[en] The computer code RETRAN is used for preliminary thermal-hydraulic analysis of a Pressurized Water Reactor. The steam generator is modeled into an optimum number of nodes for a detailed calculation. A sensitivity analysis is done on the number of nodes necessary to model a steam generator in RETRAN. A total loss of all feedwater transient with pump coastdown is analyzed with the steam generator detailed model. The analysis shows that a large amount of heat can be transferred from the primary to the secondary fluid, under certain conditions, which will result in a rapid cooldown in the primary loop