Tracking the technology

31 August 2004

Compared to other enrichment processes, centrifuges are simpler to build, operate, and hide from detection. This makes them preferable for use in clandestine weapons programmes. By Jack Boureston

Although we can thank the German scientist Gernot Zippe for developing the basic designs found in modern day centrifuges, the original concept and application of centrifuges for uranium enrichment was pioneered during the 1930s and 1940s by Jesse W Beams, a specialist in high speed devices at the University of Virginia. In the late 1930s he was able to partially separate the isotopes of chlorine, and later in 1941, he and his colleagues conducted the first known successful application of centrifuges to separate uranium isotopes. In undertaking his endeavors, Beams developed three basic designs. These included a small subcritical rotor 76.2mm wide by 355mm long; a subcritical rotor 183mm wide by 1000mm long; and a larger supercritical rotor that was 183mm wide by 3300mm long. Beams’ work attracted much attention at the ongoing Manhattan Project, but centrifuges were ultimately rejected there in favour of diffusion methods of enrichment.

The Americans were not the only ones interested in Beams’ work. Enrichment research played a peripheral role in Germany’s wartime nuclear weapons development activities. At the end of World War II, the Soviet Union combed Germany, Austria, and Czechoslovakia for experts and know-how, seized whole laboratories, nuclear related equipment, uranium, and scientists, and sent them to various locations in Russia to support Soviet efforts to build an atomic bomb.

German scientists that moved to Soviet facilities included Manfred von Ardenne, Gustav Hertz, Adolf Thiessen, and Max Vollmer. Each had previously headed their own laboratories and had worked on various aspects of Germany’s nuclear weapons programme. Other scientists and technicians who worked for the military during the war, or may have worked in companies supporting the war, ended up in detention camps located in the USSR. As the Soviet nuclear weapons effort gained momentum, these people were sent to Soviet research facilities to continue their work.

In the summer of 1946, von Ardenne saved Zippe from the Krasnogorsk detention camp where he was being held, and took him to ‘Institute A’ in Sukhumi, where he worked directly under Max Steenbeck as the head of all experimental development of rotor dynamics. Work at Institute A included research on electromagnetic separation of uranium isotopes (under von Ardenne); techniques for manufacturing porous barriers (under Thiessen); and molecular techniques for separation of uranium isotopes (under Steenbeck). With the help of an expert formerly at the German firm AEG, Zippe’s team built a magnetically suspended centrifuge with a rotor that was approximately 25-30cm long, 22mm in diameter, and had a wall thickness of approximately 0.15mm.

In his presentation to a workshop in 2000, Zippe reported that in 1952 he and his team, while working in the Soviet Union, learned how to combine several short subcritical centrifuges with flexible bellows to design supercritical rotors. According to Zippe, during this time he conducted endurance tests in which his team operated a group of six 3m-long centrifuges, containing 10 short tubes of 58mm diameter and connected by flexible bellows, for more than 1000 hours. Later Zippe included pitot tubes inside the centrifuges to extract separated uranium, used molecular pumps to maintain the necessary vacuum around the spinning rotor, and added aluminium-nickel ring magnets to the top of the centrifuge to serve as suspension bearings. In 1956, after he left the Soviet Union, Zippe began to work with the German firm Degussa on the mechanics of high speed rotors, and improve his centrifuge designs. Some analysts have estimated that the German scientific contribution in centrifuges and other aspects of nuclear technology accelerated the Soviet Union’s nuclear weapons effort perhaps by as much as five to ten years.

While attending a conference in 1957, Zippe realised how advanced his Soviet team’s designs were, and he subsequently patented what he termed the ‘short bowl gas centrifuge’ technology. Later he continued his design work and further advanced the combining of centrifuges into more powerful supercritical machines with rotors manufactured out of maraging steel to minimise material creep at high speeds.

