
An early U.S. intelligence assessments shows that the thermonuclear device North Korea tested on Sunday at its regular nuclear testing site at Punggye-ri demonstrated a yield of 140 kilotons of TNT equivalent. Japan, has meanwhile, revised its official yield estimate of that nuclear test up to 160 kilotons. Some independent experts, meanwhile, believe that the depth of the test may indicate a higher yield yet—potentially in the 200+ kiloton range. Either way, this was a big test.

While it remains unclear if the device tested was a true two-stage thermonuclear bomb, like North Korea claimed in its state media, or a different type of weapon—perhaps a boosted fission device—the demonstrated yield alone makes it clear that, if mountable on its intercontinental-range ballistic missiles, Pyongyang now possesses a considerably powerful long-range nuclear strike capability. A 140 kiloton yield, delivered in an airburst, would be enough to level large swathes of most American cities.

That alone was reason enough for celebration in Pyongyang, where the nuclear weapons program, along with the country’s increasingly potent and diversified arsenal of ballistic missiles, form one proud leg of Kim Jong-un’s guiding ideological framework of byungjin. (The other leg of byungjin is economic prosperity.) On Wednesday, thousands gathered in Pyongyang’s Kim Il-sung square to fete the engineers and scientists who made Sunday’s nuclear test possible.

Sunday’s test represented North Korea’s sixth. The country tested a nuclear device for the first time in 2006, after indications of nuclear technology transfers from Pakistan to North Korea via Abdul Qadeer Khan’s infamous proliferation network. After 2006, Pyongyang conducted another test in 2009 under Kim Jong-il, current leader Kim Jong-un’s father. After Kim Jong-un’s takeover in 2011, North Korea carried out four more tests: in 2013, twice in 2016, and last weekend. To this day, North Korea remains the only country to have carried out nuclear tests in the 21st century.

The 20th century, however, accounted for more than 2,000 nuclear detonations—from the birth of the atom bomb at 5:29 a.m. on July 16, 1945, at the Trinity Site in New Mexico in the United States to Pakistan’s nuclear breakout, just a little more than two weeks after India’s own first weaponized tests in May 1998. The Soviet Union, China, the United Kingdom, and France carried out tests of their own, with China and France continuing testing until the mid-1990s, before the signing of the Comprehensive Test-Ban Treaty (CTBT).

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After the signing of the CTBT, which has yet to enter into force more than twenty years later, only India, Pakistan, and North Korea have carried out tests. The last test prior to the signing of the CTBT came in July 1996, when China tested a 3 to 12 kiloton device at its Lop Nur test site, seeking to advance its compact nuclear weapons design before the treaty would be signed.

North Korea’s latest nuclear test, at 140 kilotons, is certainly a notable detonation in the overall history of human nuclear testing. Since the United States’ undergound test in 1986 of a weapon with a yield of 119 kilotons, only a handful of other tests had crossed the 100 kiloton mark. Three of these tests were carried out by China and the others by France.

The first of the Chinese tests was on May 21, 1992, at the Lop Nur site and involved a 660 kiloton detonation underground—China’s largest nuclear test ever. The second of these came on August 18, 1995, at the Lop Nur site. The exact yield of this test is not known, but the Stockholm International Peace Research Institute (SIPRI) estimates that the energy released fell within 40 and 160 kilotons. A third Chinese test—its penultimate test prior to the CTBT’s signing—may have yielded up to 120 kilotons, according to the SIPRI.


A series of French nuclear tests in 1995 also crossed the 100 kiloton mark. The tests codenamed Ploutos and Xouthos each are estimated by the SIPRI to have yielded a maximum of 110 and 120 kilotons respectively. The latter was France’s last nuclear test.

Fast-forward to 2017 and it’s clear that if North Korea’s sixth nuclear test did indeed yield 140 kilotons as the early U.S. intelligence assessment suggests, it was without a doubt the largest man-made explosion on Earth in at least 21 years, since China’s 1996 test. Accounting for the uncertainty around the 1996 test’s yield, North Korea’s test could have been the largest man-made explosion in 25 years—since the 660 kiloton Chinese test in 1992.

The question of further nuclear testing by North Korea cannot be ruled out, but will depend on a range of factors. First, if North Korea did test a two-stage thermonuclear device and is satisfied with its design, it may decide to forgo another provocative nuclear test and standardize its existing designs. Indeed, after its fifth test, it claimed to have tested a “standardized” device—likely a comparatively lower-yield fission bomb for non-ICBM use.

Second, North Korea’s chosen test site at Punggye-ri has to date hosted six tests, but there are questions regarding its ability to contain tests of higher-yield weapons without causing a major cave-in that would release radiation into the atmosphere. A release of contaminants would not only cause a major crisis, certainly with China, the closest country to the testing site, but also allow the United States, Japan, and South Korea to gain considerable knowledge about North Korea’s weapon’s design. (U.S. efforts to detect evidence of this sort of “venting” are underway this week.)

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A seventh North Korean nuclear test may or may not happen. The “nightmare” scenario on several analysts’ minds lately—especially after the August 29 launch of a Hwasong-12 intermediate-range ballistic missile over Japan—is if North Korea chooses to conduct a final nuclear test to remove any and all lingering doubt that it could deliver a thermonuclear weapon with a city-busting yield to intercontinental ranges. To do this, it would mount its presumably compact two-stage weapon on its Hwasong-14 ICBM and launch it into the Pacific Ocean on a minimum energy trajectory and detonate the nuclear payload in the atmosphere.

This would have the desired effect of removing any doubt that it had the full package of capabilities necessary to credibly deter the United States with its thermonuclear-tipped ICBMs, but would be a provocation unlike anything seen to date out of North Korea. It would pose a serious risk to civil aviation and maritime activity—especially since North Korea does not take precautions to notify mariners and aviators prior to its ballistic missile tests. Finally, it would represent the first atmospheric nuclear test since China’s last test of this kind in October 1980.

North Korea’s reasons for carrying out a highly provocative test like this primarily in its deterrence requirements; it has to credibly convince the United States that it can accomplish what it claims today that it can accomplish. Daring Kim Jong-un to prove that he can deliver his 140+ kiloton thermonuclear weapons to intercontinental ranges, thus, might lead to him taking the bait. Let’s all just hope things don’t come to that.