A standard commercial reactor is ~5% enriched. That means that 5% of the Uranium is U235, and the rest U238. U238 absorbs neutrons, however, eventually becomi

A standard commercial reactor is ~5% enriched. That means that 5% of the Uranium is U235, and the rest U238. U238 absorbs neutrons, however, eventually becoming plutonium. This plutonium is where a majority of the energy comes from. It's just too damn hard to separate it out into something that would be weapons grade, unless the reactor was specifically built that way from the start.