(07-12-2016, 06:15 PM)UniqueStranger Wrote: Again, not finding 238 as fissible, but rather fissionable, that is, if Jupiter can provide enough high energy neurons, which we don't know, right?
Quote:the even isotopes, plutonium-238, -240, and -242 are not fissile but yet are fissionable–that is, they can only be split by high energy neutrons. Generally, fissionable but non-fissile isotopes cannot sustain chain reactions; plutonium-240 is an exception to that rule.
The minimum amount of material necessary to sustain a chain reaction is called the critical mass. A supercritical mass is bigger than a critical mass, and is capable of achieving a growing chain reaction where the amount of energy released increases with time.
The amount of material necessary to achieve a critical mass depends on the geometry and the density of the material, among other factors. The critical mass of a bare sphere of plutonium-239 metal is about 10 kilograms. It can be considerably lowered in various ways.
The amount of plutonium used in fission weapons is in the 3 to 5 kilograms range. According to a recent Natural Resources Defense Council report(1), nuclear weapons with a destructive power of 1 kiloton can be built with as little as 1 kilogram of weapon grade plutonium(2). The smallest theoretical critical mass of plutonium-239 is only a few hundred grams.
In contrast to nuclear weapons, nuclear reactors are designed to release energy in a sustained fashion over a long period of time. This means that the chain reaction must be controlled–that is, the number of neutrons produced needs to equal the number of neutrons absorbed. This balance is achieved by ensuring that each fission produces exactly one other fission.
Yet, a similar mix of Pu-238/239 oxide as was provided with Galileo has shown to be both fissile and fissionable. Richard Garwin concluded this also is his report. An 80/20 mix is a viable weapon. Which is what Galileo had.
Once again, it's not DESIRABLE yet is does provide a low yield.