Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Corrosion or damage prevention
Reexamination Certificate
1999-08-04
2001-05-29
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Corrosion or damage prevention
C376S249000, C376S260000, C376S261000, C376S262000, C376S263000, C376S310000, C376S316000
Reexamination Certificate
active
06240155
ABSTRACT:
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 charac
Chiba Noboru
Enomoto Kunio
Hayashi Eisaku
Kurosawa Koichi
Morinaka Ren
Carone Michael J.
Hitachi , Ltd.
Keith Jack
Mattingly, Stanger & Malur
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