People in Seoul, South Korea, watch a news report showing North Korean army tanks.

Ahn Young-joon, Associated Press

N. Korea won't let nuclear arms go, even for cash

  • Article by: JoWarrick
  • Washington Post
  • March 31, 2013 - 11:26 PM

– U.S. officials and independent experts say North Korea appears to have taken unusual steps to conceal details about the nuclear weapon it tested last month, 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 and analyzed, 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.

A successful test of a uranium-based bomb would confirm that Pyongyang has achieved a second pathway to nuclear weapons, using its plentiful supply of natural uranium and new enrichment technology. A device based on highly enriched uranium, HEU, also would deepen concerns about cooperation between the hermetic regime and Iran.

There are two paths to a nuclear weapon. The bomb that the United States dropped on Hiroshima in 1945 used HEU as its core, and the one dropped three days later on Nagasaki was a plutonium device. North Korea has long possessed plutonium, but its enrichment of uranium is a more recent development. Iran has been concentrating on uranium enrichment, which it says is for civilian purposes.

Although North Korea and Iran have cooperated on missile technology, U.S. officials said there is no direct evidence of nuclear cooperation.

The prospect of a third nuclear test prompted heightened scrutiny of the Korean Peninsula by intelligence agencies. Despite the intense focus, U.S. analysts said they did not pick up enough physical evidence to draw firm conclusions about the fissile material used in the device.

In fact, in the days following the detonation, U.S. and South Korean sensors did not detect even a trace of the usual radioactive gases in any of the 120 monitoring stations along the border and downwind from the test site, the officials said. A Japanese aircraft recorded a brief spike of one radioactive isotope, but it was seen as inconclusive, the analysts said.

Officials and analysts said North Korea’s second nuclear test, which occurred in 2009, also left no detectable traces. It would not be surprising for North Korea to take extra steps to prevent outsiders from gaining insights into its nuclear capability, said a third U.S. official.

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