Simplify: like the steam machines of early industry, a nuclear power plant mainly produces steam, which is itself transformed into electricity.
To obtain this steam, water is heated to very high temperatures with uranium. Inside the nuclear reactor, uranium rods are bombarded by neutrons, microscopically small particles. During this bombardment, the material that makes up uranium - the atoms - break apart; this is known as nuclear fission.
As these uranium atoms break up, they release not only radioactivity, but also a large amount of energy in the form of heat, which heats the water in the primary circuit. This closed circuit in turn heats the water circulating in the secondary circuit, also closed. The heat transforms the water into steam, which drives turbines that start up an alternator, just like in a car engine; it's the operation of this alternator that generates electricity.
The steam is immediately cooled by a third circuit, which pumps water from a nearby stream. This water can be cooled in air-cooling towers, the iconic chimneys of nuclear power plants, or directly into the sea.
Fission also releases a few neutrons, which again bombard other uranium atoms, maintaining the chain reaction.
Power plants in Normandy
Have you ever noticed the absence of cooling towers in Normandy?
The cooling circuits of the Penly, Paluel and Flamanville power plants are fed directly from the English Channel ; the quantity and flow rate of water are very high! Steam cooling is therefore much faster, and after temperature control, the water is discharged directly into the sea.