A long cladding tube sealed on the ends containing stacked nuclear fuel pellets. Each type of reactor has its own type. Fuel rods are usually coupled together to form a fuel assembly. The fuel rod simplifies the handling of nuclear fuel and also provides the first barrier against radionuclide leakage. The gap between the pellets and the cladding is usually filled with helium to improve thermal conductivity, which also provides space for the swelling of the pellets due to the fission products produced.