The Investigation in the Oscillation Modes of In-Phase and Out-of-Phase in a Two-Phase Forced Circulation Loop With Multiple Nuclear-Coupled Boiling Channels

This study develops the nonlinear dynamic model for a multi-channel nuclear-coupled boiling forced circulation loop. Stability maps and nonlinear dynamics of a three nuclear-coupled boiling channel forced circulation loop are investigated. The possible mechanisms of in-phase and out-of-phase oscillations, and their distributions along with the stability boundaries are also discussed. For the pure thermal-hydraulic system without considering the nuclear coupling, the thermal-hydraulic feedback gain tends to dominate the system with different radial heat flux ratios to present out-of-phase mode of oscillations in the boundary states. The difference in heat fluxes among channels would enhance the channel interactions to destabilize the system and cause the phase difference among channel oscillations. The effects of neutron interaction and void-reactivity feedback have a great influence on the oscillation mode among three channels in the nuclear-coupled boiling forced circulation loop system. For the asymmetric heating system with a reference void-reactivity coefficient, the case with a relatively weak neutron interaction remains presenting out-of-phase mode with phase difference along with the stability boundary. However, the system with a relatively strong neutron interaction, the in-phase mode with phase difference distribute over the boundary states in the medium to high subcooling number region while the neutronic feedback gain may be dominant. On the other hand, it would favor out-of-phase mode with phase difference at the boundary states of low subcooling numbers if the thermal-hydraulic feedback gain is stronger and the two-phase frictional pressure drop is dominant in all channels. Moreover, the out-of-phase mode among unequal-heating channels in the weak neutron interaction system may turn into in-phase mode as strengthening the neutronic feedback gain through the reference void-reactivity coefficient doubled.