This study extended the capabilities of PCTRAN by including the Gaussian puff model and the basic numerical techniques, to efficiently calculate the radioactive effluent dispersion. PCTRAN is a PCbased nuclear power plant simulation code, and capable of running faster than real time. Accident initiation events can be activated in the simulationsoftware, and the radioactive materials are assumed to be released from the plant to the atmosphere atevery time of interval. During a transient or anaccident simulation, puffs are sequentially generated and dispersed in all directions governed by the Pasquill stability category, wind velocity and winddirection. The thyroid dose rate and whole body doserate (as well as their accumulations) at every spatiallocation in the emergency planning zone (EPZ) are shown as a color-shaded plot. A postulated scenario was given in PCTRAN to demonstrate that the codepredict the time-varying distributions of thyroid and whole body dose rates (and their accumulateddosages) efficiently. Therefore, the urgent decision- making on protective actions can be determined ahead of the accident time.