Gas Centrifuge Uranium Enrichment
A method in widespread use is the gas centrifuge [Urenco (Netherlands, Germany, UK), Russia, Japan] in which UF6 [Uranium Hexafluoride]gas is whirled inside complex rotor assemblies and centrifugal force pushes molecules containing the heavier isotope to the outside. Again, many stages are needed to produce the highly enriched uranium needed for a weapon, but centrifuge enrichment requires much less electricity than either of the older technologies.
The use of centrifugal fields for isotope separation was first suggested in 1919; but efforts in this direction were unsuccessful until 1934, when J.W. Beams and co-workers at the University of Virginia applied a vacuum ultracentrifuge to the separation of chlorine isotopes. Although abandoned midway through the Manhattan Project, the gas centrifuge uranium-enrichment process has been highly developed and used to produce both HEU and LEU. It is likely to be the preferred technology of the future due to its relatively low-energy consumption, short equilibrium time, and modular design features.
In the gas centrifuge uranium-enrichment process, gaseous UF6 is fed into a cylindrical rotor that spins at high speed inside an evacuated casing. Because the rotor spins so rapidly, centrifugal force results in the gas occupying only a thin layer next to the rotor wall, with the gas moving at approximately the speed of the wall. Centrifugal force also causes the heavier 238 UF6 molecules to tend to move closer to the wall than the lighter 235 UF6 molecules, thus partially separating the uranium isotopes. This separation is increased by a relatively slow axial countercurrent flow of gas within the centrifuge that concentrates enriched gas at one end and depleted gas at the other. This flow can be driven mechanically by scoops and baffles or thermally by heating one of the end caps.
The main subsystems of the centrifuge are (1) rotor and end caps; (2) top and bottom bearing/suspension system; (3) electric motor and power supply (frequency changer); (4) center post, scoops and baffles; (5) vacuum system; and (6) casing. Because of the corrosive nature of UF6 , all components that come in direct contact with UF6 must be must be fabricated from, or lined with, corrosion-resistant materials. The separative capacity of a single centrifuge increases with the length of the rotor and the rotor wall speed. Consequently, centrifuges containing long, high-speed rotors are the goal of centrifuge development programs (subject to mechanical constraints).
The primary limitation on rotor wall speed is the strength-to-weight ratio of the rotor material. Suitable rotor materials include alloys of aluminum or titanium, maraging steel, or composites reinforced by certain glass, aramid, or carbon fibers. At present, maraging steel is the most popular rotor material for proliferants. With maraging steel, the maximum rotor wall speed is approximately 500 m/s. Fiber-reinforced composite rotors may achieve even higher speeds [advanced design carbon fiber rotors can exceed 600 m/sec]. However, the needed composite technology is not within the grasp of many potential proliferants. Another limitation on rotor speed is the lifetime of the bearings at either end of the rotor. Rotor length is limited by the vibrations a rotor experiences as it spins. The rotors can undergo vibrations similar to those of a guitar string, with characteristic frequencies of vibration. Balancing of rotors to minimize their vibrations is especially critical to avoid early failure of the bearing and suspension systems. Because perfect balancing is not possible, the suspension system must be capable of damping some amount of vibration.
One of the key components of a gas centrifuge enrichment plant is the power supply (frequency converter) for the gas centrifuge machines. The power supply must accept alternating current (ac) input at the 50- or 60-Hz line frequency available from the electric power grid and provide an ac output at a much higher frequency (typically 600 Hz or more). The high-frequency output from the frequency changer is fed to the high-speed gas centrifuge drive motors (the speed of an ac motor is proportional to the frequency of the supplied current). The centrifuge power supplies must operate at high efficiency, provide low harmonic distortion, and provide precise control of the output frequency.
The casing is needed both to maintain a vacuum and to contain the rapidly spinning components in the event of a failure. If the shrapnel from a single centrifuge failure is not contained, a “domino effect” may result and destroy adjacent centrifuges. A single casing may enclose one or several rotors.
Although the separation factors obtainable from a centrifuge are large compared to gaseous diffusion [ranging from 1.01 to over 1.10], several cascade stages are still required to produce even LEU material. Furthermore, the throughput of a single centrifuge is usually small, which leads to rather small separative capacities for typical proliferator centrifuges. Separation factors depend on the absolute mass difference between isotopes (not the mass ratio) and the square of the peripheral speed. Separation factors for U-235/238 range from 1.026 for a 250 m/sec centrifuge to over 1.233 for a 600 m/sec centrifuge.
A single centrifuge might produce about 30 grams of HEU per year, about the equivalent of five Separative Work Unit (SWU). As as a general rule of thumb, a cascade of 850 to 1,000 centrifuges, each 1.5 meters long, operating continuously at 400 m/sec, would be able to produce about 20-25 kilograms of HEU in a year, enough for one weapon. One such bomb would require about 6,000 SWU.
A typical centrifuge facility appears to have a capacity of 10-20 SWU/meter square, and to consume in the range of 40-50 kWh per SWU. A facility capable of producing one bomb per year would thus require about 600 square meters of floor space, and consume in the range of about 100 kWe.
