In relation to the thermal-hydraulic design of an innovative Flexible-fuel-cycle Water Reactor (FLWR), this study investigates inter-subchannel cross flow phenomena in a tight-lattice rod bundle. Numerical simulations of cross flow using advance interface tracking method were performed and the results were analyzed by a statistical method to clarify the characteristics of inter-subchannel two phase cross flow in the FLWR reactor core. It was revealed that strong correlation exists between differential pressure and gas/liquid mixing coefficients, and cross flow results mechanistically from differential pressure between subchannels. An approximate model is presented which permits the prediction in detail of the components of the inter-subchannel fluctuation differential pressure. The instantaneous fluctuation of differential pressure between two subchannels in gas-liquid slug flow regime is deemed as a result of the intermittent nature of slug flow in each subchannel. The model is based on the detailed numerical simulation results that pressure drop occurs mainly in liquid slug region and in the bubble region it is negligibly small. The instantaneous fluctuation of differential pressure is associated with pressure gradient in the liquid slug for each channel. In addition to a hydrostatic gradient, acceleration and frictional gradients are taken into account to predict pressure gradient in the liquid slug. This model used in conjunction with the numerical simulation code works satisfactorily to reproduce numerical simulation results for instantaneous fluctuation of differential pressure between two modeled subchannels.

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