The most important function of grid springs in a spacer grid is to position the fuel rods appropriately using a friction force and to prohibit the fuel rods from dropping during a normal reactor operation. While the fuel burn-up increases, spring force of grid springs decreases because the spring stiffness of grid springs is degraded by a high temperature and an irradiation effect in a reactor core. Therefore this phenomenon has to be reflected when the initial spring force of a grid spring is being designed. Usually, characterization tests of grid springs are conducted to verify designed grid springs have suitable spring forces. (Kim, 2007) FE model using a contact definition and rigid beam elements for a welding bead for predicting the spring stiffness without any spring test was established and verified in a previous research. In those days, two assumptions were used and the analysis for an unloading process was not included in spite of its importance for a spring design. In this paper, the fact that the assumptions have no effect on the analysis results is proven and a realistically improved FE analysis method using coupling constraints for a welding bead is suggested for investigating the spring’s characteristics under an in-grid boundary condition.

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