On 3 September, North Korea conducted its sixth nuclear weapons test, afterward declaring the device had been a thermonuclear variant. Preliminary estimates from seismic monitoring put the test (called Kim-6) at a yield of more than 100-120 kilotons. This is at least 10 times as powerful as tests conducted between 2006 and 2016, which adds to the credibility of claims that a hydrogen device was used.
The Kim-6 test confirms North Korea’s determination to master and expand its nuclear arsenal. The history of the five nuclear powers shows that thermonuclear warheads are preferred over other nuclear types, mostly because of the powerful yield, meaning greater destruction capability. North Korea is likely to follow the same path, even if at a smaller scale. It possesses enough scientific and industrial capacity to produce plutonium and highly enriched uranium to build all the main types of nuclear and thermonuclear warheads.
Under Kim Jong Un, North Korea has made qualitative steps in ballistic missile technology. The missiles already at its disposal are not only capable of reaching American allies in Asia and the U.S. territory of Guam, but sooner or later even the West Coast of the United States. Because traditional and simple nuclear warheads have a relatively small yield, there is no sense in mounting them on intercontinental ballistic missiles (ICBM). Only a thermonuclear warhead is capable of destroying American cities and effectively deterring the U.S. It’s clear that North Korea will continue to try to master the production of reliable thermonuclear warheads and their delivery means, including not just ICBMs but also submarine-launched ballistic missiles (SLBMs).
ICBM-mounted thermonuclear warheads might serve as a deterrent and threat to the continental U.S., but they are not necessary to terrorize South Korea or to strike the region. The North is capable of hitting Seoul—just 40 km from the border—with artillery and rockets as well as with short and medium-range ballistic missiles (SRBM Scud, MRBM Nodong). They might be armed with nuclear implosion warheads that originated in Pakistan and are based on a 1966 Chinese warhead with a yield of 12-20 kilotons. The range of these missiles permits North Korea to hit U.S. bases in both South Korea and Japan. These same standardized nuclear warheads might also be installed on medium-range ballistic missiles (MRBM Musudan) targeting U.S. bases on Guam.
North Korea will continue to expand and diversify its nuclear arsenal and means of delivery. Paradoxically, it is much easier to predict its nuclear technology path than the political calculations of Kim Jong Un. The optimal strategy for the U.S. and its allies to mitigate the crisis or further escalation is a combination of nuclear deterrence and enhanced missile defence of South Korea, Japan and U.S. Navy ships. Despite this strategy, North Korea is likely to continue the scale and tempo of building up its thermonuclear and ICBM arsenals, probably counting on forcing the U.S. into direct negotiations. Contrary to rhetoric from Seoul and Tokyo, it is unlikely they would unilaterally begin military actions without consulting the U.S. administration.