What is critical mass in uranium nuclear. The phenomenon of radioactivity, discovered by Becquerel, indicates that... A

1. What is a chain reaction?A. A reaction in which a chain of atomic nuclei is formed. B. Nuclear fission reaction. IN. Nuclear fusion reaction. G. A reaction in which nuclei decay. D. A reaction in which the particles causing it are formed as products of this reaction.

2. The fission reaction of heavy nuclei occurs as a chain reaction due to the emission of some particles. Indicate which particles are in the given reaction:. A. Two protons. B. One proton and one neutron. IN. Three neutrons. G. Two neutrons. D. One proton and two neutrons.

3. The atomic nucleus of bismuth, as a result of a series of radioactive transformations, turned into a lead nucleus . What types of radioactive transformations has it experienced?A. Alpha decay. B. Beta plus decay. IN. Beta minus decay. G Beta plus decay and alpha decay. D. Beta minus decay and alpha decay.

4. The nucleus emits g-quantum. Choose the correct one from the statements below. Element serial number: A. Increasing. B. Decreases. IN. Doesn't change.

5. The nucleus emits an electron. Choose the correct one from the statements listed in the table.

Cross out what is unnecessary.

6. During the nuclear fission reaction of uranium nuclei, about 165 MeV is released in the form kinetic energy movement of core fragments. What forces impart acceleration to nuclear fragments, increasing their kinetic energy?A. Coulomb forces. B. Gravitational forces. IN. Nuclear forces. G. Forces of weak interaction. D. Forces of unknown nature. E. Electromagnetic forces.

7. What condition is necessary for a nuclear chain reaction to occur: 1) the mass of uranium or plutonium must be no less than the critical mass; 2) the presence of high temperature; 3) should the mass of uranium or plutonium be less than the critical mass?A. Only 1. B. Only 2. IN. 1 and 2. G. Only 3. D. 2 and 3.

8. What is the critical mass in a uranium nuclear reactor?A. The maximum mass of uranium in a reactor at which it can operate without explosion. B. The minimum mass of uranium in a reactor at which a chain reaction can occur. IN. Additional mass of uranium introduced into the reactor to start it. G. An additional mass of substance introduced into the reactor to stop it in critical cases.

9. Which substances from the following are usually used in nuclear reactors as neutron absorbers: 1) uranium; 2) graphite; 3) cadmium; 4) heavy water; 5) boron; 6) plutonium.(Choose the correct answer).

10. Which substances from the following are usually used in nuclear reactors as neutron moderators: 1) uranium; 2) graphite; 3) cadmium; 4) heavy water; 5) boron; 6) plutonium.(Choose the correct answer).

11. Which substances from the following are usually used in nuclear reactors as nuclear fuel: 1) uranium; 2) graphite; 3) cadmium; 4) heavy water; 5) boron; 6) plutonium.(Choose the correct answer).

12. Which substances from the following are usually used in nuclear reactors as coolants: 1) uranium; 2) graphite; 3) cadmium; 4) plain water; 5) liquid sodium; 6) plutonium; 7) heavy water.(Choose the correct answer).

13. What is it called? A nuclear reactor is a device in which... A. nuclear energy turns into electric. B. a controlled nuclear fission reaction takes place. IN. nuclear synthesis occurs. G. nuclear decay occurs. D. a chemical reaction takes place.

On the next anniversary of the badabum on Hiroshima and Nagasaki, I decided to scour the Internet on questions of nuclear weapons, where why and how they were created was of little interest to me (I already knew) - I was more interested in how 2 pieces of plutonium do not melt but make a big bang.

Keep an eye on the engineers - they start with a seeder and end with an atomic bomb.

Nuclear physics is one of the most scandalous areas of the venerable natural science. It is into this area that humanity has been throwing billions of dollars, pounds, francs and rubles for half a century, like into the locomotive furnace of a late train. Now the train doesn't seem to be late anymore. The raging flames of burning funds and man-hours subsided. Let’s try to briefly figure out what kind of train this is called “nuclear physics”.

Isotopes and radioactivity

As you know, everything that exists is made up of atoms. Atoms, in turn, consist of electronic shells, living according to their own mind-blowing laws, and the core. Classical chemistry is not at all interested in the nucleus and its personal life. For her, an atom is its electrons and their ability to exchange interaction. And from the chemistry nucleus you only need its mass to calculate the proportions of the reagents. In turn, nuclear physics I don't care about electrons. She is interested in a tiny (100 thousand times smaller than the radius of electron orbits) speck of dust inside an atom, in which almost all of its mass is concentrated.

