Back to the future

3 September 2003

The demonstration of USEC's gas centrifuge uranium enrichment technology is entering its licensing phase. By the end of the decade, USEC expects to build and operate a commercial centrifuge uranium enrichment plant. By Ron Green

In June 2002, USEC signed an agreement with the US Department of Energy (DoE) to facilitate the company's demonstration and deployment of gas centrifuge uranium enrichment technology, which is referred to as the American Centrifuge™ technology. DoE spent more than $3 billion over 20 years developing the technology.
This past March, the US Nuclear Regulatory Commission (NRC) formally accepted USEC's licence application for the American Centrifuge Demonstration Facility. The NRC anticipates concluding its review by next February. USEC has completed several key milestones ahead of schedule and plans to begin operation of the facility in 2005. This facility will yield important cost, schedule and performance data before the company expects to begin construction of a commercial plant.
History of US Enrichment
By the 1940s, the physics of several methods for isotopic separation were well known, including electromagnetic separation, gaseous diffusion and gas centrifuge. The US government opted to focus its resources, due to the urgency of the Manhattan Project, on the more developed gaseous diffusion technology. Despite its mammoth size and substantial electric power requirements, gaseous diffusion proved to be the best available technology for producing large quantities of enriched uranium. US gas centrifuge research and development (R&D) was focused at the University of Virginia, where Professor Jesse W Beams and his colleagues conducted the first known separation of uranium isotopes using a gas centrifuge in 1941.
Oak Ridge, Tennessee became the site of the world's first gaseous diffusion uranium enrichment plant (GDP), known as K-25. The plant began to ship enriched uranium product for military uses in March 1945. By 1956, the US government had expanded enrichment operations in Oak Ridge and built two more gaseous diffusion plants in Paducah, Kentucky and near Portsmouth, Ohio. Over the next two decades, the emergence of the nuclear power industry prompted an accelerated demand for low enriched uranium.
Projections in the late 1960s and early 1970s for the construction of many new commercial nuclear power plants drove the need for greater quantities of enriched uranium fuel. In 1976, Congress responded by authorising the development of additional enrichment capacity at Portsmouth. Multiple technologies were considered, and the Carter administration announced the next year that Portsmouth would host a gas centrifuge enrichment plant (GCEP).
By the late 1970s, the US gas centrifuge programme had made substantial progress. GCEP consisted of third generation machines, called 'Set III', which were achieving an annual performance of approximately 200 separative work units (SWU) per machine and had accumulated nearly 200 machine-years of operating experience (see Figure 1).
Later the DoE developed 'Set V' machines, known as advanced gas centrifuge (AGC), with an enrichment goal three times greater than Set III. At the same time, the DoE was yielding promising results from another enrichment technology atomic vapour laser isotope separation (AVLIS).
By the late 1970s, the wave of nuclear power plants ordered by electric utilities began to be substantially reduced. Concerns about nuclear waste disposal, regulatory uncertainty and rising construction costs, due to higher interest rates, prompted widespread cancellation of nuclear plant orders. Demand for new enrichment capacity also declined. Dwindling federal R&D funding required that the government choose between centrifuge and laser enrichment technology.
AVLIS laser technology appeared more versatile than the AGC or GCEP projects, and required less funding given its earlier stage of development. In 1985, the DoE announced a revised uranium enrichment strategy that terminated GCEP and AGC activities, and committed to further AVLIS development.
In 1992, the Energy Policy Act created a government corporation, called the United States Enrichment Corporation (USEC), and directed the transfer of DoE's uranium enrichment enterprise to the new government corporation. At the time, uranium enrichment was the only sector of the nuclear fuel cycle still performed by the government. The act directed USEC to establish and run the enrichment enterprise on a commercial basis and then sell the business to the private sector. The act also transferred to USEC the exclusive rights to complete AVLIS development and to deploy the laser enrichment technology as a lower cost replacement to the ageing GDPs. USEC was privatised in July 1998, after five years of operating as a government corporation.
