Most reactors use somewhere in the range of 3-5% U235 (compared to .7% naturally). That by itself won't let you make a bomb, but it gets you about 1/8th of the

Most reactors use somewhere in the range of 3-5% U235 (compared to .7% naturally). That by itself won't let you make a bomb, but it gets you about 1/8th of the way to 20% which is about where a bomb is possible.