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Brighter than the Sun: The Atomic Bomb
A mysterious device that can release gigajoules of energy for an indescribably short period of time is surrounded by a sinister romance.
Needless to say, all over the world, work on nuclear weapons was deeply classified, and the bomb itself was overgrown with a lot of legends and myths.
Let's try to deal with them in order.
Andrey Suvorov
18.11.2008
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Nothing arouses such interest as an atomic bomb
The structure of the bomb charge
August 9, 1945.
Nagasaki.
Nuclear mushroom
September 24, 1949.
New York newspapers announced the explosion of the Soviet atomic bomb
August 1945.
Ernest Orlando Lawrence at the Atomic Bomb Development Laboratory
July 25, 1946.
Underwater nuclear explosion at Bikini Atoll in the Pacific Ocean
the year is 1954.
Eight years after the explosion at Bikini Atoll, Japanese scientists found high levels of radiation in fish caught in local waters
The atomic bomb "Fatman" (Fatman) is the same as the one dropped on Nagasaki on August 9, 1945
Critical mass
Everyone has heard that there is a certain critical mass that needs to be gained in order for a chain nuclear reaction to begin But in order for a real nuclear explosion to occur, one critical mass is not enough — the reaction will stop almost instantly, before any noticeable energy has time to be released For a full scale explosion of several kilotons or tens of kilotons, you need to simultaneously collect two or three, or preferably four or five critical masses.
It seems obvious that you need to make two or more parts from uranium or plutonium and connect them at the required moment.
In fairness, I must say that physicists thought the same when they took up the construction of a nuclear bomb.
But reality has made its own adjustments.
The fact is that if we had very pure uranium 235 or plutonium 239, we could have done so, but scientists had to deal with real metals.
Enriching natural uranium, it is possible to make a mixture containing 90% of uranium 235 and 10% of uranium 238, attempts to get rid of the remainder of uranium 238 lead to a very rapid increase in the cost of this material (it is called highly enriched uranium) Plutonium 239, which is obtained in a nuclear reactor from uranium 238 during the fission of uranium 235, necessarily contains an admixture of plutonium 240.
The isotopes uran235 and plutonium 239 are called even odd, since 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 divide spontaneously (scientists say: "spontaneously"), but they easily divide when a neutron hits the nucleus.
Uranium 238 and plutonium 240 are even even.
On the contrary, they practically do not share neutrons of low and moderate energies, which fly out of fissionable nuclei, but they divide 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 a premature start of the reaction before the two parts of the charge meet.
Because of this, in a device prepared for an explosion, parts of the critical mass must be located far enough away from each other, and connect at a high speed.
Nevertheless, 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, the inner layer of which was made of tungsten carbide, the outer layer 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 some of the neutrons flying out of the uranium back.
Taking into account the neutron reflector, 64 kg made up 2.3 critical masses.
How did it work out, because 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: a target made of several rings of low height, and a bullet made of six washers.
The reason is simple — the uranium blanks should have been 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 enclosed in a thin walled stainless steel shell, with a lid made of tungsten carbide, like the target shell.
In order to direct the bullet to the center of the target, we decided to use the barrel of a conventional 76.2 mm anti aircraft gun.
That is why a bomb of this type is sometimes called a cannon assembly bomb.
The barrel was bored from the inside to 100 mm, so that such an unusual projectile entered it.
The length of the trunk was 180 cm.
Ordinary smokeless powder was loaded into its charging chamber, which fired a bullet at a speed of about 300 m / s.
And the other end of the barrel was pressed into the hole in the shell of the target.
This design had a lot of disadvantages.
It was monstrously dangerous: after the powder was loaded into the charging chamber, any accident that could ignite it would lead to the explosion of the bomb at full power.
Because of this, the charging of pyroxylin occurred already in the air when the plane was approaching the target.
When the plane crashed, the uranium parts could connect without gunpowder, just 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 the water, then due to the slowing down of neutrons in the water, the reaction could begin even without connecting the parts.
True, a nuclear explosion is unlikely, but there would be a thermal explosion, with the spraying of uranium over a large area and radioactive contamination.
The length of the bomb of this design exceeded two meters, and this is actually insurmountable.
After all, the critical state was reached, and the reaction began when the bullet was still a good half meter away from stopping!
Finally, this bomb was very wasteful: less than 1% of uranium had time to react in it!
The advantage of a cannon bomb was exactly one: it could not fail to work.
They werenot even going to test it!
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 one (less than 1%!) the 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" - the British physicist Klaus Fuchs.
His idea, which later became known as" implosion", was to form a converging spherical shock wave from a diverging one, using so called explosive lenses.
This shock wave was supposed to compress a piece of plutonium so that its density doubled.
If a decrease in density causes an increase in 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 cools from its melting point to room temperature, it undergoes four phase transitions.
At the latter (about 122 degrees) its density increases by a jump of 10%.
