A PWR fuel assembly comprises a bottom nozzle into which rods are fixed through the lattice and to finish the whole assembly it is ended by a top nozzle. There are spacing grids between these nozzles. These grids ensure an exact guiding of the fuel rods. The bottom and top nozzles are usually made of stainless steel, they are heavily constructed as they provide much of the mechanical support for the fuel assembly structure. The top nozzle also ensures the assembly handling function. Top nozzle may be designed as removable to enable fuel rod repair and equipped with deflectors to ease the core loading.
Bottom nozzle provides the mechanical support for the fuel assembly structure. The bottom nozzle features a debris mitigation device, to catch traveling foreign bodies, which had formed, at one time, the chief cause of cladding failure.
The fuel assembly constitute the base element of the nuclear reactor core. The reactor core (PWR type) contains about 157 fuel assemblies (depending on a reactor type). Western PWRs use a square lattice arrangement and assemblies are characterized by the number of rods they contain, typically, 17×17 in current designs. The enrichment of fuel rods is never uniformed. The enrichment is differentiated in radial direction but also in axial direction. This arrangement improves power distribution and improves fuel economy.
Russian VVER-type reactors use a fuel that is characterized by their hexagonal arrangement, but is otherwise of similar length and structure to other PWR fuel assemblies.
A PWR fuel assemblies stand between four and five metres high,are about 20 cm across and weighs about 800 kg (of which about 500kg is uranium). The assemblies have vacant rod positions for control rods or in-core instrumentation. Control rods, in-core instrumentation, neutron source, or a test segment can be vertically inserted into a vacant tube called the guide thimble tube.
Special Reference: CEA, Nuclear Energy Division. Nuclear Fuels, ISBN 978-2-281-11345-7