1. Nuclear power is a field of science and industrial technology in which methods and means of converting nuclear energy into thermal and electrical energy are developed and used in practice. The foundations of nuclear energy are nuclear power plants (NPPs). The source of energy at nuclear power plants is nuclear reactors, in which a controlled chain reaction of fission of nuclei of heavy elements occurs, mainly U-235 and Pu-239.
Nuclear reactors are of two types: slow neutron reactors and fast neutron reactors. Most nuclear power plants in the world are built on the basis of slow neutron reactors. The first reactors built in the USA (1942), the USSR (1946) and other developed countries were intended to produce weapons-grade plutonium Pu-239. The heat released in them was a by-product. This heat was removed from the reactor using a cooling system and simply released into the environment.
The mechanism of heat release in the reactor is as follows. The two fragments that arise during the fission of a uranium nucleus carry away enormous kinetic energy of about 200 MeV. Their initial speed reaches 5000 km/s. Moving among uranium, moderator or structural elements, these fragments, colliding with atoms, transfer their energy to them and gradually slow down to thermal speeds. The reactor core is heating up. By increasing the intensity of the nuclear reaction, it is possible to achieve greater thermal powers.
The heat generated in the reactor is removed using a liquid or gaseous coolant. In general, a coolant reactor resembles a steam tube boiler (water flows through pipes inside the furnace and heats up). Therefore, along with the concept of “nuclear reactor”, the synonym “nuclear boiler” is often used.
In Fig. Figure 144 shows a diagram of a nuclear power plant in reactor 1. The neutron flux density inside the operating reactor reaches 10 14 particles every 1 cm 2 per second.
A distinction is made between thermal and electrical power of the reactor. Electrical power is no more than 30% of thermal power. The world's first nuclear power plant was built in 1954 in the USSR in Obninsk. Its thermal power is 30 MW, electrical power is 5 MW. The active zone of a uranium-graphite slow neutron reactor has the shape of a cylinder with a diameter of 1.5 m and a height of 1.7 m. The coolant is water. Water temperature at the reactor inlet is + 190°C, at the outlet + 280°C, pressure 100 atm.
The reactor load is 550 kg of uranium enriched to 5%. Duration of operation at rated power is 100 days. The design burnup of U-235 is 15%. The reactor contains 128 fuel elements (fuel elements). The Obninsk NPP was built with the aim of developing technological solutions for nuclear energy. In later serial nuclear power plants, the load and power of reactors increases hundreds of times.
2. Slow neutron nuclear reactor. As already mentioned in §21, the main task in the development of nuclear reactors was that the reactor could operate on natural uranium, i.e. extracted chemically from ores and containing a natural mixture of isotopes: U-238 (99.282%), U-235 (0.712%), U-234 (0.006%), or on relatively cheap low-enriched uranium, in which the isotope content is U-235 or Pu-239 increased to 2-5%.
To do this, three conditions must be met: firstly, the mass of fissile material in the reactor (U-235 or Pu-239) must be no less than critical for its given configuration. This means that, on average, one neutron from the number produced in each nuclear fission event could cause the next fission event. Secondly, neutrons need to be slowed down to thermal speeds, and this must be done in such a way as to minimize their losses due to radiation capture by the nuclei of non-fissile materials. Third, develop principles and create means of controlling a nuclear chain reaction. Although all these conditions are interrelated, for each of them it is possible to identify the main ways of their implementation.
A. Achieving a critical mass of fissile material is possible in two ways: simply increasing the mass of uranium and enriching uranium. Due to the low concentration of fissile material, its critical mass in the reactor is much greater than in an atomic bomb. For example, in the Obninsk NPP /m cr U-235 is about 25 kg. In more modern high-power reactors, m cr reaches several tons. To reduce losses due to neutron leakage from the reactor, its core is surrounded by a neutron reflector. This is a substance with light nuclei that weakly absorbs neutrons (graphite, beryllium).
b. Neutron moderation. Figure 145 shows the energy spectrum of neutrons emitted by fissile nuclei of U-235. The abscissa axis shows the kinetic energy E of neutrons, and the ordinate axis shows the relative frequency ΔN/N of repetition of such energy in conventional units. The curve has a maximum at E = 0.645 MeV. The figure shows that the fission of U-235 nuclei produces predominantly fast neutrons with energy E > 1 MeV.
As mentioned earlier, the effective cross section for neutron capture by U-235 nuclei is maximum for thermal neutrons, when their energy E< 1 Мэв. Поэтому для наиболее эффективного использования нейтронов их надо замедлять до тепловых скоростей. Казалось бы, это можно сделать простым наращиванием массы естественного урана. В этом случае нейтроны, последовательно сталкиваясь с ядрами урана, должны постепенно уменьшать свою энергию и приходить к тепловому равновесию с массой урана. Но в естественном уране на 1 ядро U-235 приходиться 140 ядер U-238. Сечение радиационного захвата быстрых нейтронов ядрами U-238 невелико (σ=0,3 барна), и этот путь был бы возможен, если бы не резонансная область (см. рис.139), где σ возрастает в тысячи раз. Например, при энергии нейтронов E=7эВ σ достигает 5000 барн. Нейтроны этот диапазон энергий в уране не пройдут. Они почти все будут захвачены ядрами U-238
To prevent such absorption from occurring, neutrons must be removed from the uranium mass, slowed down in a moderator that weakly absorbs neutrons (graphite, heavy water, beryllium) and returned back to the uranium mass (diffuse). This is achieved by loading uranium into thin tubes of fuel elements (fuel rods) . And the fuel rods are immersed in the moderator channels.
Typically, fuel rods are thin-walled tubes with a diameter of 15-20 mm made of zirconium alloy. Nuclear fuel is placed inside the fuel rods in the form of tablets compressed from uranium oxide U0 2. The oxide does not sinter at high temperatures and is easily removed when recharging fuel rods. Depending on the size of the reactor core, the length of the fuel rods can reach 7-8 m. Several fuel rods are mounted in containers, which are pipes with a diameter of 10-20 cm or prisms. When reactors are recharged, these containers are replaced, and their disassembly and replacement of fuel rods is carried out at the plant.
