Karlstein: fabrication of spacer grids

The Karlstein site specializes in the fabrication of customized spacer grids and end grids for pressurized water reactors (PWR) and boiling water reactors (BWR). These include spacers, upper and lower tier plates and cages for BWR, as well as assembly parts such as fuel rod end plugs, sleeves, screws and other small components.

Main stages of the fabrication process

Water channel end-plug welding:

  • All welding tasks performed with a single welding optic
  • A flexible system due to sliding plate
  • Laser-sharing between end-plug and cage welder
  • In-line vision inspection performed prior to welding (edge detection with automated positioning of welding optic)

Electrical discharge machining:

  • Used in Fuelguard™ manufacturing for pressurized water reactors (PWR) and boiling water reactors (BWR), based on the forceless insertion of holes in filigree structure (curve blades)
  • Integrated in iterative measurement and adjustment stages to optimize the absolute positioning of electrodes
  • Improves manufacturing accuracy and tooling cost
  • New electrode concept developed by R&D, based on designing electrodes with graphite tips instead of full-length electrodes in order to reduce wear-and-tear
  • Reduces possible angular deviation through the application of precision adaptors
  • Minimizes form deviation at electrode vertices

Electron beam welding:

  • Stainless steel welding up to 15 mm depth
  • Thermal rounding of boiling water reactors spacer strips

Heat treatment and brazing:

  • Performed by three furnaces at temperatures up to 1,100°C (2,000°F)
  • Vacuum down to 10-5 mbar; fast cool-down achieved by using the inert gas Argon


  • Surface treatment—long parts (up to four meters in length):
          - Cleaning of U-profiles and water channels
          - Cleaning of stainless steel guide tubes
          - Autoclaving of BWR spacers
  • Small parts:
          - Cleaning of all small components, e.g. spacers (except HTP™/HMP™), tie plates, end plugs
          - Hot-air drying
          - Ferrite contamination testing

Key inspection processes

  • Automated contact-free spacers measurement
  • Automated contact-free cages measurement
  • Tactile spring force measurement

Products manufactured at Karlstein site

Components for nuclear fuel assemblies: array geometry of 10x10 for BWR, and array geometries of 14x14, 15x15, 16x16, 17x17, 18x18 for PWR.

Spacer grids: used in rod bundles to provide mechanical support to the rods and reduce flow-induced vibration in nuclear reactor fuel assemblies.

Lower and upper tie plates: used to mount fuel bundles including an array of fuel rods and water rods.

Water channel: a component of the BWR cage.

Fuel rod end-plugs and other small components for fuel element assembly.