Abstract
A spent nuclear fuel assembly is put in a spent fuel rack and stored in a spent fuel pool filled with water to be cooled down. A free-standing spent fuel rack (FS rack) is a kind of spent fuel rack, which is not fixed to a floor or a wall of the pool. Since it moves freely in water, seismic inertial force is offset by fluid force and friction force; thus earthquake-resistance of spent fuel racks is expected to improve. However, collision between a rack and another rack or a wall of the pool leads a severe accident, so that the motion of FS racks under earthquake excitations has to be investigated in designing FS racks. In this paper, a 4-DOF dynamic model of FS rack is constructed. It consists of two sub-models: a translational model which computes translational movement (X and Y) and planar rotation (θ) regarding water around the racks as a clearance-flow network; and a rocking model that computes rocking angle (φ) considering deviation of the center of gravity and the center of buoyancy, fluid force around the racks and direction of friction force caused by translational motion. Simulation of the motion of an FS rack under actually measured seismic acceleration with the constructed model revealed that the rocking motion is the main cause of collision between a rack and a wall. Moreover, it is found that rocking motion induces planar rotation. Therefore, the following suggestion is derived as a conclusion of this paper: preventing rocking motion is important for the design of FS racks, and its countermeasure is applying a rack with wider width.
