Nuclear weapons began to cause people from people from the very moment when the restoration of its creation was theoretically proved. And for more than half a century, the world lives in this fear, only its magnitude changes: from paranoia 50-60s to permanent alarm now. But how did such a situation become possible? How could the very idea of \u200b\u200bcreating such a terrible weapon could come to the human mind? After all, we know that the nuclear bomb was actually created by the hands of the greatest physicists of those times, many of them were at that time Nobel laureates or became subsequently.

The author tried to give a clear and affordable answer to these and many other questions, told about the race for the possession of nuclear weapons. The main attention is paid to the fate of individual physicists directly involved in the events under consideration.

Chapter 3 Critical Mass

In January 1939 Otto Frish finally got good news. He found out that his father, although he remained while Dakhau's concentration camp, after all, got a Swedish visa. Soon he was released and in Vienna he was able to meet with his mother Frisch. Together, they moved there, where nothing threatened to them - in Stockholm.

But even so joyful news could not save Otto from the premonition of a close big misfortune, from recently his filling. Waiting for the beginning of the war, which was no longer around the corner, immersed him and deeper into the depression puchin. Frish did not see any sense to continue those studies in Copenhagen. The feeling of insecurity grew up. When Britan Patrick Blakette and Australian Mark Oliphant arrived in the bara laboratory, Otto asked them for help.

Oliphant grew up in Adelaide. At first, he was interested in medicine and, in particular, dentistry, but at the university he became interested in physics. Having heard Ehrensta Rostford, New Zealander in origin, the impressionable student decided to do nuclear physics. In 1927, he joined the Rutherford led by a group of researchers who worked in the Cavendish laboratory in Cambridge. There, in the early 1930s, he became a direct witness to many wonderful discoveries in the field of nuclear physics. In 1934, in collaboration with Rutherford (as well as the German chemist Paul Garkov), Oliphant published an article in which the reaction of nuclear synthesis was described with the participation of heavy hydrogen - deuterium.

In 1937, Oliphant received a professorship at the University of Birmingem, becoming a dean of the Faculty of Physics. He very sadly reacted to the request of Frisch about help and soon sent him a letter in which he invited Otto to visit Birmingham in the summer of 1939 and at the place of seeing what could be done for him. The calm and confidence of Oliphant was very impressed by Frisch, who could not get out of depression, and he did not wait for another invitation. Packing two small suitcase, he left in England, "no different from other tourists."

Australian arranged Otto to the post of a junior teacher. He now worked in a rather informal setting. Oliphant read students lectures and sent to Frosha to those who have experienced difficulties with the development of a new material. Otto worked with several dozen students who asked him a huge number of questions, and so a very lively discussion was tied. Frusha really liked such a job.

In Birmingham, Frish met with another emigrant, his countryman - Rudolf Pyerles. Rudolf was born in Berlin, in the family of assimilated Jews. He studied physics in Berlin, Munich and Leipzig, where he defended in 1928 in Heisenberg. Then Payerls moved to Swiss Zurich and already there in 1932 a rockefeller scholarship was awarded. He should have learned first in Rome, in Fermi, and then in the English Cambridge - the physicist-theority of Ralph Fowler. When in 1933, Hitler came to power, Payerls was just in England. Soon he became clear that the return route to Germany was closed. Having completed the training, Rudolf went to Manchester, where he worked with Lorenis Brang, and then returned to Cambridge again, where he stayed a couple of years. In 1937, he became a professor of mathematics at the University of Birmingem.

Since September 1939, after the start of the war, the laboratory in Birmingham began to mainly engage in extremely important - and classified - studies for the military.

The work of scientists was associated with a resonant magnetron - the device necessary to generate intensive microwave radiation in terrestrial and side aircraft radar. Later, Ch. P. Snow called these devices "the most valuable scientific inventions of the British, made during the war with Hitler."

Being citizens of a hostile state, Frish and Payerls should not have known anything about these works. However, the secrecy of the project had some incomprehensible. Sometimes Oliphant asked Pyerles hypothetical questions that began with words: "If you were faced with the following problem ...". As Frish, "Oliphant know that Payerls knows, and, I think, Payerls knew that the Olyolone knows what he knows. However, none of them also showed. "

Frish worked with students not constantly, so, having enough free time, he could again do the problem of dividing nuclei. Using the laboratory in those moments when she was not busy, Otto spent several small experiments. Bor with Wheeler argued that uranium split mainly due to the isotope U 235, which has not very high stability. Frish decided to prove it with an experimental way, having received data on samples with a slightly increased content of rare isotope. To distinguish a small amount of uranium-235, he collected a small apparatus, in which the thermal diffusion method was used invented by Clusus and Diekel. Progress, however, was extremely slow.

Meanwhile, the British Chemical Society applied to Frosha with a request to write a review material for them and highlight all recent successes in the study of the atomic nucleus for them so that it was understandable and interesting to chemists. An article Otto wrote in his removable room. Without removing the coat, he was sitting, holding a machine on his knees, near the gas burner, trying to at least warm up: the temperature of that winter was lowered to -18 ° C. At night, the water was frozen in a glass.

Talking about the splitting of the nucleus, he repeated the opinion that was generally accepted at that time: if one day it would be possible to carry out a self-sustaining chain reaction, then taking into account the fact that slow neutrons should be used, the atomic bomb in which the chain reaction will occur to explode almost impossible. "At least a similar result we would have achieved if a similar amount of gunpowders were simply set on fire," he wrote in the final part. Frish did not believe in the possibility of creating an atomic bomb.

However, by completing the article, he thought. The main problem at the moment, according to Bora and Wheelera, was slow neutrons. The uranium-238 kernel has always captured rapid neutrons that had a certain "resonant" energy, or speed, for the reaction with natural uranium only slow neutrons are needed. However, their use meant that the resulting energy would accumulate very slowly. If you build a reaction to slow neutrons, then released energy will heat uranium and may melt it or even evaporate long before he can explode. As uranium heats up, less neutrons will join the reaction, and in the end it will simply fit.

The physics of the "uranium society" came to the same opinion. However, Frisha is now very interested in the answer to the question: what will happen if used fast Neutron? It was believed that uranium-235 was split by neutron of both types. However, if there are too many U 238 in a breaking uranium, then from fast secondary neutrons emitted by U 235 during decay, there will be little good: apparently, these rapid secondary neutrons will come out of the reaction due to the resonant capture of the uranium-238 core. But this obstacle is easy to get around, if you use pure or almost pure uranium-235. Frish without much difficulty gathered a small apparatus of club dikel for separating U 235. It was clear that in this way to get large volumes of pure uranium-235, for example, several tons, it is impossible. But suddenly, for the chain reaction on fast neutrons, there will be enough and much smaller number?

Chain reaction on fast neutrons using pure uranium-235 - if we assume that atomic bomb initially was some secret, now he became known to Frosh.

Otto shared his thoughts with Payerls, who at the beginning of June 1939 finalized the formula for calculating the critical mass of the material necessary to maintain the chain nuclear reaction. This formula was compiled was the French Physico-theorist Francis Perenom. For a mixture of isotopes with a high content of U 238, Pyerles used its modified formula, but since the account was conducted on tons, it was not suitable for creating weapons.

Now, Frosh was necessary to calculate a completely different order - with the participation of pure uranium-235 and not slow, and fast neutrons. The problem was that no one had yet knew what should be the share of U 235 to ensure successful participation in the reaction of fast neurons. And they did not know this scientists because it was not yet possible to obtain a sufficient amount of uranium-235 in its pure form.

