In certain passive, future boiling water reactor concepts, during emergency cooling conditions, mixtures of steam and nitrogen are blown into a pool of water via a downward-facing open pipe; at its end, large gas bubbles form, break up, and rise in the water. We have developed a computer simulation program for the hydrodynamics of the process using an isothermal piecewise linear interface construction-volume of fluid method and carried out an experiment with flow rates up to 50 l/s into a tank of 1 m3 volume. Bubble frequencies and volumes can be predicted fairly well for the case of air injection. The experiments show that most of the condensation takes place before the bubble detaches from the pipe exit. The phenomena depend mainly on the volumetric flow rate of the gas and on a parameter measuring the shrinkage due to condensation. The rates of condensation were estimated to be very high.