The Latest North Korean Nuclear Test

On September 2, North Korea conducted its sixth underground nuclear weapons test at Mount Mantap. The earthquake tremor measured 6.3 on the Richter scale, leading experts to conclude that the test was in the range of 50 to 140 kilotons of TNT. A second smaller tremor followed, suggesting cratering at the site. Satellite photos show multiple landslides on Mount Mantap, causing observers to speculate about the possibility of environmental contamination. If the test did “vent,” it may be possible to ascertain whether it was a true two-stage thermonuclear weapon.

Q1: North Korea claimed in 2016 that it tested a hydrogen bomb, but experts were skeptical. What’s different this time?

A1: In December 2015, Kim Jong-un claimed that North Korea had a hydrogen bomb. Actions followed words, and in January 2016, scientists conducted a test that was estimated to have a yield of about 10 kilotons (KTs). At the time, experts were skeptical that a thermonuclear weapon—one that uses a primary fission device to trigger a fusion reaction in the secondary device—would have such a low yield. (For comparison, the first U.S. thermonuclear weapon test had a yield of 10.4 megatons (MTs), the first Russian two-stage thermonuclear weapon a yield of 1.6 MTs, and the first similar Chinese device a yield of 3.3 MTs). Some Western experts suggested it was either a failed thermonuclear test or what is known as a boosted fission device (where tritium and deuterium are injected into the core to increase the yield). In September 2016, North Korea tested again, with a yield of between 15 and 25 KTs.

The larger yield of the September 2 test indicates that the North Koreans are learning from their mistakes. The only sure way to know if it was a thermonuclear device is through environmental sampling for certain radioisotopes. The cratering at the test site may facilitate that sampling.

Q2: How close is North Korea to fielding a nuclear weapon capable of hitting the United States?

A2: North Korea’s ability to target U.S. territory with a nuclear missile depends on several variables. It needs a missile with enough range, obviously. But that range is often compromised by the weight of the payload. Smaller and lighter warheads (often termed “miniaturized” because the first nuclear warheads were quite large) help extend the missile’s range. Depending on the targets, the missiles must also be accurate. Their accuracy depends on many factors, including how well the reentry vehicle withstands the temperatures and pressures of reentering the atmosphere. Having a more powerful warhead reduces the burden of accuracy but only for soft targets. That said, there are no indications so far that North Korea envisions anything beyond a deterrence role for its nuclear weapons, so it’s not clear how accurate its missiles need to be.

North Korea is obviously intent on improving its missile capabilities across the board, as evidenced by its tests of 18 missiles between February and August of this year. The September 2 nuclear test also demonstrates its intention to move forward in improving its design to fit atop a missile. The U.S. Defense Intelligence Agency recently concluded that North Korea may advance from a prototype to production of a reliable, nuclear-capable intercontinental ballistic missile (ICBM) as early as next year. This likely assumes that North Korea improves its reentry vehicle and has a truly viable thermonuclear warhead that does not weigh so much that it reduces the range of the missile. Operationalizing such missiles will also require integration of the warheads and missiles, as well as development of command and control systems.

Q3: Is North Korean denuclearization still a viable policy objective for the United States?

A3: The United States and most other countries around the world continue to call for the denuclearization of the Korean peninsula, something to which that both North and South Korea committed in 1992. Obviously, North Korea has violated its joint declaration with South Korea, but the long-term goal of peace and security on the Korean peninsula and in Northeast Asia is impossible without denuclearization. As North Korea’s nuclear weapons capabilities increase, it has more invested in the program and therefore more to lose. It seems increasingly unlikely that the North will agree to denuclearization anytime soon. However, a halt in activities could be a useful first step that could lead to additional measures to reduce the nuclear threat.

The most pressing question therefore is how to bring North Korea back to the negotiating table. In July, Russia and China recommended a double freeze (a freeze on missile and nuclear tests in exchange for a freeze on military exercises) as a way to get talks started. The United States outlined its approach of strategic accountability in an op-ed by Secretary of State Rex Tillerson and Secretary of Defense Jim Mattis in the Wall Street Journal on August 13, 2017. The recommendation there was for North Korea to cease its nuclear and missile tests as a gesture of goodwill and indication of interest in negotiations.

Sharon Squassoni is a senior fellow and director of the Proliferation Prevention Program at the Center for Strategic and International Studies in Washington, D.C.

Critical Questions is produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).

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Sharon Squassoni