In such a situation, it remained only to nominate assumptions. The results obtained by Bor and Wheeler were clearly made to understand that the kernel U 235 is easily cleaved by slow neutrons. Next, it was logical to assume that the impact of fast neutrons is no less efficient, and it is even possible that the uranium-235 core is divided with any contact with them. Subsequently, Payerls wrote so much about this hypothesis: "Apparently, from the data that Bor and Wheeler received, it was precisely such a conclusion: each neutron, which falls into the nucleus of the 235th [uranium], causes his decay." Such an assumption was extremely simplified by calculations. Now it remains only to calculate how much uranium-235 is necessary in order for it to be easily cleaned with rapid neutrons.

Scientists substantiated new numbers in the Payerls formula and were fighting on the result obtained. About the tons of uranium now and speech could not be. Critical mass, according to calculations, was everything a few kilograms. For a substance with a density, like uranium, the volume of such a quantity would not exceed the magnitude of the golf ball. According to Frisha estimates, so much u 235 can be obtained in a few weeks, using the order of one hundred thousand tubes of the Klusius dikel devices, similar to the one he collected in the Birmingham laboratory.

"We all overwhelmed, realizing that it was still possible to create an atomic bomb."

Critical mass, the minimum mass of a material capable of dividing necessary to start a chain reaction in an atomic bomb or an atomic reactor. In the atomic bomb, the exploding material is divided into parts, each of which is less critical ... ... Scientific and Technical Encyclopedic Dictionary

See Critical Mass. Rayzberg BA, Lozovsky L.Sh., Starodubtseva E.B .. Modern Economic Dictionary. 2 e ed., Act. M.: Infra M. 479 s .. 1999 ... Economic Dictionary

CRITICAL MASS - the smallest (see) dividing substance (uranium 233 or 235, plutonium 239, etc.), at which a self-sustaining chain reaction of division of atomic nuclei may occur and leak. The value of the critical mass depends on the type of the dividing substance, its ... ... Large polytechnic encyclopedia

Critical mass, the minimum mass of a felting substance (nuclear fuel), which ensures the flow of self-sustaining nuclear chain reaction of the division. The critical mass (MKR) depends on the type of nuclear fuel and its geometric ... ... Modern encyclopedia

The minimum mass of the dividing substance that ensures the flow of a self-sustaining nuclear chain reaction of fission ... Big Encyclopedic Dictionary

Critical Mass The smallest mass of fuel in which a self-sustaining chain reaction of core division can occur with a certain design and composition of the active zone (depends on many factors, for example: fuel composition, moderator, forms ... ... Terms of atomic energy

critical mass - the smallest fuel mass in which a self-sustaining chain reaction of nuclei dividing under a certain design and composition of the active zone (depends on many factors, for example: fuel composition, retarder, forms of the active zone and ... ... Technical translator directory

Critical mass - Critical mass, the minimum mass of a fusional substance (nuclear fuel), which ensures the flow of a self-sustaining nuclear chain reaction of division. The critical mass (MKR) depends on the type of nuclear fuel and its geometric ... ... Illustrated Encyclopedic Dictionary

The minimum number of nuclear fuel containing the dividing nuclides (233U, 235U, 239PU, 251CF) is possible, with a nuclear chain fission reaction (see the core division. Nuclear reactor, nuclear explosion). K. m. Depends on the size and shape ... ... Physical encyclopedia

The minimum mass of the dividing substance that ensures the flow of a self-sustaining nuclear chain fission reaction. * * * Critical mass Critical mass, the minimum mass of a fissile substance that ensures the flow of self-sustaining ... encyclopedic Dictionary

In the next anniversary of Badabum on Hiroshima and Nagasaki, I decided to save the Internet for the questions of nuclear weapons, where and how it was created little interested (I already knew) - I was more interested in how 2 pieces of plutonium do not melt and make big babes.

Looking at the engineers - they start with a seeder, and finish atomic bomb.

Nuclear physics is one of the most scandalous areas of venerable natural science. It is in this area that mankind has thrown billions of dollars, pounds, francs and rubles, as in the locomotive furnace of the fallen train. Now the train seems no longer late. A raging flame burning means and man-hours subsided. Let's try briefly figure out what is the train called "Nuclear Physics".

Isotopes and radioactivity

As is known, all the existence consists of atoms. Atoms, in turn, consist of electronic shells living in their breathtaking laws, and kernels. Classical chemistry is absolutely not interested in the core and his personal life. For her, atom is its electrons and their ability to exchange interaction. And from the kernel of chemistry, only his mass is needed to calculate the proportions of reagents. In turn, nuclear physics care deeply on electrons. It is interested in tiny (100 thousand times less than the radius of the electron orbits) of the dusting inside the atom, in which almost all of its mass is concentrated.

What do we know about the kernel? Yes, it consists of positively charged protons and non-electrical neutron charge. However, it is not quite true. The kernel is not a handful of two colors balls, as in the illustration of the school textbook. It employs very different laws under namescent interaction, transforming and protons, and neutrons into some kind of indistinguishable messenger. However, the charge of this mole is exactly equal to the total charge of protons included in it, and the mass - almost (I repeat, almost) coincides with the mass of neutrons and protons, of which the kernel consists.

By the way, the number of protons of the non-ionized atom always coincides with the number of electrons having the honor to surround it. But with neutron, it's not so easy. Actually, the neutron task is to stabilize the kernel, because without them the same name, the protons would not have fallen together together and microseconds.

Take for definiteness hydrogen. The most ordinary hydrogen. Its device to laughter is simply one proton, surrounded by one orbital electron. Hydrogen in the universe in bulk. It can be said that the universe consists mainly of hydrogen.

Now we carefully add a neutron to Proton. From the point of view of chemistry it is all the same hydrogen. But from the point of view of physics is no longer. Finding two different hydrogen, physicists were worried and immediately came up with the name of ordinary hydrogen by the capital, and hydrogen with a neutron with a proton - deuterium.

Cheer the arrogance and the rapid kernel another neutron. Now we have another hydrogen, even more hard - tritium. He, again, from the point of view of chemistry, is practically no different from two other hydrogens (well, except that the reaction now comes a little less eagerly). I immediately want to warn - no effort, threats and admonitions you will be able to add another neutron to the core of tritium. Local laws are more strict than human.

So, participants, deuterium and tritium are isotopes of hydrogen. Their atomic mass is different, and the charge is no. But it is the charge of the nucleus that the location in the periodic system of elements is determined. Therefore, called isotopes areotopes. Translated from Greek this means "occupying the same place." By the way, well-known heavy water is the same water, but with two deuterium atoms instead of time. Accordingly, superheavy water contains instead of Tritting.

Let's take a look again at our hydrogens. So ... Ftours in place, deuterium on the spot ... And this is more? Where did my tritium go and where did Helium-3 come from here? Our tritium, one of the neutrons clearly missed, decided to change the profession and became a proton. At the same time, it spawned an electron and antineutrino. The loss of tritium is, of course, is pretty, but we now know that he is unstable. I feed the neutron gift did not pass.

So, as you understand, isotopes are stable and unstable. Stable isotopes around us are fully, but unstable, thank God, practically no. That is, they are, but in such a scattered state, which is at the cost of very large labor. For example, Uranus-235, which delivered so much an Oppenheimer's nervousness, constitutes only 0.7% in natural uranium.

Half life

Everything is simple here. The half-life of an unstable isotope is called a period of time, for which exactly half of the isotope atoms will break up and turn into some other atoms. Already familiar to us tritium has a half-life of 12.32 years. This is a sufficiently short-lived isotope, although compared with Francium-223, which has a half-life of 22.3 minutes, triumph will seem to be a gray-robed aksakal.

None macroscopic external factors (pressure, temperature, humidity, the mood of the researcher, the number of allocations, the location of the stars) do not affect the half-life. Quantum mechanics are insensitive to such nonsense.

