Nuclear reactions between light atomic nuclei occurring at very high temperatures (10 7 10 8 K) are called thermonuclear reactions. In these reactions, the nuclei experiencing mutual Coulomb repulsion manage, having overcome the corresponding electrostatic barrier (Fig. 1), to approach a distance of the order of the radius of action of the nuclear forces of attraction and, having fallen into the deep potential well formed by them, to perform one or another exoenergetic (i.e., accompanied by the release of energy) nuclear restructuring. By "energy release" is meant the release of excess kinetic energy in the reaction products, equal to the increase in the total binding energy. Thus, relatively loose nuclei rearrange into more tightly bound ones, and since the nuclei with the highest binding energy per nucleon are in the middle part of the Mendeleev periodic system, the most typical exoenergetic reaction mechanism is merger(fusion) of the lightest nuclei into heavier ones. Although there are also exoenergetic reactions of fission of light nuclei. Due to the special strength of the 4 He nucleus, for example, the reaction
The processes described above are called nuclear fusion reactions (NF).
According to the mechanism of overcoming the Coulomb barrier, nuclear reactions can be divided into two main classes: A - reactions with an undistorted barrier, requiring a sufficiently large relative energy of colliding nuclei, which is imparted to them as a result of acceleration or strong heating; B - reactions of the so-called cold fusion, which become possible as a result of a strong distortion of the barrier itself - first of all, its narrowing, due to the "cutting off" of the outer, widest part.
Class A reactions can be realized either in some accelerator, or in high-temperature plasma of stellar interiors, a nuclear explosion, a powerful gas discharge, or in a plasma of matter heated by a giant laser radiation pulse, bombardment by an intense particle beam, etc.
Type B reactions are the result of such phenomena as:
The enduring interest in nuclear reactions, and first of all in thermonuclear reactions, is due to the fact that they are:
- the main source of the Sun and stars, as well as the mechanism of prestellar and stellar processes of synthesis of atomic nuclei of chemical elements;
– one of the physical foundations of a nuclear explosion and (thermo-)nuclear weapons;
- the basis of controlled thermonuclear fusion (CTF) - an economically and environmentally promising direction in the energy sector of the future.
Table 1 lists a number of reactions of interest for CTS.
Table 1
Exoenergetic reactions between light nuclei
|
energy release, |
(in the energy region |
The energy of the incident particles, resp. , MeV |
||
|
0.16 at 2 MeV | ||||
|
0.69 at 1.2 MeV |
P – proton, d– deuteron (deuterium nucleus 2 H), t– triton (tritium nucleus 3 H), n- neutron, e + - positron, ν - nittrino, γ - photon. The distribution of energy between the reaction products is usually inversely proportional to their masses.
When analyzing the results, one must keep in mind that the cross section σ of any of the reactions is, roughly speaking, the product of the cross section for passage through the Coulomb barrier and the probability of subsequent nuclear transformation. The first, "Coulomb" factor is by its nature universal for all thermonuclear reactions. barrier height E δ
where and are the nuclear charges, and R is the sum of their "radii". Even for combinations of nuclei with the smallest , for example, is 200 keV. The average energy of particles for the plasma of stellar interiors or modern directions of CTS, where temperatures (10 7 10 8) K are most typical, is about (110) keV. Consequently, overcoming a potential barrier is, as a rule, in the nature of a tunnel, moreover, deeply under the barrier, passage. Probability of tunneling when the relative energy E colliding nuclei is much less than the barrier height (), can be described by the limiting form of the known exponent, namely:
where is the relative velocity of nuclei,
is their reduced mass.
The second, "nuclear" factor, which determines the basic order of the cross section of a thermonuclear reaction, is specific for each specific reaction. Thus, for reactions with the formation of the most strongly bound 4 He nucleus, it is large and usually resonantly depends on the energy. This applies, for example, to reactions 7 and 10, which are the most important for CTS, and to one of the hypothetically promising "pure", i.e., without neutron reactions, reaction 20. For reactions due to weak interaction, it is extremely small. For example, reaction 1, which is fundamental for the energy release of the Sun, has not been observed directly (in the laboratory) at all.
The intensity of a thermonuclear reaction depends on the plasma density and temperature. The density dependence is determined by the fact that reactions occur as a result of pairwise collisions between nuclei. The number of reactions per unit volume per unit time is , where n 1 , n 2 – concentrations of kernels of sorts 1 and 2; angle brackets denote averaging over the distribution of relative velocities , which is further taken to be Maxwellian. In the area of "not very high" temperatures T ≤ (10 7 ÷10 8)K and in the absence of resonance can be approximately expressed in a form that is universal for all non-resonant reactions:
where is a constant characteristic of this reaction. This formula is valid only for large (1) values of the exponent. The resulting temperature dependence is strong enough in itself, but still not as sharp as, for example, the typical temperature dependence of the rate of chemical reactions.
