Computational fluid dynamics (CFD) is used to predict steam generator inlet plenum mixing during a particular phase of a severe accident in a pressurized-water reactor. Boundary conditions are obtained from SCDAP/RELAP5 predictions of a TMLB' station blackout. Full- scale CFD predictions are completed for the scaled-up geometry of a 1/7th scale test facility to isolate the scaleup effect. These predictions are repeated with a Westinghouse model 44 steam generator design. The effect of tube leakage on the mixing is also considered. Finally, predictions are completed for a steam generator from a Combustion Engineering (CE) nuclear power plant. Scaleup predictions indicate that data at 1/7th scale are indicative of the full-scale behavior for similar geometries. Predictions for a model 44 steam generator design indicate slightly less mixing and increased plume oscillations and indicate that the geometry is an important parameter. Tube leakage does not show a significant impact on the mixing for leakage rates below 1.4 kg/s at these severe accident conditions. A CE steam generator design results in significantly less inlet plenum mixing. The highest tube entrance temperatures approach the hot leg temperatures in this case. Heat transfer rates to the secondary side are determined to be a dominant governing parameter.