Critical mass of uranium 238. Electroplating, chrome plating
Manual for citizens "Caution! Radiation"
Fission of atomic nuclei
The fission of atomic nuclei is a spontaneous, or under the influence of neutrons, splitting of the atomic nucleus into 2 approximately equal parts, into two “fragments”.
The fragments are two radioactive isotopes of the elements in the central part of D.I. Mendeleev’s table, approximately from copper to the middle of the lanthanide elements (samarium, europium).
During fission, 2-3 extra neutrons are emitted and excess energy is released in the form of gamma quanta, much greater than during radioactive decay. If for one act of radioactive decay there is usually one gamma ray, then for 1 act of fission there are 8–10 gamma quanta! In addition, flying fragments have high kinetic energy (speed), which turns into thermal energy.
Emitted neutrons can cause the fission of two or three similar nuclei if they are nearby and if the neutrons hit them.
Thus, it becomes possible to carry out a branching, accelerating chain reaction of fission of atomic nuclei with the release of a huge amount of energy.
If the chain reaction is kept under control, its development is controlled, it is not allowed to accelerate and the released energy (heat) is constantly removed, then this energy (“nuclear energy”) can be used either for heating or to generate electricity. This is done in nuclear reactors and nuclear power plants.
If the chain reaction is allowed to develop uncontrollably, an atomic (nuclear) explosion will occur. These are already nuclear weapons.
There is only one chemical element in nature - uranium, which has only one fissile isotope - uranium-235. This weapons grade uranium. And this isotope in natural uranium is 0.7%, that is, only 7 kg per ton! The remaining 99.3% (993 kg per ton) is a non-fissile isotope - uranium-238. There is, however, one more isotope - uranium-234, but it is only 0.006% (60 grams per ton).
But in a conventional uranium nuclear reactor, from non-fissile (“non-weapon-grade”) uranium-238, under the influence of neutrons (neutron activation!), a new isotope of uranium is formed - uranium-239, and from it (by double beta minus decay) a new, artificial, not The naturally occurring element plutonium. In this case, a fissile isotope of plutonium is immediately formed - plutonium-239. This weapons-grade plutonium.
The fission of atomic nuclei is the essence, the basis of atomic weapons and nuclear energy.
The critical mass is the amount of a weapons-grade isotope at which neutrons released during spontaneous fission of nuclei do not fly out, but enter neighboring nuclei and cause their artificial fission.
The critical mass of metallic uranium-235 is 52 kg. This is a ball with a diameter of 18 cm.
The critical mass of metallic plutonium-239 is 11 kg (and according to some publications - 9 and even 6 kg). This is a ball with a diameter of about 9-10 cm.
Thus, humanity now has two fissile, weapons-grade isotopes: uranium-235 and plutonium-239. The only difference between them is that uranium, firstly, is more suitable for use in nuclear energy: it allows you to control its chain reaction, and secondly, it is less effective for carrying out an uncontrolled chain reaction - an atomic explosion: it has a lower speed spontaneous fission of nuclei and a greater critical mass. Weapons-grade plutonium, on the contrary, is more suitable for nuclear weapons: it has a high rate of spontaneous nuclear fission and a much lower critical mass. Plutonium-239 does not allow one to reliably control its chain reaction and therefore has not yet found widespread use in nuclear energy or in nuclear reactors.
That is why all the problems with weapons-grade uranium were solved in a matter of years, and attempts to use plutonium in nuclear energy continue to this day - for more than 60 years.
Thus, two years after the discovery of uranium nuclear fission, the world's first uranium nuclear reactor was launched (December 1942, Enrico Fermi, USA), and two and a half years later (in 1945) the Americans detonated the first uranium bomb.
And with plutonium... The first plutonium bomb was detonated in 1945, that is, about four years after its discovery as a chemical element and the discovery of its fission. Moreover, for this it was necessary to first build a uranium nuclear reactor, produce plutonium in this reactor from uranium-238, then isolate it from irradiated uranium, study its properties well, and make a bomb. Developed, allocated, manufactured. But talk about the possibility of using plutonium as nuclear fuel in plutonium nuclear reactors has remained talk, and has remained so for more than 60 years.
The fission process can be characterized by a "half-life".
Half-division periods were first assessed by K. A. Petrzhak and G. I. Flerov in 1940.
For both uranium and plutonium they are extremely large. So, according to various estimates, the half-life of uranium-235 is approximately 10^17 (or 10^18 years (Physical Encyclopedic Dictionary); according to other data - 1.8·10^17 years. And for plutonium-239 (according to that same dictionary) is significantly less - approximately 10^15.5 years; according to other data - 4·10^15 years.
For comparison, recall the half-lives (T 1/2). So for U-235 it is “only” 7.038·10^8 years, and for Pu-239 it is even less - 2.4·10^4 years
In general, the nuclei of many heavy atoms, starting with uranium, can fission. But we are talking about two main ones, which have been of great practical importance for more than 60 years. Others are rather of purely scientific interest.
Where do radionuclides come from?
Radionuclides are obtained from three sources (in three ways).
The first source is nature. This natural radionuclides, which have survived, have survived to our time from the moment of their formation (possibly from the time of the formation of the solar system or the Universe), since they have long half-lives, which means a long lifetime. Naturally, there are much fewer of them left than there were at the beginning. They are extracted from natural raw materials.
The second and third sources are artificial.