Popular explosion mechanic

The essence of any explosion is the rapid release of the energy previously located in a non-free, associated condition. The released energy dissipates, mainly moving to heat (the kinetic energy of the unordered movement of molecules), the shock wave (here is also a movement, but already ordered, in the direction of the explosion center) and radiation - from soft infrared to rigid short-wave quantum.

With a chemical explosion, everything is relatively simple. An energy-advantageous reaction occurs when some substances interact with each other. Only the upper electronic layers of certain atoms participate in the reaction, and the interaction does not go deeper. It is easy to guess that hidden energy in any substance is much more. But what would be the conditions for the experience, however, we picked up, however, we picked up, no matter how the proportions appear - the chemistry is deeper into atom. The chemical explosion is a primitive, ineffective phenomenon, and from the point of view of physics, to the indecent one.

The nuclear chain reaction allows you to dig a little deeper, including the game not only electrons, but also the kernel. This is truly weighty it sounds, perhaps, only for physics, and the rest will bring a simple analogy. Imagine a giant weight, around which electrified dust fluxes flutter at a distance of several kilometers. This is an atom, "weight" - core, and "dust" - electrons. With these dusting, they do not give a hundredth share of that energy that can be obtained from weighting weights. Especially if, due to some reasons, it will split, and massive debris at huge speed will be broken down in different directions.

The nuclear explosion involves the potential of communication of heavy particles, of which the kernel consists. But this is still not the limit: hidden energy in the substance is much larger. And the name of this energy is mass. Again, for non-physics, it sounds a bit unusual, but the mass is energy, only the extremely concentrated. Each particle: electron, proton, neutron - all this meager bunches are incredibly dense energy, until the time being staying alone. You probably know the formula E \u003d MC2, which the authors of the anecdotes, editors of the stepsheet and designers of school offices loved. She exactly about this, and it is she who postulates the mass as nothing more than one of the forms of energy. And she gives an answer to the question how much energy can be obtained from the substance to the maximum.

The process of a complete transition of mass, that is, the energy is associated, into the energy is free, called annhydration. In the Latin root of "Nihil" it is easy to guess about its essence - this is the transformation into "nothing", or rather - to radiation. For clarity - a little numbers.

Burst Trootil Equivalent Energy (J)

Grenade F-1 60 grams 2.50 * 105

Bomb dropped on Hiroshima 16 Kilotonn 6.70 * 1013

Annihilation of one gram of 21.5 kilotonne 8,99 * 1013

One gram of any matter (only mass is important) with annihilation will give more energy than a small nuclear bomb. Compared with this intavo, it seems funny and exercise of physicists over the splitting of the core, and even more so experiments of chemists with active reagents.

For annihlation, the relevant conditions are needed, namely, the contact of matter with antimatter. And, in contrast to the "red mercury" or "philosophical stone", antimatterium is more than real - similar anti-particles exist for our known particles, and the experiments on the annihilation of the "electron + positron" are repeatedly carried out in practice. But in order to create an annihilation weapon, it is necessary to collect together some weighty volume of antiparticles, as well as limit them from contact with any matter right up to, actually, combat use. This, pah-pah, still distant perspective.

Massage defect

The last question that remains to understand regarding the explosion mechanics is where the energy is taken from: the very, which is released during the chain reaction? Here again it did not cost without mass. Rather, without its "defect".

Up to last century, scientists believed that the mass remains under any conditions, and were right in their own way. So we lowered the metal into acid - in the retort it closed and through the thickness of the liquid, gas bubbles rushed to the top. But if weighing the reagents before and after the reaction, without forgetting the gas distinguished, the mass converges. And so will always be as long as we operate kilograms, meters and chemical reactions.

But it is necessary to deepen in the area of \u200b\u200bthe microparticles, as well as the mass also presents a surprise. It turns out that the mass of the atom can not exactly equal to the sum of the masses of the particles, its components. When divided into part of a heavy nucleus (for example, the same uranium) "fragments" in the amount weigh less than the kernel before division. For the "difference", also called the defect mass, corresponds to the bonding energy inside the nucleus. And it is this difference that goes into heat and radiation during an explosion, and all for the same simplicity formula: E \u003d Mc2.

It is interesting: it so happened that heavy kernels are energy to share, and the lungs - to unite. The first mechanism works in a uranium or plutonium bomb, the second - in hydrogen. And from iron a bomb does not do at all desire: it stands in this line exactly in the middle.

Nuclear bomb

Observing the historical sequence, consider first nuclear bombs and implement your small "Manhattan project". I will not tire you with boring methods for separating isotopes and mathematical calculations of the theory of chain fission reaction. We have uranium, plutonium with you, other materials, assembly instructions and the necessary proportion of scientific curiosity.

All uranium isotopes are unstable to one degree or another. But uranium-235 - in a special position. With the spontaneous decay of the uranium-235 kernel (it is also called alpha decay) two fragments are formed (the kernels of others, much easier elements) and several neutrons (usually 2-3). If the neutron formed during the decay hit the core of the other uranium atom, there will be a normal elastic collision, the neutron will bounce and continue the search for adventures. But after some time it will waste energy (the perfectly elastic collision is only in spherical horses in vacuum), and the next core will be a trap - neutron will absorb them. By the way, such a neutron is called physics.

Look at the list of well-known uranium isotopes. Among them there is no isotope with the atomic mass of 236. And you know why? Such a core lives the share of microseconds, and then disintegrates with the allocation of a huge amount of energy. This is called forced decay. Isotope with this time of life even somehow awkwardly call the isotope.

The energy elected during the decay of the uranium-235 kernel is the kinetic energy of fragments and neutrons. If you calculate the total mass of the spree of uranium core products, and then compare it with a mass of the original kernel, it turns out that these masses do not coincide - the initial kernel was larger. This phenomenon is called a defect of mass, and its explanation is laid in the formula E0 \u003d MC2. The kinetic energy of fragments, divided into the square of the speed of light, will exactly equal to the difference of the masses. Shards are inhibited in the crystal grille of uranium, giving birth to X-ray radiation, and neutrons, the concerning, are absorbed by other uranium cores or leave the uranium casting, where all events occur.

If the uranium casting is small, then most of the neutrons leave it, not having time to slow down. But if each act of forced decay causes at least one more actual act due to the emitted neutron - this is already a self-sustaining chain fission response.

Accordingly, if you increase the size of the casting, an increasing number of neutrons cause acts of forced division. And at some point, the chain reaction will become unmanageable. But this is still not a nuclear explosion. Just a very "dirty" thermal explosion, in which a large number of very active and poisonous isotopes will be distinguished.

A completely natural question - how much uranium-235 is needed so that the chain fission reaction has become avalanche-like? In fact, not everything is so simple. Here they play the role of the properties of a splitting material and the ratio of volume to the surface. Imagine a ton of uranium-235 (I will immediately make a reservation - it is a lot), which exists in the form of fine and very long wire. Yes, neutron, flying along it, of course, will cause an act of forced decay. But the share of neutrons flying along the wire will be so small that it is just ridiculous about the self-sustaining chain reaction.

Therefore, it was agreed to consider the critical mass for spherical casting. For pure uranium-235, the critical mass is 50 kg (this is a ball with a radius of 9 cm). You understand, such a ball does not exist for a long time, however, as those who cast it.

If the ball of the smaller mass surround the neutron reflector (beryllium is perfectly suitable), and in the ball introduce material - the neutron retarder (water, heavy water, graphite, the same beryllium), then the critical mass will become much smaller. Applying the most effective reflectors and neutron retarders, you can bring the critical mass to 250 grams. This, for example, can be achieved if placed in a spherical beryllium container saturated solution of uranium-235 salt in heavy water.

Critical mass exists not only for uranium-235. There are still a number of isotopes capable of a chain fission reaction. The main condition - the spree products of the nucleus should cause acts of decay of other cores.

