WASHINGTON — In late March, when the U.N. Security Council condemned North Korea for test firing two medium-range missiles into the Sea of Japan, Pyongyang shot back, warning of “next-stage steps, which the enemy can hardly imagine” – including “a new form of nuclear test for bolstering up its nuclear deterrence.”
Golly, that sounds awfully hostile.
A “new form” of nuclear test? My thoughts immediately turned to Pyongyang’s next step in its nuclear weapons development. North Korea might test a device using highly enriched uranium (if it hasn’t done that already), or start down the path toward tactical nuclear weapons or perhaps burning thermonuclear fuel. I suspect that these are their ultimate objectives, although it is hard to know Pyongyang’s near- and long-term technical goals for its nuclear arsenal.
The phrasing of the statement in the original Korean, however, suggests that there is something new about how North Korea tests, not what it tests. After checking with a number of Korean speakers, the phrase appears to refer to a new form of testing, as opposed to simply a new device.
North Korea says we can hardly imagine what that might be, but I think we can try. The simplest explanation is that North Korea may conduct simultaneous detonations of two or more nuclear devices. Most nuclear powers use these “salvo tests” to test more weapons in less time. None of the test personnel live year-round at the nuclear sites, which are often located in remote areas. These areas are often remote because the weather stinks. So the idea of getting two-for-one (or even three-, four- or five-for-one) while the scientists are on site has plenty of appeal, even if it makes the tests more complex. Here is how the Russians explained it:
For underground nuclear explosions the special technology of salvo nuclear tests was developed, when two or more nuclear devices were simultaneously detonated within one nuclear test. This technology represented an essential step forward in comparison with testing of individual nuclear devices because it allowed 1 / 8the Soviet Union 3 / 8 to intensify testing activities, even if its realization required some increase in the complexity of the experiments. This approach was advanced both in the USSR, and in the U.S., but was more widely practiced in the USSR, apparently due to the more severe weather conditions at the nuclear sites and lower financial and material capabilities. The Soviet Union conducted 146 salvo nuclear tests in which 400 nuclear devices were detonated, while the United States conducted 63 salvo nuclear tests in which 158 nuclear devices were detonated.
Lower financial and material capabilities and severe weather are also factors for Pyongyang. In particular, the North Korean test site has lousy winters, followed by spring floods. This would fit well with my hypothesis that North Korea’s tunneling at Punggye is intended to support more intense nuclear testing than the country has conducted to date. On March 28, North Korea hinted at this, warning that the United States should expect “more annual and regular” efforts to bolster and demonstrate Pyongyang’s “war deterrent.” That general statement suggests we should expect more missile launches and nuclear tests in the coming years.
There are other, more speculative possibilities. North Korea has conducted its previous tests in tunnels drilled horizontally into the mountains around the test site. The size of bombs that the North Koreans can test in these tunnels is limited by the size of the mountain and the resulting overburden. A back-of-the-envelope calculation suggests that the current test site can only accommodate a few tens of kilotons.
Larger tests, then, would need to be conducted in shafts drilled horizontally much, much deeper into the ground. Shaft tests are usually limited to several hundred kilotons – although you can always dig deeper. In 1971, for example, the United States conducted a 5-megaton test in Alaska, Cannikin, in a 1,860 meter shaft. At some point, though, drilling deeply is harder than simply scaling the test, given the costs of drilling and challenges of testing below the water table. Thus, this would probably require a different test site than Punggye-ri. So far, there are no reliable reports of a second test site.
Of course, one needs to be careful: Underground tests can release radioactivity into the atmosphere. This happened to the United States with its 1970 Baneberry test, which, much to the chagrin of its team, vented radioactivity. Oops.
Atmospheric testing is tricky though: For a rich example, look at China’s nuclear testing program, which is largely a story of managing international opposition to atmospheric nuclear tests while trying to master the technology of testing underground. North Korea has the same constraints, only in Pyongyang’s case, the downwinders are its primary lifeline.
I suspect, for the moment, that Chinese public opinion is enough to keep North Korea’s nuclear tests underground.
North Korea might conduct atmospheric testing if Kim Jong Un feels the need to develop thermonuclear weapons or perhaps, like China in 1966, wants to demonstrate the ability to deploy nuclear-armed missiles or artillery. A nuclear-armed Musudan, an intermediate-range ballistic missile, would get my attention. Kim Jong Un is only likely to do that, however, if he no longer cares what Beijing thinks.
For now, I am inclined to believe the “new form” of nuclear testing most likely means simultaneous tests, part of a program of more intense nuclear testing that we are likely to see over the next few years. Still, it is useful to remember that Kim Jong Un has a number of other unpleasant provocations from which he might choose.
Lewis is director the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies.