Pyongyang lashed out harshly at the United States following the most recent U.N. Security Council resolution condemning its December missile test. The Kim Jong Un regime threatened to increase its nuclear deterrent both quantitatively and qualitatively and vowed to conduct a third nuclear test at a "higher level." So what might we expect from another test? Why, what, how will we know, when, and what difference will it make?
First, why test? Without additional nuclear tests, North Korea is greatly limited in its ability to miniaturize a nuclear device to fit on one of its missiles. The 2006 and 2009 tests demonstrated that North Korea can build a nuclear device, but that its nuclear arsenal is likely limited to bulky devices that would need to be delivered by plane, boat, or van, thereby greatly limiting their deterrent value. To make its nuclear arsenal more menacing and provide the deterrent power Pyongyang's vitriolic pronouncements are aimed to achieve, North Korea must demonstrate that it can deliver the weapons on missiles at a distance.
During my previous visits to the Yongbyon nuclear complex, which housed both its plutonium production and its uranium enrichment facility, North Korea's nuclear specialists told me that the first two nuclear devices tested used plutonium as the bomb fuel. Pyongyang voluntarily suspended its plutonium production in 2008 and I estimate it has only 24 to 42 kilograms of plutonium, sufficient for 4 to 8 primitive nuclear devices, with no more in the pipeline. Yet with only two plutonium tests, one successful and one only partially successful, they need more tests to have confidence that they can build a smaller nuclear warhead.
The next test, however, could just as well be designed to demonstrate a highly enriched uranium (HEU)-fueled bomb. For years, Pyongyang had consistently denied having a uranium enrichment program, but in 2010 North Korean officials showed my Stanford University colleagues and me a modern centrifuge facility for uranium enrichment, ostensibly dedicated to making low-enriched uranium reactor fuel for electricity production. Based on what we were shown and our subsequent analysis of the time scales for constructing this facility, I concluded that Pyongyang must have a covert centrifuge facility, and that it has likely also produced HEU. I believe the amount of HEU produced to date is relatively small, but quite likely sufficient for a nuclear test.
What will they test? The most likely choice is an HEU device. Pyongyang threatened to increase the size of its nuclear arsenal; it can only do so with HEU, but it has a limited plutonium inventory and has decided to suspend plutonium operations. One can only speculate why it made that choice. Its plutonium facilities could have continued to produce one bomb's worth of plutonium per year. It is possible that the North Koreans believe they can develop a significantly larger HEU production capacity. In addition, the reactor operations necessary to produce plutonium are fully visible from satellite imagery because the reactor's cooling tower emits a visible steam plume, whereas the location and operations of uranium centrifuge facilities cannot be monitored from a distance, as was clearly demonstrated when we were shown the previously undiscovered Yongbyon centrifuge facility.
The apparent decision to pursue HEU devices is also puzzling because plutonium bomb fuel is more suitable for miniaturized nuclear devices than HEU (which is why the modern nuclear arsenals of established nuclear powers use plutonium). Yet Pyongyang may have decided it would require too many tests and too much plutonium, which is in short supply, to demonstrate a miniaturized plutonium device. And, it is likely that A.Q. Khan sold the North Koreans a Pakistani HEU design that could be mounted on some of North Korea's short or medium-range missiles. If Khan provided both design and test-performance data, Pyongyang may have decided that HEU, albeit less effective than plutonium, was a quicker and more certain route to miniaturized nuclear devices.
In an article co-authored last summer with Frank Pabian in the Bulletin of the Atomic Scientists, we speculated that it is possible that the North Koreans may decide to test both plutonium and HEU devices -- simultaneously in one test tunnel. One more plutonium test provides valuable information on the yield-to-weight ratio, critical for miniaturized designs. An HEU test allows them to move to a possibly expanded future arsenal. Multiple simultaneous tests have been conducted by the United States and the Soviet Union, and most recently in 1998 by India and Pakistan. Such tests have some technical limitations and are more challenging to conduct, but they have the huge advantage of not incurring additional political cost -- in other words, they can get two for the price of one.
Pyongyang had previously announced that it has mastered nuclear fusion technologies, prompting some observers to predict that the next test could be a fusion-boosted device or possibly even a thermonuclear device, typically referred to as a hydrogen bomb. North Korean nuclear specialists are undoubtedly familiar with these technological advancements and likely have tried their hand at designing such devices, but I consider application of these concepts to be still out of reach of their specialists, unless they are prepared to conduct multiple nuclear testing campaigns.