Thermonuclear reactions are exothermic reactions of the fusion of light nuclei that efficiently proceed at ultrahigh temperatures (of the order of 10 7 - 10 9 K), self-continuing due to a significant release of energy in them. High temperatures in them are necessary in order for the kinetic energy of the thermal motion of the nuclei to be sufficient to overcome the Coulomb potential barrier of the nuclei, approach a distance of the order of the action of nuclear forces, and then initiate the fusion reaction, accompanied by the release of energy in the form of excess kinetic energy of the reaction products.
During the fusion of light nuclei and the formation of a new nucleus, a large amount of energy should be released. This can be seen from the curve of specific binding energy versus mass number A (see 8.1.2). Up to nuclei with a mass number of about 60, the specific binding energy of nucleons increases with increasing A. Therefore, the synthesis of any nucleus with A< 60 из более лёгких ядер должен сопровождаться выделением энергии. Общая масса продуктов реакции синтеза будет в этом случае меньше массы первоначальных частиц.
In order for two nuclei to enter into a fusion reaction, they must approach at a distance of action of nuclear forces of the order of 2·10 -15 m, overcoming the electrical repulsion of their positive charges. For this, the average kinetic energy of the thermal motion of molecules must exceed the potential energy of the Coulomb interaction. The calculation of the required temperature T for this leads to a value of the order of 10 8 – 10 9 K. This is an extremely high temperature. At this temperature, the substance is in a fully ionized state, which is called plasma.
The energy released in thermonuclear reactions per nucleon is several times higher than the specific energy released in chain reactions of nuclear fission. As an example, consider some synthesis reactions:
(Q=3.3 MeV); (8.48.55)
(Q=17.6 MeV);
(Q=22.4 MeV),
where is the released energy. For example, in the fusion reaction of deuterium and tritium nuclei, 3.5 MeV/nucleon is released. In total, 17.6 MeV is released in this reaction. This is one of the most promising thermonuclear reactions. Thermonuclear reactions give the greatest contribution of energy per unit mass of "fuel" than any other transformations. For example, the amount of deuterium in a glass of plain water is energetically equivalent to about 60 liters of gasoline. Interest in the implementation of a controlled thermonuclear reaction is understandable.
Controlled thermonuclear fusion, which is based on thermonuclear reactions, is potentially an inexhaustible source of energy and is an environmentally and economically promising direction in the energy sector of the future. For controlled thermonuclear fusion, the most important reaction is the fusion of deuterium and tritium nuclei with the formation of a helium nucleus and the release of 17.6 MeV of energy per fusion event. To initiate a fusion reaction, it is necessary to heat a mixture of deuterium and tritium to a temperature of more than 100 million degrees. At this temperature, the mixture is a completely ionized plasma, and the problem arises of confining the plasma and effectively isolating it from the walls of the working volume. In 1950, academicians I.E. Tamm and A.D. Sakharov proposed the idea of confining and thermally insulating plasma by a strong magnetic field of a special configuration created in a toroidal chamber by magnetic coils. This idea was the basis for the design of thermonuclear installations, called tokamaks (short for "toroidal chamber with magnetic coils").
The first experimental studies of these systems in the USSR began in 1956 under the guidance of Acad. L.A. Artsimovich. The beginning of the modern era in the study of thermonuclear fusion should be considered 1969, when a temperature of 3 million K was reached in the Russian thermonuclear installation "Tokamak-3" in a plasma with a volume of 1 m3. installation "Tokamak-10", in which plasma was obtained with a temperature of 7-8 million K. in a volume of 5 m 3. At present, temperatures of the order of 150 million K have been reached at existing tokamak-type installations (European installation JET - Joint Europpean Torus). Since 1988, the USSR (since 1992 - Russia), the USA, European countries and Japan have been jointly developing the project of the International Thermonuclear Experimental Reactor - ITER tokamak, which should become the first large-scale power plant designed for long-term operation. The reactor power must be at least 500 MW. The launch of the reactor is planned to be carried out in 2018, and the production of hydrogen-deuterium plasma - in 2026.
Thermonuclear reactions play an extremely important role in the evolution of the universe. First, the radiation energy of the Sun and stars is of thermonuclear origin. Secondly, thermonuclear reactions are one of the main mechanisms of nucleosynthesis.
