RDS-6s: Andrei Sakharov hydrogen bomb
The old man’s monastery at the confluence of two small rivers, Satis and Sarovka, became famous thanks to Seraphim, who was canonized in 1903 because an heir was born to the royal couple after a moleben of monarchs during a pilgrimage to this desert. The fate of the monastery, however, was unenviable: in 1917 it was looted. And after three decades, work on completely different “children” began to boil here: in 1946, KB-11, the forge of the Soviet atomic project, was located directly in the monastery buildings.
Something of my own
Now, in Internet times, everyone knows about Klaus Fuchs and other "atomic spies", and it seems to be taken for granted that the first atomic bomb of the USSR - RDS-1 - was copied from "Fat Man", a bomb dropped on Nagasaki. But it is not so. Indeed, the size of the plutonium nucleus, the shape and design of the polonium-beryllium neutron source, and the idea of implosion - an inwardly directed explosion - were precisely borrowed from the American project. This is a lot, but not all. The fact is that two types of explosives were used in the American bomb, Composition B and Baratol, their exact composition was not known to Klaus Fuchs, and they were not made in the USSR. And the size and shape of the explosive lenses that formed a spherical converging shock wave depend on these types, and when the composition is changed by literally a few percent, the geometry of these lenses needs to be adjusted.
In RDS-1, instead of Composition B, containing 59.5% RDX, 39.5% TNT, and 1% wax or ceresin, the domestic alloy TG-50 (equal to RDX and TNT) was used. Instead of barratol, it is an inert material that does not explode at all, only transmitting a shock wave with minimal losses. So our bomb builders had to refine the shape of the transition surface themselves, using the same methods - high-speed X-ray photography - and inventing their own, no less witty. The auxiliary systems of the bomb, such as a radio altimeter, barometric altimeter, automatic bombing, were also their own, so the scope of design work was very large even with the well-known operating principle.
But as work progressed on the first "product", RDS-1, intelligence began to receive information that the Americans were working on an even more powerful bomb. Much more powerful than atomic. Our nuclear scientists managed to ask a question to the Nobel laureate Niels Bohr about the essence of the device of this bomb, but the great physicist, known for his sympathies for the USSR, was not aware of the details, and his answer could not reassure our nuclear scientists. Therefore, two groups were created whose task was to determine the possibility of an explosion in a bomb not due to fission of heavy nuclei, but due to the synthesis of lungs: helium and neutron should be obtained from deuterium and tritium. The group, led by Igor Evgenievich Tamm, included a young Ph.D. Andrei Dmitrievich Sakharov.
In Sakharov’s puff, only 15–20% of the energy released is generated by thermonuclear reactions, and the rest is provided by the fission reaction of both the uranium-235 or plutonium-239 of the original bomb and the uranium-238 of the outer layers. But this is more a virtue than a disadvantage, because uranium-238 is cheap. The fusion of one nucleus of deuterium and one tritium gives 18 MeV of energy, and the fission of one nucleus of uranium-238 gives 200 MeV. In the first reaction, one neutron is released, which takes 14 MeV out of 18, and in the second it is consumed, but three less energetic neutrons are released, suitable only for obtaining tritium from lithium-6. The result is a gain of more than 10 times! And this means that almost all thermonuclear neutrons are used "for their intended purpose", and a small part flies away.
From the very beginning, Sakharov did not want to deal with an absolutely secret military topic - he refused to join the group, although he was promised to help with housing and other material goods, which were desperately lacking in post-war Moscow. In his memoirs, he writes that he twice refused the offers of the military and Kurchatov, but for the third time, in 1948, his consent was no longer asked. The main feature of Sakharov’s talent, which was mentioned by everyone who worked with him, was that he could see the workability or the correctness of a physical idea without accurate calculations, intuitively, therefore it is not surprising that only a few months after he saw the device diagram future RDS-1, he came up with the idea to add a layer of light elements to the uranium container surrounding the plutonium core. This happened back in the fall of 1948, before the first test. True, it was originally intended to use heavy water - as a substance containing deuterium in the maximum volume concentration.