Artificial radionuclides are formed in two ways.
The first - radionuclides of fragmentation origin, which are formed as a result of fission of atomic nuclei. These are "fission fragments". Naturally, the bulk of them are formed in nuclear reactors for various purposes, in which a controlled chain reaction is carried out, as well as during testing of nuclear weapons (uncontrolled chain reaction). They are found in irradiated uranium extracted from military reactors (from "industrial reactors"), and in huge quantities in spent nuclear fuel (SNF) extracted from nuclear power plant reactors.
Previously, they were released into the natural environment during nuclear testing and the processing of irradiated uranium. Nowadays they continue to fall during the reprocessing (regeneration) of spent fuel, as well as during accidents at nuclear power plants and reactors. If necessary, they were extracted from irradiated uranium, and now from spent nuclear fuel.
The second ones are radionuclides of activation origin. They are formed from ordinary stable isotopes as a result of activation, that is, when some subatomic particle enters the nucleus of a stable atom, as a result of which the stable atom becomes radioactive. In the vast majority of cases, such a projectile particle is a neutron. Therefore, to obtain artificial radionuclides, the neutron activation method is usually used. It consists of placing a stable isotope of any chemical element in any form (metal, salt, chemical compound) into the reactor core for a certain time. And since a colossal amount of neutrons are formed in the reactor core every second, therefore everything chemical elements, which are in the active zone or near it gradually become radioactive. Those elements that are dissolved in the reactor cooling water are also activated.
A less commonly used method is stable isotope bombardment in accelerators. elementary particles protons, electrons, etc.
Radionuclides are natural - of natural origin and artificial - of fragmentation and activation origin. An insignificant amount of radionuclides of fragmentation origin has always been present in natural environment, because they are formed as a result of the spontaneous fission of uranium-235 nuclei. But there are so few of them that they cannot be detected by modern means of analysis.
The number of neutrons in the core of various types of reactors is such that about 10^14 neutrons fly through any cross section of 1 cm^2 at any point in the core in 1 second.
Measurement of ionizing radiation. Definitions
It is not always convenient or advisable to characterize only the sources of ionizing radiation (IIR) themselves and only their activity (the number of decay events). And the point is not only that activity can be measured, as a rule, only under stationary conditions in very complex installations. The main thing is that during a single act of decay of different isotopes, particles of different nature can be formed, and several particles and gamma rays can be formed simultaneously. In this case, the energy, and therefore the ionizing ability of different particles, will be different. Therefore, the main indicator for characterizing radiation sources is the assessment of their ionizing ability, that is, (ultimately) the energy that they lose when passing through a substance (medium) and which is absorbed by this substance.
When measuring ionizing radiation, the concept of dose is used, and when assessing their effect on biological objects, correction factors are used. Let's name them and give a number of definitions.
Dose, absorbed dose (from Greek - share, portion) - the energy of ionizing radiation (IR), absorbed by the irradiated substance and often calculated per unit of its mass (see "rad", "Gray"). That is, the dose is measured in units of energy that is released in a substance (absorbed by the substance) when ionizing radiation passes through it.
There are several types of doses.
Exposure dose(for X-ray and gamma radiation) - determined by air ionization. The SI unit of measurement is “coulomb per kg” (C/kg), which corresponds to the formation in 1 kg of air of such a number of ions, the total charge of which is 1 C (of each sign). The non-systemic unit of measurement is the “roentgen” (see “C/kg” and “roentgen”).
To assess the impact of AI on humans, they are used correction factors.
Until recently, when calculating the "equivalent dose" we used "radiation quality factors "(K) - correction factors that take into account the different effects on biological objects (different abilities to damage body tissues) of different radiations at the same absorbed dose. They are used when calculating the “equivalent dose”. Now these coefficients are in the Radiation Safety Standards (NRB-99 ) was called very “scientifically” - “Weighting coefficients for individual types of radiation when calculating the equivalent dose (W R radiation risk coefficient
Dose rate- dose received per unit of time (second, hour).
Background- the exposure dose rate of ionizing radiation in a given location.
Natural background- the exposure dose rate of ionizing radiation created by all natural sources of radiation (see "Background radiation").
Many of our readers associate the hydrogen bomb with an atomic one, only much more powerful. In fact, this is a fundamentally new weapon, which required disproportionately large intellectual efforts for its creation and works on fundamentally different physical principles.
The only thing that the atomic and hydrogen bombs have in common is that both release colossal energy hidden in the atomic nucleus. This can be done in two ways: to divide heavy nuclei, for example, uranium or plutonium, into lighter ones (fission reaction) or to force the lightest isotopes of hydrogen to merge (fusion reaction). As a result of both reactions, the mass of the resulting material is always less than the mass of the original atoms. But mass cannot disappear without a trace - it turns into energy according to Einstein’s famous formula E=mc 2.
To create an atomic bomb, a necessary and sufficient condition is to obtain fissile material in sufficient quantities. The work is quite labor-intensive, but low-intellectual, lying closer to the mining industry than to high science. The main resources for the creation of such weapons are spent on the construction of giant uranium mines and enrichment plants. Evidence of the simplicity of the device is the fact that less than a month passed between the production of the plutonium needed for the first bomb and the first Soviet nuclear explosion.
Let us briefly recall the operating principle of such a bomb, known from school physics courses. It is based on the property of uranium and some transuranium elements, for example, plutonium, to release more than one neutron during decay. These elements can decay either spontaneously or under the influence of other neutrons.
