Patent Number: 056549920
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of repairing a structure or a component composing internals of a nuclear reactor pressure vessel in a nuclear power plant in service and, more particularly, to a method of repairing a structure or a component irradiated with neutrons and having a crack defect by covering the defect portion of the structure with a cover plate and welding the structure and the cover plate to realize a high reliable repair for the integrity after the repair. It is worried to occur a crack defect such as a stress corrosion cracking in a structure or a component under a high temperature and high pressure environment in a reactor pressure vessel as time passes. The stress corrosion cracking occurs in a superposed condition of a deterioration factor such as local change in the composition of a material itself, a stress factor of tensile remaining stress applied to the structure due to welding or the like and a corrosion environment factor under a high temperature and high pressure environment, and the crack grows. When the crack grows through the structure or the component, a serious accident may occur in the nuclear plant by some possibility. Therefore, a repairing technology to prevent a through-crack in the structure having the crack is necessary. As for such a repairing technology to prevent a through-crack, there is a repairing method shown in FIG. 19a, wherein the growth of crack is prevented by covering a region containing a crack 1 with a plate 3 and by filler-welding the edge portion of the plate 3 and the structure 2 utilizing heat input generated by arc or welding arc 6 while a filler metal 5 is being added to isolate the crack 1 from the corrosion environment. However, the structure irradiated with high energy particle rays such as .alpha.-rays, .beta.-rays, neutron rays or the like contains He generated by nuclear transformation of the component elements of the structure. In a case wherein a structure of such a material containing He is covered with a plate and the edge of the plate and the structure are filler-welded as described above, heat is inevitably input large enough to melt the filler material, a part of the base material and a part of the plate in a conventional welding technology such as arc welding. Therefore, when the structure 2 irradiated with neutrons and having a crack 1 is repaired by filler-welding the cover plate 3 and the structure 2 as shown in FIG. 19a, the heat affected zone 8 due to the weld in the structure around the filler-welded portion 7 of the repaired portion possibly may become a new heat affected zone to produce a crack 9, as shown in FIG. 19b. The above problem for austenitic stainless steel is described in, for example, Journal of Material Science, Vol. 26(1991), pp 2063-2070. The mechanism of occurrence of a crack is that when welding work is performed with adding heat on the above material of an alloy containing generated He and having a cumulative total amount of neutron irradiation more than 1.0.times.10.sup.20 n/m.sup.2, the He easily diffuses to the crystal grain boundaries of the material by thermal activation due to heating of the welding heat affected zone near the welded portion, and the He voids collected in the grain boundaries gathers to form bubbles having sizes of .mu.m order to decrease the strength of grain boundaries of the material. Further when a tensile stress due to solidification and shrinkage is added to the material after the welding, cracks occur in the grain boundaries in the heat affected zone of non-melted portion. As for repairing of a structure with a crack which is the object of the present invention, in the repairing method according to the conventional technology of covering the structure with a plate and filler-welding it to the structure, the plate and the structure are closely contacted so that the side surface of the edge portion of the plate and the surface of the structure are in a face-contact state, and the filler wire, the side surface of the edge portion of a plate and the surface of the structure are melted to be welded while the filler wire is being fed and thermal energy is being input. Therefore, there are some cases where the amount of heat input added to the structure becomes locally high. Under such a welding condition, there is a possibility that the He easily diffuses to the crystal grain boundaries of the material by thermal activation due to heating of the welding heat affected zone, and the heat affected zone in the structure becomes a new crack generating portion. There are various disclosures concerning relations between welding of steel containing He and cracking. They are as follows: Metallurgical Transaction A. Vol. 21A (1990/9) pp. 2585-2596 [H. T. Lin etc.], which discloses that when GTA arc welding is applied to He containing stainless steel, intergranular cracking occurs in a welding heat affected zone; decrease in welding heat input tends to decrease in cracking sensibility. Welding Journal (1991/5) pp. 123-132 [S. H. Goods etc.], which discloses relations between He amount and cracking property when large heat input GMA welding is applied to 304 stainless steel containing He, wherein as the heat input becomes smaller, scale of cracking becomes small, as in FIG. 12. Metallurgical Transaction A Vol. 23A (1992/5) pp. 1021-1032 [S. H. Goods etc.], which discloses welding cracking of irradiated material is due to formation/growth/joining of He bubbles, and wherein mechanism of cracking caused by He is discussed. Welding Journal (1992/4) pp. 43-51 [E. A. Franco-Ferreira etc.], which discloses relations between welding heat input and cracking in case various He amounts are taken in GTA/GMA welding. DP-MS-89-41 (DE90-001312) Herium Induced Weld Cracking In Irradiated 304 Stainless Steel (U) by A. K. Birchenall, which discloses a relation between welding heat input and cracking in case various He amount are taken. SUMMARY OF THE INVENTION The object of the present invention is to provide a repairing method securing the integrity of repaired portion with considering the problems in the conventional repairing method described above when repairing is performed on a crack such as a stress corrosion cracking occurred in a structure or a component made of stainless steel, a Ni based alloy or low alloy steel inside a reactor pressure vessel. The above object of the present invention can be attained by providing a method of repairing a structural material such as stainless steel, a Ni based alloy or low alloy steel of a nuclear reactor internals having a crack and irradiated with .alpha.-rays, .beta.