Studies on Boiling Two-phase Flow and Burnout Phenomena in Narrow Channel Simulated Subassembly of High Conversion Reactor

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High conversion LWRs with a high density hexagonal lattice are expected as one of next generation reactors because of their high utilization of uranium fuel and effective burning of MOX fuel. One of important considerations for the reactors may be heat removal from the core in both normal and off-normal conditions due to low water inventory in the core with long narrow coolant channels. As hydraulic diameter of the heated channels decrease, the fluid flow is strongly influenced by the power input due to increase of power input rate per unit fluid volume. In this study,boiling heat transfer,critical heat flux and pressure drop for two-phase flow both (a)in a narrow annula channel with a heated rod and (b)in a narrow subassembly channel with seven rods with spiral wires were investigated by using Freon-113 thermal loop at 0.3 MPa.The main results are : 1)Transitions of flow patterns both from the bubble to the froth and from the froth to the annular cause in lower quality regions in the narrow channels compared with these in the normal cahnnel. 2)Boiling heat transfer coefficients in the narrow channels become larger than those in the normal channel,because of flow velocity change induced by the boiling and enhancement of turbulent mixint and cross flow by spiral wires. 3)Critical heat fluxes in the narrow channel decrease about 20 percent less that Katto's correlation obtained from the data in the normal size channel. However,in the seven subassembly channel,the critical flux become the same values as the correlation,because of the enhancement by the cross flow. 4)The total pressure drop is well estimated by a drift flux model of two-phase flow with modified friction loss multiplier and modified quality model in the subcooled flow regin,which are newly introduced.