A flashing droplet model was developed to examine the rupture flow of reactor coolant and its transport phenomena through the stream generator during a steam generator tube rupture accident. The model includes flashing flow; droplet formation; droplet removal by tube bundles, bubble scrubbing, steam separators, and steam dryers; and droplet size change by evaporation and condensation. The calculation follows the actual sequence of events during the accident. Those reactor coolant droplets escaping from the steam generator are used to estimate the radioactivity released into the environment. The steam generator tube rupture accident that occurred at the Prairie Island Plant on October 2, 1979, was studied using the model. The model estimated a release of 204 µCi of 131 I equivalent activity. The U.S. Nuclear Regulatory Commission estimated a 210-µCi release, assuming an iodine partition factor of 1/100 in the steam generator. The model was also used to analyze a hypothetical steam generator tube rupture accident coupled with loss of off-site power in a large 1100-MW(electric) Westinghouse four-loop plant. The model estimated that 45 Ci of 131 I equivalent activity could be released through the relief valves, which were stuck open for 30 min. The number is eight times higher than the estimate from the Westinghouse safety analysis report using a uniform mixing model.