The reactor itself is most often a cylinder, through the upper base of which vertical channels are made in a checkerboard pattern. Containers with fuel rods and absorber control rods are placed in these channels.
V. Nuclear chain reaction control carried out using rods made of materials that strongly absorb neutrons - cadmium 48 113 Cd and boron 5 10 V. The latter is often in the form of carbide B 4 C (melting point for cadmium 321 ° C, for boron 2075 ° C). Their absorption cross sections are σ = 20,000 and 4,000 barn, respectively. The parameters of the absorber rods are calculated so that when the rods are fully inserted, a nuclear reaction certainly does not occur in the reactor. With the gradual removal of the rods, the multiplication factor K in the core increases and at a certain position of the rod reaches unity. At this moment the reactor begins to operate. During operation, the K coefficient gradually decreases due to contamination of the reactor with fission fragments. This decrease in K is compensated by the extension of the rods. In case of a sudden increase in the intensity of the reaction, there are additional rods. Their rapid release into the core immediately stops the reaction.
Reactor control is made easier by the presence of delayed neutrons. Their share for different isotopes ranges from 0.6 to 0.8%; for U-235 it is approximately 0.64%. The average half-life of fission fragments producing delayed neutrons is T = 9 s, the average lifetime of one generation of delayed neutrons is τ = T/ln2 = 13 s.
During stationary operation of the reactor, the multiplication factor of fast neutrons is K b = 1. The total coefficient K = K b + K differs from unity by the fraction of delayed neutrons and can reach K = 1 + 0.006. In the second generation, after 13 seconds, the number of neutrons is N = N 0 K 2 = N 0 (1.006)2 = 1.012MN 0. In the tenth generation, after 130 s, their number will be N 0 K 10 = 1.062 MN 0, which is still far from an emergency situation. Therefore, the automatic control system, based on monitoring the neutron flux density in the core, is quite capable of monitoring the slightest nuances in the operation of the reactor and responding to them by moving the control rods.
3. Reactor poisoning- this is the accumulation of radioactive products in it. The accumulation of stable products in it is called slagging of the reactor. In both cases, nuclei accumulate, intensively absorbing neutrons. The capture cross section of the most powerful xenon-135 poisoner reaches 2.6 * 10 6 barn.
The mechanism of Xe-135 formation is as follows. When U-235 or Pu-239 is fissioned by slow neutrons, with a probability of 6%, a fragment is obtained - a tellurium nucleus of 52,135 Te. With a period of 0.5 minutes, Te-135 undergoes β - decay, turning into the nucleus of the iodine isotope I. This isotope is also β - active with a period of 6.7 hours. The decay product of I-135 is the xenon isotope 54 135 Xe. With a period of T = 9.2 hours, Xe-135 undergoes β - decay, turning into a practically stable cesium isotope 55 135 Cz. (/T= 3*10 6 years).
Other decay patterns produce other harmful nuclei, such as samarium 62,139 Sm. Poisoning occurs especially quickly during the initial period of reactor operation. Over time, radioactive equilibrium is established between the decay products. From this moment, the slagging of the reactor begins to increase.
A reactor in which the fissile material (uranium), moderator (graphite) and absorber (cadmium) are separate phases and have interfaces is called heterogeneous. If all these elements in a liquid or gaseous state represent one common phase, the reactor is called homogeneous. For energy chains, exclusively heterogeneous reactors are built.
5. Fast neutron reactors. The nuclei of U-235, Pu-239 and U-233 are fissioned by all neutrons. Therefore, if you increase the enrichment of uranium, for example, with the U-235 isotope, then due to the increase in the concentration of fissile nuclei, an increasingly larger proportion of neutrons will fission the U-235 nuclei without leaving the uranium mass. At a certain concentration of fissile nuclei and with a sufficient mass of uranium in the core, the neutron multiplication factor reaches unity even without moderating them. The reactor will operate on fast neutrons (abbreviated as fast reaction).
The advantage of a fast reaction over a slow reaction (that is, over a reaction with slow neutrons) is that neutrons are used more efficiently. As a result, the reproduction of nuclear fuel increases. In a slow reaction of 2.5 neutrons, 1 also goes to the U-235 nucleus, maintaining the reaction, approximately 1 goes to the U-238 nucleus, then forming Pu-239 (nuclear fuel), and 0.5 neutrons are lost. One core of “burnt” U-235 produces approximately 1 core of Pu-239. In a fast reaction, out of 2.5 neutrons, 1 is also used to maintain the reaction. But less than 0.5 neutrons are lost. Therefore, more neutrons enter the U-238 nuclei. As a result, more than 1 Pu-239 nucleus is formed per one core of “burnt” U-235. Expanded reproduction of nuclear fuel is taking place. The creation and operation of fast neutron reactors is more difficult than slow neutron reactors. Firstly, the volume of the active zone decreases sharply. This increases the energy density, which leads to an increase in temperature and tightens the requirements for structural materials and coolant. Secondly, the requirements for the reactor control system are increasing, that is, for the speed of operations performed by the control system.
6. Prospects for nuclear energy. Today, normally operating nuclear power plants are the cleanest of all energy sources. They do not emit C0 2 and S0 2, like thermal plants, and therefore do not aggravate the greenhouse effect and do not flood arable land with water, like hydroelectric power plants. Taking into account the possibility of processing U-238 into Pu-239 and Th-232 into U-233, the reserves of readily available nuclear fuel will last for hundreds of years. The use of nuclear power plants will save oil, gas and coal for the chemical industry. There are two difficulties with expanding the nuclear power plant fleet. One is objective, its essence is that the problems associated with the disposal and disposal of waste nuclear fuel and structural elements that have spent their reactor life have not been fully resolved.
The second difficulty is subjective. Compared to thermal and hydropower plants, servicing nuclear power plants requires a higher technical culture and imposes enormous responsibility on a person. The slightest deviation from technological discipline can result in tragedy for thousands of people.
