End plugs or end caps close the cladding tube at both ends of the fuel rod and are made of a zirconium alloy. They engage with the end plates of the assembly and are exposed to less severe operation conditions since they are not subjected to mechanical stress by expanding pellets and run at lower temperature than the cladding (no heat flux). End plugs are forged and machined from blanks, which are cut from plate or barstock material. Depending on the rod assembly process, the upper end plug may have a hole that is welded tight after the rod is filled with helium.

9.4.1 Plenum spring

The plenum spring secures the fuel pellet stack against axial movement during transport and handling. It is made of one piece of stainless steel (BWR) or Inconel (PWR) wire. During initial operation, it counteracts the formation of axial gaps in the pellet stack by pressing the pellets downwards. The hold-down force is of the order of 4 to 5 times the weight of the fuel stack.