Patent Number: 060581530
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a preventive maintenance apparatus for structural members in a nuclear pressure vessel and, more particularly, to a preventive maintenance apparatus for structural members in a nuclear pressure vessel capable of preventing occurrence of stress corrosion cracks of structural members by adding compressive remaining stress to surfaces of the structural members. The present invention relates to an apparatus of preventive maintenance for structural members in a nuclear pressure vessel suitable for adding compressive residual stress to a surface of a welded portion and a heat affected zone in each of core internals of, preferably, a boiling water reactor (BWR) such as a core shroud, a shroud support cylinder, a shroud support leg, a shroud support plate and a jet pump diffuser. Japanese Patent Application Laid-Open No.62-63614 discloses a method of releasing tensile remaining stress in a welded portion which may become a cause of occurrence of stress corrosion cracks. In the method, a high pressure water shot peening apparatus is inserted inside of a heat transfer tube of a heat exchanger to peen an inner surface of the heat transfer tube by axial kinetic pressure energy of a high pressure water jet (kinetic pressure energy of a confined water jet in the axial direction). Tensile remaining stress having existed near the inner surface of the heat transfer tube is converted into compressive remaining stress by the peening. The high pressure water shot peening apparatus comprises a rotating nozzle portion for discharging a high pressure liquid jet. Further, Japanese Patent Application Laid-Open No.5-78738 discloses an improving method of converting tensile remaining stress on a surface of a core shroud in a reactor pressure vessel into compressive remaining stress by water jet peening. The water jet peening is performed by arranging a traveling cart mounting a vertical driving apparatus on a flange in a top end portion of the reactor pressure vessel. An upper mast and a lower mast having a water jet discharging head in the top end are mounted onto the vertical driving apparatus. A high pressure water jet is discharged from a water jet discharging nozzle of the water jet discharging head to generate cavitation. Air bubbles generated by the cavitation are hit on a surface of the shroud. Furthermore, Japanese Patent Application Laid-Open No.7-270591 discloses a method in which preventive maintenance apparatuses comprising a nozzle unit having an upper attachment, a lower attachment and a drive mechanism for a discharging nozzle and a main apparatus body are arranged in a top end portion of a CRD housing and a lower core support plate inside a reactor pressure vessel to generate cavitation bubbles by discharging a high pressure jet from the discharging nozzle. The method also discloses a method of improving remaining stress by water jet peening. The cavitation bubbles are hit onto the surfaces of a lower barrel of the core shroud, a core shroud support cylinder and so on. Tensile remaining stress in the surfaces of the lower barrel of the core shroud, the core shroud support cylinder and so on is converted to compressive remaining stress. The method of the prior art disclosed in Japanese Patent Application Laid-Open No.62-63614 is effective as a method of releasing the remaining stress in a heat exchanger and the like. The axial kinetic pressure of the water jet in this method can be effectively used in the work under atmospheric pressure. However, when the high pressure shot peening apparatus of the prior art is used under water, an effective peening effect cannot be obtained because the axial kinetic pressure of the water jet is substantially decayed under water. In order to obtain an axial kinetic pressure equivalent to that under a condition of air atmosphere under a condition of water using the high pressure shot peening apparatus, a water jet of ultra high pressure discharge is necessary. Accordingly, the pump and the related components used need to have structures capable of withstanding the ultra high pressure. In order to avoid such structures, it is required to discharge the high pressure liquid jet under air atmosphere by lowering a core water level inside the reactor pressure vessel. Since lowering of the core water level causes an increase in the environmental radiation dose, radiation exposure to workers may be increased. On the other hand, the method of improving remaining stress by the water jet peening disclosed in Japanese Patent Application Laid-Open No.5-78738 is effective as a method of improving remaining stress in core internals such as a core shroud. However, since the traveling cart having the mast is placed on the top end portion of the reactor pressure vessel, the mast becomes long in order to apply the method of improving remaining stress by water jet peening to the lower barrel of the core shroud, the core shroud support cylinder and so on. In addition to this, the apparatus is difficult to be handled. It cannot be said that this is preferable from the viewpoint of workability. The method of improving remaining stress by the water jet peening disclosed in Japanese Patent Application Laid-Open No.7-270591 is an effective technology aiming to improve the workability which is the problem in the method of improving remaining stress described in Japanese Patent Application Laid-Open No.5-78738 since the apparatus does not have any long mast. However, application of the method in Japanese Patent Application Laid-Open No.7-270591 is limited within a small field of preventive maintenance work since the preventive maintenance apparatus is attached to the top end portion of the CRD housing and the lower core support plate inside the reactor pressure vessel. Therefore, it is necessary that the preventive maintenance apparatus is detached and moved from one CRD housing after completion of the preventive maintenance work to a portion existing in the inner surface of the core shroud to be set to another CRD housing. SUMMARY OF THE INVENTION An object of the present invention is to provide a preventive maintenance apparatus for structural members in a reactor pressure vessel which is capable of being easily arranged on a core shroud and easily moving a discharging nozzle to a portion to perform preventive maintenance. A first invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a ring-shaped guide rail having a plurality of lugs, the guide rail being placed on an upper flange of a core shroud provided inside a reactor pressure vessel, at least of the plurality of lugs engaging a plurality of guide rods provided on an inner surface of the reactor pressure vessel; a turntable rotatable on the guide rail; a discharging nozzle moving apparatus for moving a discharging nozzle in a radial direction of the core shroud and in an axial direction of the core shroud, the discharging nozzle moving apparatus being placed on the turntable; and a high pressure water supply apparatus for supplying high pressure water to the discharging nozzle. Since the guide rail has the plurality of lugs engaging with the plurality of guide rods provided in the inner surface of the reactor pressure vessel, the guide rail can be easily moved downward up to the upper portion of the core shroud along the guide rods. Therefore, the guide rail can be easily placed on the upper flange without being interfered with main steam line plugs which are inserted into opening portions of main steam pipes. In addition to this, since the turntable can be rotated, the