A passive containment cooling system, PCCS, for a nuclear reactor, the PCCS comprising: a first stage condenser including a first inlet header, a first outlet header defining a first outlet opening, and first channels in fluid communication between the first inlet header and the first outlet header, the first inlet header defining a first inlet opening for receiving a fluid mixture, the first channels being configured to condense a first condensate portion from a first gas in the fluid mixture, based on heat exchange between the fluid mixture and an external environment through the first channels, as the fluid mixture flows through the first channels from the first inlet header to the first outlet header; a catalyst in at least one of the first outlet header or a second inlet header of a second stage condenser, the catalyst being configured to catalyze a reaction for forming the first gas from a second gas and a third gas in the fluid mixture as the fluid mixture flows through at least one of the first outlet header or the second inlet header; and the second stage condenser, the second stage condenser including the second inlet header, a second outlet header, and second channels in fluid communication between the second inlet header and the second outlet header, the second inlet header defining a second inlet opening in fluid communication with the first outlet opening for receiving the fluid mixture from the first outlet header, the second channels being configured to condense a second condensate portion from the first gas, based on heat exchange between the fluid mixture and the external environment through the second channels, as the fluid mixture flows through the second channels from the second inlet header to the second outlet header, the second outlet header defining a second outlet opening for providing the second condensate portion to a drain line, and the second outlet header defining a vent opening for transmitting a portion of the fluid mixture to a vent line. a first stage condenser including a first inlet header, a first outlet header defining a first outlet opening, and first channels in fluid communication between the first inlet header and the first outlet header, the first inlet header defining a first inlet opening for receiving a fluid mixture, the first channels being configured to condense a first condensate portion from a first gas in the fluid mixture, based on heat exchange between the fluid mixture and an external environment through the first channels, as the fluid mixture flows through the first channels from the first inlet header to the first outlet header; a catalyst in at least one of the first outlet header or a second inlet header of a second stage condenser, the catalyst being configured to catalyze a reaction for forming the first gas from a second gas and a third gas in the fluid mixture as the fluid mixture flows through at least one of the first outlet header or the second inlet header; and the second stage condenser, the second stage condenser including the second inlet header, a second outlet header, and second channels in fluid communication between the second inlet header and the second outlet header, the second inlet header defining a second inlet opening in fluid communication with the first outlet opening for receiving the fluid mixture from the first outlet header, the second channels being configured to condense a second condensate portion from the first gas, based on heat exchange between the fluid mixture and the external environment through the second channels, as the fluid mixture flows through the second channels from the second inlet header to the second outlet header, the second outlet header defining a second outlet opening for providing the second condensate portion to a drain line, and the second outlet header defining a vent opening for transmitting a portion of the fluid mixture to a vent line. the second stage condenser including the second inlet header, a second outlet header, and second channels in fluid communication between the second inlet header and the second outlet header, the second inlet header defining a second inlet opening in fluid communication with the first outlet opening for receiving the fluid mixture from the first outlet header, the second channels being configured to condense a second condensate portion from the first gas, based on heat exchange between the fluid mixture and the external environment through the second channels, as the fluid mixture flows through the second channels from the second inlet header to the second outlet header, the second outlet header defining a second outlet opening for providing the second condensate portion to a drain line, and the second outlet header defining a vent opening for transmitting a portion of the fluid mixture to a vent line.