Fuel assembly (PWR)

Fuel assemblies contain the nuclear fuel with which a nuclear power plant is operated. Depending on the reactor type, fuel assemblies differ in their structure.

The fuel assemblies of pressurised water reactors (PWRs) consist of a support frame in which so-called fuel rods are installed. The number of fuel rods varies depending on the type of fuel assembly. As this is a square pattern, it ranges from 13 x 13 to 18 x 18 arrangements.  The fuel rods are tubes made of a zirconium alloy with an outer diameter of around 10 millimetres.  The fuel rods contain uranium fuel in the form of heat-resistant pellets of uranium dioxide. The proportion of fissile uranium-235 can be up to five weight per cent. The fuel rods of so-called mixed-oxide or MOX fuel elements also contain plutonium dioxide as fissile material in addition to uranium dioxide.

 The support structure of a PWR fuel assembly consists of up to 24 guide tubes for the control rods of the reactor, the bottom end and top end pieces and the spacers. The support frame gives the fuel assembly mechanical stability and serves to facilitate handling and fastening in the reactor. A PWR fuel assembly of this type is almost five metres long and weighs around 800 kilograms. The so-called reactor core of a modern PWR contains almost 200 fuel assemblies. The fuel assemblies are usually used in nuclear power plants for four to five years.

In contrast to new fuel assemblies that have not yet been used in the reactor, "irradiated" or spent fuel assemblies are highly radioactive and generate heat. After use, they must first be stored in a decay pool in the nuclear power plant for 'aftercooling'. The cooling time in the decay pool depends on how long the reactor has been in use. It is currently around five years. After this, the fuel assemblies are stored in special casks in temporary storage facilities in Germany until their disposal.