International attention to North Korea's October 9th test of a nuclear device has been centered on security and international politics. But despite attention after the fact, the test was conducted in secret and many of its details remain undisclosed. External evidence permits some quantitative estimates of the device tested and the North Korean nuclear weapons program.

Seismic Evidence
Nuclear and non-nuclear explosions produce shock waves which can read like earthquakes. Thus, following the North Korean test, a South Korean geological institute registered a tremor of roughly 3.6-3.8 on the Richter scale. The US Geological Survey estimates the magnitude significantly higher, at 4.2.

The Richter scale is intuitively tricky because its shape is logarithmic rather than linear. For some tangible comparison, on October 2nd a minor tremor in coastal Maine registered about 3.8-4.2 in magnitude. The quake broke some water pipes, closed a road in Acadia National Park, and lowered the water level of local wells. But no injuries were reported.

Using Richter magnitude to measure the North Korean device is trickier still because shockwaves must be interpreted in the context of blast shielding at the test site and the particulars of local geology, both of which are incompletely known to outside observers. Even so, the recorded magnitudes suggest that the explosive yield of the test device was roughly 1 kiloton of TNT.

1 kiloton is small for a nuclear device. Early fission devices tested in the US, USSR, UK, and China were about 20 times larger. The largest fission-fusion bomb ever tested, by the USSR, was 50,000 times larger.

The very small yield estimate begs speculation. The energy released is well within the compass of cheap, non-nuclear explosives like fertilizer. So the explosion might have been a bluff, except that the US Air Force detected airborne radioactive debris near the site. If a plutonium bomb, the device may have contained flaws in its geometry or in the quality of its materials which caused incomplete plutonium detonation -- a "fizzle." Indeed, North Korea's pre-test warning to China predicted a yield four times larger. If flaws prevented an ideal result, tests provide data, and North Korea might simply correct their designs to achieve larger explosive yields.

The Yongbyon Nuclear Facility
Even 4 kilotons would be a relatively small nuclear weapon. Such a small design could reflect plans for tactical nuclear weapons, but they are technically challenging. A small design may instead reflect North Korea's limited capacity to produce plutonium.

The International Atomic Energy Agency lists only one nuclear facility in North Korea. It is called Yongbyon and located in the west about 100 kilometers north of Pyongyang. The single operational reactor there produces 5 megawatts of electrical power, which is small compared to the hundred- and thousand-megawatt reactors typical around the world. The Magnox design of the reactor is obsolete, but nevertheless makes it well suited for creating plutonium-239 out of locally occurring un-enriched uranium -- in this reactor some 6 kilograms of plutonium result per year. This amount might be enough to produce one or two nuclear weapons every two years, given a successful fuel reprocessing program.

North Korea had begun construction on two other Magnox reactors, which together might have produced 40 times as much electricity and plutonium, but in 1994, as part of the Agreed Framework negotiated with the Clinton administration, the North Korean government shut down its small Yongbyon reactor and stopped construction of the two others. For its part, the US would have provided Korea with two light water reactors, each capable of producing 1000 megawatts of electricity -- in total an 8-fold increase in North Korea's nuclear power capacity.

The US offer of light water reactors was significant. Light water reactors require enriched uranium fuel, which might have been imported and therefore facilitated oversight by the IAEA. Furthermore, plutonium production in light water reactors is self-defeating from a weapons perspective. Along with plutonium-239, enriched uranium fuel accumulates plutonium-240, which stifles explosive fission reactions. The two plutonium isotopes are difficult to separate.

But the Agreed Framework broke down: in 2002 the Bush administration accused North Korea of embarking on a weapons-grade uranium enrichment project. In 2003, North Korea withdrew from the Nuclear Non-Proliferation Treaty, violating the Agreed Framework, and construction of the two light water reactors was suspended.

The small Magnox reactor at Yongbyon went online again in 2003 and has since been producing plutonium. This, combined with plutonium observed by inspectors in 1994, suggests that North Korea may posses some 50 kilograms of plutonium, enough for some 10 nuclear weapons.

Although modest compared to the nuclear capabilities of other countries, the results of North Korea's recent test and its nuclear facilities in general appear authentic and significant.

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