Forgive me if this question has been asked before or is a FAQ. Can somebody tell me to what percentage uranium has to be enriched in order to build a bomb? Believe me, I'm not trying to build one!! I'm just trying to inject some physics into the current Israel/Iran debate. (Let's confine the discussion to physics, not politics!) Also, what percentage enrichment is necessary for commercial reactors, and is that a percentage of U-235 or U-238? Thanks.