Cooling towers are towers that are used to transfer waste heat to the atmosphere. Cooling towers may use the evaporation of water to remove heat and cool the working fluid to near the wet-bulb air temperature. They may also use only air to cool the working fluid to near the dry-bulb air temperature. Cooling towers are used in oil refineries, petrochemical plants and power stations, especially nuclear power stations. In a nuclear power plant, the cooling tower is isolated from the nuclear reactor by a heat exchanger, and the steam from the cooling tower is not radioactive. The cooling tower is usually the tallest and most visible part of a nuclear power plant, much taller than the reactor building or turbine hall. 