When the email notification of a shallow 4.9 tremor near the North Korean nuclear test site landed in my inbox, courtesy of the United States Geological Survey, I went to Twitter:
The magnitude of the explosion, now revised to 5.1, is significantly larger than North Korea's previous nuclear tests in 2006 (4.3) and 2009 (4.7). Seismic scales are logarithmic, so 5.1 is much larger. The corresponding size of the explosion or "yield" is several kilotons, or thousands of tons of TNT -- although we should treat all yield estimates at this point as rough approximations. (I've posted a long discussion on the perils of estimating yield based on the simple seismic data at ArmsControlWonk.com.)
The point is, this one is bigger. Although it's not nearly so big as the modern thermonuclear weapons in the U.S. arsenal -- which range from hundreds of kilotons to a megaton -- you really wouldn't want this dropped in your neighborhood.
More importantly, however, North Korea has announced that the device was a "miniaturized and lighter nuclear device with greater explosive force than previously." What this boils down to is a North Korean claim that this nuclear weapon will fit on a missile like the Nodong. Or maybe even the KN-08, which the North Koreans say over and over is intended for us. Like a valentine.
The question is, do we believe them? And if we do believe them, does it mean we have to think about this problem differently?
There are basically two approaches to these questions, which arise from differing interpretations of the rather small yields of the first two North Korean nuclear tests -- a few hundred tons in 2006 and about two kilotons in 2009.
One view is to insist the North Koreans prove they are not terrible at building nuclear weapons. Here, I need to introduce you to a term of art: "simple fission device." This is a bulky nuclear weapon of the sort that the United States dropped on Nagasaki. That bomb weighed 4,000 kilograms (over four tons!) and contained six kilograms (about 13 pounds) of plutonium. The yield was about 20 kilotons. It was affectionately called "The Gadget," which is funny in precisely the opposite way that it's amusing to name a Chihuahua "Bruiser." The massive Gadget would have been far too large to be delivered by a missile, if the United States had any at the time. No country has ever built a simple fission device and discovered it did not work. Hence, "simple," as in anyone can do it.
Until the North Koreans, that is. In this telling, the North Korean nuclear program, like the North Korean missile program, required multiple tries to achieve success: in this case, building a Gadget-sized simple fission device. The third time was a charm -- but North Korea is still stuck with 1945 technology, wondering whether the damn thing will fit in the bomb bay.
The second view, to which I am inclined, is that we ought to take the North Koreans at their word. This view arises from the judgment that North Korea's disappointing yields in 2006 and 2009 are not the result of technical incompetence so much as outsized ambition. The North Koreans tried to skip some steps and go directly to miniaturized devices.
The United States quickly reduced the mass and dimensions of its nuclear weapons, largely by making the process of implosion more efficient. When I visited the historical collection at the Atomic Weapons Establishment in Aldermaston, England, I was surprised to see a rather explicit description of the use of air pockets among the lenses to improve the efficiency of explosions. There are other tricks, too, like suspending or "levitating" the plutonium pit. (The physicist Ted Taylor elegantly illustrated the idea by asking, "When you hammer a nail, what do you do? Do you put the hammer on the nail and push?") The result was a steady reduction in size, without the loss of explosive power. One of the first miniaturized U.S. fission weapons, the Mark 7, weighed just 750 kilograms and used considerably less plutonium than the Gadget.
It's no coincidence that the U.S. intelligence community, in the late 1990s, estimated that 650-750 kilograms was probably the best that North Korea could do without testing. The Mark 7 is used as a proxy for what a new nuclear state might attempt to build. China gave Pakistan a roughly comparable design, which Pakistan may have further shrunk, as well as sold to Libya and god knows who else. There are reports that Iran had access to a still more compact Soviet design.