Safety concerns of nuclear reactors have attracted the attention of researchers on flow instabilities in natural circulation boiling loops. In this theoretical study, a drift flux model which solves the conservation equations of mass, momentum and energy applicable to boiling two-phase natural circulation systems is adopted. The influence of two-phase flow parameters such as drift velocity and void distribution parameter on the loop flow rate is weak. The model is used to analysis the effects of heat flux and inlet subcooling on steady circulation flow rate. High circulation flow rate is accompanied by high heat flux and low inlet subcooling. According to the region and number of meeting points which connects the resistance pressure drop curve and the driving pressure drop curve, flow excursion and density-wave instability sometimes may occur. Further, investigations are carried out to study the effect of heat flux and system pressure on the instabilities region in natural circulation.

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