Laser Welding Unit for Intersection Line Welding of Spacer Grid Inner Straps and its Application

Abstract

A spacer grid assembly is one of the most important structural components in a Pressurized Wa-ter Reactor fuel assembly. The spacer grid assembly, which supports nuclear fuel rods laterally and vertically with a friction grip, is an interconnected array of slotted grid straps welded at the intersec-tions to form an egg-crate structure. From a structural point of view, the spacer grid assembly is re-quired to have enough crush strength under lateral loads so that the nuclear fuel rods are maintained in a coolable geometry, and that control rods are able to be inserted. The capacity of a spacer grid assembly to resist lateral loads is usually characterized in terms of its crush strength. Zircaloy is prevailing as the material of a spacer grid because of its low neutron absorption characteristic and its extensive successful in-reactor use. One of the primary considerations is to provide a Zircaloy spacer grid with crush strength sufficient enough to resist design basis loads, without significantly increasing the pressure drop across the reactor core. Based on the fact that the crush strength of a spacer grid assembly is known to be proportional to a weld penetration at the intersections of the slotted grid straps, a laser welding unit which is able to weld along the intersection line of spacer grid inner straps was proposed, and spacer grid specimens were fabricated using the proposed laser welding unit in this study. Also, a crush strength test and analysis of the spacer grid specimens were carried out to confirm the effect of the resultant crush strength enhancement.