How a nuclear reactor works ?

The chain reaction is produced and maintained in a nuclear reactor, commonly called a nuclear power station.
The principle: the heat given off by fission heats water, which is transformed into steam. The steam turns a turbine linked to an alternator, producing electricity.

It is the principle of any thermal power station: In a fossil fuel power station, water is heated by burning coal, oil or gas. The water is transformed into steam that drives a turbine linked to an alternator. This produces the electricity.

In a nuclear power station, there is no chemical combustion, but we speak nevertheless of nuclear combustion.

The uranium must first undergo several preparatory processes:
- It is enriched, that is to say the proportion of the fissile element U235 is increased. This proportion, which is only 0.7% in natural uranium, must rise to between 3 and 5%;
- It undergoes several chemical treatments;
- It is “packaged” in the form of pellets, inserted in long metal tubes called fuel tubes;
- These tubes are grouped into “bundles”, called fuel assemblies, which are placed in the tank of a nuclear reactor, filled with water: this is what is called the reactor core.

The reactor consists of three separate circuits: the primary circuit, the secondary circuit and the cooling circuit.
Under bombardment by neutrons, the fissile atoms of uranium 235 split, liberating heat that warms water in a closed circuit, the primary circuit. The water is maintained in its liquid state under pressure (155 bars at 300°C).

Heat is recovered by thermal exchange in a steam generator, the heat of the water in the primary circuit being transmitted through a multitude of tubes to water in the secondary circuit. It is this latter water that is transformed into steam that will drive the turbine.

The third circuit is the cooling circuit, which condenses the steam and recycles the resulting cold water into the steam generator.
The coolant in the third circuit is pumped into the sea or into a river. If the rate of flow is insufficient, cooling towers are used which discharge into the atmosphere those clouds of steam so easily identified in the landscape! At the exit from the alternator, a transformer increases the voltage of the electricity, so that it can be dispatched into the distribution network at 400 000 volts.
The nuclear reaction is controlled by means of control rods, which have the property of being able to absorb neutrons. The chain reaction can therefore be slowed down or stopped completely by lowering these rods into the reactor core.
In the case of an incident, the rods are lowered automatically, leading to the halting of the nuclear reaction.