CF (Chinese Fuel) series fuel assemblies are the core components of nuclear power independently developed by China National Nuclear Corporation (CNNC), which together with the drive mechanism, constitute an important part of a large pressurized water reactor which is related to the stability and safety of nuclear reactor operation. Development of any fuel assembly and related equipment must undergo friction and rod drop test to verify its structural integrity and the property. The traditional friction and rod drop performance test can only be carried out with small eccentricity to obtain the test data such as friction force and rod drop curve. It can't be achieved to analyze and study the performance with multiple eccentricities, especially the larger eccentricity. And there are no more studies on comparative mechanical analysis of the friction force of the driving mechanism through full displacement and in the two media of water and air.

This study aims to analyze the friction and rod drop performance of the drive line's moving parts of the self-developed CF2 fuel assembly under water and air working conditions with different eccentricity.

A 1:1 simulated CF2 fuel assembly was used in the test with an independently-developed rotatable top cap. The integration of multiple eccentricity was initially implemented for scientific and accurate regulation. The method to study the performance of the driving mechanism was optimized. The friction force and rod drop performance data were obtained through the experiments of the driving mechanism in water and air through full displacement and under different eccentric conditions.

Experimental results show that the friction force of control rod drop in the air at the lowest and highest positions is 26.5 N and 32.1 N, respectively when the eccentricity is 4.67 mm, whilst the two force values in the air at the lowest and highest position is 37.1 N and 39.2 N, respectively when the eccentricity is 9.4 mm. The friction force control rod drop in the motionless water at the lowest position and the highest position is 54.8 N and 39.5 N, respectively when the eccentricity is 4.67 mm whilst the two force values in the water are 62.8 N and 44.1 N, respectively when the eccentricity is 9.4 mm. As the control rod is gradually raised, the friction force in the air increases much more than that in static water. The maximum value in the air occurs at the highest position, while the maximum value in static water occurs at the lowest position. By comparing and analyzing the rod drop performance and parameters like velocity, displacement and vibration, it can be seen that with the continuous increase of the eccentricity, the friction during rod operation also increases correspondingly, and the maximum speed during rod drop gradually decreases. The data shows that the impact of the various eccentricities on the buffer time is small and the value can be basically consistent. The minimum rod drop time occurs when the eccentricity is 0 mm. The time for control rod reaching to the buffer port is 1.049 s, and the time reaching to the bottom is 1.477 s.

The numerical comparison of friction force between in the air and static water through full displacement and under multiple eccentricities has been carried out, the fuel assembly and control rod run well, the friction does not exceed the limit value, and no jamming of the rod occurs under the maximum eccentric condition. It can verify the rationality of the design of CF2 fuel assembly, providing an important test basis for the design, safety evaluation and software development of this series fuel assemblies.