[en] In a nuclear reactor installation a live-steam line leading from each steam generator through the containment is equipped with a fast-acting valve. If the pressure in the generator rises the valve operates as a safety valve and releases up to one-half of the aperture cross section of the valve, so that there is no danger of damage to the steam generators. According to the invention, the valve is operated for this purpose by two pistons by means of steam. The invention is of interest particularly for light-water reactors, e.g., pressurized-water reactors