For normal homogeneous stars, including the Sun, nuclear fusion is carried out according to the so-called proton-proton, or hydrogen cycle. Hydrogen cycle(proton-proton chain) - a sequence of thermonuclear reactions in stars, leading to the conversion of hydrogen into helium without the participation of a catalyst; main source of energy for stars with mass M<1,2 M s (M s is the mass of the Sun) at the initial stage of their existence. The total result of the reactions in which the formation of helium nuclei from hydrogen can be written as follows:
4 2e ++ 2 + 26.73 MeV.
Of course, this transformation does not occur immediately, but in several stages. The most important reactions of the hydrogen cycle are the following:
The end result of this sequence of reactions (proton-proton or hydrogen cycle) is the transformation of four hydrogen nuclei into the nucleus of a helium atom. The total energy released during such a reaction is 26.73 MeV. The neutrinos formed in this reaction weakly interact with matter and leave the star, carrying away their energy - about 0.5 MeV (the so-called solar neutrinos). This reaction can proceed at temperatures of the order of 13 million K. Approximately 70% of all reactions of the hydrogen cycle in the Sun occur according to this scheme. In 30% of cases, it can connect with and then the reactions will go according to the following scheme:
On the Sun, the hydrogen cycle is more efficient than the carbon-nitrogen cycle and provides 98.4% of the energy release.
If there is a certain amount of carbon in the star, then carbon-nitrogen cycle- a series of thermonuclear reactions leading to the synthesis of helium from hydrogen with the participation of nitrogen and carbon as catalysts. The carbon-nitrogen cycle was discovered independently by G. Bethe and the German physicist and astrophysicist K. von Weizsacker. This cycle consists of six reactions:
The end result of this chain is the transformation of four protons into one helium nucleus with the release of 26.73 MeV of energy, while 1.7 MeV is carried away with neutrinos. Since carbon and nitrogen nuclei participate in this sequence of reactions, it is called the carbon-nitrogen cycle. The carbon-nitrogen cycle is the main source of energy for stars with a mass greater than 1.2 solar masses. At the center of these stars, the temperature is about 20 million K, and the carbon-nitrogen cycle is more efficient than the hydrogen cycle. The carbon-nitrogen cycle also occurs on the Sun, but it provides only about 1.6% of the energy release. In the bowels of the Sun, 3.6∙10 38 protons burn every second, i.e. about 630 million tons of hydrogen are converted into helium. At the same time, the radiation power of the Sun is 3.86∙10 26 W.
Control questions for self-training of students:
1. What types of radioactivity do you know?
2. Law of radioactive decay. Displacement rules.
3. Regularities of -decay.
4. What is a neutrino? In what decay is it emitted?
5. What phenomena accompany the passage - radiation through the substance and what is their essence?
6. Types of nuclear reactions.
7. Under the influence of what particles are nuclear reactions more efficient?
8. What is a nuclear fission reaction?
9. Why is the fission of heavy nuclei and the synthesis of atomic nuclei accompanied by the release of a large amount of energy?
10. On what grounds can nuclear reactors be classified?
Literary sources:
1. Trofimova, T.I. Physics course: textbook. allowance for universities / T.I. Trofimov. – M.: ACADEMIA, 2008.
2. Saveliev, I.V. Course of general physics: textbook. manual for technical colleges: in 3 volumes / I.V. Savelyev. - St. Petersburg: Spec. lit., 2005.
there are sufficiently large relative energies of colliding nuclei that are necessary to overcome the electrostatic barrier due to the mutual repulsion of nuclei (as like-charged particles). Without this, it is impossible for the nuclei to approach each other at a distance of the order of the radius of action of nuclear forces, and, consequently, for the "restructuring" of the nuclei that occurs when thermonuclear reactions That's why thermonuclear reactions under natural conditions, they occur only in the interiors of stars, and for their implementation on Earth, it is necessary to strongly heat the substance with a nuclear explosion, a powerful gas discharge, a giant laser radiation pulse, or bombardment with an intense particle beam.thermonuclear reactions, as a rule, are the processes of formation of strongly bound nuclei from looser ones and therefore are accompanied by the release of energy (more precisely, the release of excess kinetic energy in the reaction products, equal to an increase in the binding energy). At the same time, the very mechanism of this "exoenergetic" shift to the middle part of Mendeleev's periodic system of elements is opposite to that which takes place in the fission of heavy nuclei: almost all practically interesting thermonuclear reactions- these are fusion reactions (synthesis) of light nuclei into heavier ones. There are, however, exceptions: due to the special strength of the nucleus 4 (a-particle), exoenergetic fission reactions of light nuclei are possible (one of them, the “pure” reaction 11 + p ® 3 4 He + 8.6 mev, has attracted interest in recent years.
