Abstract:
A method of performing maintenance on a structural member inside a reactor pressure vessel includes lowering an annular guide rail having a second lug until the guide rail rests on an upper flange of a core shroud provided inside the reactor pressure vessel, detachably mounting the second lug of the guide rail to a first lug mounted on the core shroud, positioning a discharging nozzle at a position at which compressive remaining stress is added to a surface of the core shroud by moving the discharging nozzle in a radial direction and an axial direction of the core shroud by a discharging nozzle moving apparatus mounted on a turnable revolving on the guide rail and discharging water to add compressive remaining stress to a surface of the core shroud from the discharging nozzle.

Description:
This is a divisional application of U.S. Ser. No. 09/026,726, filed Feb. 20, 1998, now U.S. Pat. No. 6,058,153. 
    
    
     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 a preventive maintenance apparatus 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 one of the plurality of lugs engaging with 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 members 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become readily apparent from the following detailed description of the preferred embodiments thereof when read reference to the drawings in which: 
     FIG. 1 is a view showing the construction of a preferred embodiment of a preventive maintenance apparatus for structural members inside a reactor vessel in accordance with the present invention. 
     FIG. 2 is an enlarged view showing the vicinity of a core shroud shown in FIG.  1 . 
     FIG. 3 is an enlarged view showing a driving portion of a turntable shown in FIG.  1 . 
     FIG. 4 is an enlarged view showing a guide rail shown in FIG.  1 . 
     FIG. 5 is an enlarged view showing a discharging nozzle moving apparatus which is performing preventive maintenance work to an outer surface of the core shroud shown in FIG.  1 . 
     FIG. 6 is a transverse cross-sectional view showing the discharging nozzle moving apparatus shown in FIG.  5 . 
     FIG. 7 is an enlarged view showing the discharging nozzle moving apparatus which is performing preventive maintenance work to an inner surface of the core shroud shown in FIG.  1 . 
     FIG. 8 is a view showing the construction of another embodiment of a driving mechanism for a pole member of the discharging nozzle moving apparatus shown in FIG.  5 . 
     FIG. 9 is a view showing the construction of another embodiment of a preventive maintenance apparatus for structural members inside a reactor pressure vessel in accordance with the present invention. 
     FIG. 10 is a view showing the construction of another embodiment of a discharging nozzle moving apparatus used in a preventive maintenance apparatus for structural members inside a reactor pressure vessel in accordance with the present invention, wherein the preventive maintenance apparatus is performing preventive maintenance work to an outer surface of the core shroud. 
     FIG. 11 is a view showing the construction of another embodiment of a discharging nozzle moving apparatus used in a preventive maintenance apparatus for structural members inside reactor pressure vessel in accordance with the present invention, wherein the preventive maintenance apparatus is performing preventive maintenance work to an inner surface of the core shroud. 
     FIG. 12 is a view showing the construction of another embodiment of a preventive maintenance apparatus for structural members inside a reactor pressure vessel in accordance with the present invention. 
     FIG. 13 is a view showing the construction of another embodiment of a discharging nozzle moving apparatus used in a preventive maintenance apparatus for structural members inside a reactor pressure vessel in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of a preventive maintenance apparatus for structural members inside a reactor pressure vessel (hereinafter, simply referred to as “preventive maintenance apparatus for structural members”) in accordance with the present invention will be described below, referring to FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG.  6  and FIG.  7 . 
     A boiling water reactor (BWR) comprises a reactor pressure vessel  2 , a core shroud  3  installed inside the reactor pressure vessel  2 , an upper core grid plate  82  and a lower core support plate  121  installed inside the core shroud  3 , and jet pumps  123  arranged in a ring-shaped space  122  formed between the reactor pressure vessel  2  and the core shroud  3 . 
     The preventive maintenance apparatus for structural members in accordance with the present invention comprises a guide rail  5 , a turntable  6 , a control board  18 , a crud collecting apparatus  41 , discharging nozzle moving apparatuses  124  and  124 C, and a discharged water supply apparatus  125 . 
