A first-principle model has been developed to perform steady-state and transient analyses of pressurized water reactor plants on microcomputers. The model includes representations for all major components and control systems of the nuclear steam supply systems of plants that have U-tube steam generators. The process of inputting data is simple, and it is easy to set up for simulations of various transient scenarios. The model is accompanied by sophisticated preand postprocessors to help users define or modify input files, start model executions, and analyze output results. It has been benchmarked against plant data and RELAPS for various steady-state and transient scenarios. Satisfactory comparisons have been obtained. Results are presented for full-power steady-state conditions and for a steam generator tube rupture transient.