7. Fusion. From the distribution curve of the specific binding energy it follows that the fusion of light nuclei into one nucleus, like the fission of heavy nuclei, must be accompanied by the release of a huge amount of energy. All nuclei carry the same positive charge. To bring them closer to the distance at which fusion begins, two interacting nuclei need to be accelerated towards each other. This can be done in two ways. Firstly, with the help of accelerators. This path is cumbersome and ineffective. Secondly, simply heating the gas to the required temperature. Therefore, fusion reactions of light nuclei initiated by heating a gas are called thermonuclear reactions. Let us estimate the temperature of deuterium gas at which thermonuclear fusion of deuterium + deuterium begins. 1 2 H+ 1 2 H→ 2 3 He + 0 1 n + 3.27 MeV.
To merge nuclei, they need to be brought together at a distance of r = 2*10 -15 m. The potential energy during such a rapprochement should be equal to the kinetic energy of both nuclei in the system
center of mass (1/4πε 0)*(e 2 /r) = 2*(mυ 2 /2) = 2*(3/2)* kT. Gas temperature T=(1/3K)*(1/4πε 0)*(e 2 /r)=3*10 9 K. The energy distribution of particles is close to Maxwellian. Therefore, there are always “hotter” particles, and also due to the tunnel effect, the fusion reaction begins at lower temperatures T ≈ 10 7 K.
In addition to the reaction, two more are of particular interest: deuterium + deuterium and deuterium + tritium. 2 1 H + 1 2 H+ 1 2 p + 4.03 MeV. (22.3) and 1 2 H + 1 3 H → 2 4 He + 0 1 n +17.59 MeV. (22.4)
The latter reaction releases approximately 5 times more energy per unit mass than the fission of U-235. This energy is the kinetic energy of the movement of neutrons and the resulting helium nuclei. Under terrestrial conditions, it was possible to realize a nuclear fusion reaction in the form of an uncontrolled explosion of a thermonuclear hydrogen bomb.
8. Hydrogen bomb is a conventional atomic bomb, the nuclear charge of which (U-235 or Pu-239) is surrounded by a blanket of a substance containing light atoms. For example, lithium deuteride LiD. The high temperature that occurs when an atomic charge is detonated initiates thermonuclear fusion of light atoms. This releases additional energy, increasing the power of the bomb. In addition to reactions (22.1) and (22.3), another one can occur in a bomb with a lithium deuteride blanket. 3 6 Li+ 1 1 p→ 2 4 He + 2 3 He + 4 MeV. (22.5). (22.4). But tritium is β - an active element. With a period of 12 years it turns into He-3. Therefore, hydrogen charges with tritium have a limited shelf life and must be tested regularly. The substances involved in thermonuclear fusion do not produce radioactive products. But thanks to the intense neutron flux, radioactivity is induced in the nuclei of structural materials and surrounding bodies. Therefore, it is impossible to implement a “clean” fusion reaction without radioactive waste.
9. The problem of controlled thermonuclear fusion (U HS) has not yet been resolved. Its solution is very promising for the energy sector. The water of the seas and oceans contains approximately 0.015% deuterium (by the number of atoms). There is about 10-20 kg of water on earth. If you extract deuterium from this water, then the energy that can be obtained from it is equivalent to 6 * 10 18 K)" tons of coal, this is a gigantic amount (about 0.001 Earth masses). Therefore, deuterium in the seas and oceans is a practically inexhaustible source of energy.
The problem of CTS comes down to two tasks. Firstly, you need to learn how to create a high temperature T>10 7 K in a limited volume. Secondly, maintain the volume of plasma dressed to this temperature for a time sufficient for the nuclear fusion reaction to occur. Both of these problems are far from being solved.
10. Thermonuclear reactions in stars. According to modern concepts, a star is born from extended gas and dust clouds, consisting mainly of hydrogen. As a result of gravitational compression, the cloud becomes denser and begins to undress, turning into a protostar. When the temperature in the center of a protostar reaches 10 7 K, thermonuclear reactions of synthesis of light elements, mainly hydrogen, are excited in it. Gravitational compression is suspended by increased gas-kinetic and optical pressure. A protostar turns into a star. There are two possible cycles of converting hydrogen into helium. The main reactions that make up each cycle are listed below. In parentheses next to the reaction equations, the average reaction time τ is indicated, calculated using the effective reaction cross section for the pressures and temperatures that exist inside the star.
The fusion reaction is as follows: two or more atomic nuclei are taken and, using a certain force, brought together so close that the forces acting at such distances prevail over the forces of Coulomb repulsion between equally charged nuclei, resulting in the formation of a new nucleus. It will have a slightly smaller mass than the sum of the masses of the original nuclei, and the difference becomes the energy that is released during the reaction. The amount of energy released is described by the well-known formula E=mc². Lighter atomic nuclei are easier to bring together to the desired distance, so hydrogen - the most abundant element in the Universe - is the best fuel for the fusion reaction.
It has been found that a mixture of two isotopes of hydrogen, deuterium and tritium, requires the least amount of energy for the fusion reaction compared to the energy released during the reaction. However, although deuterium-tritium (D-T) is the subject of most fusion research, it is by no means the only potential fuel. Other mixtures may be easier to produce; their reaction can be more reliably controlled, or, more importantly, produce fewer neutrons. Of particular interest are the so-called “Neutronless” reactions, since the successful industrial use of such fuel will mean the absence of long-term radioactive contamination of the materials and reactor design, which, in turn, could have a positive impact on public opinion and the overall cost of operating the reactor, significantly reducing the costs of its decommissioning. The problem remains that synthesis reactions using alternative fuels are much more difficult to maintain, so the D-T reaction is considered only a necessary first step.
Scheme of the deuterium-tritium reaction
Controlled fusion can use different types of fusion reactions depending on the type of fuel used.
Deuterium + tritium reaction (D-T fuel)
The most easily feasible reaction is deuterium + tritium:
2 H + 3 H = 4 He + n at an energy output of 17.6 MeV (megaelectronvolt)
This reaction is the most easily feasible from the point of view of modern technologies, provides a significant energy yield, and fuel components are cheap. Its disadvantage is the release of unwanted neutron radiation.