At the same time, 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 flying out of plutonium nuclei during their decay would knock out free neutrons from aluminum nuclei, increasing the already noticeable neutron background, so gallium was used in the first atomic bomb.
A ball with a diameter of only 9 cm and a weight of about 6.5 kg was made from an alloy containing 98% plutonium 239, 0.9% plutonium 240 and 0.8% gallium.
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 — a neutron source that was triggered when a bomb exploded could be inserted into the inner cavity.
All three parts had to be nickel plated, because plutonium is very actively oxidized by air and water and is extremely dangerous when ingested into the human body.
The ball was surrounded by a neutron reflector made of natural uranium 238 with a thickness of 7 cm and a weight of 120 kg.
Uranium is a good fast neutron reflector, and the assembled system was only slightly subcritical, so instead of a plutonium plug, a cadmium one was inserted, absorbing 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.
Then there was an 11.5 cm layer of aluminum alloy weighing 120 kg.
The purpose of the layer is the same as that of the illumination on the lenses of lenses: to make the blast wave penetrate the uranium plutonium assembly, and not reflect from it.
This reflection occurs due to the large difference in the densities of explosives and uranium (about 1:10).
In addition, in the shock wave, a rarefaction wave follows the compression wave, the so called Taylor effect.
The aluminum layer weakened the rarefaction wave, which reduced the effect of the explosives.
Aluminum had to be doped with boron, which absorbed neutrons emitted from the nuclei of aluminum atoms under the influence of alpha particles arising from the decay of uranium 238.
Finally, there were those "explosive lenses" outside.
There were 32 of them (20 hexahedral and 12 pentahedral), they formed a structure similar to a football.
Each lens consisted of three parts, and the middle one was made of a special "slow" explosive, and the outer and inner ones were made of "fast".
The outer part was spherical from the outside, but inside it had a conical cavity, like on a cumulative charge, but its purpose was different.
This cone was filled with a slow explosive, and at the interface there was a refraction of the explosive wave 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 that could calculate the shape of such lenses, and most importantly, there was not even a suitable theory.
Therefore, they were made 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, registering the parameters of the blast wave.
When the reduced version was worked out, it turned out that the explosives did not scale so easily, and it was necessary to greatly adjust the old results.
The accuracy of the shape had to be observed with an error of less than a millimeter, and the composition and uniformity of the explosives had to be maintained very carefully.
It was possible to make parts only by casting, so not all explosives were suitable.
The fast explosive was a mixture of RDX and TNT, and there was twice as much RDX.
Slow — the same TNT, but with the addition of inert barium nitrate.
The velocity 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 be triggered simultaneously with an unheard of accuracy 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.
With the same accuracy, it was necessary to combine the conjugated surfaces of the lenses, and after all, the error of their manufacture 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 has become little like a theoretical model.
We had to invent new detonators: the old ones did not provide proper synchronicity.
They were made on the basis of wires exploding under a powerful pulse of electric current.
To trigger them, a battery of 32 high — voltage capacitors and the same number of high speed spark arresters was needed one for each detonator.
The entire system, together with the batteries and a charger for capacitors, weighed almost 200 kg in the first bomb.
However, compared to the weight of the explosives, which took 2.5 tons, it was not much.
Finally, the entire structure was enclosed in a duralumin spherical body, consisting of a wide belt and two covers — the upper and lower, all these parts were assembled on bolts.
The bomb's design allowed it to be assembled without a plutonium core.
In order to insert the pl into place the utonium together with a piece of a uranium reflector, unscrewed the upper cover of the case and took out one explosive lens.
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 the power previously available only to the gods.
A brilliant success
The place for the test was chosen in the state of New Mexico, in a place with the picturesque name Giornadadel Muerto (The 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, it was raised to the top of a specially built tower with a height of 30 m, wires were connected to the detonators and the last stages of preparation began, associated with a large number of measuring equipment.
On July 16, 1945, at five thirty in the morning, the device was blown up.
The temperature in 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 a few seconds, then begins to rise, darken, turns from white to orange, then purple, and the now famous nuclear mushroom is formed.
The first mushroom cloud rose to a height of 11 km.
The energy of the explosion was more than 20 kt of TNT equivalent.
Most of the measuring equipment was destroyed, because the physicists counted on 510 tons and put the equipment too close.
Otherwise, it was a success, a brilliant success!
But the Americans were faced with an unexpected radioactive contamination of the area.
The plume of radioactive fallout stretches for 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 5760 X rays.
It turned out that in order to avoid infection, the bomb must be detonated at a sufficiently high altitude, at least a kilometer and a half, then the products of radioactive decay are dispersed over an area of hundreds of thousands or even millions of square kilometers and dissolve 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 nuclear weapons use implosion technology.
The first Soviet RDS 1 bomb, tested on August 29, 1949, was made according to the same scheme.
The article was published in the journal "Popular Mechanics "(No. 13, November 2003).
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