Since I didn't really understand the difference just that there was one I found this article:
A Nuclear Primer -- What Is An Atomic Bomb - Forbes
First, no nuclear weapons program ever came out of a nuclear energy program. Commercial light water reactor nuclear power plants, and their nuclear waste, cannot really be used to make weapons, contrary to popular belief. It is theoretically possible, but not practical, and everyone knows there are easier ways to do it. We tried very hard in the 1960s and made a crappy device that demonstrated clearly thatiIf you want weapons, you develop a weapons program.
(
I hope that is true)
Commercial nuclear power is completely different from atomic weapons. There are two types of atomic weapons – a U-bomb and a Pu-bomb. Each needs the material to be almost totally the one radionuclide, over 90% of either U-235 or Pu-239. For a U-bomb, you don’t need a nuclear reactor, you just need to enrich the U-235 up to about 93%, way more then the 5% for a commercial reactor.
Anything
greater than 20% U-235 is considered highly enriched, and can theoretically be made into a device. But anything
less than 90% is too heavy and unreliable. Since using an atomic weapon is crossing quite a line, you better be sure it works since the consequences of trying may get you blown off the face of the Earth
(sounds scary, and that is why we need close monitoring and no country really should be allowed to refuse to be checked)