Formulation and application of a three-dimensional structural model for upper internal structures

Abstract

A finite element structural model is developed for the upper internals which is capable of treating material nonlinearities and geometric nonlinearities arising from large displacements when subjected to dynamic loads. This structural model can be used either by itself or in an interactive mode with a hydrodynamics code. The upper internal structures which are located above the core and below the head cover may play a significant role in the hydrodynamics of an energy excursion by mitigating its response. The upper internal structure (UIS) is essentially a massive, perforated rigid body which is connected to the reactor head by support columns with a cylindrical cross section. The major response of interest is the buckling of these support columns which results from the compressive forces they sustain when the upper internals are loaded vertically upward by dynamic loads. In addition to buckling, these columns sustain plastic deformations and changes in cross section making geometric and material nonlinearities accountable. For the case of substantial change in the shape of the column cross section, it will be more convenient to treat the behaviour of the elements stiffness in terms of moment-curvature relations which account for the changes in flexural rigidity with change in cross section. Two factors considered of importance in developing the model are: (1) whether an imperfection is necessary in the initial mesh of the columns to trigger buckling and to what extent does the magnitude of the imperfection effect the results; (2) whether the buckling pattern is symmetric or asymmetric. It was established by these studies that elastic buckling is not affected by the magnitude of the imperfection; for plastic buckling, the results are more sensitive to the magnitude. The studies also showed that the UIS mass is sufficiently large so that it cannot be laterally displaced. By checking the deformed shape, it was observed that the column always attempts to reach the symmetric buckling mode. Even if the asymmetric shape is triggered, it is always at a load higher than that required to trigger a symmetric response. The behavior of the model has also been compared with the SRI tests simulating highly energetic CDAs. The model predicts the magnitude of the axial deflection quite well.