Westinghouse realistic Large Break LOCA methodology (Frepoli, 2008) is based on the use of the WCOBRA/TRAC code (Young, et. al., 1998). The original COBRA/TRAC code (Thurgood, et. al., 1983) was developed at Pacific Northwest Laboratory by combining the COBRA-TF code (Thurgood, et. al., 1980) and the TRAC-PD2 code (Liles, et al., 1981). Later the code was significantly upgraded at Westinghouse and validated for Large Break LOCA (LBLOCA) phenomena and renamed WCOBRA/TRAC. The current WCOBRA/TRAC has been shown to adequately model LBLOCA phenomena which are mainly dominated by the thermal-hydraulic phenomena in the vessel component. Westinghouse is now looking at extending the code applicability to smaller break sizes such that the same code can be applied to a full spectrum of LOCAs, from small to intermediate break as well as large break LOCAs. Several upgrades have been made to the code which has been renamed WCOBRA/TRAC-TF2. In this new code, the TRAC-PD2 module was replaced by the 1D module of TRAC-PF1/MOD2 code (Los Alamos, 1993, Spore, et. al., 2000). TRAC-PF1 is based on a two-fluid (6-equation) formulation as opposed to the 5-equation drift-flux model of TRAC-PD2. The new formulation allows a more realistic simulation of the two-phase flow features in the PWR loops during a SBLOCA scenario. This paper also describes some of the improvements added to the code and the assessment needed to extend its applicability to the full spectrum of LOCA scenarios or break sizes.

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