The control rod drop time is one of the most important parameters for safety analysis. The calculation accuracy of control rod drop time will be affected if ignoring the transverse vibration, which is mainly caused by the fluid-structure interaction between the RCCA (rod cluster control assembly)s, guide tubes and the fluid interaction.

A new method for RCCA drop analysis is presented in this paper. The transverse equations of motion are established considering fluid-elastic structure interaction, assuming the fluid is incompressible laminar flow. And the vertical equations of motion are established considering the gravity force of RCCAs, the fluid resistance, and the friction force led by collision.

A computer program based on this method is used to calculate the control rod drop time, the impact and the dynamic response of RCCA. The analysis results are compared with those of the in-house code, which is used for commercial design. It shows the computer program using the new models provides a useful tool in the design of RCCAs and fuel assemblies.

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