A Three-Dimensional Rigid-Body Model for Seismic Analysis of the Pebble-Bed HTR Graphite Core Structure

Graphite plays an important role in the pebble-bed high temperature gas-cooled reactors (HTR) as moderator, reflector as well as internal structural material. The HTR core consists of a large number of graphite bricks interconnected with keys. It is required that the structural integrity of the HTR core be maintained when subjected to the seismic load. Hence it is important from the viewpoint of seismic design to investigate the seismic responses of the graphite bricks. Considering the pebble-bed HTR has various graphite shapes, a generalized three-dimensional model with the associated computer code is developed to treat these interconnected graphite bricks with arbitrary shapes. In this model, each brick is treated as a rigid body with six degrees-of-freedom: three translational displacements and three rotations around the brick center of gravity. A nonlinear spring dashpot model is applied to present the collision between adjacent bricks and the interaction forces through the key systems. In the numerical tests, the code is verified by comparing predicted responses with exact solutions for two cases and good agreement is observed. The model is then used for the dynamic analysis of the side reflectors of the pebble-bed HTR core under a given seismic load. The calculated response behaviour of the side reflector column is summarized and discussed.