As a private company, USEC continued to fund AVLIS, investing approximately $100 million in its development. As the need for additional research and development funds increased, USEC carried out a comprehensive evaluation of AVLIS. Although the AVLIS enrichment process worked, a comprehensive technical and commercial review led to the conclusion that commercial economic returns would not be sufficient to outweigh the risks and the additional expenditures necessary to commercialise AVLIS. In June 1999, USEC suspended development of AVLIS technology. Recognising the need to secure a replacement for AVLIS, USEC launched a worldwide search to evaluate existing and potential advanced enrichment technologies, including European, Russian, and US gas centrifuges.
US Technology Revisited
USEC's search led to familiar territory DoE's gas centrifuge programme. The DoE centrifuges had been designed, built and proven to operate reliably with an annual production rate of approximately 200SWU per machine. In the AGC programme, DoE demonstrated centrifuge performance well in excess of 300SWU annual output. Based on this data, USEC expects to build and reliably operate an American Centrifuge with approximately 300SWU performance. The American Centrifuge will offer commercial advantages compared to other technologies that were evaluated.
Much of the detail about gas centrifuge technology is either classified for national security reasons or protected by export control restrictions. However, the basic theory and description of a centrifuge are publicly available
To begin with, it is important to acknowledge that the basic centrifuge design in Russia, the USA and Europe have a common origin in the influential work of Dr Gernot Zippe. As a captured prisoner of war, Zippe began work on centrifuges in the Soviet Union in 1946. After his release, he continued his efforts and worked with Beams in the USA from 1958 to 1960. Upon the classification of the technology by the USA, Zippe returned to Europe and continued his work. Zippe's work has influenced all gas centrifuge designs.
Key features of a centrifuge include the rotor, the motor drive, the casing, the vacuum system, suspension systems, and the column that contains the feed, tails and product lines (see Figure 2). The casing encloses the centrifuge and provides two important functions leak tightness to preserve a vacuum and physical protection from the spinning rotor. To reduce friction, the rotor spins in a vacuum. The rotor is a thin-wall right circular cylinder that spins in response to a drive motor. The suspension system holds the rotor upright within the casing. The rotation of the rotor generates the centrifugal force acting on the UF6 gas to be enriched.
In the presence of this force, the slightly heavier 238-isotope in the UF6 gas is forced closer to the inside wall of the rotor than the 235-isotope. The radial separation factor is proportional to the absolute mass difference between the two isotopes. This is fundamentally different from gaseous diffusion where the relative, rather than the absolute, molecular mass difference provides the separation effect. In addition to isotopic separation along the radial dimension, there is also separation along the vertical axis.
UF6 gas is introduced into the spinning centrifuge near the centre of the rotor, where it is acted upon by two fields the centrifugal field induced by the rotor rotation and an internal countercurrent circulation flow induced by an axial thermal gradient along the length of the rotor. The thermal gradient is established by creating thermal non-uniformities of temperature at the rotor end caps or along the rotor wall. The gas near the hotter end cap rises in the centrifugal field moving radially inward while the gas at the colder end cap does the opposite. The countercurrent circulation pattern established through thermal convection is superimposed on the radial flow, resulting in a relatively large assay change between the top and bottom of the centrifuge. An important practical benefit of this axial effect is the enrichment difference at the geometric extremes of the top and bottom of the rotor. This provides convenient extraction points for enriched product and depleted tails.
In a gaseous diffusion plant, the UF6 gas passes through more than 1000 stages to obtain assays of enriched product in the 4-5% U-235 range required for commercial nuclear reactors. By contrast, commercial enrichment assay levels can be achieved in about one hundred times fewer stages in a centrifuge cascade. However, the throughput from a single cascade is relatively small and a commercial plant requires a large number of cascades. Thus, the cascade is the 'basic building block' of a uranium enrichment facility.
Bigger and faster is better
It is well known that the separative performance of a centrifuge can be described by the simplified Dirac expression:

where: r = gas density; D = coefficient of self-diffusion; M = molecular weight difference of isotopes being separated; v = peripheral rotor velocity; R = universal gas constant; T = gas temperature; L = rotor length; h = circulation efficiency.