What do we know about the nucleus? Yes, it consists of positively charged protons and without electric charge neutrons. However, this is not entirely true. The core is not a handful of balls of two colors, as in the illustration from the school textbook. There are completely different laws at work here called strong interaction, turning both protons and neutrons into some kind of indistinguishable mess. However, the charge of this mess is exactly equal to the total charge of the protons included in it, and the mass almost (I repeat, almost) coincides with the mass of the neutrons and protons that make up the nucleus.

By the way, the number of protons of a non-ionized atom always coincides with the number of electrons that have the honor of surrounding it. But with neutrons the matter is not so simple. Strictly speaking, the task of neutrons is to stabilize the nucleus, since without them similarly charged protons would not get along together even for microseconds.

Let's take hydrogen for definiteness. The most common hydrogen. Its structure is ridiculously simple - one proton surrounded by one orbital electron. There is plenty of hydrogen in the Universe. We can say that the Universe consists mainly of hydrogen.

Now let's carefully add a neutron to the proton. From a chemical point of view, it is still hydrogen. But from the point of view of physics, no longer. Having discovered two different hydrogens, physicists became worried and immediately came up with the idea of ​​calling ordinary hydrogen protium, and hydrogen with a neutron at a proton - deuterium.

Let's be bold and feed another neutron to the nucleus. Now we have another hydrogen, even heavier - tritium. Again, from a chemical point of view, it is practically no different from the other two hydrogens (well, except that it now reacts a little less readily). I want to warn you right away - no amount of effort, threats or persuasion can add another neutron to the tritium nucleus. The local laws are much stricter than human ones.

So, protium, deuterium and tritium are isotopes of hydrogen. Their atomic mass is different, but their charge is not. But it is the charge of the nucleus that determines the location in periodic table elements. That's why isotopes are called isotopes. Translated from Greek, it means “occupying the same place.” By the way, the well-known heavy water is the same water, but with two deuterium atoms instead of protium. Accordingly, superheavy water contains tritium instead of protium.

Let's take a look at our hydrogens again. So... Protium is in place, deuterium is in place... Who else is this? Where did my tritium go and where did the helium-3 come from? In our tritium, one of the neutrons clearly got bored, decided to change his profession and became a proton. In doing so, it generated an electron and an antineutrino. The loss of tritium is, of course, upsetting, but we now know that it is unstable. The feeding of neutrons was not in vain.

So, as you understand, isotopes are stable and unstable. There are plenty of stable isotopes around us, but, thank God, there are practically no unstable ones. That is, they exist, but in such a scattered state that they have to be obtained at the cost of very great labor. For example, uranium-235, which caused so much trouble for Oppenheimer, makes up only 0.7% of natural uranium.

Half life

Everything is simple here. The half-life of an unstable isotope is the period of time during which exactly half of the atoms of the isotope will decay and turn into some other atoms. Tritium, already familiar to us, has a half-life of 12.32 years. This is a fairly short-lived isotope, although compared to francium-223, which has a half-life of 22.3 minutes, tritium will seem like a grey-bearded elder.

No macroscopic external factors (pressure, temperature, humidity, the mood of the researcher, the number of allocations, the location of stars) affect the half-life. Quantum mechanics insensitive to such nonsense.

Popular explosion mechanics

The essence of any explosion is the rapid release of energy that was previously in a non-free, bound state. The released energy is dissipated, predominantly turning into heat (the kinetic energy of the disordered movement of molecules), a shock wave (there is also movement, but already ordered, in the direction from the center of the explosion) and radiation - from soft infrared to hard short-wave quanta.

In a chemical explosion, everything is relatively simple. An energetically beneficial reaction occurs when certain substances interact with each other. Only the upper electronic layers of some atoms participate in the reaction, and the interaction does not go deeper. It is easy to guess that there is much more hidden energy in any substance. But whatever the conditions of the experiment, no matter how successful the reagents we select, no matter how we check the proportions, chemistry will not let us go deeper into the atom. A chemical explosion is a primitive phenomenon, ineffective and, from the point of view of physics, indecently weak.

Nuclear chain reaction allows you to dig a little deeper, bringing into play not only electrons, but also nuclei. This sounds truly significant, perhaps, only for a physicist, but for the rest I will give a simple analogy. Imagine a giant weight with electrified dust particles fluttering around it at a distance of several kilometers. This is an atom, the “weight” is the nucleus, and the “dust particles” are electrons. Whatever you do with these specks of dust, they will not provide even a hundredth of the energy that can be obtained from a heavy weight. Especially if, for some reason, it splits, and massive fragments scatter in different directions at great speed.