From 1958 to 1960, Zippe worked in the USA with Beams. The two cooperated fully, exchanging information between Degussa, the University of Virginia, the US Atomic Energy Commission, and the German Ministry for Atomic Energy. They conducted several centrifuge endurance tests, obtaining a 30% efficiency of separative work.

In 1964, Germany formed the state-owned company Gesellschaft für Kernverfahrenstechnik (GKT) in Jülich to develop a reliable and economic centrifuge for uranium enrichment on an industrial scale. In 1970 the company was privatised. During the 1970s, Hoechst AG (25%), RWE (37.5%) and Preussenelektra/VEBA (37.5%) became shareholders of the newly founded Uranit GmbH. On 4 March 1970, Germany, Netherlands and the UK signed the Treaty of Almelo, which was the basis for collaboration between the three countries for the development and industrial exploitation of centrifuge technology to enrich uranium.

In 1971, the three industrial partners: BNFL, Ultra-Centrifuge Nederland NV (UCN) and Uranit GmbH founded the jointly-owned Urenco Ltd in Marlow, UK to market their enrichment services. By 2004, the three Urenco partners had successfully developed the gas centrifuge from early stages based on low-power subcritical aluminium and steel models with throughputs of 1SWU/y or less, to highly efficient supercritical machines based on advanced carbon fibre technology and throughputs reaching towards 100SWU/y.

White House photo by Tina Hager

President Bush and Jon Kreykes
President Bush and Jon Kreykes of the National Security Advanced Technologies group look over equipment obtained from Libya at the Oak Ridge National Laboratory


When Urenco was founded in 1971, it was the only commercial centrifuge enrichment programme set up exclusively for production of low-enriched uranium (LEU) and outside the purview of established nuclear weapons powers such as the USA, the Soviet Union and China. During the 1970s, partly on the basis of technology stolen from the Urenco programme, Pakistan joined the group of countries that aimed to enrich uranium for producing nuclear weapons.


In about 1972, Pakistan decided to embark on nuclear weapons development. President Zulfikar Ali Bhutto travelled to Egypt, Libya, and Saudi Arabia to rally support for developing an Islamic Bomb. Pakistani planners developed parallel programmes for reprocessing and uranium enrichment to obtain fissionable material. Because it lacked expertise and technology, Pakistan organised a procurement network and sent agents out to Pakistani embassies to obtain it. Pakistan may have focused its efforts on enrichment after the USA pressured other countries not to assist Pakistan in reprocessing. In addition, a Pakistani metallurgist, Abdul Qadeer Khan, during the first half of the 1970s, gained access to technology developed in Europe for the Urenco programme and he learned from pioneering scientists about centrifuge-related technologies. Khan was hired by the Physics Dynamic Research Laboratory (FDO), which did contract work for the Netherlands Urenco partner UCN. In 1974, while working at UCN, Khan obtained access to classified design documents for centrifuges capable of producing an estimated 2-5SWU/y.

On the basis of a Dutch government investigation carried out from 1975 to 1980, and corroborative evidence obtained by the International Atomic Energy Agency (IAEA) and Western governments investigating enrichment programmes in Iran and Libya during 2003 and 2004, it is now believed that Khan obtained design information for a variety of Urenco centrifuges before he left Europe for Pakistan in 1975. The Dutch investigation had concluded that Pakistan obtained the design of a UCN centrifuge called CNOR/SNOR and replicated it in Pakistan. Before 1992, Western intelligence agencies certified that Pakistan had built two German centrifuges, G-1 and G-2, and that, assuming a mix of centrifuge types with throughputs up to about 5SWU/y, Pakistan had an enrichment capacity of between 9000 and 15,000SWU/y. The capacity allowed Pakistan to produce HEU for several nuclear weapons, which were tested in 1998.


In January 2003, Nuclear Fuel reported that Western intelligence agencies had identified Khan’s laboratory, KRL, as the primary source for centrifuge design information for a hitherto undeclared enrichment programme in Iran.

