Analysis of coupled nuclear/thermal instabilities of two-phase flows in parallel boiling channels

An analytical model is proposed to study the two-phase flow instabilities o f in-phase and out-of-phase modes in parallel boiling channels. This model takes into consideration the interaction between channels with different vo id fraction variations by modeling the diffusion of neutrons. Linear stabil ity analysis in the frequency domain is performed to obtain stability bound aries and flow oscillation modes. in the case of parallel two channels. two types of stability boundaries are found to exist on the stability map; one for the instability of the in-phase mode and the other for that of the out -of-phase mode respectively. For a certain range of the void-reactivity coe fficient, the void-reactivity feedback loop is found to be unstable in a ce rtain region on the stability map, which has an effect on the stability of the in-phase mode. The interaction between channels affects the instability of the out-of-phase mode. The results are also obtained for the case of th ree and four channels. It is found that the channels having stronger intera ction between them are more likely to oscillate in-phase with Each other, w hile those having weaker interaction are more likely to oscillate out-of-ph ase with each other.