Pyongyang conducted its fourth nuclear test on 6 January. Technically, we know comparatively little about the device that it tested. In terms of seismic signature the blast seems to have been from a bomb about 7 kilotons in size. But seismic signature is an imprecise way of measuring yield, so we should probably allow a factor of two in either direction. That means the bomb’s yield was somewhere between 3.5 and 14 kilotons.

Was it—as North Korea claimed—a hydrogen bomb? We don’t know. It might have been a plain fission device or a ‘boosted’ fission device (with deuterium or tritium emplaced to ‘boost’ the yield). Frankly, unless it proves possible to detect radionuclides from air-sampling in the vicinity of the test site—and that would require leakage from the test shaft—it seems unlikely we’ll know what sort of device it was. Air-sampling aircraft didn’t detect any tell-tale radionuclides in the weeks after the third test. (The Comprehensive Nuclear Test Ban Treaty Organisation did report a finding of noble gases xenon-131m and xenon-133 about two months after the test, a finding it described as ‘consistent with a nuclear fission event occurring more than 50 days before the detection’.)

True, the yield suggests it wasn’t a full-sized hydrogen bomb. But North Korea has already said it wasn’t, so no prizes there. Even in a hydrogen bomb, the most interesting reactions take place during triggering: the detonation of the primary fission weapon and the initiation of fusion. Boy scouts learn to light fires, not to build the biggest fires they can. So too for a weapon designer—once the fire’s burning it’s comparatively easy to add more wood. So it might have been a test of a prototype hydrogen bomb with relatively little fusion fuel.

It might also have been a test of a boosted fission weapon. Pyongyang has long sought ways to miniaturise its nuclear weapons, the better to place them atop its ballistic missiles. Some media reports in recent months have suggested the North Koreans have been trying to build weapons that use less than 6kg of plutonium, and so they might have been testing a device doing so, perhaps using boosting to help the detonation along.

Of course, they might also have been testing a simple fission device. True, they know they already have a workable design for that—as the first three tests show—but perhaps they’ve made advances in shrinking the weapon. Superimposing the seismic signals for all four tests does highlight an apparent similarity to the four detonations.

What should we make of the fact that after four tests the North Koreans are still achieving yields in the single-digit kiloton range? Possibly not too much. Seven kilotons in downtown Seoul, Tokyo or Los Angeles can still make for a bad day. And perhaps the constraint is political, not technical. Maybe the North Koreans are deliberately keeping the explosions small. After all, the test site isn’t that far from China. And the outpouring of international vitriol would undoubtedly be more intense if the tests were larger.

Finally, where does this leave the North Korean nuclear arsenal? A test device isn’t automatically a deployable weapon that could survive the rigours of delivery. And the North’s long-range missiles are still a hit-and-miss affair in terms of their reliability. But how reliable do the warheads and missiles have to be in order to satisfy Kim Jong-un that he has an ace in the hole? Moreover, how reliable do they have to be to factor into US policy-making? Would a US president think a 20% chance of a North Korean warhead successfully hitting Los Angeles makes for more confident US Asian engagement in North Asia than a 50% chance or an 80% chance?

Critics of the North Korean program may deride its achievements. And yes, the program limps rather than runs. But Pyongyang is gradually putting itself into a position where it must be treated as a real nuclear weapon state. Denuclearisation is no longer an option—at least not while the Kims remain in power.

Over the past few months North Korea has undertaken a large diplomatic effort. It has reached out to traditional opponents like the United States, Japan and South Korea. Contemporaneously, it has pursued a warmer relationship with Russia. But one nation has been missing from that charm offensive: China.

The border between China and North Korea is the focus of a significant amount of investment activity. But when one looks more closely at that investment, a pattern begins to emerge: Chinese spending along the border isn’t replicated within North Korea.

Emblematic of that spending—and perhaps also of the wider relationship—is the new bridge over the Yalu River at Dandong. The bridge itself is no small feat of engineering as it spans nearly 1.5 kilometres. Complementing the bridge, is a new high-speed-rail link to Dandong, from Shenyang, intended to feed into the new Yalu River crossing point. On the North Korean side of the bridge, there’s nothing but dirt. Read more

In light of the execution of North Korea’s second most powerful man and Kim Jong-un’s step-uncle, descriptions of North Korea as ‘irrational’ have resurfaced.

While ‘irrationality’ is an easy label for a situation not completely understood by the outside world, it doesn’t accurately describe North Korea’s leadership. North Korea’s actions—while unpredictable and apparently chaotic—result from sound strategy according to desired ends that the DPRK seeks, with the limited means available to it. North Korea has few cards but plays them well.

Relying on Robert Mandel’s definition of irrational activity as ‘a decision’s incompatibility with policy goals, prevailing consensus, or preferred outcome’, Ulv Hanssen on East Asia Forum observes: ‘one is often left with few other alternatives than to explain North Korean decision-making as irrational.’ Hanssen notes that the recent North Korean missile launch, which rescinded a deal for food aid, is behaviour so far removed from sound logical policy that irrationality remains the only appropriate explanation. Read more

North Korea’s latest nuclear test has been discussed from several angles: the level of technological progress of the regime; if China should and will end its support for its neighbour; and whether tougher sanctions by the international community would have any significant impact on Pyongyang’s nuclear behaviour. Yet, equally significant is the fact that North Korea’s nuclear test is also part of a broader picture: the emergence of nuclear multipolarity in the Asia-Pacific region.

After the Cold War, there were high hopes that the role of nuclear weapons in international politics would diminish. And for a short time, things seemed to be moving in that direction as both the United States and Russia started to reduce the number of their strategic nuclear warheads during the 1990s. But more than 20 years later, Washington and Moscow still retain enough nuclear stockpiles to wipe each other off the map multiple times. France and the United Kingdom show no signs of getting rid of their minimum nuclear deterrent despite their enormous financial costs. And Israel certainly has no intention of dismantling its ‘unofficial’ nuclear arsenal in the face of Iran’s nuclear ambitions. Read more