What are the purpose of these towers when generating nuclear power?
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Nuclear power plants, like coal, oil and gas power plants, produce energy in the form of heat. For coal, oil and gas, the heat is produced by burning the fuel ; for nuclear power plants, it comes from the nuclear reactions inside the reactor.
The problem is that we generally don’t want to use energy as heat, we want electricity instead. And the laws of physics prevent us from converting all the thermal energy into electrical energy. In the case of nuclear power plants, we only retrieve about the third of the thermal energy. The rest of the heat has to be disposed of, otherwise the reactor would overheat. But this has to be done carefully, since you don’t want radioactivity to come out of the plant.
This is done in the following way : the heat from the reactor is collected by a first loop, completely closed, of highly pressurized water. This water comes from inside the reactor, so it is radioactive : you don’t want it out of the reactor. So it goes through a heat exchanger where the heat evaporates water circulating in a second loop. The water from the two circuits don’t mix : only heat is transferred. The steam of the second loop goes through a turbine that converts heat into mechanical energy, which will be later converted into electricity.
The steam then goes through another heat exchanger to dump the heat into a third water circuit, thus ensuring absolutely no radioactivity comes out. This clean heated water in the third circuit can be taken from the sea, in which case it is sent back, a little warmer, into the sea. When sea is not available, the water comes from a river, but sending this water back into the river would surely kill all the fish : not because of radioactivity, there is none, but because fish + hot water = cooked fish. So instead the water goes through a cooling tower where it is sprayed in droplets to make it evaporate and dump the heat into the air. The heated air with water vapour and microscopic droplets is then evacuated at the top of the tower : this is the white cloud that sometimes come out of the tower.
This three-circuit process is the one used in France (my country), so that’s the one I know. It may be done a bit differently in other countries, so there may only be two circuits instead of three. And this kind of cooling towers are also used for coal, oil and gas power plants. But these are recognizable because they also have another stack for combustion gas.
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