Effect of a Dimple Location in a Spacer Grid on the Impact Strength

Abstract

A spacer grid is one of the most important structural components in a PWR fuel assembly. The spacer grid, which supports nuclear fuel rods laterally and vertically with a friction grip, is an interconnected array of slotted grid straps welded at the intersections to form an egg-crate structure. Dimples and springs are stamped into each grid strap to support the fuel rods. Zircaloy is prevailing as the material of the spacer grid because of its low neutron absorption characteristic and its extensive successful in-reactor use. The primary considerations are to provide a Zircaloy spacer grid with crush strength sufficient to resist design basis loads, without significantly increasing pressure drop across the reactor core. Generally, a Zircaloy grid strap’s thickness and average height have been the main design variables in order to meet the above considerations. Recently, it was reported that a dimple location is also a design variable that affects the crush strength of a spacer grid. In this paper, based on this fact, the effect of a dimple location in a 3×3 sub-size support grid on the impact strength has been investigated as a preliminary parameter study for a 16×16 full size support grid. A 3×3 model is chosen to facilitate in a fast computation for an optimization. The optimal location of the dimple has been found and some design guidelines for a support grid have been deduced in this work.