The TRAC series of codes was developed to simulate pressurized water reactors (PWRs) and boiling water reactors (BWRs) under hypothetical accident conditions. The thermal hydraulics of these codes are based on a two-fluid formulation. These codes were applied to the Dartmouth College countercurrent flow tests to assess the ability of the interfacial momentum transfer models in the code to predict the countercurrent behavior. The TRAC-BD1 code, developed for the BWR analysis, qualitatively predicted the proper countercurrent flow behavior, but always overpredicted the liquid downflow. This led to the conclusion that interfacial momentum transfer in the annular regime was underestimated. The PWR version of the TRAC code, TRAC-PF1, had better agreement with the data but computed unusual behavior for the 0.152-m-i.d. pipe due to the use of Dukler’s correlation outside the data base. The code prediction improved when Bharathan-Wallis’ correlation was incorporated into this code. The correlations based on cocurrent data were not accurate in predicting countercurrent flows.