A uranium enrichment centrifuge does not look like your typical laboratory centrifuge. It consists of a hollow tube, typically 20 centimeters in diameter and two meters tall, delicately balanced to spin at 40,000 rotations per minute; the surface moves at close to a kilometer per second. The tube can be aluminum, although it can be spun faster if made of maraging steel, one of the strongest manufacturing materials known, a low carbon steel named after the marcasite crystals used in its manufacture. It used to be an exotic material, until it became a popular surface for expensive golf clubs. It is a controlled material, on the watch list of the International Atomic Energy Agency. Be suspicious of any rogue country that imports lots of golf clubs.

The centrifuge is filled with uranium hexafluoride, a material that vaporizes at the relatively cool temperature of 57 Celsius. When spun in a centrifuge, the effective force is 200,000 times that of gravity. The heavier U-238 diffuses toward the outside; the lighter, more fissile U-235 is concentrated at the center, where it is scooped off and sent to the next stage. North Korea has large reserves of uranium ore, estimated at four million tons. A centrifuge plant capable of separating enough of the light isotope U-235 to make one nuclear weapon per year could fit inside a small movie theater.

How do we know North Korea is building such a plant? It is too small to draw attention to itself in satellite imagery. Despite their rapid rotation, centrifuges are quiet. And energy consumption is not a helpful signature, since centrifuges consume significantly less power than other enrichment processes. So you cannot find centrifuges easily. But if you learn that a country is importing the key materials and equipment needed (finely balanced aluminum tubes, special bearings, maraging steel) then you can deduce that such a plant is under construction.
 
We know from such indications that North Korea is manufacturing these centrifuges. We suspect that Iraq is making them. So why attack Iraq and not North Korea? This question is common these days, frequently asked by people who think it is improper to attack either. What is the difference between North Korea and Iraq?

The first answer that comes to mind is: there is no difference. Former President Bill Clinton recently said that in the 1990s he had explicitly threatened to attack North Korea's nuclear facilities unless the country ended its nuclear program (New York Times, Dec 15, 2002). Just because President Bush is not openly threatening North Korea at present, don't assume that it is not on his agenda. Prior to North Korea's admission of its centrifuge program, President Bush was interviewed by Bob Woodward of the Washington Post. The president told him, "I loathe Kim Jong-il. I've got a visceral reaction to this guy, because he is starving his people." After Iraq, don't be surprised if North Korea moves up to President Bush's front burner.