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Davis Besse Alloy 600 Mitigation Shielded Work Platform

Project Scope:

  • Design and build two shielded work platforms to protect workers during an optimized weld overlay repair to be performed on two core flood nozzles simultaneously.
  • Access to the nozzles required a 3’ x 3’ cut-out to be made in the five foot thick bioshield adjacent to the core flood pipe.
  • Platforms were designed to be installed through this cut-out and cantilevered into the 28” inch wide annulus space between the reactor and the bioshield.


  • Numerous interferences, including nuclear instrumentation tubing, check valves, and primary piping supports had to be accommodated in the design.  The cut-out size limitation, as well as a tortuous access path to the cut-outs, required that the shield package be hand carried and manually installed.  These 4 ton structures were installed in eight hours, and removed in less than six hours.  The shielded platforms provide a tenth value reduction in radiation levels.
  • Structural design, including seismic, performed using FEA analysis.
  • Generated 3D virtual models of plant systems and structures to prove concept.
  • Collaborated with welding contractor to ensure clearance for welding tracks and machine, and to design extremity shields and FME barriers.
  • Provide 4” lead shield on platform deck, and 1” on walls in three layers of wall sections.
  • Components sized for man-handling and movement through plant to installation location.
  • Built full scale mockup to perfect installation methods and sequence.  Mock up later used for extensive installation and removal crew training.
  • The project was performed in parallel with the outage critical path without impact.