PLUTONIUM. The fuel that the North Koreans used for their bomb was plutonium. This is utterly unsurprising; the parallel uranium program that North Korea had developed in the 1990s was never capable of producing much in the way of bomb material. This reinforces the conclusion that the key diplomatic moments came in 1994, when the North Koreans agreed to substantially scale back their nuclear ambitions in return for aid, and in 2002 when they gave up on this agreement. To paraphrase General Buck Turgidson, the Bush administration in 2002 faced two unfortunate but clearly distinguishable realties; one in which North Korea had the material required to make one or two bombs, and one in which had the capacity to make nearly a dozen. Because of its diplomatic ineptitude, ideological commitment, and obsession with Iraq, the administration had neither the interest in dealing with North Korea nor the capacity to carry out any threats.
Hilzoy is indispensible on this question.
--Robert Farley