Abstract: Gases and entrained particulate matter are automatically vented from a nuclear containment as pressure approaches a set point of the containment design during a postulated accident scenario. A porous bed filter is submerged in a tank of water, affording a first stage of filtration of the vented gases and entrained particulate matter passing in an upward flow from the bottom of the tank through the porous filter bed and a second, pool scrubbing stage as the flow proceeds upwardly through a central region of the water above the filter with accompanying decay heat removal prior to release of the gases from the water surface and discharge to atmosphere. The water returns through a down-flow path defined by an annular boundary region surrounding the central region of the water volume, in a pumping function for repeating the mixing and two-stage flow, and purges the porous filter bed.