Monday, January 01, 2007

North Korea's Nuclear Dud

I'm always amazed at how much contemporary news one can find in Scientific American. In the late 1970s, I used a Scientific American article on the economic and military strength of the world's nations to identify four countries I thought the Soviet Union might invade partly because of its growing militarism at the time and partly because of its strangely conservative habit of nibbling at countries around its border. Afghanistan was at the top of that list. I was no hawk at the time; it was just a matter of reading clues in the public literature.

North Korea's strange performance last July 4th when it launched a series of missiles that didn't function well was a sign that their technological prowess was not what the Bush Administration said it was (no surprise after their exaggeration of Iraq's threat to the United States). When North Korea's nuclear explosion this fall showed signs of a being a fizzle, I was suspicious that the Bush Administration was trying to claim a much more serious situation (there was one report, as an example, that maybe North Korea detonated a suitcase bomb). It takes time to evaluate these things and given the seriousness of these things you have to avoid a rush to judgment—that includes the Bush Administration as well as its critics. And for all we know, North Korea may solve its technical problems in the next year or two.

Evaluating the Bush Administration's ability to judge threats is important. Here's a Scientific American article that sheds some light on the North Korea fizzle:
Soon after the news broke that North Korea claimed to have conducted a nuclear test, experts realized that the blast had been much smaller than is usual for a first device. Nuclear explosions are measured in kilotons, an energy release equivalent to that of thousands of tons of TNT. Most countries' first tests range from five to 25 kilotons. For example, the U.S.'s 1945 "Trinity" test had a yield of about 20 kilotons. Yet estimates of the North Korean test clustered around half a kiloton. Reportedly, North Korean officials had told China to expect a blast of four kilotons.

(snip)

Clearly, then, the North Koreans produced some kind of a nuclear damp squib. What could have gone wrong depends on the nuclear fuel used. Apparently, the device relied on plutonium (like Trinity and the Nagasaki bomb) and not uranium (like Hiroshima), a conclusion that is supported by the nature of the air samples, according to U.S. officials. Indeed, North Korea has ample quantities of plutonium, but outside that country no one knows its progress in enriching significant quantities of uranium to weapons-grade levels.

Plutonium weapons have several ways of misfiring. The first depends on the triggering of the plutonium by an implosion process. The implosion must be extremely symmetrical to be fully successful. Typically a combination of fast and slow conventional explosives surrounds a sphere of plutonium (the "core" or "pit"). Engineers must carefully machine all the pieces that make up this explosive shell into shapes that, when detonated simultaneously, produce a precisely spherical shock wave that compresses the plutonium to two to five times its normal density (the more compression, the greater the explosive yield). At the higher density, what was a subcritical mass of plutonium becomes supercritical--that is, one in which a sustained chain reaction takes place, producing the blast.

If the shock wave fails to be completely symmetrical--for example, if a detonator goes off 100 nanoseconds later than the rest--the compression will be less efficient because the core will tend to squirt out in the directions where the shock wave is weaker or arrives late. ... ...

The article is properly cautious about what it all means but I would note that Kim Jong Il was rather contrite towards the Chinese within a week of the test. That strongly suggests that indeed things did not go well.

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