The reactor core is the source of energy. Similar to the hearth of a boiler, it produces heat. It consists of fuel elements containing pellets of uranium enriched to 3.5% or including also plutonium (MOX). A metallic rod of “Zircaloy surrounds these elements. This first barrier traps the radioactive products that that are formed within the pellets.
The core is contained within the reactor pressure vessel, a sealed metal vessel, coated in the inside with stainless steel. It includes control devices, mainly neutrons absorbing cadmium rods, as well as safety devices. This vessel is a second containment barrier.
Water circulates inside the high pressure vessel between the fuel elements. It removes the heat generated in the fuel elements and also plays the role of moderator. The pressure of the primary water reaches 155 atmospheres.
The temperature of the primary water leaving the reactor core is about 300 °C. This water then passes through a heat exchanger, called “steam generator”, where it cools by vaporizing the water of a secondary circuit. In the steam generator, secondary water cools the tubes where the primary water circulates before returning to the reactor vessel. At the exit of the steam generator, the secondary water steam pressure is 70 atmospheres.
The secondary circuit is a closed circuit. The steam is sent through a turbine. The turbine drives an alternator that is coupled. The alternator produces electricity to be sent to the national grid at high voltage. The secondary steam is condensed at the outlet of the turbine, before being recycled into the steam generators.
The reactor core, the primary circuit and steam generators are housed in a containment building sealed by a single or double concrete wall. This building is about 50 m in diameter and 60 m in height. It is depressurized to prevent leaks to the outside. This containment provides a third protective barrier.
The steam exiting the turbine outlet is condensed by a high flow of water circulating at high rate in a third cooling circuit. The condensed steam then returns to the generators. "Tertiary" water, which itself needs to be cooled, is sent to large cooling towers (one per reactor). These towers are the most visible part of a nuclear plant. They give off plumes of water vapour.
To compensate the water lost by evaporation in towers, external water is needed. Nuclear power plants are located near the sea or rivers, whose water is not in contact with radioactive materials because of the various containment barriers.