From this expression, four relationships regarding centrifuge performance can be gleaned:

• Desirability to operate at lower temperature.
• Direct proportionality to rotor length.
• Proportionality to the fourth power of rotor speed.
• Independence to rotor diameter.

There is a practical limit on lower temperatures to ensure that the UF6 gas does not condense. However, two important degrees of freedom remain rotor length and rotor speed. By successfully optimising these two parameters, the American Centrifuge design surpasses other designs in use today. For example, the American Centrifuge design that USEC plans to deploy is more than three times longer than the TC12, which is currently used by Urenco. The TC12 provides more than 50% of Urenco's total capacity, and is the model being proposed for a commercial enrichment plant in the USA.
The maximum speed of a centrifuge is limited by the tensile strength of the rotor material and the ability to attenuate harmonic flexural vibrations. In the 1980s, DoE found that composite carbon fibre is an ideal rotor material because it provides both low weight and high tensile strength. For a given material, the maximum peripheral speed of a rotor can be estimated by the expression:

where: s = rotor tensile strength; r = rotor density.

From this expression, it can be easily concluded that a carbon fibre rotor could safely spin faster than a rotor made from other lightweight and strong materials, such as aluminum alloy or maraging steel. When DoE cancelled its centrifuge programme, carbon fibre composites were exotic materials both high in cost and difficult to make. Today, however, some composites are so commonplace and affordable that they are used in recreational applications such as tennis rackets and bicycles.
Long, small diameter, thin-walled rotors are only rigid at low speeds. As the speed increases beyond a 'critical speed', the rotor experiences flexural vibrations. Centrifuges that operate above this critical speed are referred to as 'supercritical' machines. Rotor vibrations are counteracted by precision balancing that reduces the amplitude to a level that stays within the confines of the casing. With the aid of today's faster computers and high quality manufacturing techniques, rotor balancing is no longer the technical challenge it was in the 1980s.
Of course, in practice, no process reaches its theoretical maximum performance in actual operation. Circulation inefficiency, resulting from back diffusion along the axial concentration gradient in the rotor, limits the actual performance of the centrifuge. According to public documents filed by the first Urenco-led group Louisiana Energy Services in 1991, the TC12 has an approximate annual performance of 40SWU per machine. The American Centrifuge, which will be longer and faster than its European cousin, is expected to produce approximately 300SWU per machine annually.
Design improvements
The first phase of USEC's American Centrifuge demonstration project testing and fabrication is underway in Oak Ridge, the site of DoE's original centrifuge research and development programme. The mission of the demonstration project is to update and fine-tune DoE's basic design. Most of the improvements to the basic design take advantage of engineering and material advances in the 15 years since the DoE programme was terminated. These include advances in motor drive, composite material science, automated manufacturing techniques, and digital instrumentation and control. But the basic DoE machine remains unaltered. USEC's design targets for key parameters have been conservatively selected to be well within the limits of DoE's operating experience (see Figure 3). Therefore, the first phase of USEC's programme is referred to as a 'demonstration' rather than 'development'.
The American Centrifuge design will be initially assembled and tested in the K-1600 centrifuge test facility in Oak Ridge. This facility was used until 1985 to develop and evaluate the AGC machines. It contains test stands with diagnostic instrumentation for assessing the performance of individual machines. USEC began leasing this facility from DoE in December 2002. DoE is providing regulatory oversight for activities at Oak Ridge.
To complete the first step of the demonstration phase, in Oak Ridge, USEC has recruited engineers and scientists from the original DoE programme, as well as other experts in a variety of relevant disciplines. As part of a DoE-approved cooperative research and development agreement (CRADA), key technology experts working at the Oak Ridge National Laboratory have joined USEC's centrifuge demonstration effort. This melding of experienced personnel from government and private industry are reconstituting and improving the already proven centrifuge design on a rapid schedule. As soon as acceptable centrifuge performance is demonstrated, the centrifuge team will 'freeze' the design and begin the next phase of the demonstration: initial manufacturing of a model for operation in USEC's American Centrifuge Demonstration Facility, to be constructed at the former GCEP facility in Portsmouth, Ohio.
Nuclear licensing
In November 1996, USEC obtained NRC certificates of compliance for both the Portsmouth and Paducah GDPs. Then in March 1997, the NRC assumed regulatory jurisdiction for both GDPs from DoE, successfully transitioning the GDPs from DoE to NRC oversight. Three years later in 2001, USEC received NRC approval for a major amendment that doubled the authorised enrichment limit of the Paducah GDP from 2.75% to 5.5% U-235.
American Centrifuge technology will be licensed by the NRC in two separate and distinct steps:

• Demonstration facility (application submitted to the NRC in February 2003, more than two months ahead of schedule).
• Commercial enrichment facility.

USEC has applied for a licence to operate up to 240 full-size centrifuges arranged in a closed loop cascade configuration the lead cascade in the former GCEP building in Portsmouth, Ohio. Using existing facilities and infrastructure significantly reduces project costs and saves time. The former GCEP buildings have been well maintained and can be used for the American Centrifuge Demonstration Facility.
Licensing the American Centrifuge Demonstration Facility is more simple and faster than licensing a commercial plant for three reasons:

• The facility will operate in 'recycle' mode with no continuous feed or withdrawal equipment required.
• The facility will take advantage of many applicable NRC-approved safety programmes already in place at the Portsmouth plant.
• As a demonstration facility, it is not subject to the additional requirements currently applicable to a commercial-scale uranium enrichment facility (such as environmental impact statement and a formal adjudicatory hearing).

A significant number of American Centrifuge Demonstration Facility safety programmes are based on or use existing USEC programmes and site infrastructure in Portsmouth that are independent of the enrichment technology employed (centrifuge or diffusion). These programmes include, but are not limited to: fire protection, health physics, environmental protection and emergency management. On 13 March 2003, the NRC accepted USEC's licence application for further review. The NRC anticipates concluding its licensing review by February 2004.
Successful licensing of the American Centrifuge Demonstration Facility would significantly facilitate the licensing of USEC's commercial plant. A key component of licensing a fuel cycle facility is the submittal of an integrated safety analysis (ISA), as required by NRC regulations. The ISA is a systematic examination of a facility's processes, equipment, structures and personnel activities to ensure that all relevant hazards that could result in adverse consequences have been adequately evaluated and appropriate protective measures have been identified and implemented. Once the ISA has been reviewed and approved by the NRC for the lead cascade, USEC can use many of the same analytical methodologies and lessons learned for commercial plant licensing.
In addition to facilitating the licensing of the commercial plant, the American Centrifuge Demonstration Facility has a number of other important objectives, including:

• Evaluating the reliability and efficiency of full-scale centrifuges.
• Evaluating the integrated operation of centrifuge machines.
• Providing a better basis for plant capital and operating cost estimates.
This should significantly reduce risks associated with commercial-scale deployment of American Centrifuge technology and allows USEC the opportunity to pursue and evaluate various financing and partnership options for construction of the $1 billion to $1.5 billion commercial plant by the end of this decade.
USEC's June 2002 agreement with DoE requires USEC to site the commercial plant at either the Paducah or Portsmouth GDPs, both of which have compelling schedule, regulatory and cost advantages over other sites in the USA. No other site in the country has the unique combination of: readily accessible environmental data; NRC-approved regulatory programmes related to uranium enrichment; availability of skilled labour with uranium enrichment experience; compatibility with current and past uses of the site; and communities eager and willing to host uranium enrichment operations.
The advancement in materials technology in the past decade, the resurgence in the nuclear power industry and prospects for a continued nuclear renaissance underscore the opportunity and importance today of deploying new enrichment capacity. DoE built thousands of large machines and accumulated millions of machine-hours of relevant experience at performance levels superior to other centrifuge technologies in use today. The American Centrifuge is founded upon a proven, reliable, manufacturable and operable design.

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