In February 2003, taking the lead from the National Council of Resistance of Iran, an Iranian opposition group who in 2002 had exposed an enrichment site in Natanz, inspectors from the International Atomic Energy Agency (IAEA) made several visits and viewed a centrifuge enrichment plant under construction. In August 2003, Iran disclosed to the IAEA that it had launched a centrifuge enrichment programme in 1985, and had obtained centrifuge blueprints through a foreign intermediary in ‘about 1987’. After months of denials, at the end of 2003 Pakistan confirmed the report that Khan had diverted its enrichment know-how to Iran, Libya, and North Korea.

The Atomic Energy Organisation of Iran told the IAEA that its enrichment programme consisted of three phases. During phase one (1985-1997), which centred on the Plasma Physics Laboratories in Tehran, efforts were concentrated on achieving an operating centrifuge. To do this the Iranians acquired components from abroad through foreign intermediaries, or directly by the Iranians themselves. Iran acquired high-strength aluminium, maraging steel, electron beam welders, balancing machines, vacuum pumps, computer-numerically controlled machine tools, and flow-forming machines for both aluminium and maraging steel. Many of these items were obtained in Europe, especially from Germany and Switzerland. It is believed that suppliers trained Iranians in the use of critical equipment and technologies needed in a centrifuge programme. According to a November 2003 IAEA report on Iran, the Iranians were able to acquire 2000 components and some subassemblies. In Phase 2 (1997-2002), activities were carried out at the Kalaye Electric Company, located in a suburb of Tehran. During this phase, efforts were focused on centrifuge construction, assembly, and mechanical testing. Phase 3 (since 2002), has involved research, assembly, installation, and completion of centrifuge cascades in the centrifuge facilities at Natanz. During 2003, the IAEA determined that Iran had set up centrifuges at Natanz which were based on early Urenco efforts.

While the IAEA has reported to its board of governors on Iran’s centrifuge development programme throughout 2003 and 2004, a satisfactory and consistent description of the centrifuges at Natanz in the open literature has remained elusive. Within a week of the initial IAEA visit to Natanz in late February 2003, sources told Nuclear Fuel that the centrifuges represented a supercritical aluminium model likely having several segments and a throughput considerably higher than for simple aluminium machines of only about 1SWU/y or less. According to reports appearing over half a year later, some analysts suggested that the centrifuges may have been ‘G-2 type’ machines or based on a different Urenco machine called M-4. (The G-2 is a German centrifuge with two rotor segments; M-4 is a Dutch machine with four segments). A report by the Carnegie Endowment, citing IAEA findings, described the Natanz plant as outfitted with so-called ‘P-1’ machines, an early Pakistan model. But while both Nuclear Fuel and Albright and Hinderstein have suggested that the Natanz machines are articulated, in another report to the board of governors in 2004, the IAEA identified P-1 as G-1. Likewise, experts close to the Urenco programme and Urenco government officials have invariably described P-1 as a ‘single-lidded machine’ without bellows or simply as basically identical to Urenco’s G-1.


Libya’s interest in uranium enrichment began during the early 1980s. A subsequent clandestine programme was unravelled by an investigation by US and UK intelligence which then led to a negotiated termination of the programme and the seizure of relevant components by the US government. The programme has also been subject to investigation by the IAEA, which has described it as featuring equipment for both P-1 and P-2 centrifuges.

Libya told the IAEA this year that regardless of assistance provided by a European expert, it failed to produce a working centrifuge, but it reported that Libya gained experience in the design and operation of centrifuge equipment and related technologies. During its inspections of Libyan components, the IAEA discovered several unfinished, maraging steel cylinders in Libya’s inventory of centrifuge components from the early 1980s period. The cylinders however have the same parameters as the more advanced P-2 centrifuges obtained by Libya in September 2000.

Libya has stated that in 1997 it received 20 assembled P-1 centrifuges and most of the components for another 200 P-1 centrifuges from an unnamed supplier state.

