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How does a nuclear power plant work?

A nuclear power plant produces electricity using the heat emitted by the fission of uranium, the mineral that it uses as fuel. Uranium atoms have a heavy, unstable nucleus whose fission is triggered by a collision with a free neutron. This collision results in:
- the splitting of the nucleus into two more tightly bonded nuclei;
- the release of two or three neutrons; and
- the release of energy.
The neutrons thus produced can then interact with other uranium nuclei, continuing the fission process in what is called a chain reaction. In a nuclear reactor, this phenomenon is regulated by control rods that can be used to control the chain reaction in order to vary the power or stop the reactor. Fission can also be controlled through the physical feedback phenomena, which naturally prevent the reaction from going overcritical.
In France, the 58 nuclear reactors in operation are of the PWR type, for “Pressurized Water Reactor.” The enormous quantities of heat produced by uranium fission heat a circuit of water that is kept under pressure. This primary loop heats a secondary circuit of water that transforms into steam and powers a turbine, which in turn drives an alternator. The latter generates an alternating electrical current that passes through a transformer, which raises its voltage to facilitate its transmission through very-high-tension lines.
After it leaves the turbine, the steam of the secondary circuit is reverted to water by a condenser cooled with cold water drawn from the sea or a river. This third water circuit is called the cooling circuit.
If the water flow in the river is too low, or to avoid draining warm water back into the ecosystem, the water in that circuit can itself be cooled by evaporation upon contact with air circulating in tall round towers. These cooling towers are the most visible features of a nuclear power plant.
The three water loops are completely separate and isolated from each other.