discharging nozzle can be easily moved to a portion to perform preventive maintenance. Since the turntable is rotated on the guide rail placed on the upper flange, the turntable does not contact the upper flange. Accordingly, the upper flange can be prevented from being damaged by the rotation of the turntable. A second invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a first discharging nozzle moving apparatus for moving a discharging nozzle in a radial direction of the core shroud and in an axial direction of the core shroud, the discharging nozzle discharging high pressure water to add compressive remaining stress to an outer surface of the core shroud, the discharging nozzle moving apparatus being placed on the turntable; and a second discharging nozzle moving apparatus for moving a discharging nozzle in a radial direction of the core shroud and in an axial direction of the core shroud, the discharging nozzle discharging high pressure water to add compressive remaining stress to an inner surface of the core shroud, the discharging nozzle moving apparatus being placed on the turntable. Since the first and the second discharging nozzle moving apparatuses are installed in the turntable, compressive remaining stress can be added to both of the outer surface and the inner surface of the core shroud. Therefore, it is possible to shorten the time for performing preventive maintenance to the core shroud. A third invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel in which the second discharging nozzle moving apparatus comprises an arm member movable in a horizontal direction; a pole member movable in an axial direction of the core shroud provided in the arm member; a multi-joint arm attached to the pole member; and the discharging nozzle provided in a top end portion of the multi-joint arm. Since the multi-joint arm is provided, it is possible to insert the discharging nozzle into a narrow portion formed between the core shroud and an upper core grid plate placed on the core shroud. Therefore, compressive remaining stress can be added to the inner surface of the core shroud in the narrow portion. A fourth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel in which the first discharging nozzle moving apparatus comprises an arm member movable in a horizontal direction; a pole member movable in an axial direction of the core shroud provided in the arm member, the pole member being inserted between the reactor pressure vessel and the core shroud; a vertically moved body attached to the pole member, the vertically moved body being movable in a vertical direction; and the discharging nozzle provided in a top end portion of the vertically moved body. Since the vertically moved body having the discharging nozzle can be vertically moved along the pole member, it is possible to easily add compressive remaining stress to a welded portion of the core shroud and the vicinity. A fifth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a water supply apparatus for cleaning reactor water and supplying the water to a high pressure water supply apparatus. Since the reactor water is cleaned to be supplied to the high pressure supply apparatus, the water discharged from the discharging nozzle becomes the reactor water again. Accordingly, an amount of the reactor water inside the reactor pressure vessel and the reactor well is never increased even when the high pressure water is discharged from the discharging nozzle during preventive maintenance work. Therefore, radioactive disposal liquid cannot be produced even when the high pressure water is discharged from the discharging nozzle. A sixth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a crud sucking apparatus for sucking crud suspending in reactor water. Since it is possible to remove crud suspended in the reactor water during preventive maintenance work, visibility under the reactor water can be improved. Therefore, it is possible to clearly monitor a portion under preventive maintenance work using an image in a monitoring camera. A seventh invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a bubble collecting apparatus for collecting bubbles reaching a water surface in a reactor well, the bubble collecting apparatus being placed near the water surface. Since the bubble collecting apparatus is provided, it is possible to prevent radioactive materials floating up in the reactor water accompanied by the bubbles from being dispersed. Therefore, it is possible to suppress radiation exposure to workers. An eighth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel in which the first discharging nozzle moving apparatus comprises an arm member movable in a horizontal direction; a plurality of pole members provided in the arm member, the pole member being inserted between the reactor pressure vessel and the core shroud; vertically moved bodies respectively attached to the pole members, the vertically moved body being movable in a vertical direction; and the discharging nozzles respectively provided in the vertically moved bodies. Since the vertically moved bodies capable of respectively and vertically moving the plurality of pole members are provided, it is possible to perform preventive maintenance work to different positions on the outer surface of the core shroud at the same time. Therefore, it is possible to further shorten the time required for the preventive maintenance work. A ninth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel which comprises a rotating apparatus for rotating a metal fitting for bundling a plurality of hoses and a plurality of cables, the plurality of hoses and the plurality of cables being connected to the first discharging nozzle moving apparatus and the second discharging nozzle moving apparatus. Since the rotating apparatus for rotating the metal fitting for bundling the plurality of hoses and the plurality of cables is provided, the rotating apparatus can be rotated when the turntable mounting the first discharging nozzle moving apparatus and the second discharging nozzle moving apparatus is rotated during preventive maintenance work. Therefore, it is possible to prevent the plurality of hoses and the plurality of cables from being intertwined by rotation of the turntable. A tenth invention to attain the above-mentioned object is characterized by a preventive maintenance apparatus for structural members inside a reactor pressure vessel in which the first discharging nozzle moving apparatus and the second discharging nozzle moving apparatus respectively comprise a discharging nozzle for discharging high pressure water for adding compressive remaining stress to an outer surface of the core shroud and a discharging nozzle for discharging high pressure water for adding compressive remaining stress to an inner surface of the core shroud, and the preventive maintenance apparatus further comprises an apparatus for moving the discharging nozzles. Since the first discharging nozzle moving apparatus and the second discharging nozzle moving apparatus respectively comprise the discharging nozzle for discharging high pressure water for adding compressive remaining stress to an outer surface of the core shroud and the discharging nozzle for discharging high pressure water for adding compressive remaining stress to an inner surface of the core shroud, it is possible to perform preventive maintenance work to four positions in the inner and outer surfaces of the core shroud at the same time. Therefore, it is possible to substantially shorten the time required for the preventive maintenance work to the core shroud.