AbstractThe once-through steam generator version of the personal computer transient analyzer, PCTRAN/P, was used to simulate the overcooling event that occurred on December 26, 1985, at Rancho Seco. Loss of power to the integrated control system led to a reactor trip and overfeeding by the auxiliary feedwater with excessive steam dump. As a result, the plant cooled down rapidly in a short period. PCTRAN/P has successfully reproduced the transient using its interactive control functions. Areas of the system’s design deficiencies are thus identified and modifications can be made to prevent a similar event from recurring. The computation time on an IBM-PC/XT was∼20 min for the 50-min transient. This demonstrates that PCTRAN/P can be used as a fast-turnaround tool for conducting reactor transient analyses.