This graphic of potential nuclear weapons states, while overwrought, states the problem well. If North Korea goes essentially unchecked in its nuclear weapons development, others are sure to follow. Whether it's Iran for its own reasons, or Japan or South Korea in reaction doesn't matter much. The perceived need, and the danger of actual use, goes up something like the square of those that have them.
Either the world powers put their foot down, hard, on North Korea, demonstrating that acquiring nuclear weapons decreases national security rather than enhancing it, or the world will have 20 or so nuclear states within the next two decades. Which is as sure a path to nuclear war as there is.
Problem is, China has all the cards. Short of a preemptive nuclear attack on North Korea, which is unlikely in the extreme, China has an effective veto on any attempt to bring down the North Korean state, as well as the power to accomplish it unilaterally. One would think that China sees this, and is equally aware that this is a test of its stature in world affairs. Yet they resist. Why? What do they want?
Two things come to mind. Either they want to use North Korea as a pawn to strike at the US, which I find hard to believe as the result would harm them, if indirectly.
Or they want Taiwan and hope we'll make the trade.
Something that is probably not lost on the Taiwanese. And of course, there's only one answer for that....
One of the problems in deriving plutonium from spent reactor fuel rods, as the North Koreans apparently did, is that two isotopes of plutonium (239 and 240) have long half-lives, but only one of them (Pu239) is of use in nuclear bombs; the other is simply a contaminant. Since Pu240 is created from exposure of Pu239 to additional neutrons, the US and other nuclear states obtain their plutonium from from dedicated reactors where the Pu239 is removed as it is created, before significant Pu240 is formed. In a commercial reactor this isn't done, and the spent fuel rods contain significant quantities of the Pu240 contaminant (~20%). Since the two isotopes cannot be separated by chemical means, but only by time-consuming weight discrimination methods (e.g. centrifuge) which are exceedingly difficult with plutonium, no nuclear weapon states use the commercial reactor path in creating their weapon-grade plutonium.
From Wikipedia:
The isotope Pu-240 undergoes spontaneous fission very readily, and is produced when Pu-239 is exposed to neutrons. The presence of Pu-240 in a material limits its nuclear bomb potential since it emits neutrons randomly, increasing the difficulty of initiating accurately the chain reaction at the desired instant and thus reducing the bomb's reliability and power. Plutonium consisting of more than about 90% Pu-239 is called weapon-grade plutonium; plutonium obtained from commercial reactors generally contains at least 20% Pu-240 and is called reactor-grade plutonium.It seems quite likely that the NoKo bomb fizzled, at least in part, due to contamination of this sort. Of course, a poor implosion would have exacerbated the problem, but fuel contamination seems highly probable.Pu-240, while of little importance by itself, plays a crucial role as a contaminant in plutonium used in nuclear weapons. It spontaneously fissions at a high rate, and as a 1% impurity in Pu-239 will lead to unacceptably early initiation of a fission chain reaction in gun-type atomic weapons, blowing the weapon apart before much of its material can fission. Pu-240 contamination is the reason plutonium weapons must use an implosion design.
Poor separation of U235 from U238 in a uranium bomb would have similar issues, but it would take gross incompetence to screw that up.