Spacing grids ensure an exact guiding of the fuel rods. Spacing grids are welded onto the guide tubes and ensure, by means of springs and dimples, fuel rod support, and spacing. There are usually 8 to 15 equidistant spacing grids in the typical fuel assembly. Spacing grids are welded onto the guide tubes and ensure, by means of springs and dimples, fuel rod support, and spacing. Spacing grids may carry vanes, allowing improved mixing of fluid streams, thus enhancing the assembly’s thermal–hydraulic performance.
Spacing grids are usually made of a corrosion-resistant material with low absorption cross section for thermal neutrons, usually zirconium alloy (~ 0.18 × 10–24 cm2). First and last spacing grid may be also made of low cobalt Inconel, which is an austenitic nickel-chromium-based superalloy well suited for service in extreme environments subjected to pressure and heat.
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.