A passive emergency water system for cooling the atmosphere inside containment in a water-cooled nuclear reactor comprises an elevated in-containment water reservoir connected to an in-containment heat exchanger. The heat exchanger promotes natural convection of containment atmosphere and heat transfer. Heat from containment atmosphere is transferred to the reservoir by a convective return flowpath. The heat exchanger is preferably an elevated tube bank Baffle walls can be used inside containment to promote circulation of containment atmosphere. In one embodiment, the reservoir tank is closed with respect to containment atmosphere and is vented through the containment wall to the external atmosphere. In an alternative embodiment, the reservoir tank can be of a sufficient volume to absorb heat from containment as sensible heat without boiling.