Feature Report: Understanding Nuclear Energy

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Inside a Conventional Nuclear Reactor

The nuclear reactor is the tool for collecting and converting the great quantities of energy produced by the nuclear fission processes. Its function is to control this fission and to convert the energy collected in the form of heat. As in a conventional power plant powered by gas, coal or heating oil, steam drives a turbine generator to produce electricity1.

Nuclear reactors recover the heat from nuclear fission to produce steam, which drives a turbine generator to produce electricity. © AFP PHOTO / JEAN-CHRISTOPHE VERHAEGEN

Nuclear fissionReaction that occurs when an unstable nucleus of a heavy atom splits into lighter nuclei... is the disintegration of heavy nuclei, like those of uraniumGray, very dense radioactive metal that is relatively abundant in the Earth's crust and oceans in the form of UO2..., after a collision with a neutronType of particle, along with the proton, that makes up the nucleus of an atom. Neutrons have no net electric charge.. Other neutrons are released, which in turn trigger new fissions.

As the neutrons are ejected at an extremely high speed, they risk bouncing against the nuclei without breaking them apart. They therefore need to be slowed down. Moreover, in order to avoid a runaway (an out-of-control chain reaction), the proliferation of these neutrons must be avoided by absorbing a part of them, about two out of three.

In the most common types of reactors (PWRs or BWRs - see below), the nuclear fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... is placed in a steel tank filled with water. This water slows the neutrons, thanks to its hydrogenThe simplest and lightest atom, the most abundant element in the universe. atoms, a little like the impact of two billiard balls. Graphite is another speed moderator, but it is gradually being phased out.

In order to absorb the excess neutrons, neutron-absorbing materials (rods made of boronBoron is a metalloid, semi-conducting chemical element (symbol B)...) are placed in the aqueous medium. The pellets of nuclear fuel are loaded into rods made of zirconium tubes (See Close-up: "The Nuclear Fuel Cycle"). They are grouped in a square base assembly with empty tubes positioned between them. The boron rods are slipped into these tubes. By raising or lowering the tubes slightly, the absorption of the neutrons can be reduced or increased. These regulating rods are controlled from the reactor's control room. The positioning of the tubes must be adjusted carefully because a reactor's production cannot proceed in fits and starts. In the event of an incident, the regulating rods will fall automatically and shut down the reactor. However, the heart of the reactor will remain very hot for months and must continue to be cooled. It was the lack of cooling that led to the Fukushima accident.

Water circuits

The tank's water, which has been heated to a high temperature by the nuclear reactions, circulates in a closed circuit known as the primary circuit. This hot water comes into contact, in a steam generatorA component that converts water into steam..., with the pipes of another closed circuit known as the secondary circuit. Inside the pipes, the water turns to steam that will drive the turbine.

In order for the system to operate continuously, the water in the secondary circuit must be cooled before it returns to the steam generator. For this purpose, a third circuit is put into place. Cool water is taken from a nearby river or the ocean and circulated in a condenser whose multiple pipes make it possible to return the steam in the secondary circuit to a liquid state. The condenser's water, which has not been in contact with radioactivity, is then released into the environment.

 

The white clouds that rise above nuclear power plants are made up of water vapor.

If there is an insufficient flow from the water sources located around the powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... plant, cooling towers evacuate the heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... towards the exterior. The hot water coming from the condenser is then cooled by the air current that rises in the tower, like the suction effect of a giant chimney, before returning to the condenser (this is called closed cycle cooling). Thus, the white clouds that rise above power plants are made up of water vapor.

The six types of nuclear reactors

Reactors are described according to their fuel (natural uranium enriched to varying degrees, MOXNuclear fuel manufactured from spent reactor fuel. It consists 93% of depleted uranium and 7% plutonium... or thorium), the moderator that controls the chain reaction The nuclei of uranium-235 atoms are bombarded by neutrons in order to induce a nuclear fission reaction...(water or graphite), and the heat transfer medium that carries the heat produced towards the turbine (water, gas, sodium or molten salt).

  • Most of the reactors either in operation or planned in the world are Pressurized Water Reactors (PWRs). They are fueled with enriched uranium and/or MOX. Water is both the moderator and heat transfer medium (or coolant)2.
  • One quarter of the reactors in use in the world are Boiling Water Reactors (BWRs). They are fueled with enriched uranium, using water as both the heat transfer medium and moderator. In contrast to PWRs, there is no secondary water circuit.

Three other types are much less widespread:

  • Gas-Cooled Reactors (GCRs) using enriched uranium as the fuel and graphite as the moderator.
  • Light Water Graphite-Moderated Reactors (RBMK, for Reaktor Bolshoy Moshchnosti Kanalnyy), which are fueled with low-enriched uranium and use ordinary water as the heat transfer medium.
  • Heavy Water Reactors (HWRs) using a hydrogen isotope.

Lastly, Generation IV Fast Neutron Reactors (FNRs), which would use all of the nuclear fuel, are under study but do not yet produce electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... (See Close-up: "Reactors of the Future").

 

Sources

(1) EDF

(2) Mines-ParisTech