So, we have two hemispherical uranium castings weighing 40 kg. While they are at a respectful distance from each other, everything will be calm. And if you start to slowly shift them? Contrary to popular belief, nothing molbo-shaped will happen. Just pieces as rapprochement begin to heat up, and then, if you don't get treasured in time, crack. In the end, they simply spread and spread out, and all who moved castings will give oak from irradiation with neutrons. And those who watched with interest to this, glue flippers.

And if faster? Faster is missing. Even faster? I am still faster. Cool? Yes, even in liquid helium, it will not be possible. And if you shoot one piece in another? ABOUT! Moment of truth. We just came up with a uranium cannon scheme. However, we particularly proud to be proud of us, this scheme is the simplest and unknown of all possible. Yes, and from the hemispheres will have to abandon. They, as practice showed, are not inclined to stick together with planes. The slightest break - and it will turn out a very expensive "bunch", after which you will have to clean long.

We better make a short thick-walled pipe from uranium-235 with a mass of 30-40 kg, to the hole of which, with a high-strength steel trunk of the same caliber, charged with a cylinder from the same uranium about the same mass. Surrounding the uranium target beryllium neutron reflector. Now, if the Palp Pool is a Uranium "Pipe" - there will be a complete "tube". That is, there will be a nuclear explosion. Only the palp language is necessary in serious, so that the muzzle speed of the uranium projectile was at least 1 km / s. Otherwise, will again be a "bunch", but the pogrom. The fact is that with the rapprochement of the projectile and the target, they are so warmed up that they begin to steal intensively from the surface, thoring on the oncoming gas streams. Moreover, if the speed is insufficient, that is, the chance that the projectile simply does not reach the target, but evaporates on the road.

Discalcut to such a speed of a blank mass in several tens kilograms, and on a segment of a couple of meters - the task is extremely difficult. That is why it is not a powder, but a powerful explosive capable of creating proper gas pressure in the trunk in a very short time. And then you don't have to clean the trunk, do not worry.

The MK-I "Little Boy" bomb, discarded on Hiroshima, was arranged precisely on a cannon scheme.

There are, of course, minor details that we did not take into account our project, but against the principle did not sink completely.

So. Uranium bomb We blew up. The mushroom loved. Now we will blow up plutonium. Just do not pull the target here, projectile, trunk and other trash. This room with plutonium will not pass. Even if we are a palm one piece to another at a speed of 5 km / s, it will not work out any equal to the supercritical assembly. Plutonium-239 will have time to warm up, evaporate and squeeze everything around. Its critical mass is a little more than 6 kg. You can imagine how active in the neutron capture plan.

Plutonium - metal unusual. Depending on the temperature, pressure and impurities it exists in six modifications of the crystal lattice. There are even such modifications in which it is compressed when heated. Transitions from one phase to another can be performed jumps like, while the density of plutonium may vary by 25%. Let all normal heroes, we will go around. Recall that the critical mass is determined, in particular, the ratio of the volume to the surface. Well, we have a bottom of the precritical mass having a minimum surface at a given volume. Let's say 6 kilograms. The ball radius is 4.5 cm. And if this ball is squeezed from all sides? The density increases in proportion to the linear compression cube, and the surface will decrease in proportion to its square. And this is what happens: Plutonium atoms are compacted, that is, the neutron braking path will decrease, which means that the probability of its absorption will increase. But, again, squeeze at the right speed (about 10 km / s) will not work anyway. Dead end? And here is not.

At 300 ° C, the so-called delta phase occurs - the most loose. If you degue Plutonium Gallium, heat it up to this temperature, and then slowly cool, the delta phase will be able to exist at room temperature. But it will not be stable. With a large pressure (about tens of thousands of atmospheres) there will be a jump-shaped transition to a very dense alpha phase.

Press the plutonium ball in the large (diameter 23 cm) and heavy (120 kg) hollow ball from uranium-238. Do not worry, it does not have a critical mass. But he perfectly reflects rapid neutrons. And they will be useful for us. Though, blew up? No matter how. Plutonium - damn capricious essence. We'll have to work yet. We will make two hemispheres from Plutonia in Delta phase. We form a spherical cavity in the center. And in this cavity, put the quintessence of nuclear-weapon thought - neutron initiator. This is such a small hollow beryll ball with a diameter of 20 and a thickness of 6 mm. Inside it is another ball of beryllium with a diameter of 8 mm. On the inner surface of the hollow ball - deep grooves. All this is generously nickel and covered with gold. Polonium-210 is placed in the groove, which is actively emitting alpha particles. Here is such a miracle technology. How does it work? Secrets. We still have several cases.

Surrounding the uranium shell of another one, from aluminum alloy with boron. Its thickness - about 13 cm. Total, our "Matryoshka" now stretched to half a meter and recovered from 6 to 250 kg.

Now we will produce implosion "lenses". Imagine a soccer ball. Classic, consisting of 20 hexagons and 12 pentagons. We will produce such a "ball" from explosives, and each of the segments will provide several electrodetonists. The thickness of the segment is about half a meter. In the manufacture of "lenses" there is also a mass of subtleties, but if they describe them, then the rest is not enough space. The main is the maximum accuracy of the lenses. The slightest error - and the entire assembly will crush the brisk action of explosives. Full assembly now has a diameter of about one and a half meters and a mass of 2.5 tons. The structure of the electrical circuit completes, whose task is to undermine detonators in a strictly defined sequence with an accuracy of the microsecond.

Everything. Before us is a plutonium implosion scheme.

And now - the most interesting.

When detonation, the explosive crimps the assembly, and the aluminum "pusher" does not allow the decline in the explosive wave, propagating after its front inside. Having passed through the uranium with a counter rate of about 12 km / s, the compression wave of complicate and it, and plutonium. Plutonium at pressures in the compression zone of the order of hundreds of thousands of atmospheres (effect of focusing explosive front) will take a jump in the alpha phase. For 40 microseconds described here, the uranium-plutonium assembly will become not just supercritical, but exceeding the critical mass several times.

Reaching until the initiator, the compression wave of Somnet's whole design in the monolith. At the same time, the gold-nickel insulation collapses, polonium-210 due to diffusion penetrates the beryllium emitted by them alpha particles passing through beryllium, will cause a colossal stream of neutrons, which run the chain reaction of division throughout the volume of plutonium, and the flow of "fast" neutrons born The breakup of plutonium will cause an explosion of uranium-238. Finish, we raised the second mushroom, no worse than the first.

An example of a plutonium implosion scheme is the Bomb MK-III "Fatman", discarded on Nagasaki.

All the tricks described here are needed in order to force the maximum number of atomic plutonium nuclei into the reaction. The main task is to keep the charge in a compact state as long as possible, do not give it a plasma cloud, in which the chain reaction will instantly stop. Here each won microsecond is an increase in one or two kiloton power.

Thermonuclear bomb

There is a commodity opinion that a nuclear bomb - fused for thermonuclear. In principle, everything is much more complicated, but the essence is picked up correctly. The weapon based on the principles of thermonuclear synthesis made it possible to achieve such an explosion power, which under any circumstances cannot be achieved by a chain fission response. But the only one while the source of energy allowing the "set" thermonuclear synthesis reaction is a nuclear explosion.

Remember how we "fed" with a hydrogen core with neutrons? So, if you try to combine two protons in a similar way, nothing will come out. Protons will not be held together due to the Coulomb of the repulsion forces. Either they scatter, or a beta decay will occur and one of the protons will become a neutron. But Helium-3 exists. Thanks to one-sole neutron, which makes protons more dear with each other.

