After the steam is used to power the turbine, it is cooled off to make it condense back into water. Some plants use water from rivers, lakes or the ocean to cool the steam, while others use tall cooling towers. The hourglass-shaped cooling towers are the familiar landmark of many nuclear plants. For every unit of electricity produced by a nuclear power plant, about two units of waste heat are rejected to the environment. Commercial nuclear power plants range in size from about 60 megawatts for the first generation of plants in the early 1960s, to over 1000 megawatts.
Many plants contain more than one reactor.
The Palo Verde plant in Arizona, for example, is made up of three separate reactors, each with a capacity of 1221 megawatts. Some foreign reactor designs use coolants other than water to carry the heat of fission away from the core. A few plants use liquid metal or sodium. By 1995, over 32,000 metric tons of highly radioactive waste had been produced by American nuclear reactors. That number is expected to rise to 75,000 metric tons by 2015.