The proposal to use lithium deuteride instead of deuterium water came from V.L. Ginzburg, who, due to “errors in personal data” (his wife was convicted under a political article and after serving her sentence to exile in Gorky), was not a member of the group and was not allowed into most of the atomic secrets. Lithium hydride had several decisive advantages over heavy water. Firstly, it is a solid substance, which somewhat simplifies the design and greatly facilitates the operation of a nuclear explosive device. Secondly, lithium is bombarded with neutrons to produce tritium, which enters into a thermonuclear reaction with deuterium with the release of more energy than in pure deuterium. Thirdly, lithium is involved in neutron moderation (oxygen in water is too “heavy” for this). Therefore, the idea was accepted with great enthusiasm.
Guess on the first try
The design, later called the Sakharov Puff, seems obvious, but it was not easy to calculate the correct thickness of the layers. It is determined by the fact that lithium deuteride is a good neutron moderator, and uranium-238 reflects them. Slow neutrons are more suitable for tritium production. As a result, it turned out that there should be two layers of light elements, and three of uranium-238. Not all factors could be calculated, the theoreticians' intuition had to be believed that the mixing of the layers would not be catastrophic. But at first the assembly was to be compressed by a converging blast wave from the implosion of conventional explosives, and then the light pressure from the X-ray “pushed” it back, and even at such a speed that the outer layers could not keep up with the inner ones! Due to this, the thermonuclear material is compressed and adiabatically heated. But after this, despite the expansion of the former critical assembly proceeding at a speed that is fantastic for chemical explosives, an additional isothermal compression of light elements still occurs inside the “puff” due to the ionization of uranium-238, which absorbs the x-ray radiation of the exploded atomic “lighter”. This seemingly unexpected phenomenon was called “saccharization”. And with the recent expansion of a beautifully constructed structure from embedded balls, a pack of thermonuclear reactions will begin and end just as quickly - from lithium-6 neutrons will produce tritium, it will react with deuterium, giving new thermonuclear neutrons, some of which will be used to produce new tritium, and part will divide uranium-238, from which more neutrons will fly out of the fragments, and the chain will be closed.
But this reaction is not self-sustaining, because it can occur only in a nonequilibrium state. The peak of energy release takes fractions of a microsecond, then the expansion of the atoms will do its job, the distance between them will increase so much that the neutrons will no longer fall into their "goals", and the reaction will quickly decay. Therefore, the "puff" has an optimal size, and Soviet physicists and gunsmiths actually guessed it - on the first try. Later, the British tested the “puff” at 720 kt, but its efficiency was noticeably less.
The first "puff" was not cheap and convenient. It simultaneously used all methods of increasing energy release. And the inner layer of lithium deuteride contained a large amount (of the order of hundreds of grams) of lithium tritide, so to speak, to guarantee; since it was not required to obtain this tritium during the explosion, the uncertainty was greatly reduced and calculations were simplified. And numerical calculations then had to be carried out manually, on arithmometers. The use of tritium allowed to halve the army of girls for arithmometers. But tritium is very expensive, its production requires a large consumption of uranium-235, the half-life is about 12 years, and it is very “volatile”, like ordinary hydrogen.