Furthermore, in September 2000, Libya imported two P-2 type test centrifuges and several small cylinders, containing UF6, from an unnamed supplier state through a network of foreign intermediaries. It also ordered 10,000 more P-2 centrifuges, which began to arrive from outside sources in December 2002. By the time Libya decided to dismantle its WMD programmes in December 2003, a large number of P-2 centrifuge components and supporting equipment was already in Libya’s possession. Similarly, Libya imported equipment for a large precision machine shop that it planned to use for domestic centrifuge assembly and production. The IAEA examined the centrifuge components and supporting equipment, as well as the machine shop during its inspections in January 2004, and found all of it boxed and unopened.

As for the case for centrifuges which Iran set up at Natanz, the IAEA has found HEU contamination on the first two complete P-2 centrifuges and on some of the P-2 components which were obtained by Libya. All centrifuges and related components and equipment have been removed from Libya and shipped to the USA between January and March 2004. The IAEA continues to analyse the centrifuge design drawings and documents, as well as computer data with centrifuge-related information, such as assembly and test instruction manuals that Libya had reportedly received from Khan and a network of suppliers supported by Khan. The IAEA is also investigating Libya’s participation in various centrifuge-related training programmes provided by experts at locations in Africa, Asia, Europe, the Middle East and South East Asia.


Iraq’s centrifuge programme probably started sometime in the early 1980s. A trail left by Iraqi procurement agents in Europe and elsewhere during the 1980s led intelligence analysts to suspect that Iraq was making progress in developing centrifuges for uranium enrichment, but the full scope of the programme became known only after the 1991 Gulf War, when the IAEA and the UN were empowered by the UN Security Council to investigate and then destroy Iraq’s illegal weapons programmes.

As in Pakistan, the Iraqi programme was supported by technology that was provided by experts in Europe who had been associated with the Urenco programme, principally through a partnership between Urenco and the German firm MAN Technologie AG. At least two experts from MAN aided Iraq in key areas related to centrifuge design and development, and they provided Iraq with blueprints and design know-how for G-1 type machines, as well as a nearly-complete set of blueprints for a then advanced supercritical carbon fibre centrifuge developed by Urenco called TC-11.

According to IAEA inspectors from one US national laboratory, the IAEA team found blueprints in Iraq with proprietary markings from Uranit GmbH, then Urenco’s German technology holder. In addition, the IAEA during the 1990s had concluded that Iraqi spies had penetrated another German firm, Interatom, and may have obtained cascade design information from the German centrifuge programme. According to UN inspection reports, Iraq had put together before the Gulf War a multi-staged plan designed to culminate in 1996 with the operation of a 500 machine cascade of G-1 type centrifuges.

North Korea

Over a year before Pakistan confirmed that Khan had tried to help North Korea’s secret nuclear programme, the US government accused North Korea of violating the 1994 bilateral Agreed Framework, which suspended North Korea’s independent nuclear activities, by trying to enrich uranium with centrifuges.

Compared to the level of information on centrifuge development activities available for Pakistan, Iran, Iraq, and Libya, little is known in the open literature about North Korea’s suspected enrichment effort. Lack of certainty about the extent of North Korean activities in this area this year prompted China to doubt whether North Korea had a uranium enrichment programme. But official analysts in Japan, South Korea, and the USA said they are satisfied that a centrifuge enrichment programme is there. According to US officials, since 2003, it is suspected that North Korea has tried to set up centrifuges based on either G-1 or G-2 models, on the basis of assistance provided by Khan in Pakistan.

Some information suggests that North Korea initiated its enrichment effort during the late 1990s, but it could have been started sometime in the 1980s. A 2003 report from the Nautilus Institute in Berkeley California suggests that North Korea may be constructing a facility with a capacity of about 2000 centrifuges, each with a throughput of around 1SWU/y. According to a report carried out for the Japanese government earlier this year, “if North Korea operated 2700 centrifuges for three years, using feedstock of 17 metric tons of UF6, it could produce 60kg of HEU enriched to 90% U-235.” Western officials evaluating that report concluded: “Under this assumption that (level of) production would require about 4000SWU/y, suggesting that the centrifuges might have an average throughput of 1-2SWU/y. This would be consistent with the assumption of a G-1 type centrifuge.”

