All Spun Up: Tutorial On Gas Centrifuges' Role in Proliferation
Iraq's purchase of high strength aluminum tubes "claimed to be part of an effort to build uranium gas centrifuges to enrich uranium for nuclear weapons" was presented as one of the strongest pieces of evidence for a revived Iraqi nuclear weapons program and, therefore, one of the strongest arguments for going to war. In fact, we now know that the preponderance of the pre-war intelligence suggested that the tubes were not suitable for centrifuges and were intended for conventional rocket bodies. Why was there so much concern about aluminum tubes? What role did they, or might they, play? And how do you recognize a centrifuge tube when you see it? This article discusses why centrifuges are important and their key requirements and characteristics and why they are central to concerns about nuclear proliferation.
Materials for an Atomic Bomb
A proliferator has two routes leading to a bomb, one exploiting plutonium, the other uranium. Plutonium does not occur naturally and has to be created in a nuclear reactor but, once made, it is easy to separate. But the bombs that use plutonium are much harder to design and manufacture. On the other hand, the simplest uranium bomb, in which one slug of uranium is shot into another, thus called a 'gun-assembled' bomb, is quite simple indeed. But the required bomb-grade uranium has been very difficult to prepare, requiring huge, energy-hungry gaseous diffusion plants. Thus, either route presented a would-be proliferator with at least one big technical hurdle, either the bomb or the material. Moreover, the production of either nuclear material required plants that are distinctive and difficult to conceal.
Modern gas centrifuges change this picture. They make the separation of the fissionable uranium-235 much easier and cheaper than it would be using gas diffusion, even potentially easier than producing plutonium, so the easiest route to getting bomb material has become aligned with the simplest gun-assembled bomb design. Modern centrifuges open up a nuclear option for a new group of proliferators with only moderate technical sophistication, such as Iraq, Iran, or North Korea. Moreover, centrifuge enrichment plants are modular, much smaller than gas diffusion plants, and use potentially just five percent of the electrical power of a gas diffusion plant. Thus, they not only make the development of nuclear weapons easier, they make more difficult both the monitoring of supposedly peaceful uranium enrichment for nuclear power and the detection of clandestine bomb-making programs.
Two atoms of the same element can have different masses by having a different number of neutrons. These are called 'isotopes.' Differences in the number of neutrons always have tiny effects on an element's chemical properties but can make huge differences in the nuclear properties, such as whether a nucleus is usable in a nuclear bomb or not. Uranium-235 and uranium-238 are, for all practical purposes, chemically identical but have different numbers of neutrons and wildly different nuclear properties. To separate the two isotopes, we must somehow exploit their slight difference in mass. Centrifuges are one obvious means to separate the small amount of uranium-235 from the greater quantity of uranium-238 found in natural uranium.
Designing a Centrifuge
A gas centrifuge tube spins very fast to create an artificial gravity. The tubes do not spin end over end like a cheerleader?s baton but around their long axis, like an axle. With the tube spinning at full speed, uranium, in the form of a gaseous compound, uranium hexafluoride, is introduced through a small feed tube running along the axis of the centrifuge tube. With these huge pseudo-gravitational fields, the heavier uranium-238 will be slightly more concentrated around the outside of the centrifuge tube than the lighter uranium-235, which will be slightly more concentrated along the central axis of the spinning tube. If the uranium hexafluoride were removed from the central axis of the tube, it would have a higher concentration of uranium-235, or be 'enriched,' compared to what went in. But the enrichment effect is small. There is, however, a simple but quite clever trick that dramatically increases the amount of enrichment possible in one centrifuge. If one end of the tube is heated, convection currents form. (Remember that in the spinning tube the centrifugal forces are such that 'up' is toward the central axis of the tube and 'down' is out toward the wall of the tube. So the heated gas 'rises,' going to the center, while the cooler gas fills in to replace it along the 'bottom,' that is, along the wall.) The lighter uranium-235 molecules will spend, on average, a little bit more time near the axis being swept along toward the cool end of the tube while the heavier uranium-238 molecules will spend a little bit more time near the wall of the tube, being swept toward the hot end. Now, rather than having the gas be slightly enriched toward the center of the tube, it is more enriched at one end of the tube. The degree of enrichment depends now on the length of the tube. Early experimental gas centrifuges had rotor tubes a meter long and the newest rotor tubes can be 10-12 meters long and get enrichment factors per stage a hundred times higher than the gas diffusion process.
Materials for a Gas Centrifuge
The amount of separation of uranium isotopes depends on the outside speed of the rotor, raised to the fourth power. So doubling the rotor speed yields 16 times better separation. Clearly, a centrifuge designer needs to have the tubes spin as fast as they can. Their maximum speed depends on the strength of the tube material. Actually, since the main stresses on the tube come from its own weight, the key characteristic of the tube material is its strength in relation to its weight. Typically, one of three materials is used for centrifuge tubes: aluminum, steel, and fiber-resin composites.
With the extreme stresses involved, only particular alloys of steel and aluminum are adequate. The steel must be a maraging steel, which is a special alloy containing layered crystals of steel and iron. The aluminum must be the highest strength alloy. Aluminum alloys are described by a four digit number. The first two digits indicate the major alloying elements and the second two digits have no particular meaning but uniquely identify a particular formulation. The strongest alloys are the 7000 series, with the '7' indicating that the major alloying element is zinc. Aluminum alloys containing zinc, magnesium and copper are the strongest. The tubes Iraq was trying to import were exactly these alloys. But these alloys are used in other applications where high strength to weight ratios are required, for example, for aircraft frames or aircraft landing gear, or in rockets.
When operating at full speed, the outer wall of the centrifuge tube can be going faster than the speed of sound (although the tubes spin in near vacuum to reduce drag so there is no supersonic shockwave). At these extreme speeds, the balance of the tubes must be near perfect. Even tiny flaws in the finish or irregularities in the manufacture can cause imbalances that stress the bearings.
Manufacturing centrifuge tubes precisely enough to stay in balance is hard but achieving such precision is becoming easier as modern numerically-controlled machine tools continue to improve. Indeed, as greater precision become easier to achieve, it is more likely to be requested. Thirty years ago, a centrifuge designer would demand high precision because it was absolutely essential but designers of other applications for example, tactical rockets, could accept lower precision because exacting standards would have been nice to have but they were not essential and they were expensive. Improvement in the manufacturing process reduces the cost of precision and the tactical rocket designer can ask for precision that would have been too costly before. Thus, due to advances in manufacturing, it is reasonable that the precision of a tactical rocket would be the same as the precision of a centrifuge tube.
The load on the bearings of a spinning centrifuge tube can be reduced by keeping its weight as low as practical, requiring that the walls of the rotor be as thin as practical, typically a millimeter or so thick. The tubes the Iraqis ordered had walls three millimeters thick, thicker than those of modern centrifuges but the thickness appropriate for tactical rockets.
 David Barstow, "How White House Embraced Suspect Iraq Arms Intelligence," The New York Times, 3 October 2004, p. A1.