A hypothetical accident is analyzed, in which an external (outof-plant) natural or man-made event causes a loss-of-coolant accident after penetrating the containment wall The computer codes CONTEMPT and RELAP4 have been used to study the containment thermal-hydraulic behavior during the accident. The radioactive materials outflow rate is calculated for a wide range of anticipated breach sizes, assuming perfect mixing in the air-steam-water mixture in the containment building. Typically, there is only a low pressure buildup in the initially breached containment. Therefore, the calculated fission product release rate is lower than the predicted rate in case the containment fractures in a later phase of the accident.