste disposal perspective. Lead-bismuth eutectic mixtures can often be used in this type of reactor. In this case, bismuth will present little interference to radiation because it is not completely transparent to neutrons, and can mutate into another isotope more easily than lead. The Russian Alpha-class submarine uses a lead-bismuth-cooled fast reactor as its main power generation system.

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

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

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

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

Innovative nuclear systems and projects

Advanced Reactors

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

Integrated fast neutron nuclear reactor(IFR) was built, tested and tested during the 1980s, and then retired after the Clinton Administration left office in the 1990s due to nuclear nonproliferation policies. Reprocessing spent nuclear fuel is built into its design and therefore produces only a fraction of the waste from operating reactors.

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

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

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

The scaled-down water-moderated reactor is based on the improved boiling water reactor (ABWR) currently in operation. It is not a full fast neutron reactor, but uses mainly epithermal neutrons, which have velocities intermediate between thermal and fast.

Self-regulating nuclear power module with hydrogen neutron moderator (HPM) is a design type of reactor produced by Los Alamos National Laboratory that uses uranium hydride as fuel.

Subcritical nuclear reactors are intended to be safer and more stable, but are complex in engineering and economic terms. One example is the Energy Booster.

Thorium based reactors. It is possible to convert thorium-232 to U-233 in reactors designed specifically for this purpose. In this way, thorium, which is four times more abundant than uranium, can be used to produce U-233-based nuclear fuel. U-233 is believed to have favorable nuclear properties compared to conventionally used U-235, particularly better neutron efficiency and a reduction in the amount of long-lived transuranium waste produced.

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

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

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

What are fourth generation reactors?

The fourth generation of reactors is a collection of different theoretical designs that are currently being considered. These projects are unlikely to be completed by 2030. Current reactors in operation are generally considered second or third generation systems. First generation systems have not been used for some time. Development of this fourth generation of reactors was officially launched at the Generation IV International Forum (GIF) based on eight technology goals. The main objectives were to improve nuclear safety, increase proliferation resistance, minimize waste and use of natural resources, and to reduce the costs of building and operating such plants.

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

What are fifth generation reactors?

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

• Liquid phase reactor. A closed circuit with liquid in the core of a nuclear reactor, where the fissile material is in the form of molten uranium or a uranium solution cooled by a working gas injected into through holes in the base of the holding vessel.
• Gas phase reactor in the core. A closed-cycle option for a nuclear-powered rocket, where the fissile material is uranium hexafluoride gas located in a quartz container. The working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation resulting from the nuclear reaction. Such a design could be used as a rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. In theory, using uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would result in lower energy generation costs and would also significantly reduce the size of reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled flow of neutrons, weakening the strength properties of much of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of materials that are similar to the materials used within the framework of the International Project for the Implementation of a Facility for Irradiation of Materials under Thermonuclear Reaction Conditions.
• Gas-phase electromagnetic reactor. Same as a gas-phase reactor, but with photovoltaic cells that convert ultraviolet light directly into electricity.
• Fragmentation reactor
• Hybrid nuclear fusion. The neutrons emitted during the fusion and decay of the original or "substance in the breeding zone" are used. For example, the transmutation of U-238, Th-232 or spent fuel/radioactive waste from another reactor into relatively benign isotopes.

Reactor with a gas phase in the core. A closed-cycle option for a nuclear-powered rocket, where the fissile material is uranium hexafluoride gas located in a quartz container. The working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation resulting from the nuclear reaction. Such a design could be used as a rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. In theory, using uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would result in lower energy generation costs and would also significantly reduce the size of reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled flow of neutrons, weakening the strength properties of much of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of materials that are similar to the materials used within the framework of the International Project for the Implementation of a Facility for Irradiation of Materials under Thermonuclear Reaction Conditions.

Gas-phase electromagnetic reactor. Same as a gas-phase reactor, but with photovoltaic cells that convert ultraviolet light directly into electricity.

