About two thirds of the heat generated by nuclear reactors is dumped into the cooling system. Ordinary water is a popular coolant for reactors. The water for cooling is drawn from either a large river or the ocean. While this makes it convenient to locate reactors near a river or the ocean, it also makes them more vulnerable to floods and tidal waves such as the recent disaster at Fukushima.
About two thirds of the nuclear power reactors in the United States are pressurize-water reactors. A one thousand megawatt pressurized-water reactor with a cooling tower consumes about twenty thousand gallons of water per minute for cooling and returns about five thousand gallons of water back into the environment. A one thousand megawatt pressurized-water reactor without a cooling tower can consume five hundred thousand gallons of water per minute for cooling and returns most of it back into the environment. These figures are for a core running at a temperature differential of thirty degrees Fahrenheit. In order to reduce that to a differential of 20 degrees Fahrenheit, about fifty percent more cooling water is required. For the reactor without a cooling tower, this would require over seven hundred and fifty gallons of water per minute. There are designs for new reactors that would need over one million gallons of water per minute for cooling.
The cooling water released from nuclear power plants can be more than thirty degrees Fahrenheit warmer than the water that was drawn from the river in the first place. In addition, radioactivity isotopes from the core leaks into the cooling water. While the exact type and amount of these isotopes released from a particular reactor cannot be known we know that the heated water and such isotopes have an adverse impact on the ecology of the plants, fish and other life forms in the river. Additional radioactive gases leak into the air from the reactor and wind up in soil and water near the reactor. Government regulations permit such releases during routine operation and do not require plants to monitor their normal releases. Some of these isotopes have half-lives of thousands to millions of years.
Full power production in nuclear reactors that use water for a coolant is dependant of a steady powerful flow of water. If the water is coming from a river and the flow of the river decreases substantially, then power production must be cut back or the plant must be shut down entirely. There have been many problems at nuclear power plants where something blocked the intake system and the reactors had to be shut down. Many major rivers are so depleted from drawdown for irrigation, municipal water supply and industrial production that some rivers do not reach the sea anymore. In addition, a terrorist could carry enough explosives in a small boat to destroy the intake for a nuclear power station and force it to shut down.
Even power plants with cooling towers are not immune to problems. Structural problems leading to collapse, clogging of screens that caused overflow, ice build up on nearby transmission lines from vapor release, as well as radioactive gases and isotopes released with the water vapor have all caused partial or total shutdown of reactors.
Use of water as a primary coolant for nuclear power plants will continue but there are many design and environmental problems that need will require additional research and development.