Large energy release in a number thermonuclear reactions determines the importance of their study for astrophysics, as well as for applied nuclear physics and nuclear power engineering. In addition, the role of thermonuclear reactions in prestellar and stellar processes of synthesis of atomic nuclei of chemical elements (nucleogenesis).
Speeds thermonuclear reactions In table. 1 for row thermonuclear reactions the values of energy release are given, the main quantity characterizing the probability thermonuclear reactions- its maximum effective cross section (s max, and the corresponding energy of the incident (in the reaction formula - the first from the left) particle.
The main reason for the very large spread of cross sections thermonuclear reactions- a sharp difference in the probabilities of the actual nuclear ("post-barrier") transformations. Thus, for most reactions accompanied by the formation of the most strongly bound 4 He nucleus, the cross section is large, while for reactions due to weak interaction (for example, p + p ® D + e + + n), it is very small.
thermonuclear reactions occur as a result of pair collisions between nuclei, so their number per unit volume per unit time is equal to n 1 n 2<vs(v)>, Where n 1 , n 2 - concentration of kernels of the 1st and 2nd grades (if the kernels of the same grade, then n 1 n 2 should be replaced with n 2), v- relative speed of colliding nuclei, angle brackets mean averaging over the speeds of nuclei v[the distribution of which is further taken to be Maxwellian (see Maxwell distribution )].
Temperature dependence of speed thermonuclear reactions determined by the multiplier< vs(v)>. In the practically important case of "not very high" temperatures T< (10 7 ¸10 8) K it can be approximately expressed in a form that is the same for all thermonuclear reactions In this case, the relative energies E colliding nuclei, as a rule, is much lower than the height of the Coulomb barrier (the latter even for a combination of nuclei with the smallest charge z= 1 is ~200 Kev, which corresponds, according to the ratio E = kT, T~ 2×10 9 ) and, therefore, the form s(v) is determined mainly by the probability of "tunneling" through the barrier (see Fig. tunnel effect ), and not by the actual nuclear interaction, which in some cases determines the "resonant" nature of the dependence s(v)(it is this dependence that manifests itself in the largest of the values of s max in table 1). The result looks like
< vs(v)> = const× T -2/3 exp)
where const is a constant characteristic of a given reaction, Z 1 , Z 2 -
charges of colliding nuclei, -
their reduced mass, e - electron charge, - Planck constant, k - Boltzmann constant.
Table 1
| Reaction | energy release, mev | s max, barn (in the energy range £1 mev) | The energy of the incident particle, corresponding to s max, mev |
|||||||||||||||||||||||||||||||
| 1 | p + p ® D + e + + v p + D ® 3 He + g D + D ® 3 He + n D + D ® 4 He + g D + T ® 4 He + n T + D ® 4 He + n T + T ® 4 He + 2n D + 3 He ® 4 He + p p + 6 Li ® 4 He + 3 He p + 7 Li ® 2 4 He + g D + 6 Li ® 7 Li + p D + 6 Li ® 2 4 He Tab. 2. - Hydrogen cycle
The hydrogen cycle branches into 3 variants. At sufficiently high concentrations of 4 He and T> (10 ¸ 15) million K, another branch of the pp-cycle begins to dominate in the total energy release, which differs from that given in Table 2 by replacing the reaction 3 He + 3 He with the chain: 3 He + 4 He ® 7 Be + g, 7 Be + e-® 7 Li + g, p + 7 Li ® 2 4 He, and at even higher T - third branch: 3 He + 4 He ® 7 Be + g, p + 7 Be ® 8 B + g, Tab. 3. - Carbon cycle
The power of this cycle as a source of energy is small. However, it seems to be of great importance for nucleogenesis, since one of the intermediate nuclei of the cycle (21 Ne) can serve as a source of neutrons: 21 Ne + 4 He ® 24 Mg + n (a similar role can be played by the C nucleus involved in - cycle). The subsequent "chain" capture of neutrons, alternating with the processes of b - -decay, is the mechanism for the synthesis of ever heavier nuclei. The average intensity of the energy release e in typical stellar thermonuclear reactions negligible on a terrestrial scale. So, for the Sun (on average per 1 G solar mass). This is much less, for example, than the rate of energy release in a living organism in the process of metabolism. However, due to the huge mass of the Sun (2×10 33 G) total power radiated by it (4×10 26 Tue) is extremely large (it corresponds to a decrease in the mass of the Sun by ~ 4 million km per second). T) and even an insignificant fraction of it is enough to have a decisive influence on the energy balance of the earth's surface, life, etc. Due to the colossal size and masses of the Sun and stars, they ideally solve the problem of confinement (in this case, gravitational) and thermal insulation of plasma: thermonuclear reactions flow in the hot core of the star, and heat transfer occurs from a distant and much colder surface. This is the only reason why stars can efficiently generate energy in such slow processes as pp- and -cycles (Tables 2 and 3). Under terrestrial conditions, these