Comparison of MARS-KS to COBRA-TF for models and correlations in pre-CHF regime

Abstract

To evaluate the safety of a Korean Nuclear Power Plant (NPP), one dimensional system analysis code, MARS-KS, is being used by the Korean regulator. The governing equations of MARS-KS are based on two-phase two-field model. MARS-KS code also includes COBRA-TF as well for sub-channel analysis of the reactor core. In contrast to MARS-KS, COBRA-TF is based on two-phase three-field governing equations. In this paper, the two phase flow regime map and correlations will be compared between MARS-KS and COBRA-TF while considering the difference in the governing equations. This exercise is not only important for the basic understanding of the two phase flow modeling, but also it is important for the future Korean regulatory activity for assessing the appropriateness of SPACE (Safety and Performance Analysis CodE for nuclear power plants) developed by a consortium led by Korea Hydro & Nuclear Power Co., Ltd. (KHNP). The governing equations of SPACE are also based on two-phase (liquid and gas phase) three-field (continuous liquid, gas and droplet) governing equations like COBRA-TF. The effect of the implemented two phase flow regime map and correlations will be evaluated by modeling the selected separate effect test case with both MARS-KS and COBRA-TF and this will be followed by the discussion on the assessment results.