     Preventive maintenance work to structural members inside the reactor pressure vessel  2  using the preventive maintenance apparatus for structural members is performed while operation of the BWR is stopped. After stopping the operation of the BWR, a top head of a reactor container (dry-well) and a upper vessel head of the reactor pressure vessel  2 , not shown, are removed. A steam dryer assembly and a steam separator assembly installed inside the reactor pressure vessel  2  are also detached and removed out of the reactor pressure vessel  2 . A well  126  above the reactor pressure vessel  2  is filled with water. Before filling the well with water, in order to prevent water from entering into main steam pipes  129 , openings of the main steam pipes  129  to the reactor pressure vessel are sealed with main steam line plugs  130  as shown in FIG.  1 . All fuel assemblies, not shown, supported in the core by the upper core grid plate  82  and the lower core support plate  121  are removed out of the reactor pressure vessel  2  and temporarily stored inside a fuel storage pool  46 . 
     In a state that the guide rail  5  mounts the turntable  6  mounting the discharging nozzle moving apparatuses  124  and  124 C, the guide rail  5  is suspended by a ceiling crane, not shown, installed above an operating floor  21  inside the reactor building, and then placed on an upper flange  4  of the core shroud  3 . The guide rail  5  has a fixing lug  12  in an outer peripheral surface. The lug  12  is detachably fixed to a lug  13  for fixing a shroud head, not shown, attached to the outer peripheral surface of the upper flange  4  with a bolt  14 . The turntable  6  is placed so as to be rotated on the guide rail  5 . 
     The turntable  6  has a box  53  as shown in FIG. 3. A drive mechanism for rotating the turntable  6  is installed inside the box  53 . The drive mechanism comprises a motor  54 , gears  55 A,  55 B attached to a shaft  57  and a gear  56 . The gear  55 A is engaged with a gear  55 C attached to a rotating shaft of the motor  54 . The gear  55 B is engaged with the gear  56 . The gear  56  is attached to a shaft  58  of a wheel  16  rotatably attached to the turntable  6 . The wheel  16  runs on a rail  15  installed on an upper surface of the guide rail  5 . Since both of projecting portions  16 A provided in the inner side and the outer side of the wheel  16  interpose the rail  15 , the wheel  16  does not run off the rail  15  during rotating the turntable  6 . 
     The guide rail  5  has three bent members  59 A,  59 B and  59 C, as shown in FIG.  4 . The three bent members  59 A,  59 B and  59 C are assembled into a ring shape in the operating floor  21 . The three bent members  59 A,  59 B and  59 C are arranged in a ring shape by engaging an engaging portion  61 A and an engaging portion  61 B of the bent members different to each other. The engaging portion  61 A and the engaging portion  61 B adjacent to each other are joined together with a pin  111 . A lug  62 A is attached to a side surface in the outer side of the bent member  59 A. A lug  62 B is attached to a side surface in the outer side of the bent member  59 C. Each of the lugs  62 A,  62 B has a cut portion  60 . 
     The discharging nozzle moving apparatus  124  comprises an apparatus main body  1  attached to the turntable  6 , an arm member  8  attached to the apparatus main body  1  movable in the horizontal direction, a pole member  9  attached to the arm member  8  and a discharging nozzle  10  provided in a vertically movable body  7  vertically moving along the pole member  9 . 
     The structure of the arm member  8  and the driving mechanism for driving the arm member  8  in the horizontal direction will be described below, referring to FIG.  5  and FIG.  6 . The arm member  8  is constructed by connecting both ends of four circular rods  8 A with connecting members  8 B,  8 C, and penetrates through a casing  128  of the apparatus main body  1 . Both end portions of a pole screw  67  are supported by the connecting members  8 B,  8 C. The drive mechanism installed in the casing  128  has a motor  68 , and gears  69 A and  69 B. The gear  69 A is attached to a rotating shaft of the motor  68 . The gear  69 B is rotatably attached to the casing  128 . The outer surface of the gear  69 B is engaged with the gear  69 A, and the inner surface of the gear  69 B is engaged with the pole screw  67 . A rotating force of the motor  68  is transmitted to the pole screw  67  through the gear  69 A and the gear  69 B to horizontally move the arm member  8  in one direction. By reversely rotating the motor  68 , the arm member  8  is moved in the opposite direction. Instead of using the motor  68 , the gear  69 A and the gear  69 B, it is possible that a cylinder may be provided in the casing  128  and the arm member  8  is moved in the horizontal direction. 
     A motor  120  attached to the pole member  9  rotates a pole screw, not shown, attached to the pole member  9  to vertically move the vertically moving body  7  engaging with the pole screw along the pole member  9 . The discharging nozzle  10  can be operated in such actions as to be moved forward and backward to a surface of a structural member on which preventive maintenance is to be performed, rotated in the horizontal direction, and swung in the vertical direction. In order to perform the three actions described above, the discharging nozzle drive mechanism comprises dedicated motors for the individual actions. The discharging nozzle  10  may be fixed to the vertically moving body  7  so as to have a certain angle. 
