In a recent opinion piece, Jeffrey Lewis and Elbridge Colby argue that conventional military responses aren’t sufficient to deter Kim Jung-Il from launching “limited offensive military operations against his neighbors”—such as when North Korea sank a South Korean warship and bombarded South Korea’s Yeonpyeong Island. They further argue that the current U.S. nuclear earth-penetrating warhead, the B61-11, is also inadequate to deter such “provocations” and “brazen acts” since it isn’t capable of threatening deep bunkers tunneled below “hundreds of meters of hard rock,” where Kim and other leaders might hide.
Because of this, they argue the U.S. should develop a new nuclear earth-penetrating capability by modifying the existing B83 nuclear bomb by “changing the external casing of the warhead, providing better attitude control, and confirming that the configuration of internal components would survive the rapid deceleration accompanying penetration into a few meters of hard rock.”
It’s worth reiterating the technical problems with using a nuclear weapon for this role. These are severe enough that a B83-based penetrator would not provide the “credible and effective option to hold at risk” deep underground bunkers that the authors are looking for, for several reasons.First, even with a megaton yield (compared to 340 kilotons maximum for the B61-11), the B83 can’t reliably destroy deep targets. An earth-penetrating warhead can get at most a couple tens of meters into the ground—it can’t penetrate deep enough to get anywhere near a deep target. Instead it relies on the fact that by burying itself even 10 meters, a much greater fraction of its energy will be converted to ground shock. It’s the ground shock that is supposed to destroy the target.
But a 1 megaton warhead that penetrates 10 m into the ground can only destroy bunkers that are at most a few hundred meters underground. There’s no reason to believe North Korea can’t dig deeper than that.
Second, it’s unlikely that the U.S. would know where an underground bunker actually is and be able to target it accurately. A country going to the trouble of building a high-security bunker to protect its leadership would also try to make it difficult to figure out where that bunker based on any surface features, like entrances or air ducts.
For both these reasons, it’s very unlikely this new penetrator would be any more effective than the existing B61 earth penetrating weapon.
Third, this option is unlikely to be credible because of the enormous amount of radioactive fallout a B83 earth-penetrator would create.
An earth-penetrating warhead maximizes fallout. Compared to a weapon exploding in the air or at the surface of the ground, an earth-penetrating warhead is much more efficient at digging up earth and throwing it into the air. A 1-megaton nuclear warhead that penetrated 10 m into granite before detonating would throw some 5 to 10 million tons of radioactive debris into the air. A typical wind pattern over North Korea would carry this debris cloud south and east—over South Korea and Japan.
Kim Jong-Il might wonder if there were extreme cases in which the U.S. might decide to use such a weapon, but a B83 penetrator would certainly not be a credible deterrent to the kinds of recent “provocations” and “brazen acts” North Korea has carried out in the last year and a half.
A few years ago, we made a video illustrating the results of using a megaton earth penetrator against a target in Iran. The problems illustrated by the video are the same ones that would come into play in using such a weapon against North Korea.
Other useful sources on this include a UCS backgrounder on earth-penetrating weapons and a technical article by Robert Nelson in Science and Global Security.