A flow model for a control rod drop analysis

In pressurized water reactor (PWR), the core must be shut down quickly to prevent damage of the reactor internals if any operating limits are exceeded. Therefore, the scram time of control rod is one of the most important design parameters for the safety of the nuclear plant. The control rod drop time is affected by various types of fluid resistance force, so the accurate calculation of the flow field will play a significant role in the drop time prediction. In this paper, a new flow model for a control rod drop analysis of PWR is presented. The calculation of flow between rod and thimble tube is described in detail. According to the drop progress of a control rod, the flow analysis is divided into three phases, 1) above the ventilation holes, 2) between the holes and the entrance of dashpot, 3) after the entrance of dashpot. In each phase, several non-linear equations which describe the flow field are established based on the conservation of energy and flow. Then, the Newton iterative method is used to solve these non-linear equations. A computer code is developed based on this model and several sensitivity analyses are carried out by using this code. The effects of structural design parameters changes, namely, the diameter and the length of dashpot, the diameter of the ventilation holes, on the scram time and the fluid resistance force are discussed and presented. These results show that the new model is useful and accurate in the analysis of control rod drop and the code can be an effective tool for the fuel assembly design of PWR. (author)