According to a recently unveiled CIA report, in the late 1990s, the Khan network transferred UF6, centrifuges and perhaps warhead designs to North Korea. An American official who saw the report said North Korea had received a package similar to the one that Khan provided Libya. In February 2004, CIA director George Tenet testified to the US Congress that North Korea is “pursuing a production-scale uranium enrichment programme.” The New York Times reported that Khan stated he had begun negotiating with North Korea for the sale of equipment in the 1980s, but did not begin ‘major shipments’ until the late 1990s. He reportedly admitted to shipping centrifuge designs, a small number of assembled centrifuges, depleted UF6 gas, and a ‘shopping list’ of equipment needed to produce ‘thousands’ more, to North Korea.


This history of the proliferation of centrifuge know-how for uranium enrichment shows that it was possible for rogue states to obtain this technology during the 1960s and 1970s. At this time, European countries that prompted their industry to learn how to enrich uranium, may have been preoccupied by concerns that a de facto monopoly on enriched uranium by the USA could prevent Europe from fuelling their power reactors. In this environment, it was possible for third parties to obtain access to key technologies related to uranium enrichment and, in the case of Khan, to obtain access to Urenco’s design data by tapping organisations which performed contract engineering and production work for that company. According to a veteran of the programme, Khan was given access to a wealth of design data for the G-2 centrifuge because, soon after the Treaty of Almelo was signed, UCN was ‘in a hurry’ to set up that German machine in the Netherlands.

Awareness of the proliferation risks in centrifuge development gathered slowly over time. Reacting to Khan’s activities at FDO, during the second half of the 1970s, the Dutch government probed for security weaknesses in its centrifuge programme and made adjustments. But the lessons were not generalised and Germany went through a similar experience in the early 1990s when it was confirmed that experts working for MAN had diverted know-how to Iraq. The recent revelations that European technology purloined by Pakistan has now been shared with Iran, Libya, and North Korea led the Dutch government this year to comment that security adjustments in the centrifuge programme were deemed necessary, particularly in the area of ‘need to know’.

After the findings that Iraq had tapped Europe’s centrifuge knowledge base with the help of German experts, a full-scale probe of security at Urenco was carried out during the 1990s. According to experts at the company at that time, and IAEA officials who were apprised of its results, the probe turned up no evidence that any Urenco personnel had been involved in any diversion of its technology to outsiders. But the investigation did, these sources said, prompt considerable tightening of internal technology controls in the organisation and severely limited access to third parties, especially consultants and contract organisations.

Only after Europe was stunned by revelations after 1975 that Pakistan had succeeded in stealing its technology for uranium enrichment did it confirm that there were numerous reasons why a country aiming to secretly develop nuclear weapons would target centrifuge technology as a means to produce fissionable material. Centrifuges have a large enrichment factor and can be flexibly configured to reflect the intention, urgency, and timeframe for a country to obtain weapons-grade HEU. A centrifuge cascade is also easier to hide than an enrichment plant relying on, for example, gaseous diffusion or electromagnetic separation technologies, and it requires far less electrical power to operate than the alternatives. Centrifuges are also easier to operate, once initial development difficulties are mastered. Once a centrifuge is installed, balanced, and running, it operates under minimum maintenance. This means that a country with little industrial infrastructure and manpower might develop a centrifuge capability more easily than alternative enrichment methods. The centrifuge has been with us for over half a century, and a wealth of information, some in the open literature, some classified but stolen and available in a black market, is available to budding proliferators.

Author Info:

Jack Boureston is the managing director at FirstWatch International, a research consultancy that assesses the nuclear proliferation potential of programmes, nations, and non-state actors. The author would like to thank Mark Hibbs for his research contribution, editorial comments and review, for this analysis

President Bush and Jon Kreykes President Bush and Jon Kreykes
Natanz Natanz

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