A nuclear explosion involves the binding potential of the heavy particles that make up the nucleus. But this is far from the limit: there is much more hidden energy in matter. And the name of this energy is mass. Again, this sounds a little unusual for a non-physicist, but mass is energy, only extremely concentrated. Each particle: electron, proton, neutron - all these are tiny clumps of incredibly dense energy, which for the time being remain at rest. You probably know the formula E=mc2, which is so loved by joke writers, wall newspaper editors and school classroom decorators. This is exactly what it is about, and it is what posits mass as nothing more than a form of energy. And it also gives the answer to the question of how much energy can be obtained from a substance to the maximum.

The process of complete transition of mass, that is, bound energy, into free energy is called annihilation. From the Latin root “nihil” it is easy to guess its essence - this is the transformation into “nothing”, or rather, into radiation. For clarity, here are some numbers.

Explosion TNT equivalent Energy (J)

F-1 grenade 60 grams 2.50*105

Bomb dropped on Hiroshima 16 kilotons 6.70*1013

Annihilation of one gram of matter 21.5 kilotons 8.99*1013

One gram of any matter (only mass is important) upon annihilation will give more energy than a small nuclear bomb. Compared to such returns, the exercises of physicists on nuclear fission, and even more so the experiments of chemists with active reagents, seem ridiculous.

For annihilation, appropriate conditions are needed, namely, contact of matter with antimatter. And, unlike “red mercury” or the “philosopher’s stone,” antimatter is more than real - for the particles known to us, similar antiparticles exist and have been studied, and experiments on the annihilation of “electron + positron” pairs have been repeatedly carried out in practice. But in order to create an annihilation weapon, it is necessary to collect together a certain significant volume of antiparticles, and also to limit them from contact with any matter up to, in fact, combat use. This, pah-pah, is still a distant prospect.

Mass defect

The last question that remains to be understood regarding the mechanics of an explosion is where the energy comes from: the same one that is released during the chain reaction? Here again there was some mass involved. Or rather, without its “defect”.

Until the last century, scientists believed that mass is conserved under any conditions, and they were right in their own way. So we lowered the metal into the acid - it began to bubble in the retort and gas bubbles rushed upward through the thickness of the liquid. But if you weigh the reagents before and after the reaction, not forgetting the gas released, the mass converges. And this will always be the case as long as we operate with kilograms, meters and chemical reactions.

But as soon as we delve deeper into the field of microparticles, mass also presents a surprise. It turns out that the mass of an atom may not be exactly equal to the sum of the masses of the particles that make it up. When a heavy nucleus (for example, uranium) is divided into parts, the “fragments” weigh less in total than the nucleus before fission. The “difference,” also called the mass defect, is responsible for the binding energies within the nucleus. And it is this difference that goes into heat and radiation during the explosion, all according to the same simple formula: E=mc2.

This is interesting: it so happens that it is energetically advantageous to divide heavy nuclei, and to combine light ones. The first mechanism works in a uranium or plutonium bomb, the second in a hydrogen bomb. But you can’t make a bomb out of iron, no matter how hard you try: it’s right in the middle of this line.

Nuclear bomb

Following the historical sequence, let's first consider nuclear bombs and carry out our little “Manhattan Project”. I will not bore you with boring methods of isotope separation and mathematical calculations of the theory of fission chain reaction. You and I have uranium, plutonium, other materials, assembly instructions and the necessary amount of scientific curiosity.

All isotopes of uranium are unstable to one degree or another. But uranium-235 is in a special position. During the spontaneous decay of the uranium-235 nucleus (also called alpha decay), two fragments (nuclei of other, much lighter elements) and several neutrons (usually 2-3) are formed. If the neutron formed during the decay hits the nucleus of another uranium atom, there will be an ordinary elastic collision, the neutron will bounce off and continue its search for adventure. But after some time it will waste energy (perfectly elastic collisions occur only among spherical horses in a vacuum), and the next nucleus will turn out to be a trap - the neutron will be absorbed by it. By the way, physicists call such a neutron thermal.

Look at the list of known isotopes of uranium. Among them there is no isotope with atomic mass 236. Do you know why? Such a nucleus lives for fractions of microseconds and then decays, releasing a huge amount of energy. This is called forced decay. It is somehow awkward to even call an isotope with such a lifetime an isotope.

The energy released during the decay of the uranium-235 nucleus is the kinetic energy of fragments and neutrons. If you calculate the total mass of the decay products of the uranium nucleus, and then compare it with the mass of the original nucleus, it turns out that these masses do not coincide - the original nucleus was larger. This phenomenon is called a mass defect, and its explanation is contained in the formula E0=mс2. The kinetic energy of the fragments divided by the square of the speed of light will be exactly equal to the mass difference. The fragments are decelerated in the crystal lattice of uranium, generating X-ray radiation, and the neutrons, having traveled, are absorbed by other uranium nuclei or leave the uranium casting, where all events take place.

