Patent Number: 062193996
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a maintenance method in a nuclear power plant, and more particularly, to a maintenance method for maintaining a suppression chamber and a spent fuel storage pool provided in a nuclear reactor building in a nuclear power plant. The reactor building of a nuclear power plant is provided with a reactor primary containment vessel (called containment vessel hereinlater) to enclose radioactive materials leaking from a reactor core within a nuclear reactor primary system and to prevent leakage of radiation in the event of a reactor failure accident in the primary system. All of the containment vessels provided in boiling-water reactors (BWR), including advanced boiling-water reactors (ABWR), are pressure-suppression type vessels, which are generally constructed to be provided with a dry well and a suppression chamber. FIG. 10 is a schematic cross-sectional view showing one example of such a containment vessel in the boiling-water reactor mentioned above. A primary containment vessel 1 is provided with a dry well 2 and a suppression chamber 3, and a reactor pressure vessel 5 is arranged at the central portion of the interior of the dry well 2 and supported by a reactor pressure vessel pedestal 4. A suppression pool 6 is provided in the suppression chamber 3 and is always filled with water. The dry well 2 and the suppression chamber 3 communicate with each other through a vent pipe 7. The vent pipe 7 is connected to a downcomer 8 within the suppression chamber 3. The downcomer 8 is opened at a tip end portion to the water of the suppression pool 6. FIG. 11 is a schematic cross-sectional view showing a primary containment vessel in a boiling-water reactor different in type from that of FIG. 10. As in the case of FIG. 10, the containment vessel of FIG. 11 is provided with a dry well 2 and a suppression chamber 3. A reactor pressure vessel 5 is provided at the central portion of the interior of the dry well 2 and is supported by a reactor pressure vessel pedestal 4. A suppression pool 6 is provided in the suppression chamber 3 and is always filled with water. The dry well 2 and the suppression chamber 3 communicate with each other through a vent pipe 7. The vent pipe 7 has openings at forked tip end portions into the water of the suppression pool 6 in the suppression chamber 3. The suppression chambers 3 shown in FIGS. 10 and 11 are formed by using steel plates. Since the steel plate does not have a corrosion allowance, the surface thereof is coated in viewpoints of corrosion resistance, water resistance and decontamination. Therefore, on the basis of the idea of preventive maintenance that the suppression chamber 3 is repaired before the life of a coated film applied onto the steel plate of the suppression chamber is over, the coated film of the suppression chamber 3 must be repaired almost at ten years intervals. In the case of performing such repair coating operation, the state of the coated film on the inner surface of a suppression pool wall 6a is conventionally inspected throughout the pool by using a remote-controlled underwater camera or the like provided within the suppression chamber 3 in advance. Based on the inspection result, re-coating timing and re-coating areas must be determined. In the re-coating operation, first, the suppression chamber 3 is drained off, and the suppression pool 6 is made vacant by draining off the chamber 3. In this state, although the re-coating operation is conducted. In the air, unlike in the water, radiation shielding effect is reduced or lost. For this reason, it is required to carry out the decontamination for removing radioactive materials present in the suppression chamber 3 before the re-coating operation. Since the radioactive materials involve substances or matters floating in the water of the suppression pool or those deposited on the bottom thereof which exist as residues or sludges, the inner surface of the suppression pool wall 6a is washed or the floating substances and/or sludges are removed through the remote control operation as the decontamination operation. After the decontamination is over, a scaffold is mounted, operators go downs along the suppression chamber wall 6a through the scaffold, carries out substrate treatment for the target coated film to be repaired and then starts re-coating. After all of the repair target surfaces are re-coated, the scaffold is dismounted and the suppression pool is filled up with water again, thereby completing the operation. Further, as shown in FIG. 12, a spent fuel storage pool 10 is provided in the nuclear reactor building 9 for storing the spent fuel, which was burnt in the reactor of the containment vessel 1 and which life has expired. Since the interior of the spent fuel storage pool 10 is normally lined with a stainless steel, it is not necessary to apply coating. However, in view of the deterioration of the lining and a generation of other various deposits, an internal inspection is desired. Conventionally, when inspecting and decontaminating the spent fuel storage pool 10, the pool is drained off. However, it takes considerable workload, time and cost to drain off the suppression chamber 3 and the spent fuel storage pool 10 and to perform decontamination following the above-stated internal inspection and repair coating operation. Further, if the decontamination operation is performed while the suppression chamber 3 and the spent fuel storage pool 10 are being drained off, i.e., in the air, it requires more operators due to the fact that radiation dose in the air becomes larger than that in the water, which also requires far more facility, considerable labor, time and cost. Moreover, if the repair coating operation is conducted for the local deterioration of the coated film within the suppression chamber 3, it is required to entirely drain off the suppression pool 6. It takes, therefore, considerable workload, time and cost to perform such a local repair. SUMMARY OF THE INVENTION An object of the present invention is to almost eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a maintenance method in a nuclear power plant capable of eliminating much workload, working time, cost and the like which have been conventionally required for the maintenance of a suppression pool and a spent fuel storage pool of a nuclear power plant due to the necessity for draining off the pools and capable of performing the repairing operation relatively easily for a short time and at low cost. This and other objects can be achieved according to the present invention by providing, in one aspect, a maintenance method in a nuclear power plant including a reactor primary containment vessel provided with a suppression pool of a suppression chamber, comprising the steps of: improving clearness of a water in the suppression pool and decontaminating