     The discharging nozzle moving apparatus  124 C comprises an apparatus main body  1  and an arm member  8 , similarly to the discharging nozzle moving apparatus  124 . The discharging nozzle moving apparatus  124 C further comprises a pole member  9 A inserted into a through hole (similar to a through hole  131  in FIG. 8) provided in a top end portion  8 D of the arm member  8 , a multi-joint arm portion  78  and a discharging nozzle  10 B. The pole member  9 A has a rack  77  on a side surface. A motor  71  has a drive unit for vertically moving the pole member  9 A, a gear  75  and a pinion  76  engaging with the rack  77  are provided in the top end portion  8 D, as shown in FIG. 8, though these are not shown in FIG.  7 . The multi-joint arm portion  78  is attached to a bottom end portion of the pole member  9 A with a rotating shaft  80 . The multi-joint arm portion  78  is formed by jointing a plurality of arm portions  79  with the rotating shafts  80 . The discharging nozzle  10 B is attached to a bottom end portion of the multi-joint arm portion  78  with the rotating shaft  80 . A monitoring camera, not shown, for taking a picture of preventive maintenance work is provided in the discharging nozzle  10 B. Each of the arm portions  79  and the discharging nozzle  10 B contains a motor, not shown, for applying a rotating force to the corresponding arm portion  79  or the corresponding discharging nozzle  10 B in respect to each of the rotating shafts  80  as a center. 
     The discharging nozzle moving apparatuses  124  and  124 C are installed on the turntable  6  in the both end portions of the turntable  6 . 
     The discharged water supply apparatus  125  as shown in FIG. 1 comprises a high pressure pump  30 , a filter  37 , a recirculation pump  38 , a hose  36  and high pressure hoses  19 A and  19 B. One end of the hose  36  is immersed into the reactor water  20  inside the well  126 . The hose  36  connects the filter  37  and the recirculation pump  38  and is connected to the high pressure pump  30 . The high pressure hose  19 A connected to the high pressure pump  30  is once attached to the casing  128  of the apparatus main body  1  and the arm member  8 , and is connected to the discharging nozzle  10 . The other high pressure hose  19 B connected to the high pressure pump  30  is once attached to the pole member  9 A, and is connected to the discharging nozzle  10 B. The filter  37  removes crud contained in the reactor water  20 . The high pressure pump  30  is installed on the operating floor  21 . A low pressure hose  35  connected to another water source is also connected to the high pressure pump  30 . The high pressure pump  30  pumps up water supplied from either the hose  36  or the low pressure hose  35  by switching to supply the discharging nozzles  10  and  10 B as the high pressure water. 
     The crud collecting apparatus  41  is constructed by connecting a sucking port  40 , a crud transferring pump  43  and a filter  44  using a crud transfer hose  42 . 
     The control board  18  installed on the operating floor  21  is connected to control cable  31  for transmitting and receiving electric signals by which the preventive maintenance apparatus for structural members is remotely operated, an air hose  32  for supplying air by which the preventive maintenance apparatus for structural members is remotely operated, a control cable  33  for transmitting and receiving electric signals by which the high pressure pump is remotely operated and so on. Based on the control signals transmitted through the control cables  31  and  33 , the corresponding apparatuses or units are remotely controlled. These remote controls can be performed selectively by manual mode or automatic mode. The preventive maintenance work is basically performed by the automatic mode. In addition to this, the control board  18  adjusts the speed of each of the actions such as forward and backward movements, rotation and swinging of the discharging nozzle  10 , and vertical movement of the vertically moving body  7 . 
     The operating air for remotely operating the preventive maintenance apparatus for structural materials is conducted to the control board  18  from an air supply source, not shown, through an air hose  34  to control a valve inside the control board  18 . By doing so, the preventive maintenance apparatus for structural materials can be remotely operated. It is also possible to operate the preventive maintenance apparatus for structural materials by supplying air to the air hose  34  from another compressor different from the air supply source described above. 
