Seismic model of the gas cooled fast breeder reactor code support structure

The modeling technique for the seismic analysis of the core support structure of a gas cooled fast breeder reactor is developed. The core support structure consists of the support cylinder and a perforated grid plate from which 265 fuel and blanket elements are suspended as cantilevered beams. The analysis of the core support structure consists of three steps: analysis of the grid plate, analysis of the core elements and modal synthesis. The dynamic theory of the grid plate is developed by generalizing Reissner-Mindlin thick plate theory with orthotropic constants and then modify the formulations of the rotary inertia expressions to include the rotary inertia effects of the core elements. In this general case the differential equation for this problem can be solved by numerical techniques. However, applying the effective elastic constants of the perforated plate in the customary fashion the solution can be determined analytically in terms of Bessel functions. By applying the present technique, it is shown that the grid plate fundamental frequency is in the range of the fundamental frequencies of the core elements so that a dynamic coupling effect exists. Due to this dynamic coupling effect the grid plate cannot be considered a 'rigid plate' for the purpose of seismic analysis of the gas cooled fast breeder reactor. Hence, a simple spring-mass seismic model is inadequate and a more elaborate finite element seismic model applying, plate and shell elements is used instead. The technique of modal synthesis is then applied to determine the effects of dynamic coupling and to study the seismically induced motions of the core elements in a realistic manner