processes are practically impracticable; for example, the fundamental reaction p + p ® D + e + + n has not been directly observed at all. thermonuclear reactions in earth conditions. On Earth, it makes sense to use only the most effective of thermonuclear reactions associated with the participation of the hydrogen isotopes D and T. Similar thermonuclear reactions have so far been carried out on a comparatively large scale only in test explosions of thermonuclear or hydrogen bombs (see Nuclear weapon ). The energy released by the explosion of such a bomb (10 23 - 10 24 erg), exceeds the weekly generation of electricity throughout the globe and is comparable to the energy of earthquakes and hurricanes. The probable scheme of reactions in a thermonuclear bomb includes thermonuclear reactions 12, 7, 4 and 5 (Table 1). In connection with thermonuclear explosions, etc. were discussed. thermonuclear reactions, for example 16,14, 3. By using thermonuclear reactions for peaceful purposes may be controlled thermonuclear fusion (UTS), with which they pin hopes for solving the energy problems of mankind, since the deuterium contained in the water of the oceans is an almost inexhaustible source of cheap fuel for controlled thermonuclear reactions The greatest progress in research on CTS was achieved within the framework of the Soviet Tokamak program. Similar programs by the mid-70s. 20th century began to develop vigorously in a number of other countries. For CTS, the most important thermonuclear reactions 7.5 and 4 [as well as 12 for expensive T regeneration]. Regardless of the energy goals, a thermonuclear reactor can be used as a powerful source of fast neutrons. However, "pure" thermonuclear reactions that do not give neutrons, for example, 10, 20 (Table 1). Lit.: Artsimovich L. A., Controlled thermonuclear reactions, 2nd ed., M., 1963; Frank-Kamenetsky D. A., Physical processes inside stars, M., 1959; Thermonuclear reactions, in the book: Problems of modern physics, M., 1954, c. 1; Fowler. A., Caughlan G. R., Zimmerman B. A., "Annual Review of Astronomy and Astrophysics", 1967, v. 5, p. 525. V. I. Kogan.
Article about the word thermonuclear reactions" in the Great Soviet Encyclopedia has been read 22360 times |
Scientists at the Princeton Plasma Physics Laboratory have proposed the idea of the most durable nuclear fusion device that can operate for more than 60 years. At the moment, this is a daunting task: scientists are struggling to get a fusion reactor to work for a few minutes - and then years. Despite the complexity, the construction of a fusion reactor is one of the most promising tasks of science, which can bring great benefits. We tell you what you need to know about thermonuclear fusion.
1. What is thermonuclear fusion?
Do not be afraid of this cumbersome phrase, in fact, everything is quite simple. Thermonuclear fusion is a type of nuclear reaction.
During a nuclear reaction, the nucleus of an atom interacts either with an elementary particle or with the nucleus of another atom, due to which the composition and structure of the nucleus change. A heavy atomic nucleus can decay into two or three lighter ones - this is a fission reaction. There is also a fusion reaction: this is when two light atomic nuclei merge into one heavy one.
Unlike nuclear fission, which can take place both spontaneously and forcedly, nuclear fusion is impossible without the supply of external energy. As you know, opposites attract, but atomic nuclei are positively charged - so they repel each other. This situation is called the Coulomb barrier. To overcome repulsion, it is necessary to disperse these particles to crazy speeds. This can be done at very high temperatures, on the order of several million kelvins. It is these reactions that are called thermonuclear.
2. Why do we need thermonuclear fusion?
In the course of nuclear and thermonuclear reactions, a huge amount of energy is released that can be used for various purposes - you can create the most powerful weapon, or you can convert nuclear energy into electricity and supply it to the whole world. Nuclear decay energy has long been used in nuclear power plants. But thermonuclear energy looks more promising. In a thermonuclear reaction, for each nucleon (the so-called constituent nuclei, protons and neutrons), much more energy is released than in a nuclear reaction. For example, when fission of a uranium nucleus per nucleon accounts for 0.9 MeV (megaelectronvolt), and whenIn the synthesis of a helium nucleus, an energy equal to 6 MeV is released from hydrogen nuclei. Therefore, scientists are learning to carry out thermonuclear reactions.
Fusion research and the construction of reactors allow the expansion of high-tech production, which is useful in other areas of science and high-tech.
3. What are thermonuclear reactions?
Thermonuclear reactions are divided into self-sustaining, uncontrolled (used in hydrogen bombs) and controlled (suitable for peaceful purposes).
Self-sustaining reactions take place in the interiors of stars. However, there are no conditions on Earth for such reactions to take place.
