An integral effect test on the SBLOCA (Small-Break Loss of Coolant Accident) aiming at 6-inch cold leg bottom break, SB-CL-09, was conducted with the ATLAS on November, 13, 2009 by KAERI. In this study, the calculations using MARS-KS V1.2 code were conducted for 6-inch cold leg break test of ATLAS (SB-CL-09) to assess MARS-KS code capability to simulate the transient thermal-hydraulic behavior for SBLOCA. The steady state was determined by conducting a null transient calculation and the errors between the calculated and measured values are acceptable for almost primary/secondary system parameters. The sequence of events except the location of loop seal clearing (LSC) and SIT injection time was predicted relatively well. The predicted pressurizer pressure agrees relatively well with the experimental data and the predicted break flow and mass are in good agreement with experiment. In MARS-KS calculation, the decrease of core collapsed water level is predicted well in blowdown phase, but just before LSC, water level is higher than experiment. However, the sudden decrease and increase of water level at the LSC are predicted qualitatively. After LSC, there is another water level dip at SIT injection time which is not in experiment. It is considered that this phenomenon is caused by rapid depressurization of downcomer due to significant condensation rate of vapor in downcomer when SIT water flows in it. For the downcomer water level, before the SIT injection time, water level is predicted well, however, it is significantly over-predicted at SIT injection time after SIT water flows in downcomer. Predicted cladding temperature generally agrees well with the experiment, while there is peak at SIT injection time in calculation which is not in experiment. The loop seals of 1A, 2B intermediate leg are cleared around 400 seconds in experiment, while only that of 1A is cleared in MARS-KS calculation at the same time. In conclusions, MARS-KS code has good capabilities to simulate cold leg break SBLOCA, however, there are some discrepancies in quantitatively predicting the steam generator secondary pressure, core collapsed liquid level, downcomer liquid level, and so on. Therefore, MARS-KS code including interfacial condensation model needs to be improved to predict more accurate results.

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