U.S. officials and independent experts say North Korea appears to have taken unusual steps to conceal details about the nuclear weapon it tested in February, fueling suspicions that its scientists shifted to a bomb design that uses highly enriched uranium as the core.
At least two separate analyses of the Feb. 12 detonation confirmed that the effects of the blast were remarkably well contained, with few radioactive traces escaping into the atmosphere where they could be detected, according to U.S. officials and weapons experts who have studied the data.
U.S. officials anticipated the test and monitored it closely for clues about the composition of the bomb, which was the third detonated by North Korea since 2006. The first two devices were thought to have used plutonium extracted from a dwindling stockpile of the fissile material that North Korea developed in the late 1990s.