6. The uranium can be enriched to anything from 0.9 to 97 percent U-235. Most light-water power reactors use 3 to 5 percent enriched fuel, research reactors currently running use 12.0 to 19.75 percent, and submarine reactors use 50 percent. In the old days, we used very expensive 94 percent fuel in research reactors. An atomic bomb needs about 82 percent.