People have been conducting uncontrolled or explosive thermonuclear fusion for a long time. In 1952, during Operation Evie Mike, the Americans detonated the world's first thermonuclear explosive device, which had no practical value as a weapon. And in October 1961, the world's first thermonuclear (hydrogen) bomb (Tsar Bomba, Kuzkin's Mother), developed by Soviet scientists under the leadership of Igor Kurchatov, was tested. It was the most powerful explosive device in the history of mankind: the total energy of the explosion, according to various sources, ranged from 57 to 58.6 megatons of TNT. In order to detonate a hydrogen bomb, it is first necessary to obtain a high temperature during a conventional nuclear explosion - only then will the atomic nuclei begin to react.
The power of the explosion in an uncontrolled nuclear reaction is very high, in addition, the proportion of radioactive contamination is high. Therefore, in order to use thermonuclear energy for peaceful purposes, it is necessary to learn how to manage it.
4. What is needed for a controlled thermonuclear reaction?
Hold the plasma!
Unclear? Now let's explain.
First, atomic nuclei. Nuclear energy uses isotopes - atoms that differ from each other in the number of neutrons and, accordingly, in atomic mass. The hydrogen isotope deuterium (D) is extracted from water. Superheavy hydrogen or tritium (T) is a radioactive isotope of hydrogen that is a by-product of decay reactions carried out in conventional nuclear reactors. Also in thermonuclear reactions, a light isotope of hydrogen, protium, is used: this is the only stable element that does not have neutrons in the nucleus. Helium-3 is contained on Earth in negligible amounts, but it is very abundant in the lunar soil (regolith): in the 80s, NASA developed a plan for hypothetical installations for processing regolith and isotope extraction. On the other hand, another isotope, boron-11, is widespread on our planet. 80% of the boron on Earth is an isotope necessary for nuclear scientists.
Second, the temperature is very high. The substance participating in a thermonuclear reaction must be an almost completely ionized plasma - it is a gas in which free electrons and ions of various charges float separately. To turn a substance into a plasma, a temperature of 10 7 -10 8 K is required - these are hundreds of millions of degrees Celsius! Such ultra-high temperatures can be obtained by creating high-power electric discharges in the plasma.
However, it is impossible to simply heat the necessary chemical elements. Any reactor will instantly vaporize at these temperatures. A completely different approach is required here. To date, it is possible to keep the plasma in a limited area with the help of heavy-duty electric magnets. But it has not yet been possible to fully use the energy obtained as a result of a thermonuclear reaction: even under the influence of a magnetic field, the plasma spreads in space.
5. What reactions are most promising?
The main nuclear reactions planned to be used for controlled fusion will use deuterium (2H) and tritium (3H), and in the longer term helium-3 (3He) and boron-11 (11B).
Here are the most interesting reactions.
1) 2 D+ 3 T -> 4 He (3.5 MeV) + n (14.1 MeV) - deuterium-tritium reaction.
2) 2 D+ 2 D -> 3 T (1.01 MeV) + p (3.02 MeV) 50%
2 D+ 2 D -> 3 He (0.82 MeV) + n (2.45 MeV) 50% is the so-called deuterium monopropellant.
Reactions 1 and 2 are fraught with neutron radioactive contamination. Therefore, "neutronless" reactions are the most promising.
3) 2 D+ 3 He -> 4 He (3.6 MeV) + p (14.7 MeV) - deuterium reacts with helium-3. The problem is that helium-3 is extremely rare. However, the neutron-free yield makes this reaction promising.
4) p+ 11 B -> 3 4 He + 8.7 MeV - boron-11 reacts with protium, resulting in alpha particles that can be absorbed by aluminum foil.
6. Where to conduct such a reaction?
The natural fusion reactor is the star. In it, the plasma is held under the influence of gravity, and the radiation is absorbed - thus, the core does not cool down.
On Earth, thermonuclear reactions can only be carried out in special facilities.
impulse systems. In such systems, deuterium and tritium are irradiated with ultra high power laser beams or electron/ion beams. Such irradiation causes a sequence of thermonuclear microexplosions. However, it is unprofitable to use such systems on an industrial scale: much more energy is spent on the acceleration of atoms than is obtained as a result of fusion, since not all accelerated atoms enter into a reaction. Therefore, many countries are building quasi-stationary systems.
Quasi-stationary systems. In such reactors, the plasma is held by a magnetic field at low pressure and high temperature. There are three types of reactors based on different magnetic field configurations. These are tokamaks, stellarators (torsatrons) and mirror traps.
tokamak stands for "toroidal chamber with magnetic coils". This is a camera in the form of a "donut" (torus), on which coils are wound. The main feature of the tokamak is the use of an alternating electric current that flows through the plasma, heats it up and, creating a magnetic field around itself, holds it.
