North Korea conducts its sixth and largest nuclear test yet – analysis

Gabriel Dominguez, Asia Editor at Jane’s Defence Weekly, IHS Markit

North Korea carried out on 3 September what it claimed was a successful underground test of a hydrogen bomb designed to be mounted on its intercontinental ballistic missiles (ICBMs).

The country’s sixth and largest nuclear test thus far was carried out “to examine and confirm the accuracy and credibility of the power control technology and internal structural design newly introduced into manufacturing [an] H-bomb” set to be carried by an ICBM, Pyongyang’s Central News Agency (KCNA) reported that same day.

It came less than a week after Pyongyang test-fired a Hwasong-12 intermediate-range ballistic missile (IRBM) that flew over Japan’s northern Hokkaido Prefecture before falling into the Pacific Ocean.

Jane’s understands that the regime in Pyongyang will likely need to conduct more provocative tests of its missile systems out to their full range before it can declare them operational. This is probably why the KCNA announced on 30 August that further ballistic missiles will be test-fired into the Pacific Ocean in the future.

The flight path taken by the Hwasong-12 on 29 August was almost certainly chosen to minimise the risk posed to people on the ground as well as the potential political ramifications as the missile overflew the southernmost tip of the island of Hokkaido, thereby spending as little time as possible over Japanese land.

A similar flightpath will likely be used to test North Korea’s Hwasong-14 ICBM. During a test launch conducted on 28 July a Hwasong-14 reached an altitude of 3,725 km and travelled a distance of about 1,000 km, according to the KCNA: an indication that the missile should be able to achieve a nominal range of more than 10,000 km.

Analysis – was it a nuclear weapon?

On 3 September the KCNA released images showing North Korean leader Kim Jong-un standing next to a device that the media outlet claimed is a hydrogen bomb designed to be loaded onto an ICBM. In the background of one of the photos, a schematic drawing was seen showing the device fitted inside what appears to be a re-entry vehicle.

What is unusual about this is that the shape of the re-entry vehicle shown in the drawing appears to be different from that seen on North Korea’s Hwasong-14 ICBMs, two of which were tested in July. This could indicate that the tested ICBMs might have carried a shroud, and that the actual re-entry vehicle could have been located inside that shroud, Markus Schiller, an aerospace engineer at Munich-based ST Analytics, told Jane’s.

As for the alleged miniaturised nuclear device shown in the KCNA images, it has the size and shape characteristics of a thermonuclear bomb, according to Robert Kelley, a former director of the IAEA and Los Alamos National Laboratory nuclear engineer. “The model is consistent with leaked photographs from 2016,” he told Jane’s.

Speaking about the seismic event triggered by the North’s latest nuclear test, Kelley said that the blast is consistent with a yield of 50 to 100 kilotons but that these figures could be revised higher. However, he added, the seismic yield in this range “proves nothing about the design of the nuclear explosive”.

“It is trivially easy to produce a yield of 100 kt in an underground test of a simple fission-only, unboosted device, where the size and weight of the explosive device are virtually unconstrained,” Kelley said, pointing out that it is easier to produce such a yield in a massive test device than to produce 10 kt in a “miniaturised” ICBM warhead.

“Jumping to conclusions that this was a thermonuclear bomb is premature,” Kelley said, adding that if North Korea wanted to induce “rampant speculation” that they have perfected a thermonuclear design based upon photographs of “realistic” models and a large yield, then they have largely succeeded in this.

True confirmation of a thermonuclear design, however, can only come from analysis of leaked radioactive debris from the underground site, said Kelley. “Curiously North Korea has worked very hard to completely contain four of their five nuclear tests, denying foreign military analysts clear evidence of thermonuclear reactions as well as the design of the plutonium or uranium used as nuclear fuel.”

Radioactive debris could be an unequivocal indicator of the characteristics of the nuclear device if a US sampling aircraft can collect it expeditiously. Otherwise, said Kelley, “North Korea will be able to claim whatever it wants and create uncertainty and fear”.

Kelley is of the view that the North’s “obsessive containment of nuclear debris is a sign that it is anxious to hide its true progress from the West, completely at odds with releasing photos of supposed devices”.

North Korea also said that it might use the bomb as an electromagnetic pulse (EMP) device that would aim to shut down parts of the US electricity grid. “An EMP device does not have to enter the Earth’s atmosphere and its accuracy can be as poor as several hundred miles over the US East Coast, for example. But whatever its effects, it would be a nuclear attack on the US homeland and would lead to massive retaliation,” said Kelley.

Pyongyang committed to nuclear development

Alison Evans, deputy head of Asia-Pacific Country Risk at IHS Markit, points out that North Korea’s sixth nuclear test, and reports by South Korean officials that North Korea is preparing to carry out another ballistic missile launch, highlight Pyongyang’s unwavering commitment to developing a nuclear-capable ICBM.

“Bellicose rhetoric from US President Donald Trump and high-level officials, intended to deter North Korea, probably reinforce the North Korean leadership’s belief that such a capability is essential to deterring the perceived US threat. The most likely upcoming dates for further missile tests or associated activity are the 9 September Day of Foundation of the Republic and 10 October Korean Workers’ Party Foundation Day,” said Evans.