     Each of the motors for performing each of actions of the turntable  6 , the vertically moving body  7 , the arm member  8 , the discharging nozzles  10 ,  10 A and the arm member  79  and each of detectors, not shown, for detecting a rotating position and detecting a rotating speed such as potentiometers are arranged in appropriate positions in the preventive maintenance apparatus for structural materials. Measured signals of these detectors are transmitted to the control board  18  through the control cable  17 . During preventive maintenance work, a computer, not shown, incorporated in the control board  18  calculates an optimum condition to a traveling speed of the discharging nozzle  10  and a discharging position (vertical position, rotating position, swinging angle), and transmits each of the control signals to each of the corresponding motors through the control cables  31  and  33 . The computer also calculates an optimum condition to a traveling speed of the discharging nozzle  10 B and a discharging position (vertical position, rotating position, swinging angle), and transmits each of the control signals to each of the corresponding motors through the control cables  31  and  33 . 
     When preventive maintenance work is performed to the core shroud  3 , the preventive maintenance apparatus for structural materials is set onto the upper flange  4 . This setting work will be described below. The guide rail  5  is assembled into a ring shape on the operating floor  21 . The turntable  6  is set on the guide rail  5 , and the discharging nozzle moving apparatuses  124  and  124 C are mounted on the turntable  6 . After assembling the guide rail  5 , the turntable  6  and the discharging nozzles  124  and  124 C in a unit, the guide rail  5  is hung down inside the reactor pressure vessel  2  using the ceiling crane described above. As shown in FIG. 4, the two guide rods  63  are set to the inner surface of the reactor pressure vessel  2  by supporting members  112  spaced apart by 180°. The cut portion  60  of the lug  62 A provided on the guide rail  5  is engaged with the guide rod  63 . The cut portion  60  of the lag  62 B is is engaged with the other one of the guide rods  63  not shown in the figure. The guide rail  5  is lowered on the upper flange  4  along the two guide rods  63 . After that, the guide rail  5  is detachably attached to the upper flange  4  with bolts  14 . Since the guide rail  5  is lowered on the upper flange  4  along the two guide rods  63 , the guide rail  5  can be lowered without interference with the main steam line plugs  130 . 
     The work of setting the guide rail  5  on the upper flange  4  is performed by picture-taking using the monitoring camera  24  attached to the vertically moving body  7  and a monitoring camera  25  hung inside the reactor pressure vessel  2  by the cables  28 ,  29 , and monitoring the pictures displayed on a monitor  27  placed in the control board  18 . The images taken by the monitoring camera  25  is transmitted to the monitor  27  through the cable  26 . The work setting the guide rail  5  and the turntable  6  onto the upper flange  4  is monitored by the picture from the monitoring camera  25 . Moving of the monitoring camera  25  is performed by a worker from a refueling machine  22 . The worker moves the monitoring camera  25  by operating cables  28 ,  29 . 
     Description will be made below in a case where preventive maintenance work is performed to welded portions in the outer surface and the inner surface of the core shroud  3 . The pole member  9  of the discharging nozzle moving apparatus  124  is inserted into the ring-shaped space  122 . The vertically moving body  7  is moved by driving of the motor  120 . The discharging nozzle  10  is positioned to a welded portion or which preventive maintenance work is to be performed from the outside of the core shroud  3 . On the other hand, the discharge nozzle  10 B is inserted into the narrow portion between the core shroud  3  and the upper core grid plate  82  by lowering of the pole member  9 A and rotation of each of the arm portions  79  around the rotating shaft  80  as a center by driving of each of the motors contained in each of the arm portions  79 . 
     By driving the recirculation pump  38 , the reactor water  20  is cleaned by the filter  37 , and then transferred to the high pressure pump  30 . The high pressure water pressurized by the high pressure pump  30  and sent through the high pressure hose  19 A is discharged out of the discharging nozzle  10  from the outside of the core shroud  3  to a welded portion on which preventive maintenance work is to be performed. Cavitation bubbles are generated by a pressure difference and a shear action between the discharged water flow from the discharging nozzle  10  and the environmental reactor water  20 . The cavitation bubbles move toward the welded portion together with the discharged water flow. The cavitation bubbles are collapsed on the surface of the welded portion and the surrounding vicinity. The impulsive pressures generated by the collapse of the cavitation bubbles reach the surface of the welded portion to convert the tensile remaining stress in the surface to a compressive remaining stress. Since the compressive remaining stress is added to the outside surface of the welded portion, it is possible to prevent occurrence of stress corrosion cracks in the welded portion. The discharging nozzle  10  discharges the discharged water flow while being moved zigzag near the welded portion over all the circumference of the welded portion in the circumferential direction by vertical movement of the vertically moving body  7  by the driving of the motor  120  and rotation of the turntable  6 . Therefore, the compressive remaining stress is added over all the circumference of the outside surface portion of the welded portion, and it is possible to prevent occurrence of stress corrosion crack in the welded portion. 
