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As North Korea appears set to test another nuclear device, the question remains: Does Pyongyang have enough highly enriched uranium to build a nuclear weapon? A third nuclear test could provide the answer.

“The answer to that question isn’t trivial,” said Mr. Hibbs.

Gary Milhollin, founder of the Wisconsin Project on Nuclear Arms Control, said North Korea “has had at least one enrichment plant running successfully for some time, and we have to assume that plant has made enough material for a test.”