The occurrence of secondary heat transfer tube failure due to overheating by sodium-water reaction in LMFBR steam generators has been concerned from the viewpoint of public acceptance. To evaluate the phenomena for the secondary heat transfer tube failure, a sophisticated computer code SERAPHIM has been developed by JAEA. The comparison of simulation results with experimental data to verify the adequacy of SERAPHIM code or to upgrade it are now required. In our paper at ICONE15, we report the void fraction and the entrainment pattern of water around a single rod set in a water pool using the probes developed for sodium experiment. In addition to these results, the reliability of the developed probe was verified by the experiment with gas jet velocity more than 340m/s accompanying oscillation. In the present paper, we reported the void fraction around a single rod in a diameter of 20mm set in a sodium pool without sodium-water reaction. The velocity of argon gas jet changed from 17.3m/s to 173m/s, which was lower velocity than that in the water experiment from the safety consideration and the void fraction was measured at every 30°. The void fraction was observed to somewhat increase with increasing the gas jet velocity. The increase rate was clearly smaller compared with that in the water experiment. The void fraction also showed more monotonous distribution from the stagnation point to the rear point than that in water pool. These results reflect the difference of surface tension between water and sodium. Because sodium has about three times surface tension as large as water, the argon gas jet column with oscillation in the sodium pool causes easily break-up than that in the water pool. It is concluded that the entrainment of ambient sodium is easily caused and this leads monotonous distribution of void fraction in the sodium pool.

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