Okay, one quick question. Why isn’t neptunium used? I’ve always assumed that it either has no fissile isotopes or that it must have too short a half-life to be of any use.
Six of one, half-dozen of the other, asterion. [sup]228[/sup]Np is fissile, but its half-life is about 62 seconds. [sup]237[/sup]Np is fissile (and has a half-life of 2,114,000 years, long enough to be useful), but the fraction of spontaneous fissions is less than 2x10[sup]-10[/sup] (by constant, [sup]235[/sup]U has an SF fraction of 7x10[sup]-9[/sup], and [sup]239[/sup] of 3x10[sup]-10[/sup]). I suppose it could be bred, but why bother? Uranium or plutonium are better choices as a “fuel” element.