A number of fuel rods are grouped side-by-side to form a fuel assembly.

A number of fuel assemblies are in turn grouped together to form the nuclear reactor core.

A large modern nuclear power reactor core is approximately 12 feet (3.7m) long and 12 feet in diameter. Some smaller PWRs have as few as 121 fuel assemblies while some of the larger BWRs can have up to 800 fuel assemblies.