Nuclear reactor installation

In a nuclear reactor installation having a containment shell, a live steam line leading outwardly from the containment shell, a fast-acting shut-off valve connected in the steam line and having a flow cross section corresponding to the cross section of the steam line, the shut-off valve including a normally open main valve disk which is actuatable to close in response to pressure from a leak in the steam line, pressure dependent means associated with the main valve disk for sensing a pressure higher than operating pressure of the steam line, an opening mechanism comprising a piston-actuated auxiliary valve disk disposed in a cut-out formed in the main valve disk and having an area equal at most to one-half the flow cross-sectional area of the shut-off valve, said auxiliary valve disk being operable by said pressure-dependent means to open said cut-out opening when a pressure higher than operating pressure of the steam line is sensed by said pressure dependent means, a first piston connected to said main valve disk and a second piston connected to said auxiliary valve disk, said second piston being displaceably disposed within a cylinder formed of a first cylindrical portion of said first piston, pressure-relief means for controlling the operation of said first and second piston, said first piston having two oppositely acting piston surfaces of different size, said first piston being disposed within a second cylindrical portion of said fast acting shut-off valve, the second cylindrical portion forming the larger of the two piston surfaces, a third smaller cylindrical portion forming the smaller of the piston surfaces on the opposite side of said first piston, and including a plurality of passages through said shut-off valve connecting said first, second and third cylindrical portions with said steam line.