-rays and neutron rays to the irradiation amount of 0 to 5.0.times.10.sup.27 n/m.sup.2 in a nuclear power plant in service, the method of repairing a structural material of a nuclear reactor internals comprising the steps of covering a region of the structural material having a crack defect with a plate to cover, welding the plate and the structural material to be repaired by locally applying pressure on the surface of the plate and adding energy to the portion to which the pressure is applied thereby to generate thermal energy in the contact surface between the plate and the structural material. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, as the above step of welding the plate and the repaired structural material, by welding the plate and the structural material by heating with ohmic heat generated, by allowing current to flow the portion to be repaired while applying pressure on the surface of the plate. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, as the above step of welding the plate and the structural material by heating with ohmic heat generated, by seam-welding the cover plate to the structural material using a roller electrode. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, as the above-mentioned welding of the plate and the structural material by heating with ohmic heat generated, by resistance-spot-welding the cover plate to the structural material using a roller electrode. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized in that the above-mentioned resistance-spot-welding of the plate and the repaired structural material by heating with ohmic heat generated is carried out by welding the structure and the cover plate at a plurality of positions at a time by allowing current to flow in a plurality of electrodes. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized in that the above mentioned welding of the plate and the repaired structural material is performed by generating thermal energy caused by frictional resistance by mechanically rubbing the contact surface between the structural material and the cover plate. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized in that the above mentioned welding of the plate and the repaired structural material by generating thermal energy caused by frictional resistance is performed by giving mechanical vibration while applying pressure on the portion to be welded to mechanically rub the contact surface. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized in that the above mentioned welding of the plate and the structural material by generating thermal energy caused by frictional resistance by giving mechanical vibration is performed by mechanically rubbing the contact surface by giving mechanical vibration obtained by converting high frequency energy into mechanical vibration through magneto-striction to the portion to be welded. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, in the above repairing work, by providing convex projections on the surfaces of the cover plate as welding surfaces. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized by providing concave notches on the positions of the structural material contacting to the convex projections on the cover plate, the cover plate being placed and welded to the structural material by engaging the convex projections on the welding surface of the cover plate with the concave notches. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, in the above repairing work, by using a reactor internal structure or component as a supporting portion for means for offsetting the reaction force produced when the pressure is applied. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, as means for offsetting the reaction force produced when the pressure is applied, by introducing a supporting pillar between an upper grid plate and a core support plate of a light water reactor internals, the pillar supported with the upper grid plate and the core support plate being used as a supporting portion for means for offsetting the reaction force produced when the pressure is applied. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, as means for offsetting the reaction force produced when the pressure is applied, by introducing a supporting pillar between a pressure vessel and a core shroud, the pillar supported with the inner surface of the pressure vessel being used as a supporting portion for means for offsetting the reaction force produced when the pressure is applied. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, in repairing work performing the above work to a structure of reactor internals contacting to the reactor water, by performing the above welding work by removing the oxide layer existing on a region including the weld surface of the position of the structural material to be repaired with the cover plate before placing the cover plate on a position of the structural material to be repaired, and then placing the cover plate. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, in repairing work performing the above work to a structure of reactor internals contacting to the reactor water, by performing the work for removing the oxide layer existing on a region including the weld surface of the position of the structural material to be repaired with the cover plate, performing surface finishing of the region including the weld surface of the position of the structural material to an average roughness of 0.2 to 10 .mu.m, after performing the surface finishing, placing the cover plate and welding the plate to the position of the structural material. The present invention provides a method of repairing a structural material of a nuclear reactor internals, which is characterized, in the above repairing work, by metallurgically welding a part of the outer periphery or the whole periphery in the contact surface of the cover plate and the repaired structural material. The present invention provides an apparatus of repairing a structural material of a nuclear reactor internals, which is characterized by comprising means for offsetting the reaction force produced when the pressure is applied in order to perform the above work. The present invention provides an apparatus of repairing a structural material of a nuclear reactor internals, as means for offsetting the reaction force produced when the pressure is applied in order to perform the above work, which is characterized by comprising pressure supporting mechanism supported by an upper grid plate and a core support plate of a light water reactor internals as a means for applying the pressure. The present invention provides an apparatus of repairing a structural material of a nuclear reactor internals, as means for offsetting the reaction force produced when the pressure is applied in order to perform the above work, which is characterized by comprising a pressure supporting mechanism supported by the inner surface of a pressure vessel as a means for applying the pressure.