It *can* be, in certain types of reactors (breeder reactors). However there aren't many breeder reactors around the world for commercial power generation at this time. In the United States, we're still on a "once-through" fuel cycle, which means that we don't reprocess or reuse any fuel. Once it's gone through a reactor once, it's considered waste, and just gets put into storage. It's a very inefficient system, but it's stuck around because of the potential proliferation hazards of reprocessing fuel.