Patent Number: 052672794
Section: summary

FIELD OF THE INVENTION The present invention relates to a method and a structure for repairing an elongated metal hollow member which is welded to a reactor pressure vessel (RPV) in operation, and more particularly, to a repairing 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 a housing for an incore monitor (ICM) which serves to monitor the neutron flux generating in the RPV, and a housing for a control-rod drive (CRD) 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 cladding weld formed on an inner surface of the RPV wall. Such ICM housing is made of TYPE 304 stainless steel. In general, a welding residual stress exists in the weld as a result of 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. On the contrary, as disclosed in JP-A-2-128195, an ICM housing lower half including a part thereof in which the stress corrosion cracking has occurred is cut and removed to replace it with a new ICM housing half welded to the remaining housing half. However, these repairing methods will make another new weld which presents a danger of stress corrosion cracking. SUMMARY OF THE INVENTION An object of the present invention is to provide a repairing 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 repairing an elongated metal hollow member which is welded to a wall of a pressure vessel of a nuclear reactor and extends into a coolant within said pressure vessel, said method comprising the steps of removing said coolant out of said hollow member, cutting and removing a part of the hollow member, including a portion to be repaired, welding a new hollow member element with a remaining part of the hollow member into a new hollow member through a weld. An inner peripheral surface of a wall portion of the new hollow member including the weld is smoothed, and a metal sleeve is located on the inner peripheral surface of the wall portion in a coaxial relationship therewith. The metal sleeve is fitted onto the inner peripheral surface of the wall portion along an entire length of the metal sleeve, and the metal sleeve is heated throughout to provide a .delta. ferrite molten metal portion penetrating into both the wall portion and the metal sleeve. Further, according to another aspect of the present invention, there is provided a repairing structure of a part of an elongated metal hollow member, including a portion to be repaired, welded to a wall of a pressure vessel of a nuclear reactor, with the metal hollow member extending into a coolant within the pressure vessel. The structure comprises a molten metal portion at a portion of inner peripheral surface of a remaining part of the hollow member excluding the portion to be repaired and of a part of another hollow member, which members are welded together into a new hollow member through a weld. At the weld, the molten metal portion penetrates into both a wall portion of the new hollow member and of a metal sleeve, including .delta. ferrite, disposed on the inner peripheral surfaces. 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.