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Oct 9, 2006 | 22:31 GMT

5 mins read

North Korea: The Implications of Weak Seismographic Data

Summary
Initial seismographic measurements of North Korea's reported Oct. 9 nuclear device test are not what had been expected. Though information is still preliminary, the current data are not terribly impressive.
It is too early to say exactly what the tremors that emanated from the eastern portion of North Hamgyong province at 10:36 a.m. local time Oct. 9 will mean for North Korea's future. Pyongyang is touting the seismic activity as a successful nuclear detonation. However, seismographic measurements have been substantially lower than expected. Some experts speculated before Oct. 9 that North Korea's nuclear device was a rudimentary implosion device similar to the "Fat Man" dropped on Nagasaki. That 21-kiloton (kt) detonation shook out a solid magnitude 5.0. Although only estimates are available, readings in South Korea show — at this point — a maximum magnitude of 4.2, much closer to the 1 kt range. The Korea Institute of Geoscience and Mineral Resources has estimated that the tonnage might have been as low as 0.55 kt. As far as nuclear weapons go, almost everything can be learned from less-than full-scale testing except the actual yield of the untested device. Thus, this could have been a scaled-down test. It also would fit with a geological concern particular to North Korea — the water table is so high that an underground test runs a very real risk of irradiating a portion of the nation's water supply. And, of course, the water tables of the Korean Peninsula do not split nicely along the 38th parallel. In a nation the size of Nevada — the state in which the United States has done much of its weapons testing — there will very likely be observable environmental consequences regardless of whether the test was full-scale. Ultimately, however, the possibility of a scaled-down test is unconvincing. Environmental considerations aside, hydronuclear and hydrodynamic testing — two common forms of subcritical experimentation with weapons design — both have yields much lower than those observed Oct. 9, on the order of tens of tons and lower. These experiments test the function of the weapon without using enough fissile material to create a supercritical mass. Also, scaled-down tests with yields in the hundreds of tons often indicate boosted fission and thermonuclear devices with yields measured in megatons and of substantially greater complexity. Such complexity is almost certainly beyond North Korea's current reach. But technical details aside, this test was a political act. Therefore, it would not have been designed to be such a small test — this test was supposed to be an unequivocal statement of nuclear power. A very rudimentary device of 20 kt would have accomplished that. As it is, government responses were slow in the hours after the test — not simply because much of the world was still asleep, but because military analysts were still attempting to decipher the low Richter numbers.
The test could have been a subcritical failure of a gun-type device. "Little Boy," the 15 kt gun-type weapon dropped on Hiroshima, Japan, was so simple that despite the vast resources and money dedicated to the Manhattan Project, the design was never tested before being dropped in combat (the Trinity device, tested before Hiroshima, used implosion). Nuclear weapons are hardly easy. But a gun-type device uses a very simple conventional explosion to propel one piece of highly enriched uranium down a short tube into another, thus creating a nuclear chain reaction. Though there are many weaknesses to the gun-type device and its yield is always less than that of the same weight of highly enriched uranium in an implosion device, the design is so fundamental that the idea of it failing is hard to buy, even in North Korea. On the other hand, even a rudimentary implosion device presents major technical hurdles. If North Korea had tested such a device — one expected to have a yield in the 20-kt range — a number of failures could have produced the observed results. The uranium or plutonium must be machined to a much finer tolerance. Explosive lensing, which is necessary to detonate the device, is extremely difficult and the timing must be exceptionally precise. A failure along any one of these lines could very well have produced a subcritical yield along the lines of the Oct. 9 observations. Ultimately, it is still too early to tell what happened underground in North Korea the morning of Oct. 9. But what North Korea really showed the world is the current primitiveness of its nuclear weapons development program. The technical hurdles of miniaturizing a weapon, hardening it to survive the strain of a missile launch and mounting it in a re-entry vehicle have always been thought to be well beyond North Korea's reach. However, the old saying, "It is better to stay quiet and be thought a fool than to open your mouth and remove all doubt," might apply to that assumption. The heart of North Korea's threat was the unknown, but that could still change with time. North Korea could test another device more robust than this one. Much of the world may spend the next few months fuming over Pyongyang's test. However, becoming the eighth nation on the planet to test a nuclear device is no small feat — especially from a nation that did so through sheer force of will.

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