In principle, on the basis of the composition of the helium nucleus-3, it can be concluded that one of the kernel of helium-3 can be quite assembled from the duration nuclei and deuterium. Theoretically, it is so, but such a reaction can only go in the depths of large and hot stars. Moreover, in the depths of stars, even from some protons can be collected helium, turning part of them into neutrons. But these are already questions of astrophysics, and the option achievable for us is to merge two deuterium or deuterium cores and tritium.

For the merger of the nuclei, one very specific condition is necessary. This is very high (109 K) temperature. Only with the average kinetic energy of the nuclei of 100 kiloelectromolts, they are able to get close to the distance, in which strong interaction begins to overcome the Coulomb.

A completely legal issue - why is this gardening? The fact is that in the synthesis of light nuclei, the energy of about 20 MeV is distinguished. Of course, with a forced division of uranium core, this energy is 10 times more, but there is one nuance - with the greatest tricks, the uranium charge of power even in 1 megaton is impossible. Even for a more advanced plutonium bomb, an achievable energy yield is not more than 7-8 kilotons with one kilogram of plutonium (with a theoretical maximum of 18 kilotonn). And do not forget that uranium core is almost 60 times heavier than two deuterium nuclei. If you consider the specific output of energy, then thermonuclear synthesis is noticeably ahead.

And yet - for thermonuclear charge there are no limitations on the critical mass. He simply does not have it. There is, however, other restrictions, but about them below.

In principle, launch the 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 Louuson criterion, which determines the energy profitability of the thermonuclear reaction. If the product of the density of the reacting cores and the time of their retention at a distance of the merge is greater than 1014 seconds / cm3, the energy given by the synthesis will exceed the energy introduced into the system.

It is the achievement of this criterion and all thermonuclear programs were devoted.

The first scheme of thermonuclear bomb, which came to the head of Edward Teller, was something akin to attempt to create a plutonium bomb on a cannon scheme. That is, everything seems to be right, but does not work. The device of the "classic supere" - liquid deuterium, in which a plutonium bomb is immersed - it was indeed classic, but not super.

The idea of \u200b\u200bthe explosion of a nuclear charge in the medium of liquid deuterium turned out to be a dead-end initially. Under such conditions, the little energy output of the energy of thermonuclear synthesis could be achieved when the nuclear charge is undermined with a capacity of 500 CT. And the achievement of the criterion of Louuson did not speak at all.

The idea to surround a nuclear charge-trigger layers of thermonuclear fuel, interspersed with uranium-238 as a heat insulator and an explosion amplifier, teller also occurred to the phone. And not only to him. The first Soviet thermonuclear bombs were built according to this scheme. The principle was simple enough: the nuclear charge warms the thermonuclear fuel to the temperature of the beginning of the synthesis, and fast neutrons born during synthesis are exploded by the Uranium-238 layers. However, the restriction remained the same at that temperature that a nuclear trigger could provide, only a mixture of cheap deuterium and incredibly expensive tritium could be entered into the reaction of the synthesis.

Later, Teller visited the idea to use the connection of deuteride lithium-6. Such a decision made it possible to abandon expensive and uncomfortable cryogenic tanks with liquid deuterium. In addition, as a result of irradiation with the neutron Li-6 turned into helium and tritium, which joined the deuterium into the synthesis reaction.

The disadvantage of this scheme turned out to be limited power - only a limited part of the thermonuclear fuel surrounding the trigger had time to the synthesis response. The rest, no matter how much it was, went to the wind. The maximum charge power obtained when using the "puffs" was 720 CT (British Orange Herald Bomb). Apparently, it was the "ceiling".

We have already spoken about the history of the development of the Teller-Ulam scheme. Now let's figure it out in the technical details of this scheme, which is also called a "two-stage" or "radiation compression scheme".

Our task is to heat thermonuclear fuel and keep it in a specific volume to perform the Louuson criterion. Leaving American Exercises with Cryogenic Schemes aside, take the deuteride lithium-6 as thermonuclear fuel.

As a container material for the thermalide charge, we choose uranium-238. Container - cylindrical shape. Along the axis of the container inside it, we have a cylindrical rod of uranium-235, having a subcritical mass.

Note: The neutron bomb in its time is the same Teller-Ulam scheme, but without a uranium rod along the container axis. The meaning is to provide a powerful flow of fast neutrons, but not allowing the burnout of the entire thermonuclear fuel to which neutrons will be spent.

The rest of the free space of the container is filled with deuteride lithium-6. Let's place a container in one of the ends of the body of the future bomb (this will be the second stage), and in the other end it is mounted by an ordinary plutonium charge of a slightly kiloton (first step). Between nuclear and thermonuclear charges, we will establish a Uranan-238 partition that prevents premature warming up of deuteride lithium-6. Fill the rest of the free space inside the bomb housing with a solid polymer. In principle, thermonuclear bomb is ready.

When undermining a nuclear charge, 80% of energy is released as X-ray radiation. The speed of its spread is much higher than the rate of propagation of fragments of Plutonium division. Through the hundredths of the microsecond, the uranium screen evaporates, and X-ray radiation begins to be intensively absorbed by uranium container of the container of thermonuclear charge. As a result of the so-called ablation (mass charges from the surface of the heated container), a reactive force arises, compressing the container 10 times. It is this effect that is called radiation implosion or refressing radiation. In this case, the density of thermonuclear fuel increases 1000 times. As a result of the colossal pressure of radiation implosion, the central rod from uranium-235 is also subjected to compression, albeit to a lesser extent, and enters the supercritical state. By this time, the thermonuclear unit is subjected to bombardment with rapid neutrons of the nuclear explosion. Having passed through the deuteride lithium-6, they slow down and intensively absorbed by the uranium rod.

The rod begins the chain fission response, quickly leading to a nuclear explosion inside the container. Since the deuteride of lithium-6 at the same time is exposed to ablative compression outside and the pressure of the nuclear explosion from the inside, its density and temperature increases even more. This moment is the beginning of the launch of the synthesis reaction. Its further maintenance is determined by how long the container will hold thermonuclear processes within itself, without giving the output of thermal energy. This is precisely the achievement of the Louuson criterion. The burnout of thermonuclear fuel comes from the axis of the cylinder to its edge. The temperature of the combustion front reaches 300 million Kelvin. The complete development of the explosion is up to the burnout of thermonuclear fuel and the destruction of the container takes a couple of hundred nanoseconds - twenty million times faster than you read this phrase.

The reliable response of the two-stage scheme depends on the exact assembly of the container and prevent its premature warming up.

The power of thermonuclear charge for the TELLER-ULAM scheme depends on the power of the nuclear trigger, which provides effective radiation compression. However, now there are also multistage schemes in which the energy of the previous step is used to compress the subsequent. An example of a three-step circuit - already mentioned 100 megaton "Kuzkina Mother".

For safe work with nuclear hazardous dividing substances, the equipment parameters must be less critical. As regulatory parameters of nuclear safety are used: the amount, concentration and volume of the nuclear hazardous dividing material; diameter of equipment having a cylindrical shape; Flat layer thickness for equipment having a plate shape. The normative parameter is set based on a valid parameter that is less critical and should not be exceeded during the operation of the equipment. In this case, it is necessary that the characteristics affect the critical parameters are in strictly defined limits. The following valid parameters are used: Number M Extra, Volume V Extra, diameter D Ext, Layer thickness T add.

Using the dependence of the critical parameters from the concentration of the nuclear-hazardly divided nuclide, the value of the critical parameter is determined, below which it is impossible at any concentration. For example, for solutions of plutonium salts and enriched uranium, critical mass, volume, infinite cylinder diameter, the thickness of an infinite flat layer have a minimum in the optimal deceleration area. For mixtures of metal enriched uranium with water, the critical mass, as for solutions, has a pronounced minimum in the field of optimal deceleration, and the critical volume, the diameter of the infinite cylinder, the thickness of the infinite flat layer with high enrichment (\u003e 35%) have minimal values \u200b\u200bin the absence of a moderator (R n / r 5 \u003d 0); For enrichment below 35%, the critical parameters of the mixture have a minimum at an optimal slowdown. Obviously, the parameters established on the basis of minimal critical parameters provide safety throughout the concentration change interval. These parameters are called safe, they are less than minimal critical parameters. The following secure parameters are used: quantity, concentration, volume, diameter, layer thickness.

