The qualitative analysis of vertical seismic acceleration effect on a single nuclear-coupled boiling channel natural circulation loop

Based on the external force method, the present study integrates the nonlinear dynamic model developed previously by the authors with vertical seismic accelerations to investigate the seismic-induced effect on the single nuclear-coupled boiling channel natural circulation loop. The natural frequencies of the states in the stable region are widely explored through nonlinear analysis. The results indicate that the natural frequency of initial state tends to increase as the increase in the phase change number (operating power) or as the decrease in the subcooling number (inlet subcooling). As supposing that a real seismic acceleration is directly imposed on the system, some parametric effects on the seismic-induced oscillations are performed in the present natural circulation loop system. The seismic-induced oscillations are found to be consistent with the combined results of the inherent system stability characteristics and the impact of external seismic acceleration. The system with a larger outlet loss coefficient of the heated channel, or a longer heated channel, would destabilize the seismic-induced oscillations, while the inlet loss coefficient of the heated channel has a stable effect on the system. A much stronger resonance oscillation could be induced by the increase in the core inlet subcooling. Notably, the enlargement and contraction of the heated channel diameter would move the natural circulation system closer to type-I and type-II instability regions, respectively. These both generate unstable effects on the seismic-induced oscillations due to the inherent stability characteristics of the initial states.