In the beginning tests of control rod drive mechanism (CRDM), two phase flow would come into being because there is air dissolved in the liquid and hidden in the groove of driving rod. The air would be decomposed from the water and be extruded out of the groove when there is disturbance. For example, electrifying of coil component, up-down movement of driving rod, swing in and out of gripper component would give rise to disturbance and make the bubbles appear in the liquid. There is small amount of air dissolved or hidden in the water and two — phase flow would not occur in the test after CRDM moves several days.

In the tests, there is no bubble observed and the air would not be decomposed from the water when there is no disturbance, which is shown in Fig. 21(a). There are three coil components in CRDM, which are lifting coil, moving coil and stationary coil. And bubbles begin to come into being and there appear several separate bubbles when the coil component is electrifying and the gripper swings into the driving rod, which is shown in

In the tests of rod dropping, the power of three coils is cut off, the grippers swing out of the driving rod, and the driving rod free falls in the rod travelling house. In this process, the disturbance is transitory and bubbles appear in the liquid, of which the amount is less than that of rod lifting. When the bubbles go up to the bottom of the test section and there is no disturbance again, no bubbles will generate in the liquid, as shown in Fig. 22.

In the cold tests of rod lifting, rod inserting and rod dropping, only bubbly flow comes into being due to that the amount of gas dissolved in the liquid is small. After CRDM moves several days, the gas dissolved are all driven out of the liquid and there will not appear two — phase flow.

Fig. 22. Rod dropping tests

3. Conclusion

On the basis of the experimental results, the conclusions are obtained,

(1) It is found that there are several main flow patterns, bubbly flow, bubbly-churn flow, churn flow and annular flow in the tube-bundle channel. And there are great differences between flow patterns and their transitions in a tube-bundle channel and that in a circular tube.

(2) Experiments show that there may be two different flow patterns in the same cross­section of the tube-bundle channel. And the flow pattern transitions exhibit unsynchronized in different sub-channels. This unsynchronized phenomenon is caused by the different geometric dimensions, different heat flux and different quantity of discrete bubbles generated in different sub-channels.

(3) The flow pattern map is drawn on the basis of experiments. Comparisons are conducted between flow pattern transition in a tube-bundle channel and that gained by Hewitt & Roberts. The results show that the regions of bubbly flow and churn flow in a tube-bundle channel are larger than that in a circular tube. In addition, the flow pattern transforming to annular flow is earlier in a tube-bundle channel than that in a circular tube.

(4) Three main tests are carried out to measure pressure drop of the containment sump strainers. The photographs are taken and the vortex is not observed in these tests.

(5) In the tests of control rod drive mechanism, two-phase flow is observed. The results of upwards movement and drop movement of the driving rod are compared and analyzed.

4. Acknowledgments

The authors would like to acknowledge the financial support from the National Science and Technology Program of China (Grant No. 2011BAA06B00). The authors would also like to thank Messrs. Yu Jiang., Zhou Jianming, Wu Wei, Bai Bing, Lu Zhaohui for the helpful discussions.

5. References

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Hewitt, G. F. & Roberts, D. N. (1969). Studies of two-phase flow patterns by simultaneous X-ray and flash photography, Atomic Energy Research Establishment, ISSN: 0029-5450, Harwell, England

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