With current technology, a single gas centrifuge is capable of about 4 separative work unit [SWU] annually, while advanced gas centrifuge machines can operate at a level of up to perhaps 40 SWUs annually. Separative Work Unit (SWU) is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, ie the extent of increase in the concentration of the U-235 isotope relative to the remainder. The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass F of feed of assay xf into a mass P of product assay xp and waste of mass W and assay xw is expressed in terms of the number of separative work units needed, given by the expression SWU = WV(xw) + PV(xp) - FV(xf), where V(x) is the "value function," defined as V(x) = (1 - 2x) ln((1 - x)/x).
A kilogram of LEU requires roughly 11 kilograms U as feedstock for the enrichment process and about 7 separative work units (SWUs) of enrichment services. To produce one kilogram of uranium enriched to 3.5% U-235 requires 4.3 SWU if the plant is operated at a tails assay 0.30%, or 4.8 SWU if the tails assay is 0.25% (thereby requiring only 7.0 kg instead of 7.8 kg of natural U feed).
An implosion weapon using U235 would require about 20 kg of 90% U235. Roughly 176 kg of natural uranium would be required per kg of HEU product, and about 230 SWU per kg of HEU, thus requiring a total of about 4,600 SWU per weapon. To enrich natural uranium for one gun-type uranium bomb would requires roughly 14,000 SWUs. Thus, producing one HEU weapon in a year would require between 1,100 to perhaps 3,500 centrifuges.
About 100-120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. A 20,000 kg-SWU per year centrifuge plant would fit within a typical factory building and would consume only 600 kW electrical power. The power consumption of a plant using laser isotope separation would be a factor of three smaller.
Enrichment costs are related to electrical energy used. The gaseous diffusion process consumes some 2400 kWh per SWU, while gas centrifuge plants require only about 60 kWh/SWU. At a tails assay of 0.30% U-235 in the enrichment plant, 4.3 SWU per kg of 3.5% enriched product is required, at 50 kWh/SWU for the modern centrifuge plant or up to 2400 kWh/SWU for the older gaseous diffusion plant.
The electrical consumption of a gas centrifuge facility is much less than that of a gaseous diffusion plant. Consequently, a centrifuge plant will not have the easily identified electrical and cooling systems typically required by a gaseous diffusion plant. Typically, about 100 kilowatt-hours are required per separative work unit [SWU], and each centrifuge can produce between 1 and 2 SWU per year.
The specific energy consumption is 2300-3000 kWh/SWU for Gaseous Diffusion, versus 100-300 kWh/SWU for gas centrifuge. The number of stages required to produce LEU is about 30 times larger in the diffusion plant than in the centrifuge plant. The corresponding equilibrium time is significantly longer in diffusion plants (months) as compared to centrifuge plants (hours). This effect, more intensive when the diffusion plant processes Uranium with higher enrichments, makes difficult and time consuming any significant change of the modus operandi of a gaseous diffusion plant. The large in-process inventory in the diffusion plant (a few tons in a small-scale diffusion plant) indicates the importance of closing the Uranium balance in this facility. On the other hand, for centrifuge plants, the small equilibrium time, small in-process inventory and the flexibility to change the cascade design (parallel to series) determine the importance of verifying that the plant is operating as declared.
In the 1970s, DOE revived the development of the gas centrifuge enrichment process, and built a pilot plant at K-25 in Oak Ridge. The success of this project led to the construction of a full-size gas centrifuge plant at the Portsmouth Plant in 1977. The gas centrifuge enrichment process was investigated by the United States over several decades until the mid-1980s and is currently used in a number of enrichment plants around the world. The Department and its predecessor agencies previously conducted R&D on gas centrifuge technology at Oak Ridge, Tennessee. Parts of this infrastructure and equipment, components, and expertise still exist there. Additionally, under the U.S. Gas Centrifuge Program, the government designed, constructed, and operated portions of a GCEP at Portsmouth. Although the GCEP was canceled in 1985 in favor of the AVLIS program, it was not canceled before portions of the GCEP had been successfully demonstrated. Much of the GCEP facilities, equipment, components, and expertise still exist at the Portsmouth site.
One factor in the decision to terminate development of the gas centrifuge was that the costs to enrich uranium were projected to be unfavorable when compared to projected costs for the AVLIS process. An element of the high cost of the gas centrifuge process was the cost of equipment and materials. Since 1985, there have been significant improvements in the properties of high-strength, light-weight materials, while the cost of these materials has dropped by about a factor of four. As a result there is the prospect that the use of these materials can dramatically improve the economics of gas centrifuge technology, making it attractive for commercial uranium enrichment. However, some design, development and verification testing is needed to demonstrate the projected performance and costs. The goal of the proposed gas centrifuge development activity would be to design an advanced gas centrifuge using new materials, to demonstrate the improvement in enrichment performance of the new design, and to collect and assess reliability and operability data to establish the potential economic performance of this technology on a commercial scale.