A nuclear power plant transforms heat into electricity through the fission of an atomic nucleus. It is composed of one or more reactors that are organized into nuclear units.

According to recent figures, it allows produce a large quantity of energy. Now, what about the one that the nuclear power plant consumes to carry out this transformation? SirEnergies provides you with some clarifications on this consumption.

An electrical consumption of 20 TWh/year in the French nuclear fleet

It exists several nuclear power plant technologies. Here we will focus on BWR and REP technologies.
There are power plants with a pressurized water reactor and those with a boiling water reactor. All the nuclear reactors used in France use pressurized water. They have three power levels:

• 900 MW
• 1,300 MW
• 1,450 MW

The 900 MW one produces on average 500,000 MWh of nuclear electricity per month. The higher the power of the reactor, the more this quantity of nuclear energy increases. To have such an efficiency, it first consumes electricity.

In France, all nuclear power plants consume approximately 20 TWh/year for auxiliaries (pumps, preheating, ventilation, uranium enrichment, etc.). Either 5% of French production.

Uranium-235

Inside a nuclear reactor there are fuel elements that are several meters long. These in turn are composed of fuel rod bundles in which nuclear fuel is found, uranium-235. It is in the form of pellets and a few are enough to activate the fission process that will produce heat. To create this phenomenon, the pellets are assembled and placed in a tank filled with water.

The temperature of the latter rises as soon as it comes into contact with the pellets (the water is first heated thanks to the friction of the water in the primary circuit and then fission takes over to raise the temperature of the water). However, the reactors keep the water under pressure and prevent it from boiling.

It circulates in a closed circuit which is called a primary circuit. It then goes through a steam generator and Transmits its heat to the water present in a secondary circuit. This water in turn heats up and becomes steam. It will activate the turbines that will drive the alternator that will produce electricity.

Each of these machines deploys great power, they require a lot of energy to function properly.

Enrichment of fuels

After its extraction, the uranium is precipitated, filtered, washed and dried. It is then rid of its impurities and is transformed into uranium hexafluoride (UF6). In this refined but natural metal, there are 993 g of uranium-238 compared to only 7 g of uranium-235.

Although it is the only one that is fissile, uranium-235 cannot be used in reactors with such an isotope. Its proportion must be between 30 and 50 g for it to be able to undergo nuclear fission, which will allow the production of electricity.

It is for this reason that it must be enriched before transforming it into pastilles. This step requires a lot of energy, nearly 19,000,000 MWh per year. This is explained by the multitude of machines that are involved during the operation. The methods that have been developed for enrichment are:

• gaseous diffusion
• The ultracentrifuge

When it reaches 56°C, uranium hexafluoride goes into a gaseous state. As soon as it is in this form, it is diffused through barriers that are thin partitions pierced with small holes. Being lighter than uranium-238 molecules, those of uranium-235 cross barriers more quickly.

However, after a single pass, the quantity of uranium-238 remains present in considerable quantity. To reach the right proportion of uranium-235, the process is therefore repeated at least 1,400 times.

Ultracentrifugation has gradually replaced gaseous diffusion and is used in almost all power plants in the world. With this method, enrichment takes place in a set of centrifuges.

They are each equipped with a rotor that makes between 50,000 and 70,000 rpm. This speed makes it possible to separate, in the gas, the two uranium molecules. Those that are heavy are projected onto the wall and those that are light (isotope 235) remain in the center.

The gas enriched in uranium-235 rises and is recovered then sent to another juicer. The cycle will be repeated several times to obtain the desired proportion of uranium-235. Ultracentrifugation also consumes a lot of electricity, but this is less than that spent by the first process.

The pump station

The pump station provides the nuclear power plant with the water it needs to function properly. It comes from a river, a river or a sea near the power plant. If the source has a high flow rate, the water pumped is several tens of m³ per second.

If not, only a few m³ are sampled per second. This cold water circulates through thousands of tubes that are in a condensing device. When the steam that drives the turbines to produce heat comes out, it comes into contact with the tubes. The water inside cools and condenses it until it becomes liquid again.

A pump then brings it back to the steam generator for a new cycle while the condenser water, which is now hot, is rejected. However, when river or sea flow is low, it is sent to cooling towers. The current of air that circulates in the tower cools it. Much of this water is returned to the tubes and the rest evaporates into the atmosphere.

This device makes it possible toavoid thermal pollution and to limit the heating of the water source. There is also electricity used in control rooms and for exports.

To go further, do not hesitate to read our in-depth article on” Nuclear energy: between innovation and energy transition ”.

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