Determination of the Optimum Welding Parameters for a Laser Welded Spacer Grid Assembly for PWRs

Abstract

A spacer grid assembly, which is an interconnected array of slotted grid straps and welded at the intersections to form an egg crate structure, is one of the core structural components of the nuclear fuel assemblies of a Pressurized light Water Reactor (PWR). The spacer grid assembly is structur-ally required to have enough buckling strength under lateral loads due to lateral seismic accelera-tions, lateral Loss Of Coolant Accident (LOCA) blowdown forces, and shipping and handling loads so that the nuclear fuel rods are maintained in a coolable geometry, and that control rods are able to be inserted. The ability of a spacer grid assembly to resist lateral loads is usually characterized in terms of its dynamic and static crush strengths, which are acquired from tests. The crush strengths and spacer grid dynamic stiffness of a spacer grid assembly are required for the fuel assembly seis-mic and LOCA blowdown analyses to verify that a coolable grid geometry is maintained. Since the crush strengths of the spacer grid assembly are known to depend on the weld qualities at the inter-sections of the slotted grid straps, high-tech welding methods, such as the TIG welding, LASER beam welding or Electron beam welding method, have been used recently in the nuclear fuel manu-facturing fields.