HEAT REMOVAL FROM A HEATING TUBE SURFACE NEAR THE SPACER SUPPORTING NUCLEAR FUEL RODS IN BWR

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An experimental research have been made in order to make clear the mechanism to cause the dryout of liquid film on the heating surface near the spacers which support nuclear fuel rods. We used an experimental apparatus with an electrically heated tube with flat plate type spacers simulating the circular cell type spacer which is an typical spacer used in Boiling Water Reactor. Vapor- liquid two phase flow behavior in an annular gap between the outer surface of the heating tube and the inner surface of the co-axial glass tube was carefully observed by a high speed video recorder. As a result followings are obtained : (1) The spacer caused drypatch to occur in the limited region near the upstream end of the spader. Drypatches occur in also the limited flow pattern, the annular flow, when the liquid wave instantaneously flows downward due to gravity while large gas bubble passes by the spacer. (2) The dry patch disappeared intermittently when waves reached at the dried area on the heating surface, the temperature of the heating tube surface is possible to sharply increase causing the burnout of the heating tube. (3) The spacer pin interrupted water to flow downward and promoted the thinning of water film near the upstream end of the spacer. Accordingly the drypatch appeared at fist at the location just below the spacer pin. (4) The spacer promotes the occurrence of drypatch. On the other hand It plays an important roll of cooling the heating surface as a cooling pin.Close inspection of the out put signals such as void fraction, wall pressure and the wall temperature, reveals the followings. (5) The effect of the spacer on the mixing of vapor and liquid is effective to reduce the frypatch formation. Down ward flow due to gravity is also effectively reduced by the existence of the spacer. which results in the reduction of the frequency of the drypatch formation. The distance between the two spacers was optimum when it is about 500mm.