Almost everybody nailed the discussion on North Korea and the bomb. Just for review, the answers are:

1. The North Koreans used plutonium (note the presence of plutonium plants, plus plutonium is easier to obtain than uranium).
2. Implosion design (because you have to with plutonium).
3. The explosion couldn't have been non-nuclear because of the radioactivity detected.
4. Plutonium.
5. They got a fizzle not a bang. That's a real problem with plutonium bombs. It's easier to get plutonium, but the implosion design is much trickier to get right. If the implosion isn't perfectly symmetrical you get a fizzle not a bang.

North Korea's nuclear weapons development continues to be cause for international concern. See this article in the New York times on a nuclear test in September of this year.

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