Nuclear power accounts for about 19 percent of the total electricity generated in the United States, an amount comparable to all the electricity used in California,Texas and New York, our three most populous (having the most people) states. A nuclear power plant operates basically the same way as a fossil fuel plant, with one difference: the source of heat. The process that produces the heat in a nuclear plant is the fissioning or splitting of uranium atoms. That heat boils water to make the steam that turns the turbine-generator, just as in a fossil fuel plant. The part of the plant where the heat is produced is called the reactor core.