     Similarly, the high pressure water conducted through the high pressure hose  19 B is discharged out of the discharging nozzle  10 B from the inside of the core shroud  3  to the welded portion on which preventive maintenance work is to be performed. The compressive remaining stress is added over the inside surface portion of the welded portion by the collapse of the cavitation bubbles. By vertical movement of the pole member  9 A and rotation of the turntable  6 , the compressive remaining stress is added over all the circumference of the inside surface portion of the welded portion. 
     In this embodiment, since the discharging nozzles  10  and  10 B are used, the compressive remaining stress can be added to the welded portion from the inside and the outside of the core shroud  3  at the same time. Therefore, the preventive maintenance work to the core shroud  3  can be completed in a short time. In a case where the preventive maintenance work from either the inside surface or the outside surface is not required, one of the valve  83 A arranged in the high pressure hose  19 A or the valve  83 B arranged in the high pressure hose  19 B (FIG. 1) not corresponding to the surface to be performed with the preventive maintenance work is closed to stop supplying of high pressure water to one of the discharging nozzles. 
     The discharging nozzle moving apparatus  124  in this embodiment may be constructed in such a structure that the pole member  9  is vertically moved by providing a motor  71 , a gear  75 , a pinion  76  and a rack  77 , as shown in FIG. 8 to be described later. The arm member  8  has a through hole into which the pole member  9  is to be inserted. The motor  71  is operated when the pole member  9  is lowered inside the ring-shaped space  122  after the discharging nozzle moving apparatus  124  is set in the turntable  6 . Vertical movement of the discharging nozzle  10  near the welded portion during the preventive maintenance work is performed by vertical movement of the vertically moving body  7  using the motor  120 . 
     By the action of the impulsive pressure generated at collapsing of the cavitation bubbles, crud (the main component is radioactivated iron oxide) attached near the welded portion is peeled off and suspended in the reactor water  20 . A crud transfer pump  43  is operated during the preventive maintenance work. A sucking port  40  is arranged at a place performing the preventive maintenance work. The reactor water  20  containing the crud is sucked through the sucking port  40 , and transferred to the filter  44  through the crud transfer hose  42 . The crud is removed by the filter  44 , and the cleaned reactor water  20  is discharged into the fuel storage tank  46  out of the hose  45 . Crud is settled out and accumulated mainly in a core shroud flange portion  39 . This crud is sucked through the sucking port  40  and removed by the filter  44  before initiating the preventive maintenance work. 
     Since the accumulated crud and the crud peeled off during the preventive maintenance work can be removed from the reactor water  20  by the crud collecting apparatus  41 , problems in the visibility of the monitoring cameras  24 ,  25  caused by suspending crud can be solved. Therefore, progress of the preventive maintenance work can be monitored under a good condition based on the pictures from the monitoring cameras  24 ,  25 . In addition to this, since diffusion of the crud can be suppressed, radiation exposure during the preventive maintenance work can be reduced. Radioactive contamination of the reactor water can be also suppressed. 
     The filter  44  having captured the crud is enclosed in a drum having a radiation shield or the like as a high level radioactive waste to be stored in a nuclear power plant site. 
     Part of the cavitation bubbles rise up to the water surface  47  without collapsing. These bubbles contain radioactivated crud. When these bubbles collapse on the reactor water surface  47 , the crud is dispersed and accordingly there is a possibility to expand a radioactive contamination area. In order to prevent expansion of the radioactive contamination area, the reactor water surface  47  is covered with an air collecting cover  48 . The bubbles having risen up to the reactor water surface  47  are collected by the air collecting cover  48 , and transferred to a filter  50  through a transfer hose  49 . The filter captures dust and mist. The gas cleaned by the filter  50  is exhausted to a ventilating and air conditioning exhaust duct  51  existing in the reactor building. The filter  52  having captured the dust and the mist is enclosed in a drum having a radiation shield or the like as a high level radioactive waste to be stored in a nuclear power plant site. 
     By setting the air collecting cover  48 , it is possible to suppress dispersion of radioactive substances to the zone above the operating floor  21 . Therefore, it is possible to reduce radiation exposure to workers in the operating floor  21 . 
