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Senior Fellow, Carnegie Endowment for International Peace
April 15, 2019
New Details Emerge on NK Nuclear Program

Paul Kerr

Several weeks after North Korea’s early October announcement that it had successfully tested a nuclear device, new information is emerging about the test and Pyongyang’s plutonium-based nuclear weapons program. Still, much uncertainty remains about the status and scope of that program and a suspected North Korean uranium-enrichment program.

Siegfried Hecker, a former director of the Los Alamos National Laboratory, spoke with North Korean officials about the nuclear program when he visited the country Oct. 31 to Nov. 4. He discussed his findings in a report presented Nov. 15. Hecker was accompanied by John Lewis, a historian at Stanford University; Charles “Jack” Pritchard, former special envoy for negotiations with North Korea; and Robert Carlin, former senior policy adviser at the Korean Peninsula Energy Development Organization. Lewis and Hecker also discussed the North Korean nuclear program with various Chinese officials. In addition, a Department of State official also discussed the status of these programs during two interviews with Arms Control Today.

Test Uncertainties

One week after the test, the office of Director of National Intelligence John Negroponte stated that North Korea had “conducted an underground nuclear explosion” with a yield of “less than a kiloton.”

North Korean officials maintained during their meeting that the test was successful, Hecker said. But he reported that Pyongyang told Chinese officials two hours before the test to expect a yield of about four kilotons.

A four-kiloton yield would have been far below that of the bombs the United States dropped on Hiroshima and Nagasaki. The Nagasaki bomb, for example, had a yield of 21 kilotons. Analysts have debated whether the low yield indicates that North Korea attempted to test either a simple nuclear device or a smaller, more sophisticated device that could be deployed on a missile. (See ACT, November 2006.)

According to Hecker, Chinese nuclear scientists estimated the explosion’s yield at about one kiloton. Those scientists argued that the North Koreans had tested a simple nuclear device designed for an explosive yield of four kilotons in order to ensure that they could contain the test’s radioactivity in a horizontal shaft. The Chinese assessment is “reasonable…based on the facts we have at this time,” he said. The State Department official said that U.S. intelligence agencies had not yet determined why the test yield was so low.

North Korean officials told Hecker that the device used plutonium as its fissile material. The State Department official said Oct. 31 that the United States does not yet know whether this claim is true but said that he “would be shocked if it were anything else.” Whether the United States will be able to determine when the plutonium was produced is uncertain, the official said.

It does not appear that North Korea is preparing to conduct another test at this time. “None of the officials we met gave us the impression that they are planning a second nuclear test,” Hecker said. Likewise, the State Department official said that there are “no indications” that Pyongyang is doing so.

Hecker said that North Korea will “keep the number” of future nuclear tests “to a minimum” because of its limited plutonium inventory.

Plutonium Program Continues

Hecker also noted that North Korea continues to operate its nuclear facilities at Yongbyon, including its five-megawatt nuclear reactor and a facility for reprocessing spent fuel to extract plutonium. But he said that operation of the research reactor has slowed somewhat. In addition, the construction of two larger reactors, which would have vastly expanded Pyongyang’s plutonium-production capacity, has not kept pace with North Korea’s previous estimates.

Under the 1994 Agreed Framework between the United States and North Korea, Pyongyang froze these facilities’ operation and subjected them, along with approximately 8,000 spent fuel rods, to international monitoring. North Korea also froze the construction of the two larger reactors.

But after the most recent North Korean nuclear crisis started in October 2002, Pyongyang ejected the inspectors, announced its withdrawal from the nuclear Nonproliferation Treaty, restarted the reactor, and claimed to have reprocessed the spent fuel to obtain plutonium.

Hecker said that prior to its test explosion, North Korea probably had 40-50 kilograms of plutonium, which is enough for about six to eight nuclear bombs. The test “most likely” consumed about six kilograms, Hecker said. This estimate of the total plutonium inventory appears to be somewhat lower than one from South Korea’s Ministry of Defense reported by the semi-official Yonhap News Agency Oct. 26. The reactor is accumulating about one bomb’s worth of plutonium per year, according to Hecker’s discussions with Yongbyon officials.

Hecker reported that Ri Hong Sop, director of the Yongbyon nuclear center, said that North Korea plans to unload the reactor “sometime next year” and reprocess the spent fuel. However, Pyongyang may unload the reactor sooner if the “political situation” changes, Ri said. North Korea expert Selig Harrison told reporters in September that Pyongyang intends to unload spent fuel rods from the reactor for reprocessing by the end of the year.

Pyongyang last unloaded the reactor during the spring of 2005 and restarted the reactor in June of that year. The reactor “has been operating with the current fuel load” since then, Hecker said.

The North Korean officials also provided an update regarding Pyongyang’s progress in completing the two larger reactors.

According to Hecker’s report, the officials said that “virtually nothing” has been done at a site where North Korea had been constructing a 50-megawatt reactor. Pyongyang has apparently not yet made “a political decision” to proceed with a full-on construction effort, Hecker said, adding that “technical difficulties” are slowing down the project. These difficulties will put the reactor’s completion “at least several years into the future,” he added.

After a visit last year, Hecker said that a North Korean official implied that Pyongyang is attempting to complete construction on the reactor within “a couple of years.”

Ri said that North Korea has not yet decided whether to complete a 200-megawatt reactor that had also been under construction prior to the Agreed Framework, said Hecker.

Uranium-Enrichment Program

Hecker said that the North Korean officials provided no information regarding the country’s suspected uranium-enrichment program. Gas centrifuges enrich uranium hexafluoride by spinning it at very high speeds to increase the concentration of the relevant fissile isotope. Highly enriched uranium can be used as fissile material in nuclear weapons.

Hecker said that “it is very likely” that North Korea has “at least a research-scale uranium-enrichment effort.” Pyongyang has repeatedly denied having such a program.

State Department officials told Arms Control Today about a year ago that North Korea has enough components sufficient for a “pilot” enrichment facility. But it is still unclear whether Pyongyang has an operating facility or possesses all necessary centrifuge components. (See ACT, June 2006.)

North Korea is likely continuing work on the program, the State Department official said in November, adding that the United States has little information about its progress and calling the program a “black hole.” The official indicated that North Korea’s efforts to obtain materials for the program had largely stopped but pointed out that Pyongyang may have learned to produce its own components.

The official also said that North Korea may have an advanced enrichment program but acknowledged that Pyongyang may have halted work on it, perhaps because the government ran out of money or lost access to foreign expertise.