Patent Number: 056235267
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors. More specifically, the present invention relates to a technique which enables the repair and/or reinforcement of shrouds which have developed cracks or the like types of faults. 2. Brief Description of the Related Art Nuclear reactors such as a boiling water type (BWR) 10 shown in FIG. 1, enclose a plurality of spaced fuel rods, generally denoted by the numeral 12, within a shroud 14 which is supported within the reactor pressure vessel (RPV) 16. The shroud 12 locates on the core support plate 20 and the top guide 32. It also supports a separator arrangement 22 and maintains a volume of water over the fuel 12. In this arrangement, a plurality of control rods 18 are disposed below the fuel rods 12 and are arranged to move up through the core support plate 20 into position to control the combustion of the nuclear fuel. The separator arrangement 22 is located on top of the shroud 22, while a dryer 24 is arranged at a still higher level and essentially opposite a main steam line 26. A pressure vessel head 28 is secured to the upper flange of the reactor pressure vessel in a manner which hermetically seals off the top of the RPV 16. As shown in FIG. 2, the shroud 14 is constructed of a number of cylindrical and/or annular sections which are welded together. In the shroud shown in FIG. 2, only seven (H1 to H7) of the welds, which form essential parts of the shroud structure, are shown. However, it is possible that, due to intergrannular stress corrosion cracking (IGSCC) and other metallurgical phenomenon, cracks can develop in the shroud, particularly at the sites of the welds, and lead to a situation wherein portions of the shroud can separate in an undesirable manner which may interfere with the proper operation of the reactor. Repair of such types of cracks is of course difficult and usually requires the fuel (fuel rods 12) to be offloaded and the repair carried out with the fuel stored in a separate holding pool. However, as will De appreciated, in order for the fuel rods 12 to be released, lifted out of the reactor vessel and immersed in a holding tank, all of the apparatus which is disposed in the reactor above the fuel rods must first be removed and placed in holding tanks. Accordingly, the number of operations which must be carried out before the actual repair work can actually begin is substantial and the length of the outage of the reactor is prolonged considerably. Even in the case of relatively minor cracks, the above-mentioned troublesome removal of the dryer 24, the shroud head and separator arrangement 22, and the fuel rods 12 must be carried out before repair operations can be initiated, irrespective of the fact that the repair operation itself may be relatively simple and quickly carried out. SUMMARY OF THE INVENTION It is an object of the present invention to provide a technique which enables the repair of the shroud without the need to remove the fuel from the reactor. It is a further object of the invention to provide a simple apparatus which can be used to fortify or repair a nuclear reactor shroud while the fuel is still in the reactor pressure vessel. It is yet another object of the invention to provide a repair technique which involves the use of straps which are fastened to the external surface of the shroud in a manner which supports the shroud against bending forces, shearing forces, and torsion forces. In brief, the above mentioned objects and others are achieved by an arrangement wherein straps, which are formed of the same material as the shroud, are placed in strategic positions with respect to cracks or the like types of weaknesses which have been detected in the shroud, and fastened in place using a suitable fastening technique. In the preferred embodiments of the invention, holes are formed using an EDM technique and bolt units which have an expanding portion are inserted into the holes, torqued and expanded in a manner which fastens the strap to the shroud. Welding and the like type of fastening techniques are not excluded and may be used alternatively or in combination with the bolting technique as required. The length of the straps is variable and can be selectively varied so that a suitable number of welds are spanned and the required amount of support is provided. A plurality of straps can be used. The number of straps varies with the problem that needs solving. The separation or interval between the straps need not be uniform. For example, if four straps are used they need not necessarily be arranged uniformly at 90 degree intervals. In the event that a number of straps are used, the length of each strap can be varied as necessary under the instant set of conditions and the degree of support that is required to ensure that the shroud exhibits the desired degree of structural strength for horizontal, vertical and lateral welds. While it is preferable that the straps be made of the same material as the shroud, a different material can be used as long as the material is such as to not introduce corrosion or undergo thermal or radiation induced changes which endangers the support which is intended to be provided by the straps. Electron discharge machining (EDM) is used in connection with the preferred embodiments for removing metal and cutting holes and the like. This technique is favored in that tooling loads during cutting are negligible and chips and the like type of debris is not formed or, alternatively, is easily collected. The combination of the straps with other support structure is not excluded from the scope of the invention. This support structure can be arranged to be disposed either internally or externally of the shroud. For example, U.S. Pat. No. 5,430,779 to Baversten et al. issued on Jul. 4, 1995, and co-pending U.S. patent application Ser. No. 08/241,441 filed on May 11, 1994 in the name of Baversten, could be referred to for teachings pertaining to support structures which could be used in connection with the present invention. The disclosures of these two documents are hereby incorporated by reference. More specifically, a first aspect of the present invention resides in a nuclear reactor having an essentially cylindrically-shaped shroud formed of a plurality of annular segments which are welded to one another and which features: a strap which extends essentially parallel to an axis of said cylindrically-shaped shroud and which is securely fastened to an external surface of the shroud, said strap being arranged to span at least one of the welds of said shroud, said strap withstanding bending, shearing and tension forces which are applied to said shroud. A second aspect of the present invention resides in a method of repairing an essentially cylindrically-shaped shroud formed of a plurality of annular segments which are welded to one another, comprising the steps of: placing a strap against a predetermined portion of an external surface of the shroud wherein the strap spans at least one of the welds which secure the segments of the shroud together; forming a plurality of holes through the strap and the shroud; disposing fastening means through the plurality of holes to fasten the strap to the shroud and to render the strap integral with the shroud. A third aspect of the invention resides in a method of repairing a nuclear reactor which includes a plurality of fuel rods and a shroud which is disposed about the fuel rods, comprising the steps of: placing a strap against the external surface of a nuclear reactor shroud so as to span at least one weld formed in the shroud; and securing the strap to the shroud using fastening means. Another feature of the strap allows the fabrication of EDM support fasteners. The EDM operation is shortened through the use of the prelocated tooling positions. The strap is a long flat structure which may have a channel, H beam or other cross-sectional shape which develops the required strength.