Nonlinear empirical model to simulate displacement-dependent structural behavior of a PWR fuel assembly

A nonlinear model of a pressurized water reactor (PWR) fuel assembly is developed, and its performance is evaluated. The nuclear fuel assembly experiences numerous friction forces developed in the fuel rods and supporting components. Therefore, the fuel assembly deflection generates slips between the structures and develops a nonlinear behavior. The loose joints make it impossible for the rods to share a common center of curvature when the fuel assembly is deflected. In addition, when the fuel assembly is under a high temperature coolant water condition, the slip is activated because of the associated dimensional change. Thus, local and global displacements are introduced to compensate for the slip, and the relationship between the two is explained. Considering the fuel becomes soft when it is deflected, a nonlinear beam model with the varying second moment of area is proposed. The Galerkin discretization method is utilized to solve the nonlinear equation of motion, and an incremental solution is determined based on the linearized equation. Verification examples are provided, and the results are compared to the test data. It is shown that the concept of the model is reasonable and applicable. The developed model could be helpful to understand the nonlinear behavior of a fuel assembly and to predict the responses in case of an accident.