A make-up water system for separating and removing dissolved gas from makeup water in a water-cooled nuclear reactor, the system comprising: a storage tank containing the makeup water which includes the dissolved gas; a pump positioned downstream of the storage tank; a discharge line to connect the storage tank to the pump, the discharge line further comprising, downstream of the pump: a first control valve; and an outlet; a bypass line connected to the discharge line downstream of the pump and upstream of the first control valve; a membrane system positioned in the bypass line having an inlet and an outlet, to receive the makeup water with the dissolved gas into the inlet, and to at least partially remove the dissolved gas from the makeup water to produce treated makeup water that is discharged through the outlet of the membrane system; a second control valve positioned in the bypass line downstream of the membrane system, wherein the bypass line connects back to the discharge line downstream of the second control valve; a recirculation line connected to the bypass line downstream of the membrane system and upstream of the second control valve; and a third control valve positioned in the recirculation line, wherein, when the make-up water system is in a standby alignment, the first and second control valves (3,2) are in a closed position and the third control valve is in an open position, and the recirculation line transfers the treated makeup water back to the storage tank, and wherein, when the make-up water system is in an activated alignment, the first and third control valves (3,1) are in a closed position and the second control valve is in an open position, and the treated makeup water is transported through the outlet of the discharge line to an end-user system or component in the water-cooled nuclear reactor. a storage tank containing the makeup water which includes the dissolved gas; a pump positioned downstream of the storage tank; a discharge line to connect the storage tank to the pump, the discharge line further comprising, downstream of the pump: a first control valve; and an outlet; a first control valve; and an outlet; a bypass line connected to the discharge line downstream of the pump and upstream of the first control valve; a membrane system positioned in the bypass line having an inlet and an outlet, to receive the makeup water with the dissolved gas into the inlet, and to at least partially remove the dissolved gas from the makeup water to produce treated makeup water that is discharged through the outlet of the membrane system; a second control valve positioned in the bypass line downstream of the membrane system, wherein the bypass line connects back to the discharge line downstream of the second control valve; a recirculation line connected to the bypass line downstream of the membrane system and upstream of the second control valve; and a third control valve positioned in the recirculation line, wherein, when the make-up water system is in a standby alignment, the first and second control valves (3,2) are in a closed position and the third control valve is in an open position, and the recirculation line transfers the treated makeup water back to the storage tank, and wherein, when the make-up water system is in an activated alignment, the first and third control valves (3,1) are in a closed position and the second control valve is in an open position, and the treated makeup water is transported through the outlet of the discharge line to an end-user system or component in the water-cooled nuclear reactor.