As a part of the U.S. Nuclear Regulatory Commission’s unresolved safety issue A-45 decay heat removal program, the Los Alamos National Laboratory (LANL) performed a TRAC-PF1 simulation of the Calvert Cliffs Unit 1 pressurized water reactor in a cooldown to residual heat removal (RHR) entry conditions after a loss of off-site power (LOSP). A detailed four-loop TRAC model developed for the A-49 pressurized thermal shock program was used. The LANL results indicated an inability to both cool down and depressurize the primary system sufficiently to meet RHR entry conditions using only the atmospheric dump valves and auxiliary pressurizer spray. A RETRAN-02/MOD3 analysis was performed for the same transient, using assumptions consistent with those in the LANL analysis. A fast-running one-loop RETRAN model was selected because of the inherent symmetry of the transient. The RETRAN results compared well with sensitivity analyses indicating that the pressurizer model dominates the transient signatures. A best estimate RETRAN analysis of the cooldown was performed using a more accurate set of assumptions to better understand actual plant operational responses. These results indicate that RHR entry could be achieved after an LOSP using only existing plant equipment and procedures.