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Four Kt nuclear explosion over one “sub-harbor” in Ulsan. Overpressures are for a 4 Kt nuke detonated at the optimum height to maximize blast effects. POL destruction limit is the range where blast effects cause severe damage to oil tanks. Thermal effects, i.e. starting a firestorm, are overestimated because slant ranges are used as ground ranges.

This title might be a little misleading. I certainly don’t want to imply that a 4 Kt nuke could not kill a lot of people. It certainly could. However, I have been thinking about whether or not North Korea might be able to use a few 4 kiloton (Kt) nuclear weapons—if that is their true yield—in what we in the West would consider tactical situations but in the context of the Korean peninsula would have strategic consequences. After all, it would seem that the US and South Korea would be in a very vulnerable position if the North launched an all-out invasion. In the standard war plans the US has for handling such an attack, often abbreviated as Halt, Build up, and Retake, involved a slow as possible fall back away from the DMZ with most of the US troops and supplies come in from outside the country.

It seemed, I thought, the perfect opportunity to use nukes. Blow up a few ports, I thought, and that would have a serious impact on the US strategy. This appears especially important when you look at the top six or seven ports. Two of these, Inchon and Tonghae, are fairly close to the DMZ and might be taken by ground forces in the first couple of days. Of the others, Ulsan, near the southern tip of the Korean peninsula, has 75% of the remaining berthing capacity. If Inchon and Tonghae could be taken by “conventional” forces, then destroying Ulsan could severely hamper the “Build-up” phase.

And it still might; but it would take more 4 Kt nukes than the North might want to expend on it. There are at least four (and arguably six) separate harbor areas in Ulsan. Baring a bottleneck for rail or road transportation that I haven’t seen in GoogleEarth, it turns out it would take four to six 4 Kt nukes to destroy them. Let’s consider in more detail an attack on one of those sub-ports, the one on the West side of the mouth of the main river.

There is a major oil tank farm associated with those docks, which presumably equipped with whatever specialized oil off-loading equipment is needed for that activity. Oil storage tanks are remarkably resilient to nuclear attack. This has only increased since 1977 when my edition of “The Effects of Nuclear Weapons” by Glasstone and Dolan was published. It seems that oil tanks get more resilient as they get bigger and most of the tank farm requires blast overpressures of 12 psi or greater. (Four kilotons is right in the transition region where the radius of creating a firestorm, about 0.8 km in for this yield, is just about equal to equal to the blast radius of 10 psi, which is just about capable of knocking over a large, half empty oil tank. This change over has to happen sometime since the thermal radiation must go as the fireball’s surface area, Y^2/3, while the blast radius goes as Y^1/3, where Y is the yield.) These blast radii assume the bomb went off at the optimum height. If it was a surface blast, to create the maximum amount of contamination, it is possible many of the oil tanks in the farm might survive a 4 Kt blast.

Knocking down the cranes needed to remove containers from ships would also require a 4 Kt blast to be very close. And much of the US supplies come in on roll-on/roll-off ships so perhaps the North would have to destroy quays, something that is very hard to do. Would the DPRK want to use four to six of its nukes to destroy Ulsan? It is possible of course, but that must be a large fraction of their current arsenal for only a 75% reduction in pre-existing berthing capacity. And there lots of possibilities for an imaginative, adaptive, and motivated US military to substitute for that capacity.