Fundamental Study on Mitigation of Rocking Motion of a Cask System by a Gyro System

In Japan, ensuring the structural integrity of cask systems during seismic events is becoming increasingly important. Cask systems, which are free-standing cylindrical structures that contain spent fuel assemblies, are believed to exhibit rocking motions under strong seismic excitations. Thus far, analytical studies conducted by the authors have indicated that cask systems subjected to strong seismic motions, undergo large rocking motions, and, in the worst case, may overturn and collapse. Therefore, reducing the rocking motions of casks to avoid overturning and consequent contamination of radioactive substances is critical. To suppress rocking motions for heavy free-standing structures such as cask systems, we propose a rocking motion suppression system that employs a gyro system. This system is installed in the free-standing structure. A previous analytical study showed that this system largely mitigates rocking motion. In the present study, we fabricated a fundamental cask system model and a gyro system. By using the cask system model without a gyro system, free vibration tests and shaking table tests were conducted to understand the basic characteristics of rocking motion and responses under base excitations. Analyses were also conducted to confirm the validity of the analytical model for rocking motion comparing with the experimental data. Moreover, analyses for the cask system with the gyro system were conducted. From these results, we evaluated the ability of the gyro system to mitigate rocking motion.