     When it is required to exchange the discharging nozzle  10  of the discharging nozzle moving apparatus  124  or the discharging nozzle  10 B of the discharging nozzle moving apparatus  124 C, the discharging nozzle moving apparatus  124  or the discharging nozzle moving apparatus  124 C is detached from the turntable  6  using a tong from the operating floor  21 . The detached discharging nozzle moving apparatus is hung by a wire loop  23  and lifted up to the operating floor  21  using a hoist crane  11  of the refueling machine  22 . The discharging nozzle moving apparatus after having exchanged the discharging nozzle is hung and lowered using the hoist crane  11  to be set on the turntable  6 . 
     In the present embodiment, since the discharging nozzle  10  can be continuously moved around the core shroud  3  by rotating the turntable  6 , it is unnecessary to dismount and mount the discharging nozzle moving apparatuses  124  and  124 C when the discharging nozzle  10  can be moved around the core shroud  3 . Therefore, the time required to perform the preventive maintenance work using the discharge nozzle  10  can be shortened. Further, since the discharging nozzle moving apparatus  124  is set on the upper flange  4  through the guide rail  5  and the turntable  6 , the length of the pole member  9  can be shortened. Therefore, the discharging nozzle moving apparatus  124  becomes easy to be handled when it is set onto the turntable  6 , and the pole member  9  can be easily inserted inside the narrow ring-shaped space  122 . 
     Since the discharging nozzle moving apparatus  124 C in this embodiment has the multi-joint arm portion  78 , the discharging nozzle  10 B can be easily inserted into the annular narrow portion or space between the core shroud  3  and the upper core grid plate  82 . Therefore, it is possible to easily perform the preventive maintenance work to the inside of the welding portion of the core shroud  3  in the narrow portion. 
     In this embodiment, since the turntable  6  is not moved directly on the upper flange  4  but on the guide rail  5  arranged on the upper flange  4 , the upper flange  4  cannot be damaged by moving of the turntable  6 . Further, the guide rail  5 , the turntable  6  and the discharging nozzle moving apparatuses  124  and  124 C are integrated into a unit in which the discharging nozzle moving apparatuses  124  and  124 C are in a state of being mounted on the turntable  6 , and then the unit is lowered onto the upper flange. Therefore, the discharging nozzle moving apparatuses  124  and  124 C can be set on the upper flange at the same time. Compared to a case where the discharging nozzle moving apparatuses  124  and  124 C are separately set onto the turntable  6  placed in the reactor pressure vessel  2 , the guide rail  5 , the turntable  6  and the discharging nozzle moving apparatuses  124  and  124 C can be set onto the upper flange  4  in a shorter time. 
     The structure of the pole member  9  and the vertically moving body  7  in the discharging nozzle moving apparatus  124  of the preventive maintenance apparatus for structural members in the embodiment described above may be modified as shown in FIG.  8 . This structure will be described below. 
     An arm member  8 A penetrating a casing  128  has a through hole  131  in the top end portion. A pole member  9 A extending in the vertical direction is vertically moved in the through hole  131 . A mounting plate  72  is attached to the arm member  8 A with bolts and nuts  74 . A motor  71  is attached to the mounting plate  72  with bolts and nuts  73 . A pinion  76  is also rotatably attached to the mounting plate  72 . A gear  75  attached to a rotating shaft of the motor  71  is engaged with the pinion  76 . The pinion  76  is engaged with a rack  77  provided in the pole member  9 A. A discharging nozzle  10  and a monitoring camera  24  are arranged on a bottom end portion of the pole member  9 A. A rotating force of the motor  71  is transmitted to the pinion  76  to vertically move the pole member  9 A. In the structure of FIG. 8, the discharging nozzle  10  is vertically moved by the motor  71  during preventive maintenance work. 
