CONTAMINATED PLUTONIUM

The reason why a US civilian reactor cannot produce plutonium for bombs is contained in the figure (taken from "The homemade nuclear bomb syndrome" by W. Meyer and others, Nuclear Safety, vol. 18, no.4, Jul/Aug 1977). "Burnup" refers to the extent to which U 238 has been irradiated as the fuel is used up; it is given in units of MW-days (of produced energy) per ton of fuel.

As the burnup increases (i.e., time goes on), various isotopes of plutonium (Pu) are formed. The figure shows that Pu 239, the isotope used for weapons, is produced in a power reactor in comparatively pure form only in the very first hours of fresh fuel being burned (and even then only in minute quantities). As the fuel continues to be burned up, the fraction of produced Pu 239 decreases, and contamination by the other plutonium isotopes (238, 240, 241, 242) sets in.

Why "contamination"? Because these other isotopes suffer from spontaneous (rather than chain-reaction-induced) neutron emission, which will make the plutonium ineffective as an explosive for the following reasons.

A nuclear (uranium or plutonium) bomb explodes by bringing several subcritical masses together into one critical mass, large enough so that most neutrons from split nuclei hit further nuclei in the spherical volume of the plutonium, and only a small number escape out through the surface where they are lost to the chain reaction.

Now if these subcritical segments were thrown together to form a critical sphere by, say, an arrangement of mechanical springs, the very first beginnings of the explosion would blow the subcritical parts apart again: there would be a fizzle. In a workable bomb, these parts are thrown together by explosives which must be so powerful that they will hold the critical mass of the fissile material together against the tremendous force of a nuclear explosion until all, or at least most, of the plutonium is consumed.

And that cannot be done when the plutonium (239) is contaminated by the other isotopes. Their spontaneously emitted neutrons will not only have the effect of a counterforce against the explosive shooting the subcritical parts together, but above all, they will cause a pre-detonation before these parts have been joined. There are certain sophisticated procedures that can partially overcome these obstacles, but the only one who has ever tried to use this utterly unsuited material as an explosive was the US government: in a misleading propaganda stunt intended to demonstrate the danger of reactor fuel for terrorist abuse, the Carter administration exploded a charge of reactor-grade plutonium in Nevada. The details were never revealed, but the gadget was called a nuclear "device" (rather than a bomb), perhaps because its yield was such that it wrought havoc among the cobwebs in the test tunnel and maybe seriously upset the flight of nearby flies; what damage it did beyond that the public was never told.