Effect of bi-directional excitation input on boiling water reactor scrammability

Abstract

When a large earthquake occurs, a boiling water reactor is automatically shutdown through the insertion of control rods among fuel assemblies. A fuel assembly can vibrate in-plane when two seismic waves act on it in directions at right angles to each other in a horizontal plane. Although the state of contact between a control rod and fuel assembly is different from that with one directional input, the effect of the difference on scram time is not clear. In this study, we evaluated control rod insertability under bi-directional excitation inputs using a dynamic insertion model of control rod. Sine waves were used as input waves, and their frequency corresponds to the first natural frequency of a fuel assembly. By changing the amplitude and phase difference of the input wave, we simulated the behaviors of the fuel assembly, such as vibration direction and whirling under bi-directional excitation inputs. As a result, we concluded that a comparatively long scram time was obtained when the fuel assembly vibrated in the direction orthogonal to the surface of the control rod blade. Additionally, as the phase difference between two horizontal inputs increased, there was increase in the scram time because the number of contacts between the control rod and the fuel assembly increased.