Effect of wall friction model on predicting emergency core coolant behavior in upper downcomer with direct vessel safety injection using MARS-KS

For predicting more realistic flow behaviors, nuclear safety analysis code, MARS has adopted multi-dimensional module. However, most of the constitutive models in the module were developed based on onedimensional experimental results. In this study, the wall friction models in MARS were validated with two benchmark tests which were conducted by Korea Atomic Energy Research Institute (KAERI) for describing the direct ECC bypass phenomenon in the upper downcomer of the Advanced Power React 1400 (APR1400) during the reflood phase of a large-break LOCA (LBLOCA). One is the two-dimensional film flow experiment and the other is the MIDAS (multi-dimensional investigation on downcomer annulus simulation) test. Both simulation results showed that the Wallis correlation could improve the predictability yielding a stronger wall friction force than the H.T.F.S. correlation did.