WHY THE CARGO IS NOT DANGEROUS

When a neutron of the right energy is absorbed by a plutonium (or a uranium 235) nucleus, the latter splits into all kinds of products, including further neutrons. A bomb designer hopes that several of these neutrons will hit further plutonium nuclei so that the chain reaction will continue.

Why shouldn't it? Because too many of the neutrons will escape through the surface of the plutonium instead of doing their work inside it. The geometrical shape of the body with the smallest area enclosing the biggest volume is a sphere. That is what a bomb designer will choose, but that by itself will not do the trick; a sphere with radius r has a surface proportional to r^2 and a volume proportional to r^3, so that the volume to surface ratio is proportional to r. In other words, only from a certain size upward will the volume to surface ratio of a sphere be big enough to sustain the "neutron economy." The number of neutrons hitting their target inside the sphere will now be sufficiently large that the loss across the surface can be tolerated.

This smallest size r at which this is achieved determines the critical mass of the material which we will assume to be bomb-grade plutonium, i.e., almost pure Pu 239. Suppose you had several subcritical segments of a sphere and you suddenly combined them into a greater than critical sphere by a mechanical or simple explosive trigger. Would the sphere explode?

How this is done is now well known, but let's not walk into that side street. Instead, imagine that a sabotaging demon sits on the bomb who shoots neutrons into the subcritical segments. Then he may make up for the neutrons that are lost (the loss makes the segments subcritical) and make them fizzle: they will be immediately blown apart to small pieces, and the bomb can do no more damage. As a matter of fact, one of the defenses against incoming missiles under consideration (or, for all I know, perhaps even developed) consisted in hitting the enemy missiles with a neutron beam, causing the nuclear warheads to fizzle. All of this goes for bomb-grade plutonium, Pu 239. But apart from demons and neutron beams there is a third way to achieve exactly the same fizzle: using reactor-grade plutonium, which will result in a non-bomb or fizzle-bomb. The neutrons are shot into the subcritical parts neither by demons nor by defense beams, but by the other plutonium isotopes that contaminate the Pu 239, in particular, by Pu 240. These isotopes emit neutrons spontaneously, even without absorbing a neutron of the deliberate chain reaction, and thus fulfill the job of the fictitious demon perfectly. Such contaminated plutonium cannot be used for a bomb because (1) it would "explode" in a fizzle, and (2) at a completely unpredictable time.

A power reactor must produce such contaminated plutonium for reasons apparent from the figure, which is taken from a document that has long been all too long neglected, The Homemade Nuclear Bomb Syndrome by W. Meyer and three other professors of nuclear engineering at the U. of Missouri, Columbia, at the request of the American Nuclear Society Public Information Committee, which published it in 1976.

The thickness of the curves is not caused by experimental error; the top edge corresponds to pressurized, and the bottom edge to boiling-water reactors. What little plutonium is produced in the fuel rods of a power reactor (at least of the US type) must be contaminated within a matter of hours. If you want to produce a plutonium bomb, you have to make the plutonium form U 238 by very strong neutron irradiation for a very short time, before the non-239 isotopes have a chance of accumulating to significant quantities. That is what a production reactor, such as the one in Hanford, Wash., used to do. There is no chemical way of separating isotopes of the same clement, and physical separation would be so vastly uneconomical (in specially built vast plants) that it is out of the question even for any government.

Bomb-grade plutonium, I am told, allows for less than 1% Pu 240. What is the percentage in the plutonium carried by the Akatsuki Maru? According to the US Committee for Energy Awareness in Washington, D.C., from 21.0% to 22.4% (in addition to smaller percentages of Pu 239, 241 and 242). How much more lunatic can the Greenpests get?