The thermal-hydraulic code A THLET used to analyze the Three Mile Island Unit 2 accident was developed at the Gesellschaft für Reaktorsicherheit in the Federal Republic of Germany for safety analysis of pressurized water reactors with U-tube steam generators. First calculations of phase 1 of the accident without simulating the secondary sides were preliminary and have shown the need for accurate heat flow rates in the steam generators. The available values resulted in an incorrect system pressure behavior. Second calculations with detailed simulation of the once-through steam generator provided interesting information like the influence of flow resistance in the aspirator on cold-leg temperature and the dependence of system behavior on the rate and position of auxiliary feedwater injection.