If nuclear energy is sometimes scary, it is because we do not know how it is produced. Here are some details on how nuclear power plants work and what happens to the waste produced.
Nuclear Power Plant: How Is Electricity Produced?
The production of electricity in the nuclear power station is based on fission reactions, which are self-sustaining and produce a permanent release of energy: the neutrons released during fission must, in turn, be able to strike other fissile nuclei, according to the chain reaction process.
Two processes are used to increase the chances of encounters between fissile nuclei and neutrons:
the enrichment of uranium, which we have already mentioned on the previous page;
the slowing down of neutrons by a substance that has the property of reducing their speed without absorbing them: the moderator
Slowing Down Of Neutrons By A Moderator
Therefore, the moderator’s role is to slow down the neutrons, which would otherwise be too energetic to cause new fission effectively. Because of their high energy, the neutrons produced by fission move at high speed (20,000 km/s).
They then pass too quickly near the uranium atoms, and fission reactions are difficult to obtain. Hence, the need to slow them down considerably (up to a speed of the order of 2 km/s).
Neutrons are slowed down when they pass through matter that is made up of atoms whose nuclei do not absorb them. Indeed, like balls moving on a billiard tables where other balls are located, the neutrons lose speed by bouncing off the nuclei.
This is slowing down occurs rapidly when the obstacles are light nuclei with a mass close to neutrons, such as those of hydrogen. The usual moderators are graphite, ordinary water, and heavy water.
Heavy water, made up of water molecules whose hydrogen atom is replaced by a deuterium atom, heavier than hydrogen, has the best qualities sought in a moderator: it slows down neutrons well and absorbs them little. Unfortunately, its manufacturing is expensive.
The fission of uranium in the core of the nuclear reactor provides energy in the form of heat, generating water vapor. This drives a turbine and thus converts thermal energy into mechanical energy. The turbine, in turn, drives an alternator, which converts mechanical energy into electricity.
Anatomy Of A Nuclear Power Plant
All these operations are allowed by the three water circuits of the nuclear power plant. The primary circuit in which a coolant circulates recovers the heat released by the fission of uranium.
The heat transfers fluid is a liquid (water or heavy water) or a gas (carbon dioxide or helium) which circulates at high speed in contact with the fuel elements. It must be able to conduct heat well and not absorb too many neutrons. The secondary circuit, independent of the first, receives this heat and produces water vapor to turn the turbine.
The cooling circuit, meanwhile, circulates cold water to condense the steam from the secondary circuit again. These three circuits operate thermal exchanges between them while remaining independent, which prevents any dispersion of the radioactive substance outside the plant.
The reactor core consists of a steel vessel containing the fuel and water for the primary circuit. Control rods (boron or cadmium), introduced through the vessel cover into the reactor core, allow the power of the chain reaction to be adjusted.
They indeed have the property of ‘eating’ neutrons. Thus, the total immersion of the rods in the reactor core makes it possible to stop the chain reaction in two seconds. The whole is enclosed in a sealed concrete enclosure, which ensures containment.
All human activity produces residues. Whether for the production of electricity or in the fields of health, industry, or research, the use of certain elements’ radioactive properties is no exception to this rule.
Radioactive waste (or nuclear waste) is any material that is not expected to be used later and whose level of activity does not allow direct release into the environment.
Nuclear facilities are not the only producers of radioactive waste. This is also the case for hospitals, certain industries, research centers, and universities.
In nuclear power plants, a third of the spent fuel is withdrawn and renewed during the unit shutdown. As it is very radioactive, the transfer operation is carried out underwater: directly from the vessel to a cooling pool, near the reactor.
The water makes it possible on the one hand to trap the radiation and, on the other hand, to cool the pellets. They stay there for at least a year. Their activity naturally decreases: it is twelve times lower after one month of the stay and thirty times lower five months later.
When the fuel has cooled and has lost a good part of its radioactivity, it is inserted into a concrete or steel container. This operation is also carried out underwater. The containers are then sent to the reprocessing plant in La Hague (Manche).
There are three categories of waste, depending on the half-life, the level of activity, and the type of radiation emitted. It should be noted that 90% of them have a short life and a low to medium activity (around 100 Bq per gram of material).
The remaining 10% is split between long-lived, high-level waste, and very low-level waste. All the radioactive waste produced each year represents relatively small quantities in terms of volume: 1 kilo per inhabitant in France, against 2,500 kilos for industrial and domestic waste.