When providing nuclear safety of the system based on the permissible parameter, the concentration of the defined nuclide is necessarily limited (sometimes the number of a moderator), at the same time, when using a safe parameter, no restrictions on the concentration (or by the number of retarder) is not superimposed.

2 Critical Massa

There will be no chain reaction, it depends on the result of a four-proceeds competition:

(1) Departure neutron from uranium,

(2) capture neutron uranium without division,

(3) capture neutrons impurities.

(4) capture neutron uranium with division.

If the loss of neutrons in the first three processes is less than the amount of neutrons released in the fourth, then the chain reaction occurs; Otherwise, it is impossible. It is obvious that if from the first three processes is very likely, the excess of neutrons released during division will not be able to ensure the continuation of the reaction. For example, when the probability of process (2) (uranium capture without division) is much more likely to capture with division, the chain reaction is impossible. Additional difficulty introduces isotopic natural uranium: it consists of three isotopes: 234 U, 235 U and 238 U, whose contributions 0.006, 0.7 and 99.3%, respectively. It is important that the probabilities of processes (2) and (4) are different for different isotopes and in different ways depend on neutron energy.

To assess the competition of various processes in terms of development in the substance of the chain process dividing nuclei, the concept of "critical mass" is introduced.

Critical mass- The minimum mass of the dividing substance that ensures the flow of a self-sustaining nuclear chain fission reaction. A critical mass is the less less than a period of a semi-presence of division and the higher the enrichment of the working element by the delivered isotope.

Critical mass -the minimum amount of fissile substance necessary to start a self-sustaining chain reaction of division. The coefficient of neutron reproduction in such an amount of substance is equal to one.

Critical mass- The mass of the reactor substance that is in critical condition.

Critical sizes of a nuclear reactor- the smallest dimensions of the active zone of the reactor, in which a self-sustained reaction of the division of nuclear fuel can still be carried out. Usually under the critical size take the critical volume of the active zone.

Critical volume of nuclear reactor- the volume of the active zone of the reactor in the critical state.

The relative number of neutrons that fly out of uranium can be reduced by resizing and form. In the sphere, surface effects are proportional to the square, and the bulk - cube of the radius. Departure neutrons from uranium is a surface effect depending on the value of the surface; Capture with division occurs throughout the material occupied by material, and therefore is

bulk effect. The greater the amount of uranium, the less likely that the departure of neutrons from the volume of uranium will prevail over captures with division and prevent the chain reaction. The loss of neutrons on the seizures without division is a surround effect, like the release of neutrons when capturing with division, so that the increase in dimensions does not change their relative importance.

Critical dimensions of a device containing uranium can be defined as dimensions in which the amount of neutron released in accuracy is equal to their loss due to departure and seizures that are not accompanied by division. In other words, if the dimensions are less critical, then, by definition, the chain reaction cannot develop.

Only odd isotopes can form a critical mass. Only 235 U is found in nature, and 239 PU and 233 U are artificial, they are formed in a nuclear reactor (as a result of neutron capture with nuclei 238 U

and 232 th with two subsequent β - decays).

IN natural uranium chain division reaction can not develop with any number of uranium, however, in such isotopes as235 U and 239 PU chain process is achieved relatively easily. In the presence of neutron retarder, the chain reaction goes in natural uranium.

A prerequisite for the implementation of the chain reaction is the presence of a sufficiently large amount of fissile substance, since in samples of small sizes, most neutrons flies through the sample, without hitting any nucleus. The chain reaction of the nuclear explosion occurs when reached

delated substance of some critical mass.

Let there be a piece of substance capable of division, for example, 235 U, which falls neutron. This neutron either cause division, either uselessly absorb substance, or, preproeding, will be released through the outer surface. It is important that it will be in the next step - the number of neutrons on average will decrease or decrease, i.e. We will weaken or develop a chain reaction, i.e. Will the system in subcritical or in the supercritical (explosive) state. Since the neutron departure is regulated in size (for a ball - radius), then the concept of critical size (and mass) occurs. For the development of the explosion, the size should be more critical.

The critical size of the dividing system can be estimated if the length of the neutron mileage is known in the fissile material.

Neutron, flying through the substance, occasionally faces the nucleus, he seems to see his cross-section. The size of the cross section of the kernel σ \u003d 10-24 cm2 (bar). If n is the number of nuclei in a cubic centimeter, then the combination L \u003d 1 / N Σ gives the average length of the neutron's mileage relative to the nuclear reaction. The length of the neutron is the only dimensional value that can serve as a starting point for evaluating the critrarser. In any physical theory, similarity methods are used, which, in turn, are built from the dimensionless combinations of dimensional values, the characteristics of the system and the substance. So dimensionless

the number is the ratio of the radius of a piece of felting material to the length of the mileage in it neutrons. If we assume that the dimensionless number of order of the unit, and the length of the run with the typical value n \u003d 1023, L \u003d 10 cm

(For σ \u003d 1) (usually σ is usually much higher than 1, so the critical mass is less than our assessment). Critical mass depends on the cross section of the reaction of the concrete nuclide. So, for the creation of an atomic bomb, approximately 3 kg of plutonium or 8 kg of 235 U (with an implosive diagram and in the case of pure 235 U), approximately 50 kg of weapons uranium is required (with uranium density 1,895 · 104 kg / m3 of the ball radius Such a mass is approximately 8.5 cm, which surprisingly coincides with our assessment.

R \u003d L \u003d 10 cm).

Now they will now withdraw a stricter formula for calculating the critical size of a piece of dividing material.

As is known, during the decay of the uranium core, several free neutrons are formed. Some of them leave the sample, and the part is absorbed by other nuclei, causing their division. The chain reaction occurs if the number of neutrons in the sample begins to grow avalanche-like. To determine the critical mass, you can use the neutron diffusion equation:

where C is the neutron concentration, β\u003e 0 - the rate of the reaction rate of neutron reactivity (similar to the constant radioactive decay has dimensions 1 / s, D-efficiency of the neutron diffusion,

Let the sample sharing a ball with a radius R. Then we need to find the solution of equation (1), satisfying the boundary condition: C (R, T) \u003d 0.

We will replace C \u003d ν E β T, then

The minimum value of the radius of the ball in which the chain reaction occurs is called

critical radius , and the mass of the corresponding ball -critical mass.

Substituting the value for R, we obtain a formula for calculating the critical mass:

M kr \u003d ρπ 4 4 D 2 (9) 3 β

The magnitude of the critical mass depends on the shape of the sample, the coefficient of neutron reproduction and the coefficient of neutron diffusion. Their definition is a complex experimental task, so the resulting formula is used to determine the specified coefficients, and the calculations carried out are proof of the existence of a critical mass.

The role of the sample sample is obvious: with a decrease in the size of the percentage of neutrons departing through its surface increases, so that at small (below critical!) Sample sizes, the chain reaction becomes impossible even with a favorable relationship between the processes of absorption and neutron formation processes.

For highly enriched uranium, the value of the critical mass is about 52 kg, for weapons plutonium - 11 kg. In regulatory documents on the protection of nuclear materials, critical masses are indicated: 5 kg 235 U or 2 kg of plutonium (for an implosive atomic bomb circuit). For a cannon scheme, critical masses are much larger. On the basis of these values, the intensity of protection of dividing substances from the attack of terrorists is being built.