The released neutron can leave the radioactive material, or it can collide with another atom, causing another fission reaction. When a certain concentration of a substance (critical mass) is exceeded, the number of newborn neutrons, causing further fission of the atomic nucleus, begins to exceed the number of decaying nuclei. The number of decaying atoms begins to grow like an avalanche, giving birth to new neutrons, that is, a chain reaction occurs. For uranium-235, the critical mass is about 50 kg, for plutonium-239 - 5.6 kg. That is, a ball of plutonium weighing slightly less than 5.6 kg is just a warm piece of metal, and a mass of slightly more lasts only a few nanoseconds.
The actual operation of the bomb is simple: we take two hemispheres of uranium or plutonium, each slightly less than the critical mass, place them at a distance of 45 cm, cover them with explosives and detonate. The uranium or plutonium is sintered into a piece of supercritical mass, and a nuclear reaction begins. All. There is another way to start a nuclear reaction - to compress a piece of plutonium with a powerful explosion: the distance between the atoms will decrease, and the reaction will begin at a lower critical mass. All modern atomic detonators operate on this principle.
The problems with the atomic bomb begin from the moment we want to increase the power of the explosion. Simply increasing the fissile material is not enough - as soon as its mass reaches a critical mass, it detonates. Various ingenious schemes were invented, for example, to make a bomb not from two parts, but from many, which made the bomb begin to resemble a gutted orange, and then assemble it into one piece with one explosion, but still, with a power of over 100 kilotons, the problems became insurmountable.
But fuel for thermonuclear fusion does not have a critical mass. Here the Sun, filled with thermonuclear fuel, hangs overhead, a thermonuclear reaction has been going on inside it for a billion years - and nothing explodes. In addition, during the synthesis reaction of, for example, deuterium and tritium (heavy and superheavy isotope of hydrogen), energy is released 4.2 times more than during the combustion of the same mass of uranium-235.
Making the atomic bomb was an experimental rather than a theoretical process. The creation of a hydrogen bomb required the emergence of completely new physical disciplines: the physics of high-temperature plasma and ultra-high pressures. Before starting to construct a bomb, it was necessary to thoroughly understand the nature of the phenomena that occur only in the core of stars. No experiments could help here - the researchers’ tools were only theoretical physics and higher mathematics. It is no coincidence that a gigantic role in the development of thermonuclear weapons belongs to mathematicians: Ulam, Tikhonov, Samarsky, etc.
Classic super
By the end of 1945, Edward Teller proposed the first hydrogen bomb design, called the "classic super". To create the monstrous pressure and temperature necessary to start the fusion reaction, it was supposed to use a conventional atomic bomb. The “classic super” itself was a long cylinder filled with deuterium. An intermediate “ignition” chamber with a deuterium-tritium mixture was also provided - the synthesis reaction of deuterium and tritium begins at a lower pressure. By analogy with a fire, deuterium was supposed to play the role of firewood, a mixture of deuterium and tritium - a glass of gasoline, and an atomic bomb - a match. This scheme was called a “pipe” - a kind of cigar with an atomic lighter at one end. Soviet physicists began to develop the hydrogen bomb using the same scheme.
However, mathematician Stanislav Ulam, using an ordinary slide rule, proved to Teller that the occurrence of a fusion reaction of pure deuterium in a “super” is hardly possible, and the mixture would require such an amount of tritium that to produce it it would be necessary to practically freeze the production of weapons-grade plutonium in the United States.
Puff with sugar
In mid-1946, Teller proposed another hydrogen bomb design - an “alarm clock”. It consisted of alternating spherical layers of uranium, deuterium and tritium. During the nuclear explosion of the central charge of plutonium, the necessary pressure and temperature were created for the start of a thermonuclear reaction in other layers of the bomb. However, the “alarm clock” required a high-power atomic initiator, and the United States (as well as the USSR) had problems producing weapons-grade uranium and plutonium.
In the fall of 1948, Andrei Sakharov came to a similar scheme. In the Soviet Union, the design was called “sloyka”. For the USSR, which did not have time to produce weapons-grade uranium-235 and plutonium-239 in sufficient quantities, Sakharov’s puff paste was a panacea. And that's why.
In a conventional atomic bomb, natural uranium-238 is not only useless (the neutron energy during decay is not enough to initiate fission), but also harmful because it eagerly absorbs secondary neutrons, slowing down the chain reaction. Therefore, 90% of weapons-grade uranium consists of the isotope uranium-235. However, neutrons resulting from thermonuclear fusion are 10 times more energetic than fission neutrons, and natural uranium-238 irradiated with such neutrons begins to fission excellently. The new bomb made it possible to use uranium-238, which had previously been considered a waste product, as an explosive.
The highlight of Sakharov’s “puff pastry” was also the use of a white light crystalline substance - lithium deuteride 6 LiD - instead of acutely deficient tritium.
As mentioned above, a mixture of deuterium and tritium ignites much more easily than pure deuterium. However, this is where the advantages of tritium end, and only disadvantages remain: in the normal state, tritium is a gas, which causes difficulties with storage; tritium is radioactive and decays into stable helium-3, which actively consumes much-needed fast neutrons, limiting the bomb's shelf life to a few months.
