What would happen if North Korea nuked South Korea?

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North Korea has been stepping up incendiary rhetoric in the past few days, ostensibly in response to South Korean comments that it would attack the North’s nuclear installations in the event that Pyongyang launched atomic weapons. Once the North’s preemptive attack is underway, the Korean Central News Agency declared, “everything will turn to ashes, not just a sea of flames.”

Coupled with missile tests and diplomatic maneuvers, these comments are worrisome but not necessarily out of the ordinary for Pyongyang. Nevertheless, North Korea‘s claim is worth investigating: Can it really turn “everything” into ashes, or even “just” a sea of flames, given its relatively miniscule nuclear arsenal?

If “everything” means all of South Korea, the answer is definitely no. If it means Seoul, the question requires a bit more examination. Basically we need to know how many nuclear weapons Pyongyang has and how powerful they are. Knowing these facts allows some projections that, while extremely rough, are nonetheless interesting.

Very little reliable information exists, but based on aggregated seismic data from North Korea’s 2006 nuclear test, Harvard analyst Hui Zhang estimates (pdf) that the yield of that explosion was between 0.5 and 2 kilotons (for comparison, the yield of the weapon used at Hiroshima was about 15 kilotons, while other countries’ first nuclear tests generally yielded 9 kilotons or above). For simplicity’s sake, let’s assume North Korea‘s test yielded about a kiloton. It is even more unclear what the weapon was designed to yield, but it was probably supposed to be between 4 and 20 kilotons.

As for the number of weapons, North Korea has recently declared (pdf) that it has 30 kilograms of separated, potentially weapons-usable plutonium. More almost certainly exists, but all of Pyongyang‘s usable plutonium has probably not been made into weapons. Since an implosion-type weapon requires (pdf) about 5 kg of plutonium, North Korea likely has at most six weapons.

Excluding fires and making some other simplifications, if we assume that Pyongyang can do no better than a 1-kiloton air burst, each explosion would kill about 50 percent of the population within a fifth of a mile. Six bombs would therefore kill half the population within about two thirds of a square mile — an area encompassing not even half a percent of Seoul, which is about 234 square miles.

If, on the other hand, North Korea has six 10-kiloton bombs, it could kill 50 percent of the population in just over 3 square miles of Seoul — slightly over 1 percent of the city’s area. These calculations are probably very conservative, but even if the damages wildly exceeded expectations it seems unlikely more than 10 percent of Seoul‘s area would see widespread destruction. I certainly do not intend to minimize the horror of that level of death and destruction. But if North Korea wants to burn all of Seoul to ash, it is going to need more than a tiny nuclear arsenal to do so.