an interior of the suppression pool; and PA1 inspecting a coated film applied on an inner surface of a wall portion of the suppression pool, wherein the improving, decontaminating and inspecting steps are performed while maintaining a water level in the suppression pool. PA1 improving clearness of a water in the spent fuel storage pool and decontaminating an interior of the spent fuel storage pool; PA1 inspecting a surface condition of an inner surface of a wall portion of the spent fuel storage pool, PA1 wherein the improving, decontaminating and inspecting steps are performed while maintaining a water level in the spent fuel storage pool. In preferred embodiments, a repair coating is further carried out to the coated film -through the underwater operation in the suppression pool after inspecting the coated film applied on the inner surface of the suppression pool wall. The clearness and the decontamination of the water in the suppression pool are performed by collecting substances or matters floating in the water of the suppression pool, such as chalk river unclassified deposit or substance, and removing and collecting substances deposited as sludges on an inner bottom surface of the suppression pool wall. The substances floating in the water of the suppression pool is sucked up together with the surrounding water out of the suppression pool by suction means movable in or above the water. The floating substances and the surrounding water sucked up in the suppression pool are subjected to solid-liquid separation on an outside the suppression pool. The suction means includes a rotating brush and a suction port arranged around the rotating brush so as to suck up the substances deposited as sludges on the inner bottom surface of the suppression pool together with the surrounding water therein. The decontamination of the interior of the suppression pool includes removing of sludges and/or deteriorated substances on the inner surface of the suppression pool wall. The sludges and/or deteriorated substances, called hereunder merely as sludge or sludges, on the inner surface of the suppression pool wall are removed through a sucking step by using a suction means comprising a rotating brush and a suction port arranged around the rotating brush so as to suck up the sludges together with the surrounding water in the suppression pool. The sludges and the surrounding water sucked up in the suppression pool are subjected to solid-liquid separation on an outside the suppression pool. The inspecting step of the coated film applied on the inner surface of the suppression pool wall is performed by visually observing an surface condition of the coated film by using an underwater camera, which may include at least one of a fixed camera disposed in the suppression chamber and a camera movable in the water of the suppression pool. The underwater camera may include a fixed camera disposed in the suppression chamber and a camera movable in the water of the suppression pool, the fixed camera being used to set a general inspection position in the suppression pool and the movable camera including a first movable camera used to set a fine position approaching the inspection point and a second movable camera used to observe a state of the coated film while approaching the inspection position more closely than the first movable camera. An information obtained by the underwater camera is displayed on a monitor television disposed outside the suppression pool to thereby allow observation in the air. A repair coating is carried out to the coated film through the underwater operation in the suppression pool after inspecting the coated film applied on the inner surface of the suppression pool wall. The repair coating step to the coated film on the inner surface of the suppression pool wall is carried out by peeling off a deteriorated or deformed coated film at a portion to be repaired by using one of a disc sander and a grinder provided with a suction means and a substrate treatment is carried out by sucking up the surrounding water out of the suppression pool. The repair coating step of the coated film on the inner surface of the suppression pool wall is carried out by applying an underwater coating to the inner surface of the suppression pool wall by using one of a brush having suction means arranged around the brush, a roller and other coating means and a coating splashed during the underwater coating applying step is sucked up outside the suppression pool together with the surrounding water. The maintenance method may further include the step of measuring a thickness of the coated film on the inner surface of the suppression pool wall by using a film thickness measuring device in the suppression pool. The maintenance method may further include the step of preparing an underwater plate thickness measuring device into the suppression pool and measuring a plate thickness of a plate constituting the suppression pool wall by using the plate thickness measuring device. The maintenance method may further include the steps of closing a strainer provided on the inner surface of the suppression pool wall in an underwater operation and inspecting a valve of a piping communicating with the outside of the suppression pool through the strainer. The maintenance method may further include the step of welding defect portions and portions to be repaired of the suppression poll wall, inner structure of the suppression pool, ducts, machineries and duct supports in an underwater operation in the suppression pool, and the welded portions are subjected to a nondestructive test in an underwater operation. The maintenance method may further include the step of carrying out a cutting working for repairing an inner structure of the suppression pool, ducts, machineries and duct supports in an underwater operation in the suppression pool. In another aspect of the present invention, there is provided a maintenance method in a nuclear power plant including a reactor primary containment vessel provided with a spent fuel storage pool, comprising the steps of: In this aspect, preferred embodiments similar to those mentioned above with respect to the maintenance method performed in the suppression pool water will be applicable. According to the present invention of the aspects mentioned above, the maintenance workings such as cleaning, decontaminating, inspecting workings to the inner wall surface of the suppression pool and the spent fuel storage pool in the reactor primary containment vessel can be carried out in the underwater therein without draining off the pools. Such draining working involves much labour, time and cost as in the conventional maintenance method. The maintenance method according to the present invention can be easily performed for short time and with low cost. The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.