Fuel Assembly Combined Lateral and Axial Model

Abstract

The objective of this paper is to develop a purely mechanistic fuel assembly structural model that will predict the fuel assembly’s static and dynamic characteristics from the knowledge of the fuel assembly’s geometry and component properties. This model provides a method for analyzing the static and dynamic lateral and axial properties of the fuel assembly. A comparison of various in-air fuel assembly test data such as lateral and axial stiffnesses and lateral natural frequencies is provided to demonstrate the analytical model. The fuel assembly model developed by Shah, Brenneman, etc. (1), achieved very good agreement with assembly lateral impact test data by utilizing a “3-beam” model. In that model, the fuel rod-to-spacer grid interfaces were represented by spring and friction elements. The fuel assembly was restrained at each grid position by means of rotational springs, which were benchmarked to the test frequencies. This newly developed model eliminates the need for using rotational springs at the grid locations. Hence, it fully simulates the fuel assembly lateral and axial behavior based on the fuel assembly geometric properties. The fuel assembly model is a 2-D planar model of beams in both lateral and axial directions. The grids are modeled with plate elements. At each grid location there are springs, preload, and frictional sliders representing the lateral and axial connectivity characteristics to the fuel assembly beam model. As the Zircaloy grid preloads relax from irradiation, they can be easily simulated by removing the preload. Hence, this model can represent the fuel assembly structural properties for all aspects of fuel assembly cycles. This model can be used to analyze the fuel assembly lateral static stiffness, first mode and higher order lateral natural frequencies, mode shapes, axial stiffness, in-grid stiffness, through-grid stiffness, and fuel assembly lateral and axial seismic and LOCA response. The model will also estimate the fuel rod frequencies and mode shapes. This model may eliminate the need for some expensive prototype fuel assembly testing.