Nuclear reactor technologies

A sustained and controlled chain reaction

Uranium 235, used as fuel in nuclear reactors, is said to be fissile. This means that its nucleus breaks into two smaller nuclei when struck by a neutron. This fission emits energy in the form of heat and releases two or three neutrons which in turn will go on to break other nuclei, and so the process continues... This chain reaction, generated artificially in nuclear power plants, is sustained and controlled using appropriate devices.

The fuel is conditioned in the form of small pellets stacked in the airtight metallic ducts that join together to form assemblies. Placed in a steel vessel filled with water - the nuclear steam supply system - these form the reactor core.

The heat emitted is transformed directly into steam in the vessel (boiling water reactors) or indirectly by means of a steam generator (pressurized water reactors) and conveyed to a turbine generator which produces electricity.

In order to vary the power output of the reactor, the intensity of the chain reaction is altered using control rods made of materials that are able to absorb neutrons. By pushing these rods deeper or shallower into the reactor core, its output can be adjusted, the reactor can be kept in operation or it can be stopped. In the event of an abnormal situation, the control rods automatically drop into the core instantaneously stopping the fission reactions of the reactor.

Heavy water and fast breeder reactors technologies

Heavy water reactors

This reactor technology, which uses heavy water as a moderator, has been predominantly developed in Canada with CANDU reactor types. Heavy water, D₂O, is a combination of oxygen and deuterium (heavy hydrogen atom) as opposed to normal water, H₂0, which is often called "light water".

The benefit of heavy water is that it absorbs fewer neutrons than normal water, enabling natural uranium to be used as fuel therefore not requiring the uranium enrichment process. With the other physical properties of heavy water being similar to those of normal water, it can also be used as coolant.

Fast breeder reactors

In these reactors, the neutrons do not need to be slowed down. The coolant fluid is either a liquid metal (often sodium) or an inert gas (helium). Their fuel contains plutonium, an artificial element that, like uranium 235, has fissile properties.

The fast breeder reactor technology has in essence undergone industrial experiments. It forms the development base for the next generation of reactors, on the horizon for 2040.

Light water reactor technologies

More than three quarters of the fleet of nuclear power plants in operation across the globe use one of the two types of light water reactors.

Boiling water reactors

In the BWR, the water is boiled and turns into steam inside the vessel itself. Recirculation pumps force the water that has not vaporized to return to the core, accelerating the movement of natural circulation. The steam that is produced is transported directly to the turbine generator via steam pipes. Reactor containment prevents radioactive products from dispersing in the event of damage to the core.

• AREVA has been active in the development of Boiling Water Reactor (BWR) technology since the early 1990s through its German subsidiary, AREVA GmbH.
• AREVA offers a new 1250 MWe boiling water reactor, the KERENA. It has been developed at the request of German electric utilities, in cooperation with other European countries. This Generation III+ reactor will provide the highest level of operational safety.

Pressurized water reactors

In a PWR, a pressurizer maintains the water at a pressure high enough to prevent if from boiling and keep it in liquid form. This water, referred to as primary coolant, is not sent directly to the turbine. It circulates in a closed circle between the core and the steam generator (SG). This enables the heat from the &uot;primary coolant&uot; to be transferred to the feedwater which boils as its pressure is much lower. It is the steam produced by the feedwater that is transported to the turbine generator.

• PWRs are the most commonly used across the globe (66% of the current fleet in installed capacity) and equip the entire French nuclear power fleet. This is the biggest in the world in terms of national electrical production (around 80%). Adopting a single reactor technology gives it a very homogeneous character. It benefits from continuous improvements supported by the exceptional feedback of experience acquired by AREVA from the 58 reactors in operation.
• AREVA therefore has 40 years of experience in the development of this reactor technology and related services.
• The EPR™ reactor, developed by AREVA, is a 1650 MWe pressurized water reactor. This Generation III+ reactor benefits from technological advances which make it an advanced reactor. It provides increased levels of competitiveness and safety, while reducing the impact on the environment.