Abstract: A fission product scrubbing system comprises a water tank in the containment building; a dividing wall extending into the water tank for separating it into first and second compartments; a collection plenum normally hermetically sealed from the containment space and the environment externally of the containment building and being at least in part formed by the dividing wall and the level of water stored in the second compartment and a communication passage in the dividing wall underneath the water level in the first and second compartments for maintaining communication between the water stored therein. There is further provided a standpipe which extends from the containment space into the second compartment and which has flow distribution holes situated in the second compartment below the water level and above the communication passage, whereby gases passing from the containment space into the standpipe travel upwardly from the distribution holes through the water into the collection plenum.