A pressurized water reactor plant analyzer code (NUPAC-1) has been developed to apply to an operator support system or an advanced training simulator. The simulation code must produce reasonably accurate results as well as run in a fast mode for realizing functions such as anomaly detection, estimation of unobservable plant internal states, and prediction of plant state trends. The NUPAC-1 code adopts fast computing methods, i.e., the table fitting method of the state variables, time-step control, and calculation control of heat transfer coefficients, in order to attain accuracy and fast-running capability. The NUPAC-1 results are compared with the RELAP5/MOD2 results to assess the accuracy for accident analyses such as loss of coolant, feedwater line break, and steam generator tube rupture. The fast computing methods had a negligibly small effect on accuracy and contributed to fast-running capability. The NUPAC-1 code can be applied to the operator support system and the advanced training simulator as a two-phase simulation code.