The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. The Zippe design was originally developed in the Soviet Union by a team led by 60 Austrian and German scientists and engineers captured after World War II, working in detention. In the West (and now generally) the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in (and contributions to) the Soviet program, Gernot Zippe. To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known (at least at a somewhat earlier stage of development) as a Kamenev centrifuge (after Evgeni Kamenev). [1] [2]
Natural uranium consists of three isotopes; the majority (99.274%) is U-238, while approximately 0.72% is U-235, fissile by thermal neutrons, and the remaining 0.0055% is U-234. If natural uranium is enriched to 3% U-235, it can be used as fuel for light water nuclear reactors. If it is enriched to 90% uranium-235, it can be used for nuclear weapons.
Enriching uranium is difficult because the isotopes are practically identical in chemistry and very similar in weight: U-235 is only 1.26% lighter than U-238 (note this applies only to uranium metal). Centrifuges need to work with a gas rather than a solid, and the gas used here is uranium hexafluoride. The relative mass difference between 235UF6 and 238UF6 is less than 0.86%. On the other hand, separation efficiency in a centrifuge depends on absolute mass difference. Separation of uranium isotopes requires a centrifuge that can spin at 1,500 revolutions per second (90,000 rpm). If we assume a rotor diameter of 20 cm (as in some modern centrifuges [3] ), this would correspond to a centripetal acceleration of around 900,000 x g [4] (around 42 times the max speed of a standard, lab benchtop microcentrifuge [5] and between 0.9 to 9 times the max speed of a standard lab ultracentrifuge [6] ) or a linear speed of greater than Mach 2 in air (Mach 1 = sound velocity, in air ca. 340 m/s) and much more in UF6. For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second (720–1500 rpm) during the spin cycle, while turbines in automotive turbochargers can run up to around 2500–3333 revolutions per second (150,000–200,000 rpm). [7] [8]
A Zippe-type centrifuge [9] has a hollow, cylindrical rotor filled with gaseous uranium hexafluoride (UF6) A rotating magnetic field at the bottom of the rotor, as used in an electric motor, is able to spin it quickly enough that the UF6 is thrown towards the outer wall, with the 238UF6 enriched in the outermost layer and the 235UF6 enriched at the inside of this layer. The centrifugal force creates a pressure gradient: On the axis of the centrifuge there is practically vacuum, so that no mechanical feedthrough or seal is needed for the gas inlet and outlets; near the wall the UF6 reaches its saturation pressure, which in turn limits the rotation speed, because condensation must be avoided. In the so-called countercurrent centrifuge, the bottom of the gaseous mix can be heated, producing convection currents. But the countercurrent is usually stimulated mechanically by the scoop collecting the enriched fraction. In such a way, the enrichment in each horizontal layer is repeated (and thus multiplied) in the next layer, similarly as in column distillation. One scoop is behind a perforated baffle that rotates with the centrifuge; it collects the 238UF6-rich fraction. The other scoop is without baffle. It slows down the gas rotation and thus increases the pressure towards the inside, so that also the 235UF6-rich fraction can be collected without pumping. [1] [9] Each centrifuge has one inlet on the axis and two output lines, one collecting the gas at the bottom and one at the top.
Quantitatively, the radial pressure (or density) distribution can be given by [9]
where p is the pressure, r the variable radius and R its maximum, M the molecular mass, ω the angular velocity, k the Boltzmann constant and T the temperature. (This equation is similar to the barometric formula.) Writing this equation for both isotopes and dividing, gives the (r-dependent) isotope ratio. It only contains ΔM (not the relative mass difference ΔM/M) in the exponent. The radial enrichment factor then results by dividing through the initial isotope ratio. To calculate the total enrichment in a countercurrent centrifuge of height H, one has to add a factor of H/(R√2) in the exponent.
According to Glaser, [3] early centrifuges had rotor diameters of 7.4 to 15 cm and lengths of 0.3 to 3.2 m, and the peripheral speed was 350 to 500 m/s. The modern centrifuge TC-21 of Urenco has a diameter of 20 cm and a length of more than 5 m, spinning with 770 m/s. Centrus (formerly Usec) plans a centrifuge with 60 cm diameter, 12 m height and 900 m/s peripheral speed.
A countercurrent of the gas is stimulated either mechanically or (less preferred) by a temperature gradient between the top and bottom of the rotor. With a countercurrent-to-feed ratio of 4, Glaser [3] calculates a separation factor of 1,74 for a TC-21 centrifuge of 5 m height. Lowering this ratio (by increasing the feed) decreases the separation factor but increases the throughput and thus the productivity.