Comment. The critical mass of the system of metallic uranium 93.5% of the enrichment (93.5% 235 U; 6.5% 238 U) is equal to 52 kg without a reflector and 8.9 kg, when the system is surrounded by a neutron reflector from beryllium oxide. The critical mass of the aqueous solution of uranium is about 5 kg.

The value of the critical mass depends on the properties of the substance (such as sections of division and radiation capture), from density, the number of impurities, the forms of the product, as well as from the environment. For example, the presence of neutron reflectors can greatly reduce the critical mass. For a particular dividing substance, the amount of material that is the critical mass may vary in a wide range and depends on the density, characteristics (type of material and thickness) of the reflector, as well as from nature and percentage of any inert diluent (such as oxygen in uranium oxide, 238 U in partially enriched 235 U or chemical impurities).

In order to compare, we will bring critical bulbs without a reflector for several types of materials with some standard density.

For comparison, we give the following examples of critical masses: 10 kg 239 PU, metal in alpha phase

(density of 19.86 g / cm3); 52 kg 94% 235 U (6% 238 U), metal (18.72 g / cm density); 110 kg UO2 (94% 235 U)

at a density in crystalline form 11 g / cm3; 35 kg of PuO2 (94% 239 PU) with density in crystalline

form 11.4 g / cm3. The smallest critical mass solutions of pure nucleides in water with a water reflector of neutrons have the smallest critical mass. For 235 U critical mass is 0.8 kg, for 239 PU - 0.5 kg, for 251 CF -

The critical mass M is associated with a critical length L: M l x, where X depends on the shape of the sample and lies in the range from 2 to 3. The dependence on the form is associated with the neutron leakage through the surface: the greater the surface, the greater the critical mass. A sample with a minimum critical mass has a shape of a ball. Table. 5. Basic estimated characteristics of pure isotopes capable of nuclear division

Let us dwell with more details on the critical parameters of the isotopes of some elements. Let's start with uranium.

As repeatedly mentioned, 235 U (Clark 0.72%) is of particular importance, since it is divided under the action of thermal neutrons (σ f \u003d 583 barn), highlighting the "thermal non-ventilation equivalent" 2 × 107 kW × h / k. Since, in addition to α-waist 235 U, it is also spontaneously divided (t 1/2 \u003d 3.5 × 1017 years), neutrons are always present in the mass of uranium, which means it is possible to create conditions for the occurrence of a self-sustaining chain reaction of the division. For metallic uranium with enrichment of 93.5% critical mass equal: 51 kg without reflector; 8.9 kg with reflector from beryllium oxide; 21.8 kg with a full water reflector. Critical parameters of homogeneous mixtures of uranium and its compounds are given in

Critical parameters of plutonium isotopes: 239 Pu: m kr \u003d 9.6 kg, 241 Pu: m kr \u003d 6.2 kg, 238 Pu: m Kr \u003d from 12 to 7.45 kg. Mixtures of isotopes are the greatest interest: 238 PU, 239 PU, 240 PU, 241 PU. The large specific energy release 238 PU leads to the oxidation of the metal in the air, so it is most likely to use it in the form of oxides. Upon receipt of 238 PU, the accompanying isotope is 239 PU. The ratio of these isotopes in the mixture determines both the value of critical parameters and their dependence when changing the retarder content. Various assessments of the critical mass for a naked metal sphere of 238 PU give values \u200b\u200bfrom 12 to 7.45 kg compared with the critical mass for 239 Pu, equal to 9.6 kg. Since the kernel 239 PU contains an odd number of neutrons, the critical mass when the water is added to the water system will decrease. The critical mass of 238 Pu when adding water increases. For a mixture of these isotopes, the total effect of adding water depends on the ratio of isotopes. With a massive content of 239 Pu, equal to 37% or less, the critical mass of the mixture of isotopes 239 Pu and 238 PU does not decrease when the water is added. In this case, the permissible amount of dioxides 239 PU-238 PU is 8 kg. With others

the ratios of dioxide 238 PU and 239 PU minimum value of the critical mass varies from 500 g for pure 239 PU to 24.6 kg for pure 238 PU.

Table. 6. The dependence of the critical mass and critical amount of uranium from enrichment of 235 U.

Note. I is a homogeneous mixture of metallic uranium and water; II - a homogeneous mixture of uranium and water dioxide; Iii - solution uranfluoride in water; IV - a solution of uranitrate in water. * Data obtained using graphic interpolation.

Another isotope with an odd number of neutrons is 241 PU. The minimum value of the critical mass for 241 PU is achieved in aqueous solutions at a concentration of 30 g / l and is 232 kg. Upon receipt of 241 PUs, 240 Pu is always accompanied from the irradiated fuel, which does not exceed it. With an equal ratio of nuclides in a mixture of isotopes, the minimum critical mass of 241 PU exceeds the critical mass of 239 PU. Therefore, in relation to the minimum critical mass of isotope 241 PU when

none of nuclear safety can be replaced with 239 PU if there are equal amounts in the mixture of isotopes

241 PU and 240 PU.

Table. 7. Minimum critical parameters of uranium with enrichment of 100% to 233 U.

We now consider the critical characteristics of isotopes americium. The presence in a mixture of isotopes 241 AM and 2433 am increases the critical mass of 242 m am. For aqueous solutions, there is a ratio of isotopes in which the system is always subcritical. With a mass content of 242 m am in a mixture of 241 am and 242 m am, there is less than 5% the system remains subcritical up to concentration of americium in solutions and mechanical mixtures of water dioxide equal to 2500 g / l. 243 am in the mixture with 242m AM also increases

the critical mass of the mixture, but to a lesser extent, since the cross section of the gripping of thermal neutrons for 243 am an order of magnitude lower than that of 241 am

Table. 8. Critical parameters of homogeneous plutonium (239 PU + 240 PU) spherical assemblies.

Table. 9. The dependence of critical masses and volume for plutonium compounds * from the isotopic composition of plutonium

* Main nuclide 94 239 PU.

Note. I is a homogeneous mixture of metal plutonium and water; II - a homogeneous mixture of plutonium and water dioxide; IIIGomogeneous mixture of oxalate plutonium and water; IV - Plutonium nitrate solution in water.

Table. 10. The dependence of the minimum critical mass of 242 m am from its content in the mixture 242 M AM and 241 AM (the critical mass is calculated for AMO2 + H2 O in spherical geometry with a water reflector):

With a small mass fraction of 245 cm, it should be borne in mind that 244 cm also has a finite critical mass in systems without retarders. Other Cure isotopes with an odd number of neutrons have a minimal critical mass several times greater than 245 cm. In the mixture of CMO2 + H2, 243 cm isotope has a minimal critical mass of about 108 g, A 247 cm - about 1170 compared to

critical mass can be assumed that 1 g of 245 cm is equivalent to 3 g 243 cm or 30 g 247 cm. The minimum critical mass of 245 cm, g, depending on the content of 245 cm in a mixture of isotopes 244 cm and 245 cm for CMO2 +

H2 about quite well described by the formula

where ξ is a mass fraction of 245 cm in a mixture of Cure Isotopes.

Critical mass depends on the cross section of the fission reaction. When creating weapons, all sorts of tricks can be reduced the critical mass required for the explosion. Thus, it is necessary to create 8 kg of uranium-235 to create an atomic bomb (with an implosive scheme and in the case of pure uranium-235; when using 90% of uranium-235 and at a trunny scheme of an atomic bomb requires at least 45 kg of weapon uranium). The critical mass can be significantly reduced, surrounding the sample of the dividing substance with a layer of material reflecting neutrons, for example, beryllium or natural uranium. The reflector returns a significant portion of neutrons departing through the surface of the sample. For example, if you use a 5 cm thick reflector, made from materials such as uranium, iron, graphite, the critical mass will be half of the critical mass of the "bare ball". Thicker reflectors reduce the critical mass. Berillium is particularly effective, providing a critical mass of 1/3 of the standard critical mass. The system on thermal neutrons has the largest critical volume and the minimum critical mass.

