Natural circulation is an important process for certain advanced reactor’s main loop and passive heat removal systems. However, in marine conditions the thermo-hydraulic characteristics of natural circulation will change because of the ship motions such as inclination, rolling and heaving, which introduces extra body forces in to the system. In this paper, we conducted theoretical studies on the natural circulation behaviors in a symmetrical two-circuit loop under rolling conditions. A RELAP5/MOD3.3 code is developed based on the basic control equations and empirical formulas. Based on this code the natural circulation behavior under a larger range of rolling angle and period is also investigated. It is found that with the increase of rolling angle and decrease of rolling period, the magnitude of flow fluctuation increases. The fluctuation period of mass flow rate do not always consists with the rolling period under the comprehensive actions of complex body forces. Under the rolling with large angle and short period, the fluctuation period of flow rate is only half the rolling period.

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