A phenomenological model is developed for the transient thermal-hydraulic processes on the secondary side of a once-through steam generator during auxiliary feedwater (AFW) injection. Detailed modeling of the thermal-hydraulic processes above the top tube support plate (TSP) is particularly emphasized. The nonuniform distribution of the AFW on the secondary-side tubes is represented by dividing the secondary side into a number of tube groups. For each tube group, the quasi-steady-state conservation equations representing the flow of a falling liquid film and steam on the secondary side and the primary coolant on the primary side are numerically solved for each time step, thereby providing the axial variation of flow rates and temperatures in the primary and secondary sides. Modeled processes include cooling due to the impingement of the AFW jet on the tubes, the forced convection/boiling heat transfer at the liquid film-tube interface, evaporation and condensation at the film-gas interphase, countercurrent flow limitation in the TSP passages, and the formation of a swollen two-phase pool above the top TSP. The aforementioned model for the thermal-hydraulic phenomena above the top TSP is incorporated into a transient model for the entire steam generator where the secondary side is divided into four regions. Global conservation equations representing the transient behavior of each region are numerically solved. Model predictions are compared with a typical test from the Multiloop Integral System Test experiments. Parametric and sensitivity calculations are also reported.