A PWR fuel assembly comprises a bottom nozzle into which rods are fixed through the lattice, and it is ended by a top nozzle to finish the whole assembly. There are spacing grids between these nozzles, which ensure the fuel rods’ exact guiding. The bottom and top nozzles are usually made of stainless steel and 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. The top nozzle may be designed as removable to enable fuel rod repair and equipped with deflectors to ease the core loading.
The bottom nozzle provides 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 constitutes the base element of the nuclear reactor core. The reactor core (PWR type) contains about 157 fuel assemblies (depending on the 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 uniform, and the enrichment is differentiated in the radial and axial directions. This arrangement improves power distribution and improves fuel economy.
Russian VVER-type reactors use a fuel 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 meters high, is 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