Scheme of processes in a nuclear reactor. Everyone has heard but no one knows. How does a nuclear (atomic) reactor work. Classification of nuclear reactors by type of coolant

Today we will make a short journey into the world of nuclear physics. The theme of our excursion will be a 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 Nobel laureate 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 a positive charge, so the forces of Coulomb repulsion force them to scatter in different directions at a speed of about 1/30 of the speed of light. These fragments are carriers of colossal kinetic energy.

For the practical use of 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, a nuclear explosion will occur. 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, high pressure is maintained in the core (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 taking place in the reactor are accompanied by the release of the most dangerous type of 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 to high parameters, it is pumped 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 a two-circuit 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 in natural material is extremely low, 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, the time of exhaustion of the natural resources of this isotope is estimated at only 100-120 years.

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

Along with the obvious advantages of nuclear power, 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 permafrost areas at a depth of 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.

The accident at the 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 environmental 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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Device and principle of operation

Power release mechanism

The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energies. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. Spontaneous transition is always prevented by an energy barrier, to overcome which the microparticle must receive some amount of energy from the outside - the energy of excitation. The exoenergetic reaction consists in the fact that in the transformation following the 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 the colliding particles, or due to the binding energy of the acceding particle.

If we keep in mind the macroscopic scales of the energy release, then the kinetic energy necessary for the excitation of reactions must have all or at first at least some of the particles of the substance. This can only be achieved by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvins, while in the case of nuclear reactions it is at least 10 7 due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions has been carried out in practice only in the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by the joining particles does not require a large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the particles of attractive forces. But on the other hand, the particles themselves are necessary to excite the reactions. And if again we have in mind 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 arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Design

Any nuclear reactor consists of the following parts:

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

Physical principles of operation

iodine pit

Main article: Iodine pit

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

Classification

By appointment

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

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

Often reactors are used to solve two or more different tasks, in which case they are called multipurpose. For example, some power reactors, especially at the dawn of nuclear energy, were intended mainly for experiments. Fast neutron reactors can be both power-generating and producing isotopes at the same time. Industrial reactors, in addition to their main task, often generate electrical and thermal energy.

According to the neutron spectrum

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

By fuel placement

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

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

Blocks of nuclear fuel in a heterogeneous reactor are called fuel assemblies (FA), which are placed in the core at the nodes of a regular lattice, forming cells.

By type of fuel

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

According to the degree of enrichment:

natural uranium
low enriched uranium
highly enriched uranium

By chemical composition:

metal U
UC (uranium carbide), etc.

By type of coolant

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

By type of moderator

C (graphite, see Graphite-gas reactor, Graphite-water reactor)
H 2 O (water, see Light water reactor, Pressurized water reactor, VVER)
D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)
Metal hydrides
Without moderator (see fast neutron reactor)

By design

steam generation method

Reactor with an external steam generator (See PWR, VVER)

IAEA classification

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

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

Reactor materials

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

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

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

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

Burnup and reproduction of nuclear fuel

During the operation of a nuclear reactor, due to the accumulation of fission fragments in the fuel, its isotopic and chemical composition changes, and transuranium elements, mainly isotopes, are formed. The influence of fission fragments on the reactivity of a nuclear reactor is called poisoning(for radioactive fragments) and slagging(for stable isotopes).

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

To an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of the reactor after its shutdown or power reduction (“iodine pit”), which makes it impossible for short-term shutdowns and fluctuations in output power. This effect is overcome by introducing a reactivity margin in the regulatory bodies. The depth and duration of the iodine well depend on the neutron flux Ф: at Ф = 5 10 18 neutron/(cm² sec), the duration of the iodine well is ˜ 30 h, and the depth is 2 times greater than the steady-state change in Keff caused by 135 Xe poisoning.
Due to poisoning, spatio-temporal fluctuations of the neutron flux Ф, and, consequently, of the reactor power, can occur. These fluctuations occur at Ф > 10 18 neutrons/(cm² sec) and large reactor sizes. Oscillation periods ˜ 10 h.

Nuclear fission gives rise to a large number of stable fragments, which differ in their absorption cross sections compared to the absorption cross section of a fissile isotope. The concentration of fragments with a large absorption cross section reaches saturation during the first few days of reactor operation. These are mainly TVELs of different "ages".

In the case of complete fuel replacement, the reactor has excess reactivity, which must be compensated, while in the second case, compensation is required only at the first start of the reactor. Continuous refueling makes it possible to increase the burnup depth, since the reactivity of the reactor is determined by the average concentrations of fissile isotopes.

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

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

Nuclear reactor control

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

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

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

Residual heat

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

Decay heat is a consequence of the β- and γ-decay of fission products, which have accumulated in the fuel during the operation of the reactor. The nuclei of fission products, as a result of decay, pass into a more stable or completely stable state with the release of significant energy.

Although the residual heat release rate rapidly drops to values that are small compared to stationary values, in high-power power reactors it is significant in absolute terms. For this reason, decay heat release requires a long time to provide heat removal from the reactor core after it has been shut down. This task requires the presence of cooling systems with reliable power supply in the design of the reactor facility, and also necessitates long-term (within 3-4 years) storage of spent nuclear fuel in storage facilities with a special temperature regime - spent fuel pools, which are usually located in the immediate vicinity of the reactor.

Literature

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

Notes

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

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 at the Chernobyl nuclear power plant in the Soviet Union in 1986, nuclear power seemed doomed.

