In the last post, I looked at what sort of damage North Korea can do with its Long Range Artillery, including both conventional and chemical payloads. Now we can try to estimate what the North can accomplish with nuclear weapons. The number and type of nuclear warheads and delivery systems the North Koreans possess isn’t knowable, but using some publicly available information and making up plausible-sounding numbers can give us an idea of what they can accomplish.
Effects
Presumably the North Koreans are confident in their 15kt bomb design, which they claim will (unlike the bombs detonated in 1945) fit onto a missile. There’s no reason not to believe them; if it doesn’t, they’ll just keep conducting tests until they have something that works and is small enough. If they don’t set off another 100kt+ detonation, it’s an indicator that the 2017 test was of a missile-suitable warhead, which it probably was – -they don’t have much use for something that can’t be put on a missile.
I’m going to use Alex Wellerstein’s NUKEMAP app to estimate the effects of a nuclear explosion, and as a comparison to the uniform population density method from the last post. He also has a “MISSILEMAP” app, but we’re not concerned with the ability to destroy a specific target; this is a countervalue attack against a densely populated area.
Playing around with the 15kt explosion on NUKEMAP, I get between 60-200k killed and 250-500k wounded depending on the nature of the district in which the bomb hits using Mr. Wellerstein’s model. In the .017人/m^2 uniform density model from the last post, and the 1200m 500 rem dose radius as a kill radius (this area also includes a shockwave >5psi), we get 76,867 fatalities. Using the 100% third-degree burns radius as the wound radius (1.9km) gives us 201,000 wounded. So, this is in the ballpark of Wellerstein’s more detailed model, on the low end. His model accounts for the possibility of casualties outside the radii noted above. Mine is much more conservative. I don’t put too much trust in the resolution of his population-density data, since the population shifts around the city substantially during the day and week. Still, we’re generally in agreement.
Commercial/Government vs Residential
When discussing the damage done by conventional munitions, I decided to reduce the “damage radius” in order to account for the protection provided by the broken, built-up urban environment. I don’t believe this to be the case with a nuclear detonation, because the large shockwave produced will tend to cause the total collapse of buildings, by applying the shockwave overpressure against a much larger area, compared to smaller conventional explosives. This isn’t an inherent characteristic of nuclear weapons, and emulating this effect with non-nuclear explosives is the point of weapons like the GBU-43 or Timothy McVeigh’s hobby project. However, none of the North Korean artillery warheads are anywhere near large enough to cause wholesale collapse of structures as in an earthquake. It is possible that the construction of larger buildings in Seoul would enable them to avoid collapse collapse by one or several nuclear blasts. If so, these are probably overestimates, but a lot of people are still going to die.
Aleppo, Syria in 2014. Despite heavy fighting, most of the buildings are basically intact.
Unfortunately, the paltry 15kt bomb is a firecracker compared to North Korea’s state of the art, for which I’ll use a conservative 100kt estimate. Depending, again, on the district hit, NUKEMAP estimates 245,000-480,000 dead and 800,000-1,200,000 wounded. Using our own model above — and 1600m/4500m kill/wound from the same source — gives us 140,000 dead and 1.1 million wounded. This is almost certainly lowballing the number of people killed, since many outside the radius would be killed by building collapse, burns, radiation, etc.
RIP Noise Basement
So, it looks like it would take 2-6x 100kt warheads or 5-15x 15kt warheads to inflict the same number of casualties as the conventional artillery barrage, while acknowledging that even one single warhead on target, especially the larger one, would do the job just fine.
Cost
We guessed in the last post that throwing 70 kilotons of high explosives at Seoul would cost about a billion dollars worth of munitions and require 50,000 people. The North Koreans’ job isn’t quite so simple as it appears above, because not every warhead can hit its target. If South Korean missile defenses can intercept 75% of incoming missiles, then the North need to launch 8 missiles to get a 90% chance of hitting. Even 50% interception rate means you need 3-4 missiles for a 90% probability of hit. Better, but do you feel lucky?
So, the North wants some combination of suppression and saturation of missile defenses in order to actually hit the target. If launching missiles in salvos reduces interception rates to 50%, and suppression of missile-defense sites (such as by maintaining a small fraction of the conventional artillery above, or by using small drones as improvised precision munitions) can get the interception rate to 33%, now we’re in business. Launch a five-missile salvo, 99% chance one goes off.
Nuclear disarmament advocate group Global Zero estimates that North Korea spends about $700 million per year on its nuclear program, compared to about $10 billion on its military overall. Estimating the marginal cost of a nuclear weapon is very difficult even in the best of circumstances. The 1998 book Atomic Audit estimates that China spent $28 billion in 1988 dollars ($58 billion in 2017) to build 450 nuclear warheads, about $120 million apiece. The comparison is apt given the similarities of a relatively poor state with modest aims. This article estimates a marginal cost of $50 million for an American ICBM. Given their possession of a workable warhead design and nuclear material, it is far from unreasonable to assume that North Korea can build and maintain 10x 100kt IRBMs and a flock of decoys for less than the billion dollar stock cost of its conventional deterrent munitions.
This also means that if nuclear forces allow the North Koreans to decrease spending on conventional forces by 10%, they’re coming out ahead. Spinning this shift as “conventional disarmament” even allows them to parlay the weapons program that in theory contributes to their pariah status into a diplomatic positive.
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