Non-radioactive lithium deutride, when irradiated with slow fission neutrons - the consequences of an explosion of an atomic fuse - turns into tritium. Thus, the radiation from the primary atomic explosion instantly produces a sufficient amount of tritium for a further thermonuclear reaction, and deuterium is initially present in lithium deutride.
It was just such a bomb, RDS-6s, that was successfully tested on August 12, 1953 at the tower of the Semipalatinsk test site. The power of the explosion was 400 kilotons, and there is still debate over whether it was a real thermonuclear explosion or a super-powerful atomic one. After all, the thermonuclear fusion reaction in Sakharov’s puff paste accounted for no more than 20% of the total charge power. The main contribution to the explosion was made by the decay reaction of uranium-238 irradiated with fast neutrons, thanks to which the RDS-6s ushered in the era of the so-called “dirty” bombs.
The fact is that the main radioactive contamination comes from decay products (in particular, strontium-90 and cesium-137). Essentially, Sakharov’s “puff pastry” was a giant atomic bomb, only slightly enhanced by a thermonuclear reaction. It is no coincidence that just one “puff pastry” explosion produced 82% of strontium-90 and 75% of cesium-137, which entered the atmosphere over the entire history of the Semipalatinsk test site.
American bombs
However, it was the Americans who were the first to detonate the hydrogen bomb. On November 1, 1952, the Mike thermonuclear device, with a yield of 10 megatons, was successfully tested at Elugelab Atoll in the Pacific Ocean. It would be hard to call a 74-ton American device a bomb. “Mike” was a bulky device the size of a two-story house, filled with liquid deuterium at a temperature close to absolute zero (Sakharov’s “puff pastry” was a completely transportable product). However, the highlight of “Mike” was not its size, but the ingenious principle of compressing thermonuclear explosives.
Let us recall that the main idea of a hydrogen bomb is to create conditions for fusion (ultra-high pressure and temperature) through a nuclear explosion. In the “puff” scheme, the nuclear charge is located in the center, and therefore it does not so much compress the deuterium as scatter it outward - increasing the amount of thermonuclear explosive does not lead to an increase in power - it simply does not have time to detonate. This is precisely what limits the maximum power of this scheme - the most powerful “puff” in the world, the Orange Herald, blown up by the British on May 31, 1957, yielded only 720 kilotons.
It would be ideal if we could make the atomic fuse explode inside, compressing the thermonuclear explosive. But how to do that? Edward Teller put forward a brilliant idea: to compress thermonuclear fuel not with mechanical energy and neutron flux, but with the radiation of the primary atomic fuse.
In Teller's new design, the initiating atomic unit was separated from the thermonuclear unit. When the atomic charge was triggered, X-ray radiation preceded the shock wave and spread along the walls of the cylindrical body, evaporating and turning the polyethylene inner lining of the bomb body into plasma. The plasma, in turn, re-emited softer X-rays, which were absorbed by the outer layers of the inner cylinder of uranium-238 - the “pusher”. The layers began to evaporate explosively (this phenomenon is called ablation). Hot uranium plasma can be compared to the jets of a super-powerful rocket engine, the thrust of which is directed into the cylinder with deuterium. The uranium cylinder collapsed, the pressure and temperature of the deuterium reached a critical level. The same pressure compressed the central plutonium tube to a critical mass, and it detonated. The explosion of the plutonium fuse pressed on the deuterium from the inside, further compressing and heating the thermonuclear explosive, which detonated. An intense stream of neutrons splits the uranium-238 nuclei in the “pusher”, causing a secondary decay reaction. All this managed to happen before the moment when the blast wave from the primary nuclear explosion reached the thermonuclear unit. The calculation of all these events, occurring in billionths of a second, required the brainpower of the strongest mathematicians on the planet. The creators of “Mike” experienced not horror from the 10-megaton explosion, but indescribable delight - they managed not only to understand the processes that in the real world occur only in the cores of stars, but also to experimentally test their theories by setting up their own small star on Earth.
Bravo
Having surpassed the Russians in the beauty of the design, the Americans were unable to make their device compact: they used liquid supercooled deuterium instead of Sakharov’s powdered lithium deuteride. In Los Alamos they reacted to Sakharov’s “puff pastry” with a bit of envy: “instead of a huge cow with a bucket of raw milk, the Russians use a bag of powdered milk.” However, both sides failed to hide secrets from each other. On March 1, 1954, near the Bikini Atoll, the Americans tested a 15-megaton bomb "Bravo" using lithium deutride, and on November 22, 1955, the first Soviet two-stage thermal bomb exploded over the Semipalatinsk test site. nuclear bomb RDS-37 with a capacity of 1.7 megatons, demolishing almost half of the test site. Since then, the design of the thermonuclear bomb has undergone minor changes (for example, a uranium shield appeared between the initiating bomb and the main charge) and has become canonical. And there are no more large-scale mysteries of nature left in the world that could be solved with such a spectacular experiment. Perhaps the birth of a supernova.