Fragmentation reactor

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

Fusion reactors

Controlled nuclear fusion can be used in fusion power plants to produce electricity without the complications associated with working with actinides. However, significant scientific and technological obstacles remain. Several fusion reactors have been built, but only recently have the reactors been able to release more energy than they consume. Although research began in the 1950s, it is expected that a commercial fusion reactor will not operate until 2050. Efforts to harness fusion energy are currently underway within the ITER project.

Nuclear fuel cycle

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

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

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

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

Most existing reactors are thermal reactors, which typically use water as a neutron moderator (moderator means it slows down a neutron to thermal speed) and also as a coolant. However, a fast neutron reactor uses a slightly different type of coolant that will not slow down the neutron flow too much. This allows fast neutrons to predominate, which can be effectively used to constantly replenish the fuel supply. Simply by placing cheap, unenriched uranium in the core, spontaneously non-fissionable U-238 will turn into Pu-239, “breeding” the fuel.

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

Nuclear Reactor Maintenance

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

At the end of the operating cycle, the fuel in some assemblies is "worked out", unloaded and replaced in the form of new (fresh) fuel assemblies. Also, this reaction of accumulation of decay products in nuclear fuel determines the service life of nuclear fuel in the reactor. Even long before the final process of fuel fission occurs, long-lived neutron-absorbing decay byproducts have accumulated in the reactor, preventing the chain reaction from occurring. The proportion of the reactor core replaced during reactor refueling is typically one quarter for a boiling water reactor and one third for a pressurized water reactor. Disposal and storage of this spent fuel is one of the most difficult tasks in organizing the operation of an industrial nuclear power plant. Such nuclear waste is extremely radioactive and its toxicity poses a risk for thousands of years.

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

The amount of energy extracted from a nuclear fuel is called its burnup, which is expressed in terms of the thermal energy produced by the original unit weight of the fuel. Burnup is usually expressed in terms of thermal megawatt days per ton of parent heavy metal.

Nuclear Energy Safety

Nuclear safety represents actions aimed at preventing nuclear and radiation accidents or localizing their consequences. Nuclear power has improved reactor safety and performance, and has also introduced new, safer reactor designs (which have generally not been tested). However, there is no guarantee that such reactors will be designed, built and can operate reliably. Mistakes have occurred when reactor designers at the Fukushima nuclear power plant in Japan did not expect that a tsunami generated by an earthquake would shut down the backup system that was supposed to stabilize the reactor after the earthquake, despite numerous warnings from NRG (the national research group) and the Japanese administration on nuclear safety. According to UBS AG, the Fukushima I nuclear accident calls into question whether even advanced economies like Japan can ensure nuclear safety. Catastrophic scenarios, including terrorist attacks, are also possible. An interdisciplinary team from MIT (Massachusetts Institute of Technology) estimates that given the expected growth of nuclear power, at least four serious nuclear accidents can be expected between 2005 and 2055.

Nuclear and radiation accidents

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

Nuclear reactor plants have been launched into orbit around the Earth at least 34 times. A series of incidents involving the Soviet unmanned nuclear-powered RORSAT satellite resulted in the release of spent nuclear fuel into the Earth's atmosphere from orbit.

Natural nuclear reactors

Although fission reactors are often thought to be a product of modern technology, the first nuclear reactors occur in natural environments. A natural nuclear reactor can be formed under certain conditions that mimic those in a constructed reactor. To date, up to fifteen natural nuclear reactors have been discovered within three separate ore deposits of the Oklo uranium mine in Gabon (West Africa). The well-known “dead” Okllo reactors were first discovered in 1972 by the French physicist Francis Perrin. A self-sustaining nuclear fission reaction occurred in these reactors approximately 1.5 billion years ago, and was maintained for several hundred thousand years, producing an average of 100 kW of power output during this period. The concept of a natural nuclear reactor was explained in theoretical terms back in 1956 by Paul Kuroda at the University of Arkansas.