But in the early 2000s, nuclear reactors began to make a comeback, thanks to 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, another crisis hit - 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 high school physics, we will remind you how nuclear fission works. Atoms are like tiny solar systems, with a core like the sun and electrons like planets in orbit around it. The nucleus is made up of particles called protons and neutrons 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 share some common characteristics. They all have a stockpile of radioactive fuel pellets - usually uranium oxide - that 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 different reactor designs in use, but one of the most common is pressurized water power reactor (VVER). In such a reactor, water is forced into contact with the core and then remains there under such pressure that it cannot turn into steam. This water then 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 of 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 man-made sources, ranging from chest x-rays, smoke detectors and luminous clock 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 high level radioactive waste, 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 release huge, lethal doses of radiation from even 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 waste ends up in groundwater or rivers, it 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 power plants close to major 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 amount of radiation spread over an area of 323,749 square kilometers. The number of deaths caused by radiation is still unclear, but the World Health Organization says it may have caused 9,000 cancer deaths.

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 resorted to the last resort - flooding the reactors with a huge amount of sea water with the addition of boric acid, which was able to prevent a catastrophe, but destroyed the reactor equipment. Eventually, with the help of fire engines 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.

Nuclear reactor

A nuclear reactor is a device in which a controlled nuclear chain reaction is carried out, accompanied by the release of energy. The first nuclear reactor was built and launched in December 1942 in the USA under the leadership of E. Fermi. The first reactor built outside the United States was ZEEP, launched in Canada in September 1945. In Europe, the first nuclear reactor was the F-1 installation, which was launched on December 25, 1946 in Moscow under the leadership of I. V. Kurchatov.

By 1978, about a hundred nuclear reactors of various types were already operating in the world. The components of any nuclear reactor are: a core with nuclear fuel, usually surrounded by a neutron reflector, coolant, a chain reaction control system, radiation protection, a remote control system. The reactor vessel is subject to wear (especially under the action of ionizing radiation). The main characteristic of a nuclear reactor is its power. A power of 1 MW corresponds to a chain reaction in which 3·10 16 fission events occur in 1 sec.

Story

Nazi Germany's "Uranium Project" theoretical group, working in the Kaiser Wilhelm Society, was headed by Weizsäcker, but only formally. Heisenberg, who developed the theoretical foundations of the chain reaction, became the actual leader, while Weizsacker, with a group of participants, focused on creating the "uranium machine" - the first reactor. In the late spring of 1940, one of the scientists of the group - Harteck - conducted the first experiment with an attempt to create a chain reaction using uranium oxide and a solid graphite moderator. However, the available fissile material was not enough to achieve this goal. In 1941, at the University of Leipzig, Döpel, a member of the Heisenberg group, built a stand with a heavy water moderator, in experiments on which, by May 1942, it was possible to achieve the production of neutrons in excess of their absorption. A full-fledged chain reaction was achieved by German scientists in February 1945 in an experiment conducted in a mine working near Haigerloch. However, a few weeks later, Germany's nuclear program ceased to exist.

The nuclear fission chain reaction (short chain reaction) was first carried out in December 1942. A group of physicists at the University of Chicago, headed by E. Fermi, created the world's first nuclear reactor, called the Chicago Pile-1, CP-1. It consisted of graphite blocks, between which were located balls of natural uranium and its dioxide. Fast neutrons that appear after the fission of 235U nuclei were slowed down by graphite to thermal energies, and then caused new nuclear fissions. Reactors like SR-1, in which the main share of fissions occurs under the action of thermal neutrons, are called thermal neutron reactors. They contain a lot of moderator compared to nuclear fuel.

In the USSR, theoretical and experimental studies of the features of the start-up, operation and control of reactors were carried out by a group of physicists and engineers led by Academician I. V. Kurchatov. The first Soviet F-1 reactor was built at Laboratory No. 2 of the USSR Academy of Sciences (Moscow). This reactor was put into critical condition on December 25, 1946. The F-1 reactor was assembled from graphite blocks and had the shape of a ball with a diameter of about 7.5 m. In the central part of the ball with a diameter of 6 m, uranium rods were placed through holes in the graphite blocks. The F-1 reactor, like the CP-1 reactor, did not have a cooling system, so it operated at very low power levels (fractions of a watt, rarely a few watts). The results of research at the F-1 reactor became the basis for projects of more complex industrial reactors. In 1948, the I-1 reactor (according to other sources it was called A-1) was put into operation for the production of plutonium, and on June 27, 1954, the world's first nuclear power plant with an electric power of 5 MW was put into operation in the city of Obninsk.

Device and principle of operation

Power release mechanism The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energies. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. Spontaneous transition is always hindered by an energy barrier, to overcome which the microparticle must receive some amount of energy from the outside - the energy of excitation. The exoenergetic reaction consists in the fact that in the transformation following the 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 the colliding particles, or due to the binding energy of the acceding particle.

If we keep in mind the macroscopic scales of the energy release, then the kinetic energy necessary for the excitation of reactions must have all or at first at least some of the particles of the substance. This can only be achieved by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvins, while 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 has been carried out in practice only in the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Design

Any nuclear reactor consists of the following parts:

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

iodine pit

Iodine pit - the state of a nuclear reactor after it is turned off, characterized by the accumulation of the short-lived xenon isotope 135Xe. This process leads to the temporary appearance of significant negative reactivity, which, in turn, makes it impossible to bring the reactor to its design capacity for a certain period (about 1-2 days).