If the uranium casting is small, then most of the neutrons will leave it without having time to slow down. But if each act of forced decay causes at least one more similar act due to the emitted neutron, this is already a self-sustaining chain reaction of fission.

Accordingly, if you increase the size of the casting, an increasing number of neutrons will cause acts of forced fission. And at some point the chain reaction will become uncontrollable. But this is far from a nuclear explosion. Just a very “dirty” thermal explosion that will release large number very active and poisonous isotopes.

A completely logical question is: how much uranium-235 is needed for the fission chain reaction to become an avalanche? It's actually not that simple. The properties of the fissile material and the volume-to-surface ratio play a role here. Imagine a ton of uranium-235 (I’ll make a reservation right away - this is a lot), which exists in the form of a thin and very long wire. Yes, a neutron flying along it, of course, will cause an act of forced decay. But the fraction of neutrons flying along the wire will be so small that it is simply ridiculous to talk about a self-sustaining chain reaction.

Therefore, we agreed to calculate the critical mass for a spherical casting. For pure uranium-235 critical mass is 50 kg (this is a ball with a radius of 9 cm). You understand that such a ball will not last long, however, like those who cast it.

If a ball of smaller mass is surrounded by a neutron reflector (beryllium is perfect for it), and a neutron moderator material (water, heavy water, graphite, the same beryllium) is introduced into the ball, then the critical mass will become much smaller. By using the most effective reflectors and neutron moderators, the critical mass can be increased to 250 grams. This, for example, can be achieved by placing a saturated solution of uranium-235 salt in heavy water in a spherical beryllium container.

Critical mass exists not only for uranium-235. There are also a number of isotopes capable of fission chain reactions. The main condition is that the decay products of a nucleus must cause acts of decay of other nuclei.

So, we have two hemispherical uranium castings weighing 40 kg each. As long as they remain at a respectful distance from each other, everything will be calm. What if you start moving them slowly? Contrary to popular belief, nothing mushroom-like will happen. It’s just that the pieces will begin to heat up as they get closer, and then, if you don’t come to your senses in time, they will become red-hot. In the end, they will simply melt and spread, and everyone who moved the castings will die from neutron irradiation. And those who watched this with interest will glue their fins together.

What if it's faster? They will melt faster. Even faster? They will melt even faster. Cool? Even if you put it in liquid helium, it won’t do any good. What if you shoot one piece at another? ABOUT! The moment of truth. We just came up with a uranium cannon design. However, we have nothing particularly to be proud of; this scheme is the simplest and most artless of all possible. Yes, and the hemispheres will have to be abandoned. They, as practice has shown, do not tend to stick together smoothly. The slightest distortion - and you get a very expensive “fart”, after which you will have to clean up for a long time.

It’s better to make a short, thick-walled pipe of uranium-235 with a mass of 30-40 kg, to the opening of which we will attach a high-strength steel barrel of the same caliber, charged with a cylinder of the same uranium of approximately the same mass. Let's surround the uranium target with a beryllium neutron reflector. Now, if you shoot a uranium “bullet” at a uranium “pipe”, the “pipe” will be full. That is, there will be a nuclear explosion. You just need to shoot seriously, so that the muzzle velocity of the uranium projectile is at least 1 km/s. Otherwise, there will be a fart again, but louder. The fact is that when the projectile and target approach each other, they heat up so much that they begin to intensively evaporate from the surface, slowed down by oncoming gas flows. Moreover, if the speed is insufficient, then there is a chance that the projectile simply will not reach the target, but will evaporate along the way.

Accelerating a blank weighing several tens of kilograms to such a speed, and over a distance of a couple of meters, is an extremely difficult task. That is why you will need not gunpowder, but a powerful explosive capable of creating the proper gas pressure in the barrel in a very short time. And you won’t have to clean the barrel later, don’t worry.

The Mk-I "Little Boy" bomb dropped on Hiroshima was designed exactly according to the cannon design.

There are, of course, minor details that we did not take into account in our project, but we did not sin at all against the principle itself.

So. We detonated the uranium bomb. We admired the mushroom. Now we will explode the plutonium. Just don’t drag a target, a projectile, a barrel and other rubbish here. This trick won't work with plutonium. Even if we shoot one piece into another at a speed of 5 km/s, a supercritical assembly will still not work. Plutonium-239 will have time to heat up, evaporate and ruin everything around. Its critical mass is a little more than 6 kg. You can imagine how much more active it is in terms of capturing neutrons.

