The development of the Oyster Creek RETRAN licensing model is presented. A three-step program was followed, namely:Step 1. Model benchmark against start-up tests: The objective here was to assure model stability and to qualify different segments of the model against plant data. A best-estimate model was thus established at this stage. Nine start-up tests were used.Step 2. Assurance that the model has built-in conservatism with respect to the reload transients it is designed to analyze. Here, a sensitivity study was carried out on a number of parameters for the limiting reload transient for Oyster Creek [turbine trip without bypass (TTWOB)]. The impact on the critical power ratio was used as the primary measure. Results from this study were used in establishing a conservative set of parameters, an uncertainty margin, and a proper choice of code options. The implementation of these results established the licensing model.Step 3. Testing the licensing model response against vendor’s analyses for typical reload transients, namely, TTWOB, main steam isolation valve closure without scram, and feedwater controller failure in maximum demand. This methodology has been submitted for U.S. Nuclear Regulatory Commission approval.