Nobody makes bombs with power reactors. Plutonium production requires the reaction U238 + n => U239, decays to Np239 and then to Pu239. However, there is also

Nobody makes bombs with power reactors. Plutonium production requires the reaction U238 + n => U239, decays to Np239 and then to Pu239. However, there is also the reaction Pu239 + n => Pu240. If you change your fuel out frequently you mostly get the first reaction, little of the second. If you leave the fuel in for too long you get a lot of the second. Pu240 is a big headache for bomb makers. The problem is that you need to be able to *fully* assemble your bomb before it produces any substantial amount of heat that will interfere. It's not merely a matter of getting it past the critical point, you need to get it far past that point so the whole reaction goes before the heat blows it apart. There have been multiple criticality accidents in the atomic era--too much material in one place but assembled by ordinary forces. They're deadly to those nearby but they don't blow up much of anything. Pu240 emits a lot of neutrons, it's prone to setting the mass off before full assembly is attained. If that happens you would have gotten a lot more boom for your buck simply piling up ordinary explosives.