Two nuclei: deuterium and tritium fuse to form a helium nucleus (alpha particle) and a high-energy neutron.
²H + ³He = 4 He + . with an energy output of 18.4 MeV
The conditions for achieving it are much more complicated. Helium-3 is also a rare and extremely expensive isotope. It is not currently produced on an industrial scale. However, it can be obtained from tritium, which is produced in turn at nuclear power plants.
The complexity of carrying out a thermonuclear reaction can be characterized by the triple product nTt (density by temperature by confinement time). By this parameter, the D-3He reaction is approximately 100 times more complex than the D-T reaction.
Reaction between deuterium nuclei (D-D, monopropellant)
Reactions between deuterium nuclei are also possible, they are a little more difficult than reactions involving helium-3:
As a result, in addition to the main reaction in DD plasma, the following also occurs:
These reactions proceed slowly in parallel with the deuterium + helium-3 reaction, and the tritium and helium-3 formed during them are likely to immediately react with deuterium.
Other types of reactions
Some other types of reactions are also possible. The choice of fuel depends on many factors - its availability and low cost, energy output, ease of achieving the conditions required for the thermonuclear fusion reaction (primarily temperature), the necessary design characteristics of the reactor, etc.
"Neutronless" reactions
The most promising are the so-called. “neutron-free” reactions, since the neutron flux generated by thermonuclear fusion (for example, in the deuterium-tritium reaction) carries away a significant part of the power and generates induced radioactivity in the reactor design. The deuterium-helium-3 reaction is promising due to the lack of neutron yield.
Conditions
Nuclear reaction of lithium-6 with deuterium 6 Li(d,α)α
TCB is possible if two criteria are met simultaneously:
- Plasma temperature:
- Compliance with Lawson's criterion:
where is the density of high-temperature plasma, is the plasma retention time in the system.
It is on the value of these two criteria that the rate of occurrence of a particular thermonuclear reaction mainly depends.
At present, controlled thermonuclear fusion has not yet been implemented on an industrial scale. Construction of the international research reactor ITER is in its early stages.
Fusion energy and helium-3
Helium-3 reserves on Earth range from 500 kg to 1 ton, but on the Moon it is found in significant quantities: up to 10 million tons (according to minimum estimates - 500 thousand tons). Currently, a controlled thermonuclear reaction is carried out by the synthesis of deuterium ²H and tritium ³H with the release of helium-4 4 He and the “fast” neutron n:
However, the majority (more than 80%) of the released kinetic energy comes from the neutron. As a result of collisions of fragments with other atoms, this energy is converted into thermal energy. In addition, fast neutrons create significant amounts of radioactive waste. In contrast, the synthesis of deuterium and helium-3³He does not produce (almost) radioactive products:
Where p is proton
This allows the use of simpler and more efficient systems for converting the kinetic synthesis reaction, such as a magnetohydrodynamic generator.
Reactor designs
Two basic schemes for implementing controlled thermonuclear fusion are considered.
Research on the first type of thermonuclear reactor is significantly more developed than on the second. In nuclear physics, when studying thermonuclear fusion, a magnetic trap is used to contain plasma in a certain volume. The magnetic trap is designed to keep the plasma from contact with the elements of the thermonuclear reactor, i.e. used primarily as a heat insulator. The principle of confinement is based on the interaction of charged particles with a magnetic field, namely on the rotation of charged particles around magnetic field lines. Unfortunately, magnetized plasma is very unstable and tends to leave the magnetic field. Therefore, to create an effective magnetic trap, the most powerful electromagnets are used, consuming a huge amount of energy.
It is possible to reduce the size of a fusion reactor if it uses three methods of creating a fusion reaction simultaneously.
A. Inertial synthesis. Irradiate tiny capsules of deuterium-tritium fuel with a 500 trillion-watt laser:5. 10^14 W. This gigantic, very brief 10^-8 sec laser pulse causes the fuel capsules to explode, resulting in the birth of a mini-star for a split second. But a thermonuclear reaction cannot be achieved on it.
B. Simultaneously use the Z-machine with the Tokamak.
The Z-Machine operates differently than a laser. It passes through a web of tiny wires surrounding the fuel capsule a charge with a power of half a trillion watts 5.10^11 watts.
Next, approximately the same thing happens as with the laser: as a result of the Z-impact, a star is formed. During tests on the Z-Machine, it was already possible to launch a fusion reaction. http://www.sandia.gov/media/z290.htm Cover the capsules with silver and connect them with a silver or graphite thread. The ignition process looks like this: Shoot a filament (attached to a group of silver balls containing a mixture of deuterium and tritium) into a vacuum chamber. During a breakdown (discharge), form a lightning channel through them and supply current through the plasma. Simultaneously irradiate the capsules and plasma with laser radiation. And at the same time or earlier turn on the Tokamak. use three plasma heating processes simultaneously. That is, place the Z-machine and laser heating together inside the Tokamak. It may be possible to create an oscillatory circuit from Tokamak coils and organize resonance. Then it would work in an economical oscillatory mode.
Fuel cycle
First generation reactors will most likely run on a mixture of deuterium and tritium. Neutrons that appear during the reaction will be absorbed by the reactor protection, and the generated heat will be used to heat the coolant in the heat exchanger, and this energy, in turn, will be used to rotate the generator.
. .The reaction with Li6 is exothermic, providing little energy for the reactor. The reaction with Li7 is endothermic - but does not consume neutrons. At least some reactions of Li7 are necessary to replace neutrons lost in reactions with other elements. Most reactor designs use natural mixtures of lithium isotopes.
This fuel has a number of disadvantages:
The reaction produces a significant number of neutrons, which activate (radioactively contaminate) the reactor and heat exchanger. Measures are also required to protect against a possible source of radioactive tritium.
Only about 20% of fusion energy is in the form of charged particles (the rest are neutrons), which limits the ability to directly convert fusion energy into electricity. The use of the D-T reaction depends on the available lithium reserves, which are significantly less than the deuterium reserves. Neutron exposure during the D-T reaction is so significant that after the first series of tests at JET, the largest reactor to date using this fuel, the reactor became so radioactive that a robotic remote maintenance system had to be added to complete the annual test cycle.
There are, in theory, alternative types of fuel that do not have these disadvantages. But their use is hampered by a fundamental physical limitation. To obtain sufficient energy from the fusion reaction, it is necessary to maintain a sufficiently dense plasma at the fusion temperature (10 8 K) for a certain time. This fundamental aspect of fusion is described by the product of the plasma density, n, and the heated plasma holding time, τ, required to reach the equilibrium point. The product, nτ, depends on the type of fuel and is a function of the plasma temperature. Of all types of fuel, the deuterium-tritium mixture requires the lowest nτ value by at least an order of magnitude, and the lowest reaction temperature by at least 5 times. Thus, the D-T reaction is a necessary first step, but the use of other fuels remains an important research goal.
Fusion reaction as an industrial source of electricity
Fusion energy is considered by many researchers as a "natural" energy source in the long term. Proponents of the commercial use of fusion reactors for electricity production cite the following arguments in their favor:
- Virtually inexhaustible fuel reserves (hydrogen)
- Fuel can be extracted from sea water on any coast of the world, which makes it impossible for one or a group of countries to monopolize fuel
- Impossibility of an uncontrolled fusion reaction
- No combustion products
- There is no need to use materials that can be used to produce nuclear weapons, thus eliminating cases of sabotage and terrorism
- Compared to nuclear reactors, negligible amounts of radioactive waste are produced with a short half-life.
- A thimble filled with deuterium is estimated to produce energy equivalent to 20 tons of coal. A medium-sized lake can provide any country with energy for hundreds of years. However, it should be noted that existing research reactors are designed to achieve a direct deuterium-tritium (DT) reaction, the fuel cycle of which requires the use of lithium to produce tritium, while claims of inexhaustible energy refer to the use of deuterium-deuterium (DD) reaction in the second generation of reactors.
- Just like the fission reaction, the fusion reaction does not produce atmospheric carbon dioxide emissions, which is a major contributor to global warming. This is a significant advantage, since the use of fossil fuels to produce electricity results in, for example, the US producing 29 kg of CO 2 (one of the main gases that can be considered a cause of global warming) per US resident per day.
Cost of electricity compared to traditional sources
Critics point out that the economic feasibility of using nuclear fusion to produce electricity remains an open question. The same study commissioned by the British Parliament's Office of Science and Technology Records indicates that the cost of generating electricity using a fusion reactor would likely be at the higher end of the cost spectrum of conventional energy sources. Much will depend on future technology, market structure and regulation. The cost of electricity directly depends on the efficiency of use, the duration of operation and the cost of reactor decommissioning. Critics of the commercial use of nuclear fusion energy deny that hydrocarbon fuels are heavily subsidized by the government, both directly and indirectly, such as through the use of the military to ensure an uninterrupted supply; the Iraq War is often cited as a controversial example of this type of subsidization. Accounting for such indirect subsidies is very complex and makes accurate cost comparisons nearly impossible.
A separate issue is the cost of research. The countries of the European Community spend about €200 million annually on research, and it is predicted that it will take several more decades before the industrial use of nuclear fusion will be possible. Proponents of alternative sources of electricity believe that it would be more appropriate to use these funds to introduce renewable sources of electricity.
Availability of commercial fusion energy
Unfortunately, despite widespread optimism (since the 1950s, when the first research began), significant obstacles between today's understanding of nuclear fusion processes, technological capabilities and the practical use of nuclear fusion have not yet been overcome, it is unclear even to what extent there may be It is economically profitable to produce electricity using thermonuclear fusion. Although progress in research is constant, researchers are faced with new challenges every now and then. For example, the challenge is developing a material that can withstand neutron bombardment, which is estimated to be 100 times more intense than traditional nuclear reactors.
The following stages are distinguished in research:
1.Equilibrium or “pass” mode(Break-even): when the total energy released during the synthesis process is equal to the total energy spent on starting and maintaining the reaction. This relationship is marked with the symbol Q. The reaction equilibrium was demonstrated at JET (Joint European Torus) in the UK in 1997. (Having spent 52 MW of electricity to heat it up, the scientists obtained a power output that was 0.2 MW higher than what was expended.)
2.Blazing Plasma(Burning Plasma): An intermediate stage in which the reaction will be supported primarily by alpha particles that are produced during the reaction, rather than by external heating. Q ≈ 5. Still not achieved.
3. Ignition(Ignition): a stable reaction that maintains itself. Should be achieved at large values of Q. Still not achieved.
The next step in research should be ITER (International Thermonuclear Experimental Reactor), the International Thermonuclear Experimental Reactor. At this reactor it is planned to study the behavior of high-temperature plasma (flaming plasma with Q ~ 30) and structural materials for an industrial reactor. The final phase of the research will be DEMO: a prototype industrial reactor in which ignition will be achieved and the practical suitability of the new materials will be demonstrated. The most optimistic forecast for the completion of the DEMO phase: 30 years. Considering the estimated time for construction and commissioning of an industrial reactor, we are ~40 years away from the industrial use of thermonuclear energy.
Existing tokamaks
In total, about 300 tokamaks were built in the world. The largest of them are listed below.
- USSR and Russia
- T-3 is the first functional device.
- T-4 - enlarged version of T-3
- T-7 is a unique installation in which, for the first time in the world, a relatively large magnetic system with a superconducting solenoid based on tin niobate cooled by liquid helium is implemented. The main task of T-7 was completed: the prospect for the next generation of superconducting solenoids for thermonuclear power was prepared.
- T-10 and PLT are the next step in world thermonuclear research, they are almost the same size, equal power, with the same confinement factor. And the results obtained are identical: both reactors achieved the desired temperature of thermonuclear fusion, and the lag according to the Lawson criterion is only two hundred times.
- T-15 is a reactor of today with a superconducting solenoid giving a field strength of 3.6 Tesla.
- Libya
- TM-4A
- Europe and UK
- JET (English) (Joint Europeus Tor) is the world's largest tokamak, created by the Euratom organization in the UK. It uses combined heating: 20 MW - neutral injection, 32 MW - ion cyclotron resonance. As a result, the Lawson criterion is only 4-5 times lower than the ignition level.
