On September 9, 2016, seismic stations around the world picked up the unmistakable signals of another North Korean underground nuclear test in the vicinity of Punggye-ri. The technical details about the test will be sorted out over the next few weeks, but the political message is already loud and clear: North Korea will continue to expand its dangerous nuclear arsenal so long as Washington stays on its current path.

Preliminary indications are that the test registered at 5.2 to 5.3 on the Richter scale, which translates to an explosion yield of approximately 15 to 20 kilotons, possibly twice the magnitude of the largest previous test. It appears to have been conducted in the same network of tunnels as the last three tests, just buried deeper into the mountain. This was the fifth known North Korean nuclear explosion; the second this year, and the third since Kim Jong Un took over the country’s leadership in December 2011.

Unlike previous announcements, such as the claim of having detonated a hydrogen bomb in January 2016, the current statement can no longer be dismissed. This time, KCNA reported North Korea’s Nuclear Weapons Institute claiming:

The standardization of the nuclear warhead will enable the D.P.R.K. to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power with a firm hold on the technology for producing and using various fissile materials. This has definitely put on a higher level the D.P.R.K.’s technology of mounting nuclear warheads on ballistic rockets.[1]

This statement brings up some fundamental questions.

How many nuclear warheads can the DPRK produce? North Korea’s capacity to produce plutonium remains limited to 6 kilograms, or approximately one bomb’s worth, per year. We estimate that it may have a stockpile of 32 to 54 kilograms (roughly 6 to 8 bombs’ worth) of plutonium now. Whereas plutonium production can be estimated reasonably accurately because of telltale signals resulting from reactor operation, production of highly enriched uranium (HEU) remains highly uncertain. However, based on what I saw at the Yongbyon centrifuge facility during my last visit in November 2010, the expanded footprint of the facility since, and our probabilistic estimates of how much it could make in covert facilities,[2] it is possible that the DPRK could add 150 kg of HEU (roughly 6 bombs’ worth) to a current stockpile of perhaps 300 to 400 kg.[3] In other words, a stockpile of sufficient fissile material for approximately 20 bombs by the end of this year and a capacity of adding approximately 7 per year makes the DPRK claim sound plausible.

Can the DPRK produce lighter, smaller warheads and as claimed by the DPRK Nuclear Weapons Institute, ones that have been “standardized to be able to be mounted on its ballistic missiles?” With the two successful nuclear tests this year, we must assume that the DPRK has designed and demonstrated nuclear warheads that can be mounted on some of its short-range and perhaps medium-range missiles. Its ability to field an ICBM fitted with a nuclear warhead capable of reaching the United States is still a long way off—perhaps 5 to 10 years, but likely doable if the program is unconstrained.

As much as a doomsday nuclear shot at the United States worries Americans, it is not what I consider to be the primary threat from Pyongyang’s unrelenting drive to more bombs and better bombs. At a minimum, the current state of the North’s nuclear arsenal is an effective deterrent to potential hostile external intervention. It has reinforced Kim Jong Un’s legitimacy as leader of what the North considers a beleaguered nation. The current situation is very different from what my Stanford colleagues and I encountered during our November 2006 visit a few weeks after the North’s first nuclear test. At each of our stops—the Ministry of Foreign Affairs, the Department of Atomic Energy, and the military—we found their pronouncements of having achieved a deterrent against the United States to ring hollow.

Nevertheless, Kim Jong Il appeared sufficiently confident of the state’s security that he accepted the Bush administration’s change of heart and new willingness to find a diplomatic solution. The years 2007 and 2008 marked significant diplomatic progress, which resulted in a rollback of operations at the Yongbyon nuclear complex, the return of international inspectors, and the presence of an American technical team. However, in the summer of 2008, the Bush administration pulled back followed by a similar response by the North in August. Kim Jong Il suffered a stroke shortly thereafter. Succession planning in the North was done in an environment greatly influenced by the hardline approach of South Korea’s new president Lee Myung-bak. All of these events likely contributed to diplomacy not getting back on track before time ran out on the Bush administration.

By the time our Stanford team visited Pyongyang in February 2009, we were told that times had changed—the North was going to launch a long-range rocket and that matters would get much worse. And, so they did. The launch was followed by UN condemnation and Pyongyang’s expulsion of the international inspectors and American team. Then in May, the North conducted its second nuclear test, which, unlike the first, appeared successful. Seoul and Washington apparently rebuffed Pyongyang’s overtures in the summer of 2009 to get back on a diplomatic track and the DPRK was off and running on a determined path for more and better bombs, combined with greatly increasing its missile threat.

