How works a Pressurized Water Reactor (PWR) like the EPR reactor

In a nuclear power plant, the reactor is the part of the facility in which the heat, necessary to produce steam, is generated by fission of the nuclei of uranium atom.

The produced steam drives a turbine generator, which generates electricity.

The nuclear steam supply system is therefore the counterpart of coal, gas or oil-fired boilers of fossil-fuelled plants.

A pressurized water reactor (PWR) nuclear power unit is a thermal power plant that is called "nuclear" because it uses the heat released by the fission of the nuclei of uranium atoms to produce steam.

This fission is caused in a reactor by bombarding the atoms of nuclear fuel contained in the reactor core (1) with free neutrons. Each time that an atomic nucleus fissions - or breaks up into smaller parts -, it emits in turn new neutrons, which keep the chain reaction going.

The rate of this reaction is decreased or increased by lowering or raising the control rods (2) inside the core, which absorb free neutrons. Simultaneously dropping all the rods into the core instantly "extinguishes" the chain reaction.

How is this heat converted into electricity, which is the end purpose of the power plant?

In the primary system (in red), the water is heated inside the reactor vessel (3) in contact with the hot fuel assemblies that constitute the reactor core (1), then passes through thousands tubes of steam generator (5), and, driven by the reactor coolant pumps (6), returns to the reactor vessel.

A pressurizer (4) keeps the primary system under high pressure, 155 bar, to force the water to remain in the liquid phase and therefore ensure the most efficient heat transfer.

Hence the name PWR for pressurized water reactor.

The water in the secondary system (in blue/green) is heated by the primary system water outside the steam generators tubes and then is transformed into steam at a pressure of 72 bar.

A set of pipes carries this steam outside the reactor containment (7) to the turbine (8), which is therefore set in motion, transforming part of the pressurized steam's heat energy to mechanical energy.

At the turbine outlet, the steam is "sucked in" by the condenser (10), where it finishes cooling down sufficiently to be converted back into liquid water. This water is then carried back to the steam generators by the condensate extraction pumps (11), so the secondary system is a closed loop.

The turbine's mechanical energy drives the generator (9), which generates medium-voltage electrical energy.

The electric current is fed to a transformer (14), where the voltage is raised for long-distance transport over the grid's high-voltage power lines.

The cooling system (13) ensures the cooling of the secondary water in the condenser, with cool water taken from a nearby river or ocean. When the river's flow is not sufficient, cooling towers are added. The turbine's mechanical energy drives the generator (9), which generates medium-voltage electrical energy.