IN stellarator (torsatron) the magnetic field is completely contained by magnetic coils and, unlike a tokamak, can be operated continuously.
W mirror (open) traps the principle of reflection is used. The chamber is closed on both sides by magnetic "plugs" that reflect the plasma, keeping it in the reactor.
For a long time, mirror traps and tokamaks fought for supremacy. Initially, the concept of a trap seemed simpler and therefore cheaper. In the early 60s, open traps were heavily funded, but the instability of the plasma and unsuccessful attempts to contain it with a magnetic field forced these installations to complicate - simple-looking designs turned into infernal machines, and it did not work out to achieve a stable result. Therefore, tokamaks came to the fore in the 1980s. In 1984, the European JET tokamak was launched, the cost of which was only 180 million dollars and the parameters of which made it possible to carry out a thermonuclear reaction. In the USSR and France, superconducting tokamaks were designed, which spent almost no energy on the operation of the magnetic system.
7. Who is now learning to carry out thermonuclear reactions?
Many countries are building their own fusion reactors. There are experimental reactors in Kazakhstan, China, the USA and Japan. The Kurchatov Institute is working on the IGNITOR reactor. Germany launched the Wendelstein 7-X stellarator fusion reactor.
The most famous international project is the ITER tokamak (ITER, International Thermonuclear Experimental Reactor) at the Cadarache Research Center (France). Its construction was supposed to be completed in 2016, but the amount of necessary financial support has grown, and the timing of the experiments has shifted to 2025. The European Union, the USA, China, India, Japan, South Korea and Russia participate in the activities of ITER. The main share in financing is played by the EU (45%), the rest of the participants supply high-tech equipment. In particular, Russia produces superconducting materials and cables, radio tubes for plasma heating (gyrotrons) and fuses for superconducting coils, as well as components for the most complex part of the reactor - the first wall, which must withstand electromagnetic forces, neutron radiation and plasma radiation.
8. Why do we still not use thermonuclear reactors?
Modern tokamak installations are not thermonuclear reactors, but research installations in which the existence and preservation of plasma is possible only for a while. The fact is that scientists have not yet learned how to keep the plasma in the reactor for a long time.
At the moment, one of the biggest achievements in the field of nuclear fusion is the success of German scientists who managed to heat hydrogen gas to 80 million degrees Celsius and maintain a cloud of hydrogen plasma for a quarter of a second. And in China, hydrogen plasma was heated to 49.999 million degrees and held for 102 seconds. Russian scientists from the (G. I. Budker Institute of Nuclear Physics, Novosibirsk) managed to achieve stable plasma heating up to ten million degrees Celsius. However, the Americans have recently proposed a method for confining plasma for 60 years - and this inspires optimism.
In addition, there is controversy regarding the profitability of fusion in industry. It is not known whether the benefits of electricity generation will offset the costs of fusion. It is proposed to experiment with reactions (for example, abandon the traditional deuterium-tritium or monopropellant reaction in favor of other reactions), structural materials - or even abandon the idea of industrial thermonuclear fusion, using it only for individual reactions in fission reactions. However, scientists still continue to experiment.
9. Are fusion reactors safe?
Relatively. Tritium, which is used in thermonuclear reactions, is radioactive. In addition, neurons released as a result of fusion irradiate the reactor structure. The elements of the reactor themselves are covered with radioactive dust due to exposure to plasma.
However, a fusion reactor is much safer than a nuclear reactor in terms of radiation. There are relatively few radioactive substances in the reactor. In addition, the design of the reactor itself assumes the absence of "holes" through which radiation can leak. The vacuum chamber of the reactor must be sealed, otherwise the reactor simply cannot work. During the construction of thermonuclear reactors, materials tested by nuclear power are used, and reduced pressure is maintained in the rooms.
When will fusion power plants appear?
Scientists most often say something like “in 20 years we will solve all the fundamental issues”. Nuclear engineers are talking about the second half of the 21st century. Politicians talk about a sea of clean energy for a penny, without bothering with dates.
How scientists are looking for dark matter in the bowels of the Earth
Hundreds of millions of years ago, minerals under the earth's surface could retain traces of a mysterious substance. It remains only to get to them. More than two dozen underground laboratories scattered around the world are busy searching for dark matter.
What hinders the development of the internal market of radiation technologies?
Scientists from the institutes of the SB RAS, who visited the countries of Southeast Asia, talked about how simple fish sellers in the local bazaars extended the shelf life of their goods with the help of simple Chinese "technology".