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

Natural nuclear reactors formed when rich uranium mineral deposits began to fill with underground water, which acted as a neutron moderator and set off a significant chain reaction. The neutron moderator, in the form of water, evaporated, causing the reaction to speed up, and then condensed back, causing the nuclear reaction to slow down and meltdown prevented. The fission reaction persisted for hundreds of thousands of years.

Such natural reactors have been extensively studied by scientists interested in the disposal of radioactive waste in a geological setting. They propose a case study of how radioactive isotopes would migrate through a layer of the Earth's crust. This is a key point for critics of geological waste disposal, who fear that isotopes contained in the waste could end up in water supplies or migrate into the environment.

Environmental problems of nuclear energy

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

For an ordinary person, modern high-tech devices are so mysterious and enigmatic that it is time to worship them, just as 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 structure

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 - converts the thermal energy of steam into rotational mechanical energy;
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 the design documentation, reactors with a temperature of 850 °C can be cooled in this way. To operate at such high temperatures, specific raw materials will 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 in natural resources, but industrialized and overpopulated, the 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 the total production will be more than 40 gigawatts;
• The country does not have atomic weapons, but research on 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 in 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 stations 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 the production of a huge amount of heat. The water in the system takes this heat and turns into steam, which turns turbines that produce electricity. Here is a simple diagram of the 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:

: ... quite banal, but nevertheless I still haven’t found the information in a digestible form - how a nuclear reactor STARTS to work. Everything about the principle and structure of work has already been chewed over 300 times and is clear, but here’s how the fuel is obtained and from what and why it is not so dangerous until it is in the reactor and why it does not react before being immersed in the reactor! - after all, it heats up only inside, nevertheless, before loading the fuel is cold and everything is fine, so what causes the heating of the elements is not entirely clear, how they are affected, and so on, preferably not scientifically).

It’s difficult, of course, to frame such a topic in a non-scientific way, but I’ll try. Let's first figure out what these fuel rods are.

Nuclear fuel is black tablets with a diameter of about 1 cm and a height of about 1.5 cm. They contain 2% uranium dioxide 235, and 98% uranium 238, 236, 239. In all cases, with any amount of nuclear fuel, a nuclear explosion cannot develop , because for an avalanche-like rapid fission reaction characteristic of a nuclear explosion, a concentration of uranium 235 of more than 60% is required.

Two hundred nuclear fuel pellets are loaded into a tube made of zirconium metal. The length of this tube is 3.5m. diameter 1.35 cm. This tube is called fuel element - fuel element. 36 fuel rods are assembled into a cassette (another name is “assembly”).

RBMK reactor fuel element design: 1 - plug; 2 - uranium dioxide tablets; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.

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

If we keep in mind the macroscopic scale of energy release, then all or initially at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of degrees Kelvin, but in the case of nuclear reactions it is at least 107 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

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

To control and protect a nuclear reactor, control rods are used that can be moved along the entire height of the core. The rods are made of substances that strongly absorb neutrons - for example, boron or cadmium. When the rods are inserted deeply, a chain reaction becomes impossible, since neutrons are strongly absorbed and removed from the reaction zone.

The rods are moved remotely from the control panel. With a slight movement of the rods, the chain process will either develop or fade. In this way the power of the reactor is regulated.

Leningrad NPP, RBMK reactor

Start of reactor operation:

At the initial moment of time after the first loading of fuel, there is no fission chain reaction in the reactor, the reactor is in a subcritical state. The coolant temperature is significantly less than the operating temperature.

As we have already mentioned here, for a chain reaction to begin, the fissile material must form a critical mass - a sufficient amount of spontaneously fissile material in a sufficiently small space, a condition under which the number of neutrons released during nuclear fission must be greater than the number of absorbed neutrons. This can be done by increasing the uranium-235 content (the amount of fuel rods loaded), or by slowing down the speed of neutrons so that they do not fly past the uranium-235 nuclei.

