On January 6, North Korea announced that it had completed a hydrogen bomb test. In all likelihood, however, this exercise was not the complete success that Pyongyang has claimed.

Here are some important factors to bear in mind. According to U.S. and Chinese measurements, the seismic magnitude of yesterday’s detonation is roughly the same as that of the last bomb test that North Korea conducted in 2013. Some sources suggest that yesterday’s blast registered as a magnitude 5.1 event on the Richter scale, while others maintain that it measured 4.9, the same as the country’s 2013 test. Even if one accepts the higher figure, that would suggest that the yield of this explosion was only about 1.5 times greater than the one three years ago. Moreover, the shape of the seismic waves that this detonation produced is similar to that of Pyongyang’s last test—another indication that the yield of this bomb and the last one tested are about the same, although this says nothing about whether the designs of the bombs are similar or even related.  

Tong Zhao
Tong Zhao is a senior fellow in Carnegie’s Nuclear Policy Program based at the Carnegie–Tsinghua Center for Global Policy.
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No one knows how deep underground North Korea’s nuclear tests were conducted in either 2013 or 2016, so precisely assessing the bombs’ yields based on seismic magnitude is relatively difficult. International scholars judge the scale of North Korea’s 2013 test to have been approximately 10 kilotons of TNT. (By comparison, the bombs that the United States dropped on Hiroshima and Nagasaki each had payloads of approximately 20 kilotons.) The yield of yesterday’s test was also most likely around 10 kilotons, although a certain margin for error is unavoidable. Triangulated measurements from global seismographic sensors indicate that this latest bomb test occurred beneath the same mountain range as the previous three, although this most recent detonation took place beneath a higher peak and thus was probably deeper underground compared to previous tests. If this is accurate, and given the similar scope of seismic magnitude generated by yesterday’s test compared to previous ones, it seems likely that the actual yield of yesterday’s detonation was slightly larger than last time.

That said, on the whole, the yield of this test differed drastically from that of the typical hydrogen bomb. When the United States detonated the world’s first such weapon in 1952, its yield was 10 megatons or a full three orders of magnitude larger than that of North Korea’s recent test. The first Soviet, British, Chinese, and French H-bomb tests also had yields measuring hundreds of kilotons or more. The fact that yesterday’s explosion produced such a small energy yield probably indicates that one of three things is true.

First, it could mean that North Korea is indeed conducting tests for a hydrogen bomb, although they have not been successful. Hydrogen bombs, also known as thermonuclear weapons, generally consist of at least two stages. The primary stage is a fission device, or more precisely, a nuclear device that derives its energy yield from a fission reaction. This initial explosion then triggers and sustains the high temperature and high pressure conditions needed to prompt the secondary stage—a fusion reaction. Both of these reactions—especially the fusion stage—can release vast amounts of energy, so the collective yield of a hydrogen bomb is enormous. 

Judging from the small total yield of North Korea’s recent test, it’s quite possible that the primary reaction did not successfully trigger the secondary one—in other words, no high-yield nuclear fusion reaction took place. If that is the case, it means that key components of North Korea’s hydrogen bomb technology are not yet up to snuff. 

Alternatively, North Korea could just have tested a boosted or enhanced fission bomb containing some fusion material. In a purely fission-based nuclear device, it is possible to add some fusion material (specifically deuterium and tritium) to produce a fusion reaction so as to increase the amount of energy that the device produces. But in light of the test’s modest yield, even if North Korea is trying to test a boosted nuclear device, there is currently no way to determine whether or not the boost attempt was successful.  

A third possibility is that North Korea only tested a purely fission device. North Korea’s nuclear weapons program began by using plutonium to make a simple, fission-fueled nuclear bomb. The composition of radioactive isotopes released after North Korea’s first underground nuclear test in 2006 confirmed that this first detonation used plutonium. But during its second nuclear test in 2009, Pyongyang successfully contained the fallout and no discernable radioactive isotopes were released. That said, most researchers agree that North Korea also used plutonium as fissile material for this nuclear test. Only small traces of radioactive particles were emitted from Pyongyang’s third test in 2013. Monitoring stations affiliated with the Comprehensive Nuclear Test Ban Treaty Organization detected only minute traces of radioactive fallout—not enough of it to conclusively determine what materials North Korea had used. Therefore, the international community cannot tell what Pyongyang used in its 2013 bomb test. Seeing as how North Korea’s plutonium supply and production capacity are limited, the country has already begun earnest efforts to produce enriched uranium to serve as fissile material. Based on this, the international community suspects that North Korea has begun using enriched uranium to fuel its latest nuclear weapons. It follows that North Korea may have used enriched uranium in a purely fission nuclear device for yesterday’s test. This would imply that the design and manufacturing of North Korea’s nuclear weapons is still relatively limited.   

At the end of the day, the radioactive particles and gases released after the explosion must undergo further analysis before any conclusive verdict can be reached. Allegedly, the U.S. government detected signs that the test was imminent before it took place and dispatched sniffer aircraft to conduct atmospheric sampling. But considering the fact that this test may have taken place even deeper underground than previous ones, if adequate sealing measures were taken, it may be that radioactive particles from the test will not be detected. However, it is also possible that over the next few weeks some radioactive inert gases may slowly seep out of the ground and enter the atmosphere.  

In addition, satellite images taken before the latest test show signs of activity near the northern tunnel at Punggye-ri, the nuclear test site that North Korea used for its 2009 and 2013 tests. In addition to this, North Korea began constructing a southern tunnel at the site in 2009 before recently breaking ground on a new western tunnel as well. Although it is not yet possible to confirm which tunnel was used for the most recent test, the fact that multiple tunnels are being built simultaneously and may be in serviceable condition suggests that North Korea conducting further tests soon is not beyond the realm of possibility. This risk warrants the international community’s careful attention.    

On the whole, since North Korea successfully conducted a fission-based test in 2013, the country has continued moving toward the goal of developing a hydrogen bomb—a highly predictable strategic trajectory that has been followed by other nuclear-weapons states. Developing a boosted fissile-based device before successfully mastering the technology for a hydrogen bomb is also par for the course. The most likely explanation for this latest test appears that its primary technical design involved a boosted fission device.

If the reason for yesterday’s modest yield is not a failed test, North Korea may have had other considerations at play. These could include conserving its fissile materials, reducing the risk of nuclear fallout (the leaking of large quantities of radioactive materials), or conducting research on miniaturizing a nuclear warhead. Among these possibilities, the issue of warhead miniaturization must be followed closely. It relates directly to whether or not North Korea is close to being able to load a nuclear warhead onto a missile and thus achieve an operational nuclear capability. Only once it has attained this objective can North Korea rightly be considered to have a genuine measure of nuclear deterrence.

Although yesterday’s test does not mean with any degree of certainty that North Korea’s nuclear capabilities have taken a qualitative leap forward, in the end it is at least a sign of incremental improvement. This would seem to pose an enormous problem for the current U.S. policy of strategic patience. Other than racketing up punitive sanctions against North Korea’s nuclear and missile programs, the international community may have no choice but to earnestly consider how a shift to positive diplomatic engagement may serve to soften what Pyongyang perceives as existential threats to its security and in so doing slow down the progress of North Korea’s nuclear program at its root.   

A version of this article was originally published in Chinese by the Financial Times. Parts of the text have been updated by the author to clarify his intended meaning.