- Tore Supra (French) (English) - a tokamak with superconducting coils, one of the largest in the world. Located at the Cadarache research center (France).
- USA
- TFTR (English) (Test Fusion Tokamak Reactor) - the largest tokamak in the USA (at Princeton University) with additional heating by fast neutral particles. A high result has been achieved: the Lawson criterion at a true thermonuclear temperature is only 5.5 times lower than the ignition threshold. Closed 1997
- NSTX (English) (National Spherical Torus Experiment) is a spherical tokamak (spheromak) currently operating at Princeton University. The first plasma in the reactor was produced in 1999, two years after TFTR was closed.
You already know that in the middle of the 20th century. the problem arose of finding new sources of energy. In this regard, thermonuclear reactions attracted the attention of scientists.
- Thermonuclear reaction is the fusion reaction of light nuclei (such as hydrogen, helium, etc.), occurring at temperatures from tens to hundreds of millions of degrees.
Creating a high temperature is necessary to give the nuclei a sufficiently large kinetic energy - only under this condition will the nuclei be able to overcome the forces of electrical repulsion and get close enough to fall into the zone of action of nuclear forces. At such small distances, the forces of nuclear attraction significantly exceed the forces of electrical repulsion, due to which synthesis (i.e., fusion, association) of nuclei is possible.
In § 58, using the example of uranium, it was shown that energy can be released during the fission of heavy nuclei. In the case of light nuclei, energy can be released during the reverse process - during their fusion. Moreover, the reaction of fusion of light nuclei is energetically more favorable than the reaction of fission of heavy nuclei (if we compare the released energy per nucleon).
An example of a thermonuclear reaction is the fusion of hydrogen isotopes (deuterium and tritium), resulting in the formation of helium and the emission of a neutron:
This is the first thermonuclear reaction that scientists have managed to carry out. It was implemented in a thermonuclear bomb and was of an uncontrollable (explosive) nature.
As already noted, thermonuclear reactions can occur with the release of large amounts of energy. But in order for this energy to be used for peaceful purposes, it is necessary to learn how to conduct controlled thermonuclear reactions. One of the main difficulties in carrying out such reactions is to contain high-temperature plasma (almost completely ionized gas) inside the installation, in which nuclear fusion occurs. The plasma should not come into contact with the walls of the installation in which it is located, otherwise the walls will turn into steam. Currently, very strong magnetic fields are used to confine plasma in a confined space at an appropriate distance from the walls.
Thermonuclear reactions play an important role in the evolution of the Universe, in particular in the transformation of chemical substances in it.
Thanks to thermonuclear reactions occurring in the depths of the Sun, energy is released that gives life to the inhabitants of the Earth.
Our Sun has been radiating light and heat into space for almost 4.6 billion years. Naturally, at all times, scientists have been interested in the question of what is the “fuel” due to which the Sun produces huge amounts of energy for such a long time.
There were different hypotheses on this matter. One of them was that energy in the Sun is released as a result of a chemical combustion reaction. But in this case, as calculations show, the Sun could exist for only a few thousand years, which contradicts reality.
The original hypothesis was put forward in the middle of the 19th century. It was that the increase in internal energy and the corresponding increase in the temperature of the Sun occurs due to a decrease in its potential energy during gravitational compression. It also turned out to be untenable, since in this case the lifespan of the Sun increases to millions of years, but not to billions.
The assumption that the release of energy in the Sun occurs as a result of thermonuclear reactions occurring on it was made in 1939 by the American physicist Hans Bethe.
They also proposed the so-called hydrogen cycle, i.e. a chain of three thermonuclear reactions leading to the formation of helium from hydrogen:
where is a particle called a “neutrino”, which means “little neutron” in Italian.
To produce the two nuclei needed for the third reaction, the first two must occur twice.
You already know that, in accordance with the formula E = mс 2, as the internal energy of a body decreases, its mass also decreases.
To imagine the colossal amount of energy the Sun loses as a result of the conversion of hydrogen into helium, it is enough to know that the mass of the Sun decreases by several million tons every second. But, despite the losses, the hydrogen reserves on the Sun should last for another 5-6 billion years.
The same reactions occur in the interiors of other stars, the mass and age of which are comparable to the mass and age of the Sun.
Questions
- What reaction is called thermonuclear? Give an example of a reaction.
- Why are thermonuclear reactions only possible at very high temperatures?
- Which reaction is energetically more favorable (per nucleon): the fusion of light nuclei or the fission of heavy ones?
- What is one of the main difficulties in carrying out thermonuclear reactions?
- What is the role of thermonuclear reactions in the existence of life on Earth?
- What is the source of solar energy according to modern ideas?
- How long should the supply of hydrogen on the Sun last, according to scientists’ calculations?
This is interesting...
Elementary particles. Antiparticles
The particles that make up the atoms of various substances - electron, proton and neutron - are called elementary. The word "elementary" implied that these particles are primary, simplest, further indivisible and unchangeable. But it soon turned out that these particles are not immutable at all. They all have the ability to transform into each other when interacting.
Therefore, in modern physics, the term “elementary particles” is usually used not in its exact meaning, but to name a large group of smallest particles of matter that are not atoms or atomic nuclei (the exception is the proton, which is the nucleus of a hydrogen atom and at the same time belongs to the elementary particles).
Currently, more than 350 different elementary particles are known. These particles are very diverse in their properties. They may differ from each other in mass, sign and magnitude of the electric charge, lifetime (i.e., the time from the moment the particle is formed until the moment it is transformed into some other particle), penetrating ability (i.e., the ability to pass through matter ) and other characteristics. For example, most particles are “short-lived” - they live no more than two millionths of a second, while the average lifetime of a neutron outside the atomic nucleus is 15 minutes.
The most important discovery in the field of elementary particle research was made in 1932, when the American physicist Carl David Anderson discovered a trace of an unknown particle in a cloud chamber placed in a magnetic field. Based on the nature of this trace (radius of curvature, direction of bending, etc.), scientists determined that it was left by a particle, which is like an electron with a positive electric charge. This particle was called a positron.