To reduce friction, the rotor spins in a vacuum. Part of the rotor with the near-by housing acts as a molecular pump, which maintains the vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact (necessary only during start-up) is the conical jewel bearing on which the rotor sits. [1] [9] Both bearings contain measures for damping vibrations. The three gas lines enter the rotor on its axis.
After the scientists were released from Soviet captivity in 1956, [1] Gernot Zippe was surprised to find that engineers in the West were years behind in their centrifuge technology. He was able to reproduce his design at the University of Virginia in the United States, publishing the results, even though the Soviets had confiscated his notes. Zippe left the United States when he was effectively barred from continuing his research: The Americans classified the work as secret, requiring him either to become a U.S. citizen (he refused), return to Europe, or abandon his research. [1] He returned to Europe where, during the 1960s, he and his colleagues made the centrifuge more efficient by changing the material of the rotor from aluminium to maraging steel, an alloy with a longer fatigue life and longer breaking length, which allowed higher speed. This improved centrifuge design was long used by the commercial company Urenco to produce enriched uranium fuel for nuclear power stations. [1] More recently, they use (e.g. in their model TC-21) carbon fiber reinforced walls. [3]
The exact details of advanced Zippe-type centrifuges are closely guarded secrets. For example, the efficiency of the centrifuges is improved by increasing their speed of rotation. To do so, stronger materials, such as carbon fiber-reinforced composite materials, are used; but details of the material and its protection against chemical attacks are proprietary. Such are also the various techniques that are used to avoid forces causing destructive (bending) vibrations: Lengthening of a (countercurrent) centrifuge improves the enrichment exponentially. [9] But it also decreases the vibrational frequency of mechanical resonances, which increases the danger of catastrophic failure during start-up (as it happened during the Stuxnet event in Iran). Interrupting the cylindrical rotor by flexible bellows controls the low-frequency vibrations, and careful speed control during start-up helps to ensure that the centrifuge does not operate too long at speeds where resonance is a problem. But more (proprietary) measures seem necessary. Therefore Russia stayed with "subcritical" centrifuges (i.e., with small lengths around 0.5–1 m), whereas those of Urenco have lengths up to 10 m.
The Zippe-type centrifuge is difficult to build successfully and requires carefully machined parts. However, compared to other enrichment methods, it is much cheaper and is faster to set up, consumes much less energy and requires little area for the plant. Therefore it can be built in relative secrecy. This makes it ideal for covert nuclear-weapons programs and increases the risk of nuclear proliferation. [3] Centrifuge cascades also have much less material held in the machine at any time than gaseous diffusion plants.
Pakistan's atomic bomb program developed the P1 and P2 centrifuges based on early designs of Urenco; [3] the first two centrifuges that Pakistan deployed in larger numbers but reduce it after 1981 based on estimation require for critical mass. The P1 centrifuge uses an aluminum rotor, and the P2 centrifuge uses a maraging steel rotor, [3] which is stronger, spins faster, and enriches more uranium per machine than the P1. In Pakistan, the Zippe-type centrifuge had a local designation and was known as Centrifuge Khan (after Abdul Qadeer Khan).: 151 [10]
Russian sources dispute the account of Soviet centrifuge development given by Gernot Zippe. They cite Max Steenbeck as the German scientist in charge of the German part of the Soviet centrifuge effort, which was started by German refugee Fritz Lange in the 1930s. The Soviets credit Steenbeck, Isaac Kikoin and Evgeni Kamenev with originating different valuable aspects of the design. They state Zippe was engaged in building prototypes for the project for two years from 1953. Since the centrifuge project was top secret the Soviets did not challenge any of Zippe's claims at the time. [2]
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research. By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants, and is also required for the creation of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade.
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
A centrifuge is a device that uses centrifugal force to subject a specimen to a specified constant force, for example to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities or liquids from solids. It works by causing denser substances and particles to move outward in the radial direction. At the same time, objects that are less dense are displaced and moved to the centre. In a laboratory centrifuge that uses sample tubes, the radial acceleration causes denser particles to settle to the bottom of the tube, while low-density substances rise to the top. A centrifuge can be a very effective filter that separates contaminants from the main body of fluid.
An ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 1 000 000 g. There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Both classes of instruments find important uses in molecular biology, biochemistry, and polymer science.
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate away from the axis of the centrifuge, while the less dense components of the mixture migrate towards the axis. Chemists and biologists may increase the effective gravitational force of the test tube so that the precipitate (pellet) will travel quickly and fully to the bottom of the tube. The remaining liquid that lies above the precipitate is called a supernatant or supernate.
Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF3 that resists any further reaction from the compound. UF6 is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.
Abdul Qadeer Khan,, known as A. Q. Khan, was a Pakistani nuclear physicist and metallurgical engineer who is colloquially known as the "father of Pakistan's atomic weapons program".
Yellowcake is a type of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. It is a step in the processing of uranium after it has been mined but before fuel fabrication or uranium enrichment. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. Typically, yellowcakes are obtained through the milling and chemical processing of uranium ore, forming a coarse powder that has a pungent odor, is insoluble in water, and contains about 80% uranium oxide, which melts at approximately 2880 °C.
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container. A prominent use of gas centrifuges is for the separation of uranium-235 (235U) from uranium-238 (238U). The gas centrifuge was developed to replace the gaseous diffusion method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in series, that achieve successively higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.
Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containing uranium-235 (235U) and uranium-238 (238U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2).
Gernot Zippe was an Austrian born German mechanical engineer and a nuclear physicist who is widely credited with leading the team which developed the Zippe-type centrifuge, a centrifuge machine for the enrichment and collection of Uranium-235, during his time in the Soviet program of nuclear weapons.
Atomic vapor laser isotope separation, or AVLIS, is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions. A similar technology, using molecules instead of atoms, is molecular laser isotope separation (MLIS).
The Dr. A. Q. Khan Research Laboratories, or KRL for short, is a federally funded, multi-program national research institute and national laboratory site primarily dedicated to uranium enrichment, supercomputing and fluid mechanics. It is managed by the Ministry of Energy for the Government of Pakistan via partnership between the universities through the security contractor Strategic Plans Division Force due to its sensitivity. The site is located in Kahuta, a short distance north-east of Rawalpindi, Punjab, Pakistan.
Separation of isotopes by laser excitation (SILEX) is a process under development to enrich uranium on an industrial scale for nuclear reactors. It is strongly suspected that it utilizes laser condensation repression to excite the uranium-235 isotope in uranium hexafluoride (UF6), allowing this lighter molecule to move more rapidly to the outer rim of a gaseous jet and resist condensing compared to the heavier, unexcited 238UF6. This differs greatly from previous methods of laser enrichment explored for their commercial prospects: one using atomic uranium (Atomic Vapor Laser Isotope Separation (AVLIS)) and another molecular method that uses lasers to dissociate a fluorine atom from 235UF6 (Molecular Laser Isotope Separation (MLIS)), allowing the enriched product to precipitate out as a solid.
The National Enrichment Facility (NEF) is a nuclear facility for the enrichment of uranium associated with the Los Alamos National Laboratory. The plant uses a gas centrifuge technology known as Zippe-type centrifuges. It is located 5 miles (8.0 km) east of Eunice, New Mexico. The NEF is operated by Louisiana Energy Services (LES), which is in turn owned by the Urenco Group. As of 2011, LES operates as URENCO USA.
Project-706, also known as Project-786 was the codename of a research and development program to develop Pakistan's first nuclear weapons. The program was initiated by Prime Minister Zulfiqar Ali Bhutto in 1974 in response to the Indian nuclear tests conducted in May 1974. During the course of this program, Pakistani nuclear scientists and engineers developed the requisite nuclear infrastructure and gained expertise in the extraction, refining, processing and handling of fissile material with the ultimate goal of designing a nuclear device. These objectives were achieved by the early 1980s with the first successful cold test of a Pakistani nuclear device in 1983. The two institutions responsible for the execution of the program were the Pakistan Atomic Energy Commission and the Kahuta Research Laboratories, led by Munir Ahmed Khan and Abdul Qadeer Khan respectively. In 1976 an organization called Special Development Works (SDW) was created within the Pakistan Army, directly under the Chief of the Army Staff (Pakistan) (COAS). This organization worked closely with PAEC and KRL to secretly prepare the nuclear test sites in Baluchistan and other required civil infrastructure.
Depleted uranium hexafluoride (DUHF; also referred to as depleted uranium tails, depleted uranium tailings or DUF6) is a byproduct of the processing of uranium hexafluoride into enriched uranium. It is one of the chemical forms of depleted uranium (up to 73-75%), along with depleted triuranium octoxide (up to 25%) and depleted uranium metal (up to 2%). DUHF is 1.7 times less radioactive than uranium hexafluoride and natural uranium.
A centrifuge is a device that uses centrifugal force to separate its components.
