This study aims to assess the multi-dimensional fluid model of MARS-KS (KINS standard version of MARS) code. The UPTF upper plenum injection test (UPI Test, Test No. 20) is modeled with the MARS in order to assess the code capability for predicting the multi-dimensional behavior of UPI water in the upper plenum during a reflood phase of large break loss-of-coolant accident. The coolant piping is simulated by one-dimensional components, while the reactor vessel is modeled by the cylindrical multi-dimensional component with five rings in radial direction, eight sectors in azimuthal direction, and fourteen axial levels. The results of MARS calculation are compared with the experimental data of UPTF test, particularly the ECC downflow area in the core flow area, the water carryover rate to hot legs, and the water level distribution in upper plenum. The calculation results indicate that the size of the downflow area is about 16.7% of the core flow area, comparable to the result of 10% in UPTF test. The MARS overestimates the amount of water carryover to the hot legs. As seen in the UPTF UPI test, the collapsed water level formed in the upper plenum is uniform at the entire core flow area, except at the down flow region where more water accumulates. These results demonstrate that MARS can provide a reasonable prediction of multi-dimensional behavior of UPI water in the upper plenum.

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