Plutonium is an unusual metal. Depending on temperature, pressure and impurities, it exists in six modifications crystal lattice. There are even modifications in which it shrinks when heated. Transitions from one phase to another can occur abruptly, while the density of plutonium can change by 25%. Let's, like all normal heroes, take a detour. Let us remember that the critical mass is determined, in particular, by the ratio of volume to surface. Okay, we have a ball of subcritical mass that has a minimum surface area for a given volume. Let's say 6 kilograms. The radius of the ball is 4.5 cm. What if this ball is compressed from all sides? The density will increase in proportion to the cube of linear compression, and the surface will decrease in proportion to its square. And this is what happens: the plutonium atoms will become denser, that is, the stopping distance of the neutron will be shortened, which means the probability of its absorption will increase. But, again, it still won’t work to compress at the required speed (about 10 km/s). Dead end? But no.

At 300°C, the so-called delta phase begins - the loosest. If plutonium is doped with gallium, heated to this temperature, and then slowly cooled, the delta phase can exist at room temperature. But it won't be stable. At high pressure (on the order of tens of thousands of atmospheres), an abrupt transition to a very dense alpha phase will occur.

Let's place a plutonium ball in a large (diameter 23 cm) and heavy (120 kg) hollow ball made of uranium-238. Don't worry, it doesn't have critical mass. But it perfectly reflects fast neutrons. And they will still be useful to us. Do you think they blew it up? No matter how it is. Plutonium is a damn capricious entity. We'll have to do some more work. Let's make two hemispheres from plutonium in the delta phase. Let's form a spherical cavity in the center. And in this cavity we will place the quintessence of nuclear weapons thought - the neutron initiator. This is a small hollow beryllium ball with a diameter of 20 and a thickness of 6 mm. Inside it is another beryllium ball with a diameter of 8 mm. There are deep grooves on the inner surface of the hollow ball. The whole thing is generously nickel plated and gold plated. Polonium-210 is placed in the grooves, which actively emits alpha particles. This is such a miracle of technology. How does it work? Just a second. We still have a few things to do.

Let's surround the uranium shell with another one, made of an aluminum alloy with boron. Its thickness is about 13 cm. In total, our “matryoshka” has now grown up to half a meter thick and has gained weight from 6 to 250 kg.

Now let's make implosion “lenses”. Imagine a soccer ball. Classic, consisting of 20 hexagons and 12 pentagons. We will make such a “ball” from explosives, and each of the segments will be equipped with several electric detonators. The thickness of the segment is about half a meter. There are also a lot of subtleties in the manufacture of “lenses,” but if we describe them, there won’t be enough space for everything else. The main thing is maximum lens accuracy. The slightest mistake - and the entire assembly will be crushed by the blasting action of the explosive. Full assembly now has a diameter of about one and a half meters and a mass of 2.5 tons. The design is completed by an electrical circuit whose task is to detonate the detonators in a strictly defined sequence with microsecond accuracy.

All. Before us is a plutonium implosion circuit.

And now - the most interesting part.

During detonation, the explosive compresses the assembly, and the aluminum “pusher” prevents the decay of the blast wave from spreading inward following its front. Having passed through uranium with a counter velocity of about 12 km/s, the compression wave will compact both it and the plutonium. Plutonium at pressures in the compression zone of the order of hundreds of thousands of atmospheres (the effect of focusing the explosion front) will jump abruptly into the alpha phase. In 40 microseconds, the uranium-plutonium assembly described here will become not just supercritical, but several times greater than the critical mass.

Having reached the initiator, the compression wave will crush its entire structure into a monolith. In this case, the gold-nickel insulation will be destroyed, polonium-210 will penetrate into beryllium due to diffusion, the alpha particles emitted by it and passing through beryllium will cause a colossal flow of neutrons, triggering a fission chain reaction throughout the entire volume of plutonium, and the flow of “fast” neutrons generated the decay of plutonium will cause an explosion of uranium-238. Done, we have grown a second mushroom, no worse than the first.

An example of a plutonium implosion design is the Mk-III "Fatman" bomb dropped on Nagasaki.

All the tricks described here are needed in order to force the maximum number of plutonium atomic nuclei to react. The main task is to keep the charge in a compact state for as long as possible and prevent it from scattering into a plasma cloud, in which the chain reaction will instantly stop. Here, every microsecond gained is an increase in one or two kilotons of power.

Thermonuclear bomb

There is a common belief that a nuclear bomb is a fuse for a thermonuclear one. In principle, everything is much more complicated, but the essence is captured correctly. Weapons based on the principles of thermonuclear fusion have made it possible to achieve such an explosion power that under no circumstances can be achieved by a fission chain reaction. But the only source of energy so far that can “ignite” a thermonuclear fusion reaction is a nuclear explosion.

Remember how you and I “fed” the hydrogen nucleus with neutrons? So, if you try to connect two protons together in this way, nothing will work. The protons will not stick together due to Coulomb repulsive forces. Either they will fly apart, or beta decay will occur and one of the protons will become a neutron. But helium-3 exists. Thanks to a single neutron, which makes protons more compatible with each other.

