Effect of Weld Properties on the Crush Strength of the PWR Spacer Grid

Mechanical properties in a weld zone are different from those in the base material because of different microstructures. A spacer grid in PWR fuel is a structural component with an interconnected and welded array of slotted grid straps. Previous research on the strength analyses of the spacer grid was performed using base material properties owing to a lack of mechanical properties in the weld zone. In this study, based on the mechanical properties in the weld zone of the spacer grid recently obtained by an instrumented indentation technique, the strength analyses considering the mechanical properties in the weld zone were performed, and the analysis results were compared with the previous research. 1. Introduction A PWR fuel assembly consists of spacer grids, fuel rods, a top nozzle, a bottom nozzle, guide tubes, and an instrumentation tube as shown in Figure 1. Among them, the spacer grid is a structural component which is an interconnected array of slotted grid straps and is welded at intersections to form an egg-crate structure. From a structural point of view, the spacer grid is required to have enough crush strength under lateral loads so that nuclear fuel rods are maintained in a coolable geometry, allowing control rods to be inserted [1]. The capacity of a spacer grid to resist lateral loads is usually characterized in terms of its crush strength, and it was reported [2] that the lateral crush strength of the spacer grid is closely related with the welding quality of the spacer grid. Figure 1: PWR fuel assembly and spacer grid. Welding is a very convenient and widely used method to join simple metallic parts with a complicated structure by the use of adhesive and cohesive attractive forces between metals [3]. Microstructures in the weld zone, including a weld (or fusion zone) and a heat affected zone (HAZ), are different from that in a base material, as shown in Figure 2 [3]. Consequently, the mechanical properties in the weld zone are different from those in the base material to some extent. When a welded structure is loaded, the mechanical behavior of the welded structure might be different from the case of a structure with homogeneous mechanical properties. Nonetheless, mechanical properties in the welded structure have been neglected in many structural analyses [4–7] of spacer grids due to a lack of mechanical properties in the weld zone. Usually, the general way to obtain the mechanical properties in the weld zone is by taking tensile test specimens in the fusion zone and HAZ, and by performing a standard tensile test. However, when the