Experimental study of coupled nuclear and thermal instabilities of two-phase flows in a boiling channel

An experimental study is performed to reveal the mechanism of the two-phase flow instability in a boiling channel which involves coupling of the void-reactivity feedback and the thermohydraulic feedback. In the experiment, the thermohydraulic feedback is dealt with using a conventional two-phase flow heat transfer loop in which the coolant is circulated, while the void-reactivity feedback is simulated with a computer using the point reactor model to obtain the relationship between the average void fraction and the heating rate of the two-phase flow channel. The heat input to the channel is then controlled in accordance with the variation of the measured average void fraction. It is found that the oscillation of the heating rate decreases as the fuel-time-constant τM increases. It is confirmed that coupled nuclear-thermal effect on the density-wave instability becomes stronger as the absolute value of the void-reactivity coefficient Cα increases, resulting in increased oscillation of the heating rate. (author)