Abstract: A depressurization system (1) for pressurized steam operated plant employing injection of cold water contained in a superelevated tank (5), said injection being triggered rapidly and in a manner independent of the magnitude of the damage and of the presence of incondensible gases, by means of a natural circulation of steam in a closed loop of pipes (19, 20, 21, 22) around said plant (2, 3, 4), caused by the condensation of steam in a condenser (10), placed on said pipes, during the occurence of an accident, comprises a first siphon-type hydraulic shutoff (32), on said pipe loop (19, 20, 21, 22) and arranged close to the inlet collector (14) of said condenser (10), a second siphon-type hydraulic shutoff (29), on the cold water delivery pipe (26) and placed close to the pressurized steam operated plant (2, 3, 4), said second siphon-type hydraulic shutoff (29) being below said first siphon-type hydraulic shutoff (32).