How uranium is enriched: centrifugal isotope separation method

At the dawn of the creation of nuclear weapons, one of the main key problems was the separation of uranium isotopes. This heavy radioactive metal is found in nature as a mixture of two main isotopes. The main share (slightly less than 99.3%) is uranium-238. The content of the lighter isotope - uranium-235 - is only 0.7%, but it is it that is necessary for the creation of nuclear weapons and the operation of reactors.

Separating isotopes is far from easy. Their chemical properties are identical (after all, this is the same chemical element), and the difference in atomic mass is just over 1%, so physical methods for separation must have very high selectivity. This issue in the 1950s was one of the decisive moments that determined the success of the Soviet nuclear industry and laid the foundation for the modern competitiveness of the Russian nuclear industry in the world market.

Through a sieve

The simplest method of separation is gas diffusion - “pushing” of a gaseous feed (uranium hexafluoride) through a finely porous membrane, while different isotopes diffuse through the pores at different speeds. It was gas diffusion that became the first method that was used to obtain industrial quantities of uranium-235 at the first enrichment plants. In the United States, gas diffusion developments for the Manhattan project were led by Nobel Prize winner Harold Urey. In the USSR, until 1954, this direction was headed by academician Boris Konstantinov, then he was replaced by Isaac Kikoin.

At first, as often happens, the gas diffusion method seemed more affordable to implement. But he demanded huge expenditures of electricity - the Sayano-Shushenskaya hydroelectric power station and the first stage of the Beloyarsk nuclear, as it now turns out, were built primarily for these purposes. In addition to the general high cost and low efficiency, the gas diffusion method was unsafe for workers - mainly due to high temperatures and noise in the workshops. Plus, large volumes of chemically active mixtures under pressure, and these are potential emissions and environmental pollution. Meanwhile, the alternative to the gas diffusion method has been known since the end of the 19th century - this is a centrifuge method that promises very significant savings: when the plant in Verkh-Neivinsk entered the settlement mode in 1958, it turned out that the energy consumption per unit of separation is 20 (!) Times less than the diffusion method, and the cost is half that. True, on the way to creating centrifuges for designers, there were numerous technological difficulties.

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Electromagnetic separation. Based on the movement of charged particles (ions) in a magnetic field. The curvature of their trajectory is different depending on the mass of the particles, and even a small difference in the atomic mass of the nuclei of uranium isotopes makes it possible to separate them. Such installations, called kalyutrons, were used in the American Manhattan project, since they made it possible to obtain a very high degree of uranium enrichment in a few passes. However, calutrons are very bulky, expensive to maintain, consume a lot of energy and have low productivity, so now they are not used for industrial enrichment.

German roots

The origins of Soviet centrifuge technology originate in Nazi Germany, where uranium separation experiments were conducted as part of an atomic project. One of the participants in this project, physicist engineer Geront Zippe, was among other German prisoners of war sent to the USSR. Under the direction of Max Steenbeck, his compatriot and father-in-law, Zippe was engaged in experimental research until 1954, first at Laboratory A in Sukhumi (the future Sukhumi Institute of Physics and Technology), and for the last two years at a special design bureau at the Kirov Plant in Leningrad.

As participants and eyewitnesses of those events testify, German scientists did not know the refusal of research materials. And their regime was almost the same as that of our secret nuclear scientists, who were just as closely guarded by the Beria department. In July 1952, a special decree of the government of Steenbek and his assistants was transferred from the Sukhumi Institute to Leningrad, in the Design Bureau of the Kirov Plant. Moreover, they reinforced the group with graduates of the Polytechnic Institute from the specialized department of nuclear research. The task was to manufacture and test two units according to the Zippe-Steenbeck scheme. They got down to business hotly, but already in the first quarter of 1953 they stopped work, not leading up to the tests: it became clear that the proposed design was not suitable for mass production.

Gas diffusion. Uses the difference in the speeds of motion of gas molecules containing various isotopes of uranium (uranium hexafluoride). Different masses cause different speeds of molecules, so that the lungs pass through a membrane with thin pores (comparable in diameter to the size of the molecules) faster than heavy ones. The method is simple to implement and has been used at the dawn of the nuclear industry in the USSR; in the USA it is still used. The degree of enrichment of each step is very small, so thousands of steps are needed. This leads to huge energy consumption and high separation costs.

