See Inside January 2007

Kim's Big Fizzle

The Physics Behind A Nuclear Dud


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.

Some commentators immediately speculated that the explosion had not been nuclear, but air samples collected two days after the explosion confirmed the nuclear nature of the explosion. One such signal is the detection of radioactive isotopes of the element xenon, a chemically inert gas produced by the atom splitting that takes place in these blasts, which readily seeps out even from underground tests.

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. Another potential troublemaker is the initiator, a small device at the center of the core that emits a burst of neutrons to start the chain reaction reliably at a precise stage of the implosion. An initiator going off early or late--or not at all--reduces the yield.

Early triggering of the explosion, a kind of fizzle called a predetonation, can also occur when too much of the isotope plutonium 240 is present. The fuel rods of nuclear reactors produce the desired isotope, plutonium 239, but the longer it remains in the reactor, the more of it becomes plutonium 240. Plutonium 240 constantly emits tens of thousands of times more neutrons a second than plutonium 239. Although neutrons are the key particles in producing a nuclear chain reaction, an excess of them early in the implosion is a recipe for predetonation.

Whatever happened near P'unggye on October 9, probably only Kim Jong Il's scientists know. In any case, the technological shortcomings of the blast have not significantly altered the political implications.

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