It is interesting that a year before the experimental discovery of the positron, its existence was theoretically predicted by the English physicist Paul Dirac (the existence of just such a particle followed from the equation he derived). Moreover, Dirac predicted the so-called processes of annihilation (disappearance) and the birth of an electron-positron pair. Annihilation is that an electron and a positron disappear upon meeting, turning into γ-quanta (photons). And when a γ-quantum collides with any massive nucleus, an electron-positron pair is born.
Both of these processes were first observed experimentally in 1933. Figure 166 shows the tracks of an electron and a positron formed as a result of the collision of a γ-quantum with a lead atom during the passage of γ-rays through a lead plate. The experiment was carried out in a cloud chamber placed in a magnetic field. The same curvature of the tracks indicates the same mass of particles, and curvature in different directions indicates opposite signs of the electric charge.
Rice. 166. Tracks of an electron-positron pair in a magnetic field
In 1955, another antiparticle was discovered - the antiproton (the existence of which also followed from Dirac's theory), and a little later - the antineutron. An antineutron, like a neutron, has no electrical charge, but it undoubtedly belongs to antiparticles, since it participates in the process of annihilation and the birth of a neutron-antineutron pair.
The possibility of obtaining antiparticles led scientists to the idea of creating antimatter. Antimatter atoms should be built in this way: in the center of the atom there is a negatively charged nucleus, consisting of antiprotons and antineutrons, and positrons revolve around the nucleus. In general, the atom is neutral. This idea also received brilliant experimental confirmation. In 1969, at the proton accelerator in Serpukhov, Soviet physicists obtained nuclei of antihelium atoms.
At present, antiparticles of almost all known elementary particles have been experimentally discovered.
Chapter summary. The most important
Below are physical concepts and phenomena. The sequence of presentation of definitions and formulations does not correspond to the sequence of concepts, etc.
Transfer the names of the concepts into your notebook and enter the serial number of the definition (wording) corresponding to this concept in square brackets.
- Radioactivity;
- nuclear (planetary) model of the structure of the atom;
- atomic nucleus;
- radioactive transformations of atomic nuclei;
- experimental methods for studying particles in atomic and nuclear physics;
- nuclear forces;
- nuclear binding energy;
- mass defect of the atomic nucleus;
- chain reaction ;
- nuclear reactor ;
- environmental and social problems arising from the use of nuclear power plants;
- absorbed dose of radiation.
- Registration of particles using a Geiger counter, studying and photographing particle tracks (including those involved in nuclear reactions) in a cloud chamber and a bubble chamber.
- The forces of attraction acting between nucleons in the nuclei of atoms and significantly exceeding the forces of electrostatic repulsion between protons.
- The minimum energy required to split a nucleus into individual nucleons.
- Spontaneous emission of radioactive rays by atoms of certain elements.
- A device designed to carry out a controlled nuclear reaction.
- Consists of nucleons (i.e. protons and neutrons).
- Radioactive waste, the possibility of accidents, promotion of the proliferation of nuclear weapons.
- An atom consists of a positively charged nucleus located at its center, around which electrons orbit at a distance significantly greater than the size of the nucleus.
- The transformation of one chemical element into another through α- or β-decay, as a result of which the nucleus of the original atom undergoes changes.
- The difference between the sum of the masses of the nucleons forming a nucleus and the mass of this nucleus.
- A self-sustaining fission reaction of heavy nuclei, in which neutrons are continuously produced, dividing more and more new nuclei.
- The energy of ionizing radiation absorbed by the emitted substance (in particular, body tissues) and calculated per unit mass.
check yourself
The nuclear reactor works smoothly and efficiently. Otherwise, as you know, there will be trouble. But what's going on inside? Let's try to formulate the principle of operation of a nuclear (nuclear) reactor briefly, clearly, with stops.
In essence, the same process is happening there as during a nuclear explosion. Only the explosion happens very quickly, but in the reactor all this stretches out for a long time. As a result, everything remains safe and sound, and we receive energy. Not so much that everything around would be destroyed at once, but quite sufficient to provide electricity to the city.
Before you understand how a controlled nuclear reaction occurs, you need to know what it is. nuclear reaction at all.
Nuclear reaction is the process of transformation (fission) of atomic nuclei when they interact with elementary particles and gamma quanta.
Nuclear reactions can occur with both absorption and release of energy. The reactor uses the second reactions.
Nuclear reactor is a device whose purpose is to maintain a controlled nuclear reaction with the release of energy.
Often a nuclear reactor is also called an atomic reactor. Let us note that there is no fundamental difference here, but from the point of view of science it is more correct to use the word “nuclear”. There are now many types of nuclear reactors. These are huge industrial reactors designed to generate energy in power plants, nuclear reactors of submarines, small experimental reactors used in scientific experiments. There are even reactors used to desalinate seawater.
The history of the creation of a nuclear reactor
The first nuclear reactor was launched in the not-so-distant 1942. This happened in the USA under the leadership of Fermi. This reactor was called the "Chicago Woodpile".
In 1946, the first Soviet reactor, launched under the leadership of Kurchatov, began operating. The body of this reactor was a ball of seven meters in diameter. The first reactors did not have a cooling system, and their power was minimal. By the way, the Soviet reactor had an average power of 20 Watts, and the American one - only 1 Watt. For comparison: the average power of modern power reactors is 5 Gigawatts. Less than ten years after the launch of the first reactor, the world's first industrial nuclear power plant was opened in the city of Obninsk.
The principle of operation of a nuclear (nuclear) reactor
Any nuclear reactor has several parts: core With fuel And moderator , neutron reflector , coolant , control and protection system . Isotopes are most often used as fuel in reactors. uranium (235, 238, 233), plutonium (239) and thorium (232). The core is a boiler through which ordinary water (coolant) flows. Among other coolants, “heavy water” and liquid graphite are less commonly used. If we talk about the operation of nuclear power plants, then a nuclear reactor is used to produce heat. Electricity itself is generated using the same method as in other types of power plants - steam rotates a turbine, and the energy of movement is converted into electrical energy.
