Patent Number: 046876251
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 shows one embodiment of the invention, in which a main steam line 2 extends through a containment vessel 3 of a nuclear reactor. Usually, welds of the main steam line 2, including butt welded joints between pipe and pipe and between pipe and shut-off valve, have a higher possibility of crack growth in comparision with the parent metal because of defects in weld produced on welding, thermal influences exerted on welding and the difference in mechanical properties between the welded metal and the parent metal. Thus, steam leaks have tended to occur mainly in these welded portions. Therefore, in this embodiment, a plurality of isolating vessels 7 are provided to cover welded portions of the main steam pipe 2 which is located between an outer wall surface of the containment vessel 3 and a portion including an outer main steam shut-off valve 5 provided at the outside of the containment vessel 3 to isolate them from the atmosphere. The provision of the isolating vessel 7 enables a steam leak through the weld of the main steam pipe 2 to be confined in the isolating vessel 7, thereby preventing the steam from spreading to the atmosphere. The isolating vessel 7 may be equipped with a steam leak detectable transducer 8 such as a moisture detector. The provision of a steam detectable transducer enables a steam leak to be detected quickly and permits the plant to be shut down earlier than would be the case if no transducer were provided, thereby increasing safety and reliability. FIG. 4 shows another embodiment of the invention, in which the entire outer surface of a portion of the main steam pipe 2 which is located between the outer wall surface of the containment vessel 3 and a portion including the outer main steam shut-off valve 5 is covered with one isolating vessel 7 which is equipped with a steam detectable transducer 8. A space defined between the isolating vessel 7 and the main steam pipe 2 may be communicated with the interior of the containment vessel 3. By this arrangement, leaked steam can be confined in the isolating vessel 7 and is prevented from spreading to the atmosphere in the same way as the aforementioned embodiment even if leaks occur at other portions than the abovementioned welded portions of the main steam pipe 2 which is located between the outer wall surface of the containment vessel 3 and the portion including the outer main steam shut-off valves 5. In a containment vessel of the type in which more than two main steam pipes 2 of a plurality of systems are located not so far apart from each other, portions of the main steam pipes 2 which are located between the outer wall surface of the containment vessel 3 and portions including the respective outer main steam shut-off valves 5 may be covered with one isolating vessel 7. In this case, the interior of the isolating vessel 7 may be divided into sections according to the respective main steam pipes 2, and each section may be provided with a steam leak transducer 8. FIG. 5 shows another embodiment of the invention in which an isolating member 7 including a bellows structure having flexibility covers the entire outer surface of a portion of the main steam pipe 2 which is located between the outer wall surface of the containment vessel 3 and the portion including the outer main steam shut-off valve 5. In normal operations, the main steam pipe 2 rises in temperature to about 270.degree. C. Therefore, if the isolating vessel 7 were formed only of rigid material and firmly fixed to the main steam pipe 2, inordinately high thermal stresses might occur in the main steam pipe 2. To avoid this trouble, the embodiment shown in FIG. 5 provides the isolating vessel 7 with a bellows construction. The invention is not limited to this specific form of isolating vessel 7 and the isolating vessel 7 may be formed entirely of a bellows structure. The embodiments of the invention shown and described hereinabove provide at least one isolating vessel 7 to cover the portion of the main steam pipe 2 located outside the containment vessel 3. Thus, even if a steam leak occurs in any portions of the main steam pipe 2 which are located between the outer wall surface of the containment vessel 3 and the portion including the outer main steam shut-off valve 5, the radioactive steam is prevented from spreading to the atmosphere. This eliminates the need to mount the inner main steam shut-off valve 4 usually provided at the inside of the containment vessel of the prior art, thereby offering the following advantages: The main steam shut-off valves which are usually about 4 m in length are mounted obliquely to the main steam pipe. Thus, each main steam shut-off valve is dimensioned such that it extends about 3 m both vertically and horizontally. If the inner main steam shut-off valve can be omitted, then the diameter of the containment vessel at its bottom can be reduced from the range between 25 and 29 m in the prior art to the range between about 20 and 24 m, thereby enabling a compact size and a light weight to be obtained in a containment vessel. This advantage will be further described by referring to the ABWR shown in FIG. 6. The ABWR comprises the isolating vessel 7 mounted to the portion of the main steam pipe 2 which is located between the outer wall surface of the containment vessel 3 and the portion including the outer main steam shut-off valve 5, and the inner main steam shutoff valve 4 which has hitherto been required in a containment vessel of the prior art as shown in FIG. 2 is omitted. By providing the main steam pipe 2 with the isolating vessel 7 and eliminating the inner main steam shut-off valve 4, the diameter of the containment vessel 3 at its bottom can be reduced to a diameter smaller than the diameter of a containment vessel of the prior art having the inner main steam shut-off valve, thereby enabling the building cost to be greatly reduced. The invention provides a solution to the problem which the containment vessel of the prior art was faced with that when a steam leak occurs in the portion of the main steam pipe which is located between the outer wall surface of the containment vessel and the portion including the outer main steam shut-off valve, the steam leak would be released to the atmosphere before the inner main steam shut-off valve is closed. The invention is capable of avoiding the spread of radioactive materials to the atmos- phere. The elimination of the inner main steam shut-off valve enables the size of the containment vessel of a nuclear reactor to be reduced, thereby contributing to a reduction in building cost. Further, leaked steam is trapped in the isolating vessel, the detection of the leaked steam becomes fast and accurate by providing a steam leak detectable transducer and thus the invention can contribute to the emergency stop of the plant. While the invention has been shown and described by referring to a main steam pipe extending from the containment vessel of a nuclear reactor to outside through an aperture formed in the wall of the containment vessel, it is to be understood that the invention is not limited to the main steam pipe and can have application any pipes connected to the containment vessel. As described hereinabove, the piping of the containment vessel includes a multiplicity of pipes including a feedwater pipe, a main steam drain pipe, reactor core spray water pipes, a residual heat removing pipe, a drywell ventilating pipe, an off-gas pipe, etc., and the invention may have application in any one of these pipes.