A little theory There are 4 reactions in a thermonuclear bomb, and they proceed very quickly. The first two reactions serve as a source of material for the third and fourth, which at the temperatures of a thermonuclear explosion proceed 30-100 times faster and give a greater energy yield. Therefore, the resulting helium-3 and tritium are immediately consumed. The nuclei of atoms are positively charged and therefore repel each other. In order for them to react, they need to be pushed head-on, overcoming the electrical repulsion. This is only possible if they move at high speed. The speed of atoms is directly related to the temperature, which should reach 50 million degrees! But heating deuterium to such a temperature is not enough; it must also be kept from scattering by the monstrous pressure of about a billion atmospheres! In nature, such temperatures at such densities are found only in the core of stars. |
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
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- Critical mass, Natalya Veselova, In the book by Natalya Veselova, member of the Russian Interregional Union of Writers, full member of the Academy of Russian Literature and Fine Arts. G.R. Derzhavin, selected stories included... Category:
A mysterious device capable of releasing gigajoules of energy over an indescribably short period of time is surrounded by sinister romance. Needless to say, all over the world, work on nuclear weapons was deeply classified, and the bomb itself was overgrown with a mass of legends and myths. Let's try to deal with them in order.
Andrey Suvorov
Nothing sparks interest like the atomic bomb
August 1945. Ernest Orlando Lawrence at the atomic bomb laboratory
1954 Eight years after the explosion at Bikini Atoll, Japanese scientists discovered high levels of radiation in fish caught in local waters
Critical mass
Everyone has heard that there is a certain critical mass that needs to be reached in order for a nuclear chain reaction to begin. But for a real nuclear explosion to occur, critical mass alone is not enough - the reaction will stop almost instantly, before noticeable energy has time to be released. For a full-scale explosion of several kilotons or tens of kilotons, two or three, or better yet four or five, critical masses must be collected simultaneously.
It seems obvious that you need to make two or more parts from uranium or plutonium and connect them at the required moment. To be fair, it must be said that physicists thought the same thing when they took on the construction of a nuclear bomb. But reality made its own adjustments.
The point is that if we had very pure uranium-235 or plutonium-239, then we could do this, but scientists had to deal with real metals. By enriching natural uranium, you can make a mixture containing 90% uranium-235 and 10% uranium-238; attempts to get rid of the remainder of uranium-238 lead to a very rapid rise in price of this material (it is called highly enriched uranium). Plutonium-239, which is produced in a nuclear reactor from uranium-238 by fission of uranium-235, necessarily contains an admixture of plutonium-240.
The isotopes uranium235 and plutonium239 are called even-odd because the nuclei of their atoms contain an even number of protons (92 for uranium and 94 for plutonium) and an odd number of neutrons (143 and 145, respectively). All even-odd nuclei of heavy elements have a common property: they rarely fission spontaneously (scientists say: “spontaneously”), but easily fission when a neutron hits the nucleus.
Uranium-238 and plutonium-240 are even-even. They, on the contrary, practically do not fission with neutrons of low and moderate energies, which fly out from fissile nuclei, but they fission spontaneously hundreds or tens of thousands of times more often, forming a neutron background. This background makes it very difficult to create nuclear weapons because it causes the reaction to start prematurely before the two parts of the charge meet. Because of this, in a device prepared for explosion, parts of the critical mass must be located far enough from each other and connected at high speed.
Cannon bomb
However, the bomb dropped on Hiroshima on August 6, 1945 was made exactly according to the scheme described above. Two of its parts, the target and the bullet, were made of highly enriched uranium. The target was a cylinder with a diameter of 16 cm and a height of 16 cm. In its center there was a hole with a diameter of 10 cm. The bullet was made in accordance with this hole. In total, the bomb contained 64 kg of uranium.
The target was surrounded by a shell, inner layer which was made of tungsten carbide, the outer one was made of steel. The purpose of the shell was twofold: to hold the bullet when it stuck into the target, and to reflect at least part of the neutrons escaping from the uranium back. Taking into account the neutron reflector, 64 kg was 2.3 critical masses. How did this work out, since each of the pieces was subcritical? The fact is that by removing the middle part from the cylinder, we reduce its average density and the value of the critical mass increases. Thus, the mass of this part may exceed the critical mass for a solid piece of metal. But it is impossible to increase the mass of the bullet in this way, because it must be solid.
Both the target and the bullet were assembled from pieces: the target from several low-height rings, and the bullet from six washers. The reason is simple - the uranium billets had to be small in size, because during the manufacture (casting, pressing) of the billet, the total amount of uranium should not approach the critical mass. The bullet was encased in a thin-walled stainless steel jacket, with a tungsten carbide cap similar to a target jacket.
In order to direct the bullet to the center of the target, they decided to use the barrel of a conventional 76.2 mm anti-aircraft gun. This is why this type of bomb is sometimes called a cannon-assembled bomb. The barrel was bored from the inside to 100 mm to accommodate such an unusual projectile. The barrel length was 180 cm. Ordinary smokeless gunpowder was loaded into its charging chamber, which fired a bullet at a speed of approximately 300 m/s. And the other end of the barrel was pressed into a hole in the target shell.
This design had a lot of shortcomings.
It was monstrously dangerous: once the gunpowder was loaded into the charging chamber, any accident that could ignite it would cause the bomb to explode at full power. Because of this, pyroxylin was charged in the air when the plane approached the target.
In the event of an airplane accident, uranium parts could come together without gunpowder, simply from a strong impact on the ground. To avoid this, the diameter of the bullet was a fraction of a millimeter larger than the diameter of the bore in the barrel.
If the bomb fell into water, then due to the moderation of neutrons in water, the reaction could begin even without connecting the parts. True, in this case a nuclear explosion is unlikely, but a thermal explosion would occur, with the spraying of uranium over a large area and radioactive contamination.