     Another embodiment of a preventive maintenance apparatus for structural members in accordance with the present invention will be described below, referring to FIG.  9 . The preventive maintenance apparatus for structural members of this embodiment comprises a guide rail  5 , a turntable  6 , a control board  18 , a crud collecting apparatus  41 , and discharging nozzle moving apparatuses  124  and  124 A. The same components in this embodiment as in the embodiment of FIG. 1 are indicated by the same reference characters. This embodiment has the discharging nozzle moving apparatus  124 A instead of the discharging nozzle moving apparatus  124 C in the embodiment of FIG.  1 . The discharging nozzle moving apparatus  124 A has the same construction as the discharging nozzle moving apparatus  124 . The discharging nozzle moving apparatuses  124  and  124 A are set onto the turntable  6  on both end portions of the turntable  6 . Each of the pole members  9  of the discharging nozzle moving apparatuses  124  and  124 A is inserted into the ring-shaped space  122 . In this embodiment, by arranging the discharging nozzle moving apparatuses  124  and  124 A on both end portions of the turntable  6 , the preventive maintenance work to the outside surface of the core shroud  3  using the discharging nozzle moving apparatus  124  described in the embodiment of FIG. 1 can be performed at two positions on the core shroud  3  spaced apart by 180° at the same time also using the discharging nozzle  10  of the discharging nozzle moving apparatus  124 A. 
     In this embodiment, when the preventive maintenance work is performed to the inside surface of the core shroud  3  in the narrow portion between the core shroud  3  and the upper core grid plate  82 , the discharging nozzle moving apparatuses  124  and  124 A are detached from the turntable  6  using the tong, as described above. Then, the discharging nozzle moving apparatuses are successively lifted up to the operating floor  21  using the hoist crane  11 . Instead, two discharging nozzle moving apparatuses  124 C are lowered and set on the turntable  6 . Each of the discharging nozzles  10 B of the discharging nozzle moving apparatuses  124 C is inserted into the narrow portion by operating the pole member  9 A and the multi-joint arm portion  78 . By using the two discharging nozzle moving apparatuses  124 C, the preventive maintenance work to the inside surface of the core shroud  3  using the discharging nozzle moving apparatus  124 C described in the embodiment of FIG. 1 can be performed at two positions on the core shroud  3  spaced apart by 180° at a time. 
     This embodiment can attain the same effect as that attained in the embodiment of FIG.  1 . However, compared to the embodiment of FIG. 1, this embodiment needs to additionally prepare the discharging nozzle moving apparatus  124  and the discharging nozzle moving apparatus  124 C by one more each. Further, this embodiment requires work to exchange between the discharging nozzle moving apparatuses  124  and the discharging nozzle moving apparatuses  124 C. 
     A further embodiment of a preventive maintenance apparatus for structural members in accordance with the present invention will be described below, referring to FIG.  10 . The preventive maintenance apparatus for structural members of this embodiment is different from the embodiment of FIG. 1 in that a discharging nozzle moving apparatus  124 B is provided instead of the discharging nozzle moving apparatus  124 . The other constructions of this embodiment are the same as those in the embodiment of FIG.  1 . The discharging nozzle moving apparatus  124 B is constructed by further adding a vertically moving body  7 A, a pole member  9 B, a discharging nozzle  10 A and a motor  120 A to the construction of the discharging nozzle moving apparatus  124 . The pole member  9 B is installed in the arm member  8  so as to be in parallel to the pole member  9 . The vertically moving body  7 A is vertically moved by the motor  120 A similarly to the vertically moving body  7 . The discharging nozzle  10 A is fixed to the vertically moving body  7 A at a preset angle so as to be directed upward. In this embodiment, the discharging nozzle  10  is fixed to the vertically moving body  7  at a preset angle so as to be directed downward. In this embodiment, the preventive maintenance work is performed using the discharging nozzles  10 ,  10 A. At that time, the discharging nozzles  10  and  10 A perform the preventive maintenance work to different positions in the vertical direction of the core shroud  3 . Since this embodiment can perform the preventive maintenance work to a plurality of positions at the same time, the time required for the work can be shortened. 
     A still further embodiment of a preventive maintenance apparatus for structural members in accordance with the present invention will be described below, referring to FIG.  11 . The preventive maintenance apparatus for structural members of this embodiment is different from the embodiment of FIG. 1 in that a discharging nozzle moving apparatus  124 D is provided instead of the discharging nozzle moving apparatus  124 . The other constructions of this embodiment are the same as those in the embodiment of FIG.  1 . Spray headers  83 A,  83 B of a core spray system are installed in the inside surface of the core shroud  3  under the upper flange  4 . 
     The discharging nozzle moving apparatus  124 D comprises a horizontal arm  90 , a box  91  and a rotating cover  93  instead of the multi-arm portion  78  in the discharging nozzle moving apparatus  124 C. The horizontal arm  90  is attached to a bottom end portion of the pole member  9 A. The box  91  is attached to the horizontal arm  90 . The rotating cover  93  is rotatably attached to the box  91 . The discharging nozzle  10 B is attached to the top end portion of the rotating cover  93 . When a motor  92  provided in the horizontal arm  90  is operated, a gear, not shown, is rotated to move the rotating cover  93  in directions indicated by an arrow  94 . 