An important role is played by the degree of enrichment on the dividing nuclide. The natural uranium with a content of 235 U 0.7% cannot be used for the manufacture of atomic weapons, since the remaining uranium (238 U) is intensively absorbs neutrons, preventing the development of the chain process. Therefore, uranium isotopes must be divided, which represents a complex and time-consuming task. The separation has to be conducted to 235 U enrichment degrees above 95%. In general, it is necessary to get rid of impurities of elements with a high cross section of neutron capture.

Comment. In the preparation of weapons, uranium, not just get rid of unnecessary impurities, and replace them on other impurities that contribute to the chain process, for example, elements are introduced - neutron multiplers.

The level of uranium enrichment has a significant effect on the magnitude of the critical mass. For example, the critical uranium mass with enrichment 235 U 50% is 160 kg (3 times more than 94% uranium), and the critical mass of 20% of uranium is 800 kg (that is, ~ 15 times more than the critical mass 94% of uranium). Similar coefficients of dependence on the level of enrichment are applicable to uranium oxide.

The critical mass is inversely proportional to the square of the material density, M to ~ 1 / ρ 2 ,. Thus, the critical mass of metallic plutonium in the delta phase (density of 15.6 g / cm3) is 16 kg. This circumstance is taken into account when constructing a compact atomic bomb. Since the probability of neutron capture is proportional to the concentration of nuclei, an increase in the density of the sample, for example, as a result of its compression, can lead to an occurrence of a critical state in a sample. In nuclear explosive devices, the mass of a fissile substance that is in a safe subcritical state is translated into explosive supercritical using a directional explosion subjected to a strong compression charge.

Since the end of the most terrible in the history of mankind of war, just over two months have passed. And on July 16, 1945, the first nuclear bomb was tested by the American military, and thousands of residents of Japanese cities were dying in atomic pecle. With those weapons, as well as the means of delivering it to goals, continuously improved for more than half a century.

Military wanted to get at his disposal as heavy-duty ammunition, with one blow sweeping whole cities and countries and ultra-low, which fit into the portfolio. Such a device would deliver a sabotage war on an unprecedented level. With the first and the second, irresistible difficulties arose. Wine everything, the so-called critical mass. However, about everything in order.

Such an explosive kernel

To sort out the order of the nuclear devices and understand what is called a critical mass, we will return to the desk for a while. From the school course of physics, we remember a simple rule: the charges of the same name are repelled. In the same place, in high school, students describe the structure of the atomic nucleus consisting of neutrons, neutral particles and protons charged positively. But how is it possible? Positively charged particles are so close to each other, repulsion strength must be enormous.

The nature of the internal forces who hold together the protons are not known to the end, although the properties of these forces have been studied quite well. Forces act only at a very close distance. But it is worth at least a little bit of separating protons in space, as repulsion strength begin to prevail, and the kernel will fly into pieces. And the power of this spool is truly colossal. It is known that the forces of an adult man would not have enough for the hold of protons of just one single core of the lead atom.

What Rangeford was frightened

The cores of most elements of the Mendeleev table are stable. However, with an increase in atomic number, this stability decreases. Business in the size of the nuclei. Imagine the core of an uranium atom, consisting of 238 nuclides, of which 92 are protons. Yes, protons are in close contact with each other, and internal tenant forces reliably cement the whole design. But the strength of repulsion of protons, which are at opposite ends of the kernel becomes noticeable.

What did Rootford have done? He produced a bombardment of neutron atoms (an electron will not pass through an electronic shell of an atom, and a positively charged proton will not be able to approach the kernel due to the repulsive force). Neutron, falling into the core of the atom, caused its division. On the sides, two separate halves and two or three free neutron were spilled.

This disintegration, due to the huge velocities of the spilled particles, was accompanied by an emission of enormous energy. We had a rumor that Rutherford even wanted to hide his discovery, frightened his possible consequences for humanity, but it is most likely nothing more than fairy tales.

So what's the mass and why it is critical

So what? How can the flow of protons can be irradiated with a sufficient amount of radioactive metal to get a powerful explosion? And what is the critical mass? It's all about those several free electrons that fly out of the "bombable" nuclear kernel, they, in turn, also faced with other nuclei, will cause their division. The so-called will begin, however, it will be extremely difficult to run it.

Claim the scale. If you take an apple for the kernel of an atom on our table, then in order to imagine a nucleus of a neighboring atom, the same apple will have to be attributed and put on the table not even in the next room, and ... in a nearby house. Neutron will be the size of a cherry bone.

In order for the nitrogenous neutrons not to fly beyond the limits of the uranium ingot, and more than 50% of them would find themselves targets in the form of atomic nuclei, this ingot must have the corresponding dimensions. This is what is called the critical mass of uranium - the mass at which more than half of the emitted neutrons face other nuclei.

In fact, this happens in an instant. The number of split cores increases as avalanche, their fragments rush in all directions with speeds comparable at the speed of light, plowing air, water, any other environment. From their collisions with environmental molecules, the explosion area instantly heats up to millions of degrees, radiating heat that creates everything in the district of several kilometers.

The sharply heated air instantly increases in size, creating a powerful shock wave, which demoloses the building from the foundations, turns and crashes everything in its path ... Such is the picture of the atomic explosion.

How it looks in practice

The device of the atomic bomb is surprisingly simple. There are two ingots of uranium (or the other mass of each of which is slightly less critical. One of the ingots is made in the form of a cone, the other - a bowl with a cone-shaped hole. As it is not difficult to guess, when combining both halves, a ball that has a critical mass is achieved. This is the standard simplest Nuclear bomb. Two halves are connected using a typical TNT charge (the cone is shot into the ball).

But you should not think that anyone will be able to collect "on the knee". The whole focus is that Uranus, so that the bomb from it exploded should be very clean, the presence of impurities is practically zero.

Why there is no atomic bomb size with a pack of cigarettes

All for the same reason. The critical weight of the most common isotope of uranium 235 is about 45 kg. An explosion of such a number of nuclear fuel is already a catastrophe. And it is impossible to make with fewer substances - it simply will not work.

For the same reason, it did not work and create heavy-duty atomic charges from uranium or other radioactive metals. In order for the bomb to be very powerful, it was made of a dozen ingots, which, when undermining, detonating charges rushed to the center, connecting as orange slices.

But what happened in practice? If for some reason, two elements were found for thousandth fraction of a second earlier than the rest, the critical mass was achieved faster than the "subsidiary" the remaining, the explosion occurred not the power to which the designers were calculated. The problem of heavy duty nuclear ammunition was solved only with the advent of thermonuclear weapons. But this is a little different story.

But how is a peaceful atom

Nuclear power plant - essentially the same nuclear bomb. Only this "bomb" will beweed (fuel elements) made from uranium, are at some distance from each other, which does not prevent them from exchange neutron "blows".

Twisters are made in the form of rods, between which the control rods are made of the material well absorbing neutrons. The principle of operation is simple:

• regulating (absorbing) rods are entered into the space between the rods of uranium - the reaction slows down or stops at all;
• adjusting rods are outlined from the zone - radioactive elements are actively exchanged by neutrons, the nuclear reaction proceeds more intense.

Indeed, the same atomic bomb is obtained, in which the critical mass is achieved so smoothly and is regulated so clearly that it does not lead to an explosion, but only to heat the coolant.

Although, unfortunately, as practice shows, the human genius is not always able to curb this huge and destructive energy - the energy of the decay of the atomic nucleus.