The reactor is brought up to power in several stages. With the help of reactivity regulators, the reactor is transferred to the supercritical state Kef>1 and the reactor power increases to a level of 1-2% of the nominal one. At this stage, the reactor is heated to the operating parameters of the coolant, and the heating rate is limited. During the heating process, the controls maintain the power at a constant level. Then the circulation pumps are started and the heat removal system is put into operation. After this, the reactor power can be increased to any level in the range from 2 to 100% of the rated power.

When the reactor heats up, the reactivity changes due to changes in the temperature and density of the core materials. Sometimes, during heating, the relative position of the core and the control elements that enter or exit the core changes, causing a reactivity effect in the absence of active movement of the control elements.

Regulation by solid, moving absorbent elements

To quickly change reactivity, in the vast majority of cases, solid movable absorbers are used. In the RBMK reactor, the control rods contain boron carbide bushings enclosed in an aluminum alloy tube with a diameter of 50 or 70 mm. Each control rod is placed in a separate channel and is cooled by water from the control and protection system (control and protection system) circuit at an average temperature of 50 ° C. According to their purpose, the rods are divided into AZ (emergency protection) rods; there are 24 such rods in the RBMK. Automatic control rods - 12 pieces, local automatic control rods - 12 pieces, manual control rods - 131, and 32 shortened absorber rods (USP). There are 211 rods in total. Moreover, the shortened rods are inserted into the core from the bottom, the rest from the top.

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

Burnable absorbing elements.

To compensate for excess reactivity after loading fresh fuel, burnable absorbers are often used. The operating principle of which is that they, like fuel, after capturing a neutron, subsequently cease to absorb neutrons (burn out). Moreover, the rate of decrease as a result of the absorption of neutrons by absorber nuclei is less than or equal to the rate of decrease as a result of fission of fuel nuclei. If we load a reactor core with fuel designed to operate for a year, then it is obvious that the number of fissile fuel nuclei at the beginning of operation will be greater than at the end, and we must compensate for the excess reactivity by placing absorbers in the core. If control rods are used for this purpose, we must continually move them as the number of fuel nuclei decreases. The use of burnable absorbers reduces the use of moving rods. Nowadays, burnable absorbents are often added directly to fuel pellets during their manufacture.

Fluid reactivity control.

Such regulation is used, in particular, during the operation of a VVER-type reactor, boric acid H3BO3 containing 10B neutron-absorbing nuclei is introduced into the coolant. By changing the concentration of boric acid in the coolant path, we thereby change the reactivity in the core. During the initial period of reactor operation, when there are many fuel nuclei, the acid concentration is maximum. As the fuel burns out, the acid concentration decreases.

Chain reaction mechanism

A nuclear reactor can operate at a given power for a long time only if it has a reactivity reserve at the beginning of operation. The exception is subcritical reactors with an external source of thermal neutrons. The release of bound reactivity as it decreases due to natural reasons ensures the maintenance of the critical state of the reactor at every moment of its operation. The initial reactivity reserve is created by constructing a core with dimensions significantly exceeding the critical ones. To prevent the reactor from becoming supercritical, k0 of the breeding medium is simultaneously artificially reduced. This is achieved by introducing neutron absorber substances into the core, which can be subsequently removed from the core. As in the chain reaction control elements, absorbent substances are included in the material of rods of one or another cross-section moving through the corresponding channels in the core. But if one or two or several rods are enough for regulation, then to compensate for the initial excess reactivity the number of rods can reach hundreds. These rods are called compensating rods. Control and compensating rods do not necessarily represent different design elements. A number of compensating rods can be control rods, but the functions of both are different. Control rods are designed to maintain a critical state at any time, to stop and start the reactor, and to transition from one power level to another. All these operations require small changes in reactivity. Compensating rods are gradually removed from the reactor core, ensuring a critical state during the entire time of its operation.

