Although many people worry about what to do with the resulting waste products, nuclear energy has the advantage of not generating global-warming emissions. This fact has helped give a tremendous boost to U.S. nuclear reactors. But while the reactors themselves do not create emissions, such plants still leave a carbon footprint. This is because the uranium used to power them needs to be mined, the ore has to be shipped and processed, and plants have to be built and maintained. All of these things result in energy being used from sources such as gasoline and coal. As of 2008, there are 104 nuclear power plants in the United States providing a little over 20 percent of the nation’s electricity needs.