In principle, based on the composition of the helium-3 nucleus, we can conclude that it is quite possible to assemble one helium-3 nucleus from the nuclei of protium and deuterium. Theoretically, this is true, but such a reaction can only occur in the depths of large and hot stars. Moreover, in the depths of stars, even from protons alone, helium can be collected, turning some of them into neutrons. But these are already questions of astrophysics, and the option achievable for us is to merge two deuterium nuclei or deuterium and tritium.

Nuclear fusion requires one very specific condition. This is a very high (109 K) temperature. Only with an average kinetic energy of nuclei of 100 kiloelectronvolts are they able to approach each other to a distance at which the strong interaction begins to overcome the Coulomb interaction.

A completely legitimate question - why fence this garden? The fact is that during the fusion of light nuclei, energy of the order of 20 MeV is released. Of course, with the forced fission of a uranium nucleus, this energy is 10 times greater, but there is one caveat - with the greatest tricks, a uranium charge with a power of even 1 megaton is impossible. Even for a more advanced plutonium bomb, the achievable energy output is no more than 7-8 kilotons per kilogram of plutonium (with a theoretical maximum of 18 kilotons). And don't forget that a uranium nucleus is almost 60 times heavier than two deuterium nuclei. If we consider the specific energy yield, then thermonuclear fusion is noticeably ahead.

And one more thing - for a thermonuclear charge there are no restrictions on the critical mass. He simply doesn't have it. There are, however, other restrictions, but more about them below.

In principle, starting a thermonuclear reaction as a source of neutrons is quite simple. It is much more difficult to launch it as a source of energy. Here we are faced with the so-called Lawson criterion, which determines the energy benefit of a thermonuclear reaction. If the product of the density of the reacting nuclei and the time of their retention at the fusion distance is greater than 1014 sec/cm3, the energy provided by the fusion will exceed the energy introduced into the system.

All thermonuclear programs were dedicated to achieving this criterion.

The first thermonuclear bomb design that occurred to Edward Teller was something akin to an attempt to create a plutonium bomb using a cannon design. That is, everything seems to be correct, but it does not work. The device of the “classic super” - liquid deuterium in which a plutonium bomb is immersed - was indeed classic, but far from super.

The idea of ​​exploding a nuclear charge in liquid deuterium turned out to be a dead end from the very beginning. Under such conditions, a more or less output of thermonuclear fusion energy could be achieved by detonating a nuclear charge with a power of 500 kt. And there was no need to talk about achieving Lawson’s criterion at all.

The idea of ​​surrounding a nuclear trigger charge with layers of thermonuclear fuel interspersed with uranium-238 as a heat insulator and explosion amplifier also occurred to Teller. And not only him. The first Soviet thermonuclear bombs were built precisely according to this design. The principle was quite simple: a nuclear charge heats the thermonuclear fuel to the temperature at which fusion begins, and fast neutrons generated during fusion explode layers of uranium-238. However, the limitation remained the same - at the temperature that a nuclear trigger could provide, only a mixture of cheap deuterium and incredibly expensive tritium could enter into the fusion reaction.

Teller later came up with the idea of ​​using the compound lithium-6 deuteride. This solution made it possible to abandon expensive and inconvenient cryogenic containers with liquid deuterium. In addition, as a result of irradiation with neutrons, lithium-6 was converted into helium and tritium, which entered into a fusion reaction with deuterium.

The disadvantage of this scheme was the limited power - only a limited part of the thermonuclear fuel surrounding the trigger had time to enter into the fusion reaction. The rest, no matter how much there was, went down the drain. The maximum charge power obtained using the “puff” was 720 kt (British Orange Herald bomb). Apparently, this was the “ceiling”.

We have already talked about the history of the development of the Teller-Ulam scheme. Now let's understand the technical details of this circuit, which is also called "two-stage" or "radiation compression circuit".

Our task is to heat the thermonuclear fuel and hold it in a certain volume in order to fulfill the Lawson criterion. Leaving aside American exercises with cryogenic schemes, let us take lithium-6 deuteride, already known to us, as thermonuclear fuel.

We will choose uranium-238 as the container material for the thermonuclear charge. The container is cylindrical in shape. Along the axis of the container, inside it we will place a cylindrical rod made of uranium-235, which has a subcritical mass.

Note: the neutron bomb, which was sensational in its time, is the same Teller-Ulam scheme, but without a uranium rod along the axis of the container. The point is to provide a powerful flow of fast neutrons, but to prevent the burnout of all thermonuclear fuel, which will consume neutrons.

