The purpose of this paper is to develop a nonlinear model to investigate the instabilities of a two-phase natural circulation loop under low-pressure condition. Inlet velocity oscillations and the corresponding trajectories are respectively presented in the time evolution planes and phase planes. We obtain a stability map to explore the instability regions of this natural circulation loop. The results show that the considered loop has two unstable regions, instability type-I in the low power region and instability type-II in the high power region. Then the parametric study is carried out to understand the relation between the parameters of system and two types of instability. The parametric study reveals that lengthening the riser has an unstable effect on system stability. Thus, lengthening the riser causes a reduction in the stability region in the both low power and high power levels. Also it can be observed that by increasing the form loss coefficient at the inlet of heated section or in the downcomer section, the stability region expands, however by increasing the form loss coefficient at the outlet of heated section or in the upper horizontal section, the stability region decreases consequently.

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