Super-factory S-tau
In the OTR program "Great Science. Great in the Small", Director of the G.I. Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences Academician Pavel Logachev spoke about the role of the "S-Tau Factory" in the development of scientific research and what caused its name.
During the lesson, everyone will be able to get an idea about the topic "Thermonuclear reaction". You will learn what a thermonuclear reaction, or fusion reaction, is. You will learn which elements and under what conditions can enter into this type of reaction, and get acquainted with the developments of the use of a thermonuclear reaction for peaceful purposes.
thermonuclear reactions(or simply thermonuclear) is called the fusion of light nuclei into one whole new nucleus, as a result of which a large amount of energy is released. It turns out that a lot of energy is released not only as a result of the fission of heavy nuclei, even more energy is released when light nuclei merge together, combine. This process is called synthesis. And the reactions themselves - thermonuclear fusion, thermonuclear reactions.
What elements are involved in these reactions? These are primarily hydrogen isotopes and helium isotopes. An example is the following reaction:
Two isotopes of hydrogen (deuterium and tritium) combine together to form a helium nucleus, and a neutron is also formed. When such a reaction takes place, a huge amount of energy is released. E = 17.6 MeV.
Don't forget that this is just one reaction. And one more reaction. Two deuterium nuclei fuse together to form a helium nucleus:
In this case, too, a large amount is allocated.
I draw your attention to the fact that in order for such reactions to occur, certain conditions are needed. First of all, it is necessary to bring the nuclei of these isotopes closer together. The nuclei have a positive charge, in this case, the Coulomb forces act, which push these charges apart. This means that these Coulomb forces must be overcome in order to bring one nucleus closer to another. This is possible only if the nuclei themselves have a large kinetic energy, when the speed of these nuclei is quite high. To achieve this, it is necessary to create such conditions when the nuclei of isotopes will have this speed, and this is possible only at very high temperatures. Only in this way can we accelerate the isotopes to speeds that will allow them to approach within a distance of approximately 10 -14 m.
Rice. 1. The distance to which the nuclei need to be brought together for the onset of a thermonuclear reaction
This is the distance from which nuclear forces begin to operate. The value of the required temperature is about t° = 10 7 - 10 8° C. This temperature can be reached when a nuclear explosion is made. Thus, in order to produce a thermonuclear reaction, we must first produce a fission reaction of heavy nuclei. It is in this case that we will achieve a high temperature, and only then this temperature will make it possible to bring the isotope nuclei closer together to a distance when they can combine. As you understand, this is the principle of the so-called hydrogen bomb.
Rice. 2. H-bomb explosion
We, as civilians, are primarily interested in the use of thermonuclear reaction for peaceful purposes to create the same power plants, but of the latest type.
Currently, developments are underway on how to create controlled thermonuclear fusion. Various methods are used for this, one of them is the use of lasers to obtain high energies and temperatures. With the help of lasers, they are accelerated to high speeds, and in this case, a thermonuclear reaction can occur.
As a result of a thermonuclear reaction, a huge amount of heat is released, the place in the reactor where the isotopes interacting with each other will be located must be well isolated so that the substance that will be at a high temperature does not interact with the environment, with the walls of the object where it is located. For such isolation, a magnetic field is used. At a high core temperature, the electrons that are together represent a new kind of matter - plasma. Plasma is a partially or fully ionized gas, and since the gas is ionized, it is sensitive to a magnetic field. Plasma is electrically conductive, with the help of magnetic fields it is possible to give it a certain shape and keep it in a certain volume. However, the technical solution for controlling a thermonuclear reaction remains unresolved.
Rice. 3. TOKAMAK - toroidal facility for magnetic plasma confinement
In conclusion, I would also like to note that thermonuclear reactions play an important role in the evolution of our universe. First of all, we note that thermonuclear reactions flow into the sun. It can be said that it is the energy of thermonuclear reactions that is the energy that has shaped the current appearance of our universe.
List of additional literature
1. Bronstein M.P. Atoms and electrons. "Library "Quantum"". Issue. 1. M.: Nauka, 1980
2. Kikoin I.K., Kikoin A.K. Physics: A textbook for the 9th grade of high school. M.: Enlightenment
3. Kitaygorodsky A.I. Physics for everyone. Book 4. Photons and nuclei. M.: Nauka
4. Myakishev G.Ya., Sinyakov A.Z. Physics. Optics. The quantum physics. Grade 11: textbook for in-depth study of physics. M.: Bustard
Task for the lesson.
1. As a result of the thermonuclear reaction of the combination of two protons, a deuteron and a neutrino are formed. What other particle appears?
2. Find the frequency γ - radiation generated during a thermonuclear reaction:
If α -particle acquires an energy of 19.7 MeV