The length of a bomb of this design exceeded two meters, and this is virtually insurmountable. After all, a critical state was reached, and the reaction began when there was still a good half meter before the bullet stopped!
Finally, this bomb was very wasteful: less than 1% of the uranium had time to react in it!
The cannon bomb had exactly one advantage: it could not fail to work. They weren't even going to test her! But the Americans had to test the plutonium bomb: its design was too new and complex.
Plutonium soccer ball
When it turned out that even a tiny (less than 1%!) admixture of plutonium-240 makes the cannon assembly of a plutonium bomb impossible, physicists were forced to look for other ways to gain critical mass. And the key to plutonium explosives was found by the man who later became the most famous “nuclear spy” - British physicist Klaus Fuchs.
His idea, later called “implosion,” was to form a converging spherical shock wave from a diverging one, using so-called explosive lenses. This shock wave would compress the piece of plutonium so that its density doubled.
If a decrease in density causes an increase in the critical mass, then an increase in density should reduce it! This is especially true for plutonium. Plutonium is a very specific material. When a piece of plutonium is cooled from its melting point to room temperature, it undergoes four phase transitions. At the latter (about 122 degrees), its density jumps by 10%. In this case, any casting inevitably cracks. To avoid this, plutonium is doped with some trivalent metal, then the loose state becomes stable. Aluminum can be used, but in 1945 it was feared that alpha particles emitted from plutonium nuclei as they decay would knock free neutrons out of the aluminum nuclei, increasing the already noticeable neutron background, so gallium was used in the first atomic bomb.
From an alloy containing 98% plutonium-239, 0.9% plutonium-240 and 0.8% gallium, a ball was made with a diameter of only 9 cm and a weight of about 6.5 kg. In the center of the ball there was a cavity with a diameter of 2 cm, and it consisted of three parts: two halves and a cylinder with a diameter of 2 cm. This cylinder served as a plug through which an initiator could be inserted into the internal cavity - a neutron source that was triggered when the bomb exploded. All three parts had to be nickel-plated, because plutonium is very actively oxidized by air and water and is extremely dangerous if it enters the human body.
The ball was surrounded by a neutron reflector made of natural uranium238, 7 cm thick and weighing 120 kg. Uranium is a good reflector of fast neutrons, and when assembled the system was only slightly subcritical, so instead of a plutonium plug, a cadmium plug was inserted, which absorbed neutrons. The reflector also served to hold all the parts of the critical assembly during the reaction, otherwise most of the plutonium would fly apart without having time to take part in the nuclear reaction.
Next came an 11.5-centimeter layer of aluminum alloy weighing 120 kg. The purpose of the layer is the same as that of antireflection on objective lenses: to ensure that the blast wave penetrates the uranium-plutonium assembly and does not reflect from it. This reflection occurs due to the large difference in density between the explosive and uranium (approximately 1:10). In addition, in a shock wave, after the compression wave there is a rarefaction wave, the so-called Taylor effect. The aluminum layer weakened the rarefaction wave, which reduced the effect of the explosive. Aluminum had to be doped with boron, which absorbed neutrons emitted from the nuclei of aluminum atoms under the influence of alpha particles produced during the decay of uranium-238.
Finally, there were those same “explosive lenses” outside. There were 32 of them (20 hexagonal and 12 pentagonal), they formed a structure similar to a soccer ball. Each lens consisted of three parts, with the middle one made from a special “slow” explosive, and the outer and inner ones from “fast” explosives. The outer part was spherical on the outside, but inside it had a conical depression, like on a shaped charge, but its purpose was different. This cone was filled with a slow explosive, and at the interface the blast wave was refracted like an ordinary light wave. But the similarity here is very conditional. In fact, the shape of this cone is one of the real secrets of the nuclear bomb.
In the mid-40s, there were no computers in the world on which it would be possible to calculate the shape of such lenses, and most importantly, there was not even a suitable theory. Therefore, they were done exclusively by trial and error. More than a thousand explosions had to be carried out - and not just carried out, but photographed with special high-speed cameras, recording the parameters of the blast wave. When a smaller version was tested, it turned out that explosives did not scale so easily, and it was necessary to greatly correct the old results.
The accuracy of the form had to be maintained with an error of less than a millimeter, and the composition and uniformity of the explosive had to be maintained with the utmost care. Parts could only be made by casting, so not all explosives were suitable. The fast explosive was a mixture of RDX and TNT, with twice the amount of RDX. Slow - the same TNT, but with the addition of inert barium nitrate. The speed of the detonation wave in the first explosive is 7.9 km/s, and in the second - 4.9 km/s.
Detonators were mounted in the center of the outer surface of each lens. All 32 detonators had to fire simultaneously with unheard-of precision - less than 10 nanoseconds, that is, billionths of a second! Thus, the shock wave front should not have been distorted by more than 0.1 mm. The mating surfaces of the lenses had to be aligned with the same precision, but the error in their manufacturing was ten times greater! I had to tinker and spend a lot of toilet paper and tape to compensate for the inaccuracies. But the system began to bear little resemblance to the theoretical model.
It was necessary to invent new detonators: the old ones did not provide proper synchronization. They were made on the basis of wires that exploded under a powerful impulse of electric current. To trigger them, a battery of 32 high-voltage capacitors and the same number of high-speed dischargers was needed - one for each detonator. The entire system, including batteries and charger for capacitors, the first bomb weighed almost 200 kg. However, compared to the weight of the explosives, which took 2.5 tons, this was not much.
