Your question is about as specific as "how long is a piece of rope?"
I suggest you consult a beginning nuclear physics text, as it will have some of the basics. It will also, of course, have references to more of the literature. You can also consult books on nuclear engineering, which will have empirically-derived formulas. If you want to calculate things from first principles, read up on stuff done on the Manhattan project during WW2 and after. You'll get into all kinds of neat stuff like Monte Carlo models, diffusion problems, cross sections, etc. I believe the mathematician Ulam did quite a bit on the problem (along with many others).