A modified kettle reboiler is planned to be used as an emergency condenser for high pressure steam in nuclear power plants. The condensing is achieved by the boiling-off of water at atmospheric pressure. The bubbles released at the tubular surface have a finite residence time in the liquid column above the tubular surface. This 'bubble holdup' increases the level of the two-phase boiling system. It is important to know the level change in order to determine the free space available for disengagement of the entrapped droplets and the capacity of the condenser to condense the high pressure steam. A solution to this problem is presented below alongwith an actual case study. © 1981.