A digital computer simulation program, SOPHT, was developed by the Nuclear Generation Division of Ontario Hydro to assist their operating staff to ensure safe, reliable, and economic power generation. The program is used to predict the transient performance for Canadian deuterium uranium pressurized heavy water reactor (CANDU-PHWR) heat transport and steam supply networks together with their associated control systems. The model was verified by test data obtained in an operating plant. Examples are used to review and demonstrate the applications and contributions of this simulation program to plant operations.