I'm confused. I thought a reaction needed a quantity of 235U in an area, and that smaller areas needed more 235U to sustain a chain reaction. Wouldn't very small pieces of relatively 235U rich uranium be fairly stable? One could then put them together with no technological requirements at all.
You are quite correct, small pieces of 235U are stable. The difference is that low concentrations of 235U in natural uranium (because of it's faster decay than 238U) make it harder to get to critical mass, even with chemically pure (but not isotopically pure) uranium. IIRC, reactor grade is around 5% 235U, while natural uranium is 0.7%. IIRC, pure natural uranium metal, at least by itself, doesn't have enough 235U to sustain a chain reaction, even in a large mass. (but I vaguely recall that the original reactor experiment with just the right spacing of...
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