[en] The direct ECC bypass phenomena that occur in the DVI system during the reflood phase of a large break loss-of-coolant accident (LBLOCA) are studied using a transparent 1/7.5 model of a Upper Plenum Test Facility (UPTF). The separate effect tests were performed in order to clarify the mechanism of the direct bypass and to derive the scaling parameters affecting the ECC direct bypass rate. The various flow regimes and their distribution in the downcomer have been identified and mapped. And the direct ECC bypass rate has been measured under the various air and water injection conditions. From the counterpart tests of UPTF Test 21-D in this small scale UPTF geometry downcomer, the dimensionless gas velocity j^*_g,eff was derived experimentally, which is believed to be a major scaling parameter of the direct ECC bypass. And in the UPTF air-water separate effect tests, it was found that the direct ECC bypass rate was greatly affected by the liquid film spreading width and the geometry of the downcomer