Sometimes control rods are made not from absorbent materials, but from fissile material or scattering material. In thermal reactors, these are mainly neutron absorbers; there are no effective fast neutron absorbers. Absorbers such as cadmium, hafnium and others strongly absorb only thermal neutrons due to the proximity of the first resonance to the thermal region, and outside the latter they are no different from other substances in their absorbing properties. The exception is boron, whose neutron absorption cross section decreases with energy much more slowly than that of the indicated substances, according to the l / v law. Therefore, boron absorbs fast neutrons, although weakly, but somewhat better than other substances. The absorber material in a fast neutron reactor can only be boron, if possible enriched with the 10B isotope. In addition to boron, fissile materials are also used for control rods in fast neutron reactors. A compensating rod made of fissile material performs the same function as a neutron absorber rod: it increases the reactivity of the reactor while it naturally decreases. However, unlike an absorber, such a rod is located outside the core at the beginning of the reactor operation and is then introduced into the core.

The scatterer materials used in fast reactors are nickel, which has a scattering cross section for fast neutrons that is slightly larger than the cross sections of other substances. The scatterer rods are located along the periphery of the core and their immersion in the corresponding channel causes a decrease in neutron leakage from the core and, consequently, an increase in reactivity. In some special cases, the purpose of chain reaction control is served by moving parts of neutron reflectors, which, when moved, change the leakage of neutrons from the core. Control, compensating and emergency rods, together with all the equipment that ensures their normal functioning, form the reactor control and protection system (CPS).

Emergency protection:

Emergency protection of a nuclear reactor is a set of devices designed to quickly stop a nuclear chain reaction in the reactor core.

Active emergency protection is automatically triggered when one of the parameters of a nuclear reactor reaches a value that could lead to an accident. Such parameters may include: temperature, pressure and coolant flow, level and speed of power increase.

The executive elements of emergency protection are, in most cases, rods with a substance that absorbs neutrons well (boron or cadmium). Sometimes, to shut down the reactor, a liquid absorber is injected into the coolant loop.

In addition to active protection, many modern designs also include elements of passive protection. For example, modern versions of VVER reactors include an “Emergency Core Cooling System” (ECCS) - special tanks with boric acid located above the reactor. In the event of a maximum design basis accident (rupture of the first cooling circuit of the reactor), the contents of these tanks end up inside the reactor core by gravity and the nuclear chain reaction is extinguished by a large amount of boron-containing substance, which absorbs neutrons well.

According to the “Nuclear Safety Rules for Reactor Facilities of Nuclear Power Plants”, at least one of the provided reactor shutdown systems must perform the function of emergency protection (EP). Emergency protection must have at least two independent groups of working elements. At the AZ signal, the AZ working parts must be activated from any working or intermediate positions.

The AZ equipment must consist of at least two independent sets.

Each set of AZ equipment must be designed in such a way that protection is provided in the range of changes in neutron flux density from 7% to 120% of the nominal:

1. By neutron flux density - no less than three independent channels;
2. According to the rate of increase in neutron flux density - no less than three independent channels.

Each set of emergency protection equipment must be designed in such a way that, over the entire range of changes in technological parameters established in the design of the reactor plant (RP), emergency protection is provided by at least three independent channels for each technological parameter for which protection is necessary.

Control commands of each set for AZ actuators must be transmitted through at least two channels. When one channel in one of the sets of AZ equipment is taken out of operation without taking this set out of operation, an alarm signal should be automatically generated for this channel.

Emergency protection must be triggered at least in the following cases:

1. Upon reaching the AZ setting for neutron flux density.
2. Upon reaching the AZ setting for the rate of increase in neutron flux density.
3. If the voltage disappears in any set of emergency protection equipment and the CPS power supply buses that have not been taken out of operation.
4. In case of failure of any two of the three protection channels for the neutron flux density or for the rate of increase of the neutron flux in any set of AZ equipment that has not been taken out of service.
5. When the AZ settings are reached by the technological parameters for which protection must be carried out.
6. When triggering the AZ from a key from a block control point (BCP) or a reserve control point (RCP).

Maybe someone can explain briefly in an even less scientific way how a nuclear power plant unit starts operating? :-)

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