Hydrogen control in the APR1400 containment for the hypothetical station blackout accident

Advanced power Reactor APR1400 with a 1400MW electric power designed in Korea has 26 PARs and 10 glow-type igniters in its design specification in order to control or reduce the hydrogen concentration in the containment during the design-based or hypothetical severe accidents. For the station blackout accident most of the hydrogen and steam generated in the reactor vessel is released into the IRWST through the POSRVs of the pressurizer. In order to analyze hydrogen distribution during the SBO accident in the APR1400 containment, the GASFLOW code is used. The source of the hydrogen and steam for the GASFLOW analysis is obtained from a MAAP calculation. The discharged water, steam, and hydrogen passing through the POSRVs are released into the water of the IRWST by the sparkers. Most of the discharged steam is condensed in the IRWST water because of its subcooling, and dry hydrogen is released into the free volume of the IRWST, and finally goes out to the annular compartment above the IRWST through the vent holes. From the GASFLOW analysis it is said that the gas mixture in the IRWST becomes quickly non-flammable by the oxygen starvation but the hydrogen is accumulated in the annular compartment because of the narrow ventilation gap between the operating deck and containment wall when the igniters are not operated. The current design of the APR1400 has specified that four PARs and two igniters are installed in the IRWST. When the igniters installed in the APR1400 are turned on, a short period of burning occurred in the IRWST and then the flame was extinguished by the oxygen-starvation in the IRWST. The unburned hydrogen was released into the annular compartment and goes up to the dome because no igniters are installed around the annular compartment in the base design of the APR1400. From this result it could be concluded that the control of the hydrogen concentration is difficult for the base design. In this study design modifications are proposed in view of the hydrogen mitigation strategy. One of them is to install igniters in the annular compartment in order to burn the hydrogen released from the vent holes of the IRWST. The other one is to use the concept of water confinement in the IRWST by the compartmentalization of the IRWST. By doing this the refueling water around the sparkers quickly arrives at the condition of saturation and some part of the seam discharged from the sparkers escapes the water surface in the IRWST. For the first modified design standing flames are made above the vent holes of the IRWST. Most of the released hydrogen was burnt in this case, but it is thought that thermal loads from the standing flame is severe and the equipment located in the annular compartment must be protected. With the second modified design huge amount of steam is released into the IRWST and goes to the annular compartment. By the steam-rich condition of the gas mixture DDT possibility is heavily reduced