The Interocitor

Why the NoKo bomb fizzled

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.

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.

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.

Poor separation of U₂₃₅ from U₂₃₈ in a uranium bomb would have similar issues, but it would take gross incompetence to screw that up.

Posted by Kevin Murphy at October 10, 2006 08:56 AM | TrackBack