Analysis Of The Control Rod Dropping Process

The control rod is a very important component in the reactor.When an accident occurs,the control rod can be shut down in time to prevent further expansion of the accident.Therefore,it is necessary to study the process of the falling of the control rod.In this paper,the theoretical and simulation methods are used to analyze the vertical dropping process of the control rod,respectively,and the drop mechanical behavior of the control rod is explored.Firstly,the Bernoulli equations and the continuity equations are respectively built to model the fluid flow in the guide pipe.Combining with the falling equation of the control rod,the falling process is theoretically solve.Secondly,a dynamic mesh technique and the UDF compiling technique are used to handle the data-exchange between the structure and the fluid flow,and the falling simulation of the control rod is computed.Finally,for the effects of the lateral vibration of the rod on the drop process,considering the collision and friction between the control rod and the pipe wall,the vibration equation is obtained with the finite element method.Based on the 2D model,the simulation analysis is subsequently conducted with the UDF technique.The results show that based on the 2D and 3D models,the vertical drop mechanical behavior of the control rod obtained by the theoretical method is almost the same as one obtained by simulation method,and the influence law of the parameters obtained by the 3D model on the falling process is basically the same as that obtained by the 2D model.It can be seen that the 2D model can capture the complete mechanical behavior of the control rod drop.A 2D simulation model with different eccentricity is established and solved to reveal its effects on the drop of the control rod.The results show that the effects of eccentricity on the falling process are not obvious.Therefore,in the following simulation focusing on the effects of the lateral vibration of the rod on the drop process,it is feasible to use a centering additional mass coefficient instead of the eccentric additional one.The simulation results based on the 2D model show that,compared with the vertical drop ones,the lateral vibration of the control rod brings about the more dropping time,the larger fluctuations of the dropping velocity and the vertical fluid force.In addition,the friction generated by the collision has a limited influence on the falling process.When the structure is in a violent collision,there is a "reflow" phenomenon at the flow exit sometimes.After the control rod enters the deceleration section,the collision will completely disturb the surrounding flow field,which is one of the main reason for the sharp fluctuation of the vertical fluid force.