Twenty times stronger
RDS-6s, despite the number, was only the fourth nuclear test in the USSR, structures with numbers 4 and 5 were tested later. The ballistic body of this bomb looks much more perfect than that used in RDS-1. Although the bomb was ready to be dropped from an airplane, it was decided to detonate it on a tower in the center of the same field as RDS-1. Radiy Ivanovich Ilkaev, scientific director of the Russian Federal Nuclear Center (RFNC) VNIIEF, explains the choice as follows: “When dropped from an airplane, only the power of the explosion can be reliably measured, but with all the early tests, so-called physical experiments were made, for the correct interpretation of the results of which it was important that there was no shock wave reflected from the ground - that is, that the center of energy release was at the same height as the recording equipment. ”
For the new test, the test site had to be “cleaned" - removed with bulldozers and taken away radioactive soil. Buildings, structures and observation posts were restored. And now - an explosion! Due to a twenty-fold increase in the energy yield, its appearance radically differed from previously tested atomic charges. Without any physical experiments, it was clear that the structures that had survived the previous explosions were destroyed in dust. A hundred-ton model of the railway bridge was cast back 200 m. The brightness of the flash blinded even through black glasses.
An official TASS report said that "on August 12, a type of hydrogen bomb was tested." Adherents of terminology say that it is more correct to consider it an atomic bomb with thermonuclear amplification. But the main task was successfully solved: in the dimensions and mass of RDS-1, a charge was created that has a 20 times greater energy output.
The calculated power of the new “product” was 300 +/- 100 kt, but in reality the bomb gave out, as the nuclear experts say, “along the upper boundary” - all 400 kt. Such accuracy - +/- 30% - was a matter of pride for Soviet theorists; Americans with calculation and falling into the predicted range were much worse. So, for example, at Mike, the world's first thermonuclear explosion, the calculated equivalent was 1.5–30 Mt, with the most likely estimate of 5 Mt, and the measured one was about 10.4 Mt. Modest against this background, domestic 400 kt was a more relevant achievement, because the bomb was placed on a Tu-16 aircraft and was suitable for combat use “even tomorrow”. Nevertheless, it was impossible to put the production of such bombs on stream, and a version of this charge that did not contain tritium was adopted for service - the RDS-27 with a capacity of 250 kt.
Then, in the early 1950s, the “puff” was a backup option, because the efficiency of the basic scheme of the thermonuclear charge, RDS-6t, which in American materials was called the “Classic Super”, was called into question. Klaus Fuchs worked on it, about which he informed our scouts, but he did not have time to warn that this was a dead end - Sakharov had to get to this himself. There was another fallback option - the “just big” RDS-7 atomic bomb. It made it possible to obtain a TNT equivalent of about the same as that of RDS-6s, without all the new technologies, but ... it did not fit slightly into the Tu-16. The Americans also insured in the same way, having detonated a bomb of this type two weeks after the first real Mike thermonuclear explosion.
Level the score
The merit of the "puff" is not only that it allowed, albeit with reservations, to "level the score" with American nuclear scientists. The development of RDS-6c led to the launch of an enrichment plant for lithium. Natural lithium consists of two isotopes, with masses of 6 and 7, and a light isotope is better for a thermonuclear bomb. The Americans, as Valentin Yefimovich Kostyukov, director of the RFNC VNIIEF says, were “scared” of the computational difficulties (when using lithium there are many different thermonuclear reactions possible, in the terminology of nuclear scientists - “channels”, with the same initial atoms) and did not begin to engage in lithium in the early stages thermonuclear race. Therefore, their first “dry”, without liquid deuterium, thermonuclear explosive devices contained either natural lithium, which included only 7.5% of the desired isotope, or partially enriched (up to 40%). From the very beginning, only light isotope was used in Soviet bombs and warheads, which made them lighter and more compact. The development of tritium technologies also began for the “puff” - tritium amplification has to be used in all modern small-sized warheads, but the RDS-6s developers were pioneers in the development of this capricious and treacherous nuclear material.
Yes, as soon as two-phase explosive devices appeared, the “puff” became outdated - but managed to play its role. For 60 years they have been separating us from that test, the Semipalatinsk test site has already been closed, for more than twenty years nuclear explosions have not been heard at all at either the Russian test site or the American one, and this, of course, is the merit of the long-standing success of our nuclear scientists, who made a nuclear war out of the inevitable - impossible.The article “Andrei Sakharov’s mistake” was published in the journal Popular Mechanics (No. 11, November 2013).