The major component of the reactor, the fuel element, is a sealed metal rod with cylindrical fuel pellets filled with fissile material (most often – a sintered uranium dioxide). Fuel elements are arranged to form fuel assemblies (FA). The fuel assembly consists of other elements (spacer grids, top and bottom nozzles) to provide rigidity to the assembly, allowing the coolant to flow freely up through the assemblies and facilitating loading and unloading.
The design and dimensions of fuel elements for reactors of various types differ. For example, the fuel elements for Kudankulam nuclear power plant reactor VVER-1000 are 3.5 m long with a diameter of 9.1 mm. Their cladding is made of a zirconium-niobium alloy, and is filled with fissile material of sintered pellets of uranium dioxide enriched up to 1.6 – 5% and with a uranium mass of about 1.6 kg in one fuel element.
The total loading for a VVER-1000 vessel is 163 fuel assemblies (approximately 80 tonnes of uranium dioxide fuel). The key characteristic of fuel assemblies is their uniformity, i.e. the ability to load them into the core, move around the reactor vessel, unload after fuel depletion and replace with new fuel assemblies without any change in principle layout of the reactor core.