In a nuclear power plant, heat is generated in the reactor by fissioning  uranium nuclei. The heat produced is absorbed by a "primary circuit" containing water which flows through the reactor. A pressurizer is used to keep this water under high pressure, thus preventing it from boiling. After the hot water of the primary circuit has transferred its energy in the steam generator, a primary pump  pumps it back to the reactor.

The fuel assemblies are part of the reactor core. They are installed in the cylindrical reactor vessel with a hemispherical bottom head and a removable upper head fixed with studs and nuts; the vessel contains the reactor internals designed to support the reactor core, and to provide a passageway for the reactor coolant from the bottom to the top. The reactor vessel is made of manganese molybdenum steel forgings selected for its weldability and its mechanical and radiation resistant properties. Internal surfaces are cladded with stainless steel deposited by welding.

Each reactor core consists of 157 fuel assemblies with fuel rods. These are composed of sintered uranium dioxide fuel pellets contained in a Zircaloy 4 tube, and each end of the tube is sealed by a welded end plug. This tight metal tube allows the heat to be dissipated whilst retaining the fission products. Each assembly contains the fuel rods in a square array bound together by a skeletal frame work composed of top and bottom nozzles, grids and control rod guide thimbles; the latter serve as guide for the rod cluster control assemblies. An in-core instrumentation guide thimble is located in the centre of the assembly.

The heavy metals uranium and plutonium can be used as fissile fuel

Uranium exists naturally in the form of three "isotopes" : an extremely small fraction of U-234, a small amount of U-235 (0.7%) and U-238 (nearly 99%). Whilst the nucleus of U-235 can be split! This is not the case for U-238. In order to obtain enough fissile U-235 in the nuclear fuel to sustain a controlled chain reactor, the natural uranium from the mine is enriched or concentrated to 3-4% U-235; which is the ideal concentration for a self-sustaining controlled chain reaction in a PWR-reactor.

Plutonium does not exist naturally. This heavy metal is produced in the nuclear reactor itself when U-238 becomes Pu-239 upon absorption of a neutron. 

Spent fuel is taken out of the reactor after four years and can be recycled in a reprocessing plant. The plutonium can be mixed with uranium to manufacture new fuel assemblies : MOX, "mixed oxide" or a mixture of 93% uranium and 7% plutonium oxide (by weight). This type of recycling saves energy and limits the total quantity of nuclear waste.


Control rad drive mechanism




Upper head


O-ring seals


Upper support plate


Fuel assemblies


Core barrel


Control rod guide thimble


Guide thimble support plate


Rod cluster control assemblies


Reactor vessel


Core support plate


In-core instrumentation guide thimble



The pressurizer is a cylindrical reservoir connected to the primary circuit and placed at a high level in the reactor building. Its function is to prevent the water in the primary circuit from boiling or evaporating at a temperature of 320°C. To do this, the water must be kept under extremely high pressure: the higher the pressure, the higher the boiling point. Electrical resistance heats the water in the pressurizer to a higher temperature than the operating temperature of the primary circuit. This creates a "steam cushion" at the top of the pressurizer which maintains a constant pressure of approximately 155 bar. The pressure can be decreased by spraying colder water into the pressurizer and thus gradually getting the steam condensed.

Reloading fissile fuel into the reactor

The reactor is controlled by raising or lowering in the core rod cluster control assemblies consisting of neutron absorber rods mainly composed of stainless steel tubes encapsulating silver-indium-cadmium absorber material and sliding into the tubular guide thimble of the fuel assembly. The rod cluster control assemblies are positioned by a separate electro-magnetic drive mechanism hermetically sealed to avoid reactor coolant leakage.

The action of the rod cluster control assemblies allows rapid reactivity variations required for the operation of the nuclear steam supply system, especially in the event of load transients or during reactor start-up; it also allows for immediate reactor shutdown in emergency cases.

Control of the reactor also requires compensation for slow reactivity variations due to the temperature effects, to poisoning by fission products or to fuel depletion. This compensation is obtained by adding or subtracting boric acid, a neutron absorber which is dissolved in the reactor coolant.

Finally, the in-core instrumentation allows to determine the distribution of the axial and radial neutron flux in the various assemblies, and the ex-core instrumentation, permanently monitoring the global neutron flux, supply the operator with all the information he needs for the efficient control of the reactor.