The wetwell space in a suppression pool of a nuclear reactor containment is continuously ventilated by exhausting gas therefrom, while at the same time, during normal system operation atmospheric air from a source of same is admitted to the wetwell but such admission being blocked during a LOCA. All exhaust flow from the wetwell is conveyed through a conduit that outlets at a remote elevated location in the atmosphere. All exhaust flow through the conduit is before outletting therefrom passed through gas treatment operation wherein any particulates in the gas mixture are removed. Further treatment of the gas with charcoal to adsorb noble gases can be carried out. In normal reactor operation the ventilation flow rate is at minimal level. However on occurrence of a loss-of-coolant-accident, highly heated gases from the containment drywell are passed into the suppression pool where condensables condense while non-condensable gases are cooled and vent to the wetwell. Fission products are scrubbed in the pool and much retained therein. The ventilation rate increases during LOCA to carry off the increased non-condensable gas mixture volume created by the LOCA and, because the conduit is such sized, containment drywell pressure during the LOCA, is maintained at a level not to exceed about one atmospheric gauge. This allows that containment structures can be of lighter weight than heretofore and enclose more components such as turbine units and condensers.
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