We will fill the remaining free space of the container with lithium-6 deuteride. Let's place a container at one end of the body of the future bomb (this will be the second stage), and at the other end we will mount an ordinary plutonium charge with a power of several kilotons (the first stage). Between the nuclear and thermonuclear charges we will install a partition made of uranium-238, which will prevent premature heating of lithium-6 deuteride. Let's fill the remaining free space inside the bomb body with solid polymer. In principle, the thermonuclear bomb is ready.

When a nuclear charge is detonated, 80% of the energy is released in the form of x-rays. The speed of its spread is much higher than the speed of spread of plutonium fission fragments. After hundredths of a microsecond, the uranium screen evaporates, and X-ray radiation begins to be intensively absorbed by the uranium of the thermonuclear charge container. As a result of so-called ablation (removal of mass from the surface of a heated container), a reactive force arises that compresses the container 10 times. This effect is called radiation implosion or radiation compression. In this case, the density of thermonuclear fuel increases 1000 times. As a result of the colossal pressure of radiation implosion, the central rod of uranium-235 is also compressed, although to a lesser extent, and goes into a supercritical state. By this time, the thermonuclear unit is bombed fast neutrons nuclear explosion. After passing through lithium-6 deuteride, they slow down and are intensively absorbed by the uranium rod.

A fission chain reaction begins in the rod, quickly leading to a nuclear explosion inside the container. Since lithium-6 deuteride is subjected to ablative compression from the outside and the pressure of a nuclear explosion from the inside, its density and temperature increase even more. This moment is the beginning of the synthesis reaction. Its further maintenance is determined by how long the container will retain thermonuclear processes inside itself, preventing thermal energy from escaping outside. This is precisely what determines the achievement of the Lawson criterion. The thermonuclear fuel burns out from the cylinder axis to its edge. The temperature of the combustion front reaches 300 million Kelvin. The full development of the explosion until the thermonuclear fuel burns out and the container is destroyed takes a couple of hundred nanoseconds - twenty million times faster than it took you to read this phrase.

Reliable operation of the two-stage circuit depends on the precise assembly of the container and the prevention of premature heating.

The power of the thermonuclear charge for the Teller-Ulam circuit depends on the power of the nuclear trigger, which ensures effective compression by radiation. However, now there are multi-stage circuits in which the energy of the previous stage is used to compress the next one. An example of a three-stage scheme is the already mentioned 100-megaton “Kuzkina mother”.

To carry out a fission chain reaction, it is necessary to create a breeding medium consisting of a pure fissile substance or a fissile substance and a moderator, the composition of which ensures the development of the reaction. It should be noted that structural materials will inevitably be present in this environment. However, selecting a breeding medium with the necessary parameters does not yet provide all the conditions for a chain reaction. With a small size and, accordingly, mass of the breeding medium, most of the neutrons generated in it will fly out without having time to cause fissions, and a self-sustaining chain reaction (SCR) will not occur. The leakage of neutrons from the volume with the breeding medium leads to the same result as their absorption without fission.

As the size of the breeding medium increases, the average path length of neutrons in it increases, and, consequently, the number of collisions with nuclei with subsequent fission and the creation of new neutrons increases. To describe the behavior of the reactor over time, it was multiplication factor k eff was introduced - the ratio of the number of neutrons in the next generation to the number of neutrons in the previous one. In this interpretation, as the size of the medium increases, keff increases from zero with a zero probability of fission to values ​​greater than unity with an avalanche-like increase in the number of neutrons in a series of generations.

With k eff equal to unity, the intensity of the fission process does not change over time - the process is self-sustaining, and such a system is called critical . At k eff< 1 скорость делений будет уменьшаться, и в этом случае систему называют subcritical . When k eff > 1 system supercritical.

The minimum mass of fissile material required for a self-sustaining fission reaction to occur is called critical mass . If the mass exceeds the critical one, then in each next generation more neutrons will be born than in the previous one, and the chain reaction will develop. The value of the critical mass depends on the properties of the fissile nuclide (235 U or 239 Pu), the composition of the breeding medium and its environment. The magnitude of the critical mass can vary from several hundred grams in experimental devices to tens of kilograms in nuclear warheads and several tons in large power reactors. Consider a nuclear reactor using natural uranium. A self-sustaining chain reaction can occur in it if the number of secondary neutrons produced during fission and capable of causing further fissions is sufficient to maintain the fission rate in the reactor at a constant level.

Some of the neutrons released during a fission reaction escape from the reaction sphere or are captured without producing fission. If conditions are created under which the rate of loss of neutrons is greater than the rate of release of new neutrons during fission, then the chain reaction under these conditions will cease to be self-sustaining, that is, it will stop. This will release some energy, but it will not be enough, and the rate of release of new neutrons will be too low to cause an effective explosion. Therefore, to carry out a nuclear explosion, it is necessary to create conditions under which the loss of neutrons would be minimal. In this regard, neutrons are especially important, they are emitted from the mass of fissile matter and do not take part in the fission reaction.

