Is it possible the right isotopes might be lying around?
Yes, good point, if intelligent life evolved faster on their planet. The relevant timing is how long it took after the supernova that generated the uranium for the alien civilization to arise. (since that sets the 238U/235U ratio).
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.
Here's the new thread for posting quotes, with the usual rules: