North Korea tested another nuclear device on February 12. One of the questions about that test was whether the fissionable material used was enriched uranium or plutonium. That question could only be answered by outsiders if samples of the xenon isotopes produced by the fission could be captured and analyzed. That couldn’t be done; North Korea does a very effective job of containing their tests. The xenon isotopes are short-lived, so the window of opportunity is now closed.

If we are going to learn more about North Korea’s bomb design, we will have to do it another way. Joby Warrick of the Washington Post speculates:

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.

The North Koreans showed Sigfried Hecker their uranium enrichment facility in 2011. Ever since, there has been speculation on whether the North Koreans have manufactured enough highly enriched uranium to build a bomb. It’s easier to build a bomb from uranium than from plutonium. This test was the first since the uranium enrichment facility became public.

But Warrick fails to consider other reasons the North Koreans might have for taking “unusual steps” to keep their testing secret.

  • If it was a plutonium device, that could mean that their uranium enrichment wasn’t going well. They have a limited stock of plutonium, and one more test would deplete that stock further. On the other hand, it would give them additional information about their design(s), which didn’t seem to work so well in earlier tests.
  • Nations keep their nuclear weapons information secret. Everyone else with nuclear weapons is just as secretive.
  • It is to North Korea’s advantage in negotiations to keep this information secret.
  • Allowing radioactive isotopes to escape from their test, or testing in the atmosphere, might irritate China and Russia further than current North Korean rhetoric.
  • There is a norm for containing nuclear tests that even North Korea follows.

Of course, more than one reason may apply. But Warrick chooses to focus on only one possibility.

Today, North Korea has announced that it will restart the reactor at Yongbyong that furnished its current plutonium stocks. If reporters were so inclined, this could be interpreted to mean that the nuclear test was of a plutonium device and wants to restock, but that possibility hasn’t been mentioned in any of the news stories I’ve seen.

The New York Times:It is unknown whether North Korea’s third nuclear test in February used some of its limited stockpile of plutonium or used fuel from its uranium-enrichment program, whose scale and history remain a mystery.

The Washington Post article is much more balanced than Warrick’s, allowing for the possibility that the test was of a plutonium device.

The Wall Street Journal article mentions the uranium possibility but does not emphasize it.

So Warrick’s emphasis on uranium as the material tested in February may not have legs. But all too often a speculation is what other reporters recall and repeat until it is the common wisdom. The idea that Iran is seeking nuclear weapons has developed in that way, even though US intelligence evaluations say that it is not.

It’s too early to say whether Warrick’s speculation will become the common wisdom. Something to keep an eye on.

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