The Zippe centrifuge was not the first Soviet machine of this purpose. Even during the war in Ufa, another German, Fritz Lange, who fled from Germany in 1936, made a bulky apparatus on a bearing. However, experts familiar with the ups and downs of the atomic project in the USSR and the USA note one unconditional achievement of the Steenbeck group - the original design of the support unit: the rotor rested on a steel needle, and this needle on a thrust bearing made of a superhard alloy in an oil bath. And all this ingenious design was held by a special magnetic suspension in the upper part of the rotor. Its promotion to operating speed was also carried out by means of a magnetic field.

Soviet competitor

While the project of the Steenbeck group failed, in February of the same 1953 a gas centrifuge with a rigid rotor designed by Soviet engineer Viktor Sergeyev was put into operation. The year before, Sergeyev was sent to Sukhumi with a group of specialists from a special design bureau of the Kirov Plant, where he worked then, to familiarize himself with the experiments of Steenbek and his team. “It was then that he asked Steenbeck a technical question about the location of gas sampling devices in the form of pitot tubes, ” Oleg Chernov, veteran of the centrifuge production of the Tochmash Production Center, who knew Sergeyev well and worked with him, revealed important details. “The question was purely technical and contained, in fact, a hint how to make the centrifuge design workable.” But Dr. Steenbek was categorical: “They will slow down the flow, cause turbulence, and there will be no separation!” After years, working on memoirs, he will regret it: “An idea worthy of coming from us!” But it didn’t occur to me ... "

Gas centrifugation by means of a rapidly rotating rotor spins the gas flow so that molecules containing heavier uranium isotopes, the centrifugal force casts to the outer edges, and lighter - closer to the axis of the cylinder. Centrifuges are combined into cascades, feeding partially enriched material from the exit of each stage to the entrance of the next stage - this way it is possible to obtain uranium of even a very high degree of enrichment. Centrifuges are easy to maintain, reliable and characterized by moderate energy consumption. The method is used in Russia and European countries.

According to Oleg Chernov, before leaving for Germany, Zippe had the opportunity to familiarize himself with the prototype of Sergeyev’s centrifuge and the brilliantly simple principle of its work. Once in the West, the "cunning Zippe, " as he was often called, patented the design of a centrifuge in 13 countries. Having learned about such intellectual insidiousness, the first persons in the Soviet atomic department did not make any noise - if you follow the official version, "so as not to arouse suspicion and increased interest in this topic among the US military and technical intelligence." Let, they say, they think that the Soviets are content with the gas-diffusion method, which is uneconomical, like theirs ... In 1957, having moved to the USA, Zippe built a working installation there, reproducing from memory a prototype of Sergeyev. And he called it, should be given credit, "Russian centrifuge." However, he failed to captivate the Americans. In relation to the new machine, as in its time and according to the design of Steenbeck, a verdict was issued: unsuitable for industrial use.

The degree of enrichment of one gas centrifuge is small, so they are combined into successive cascades in which enriched raw materials from the outlet of each centrifuge are fed to the inlet of the next, and depleted to the inlet of one of the previous ones. With a sufficient number of centrifuges in the cascade, a very high degree of enrichment can be obtained.

True, a quarter of a century later, in the USA, they nevertheless decided to switch from gas diffusion to centrifuges. The first attempt failed - in 1985, when the first 1, 300 vehicles developed at the Oak Ridge National Laboratory were installed, the US government closed the program. In 1999, work on the installation of a new generation of American centrifuges (10-15 times larger than Russian ones in height and two to three times in diameter) with a carbon fiber rotor began again at the re-opened site in Paikton (Ohio). According to the plan, 96 cascades of 120 “gyroscopes” were supposed to be installed back in 2005, but by the end of 2012 the project was still not put into commercial operation.

Laser separation of uranium isotopes is based on the fact that molecules containing different isotopes have slightly different excitation energies. By irradiating a mixture of isotopes with a laser beam of a strictly defined wavelength, only molecules with the desired isotope can be ionized, and then the isotopes can be separated using a magnetic field. There are several varieties of this method - affecting atomic vapor AVLIS (Atomic Vapor Laser Isotope Separation), SILVA (French analogue of AVLIS), and molecules - MLIS (Molecular Laser Isotope Separation), CRISLA (Chemical Reaction Isotope Separation) and SILEX (Separation of Isotopes by Laser EXcitation). General Electric is currently trying to commercialize SILEX technology, developed by specialists from South Africa and Australia. Laser separation has low energy consumption, low cost, and a high degree of enrichment (therefore, it is now used to produce small amounts of ultra-pure isotopes), but so far there are problems with productivity, with the laser life and the selection of enriched material without stopping the process.