     By lowering the pole member  9 A and rotating the rotating cover  93  toward the left hand side in FIG. 11, the discharging nozzle  10 B is inserted into the narrow portion between the core shroud  3  and the upper core grid plate  82 . The preventive maintenance work to the inside surface of the core shroud  3  can be performed by discharging high pressure water out of the discharging nozzle  10 B. As described above, the preventive maintenance work to the inside surface of the core shroud  3  can be performed from a portion above the upper core grid plate  82 . 
     In this embodiment, the preventive maintenance work to the inside surface of the core shroud  3  can be also performed from a portion below the upper core grid plate  82 . The pole member  9 A is lowered in the upper core grid plate  82 . When the horizontal arm  90  reaches the bottom end portion of the upper core grid plate  82 , lowering of the pole member  9 A is stopped. The box  91  is rotated by a motor, not shown, so that the discharging nozzle  10 B is directed upward. By rotating the rotating cover  93 , the discharging nozzle  10 B is inserted into the narrow portion from an opening  84  formed in the bottom end portion of the upper core grid plate  82 . Under this state, high pressure water hits onto the inside surface of the core shroud  3  from the lower side. 
     This embodiment can attain the same effect as that attained in the embodiment of FIG.  1 . Further, the present embodiment can also perform preventive maintenance work to the inside surface of the core shroud  3  from the bottom end portion in the narrow portion. 
     A further embodiment of a preventive maintenance apparatus for structural members in accordance with the present invention will be described below, referring to FIG.  12 . This embodiment is constructed by further adding a rotating apparatus  101  to the construction of the embodiment of FIG.  1 . The rotating apparatus  101  comprises a table  103  for rotation, a ring  107  and a fixing metal fitting  108 . The table  103  for rotation is set on a bulk head plate  106  in the reactor well  126  through legs  104 . The ring  107  is mounted on the table  103  for rotation through roller bearings. The ring  107  is rotatable on the table  103  for rotation. The fixing metal fitting  108  is composed of two half-divided parts which are joined together and attached to the ring  107 . The fixing metal fitting  108  bundles the high pressure hoses  19 A,  19 B connected to the discharging nozzle moving apparatuses  124  and  124 C and the various kinds of cables such as the control cable  31  and so on. When the turntable  6  is rotated during preventive maintenance work using the discharging nozzle moving apparatuses  124  and  124 C, the ring  107  is also rotated in the same direction together with the rotation of the turntable  6 . 
     This embodiment can attain the same effect as that attained in the embodiment of FIG.  1 . Further, since the fixing metal fitting  108  bundles the hoses and the control cable and is rotated in the same direction as the rotation of the turntable  6 , it is possible to prevent the hoses and the cables from being intertwined by performing the preventive maintenance work. 
     A still further embodiment of a preventive maintenance apparatus for structural members in accordance with the present invention will be described below, referring to FIG.  13 . In this embodiment, two discharging nozzle moving apparatuses  124 E are mounted in both end portions of the turntable  6  instead of the discharging nozzle moving apparatuses  124  and  124 C in the embodiment of FIG.  1 . In the discharging nozzle moving apparatus  124 E, two pole members  9 A are attached to a top end portion  8 E of the arm member  8  in such a manner as to be slidable in the vertical direction. One of the pole members  9 A is inserted into the ring-shaped space  122 . Similar to the discharging nozzle moving apparatus  124 , a vertically moving body  7  having a discharging nozzle  10  is attached to the pole member  9 A in such a manner as to be movable in the vertical direction. A multi-joint arm  78  similar to that in the discharging nozzle moving apparatus  124 C is attached to the other pole member  9 A with a rotating shaft  80 . A discharging nozzle  10 B is provided in a top end portion of the multi-joint arm  78 . 
     This embodiment can attain the same effect as that attained in the embodiment of FIG.  1 . Further, the present embodiment can perform preventive maintenance work to both the inside surface and the outside surface of the core shroud  3  at the same time by the one discharging nozzle moving apparatus  124 E. Since this embodiment comprises two sets of such discharging nozzle moving apparatuses  124 E, the time required for the preventive maintenance work to the core shroud  3  can be shortened compared to that in the embodiment of FIG.  1 .