Below is a diagram of the operation of a nuclear reactor.
As we have already said, the decay of a heavy uranium nucleus produces lighter elements and several neutrons. The resulting neutrons collide with other nuclei, also causing them to fission. At the same time, the number of neutrons grows like an avalanche.
It should be mentioned here neutron multiplication factor . So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction dies out. But if you maintain the value of the coefficient equal to one, the reaction will proceed long and stably.
The question is how to do this? In the reactor, the fuel is in the so-called fuel elements (TVELakh). These are rods that contain, in the form of small tablets, nuclear fuel . Fuel rods are connected into hexagonal-shaped cassettes, of which there can be hundreds in a reactor. Cassettes with fuel rods are arranged vertically, and each fuel rod has a system that allows you to adjust the depth of its immersion into the core. In addition to the cassettes themselves, they include control rods And emergency protection rods . The rods are made of a material that absorbs neutrons well. Thus, control rods can be lowered to different depths in the core, thereby adjusting the neutron multiplication factor. Emergency rods are designed to shut down the reactor in case of an emergency.
How is a nuclear reactor started?
We have figured out the operating principle itself, but how to start and make the reactor function? Roughly speaking, here it is - a piece of uranium, but the chain reaction does not begin in it on its own. The fact is that in nuclear physics there is a concept critical mass .
Critical mass is the mass of fissile material required to start a nuclear chain reaction.
With the help of fuel rods and control rods, a critical mass of nuclear fuel is first created in the reactor, and then the reactor is brought to the optimal power level in several stages.
In this article, we tried to give you a general idea of the structure and operating principle of a nuclear (nuclear) reactor. If you have any questions on the topic or have been asked a problem in nuclear physics at the university, please contact to the specialists of our company. As usual, we are ready to help you resolve any pressing issue regarding your studies. And while we're at it, here's another educational video for your attention!
And the ability to use nuclear energy, both for creative (nuclear energy) and destructive (atomic bomb) purposes, became, perhaps, one of the most significant inventions of the last twentieth century. Well, at the heart of all that formidable power that lurks in the depths of a tiny atom are nuclear reactions.
What are nuclear reactions
Nuclear reactions in physics mean the process of interaction of an atomic nucleus with another similar nucleus or with different elementary particles, resulting in changes in the composition and structure of the nucleus.
A little history of nuclear reactions
The first nuclear reaction in history was made by the great scientist Rutherford back in 1919 during experiments to detect protons in nuclear decay products. The scientist bombarded nitrogen atoms with alpha particles, and when the particles collided, a nuclear reaction occurred.
And this is what the equation for this nuclear reaction looked like. It was Rutherford who was credited with the discovery of nuclear reactions.
This was followed by numerous experiments by scientists in carrying out various types of nuclear reactions, for example, a very interesting and significant for science was the nuclear reaction caused by the bombardment of atomic nuclei with neutrons, which was carried out by the outstanding Italian physicist E. Fermi. In particular, Fermi discovered that nuclear transformations can be caused not only by fast neutrons, but also by slow ones, which move at thermal speeds. By the way, nuclear reactions caused by exposure to temperature are called thermonuclear reactions. As for nuclear reactions under the influence of neutrons, they very quickly gained their development in science, and what kind of reactions, read about it further.
Typical formula for a nuclear reaction.
What nuclear reactions are there in physics?
In general, nuclear reactions known today can be divided into:
- fission of atomic nuclei
- thermonuclear reactions
Below we will write in detail about each of them.
Nuclear fission
The fission reaction of atomic nuclei involves the disintegration of the actual nucleus of an atom into two parts. In 1939, German scientists O. Hahn and F. Strassmann discovered the fission of atomic nuclei, continuing the research of their scientific predecessors, they established that when uranium is bombarded with neutrons, elements of the middle part of the periodic table arise, namely radioactive isotopes of barium, krypton and some others elements. Unfortunately, this knowledge was initially used for horrific, destructive purposes, because the Second World War began and German, and on the other hand, American and Soviet scientists raced to develop nuclear weapons (which were based on the nuclear reaction of uranium), which ended in the infamous “ nuclear mushrooms" over the Japanese cities of Hiroshima and Nagasaki.
But back to physics, the nuclear reaction of uranium during the splitting of its nucleus simply has colossal energy, which science has been able to put to its service. How does such a nuclear reaction occur? As we wrote above, it occurs as a result of the bombardment of the nucleus of a uranium atom by neutrons, which causes the nucleus to split, creating a huge kinetic energy of the order of 200 MeV. But what is most interesting is that as a product of the nuclear fission reaction of a uranium nucleus from a collision with a neutron, several free new neutrons appear, which, in turn, collide with new nuclei, split them, and so on. As a result, there are even more neutrons and even more uranium nuclei are split from collisions with them - a real nuclear chain reaction occurs.
This is how it looks on the diagram.
In this case, the neutron multiplication factor must be greater than unity; this is a necessary condition for a nuclear reaction of this type. In other words, in each subsequent generation of neutrons formed after the decay of nuclei, there should be more of them than in the previous one.
It is worth noting that, according to a similar principle, nuclear reactions during bombardment can also take place during the fission of the nuclei of atoms of some other elements, with the nuances that the nuclei can be bombarded by a variety of elementary particles, and the products of such nuclear reactions will vary, so we can describe them in more detail , we need a whole scientific monograph
Thermonuclear reactions
Thermonuclear reactions are based on fusion reactions, that is, in fact, the process opposite to fission occurs, the nuclei of atoms do not split into parts, but rather merge with each other. This also releases a large amount of energy.
Thermonuclear reactions, as the name suggests (thermo - temperature), can occur exclusively at very high temperatures. After all, in order for two atomic nuclei to merge, they must approach a very close distance to each other, while overcoming the electrical repulsion of their positive charges; this is possible with the existence of high kinetic energy, which, in turn, is possible at high temperatures. It should be noted that thermonuclear reactions do not occur, however, not only on it, but also on other stars; one can even say that it lies at the very basis of their nature of any star.
Nuclear reactions, video
And finally, an educational video on the topic of our article, nuclear reactions.
