Theoretical research for natural circulation operational characteristic of ship nuclear machinery under ocean conditions

Based on the two-phase drift flux model and the multi-pressure nodes matrix solving method, natural circulation thermal hydraulic analysis models for the Nuclear Machinery (NM) under ocean conditions are developed. The neutron physical activities and the responses of the reactivity control systems are described by the two-group, 3-dimensional space and time dependent neutron kinetics model. Reactivity feedback is calculated by coupling the neutron physics and thermal hydraulic codes, and is tested by comparison with experiments. Using the models developed, the natural circulation operating characteristics of NM in rolling and pitching motions and the transitions between forced circulation (FC) to natural circulation (NC) are analyzed. The results show that the influence of the rolling motion increases as the rolling amplitude is increased, and as the rolling period becomes shorter. The results also show that for this NM, with the same rolling period and rolling angle, the influence of pitching motion on natural circulation is greater than that of rolling motion. Furthermore, the oscillation period for pitching motion is the same as the pitching period, while the oscillation period for rolling is one half of the rolling period. In the ocean environment, excessive flow oscillation of the natural circulation may cause the control rods to respond so frequently that the NM would not be able to realize the transition from the FC to NC steadily. However, the influence of ocean environment on the transition from NC to FC is limited.