Finally, the entire structure was enclosed in a duralumin spherical body, consisting of a wide belt and two covers - upper and lower, all these parts were assembled with bolts. The design of the bomb made it possible to assemble it without a plutonium core. In order to insert the plutonium into place along with a piece of the uranium reflector, the top cover of the housing was unscrewed and one explosive lens was removed.
The war with Japan was coming to an end, and the Americans were in a hurry. But the implosion bomb had to be tested. This operation was given the code name "Trinity" ("Trinity"). Yes, the atomic bomb was supposed to demonstrate power previously available only to the gods.
Brilliant success
The test site was chosen in the state of New Mexico, in a place with the picturesque name Jornadadel Muerto (Path of Death) - the territory was part of the Alamagordo artillery range. The bomb began to be assembled on July 11, 1945. On the fourteenth of July she was lifted to the top of a specially built 30 m high tower, wires were connected to the detonators and the final stages of preparation began, involving a large amount of measuring equipment. On July 16, 1945, at half past five in the morning, the device was detonated.
The temperature at the center of the explosion reaches several million degrees, so the flash of a nuclear explosion is much brighter than the Sun. The fireball lasts for several seconds, then begins to rise, darken, turns from white to orange, then crimson, and the now famous nuclear mushroom is formed. The first mushroom cloud rose to a height of 11 km.
The explosion energy was more than 20 kt of TNT equivalent. Most of the measuring equipment was destroyed because physicists counted on 510 tons and placed the equipment too close. Otherwise it was a success, a brilliant success!
But the Americans were faced with unexpected radioactive contamination of the area. The plume of radioactive fallout stretched 160 km to the northeast. Part of the population had to be evacuated from the small town of Bingham, but at least five local residents received doses of up to 5,760 roentgens.
It turned out that in order to avoid contamination, the bomb must be detonated at a sufficiently high altitude, at least a kilometer and a half, then the radioactive decay products are scattered over an area of hundreds of thousands or even millions of square kilometers and dissolved in the global radiation background.
The second bomb of this design was dropped on Nagasaki on August 9, 24 days after this test and three days after the bombing of Hiroshima. Since then, almost all atomic weapons have used implosion technology. The first Soviet bomb RDS-1, tested on August 29, 1949, was made according to the same design.
Nuclear weapons began to cause fear among people from the very moment when the possibility of their creation was theoretically proven. And for more than half a century the world has been living in this fear, only its magnitude changes: from the paranoia of the 50-60s to permanent anxiety now. But how did such a situation even become possible? How could the idea of creating such a terrible weapon come into the human mind? 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 them later.
The author tried to give a clear and accessible answer to these and many other questions by talking about the race to acquire 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 Frisch finally received good news. He learned that his father, although he remained in the Dachau concentration camp, had nevertheless received a Swedish visa. He was soon released and in Vienna he was able to meet Frisch’s mother. Together they moved to a place where nothing threatened them - to Stockholm.
But even such joyful news could not rid Otto of the premonition of imminent great trouble, which had recently overwhelmed him. The anticipation of the start of the war, which was just around the corner, plunged him deeper into the abyss of depression. Frisch saw no point in continuing the research he was doing in Copenhagen. The feeling of insecurity also grew. When Briton Patrick Blackett and Australian Mark Oliphant arrived at Bohr's 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. After listening to Erenst Rutherford, a New Zealander by birth, the impressionable student decided to take up nuclear physics. In 1927, he joined Rutherford's research team at the Cavendish Laboratory in Cambridge. There, in the early 1930s, he witnessed first-hand many remarkable discoveries in the field of nuclear physics. In 1934, co-authored with Rutherford (as well as the German chemist Paul Harteck), Oliphant published a paper describing the nuclear fusion reaction involving heavy hydrogen - deuterium.
In 1937, Oliphant received a professorship at the University of Birmingham, becoming Dean of the Faculty of Physics. He was very sympathetic to Frisch’s request for help and soon sent him a letter in which he invited Otto to visit Birmingham in the summer of 1939 and see on the spot what could be done for him. Oliphant's calm and confidence greatly impressed Frisch, who could not get out of his depression, and he did not wait for another invitation. Having packed two small suitcases, he left for England, “no different from other tourists.”
The Australian arranged for Otto to become a junior teacher. He now worked in a rather informal atmosphere. Oliphant gave lectures to students and referred those who had difficulty mastering new material to Frisch. Otto worked with several dozen students who asked him a huge number of questions, and a very lively discussion ensued. Frisch really liked this kind of work.
In Birmingham, Frisch met with another emigrant, his fellow countryman, Rudolf Peierls. Rudolf was born in Berlin, into a family of assimilated Jews. He studied physics in Berlin, Munich and Leipzig, where he completed his defense in 1928 with Heisenberg. Peierls then moved to Zurich, Switzerland, and there in 1932 he was awarded a Rockefeller Fellowship. He had to study first in Rome, with Fermi, and then in Cambridge, England, with the theoretical physicist Ralph Fowler. When Hitler came to power in 1933, Peierls was in England. It soon became clear to him that the return route to Germany was closed. Having completed his studies, Rudolph went to Manchester, where he worked with Lawrence Bragg, and then returned to Cambridge, where he stayed for a couple of years. In 1937 he became professor of mathematics at the University of Birmingham.
