The ability to model and predict the steady-state and transient thermal-hydraulic behavior of the Westinghouse model D steam generator is an important prerequisite for performing safety and licensing analyses of Commonwealth Edison’s Byron and Braidwood nuclear power plants. A RETRAN model using ten volumes to represent the primary side and ten volumes to represent the secondary side is developed. The model is validated over a range of steady-state conditions and is used to calculate the pressure and level response to a main steam isolation valve closure using operational data to drive the transient and a basis for comparison. Sensitivity studies and a sample reload licensing calculation are performed to further determine the model’s capabilities. The results of the model development show that the RETRAN model is a viable tool for analysis of the model D steam generator’s steady-state and transient behavior. Examination of the model’s behavior during rapid secondary depressurization events and confirmation of the carryover behavior is recommended.