Secret needles

Meanwhile, in the USSR, in the unobtrusive place of Verkh-Neyvinsk in the Middle Urals, in an atmosphere of strict secrecy, the first experimental line of gas separation centrifuges was mounted. As far back as 1942, Isaac Kikoin encountered a gas centrifuge designed by Lange and even tested it in his laboratory in Sverdlovsk. Then the experiments did not give the desired results, and the academician was skeptical about the very possibility of creating industrial gas centrifuges. The main trouble of the very first installations was their fragility. And although they initially rotated at a speed of “only” 10, 000 rpm, it was far from easy to cope with the enormous kinetic energy of the rotor.

“Your cars are being destroyed!” - the chief of the central board, Alexander Zverev, who had the rank of NKVD general, reproached the developers at one of the meetings in the Ministry of Environment.

“What did you want?” So that they also multiplied ?! - defiantly retorted the project manager at that time, Deputy Chief Designer Anatoly Safronov.

With the centrifugal separation method, a centrifugal force is created due to the high rotation speed, which is hundreds of thousands of times greater than the Earth's gravity. Due to this, the heavier molecules of uranium-238 hexafluoride "get stuck" on the periphery of the rotating cylinder, and the lighter molecules of uranium-235 hexafluoride are concentrated near the axis of the rotor. Through separate outlet pipelines (such as pitot tubes, which Soviet engineer Sergeyev spoke to the German Steenbeck), gas containing U-238 isotopes is discharged “to the dump”, and the enriched fraction with an increased content of uranium-235 flows into the next centrifuge. A cascade of such centrifuges, containing hundreds and thousands of machines, allows you to quickly increase the content of light isotope. Relatively speaking, they can be called separators, on which uranium feed (uranium hexafluoride, UF6) with a low content of the U-235 isotope converted into gas is sequentially transferred from the consistency of fresh milk to cream and sour cream. And if necessary, they can also bring down the "oil" - bring the enrichment to 45%, or even 60%, so that it can be used as fuel in submarine reactors and research facilities. And more recently, when this was required in large quantities, centrifuges were turned until they received an expensive “cheese” - weapons-grade uranium with enrichment of more than 90%. But by the end of the 1980s, so many weapons-grade uranium had been “separated” in four Soviet factories that its stockpiles in warehouses and finished nuclear weapons were deemed excessive, and the production of highly enriched uranium for military purposes was discontinued.

According to initial calculations, the thickness of the outer walls of the centrifuge body was supposed to be 70 mm - like tank armor. Try to spin such a colossus ... But by trial and error they found a compromise solution. A special alloy was created - stronger and lighter than steel. The cases of modern centrifuges, which one of the authors had the chance to see and hold in their hands at the Tochmash Production Center in Vladimir, do not cause any associations with tank armor: ordinary-looking hollow cylinders with an internal surface polished to a shine. From a distance they can be mistaken for pipe cuts with connecting flanges at the ends. Length - no more than a meter, in diameter - twenty centimeters. And at the Ural Electrochemical Plant, giant cascades hundreds of meters long were assembled from them. Signs on the walls and special markings on the painted concrete floor in the technological aisles indicate that it is customary to move around on a bicycle. True, not faster than 5-10 km / h.

And inside the buzzing centrifuges you can hardly hear completely different speeds - the rotor on a needle with a corundum thrust bearing, “suspended” in a magnetic field, makes 1500 revolutions per second! Compared with the first VT-3F product of 1960, it was almost ten times overclocked, and the period of non-stop operation was increased from three years to 30. It is probably hard to find another example when a technique showed such reliability with such extreme parameters. According to Valery Lempert, deputy head of centrifugal production, the plant in Novouralsk still has machines that Tochmash put there 30 years ago: “It was probably the third generation of centrifuges, and now the eighth is being mass-produced and the ninth is being put into pilot production.”

“There is nothing super complicated in the design of our centrifuge. It's all about working out the technology to the smallest detail and strict quality control, ”explains Tatyana Sorokina, who for decades has been“ leading ”the technology for manufacturing a support needle for the rotor at the plant. - Such needles are made of ordinary piano wire, from which the strings are pulled. But the way to harden the tip is our know-how. ”

One of its main creators, Viktor Sergeyev, gave his explanation to the secrets of the Russian centrifuge in his declining years. According to the testimony of an engineer Oleg Chernov, the designer answered succinctly: “People” to the question of special services, what should be protected in this product and what is its main secret.

The article “And Still It Turns” was published in the journal Popular Mechanics (No. 2, February 2013). I wonder how a nuclear reactor works and can robots build a house?

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