A computer code, FAUST (Flow Analysis of U-tube Steam generators), for U-tube steam generator design analysis is developed on the basis of the pressurized water reactor core transient analysis code, THERMIT. The original (x, y, z) coordinates used in THERMIT are transformed into the cylindrical (r, θ, z) coordinates for FAUST, which are better fitted in the geometry of steam generators. To couple the primary side with the secondary side, a one-dimensional tube representative of a computational cell in the heat transfer model is developed with a geometrical mapping between the primary and secondary sides. The special unitary group SU(2) is used to treat the complex geometry of the U-bend region for frictional wall force. A form loss model for tube support plates in two-phase flow is implemented in the code. The steam dome model developed here enables us to consider the different amounts of feedwater distributed into the hot and cold sides of the downcomer. Measured data from the steam generator at the BUGEY 4 nuclear power plant are used for the assessment of FAUST. Predicted results for the measured parameters are in good agreement with measured data: circulation ratio within 8% error and total power within 2% error. Considerable liquid recirculation is found in the U-bend region as the Combustion Engineering design code CALIPSOS shows.