THERMO-FLUID DYNAMIC RESEARCH ON THE CHARACTERISTICS OF HEAT REMOVAL FROM THE NUCLEAR FUEL RODS NEAR THE SPACER

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The effects of the geometry of the spacer, which supports the nuclear rods, on the thermo-fluid phenomena near the spacer were investigated by using two different types of the spacers. One is the ring type and the other is the grid type spacers. The special attention was focused on (a) the mechanism to cause the drypatch dormation, (b) the frequency of the drypatch formation and (c) the temperature distribution near the down side of the spacer. The results are summarized as follows.(1) Even if there is a gap between the spacer and the heating tube surface, the coolant water stagnates in between the spacer due to the surface tension force. Therefore it is mainly the drainage that causes the drypatche formation just underneath the spacer in the case of the present experiment where the spacer with the gap is used. Especially in the case of the grid type spacer the gap is not circumferentially uniform, and the surface tension acts in the circumferential direction as well. Then the water is … More kept more tightly within the smallest gap region than the ring type spacer and acts as if it is solid body. And accordingly the drypatch formation is limited in the region just beneath of the narrowest spacer gap region and more frequent in the grid spacer case than the ring type spacer.Although it has been made experimentally clear that the critical heat flux is smaller in the case of the grid type spacer than the ring type spacer, the reason of it is in the midst of a dispute. The result obtained in the present research is the most probable mechanism so far.(2) The temperature distribution shows that (a) the temperature near the bottom side of the spacer becomes lower in the case with the spacer than without the spacer because the thermal boundary layr is disturbed by the growing bubbles within the spacer. The gap space where the bubbles can move freely is limited to the wider gap region in the case of the grid type spacer but is not limited in the case of the ring type spacer. Then the mixing effect on the cooling of the heating rods is stronger in the case of thr ring type spacer than the grid type. This experimental fact also gives a ground That the ring type spacer is superior to the grid type.(3) The present experimental results show that the drainage of the liquid film even in the case of the annular flow is the more reasonable controlling factor to cause the thermally critical situation of the heating rods, i.e., the burnout, rather than the entrainment deposition theory, which is conventionally supposed to be the mechanism to cause the burnout. Less