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Monday, October 9, 2006

Curiously Low Yield in Reported North Korean Bomb Test

The United States Geological Survey is now reporting the magnitude of the claimed North Korean nuclear test as 4.2. This seems to be curiously low. Now, estimating explosive yield from the body magnitude of a seismic event is a tricky business, and requires knowledge of details such as the depth of the detonation and the geological properties of the surroundings, but a magnitude around 4.2 is what you'd expect for a detonation of around one kiloton. The “natural size” of a crude fission bomb is in excess of 10 kilotons, from which you'd expect a magnitude closer to 5 (recall that the Richter scale is logarithmic).

We'll have to wait for results from the Comprehensive Nuclear Test-Ban Organisation seismic network to refine the yield estimate, but it's worth noting that if the USGS figure of 4.2 is confirmed and no radionuclide release is detected, there is no real evidence that a successful nuclear test was conducted at all. A yield in the low kilotons could simply be the result of blowing up a big pile of high explosive, something which has been done on numerous occasions to calibrate seismic networks, or it's possible the nuclear test resulted in a fizzle yield.

It is certainly possible to build nuclear weapons with yields of one kiloton and below—the W-54 warhead of the Davy Crockett nuclear bazooka had a variable yield of only 10 to 20 tons, but that requires a much more difficult to build implosion system to compress a core so much less than a critical mass before detonation, and it is very unlikely that a low kiloton yield device would be used in an initial test.

See Fourmilab's on-line Nuclear Bomb Effects Computer for estimates of the effects of surface and airburst detonations with yields between 10 kilotons and 20 megatons.

Posted at October 9, 2006 16:52