An experimental apparatus was used to simulate the annulus formed by a single tube passing through a tube support plate (TSP) in the steam generator of a PWR nuclear power plant. It was found that the extent of thin film evaporation, compared to nucleate boiling, is larger for smaller annuli, with a transition at 0.203 mm gap width to a local dryout/rewet condition. A model was developed that predicts an increasing extent of surface dryout/rewet prior to critical heat flux (CHF). A CHF correlation was developed by modification of a pool boiling CHF correlation, and the Chen (1966) correlation was modified to allow prediction of the two-phase heat transfer coefficient.