The primary and secondary systems are used to transport the heat released in the core to the turbine, which produces electricity, without any of the water in contact with the core leaving the containment.

The primary system extracts the heat released in the core by circulating pressurised water, known as the primary water, in the cooling loops (3 loops for a 900 MWe reactor, 4 loops for a 1300 MWe or 1450 MWe reactor). Each loop, connected to the reactor vessel containing the core, comprises a circulating, or primary pump, and a steam generator. The primary water, heated to more than 300 °C, is kept at a pressure of 155 bar by the pressuriser, to prevent it boiling. The entire primary system is located inside the containment.

In each steam generator, the primary system water transfers the heat produced by the reactor core to the water in a secondary system, without coming into contact with it.

The steam generators contain thousands of tubes through which the primary water circulates. These tubes are immersed in the water of the secondary system and boil it.

Each secondary system primarily consists of a closed loop through which water runs in liquid form in one part and as steam in another part. The steam produced in the steam generators is partly expanded in a high-pressure turbine and then passes through superheater separators before final expansion in the low-pressure turbines, from which it is then routed to the condenser. The condensed water is sent back to the steam generators by the extraction pumps relayed by feed pumps through low and high pressure reheaters.

• protection of the reactor against external hazards;
• confinement, thereby protecting the public and the environment against radioactive products likely to be dispersed inside the containment in the event of an accident. The containments are therefore designed to withstand the pressure and temperature that could be reached in an accident situation, and offer satisfactory leaktightness in such conditions.

There are two types of PWR containments:
• 900 MWe type containments, which consist of a single pre-stressed concrete wall. This wall provides mechanical resistance to the most severe design accident pressure and structural integrity against an external threat. Leaktightness is assured by a thin metal liner on the inside of the concrete wall;
• the 1300 MWe and 1450 MWe PWR containments, comprising two walls, an inner wall made of pre-stressed concrete and an outer wall made of reinforced concrete. Leaktightness is provided by the inner wall and the ventilation system (EDE) which, in the annular space between the walls, collects any leakage from inside the containment. Resistance to external threats is mainly provided by the outer wall.