The nuclear fuel pellets are generally filled in thin-walled cladding tubes to hinder leaching by coolant in the eactor core and to prevent release of fission products into the coolant circuit. In light-water reactors, for example, zirconium alloy (zirkaloy) cladding is used.

The fuel rods for boiling and pressurized water reactors are constructed similarly. They are filled with helium to improve the heat transfer from the pellets to the cladding tube and to withstand better the pressure in the reactor and contain no fuel at the top end of the fuel rods to improve fission gas retention. The latter can be ensured by holding the fuel in place with the aid of a spiral spring. Both ends of the cladding tube are welded gas tight. The fuel rods for pressurized water reactors are manufactured with a helium pressure of ca. 23 bar and ca. 5 bar for fuel rods for boiling water reactors.

The actual fuel elements in pressurized water reactors consist of individual fuel rods and control rod tubes mounted in a self-supporting construction of spacers fitted with a top and feet. Fuel elements for boiling water reactors, by comparison, have no control rod tubes, the fuel element zirkaloy claddings being used to guide the control rods and the coolant.