Gas Centrifuge a Sensitive Nuclear Asset
Syed Rezwi
A great deal has been said about the recent illegal transfer of sensitive nuclear technology, especially the ‘Gas Centrifuges’. As the controversy gets deeper it becomes essential to describe and give a brief background on the development and of this nuclear component, and also explain how the equipment supports in the development of weapon of mass destruction.
Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources.
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 (highly enriched uranium) and LEU (low enriched uranium). 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 UF 6 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 UF 6 molecules to tend to move closer to the wall than the lighter 235 UF 6 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 UF 6, all components that come in direct contact with UF 6 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.
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-rein-forced composite rotors may achieve even higher speeds. 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, 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. To be able to produce only one weapon per year, several thousand centrifuges would be required.
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.
A modern centrifuge runs for more than 10 years with no maintenance. An advantage of the centrifuge process is its low energy consumption. The gas centrifuge process had already been commercially developed by the Russians and by Urenco in the United Kingdom, Germany, and The Netherlands. Gas centrifuges have been used in Europe for about 30 years for enriching uranium.
The principal hazards at an enrichment plant are the chemical hazards in handling UF6. If UF6 contacts moisture in air, it reacts to form hydrogen fluoride and uranyl fluoride. The chemical hazards of compounds of uranium in soluble form such as UF6 and uranyl fluoride are much greater than the radiological hazards of those same compounds. In addition, hydrogen fluoride can be very dangerous if inhaled and represents the principal hazard at an enrichment plant. These hazards are controlled by plant design and administrative controls to confine soluble uranium compounds. The radiological hazards are relatively low and containers of natural, enriched, and depleted uranium can be handled without additional shielding. Requirements for shipping UF6 are generally equivalent to requirements for shipping non-radioactive corrosive materials.
Enrichment processes generate a product of 3 to 5 percent U-235 for use as nuclear fuel and a product of depleted uranium (about 0.3 percent U-235). The depleted uranium has some commercial applications in counterweights and in antitank armaments.
After this recent nuclear fiasco, Pakistani Authorities need to take a very proactive approach in securing these nuclear assets and should take extreme measures that such an incident may not and should not happen again. Pakistan had been under the scope of several U.S. and international lobby’s that are actively promulgating nuclear non-proliferation. Most of these organizations consider Pakistan’s nuclear assets a threat to the region and to the rest of the world. The fact is that Pakistan is being dealt with some exception, due to its active participation in the United States war on terrorism is guarding Pakistan greatly, but this leeway will disappear in time as America will curtail its involvement in Afghanistan, and the issue of nuclear proliferation will come back to haunt Pakistan once again, forcing the country to either roll back its nuclear program or to persuade it to abide by higher degree of restrictions, which will be detrimental to countries sovereignty and its stature in the modern society.
With the recent admission by Pakistan’s most revered scientist Dr. Abdul Qadeer Khan; the fear of nuclear mayhem emerging from Pakistan has been confirmed. A statement made by President of the United States George Bush and Secretary of State Colin Powell favoring Pakistan’s stand in the investigation has indeed helped President Musharraf and his government to hold back the media onslaught and has also effectively taken Pakistan out of the limelight. But it is also not certain if the Bush Administration will receive the same level of support from the Americans in this year’s election, they are also under intense scrutiny due their misjudgments in the Iraqi incursion. Who will deliver for Pakistan then?
Democratic Congressman Frank Pallone the former co-chair of the India Caucus, and a loyal and staunch supporter of India has given some very damaging statement against Pakistan’s nuclear establishments, he has called for U.S. and international monitoring of Pakistan's nuclear program. He also suggested that the Bush administration should re-impose the Symington sanctions on Pakistan as a result of its "active yet covert nuclear exchange program". His policies against Pakistan have been very effective in past and his call for the international scrutiny could not be considered futile. India, with its uninterrupted democratic governments is seen as more responsible state at Capital Hill compared to Pakistan. Our weak democratic institution has made us vulnerable to pressure tactics by Pallone and other lawmakers like him.
Acquiring sensitive fissile technology for developing nuclear deterrence was in a way easy for Pakistan but holding on to this technology is a task of immense responsibility. The transfer of fissile related technology to Iran, Libya and North Korea has been very damaging to Pakistan’s credibility as a responsible nuclear state. Pakistani authorities should not let matter to rest. They should aggressively implement programs and strategies to secure the nuclear program of Pakistan and make sure such a debacle does not occur again.
The writer is the member of executive and editorial board of Association of Pakistani Professionals and a Mechanical Engineering Professional based in New York City. Forward your comments to syedrezwi@aopp.org.