The death of diplomacy, namely the Obama administration’s retreat to “strategic patience” and the demise of the Six Party Talks, opened the door to an unrelenting expansion of the North’s nuclear weapons program, as best as one can tell, unconstrained by international opprobrium and escalating sanctions.

Construction activities at Yongbyon picked up steadily in the latter half of 2009. Pyongyang made numerous claims of nuclear progress—declaring it had achieved nuclear fusion and that it was successful in enriching uranium, which it stated was now required to fuel the new indigenous light water reactor it was going to build. Most of these pronouncements were dismissed outside the DPRK, but then came its remarkable revelation of a modern, industrial-size uranium centrifuge facility during our Stanford delegation’s last visit in November 2010. Then, the curtain descended on foreign visitors to Yongbyon, while construction and testing picked up and has continued to the present.

Based on commercial satellite imagery, the 5 MWe Reactor in Yongbyon has operated off and on since August 2013. It is capable of producing up to 6 kg plutonium annually, but also is capable of producing tritium for fusion bombs if Lithium-6 targets are irradiated in the reactor. The Radiochemical Laboratory is operational again allowing plutonium to be separated from the spent reactor fuel. The North also has hot cell facilities near its IRT-2000 Reactor to recover tritium that may be produced in either reactor. It has rebuilt the fuel fabrication facilities to allow more metallic 5 MWe Reactor fuel to be fabricated. It has completed the external construction of the Experimental Light Water Reactor (ELWR), including having developed fuel fabrication facilities for its ceramic fuel. It has doubled the size of the Yongbyon centrifuge hall, enabling it to enrich uranium for ELWR fuel and produce the feed material that is likely taken to a covert facility to be enriched to weapons-grade HEU. The expansion of Yongbyon facilities, all visible from above, has been nothing short of breathtaking.

I was puzzled for many years by the slow pace of North Korea’s medium-to-long-range missile testing program. However, at about the same time as the Yongbyon expansion began, Pyongyang also mounted a major construction effort on a new launch facility on its west coast, from which it has launched three long-range rockets since 2012. Much of the world laughed as the North paraded what looked like dummy missiles through the streets of Pyongyang during military parades, but they are not laughing any more, as the pace of missile tests during the past year has been just as intense as its nuclear buildup. Beyond the satellite launches, the North has now demonstrated its capability to fire a submarine-launched ballistic missile and a solid-fuel, two-stage medium range missile.

This brings us full circle to the fifth nuclear test. This test must be viewed with great concern, not for any specific capabilities it may have demonstrated, but as part of this enormous buildup of North Korea’s nuclear arsenal. Five tests conducted over a ten-year period, sufficiently spaced that the test results can inform the next test, are deeply alarming.

What are the greatest threats from the rapidly expanding North Korean nuclear program? Left unchecked, Pyongyang will likely develop the capability to reach the continental United States with a nuclear tipped missile in a decade or so. Much more troubling for now is that its recent nuclear and missile successes may give Pyongyang a false sense of confidence and dramatically change regional security dynamics. The likely ability of the DPRK to put nuclear weapons on target anywhere in South Korea and Japan and even on some US assets in the Pacific greatly complicates the regional military picture. That situation would be exacerbated if Pyongyang decides to field tactical nuclear weapons as its arsenal expands and its confidence in its nuclear arsenal grows.

More bombs and better bombs also increase the potential of accidents and miscalculations with greater consequences as the number and sophistication of bombs increase. Rendering the nuclear enterprise safe and secure in case of internal turmoil or a chaotic transition in the North becomes more difficult. We also cannot rule out that a financially desperate leadership may risk the sale of fissile materials or other nuclear assets, perhaps to non-state actors.

So, what to do? The latest nuclear test demonstrates conclusively that attempting to sanction the DPRK into submission and waiting for China to exert leverage over Pyongyang’s nuclear program do not work. Increasing sanctions and adding missile defenses in South Korea to that mix will also not suffice and make China even less likely to cooperate. What’s missing is diplomacy as much as Washington may find it repugnant to deal with the Kim regime.

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[1] “DPRK Succeeds in Nuclear Warhead Explosion Test,” KCNA, September 9, 2016.

[2] John Bistline, et al. “A Bayesian Model to Assess the Size of North Korea’s Uranium Enrichment Program,” Science and Global Security 23 (2015): 71-100.

[3] Chaim Braun, Siegfried Hecker, Chris Lawrence, and Panos Papadiamantis, “North Korean Nuclear Facilities After the Agreed Framework,” Center for International Security and Cooperation, Stanford University, May 27, 2016, http://fsi.stanford.edu/publication/north-korean-nuclear-facilities-after-agreed-framework.