The emission of neutrons from the reaction sphere occurs through the outer surface of the uranium (or plutonium) mass. Consequently, the rate of loss of neutrons due to their emission from the mass of fissile material will be determined by the size of the surface of this mass. On the other hand, the fission process, as a result of which many new neutrons are released, occurs throughout the entire mass of the fissile substance, and therefore the rate of release of these neutrons depends on the size of this mass. As the volume of fissile material increases, the ratio of its surface area to mass decreases; therefore, the ratio of the number of lost (emitted) neutrons to the number of new neutrons released during the fission reaction will decrease.

This point is easier to understand if we consider the drawing on the right, which shows two spherical pieces of fissile material, one of which is larger than the other; in both cases, the fission process begins with one neutron, shown in the figure as a point in a circle. It is assumed that during each fission event three neutrons are released, that is, one neutron is captured.

If the mass of uranium or plutonium is small, that is, if the ratio of surface area to volume is large, then the number of neutrons lost as a result of emission will be so large that the creation of a nuclear fission chain reaction, and therefore the implementation of a nuclear explosion, will be impossible. But as the mass of uranium or plutonium increases, the relative loss of neutrons decreases, and there comes a point when the chain reaction can become self-sustaining. The amount of fissile material corresponding to this moment is called critical mass.

Thus, in order for a nuclear explosion to occur, the nuclear weapon must contain a sufficient amount of uranium or plutonium that exceeds the critical mass under given conditions. In reality, the critical mass depends, among other things, on the shape of the piece of fissile material, its composition and the degree of contamination by foreign impurities that can absorb neutrons without undergoing fission. By surrounding the fissile material with an appropriate shell - a neutron reflector, it is possible to reduce the loss of neutrons due to their emission, and therefore, reduce the value of the critical mass. In addition, elements with high density and good reflectivity for high-energy neutrons also provide some inertia of the fissile substance, delaying its expansion at the moment of explosion. The neutron reflector, due to its shielding effect and inertial properties, allows for more efficient use of fissile material in nuclear weapons.

CRITICAL MASS, the minimum mass of material capable of fission required to start a CHAIN ​​REACTION in an atomic bomb or atomic reactor. In an atomic bomb, the exploding material is divided into parts, each of which is less than critical... ... Scientific and technical encyclopedic dictionary

See CRITICAL MASS. Raizberg B.A., Lozovsky L.Sh., Starodubtseva E.B.. Modern economic dictionary. 2nd ed., rev. M.: INFRA M. 479 p.. 1999 ... Economic dictionary

CRITICAL MASS- the smallest (see) fissile substance (uranium 233 or 235, plutonium 239, etc.), at which a self-sustaining chain reaction of fission of atomic nuclei can arise and proceed. The value of the critical mass depends on the type of fissile substance, its... ... Big Polytechnic Encyclopedia

CRITICAL mass, the minimum mass of fissile material (nuclear fuel) that ensures the occurrence of a self-sustaining nuclear fission chain reaction. The value of the critical mass (Mcr) depends on the type of nuclear fuel and its geometric... ... Modern encyclopedia

The minimum mass of fissile material that ensures a self-sustaining nuclear fission chain reaction... Big Encyclopedic Dictionary

Critical mass is the smallest mass of fuel in which a self-sustaining chain reaction of nuclear fission can occur given a certain design and composition of the core (depends on many factors, for example: fuel composition, moderator, shape... ... Nuclear energy terms

critical mass- The smallest mass of fuel in which a self-sustaining nuclear fission chain reaction can occur given a certain design and composition of the core (depends on many factors, for example: fuel composition, moderator, core shape and... ... Technical Translator's Guide

Critical mass- CRITICAL MASS, the minimum mass of fissile material (nuclear fuel) that ensures the occurrence of a self-sustaining nuclear fission chain reaction. The value of the critical mass (Mcr) depends on the type of nuclear fuel and its geometric... ... Illustrated Encyclopedic Dictionary

The minimum amount of nuclear fuel containing fissile nuclides (233U, 235U, 239Pu, 251Cf), in which a nuclear fission chain reaction is possible (see Nuclear fission. Nuclear reactor, Nuclear explosion). K.m. depends on the size and shape... ... Physical encyclopedia

The minimum mass of fissile material that ensures a self-sustaining nuclear fission chain reaction. * * * CRITICAL MASS CRITICAL MASS, the minimum mass of fissile material that ensures the flow of self-sustaining ... Encyclopedic Dictionary