Patent Number: 052271242
Section: summary

FIELD OF THE INVENTION The present invention relates to a method and a structure for preventive maintenance of an elongated metal hollow member welded to a reactor pressure vessel (RPV) in operation, and more particularly, to a preventive maintenance method and a structure therefor which are capable of preventing occurrence of any stress corrosion cracking in a weld of a metal hollow member during operation, which cracking allows a coolant to leak out of the RPV. BACKGROUND OF THE INVENTION Examples of this kind of hollow member include an ICM housing for an incore monitor which serves to monitor the neutron flux generating in the RPV, and a control rod drive (CRD) housing for a control-rod drive which serves to drive the control rod. The hollow member, e.g. the ICM housing, extends through a wall of the RPV into the coolant while it is welded to the wall of the RPV through a padding weld formed on an inner surface of the RPV wall. Such ICM housing is made of SUS304 stainless steel. In general, there exists in the weld a welding residual stress resulting from the welding heat input. Under the presence of the welding residual stress, corrosion proceeds markedly at the weld of the ICM housing in the coolant, resulting in cracking. This phenomenon is referred to as the stress corrosion cracking. Upon the occurrence of the stress corrosion cracking in the weld of the ICM housing, there is the possibility that the coolant leaks out of the RPV. Once the stress corrosion cracking has occurred, repair may be conducted by a known method in which a shielding member is welded to the hollow member to surround the stress corrosion cracked portion as disclosed in JP-U-56-82696. However, this kind of repairing method will add another new weld which is in danger of stress corrosion cracking. Further, this repairing method cannot be applied until the position where the stress corrosion cracking occurs is identified, that is, until the leakage of the coolant is detected. Therefore, it is impossible to prevent the leakage of the coolant or the occurrence of the stress corrosion cracking. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a preventive maintenance method and a structure therefor which are capable of preventing occurrence of any stress corrosion cracking in a metal hollow member in an RPV. To this end, according to one aspect of the present invention, there is provided a method for preventive maintenance an elongated metal hollow member welded to a wall of a pressure vessel of a nuclear reactor through a padding weld formed on the wall and, extending into a coolant within the pressure vessel, with the method comprising the steps of: removing the coolant out of the hollow member; smoothing an inner peripheral surface of a wall portion of the hollow member corresponding to the padding weld; locating a metal sleeve onto the inner peripheral surface of the wall portion in coaxial relation; fitting the metal sleeve onto the inner peripheral surface of the wall portion along a whole length of the metal sleeve; and heating the metal sleeve throughout to provide a molten metal portion penetrating into both of the wall portion and the metal sleeve, in which molten metal portion .delta. ferrite is produced. Further, according to another aspect of the present invention, there is provided a preventive maintenance structure of an elongated metal hollow member welded to a wall of a pressure vessel of a nuclear reactor through a padding weld formed on the wall, with the metal hollow member comprising a molten metal portion at a portion of an inner peripheral surface thereof which corresponds to the padding weld, the molten metal portion penetrating into both of the wall portion and the metal sleeve and including .delta. ferrite. Other objects, functions and effects of the present invention will become more clear from the description of preferred embodiments to be described below in detail with reference to the accompanying drawings.