Analysis of Influence of Expansion on Control Rod Drop in Nuclear Reactor at High Temperature

The high temperature environment in the nuclear reactor will cause the thermal expansion of the control rod assembly, which will affect the falling time of the control rod assembly. In order to study the effect of thermal expansion on the drop of control rod assemblies, a fluid-structure coupling model for the drop of control rod assemblies was established considering expansion deformation, and the dynamic equations for the drop process of control rod assemblies were solved. The steady state thermodynamic coupling analysis of the guide tube and control rod is performed using finite element method to obtain the expansion deformation of the control rod and guide tube under high temperature conditions. Based on the established analytical model, the dynamic processes of control rod drop with and without expansion is compared. The analysis results show that the thermal expansion phenomenon delays the total drop time of the control rod, but has little impact on the drop time before the buffer section. Therefore, in general, the impact of thermal expansion on the drop time can be ignored, but the consequences caused by the control rod expansion still need to be paid attention to in the engineering design. The conclusion of this paper is important for the design of control rod and the analysis of in-pile control rod drop time.