From September 1939, after the outbreak of war, the laboratories in Birmingham became primarily involved in highly important - and classified - research for the military.
The scientists' work was related to a resonant magnetron - a device necessary for generating intense microwave radiation in ground-based and on-board aircraft radars. C. P. Snow later called these devices "the most valuable scientific invention of the British made during the war with Hitler."
Being citizens of a hostile state, Frisch and Peierls should not have known anything about these works. However, the secrecy of the project was of some incomprehensible nature. Oliphant sometimes asked Peierls hypothetical questions that began with the words: “If you were faced with the following problem...”. As Frisch would later write, “Oliphant knew that Peierls knew, and I think Peierls knew that Oliphant knew that he knew. However, none of them showed any sign of it.”
Frisch did not work with students constantly, so that, having enough free time, he could again take up the problem of nuclear fission. Using the laboratory when it was not occupied, Otto conducted several small experiments. Bohr and Wheeler argued that uranium is fissile mainly due to the isotope U235, which is not very stable. Frisch decided to prove this experimentally, obtaining data from samples with a slightly increased content of the rare isotope. To isolate small amounts of uranium-235, he assembled a small apparatus that used the thermal diffusion method invented by Clusius and Dickel. Progress, however, has been extremely slow.
In the meantime, the British Chemical Society approached Frisch with a request to write a review for them and highlight all the recent advances in the study of the atomic nucleus, so that it would be understandable and interesting to chemists. Otto wrote the article in his rented room. Without taking off his coat, he sat, holding the typewriter on his lap, near the gas burner, trying to warm up at least a little: the temperature that winter dropped to -18 °C. At night the water in the glass froze.
Talking about nuclear fission, he repeated the generally accepted opinion at that time: if one day it is possible to carry out a self-sustaining chain reaction, then taking into account the fact that it must use slow neutrons, an atomic bomb in which the chain reaction will occur will be practically impossible to explode. “We would have achieved at least a similar result if we had simply set fire to a similar amount of gunpowder,” he wrote in the final part. Frisch did not believe in the possibility of creating an atomic bomb at all.
However, after finishing the article, he began to think. The main problem on this moment, according to Bohr and Wheeler, consisted of slow neutrons. The uranium-238 nucleus has always captured fast neutrons that have a certain “resonance” energy, or speed, but only slow neutrons are needed to react with natural uranium. However, their use meant that the resulting energy would accumulate very slowly. If the reaction were based on slow neutrons, the energy released would heat the uranium and possibly melt it or even vaporize it long before it could explode. As the uranium heats up, fewer and fewer neutrons will enter into the reaction, and eventually it will simply die out.
The physicists of the Uranium Society came to the same opinion. However, Frisch was now very interested in the answer to the question: what would happen if you use fast neutrons? Uranium-235 was thought to be fissioned by both types of neutrons. However, if there is too much U 238 in the fissioned uranium, then the fast secondary neutrons emitted by the U 235 decay will be of little use: these fast secondary neutrons are likely to escape from the reaction due to resonant capture by the uranium-238 nucleus. But this obstacle can be easily circumvented if pure or almost pure uranium-235 is used. Frisch assembled a small Clusius-Dickel apparatus for separating U 235 without much difficulty. It was clear that it was impossible to obtain large volumes of pure uranium-235, for example several tons, in this way. But what if a much smaller amount is sufficient for a chain reaction with fast neutrons?
Chain reaction on fast neutrons using pure uranium-235 - if we assume that the atomic bomb initially had some kind of secret, then it has now become known to Frisch.
Otto shared his thoughts with Peierls, who in early June 1939 finalized the formula for calculating the critical mass of material required to maintain a nuclear chain reaction. This formula was compiled by the French theoretical physicist Francis Perrin. For a mixture of isotopes with a high content of U 238, Peierls used his modified formula, but since the count was in tons, this option was not suitable for creating weapons.
Now Frisch needed to carry out calculations of a completely different order - with the participation of pure uranium-235 and not slow, but fast neutrons. The problem was that no one yet knew what the proportion of U 235 should be to ensure successful participation in the reaction of fast neurons. But scientists did not know this because it had not yet been possible to obtain a sufficient amount of uranium-235 in its pure form.
In such a situation, all that was left was to make assumptions. The results obtained by Bohr and Wheeler made it clear that the U 235 nucleus was easily split by slow neutrons. Further, it was logical to assume that the effect of fast neutrons is no less effective, and it is even possible that the uranium-235 nucleus fissions upon any contact with them. Subsequently, Peierls wrote about this hypothesis: “Apparently, from the data obtained by Bohr and Wheeler, exactly the following conclusion should have been drawn: every neutron that enters the nucleus of 235 [uranium] causes its decay.” This assumption greatly simplified the calculations. Now all that remained was to calculate how much uranium-235 was needed so that it could be easily split by fast neutrons.
Scientists substituted new numbers into Peierls' formula and were amazed by the results obtained. Tons of uranium were now out of the question. The critical mass, according to calculations, was only several kilograms. For a substance with a density like uranium, the volume of such an amount would not exceed the size of a golf ball. Frisch estimates that this amount of U 235 can be obtained in a few weeks, using about one hundred thousand tubes of Clusius-Dickel apparatus, similar to the one he assembled in the Birmingham laboratory.
“Then we all looked at each other, realizing that it was still possible to create an atomic bomb.”
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