Patent Number: 053923260
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a boiling water reactor provided with a core structure, incore structural machineries and a recirculation system of a reactor for realizing a reactor capable of improving maintenace workings and operability of the reactor and remarkably reducing labour, load or the like of workers or operators for periodical inspections of the reactor. A boiling water reactor is a reactor of the type in which slightly enriched uranium is used as a fuel, which is directly boiled in a core by utilizing water as a moderator or coolant and steam is then generated. A boiling water reactor power plant is generally composed of a reactor system and a turbine system. The reactor system comprises a reactor primary series system including a reactor body having a core fuel, an incore structure and a pressure vessel, a recirculation system, a control rod driving system and a main steam system and also comprises a reactor auxiliary system including an emergency core cooling system. In a known technology, there has been proposed an improved boiling water reactor in which an internal pump system is utilized for a coolant recirculation system instead of an incore jet pump system. Such a boiling water reactor is shown for example in FIG. 20. Referring to FIG. 20, a core 2 is disposed at a portion slightly lower from a central portion in the reactor pressure vessel 1. A plurality of control rod guide tubes 3 are arranged below the core 2, and the core 2 is composed of a shroud 4 having an upper opening closed by a shroud head 5. Stand pipes 7 of a centrifugal separator 6 stand from the shroud head 5 and six rectangular flat type driers 8 are mounted on the centrifugal separator 6. A control rod driving mechanism 9 for driving cross-blade type control rods under guidance of inner surfaces of the control rod guide tubes 3 is disposed at a lower portion of the reactor pressure vessel 1. A plurality of internal pumps 10 are mounted to a bottom portion of the reactor pressure vessel 1 at portions between an inside of the reactor pressure vessel 1 and an outside of the shroud 4. The core is composed of a plurality of fuel assemblies arranged in a lattice structure, and in each of fuel assemblies, fuel rods in 8-row.times.8-line arrangement are supported by upper and lower tie plates and spacers. An entire structure of the fuel assemblies is surrounded by a channel box. Each of the fuel rods is formed by baking a slightly enriched uranium in the shape of a pellet which is then charged into a fuel clad. The control rod has a cross shape and acts to control a chain reaction in a fission, and the control rod is charged or inserted into the lattice arrangement of the fuel assembly from the lower side of the reactor pressure vessel 1, and the insertion or withdrawal of the control rod from the fuel assembly is performed by means of the control rod driving mechanism 9 connected to the control rod. In the core 2, the lower portions of the fuel assemblies are supported by a core support plate 11, the upper portions thereof are supported by an upper grid plate 12, and the entire structure thereof is surrounded by the shroud 4. A main steam pipe 13 is connected to an upper side wall portion of the reactor pressure vessel 1, and the steam dried by the driers 8 is transferred to a turbine through the main steam pipe 13. A water supply pipe 14 is also connected to the side wall of the reactor pressure vessel 1 for supplying the coolant into the reactor pressure vessel 1, and the coolant fed thereinto is forcibly circulated by the internal pumps 10. The boiled two-phase, water and steam, flow from the core 2 is separated by the centrifugal separator 6 into water and steam and the water content in the separated steam is further removed by the driers 8. The reactor pressure vessel 1 is fixedly mounted on a pedestal 16 through a supporting skirt 15. The attaching or detaching operation of the contol rod driving mechanism 9 is carried out in the pedestal by means of a control rod handling machine 17. An upper end opening of the pressure vessel 1 is pressure-tightly closed by an upper cover 18 and the entire structure of the reactor pressure vessel 1 is accommodated in the reactor containment vessel 19. With the nuclear power plant including the boiling water reactor of the structure described above, at a time of maintenance operation for the periodical inspection of the power plant, operators or workers enter the lower portion of the reactor pressure vessel 1 for removing the internal pumps 10 and the control rod driving mechanism 9. Although the internal pumps 10 are removed by a removing apparatus, the operators must perform preliminary removing work before the operation of the removing apparatus. During such preliminary work removing, however, there is a fear that the water coolant will down to the workers from the reactor pressure vessel 1. Under this dangerous environment, the long time working in such place of relatively high possibility of exposure of radiation dose is not desired for the workers in their physical and mental conditions. These may be also referred to for the inspection working or exchanging working of the control rod driving mechanism 9 and incore neutron detectors. One object of the present invention is to completely eliminate such dangerous maintenance working under the reactor pressure vessel 1. In another point of view, it is necessary for the operator to pay his highest attention to a water level in the core during the running operation of the reactor. In an ordinary operation, the water level in the core is automatically maintained to a predetermined level by an automatic controlling, and the control of the water level can be usually done by monitoring a display on a control board in a central operation room. However, in a case of a turbine trip or in a case where the coolant in the core changes into steam and then flows out of the core by an operation of safety valve for a main steam escape after the closing of a main steam isolation valve, the water level in the core lowers downward, and at this moment, a water supply pump, which is driven in isolation by a steam turbine, is operated to thereby start the water supply, but if this starting of the water supply is delayed, the water level in the core further lowers and an emergency core water supply system starts to operate. The emergency core water supply system operates itself automatically under a preliminarily designed safety control mode of a safety system, but such operation gives feel of strain or pressure to the operaters. On the contrary, when the core water level rises during the operation of the reactor by any accident of, for example, a water level setter, water flows towards the turbine, which may result in damage to the turbine blades. As described, a transition phenomenon in which undesired water level change is caused gives mental strain to the operators, so that a second object of the present invention is to realize a boiling water reactor having a wide allowable range against the change of the core water level. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide a boiling water reactor having a structure capable of handling control rods from an upper side of a reactor pressure vessel for eliminating workings of operators at a portion below a reactor pressure vessel in a reactor building. Another object of the present invention is to provide a boiling water reactor having a reduced vertical height capable of realizing an installation on an instable place such as on a ship or a portion being liably suffered from an earthquake. These and other objects can be achieved according to the present invention by providing a boiling water reactor comprising, a reactor pressure vessel accommodated in a reactor containment vessel in a vertical fashion, a core disposed at a low portion in the reactor pressure vessel, the core being composed of a plurality of fuel assemblies arranged with spaces from each other, a plurality of control rods to be inserted from an upper side of the core into the spaces between the fuel assemblies and withdrawn therefrom upwardly, a shroud surrounding the fuel assemblies so as to define the core and having an upper end opening, a shroud head which closes the upper end opening of the shroud and through which the control rods are inserted or withdrawn, separator means standing upward from the shroud head to carry out Gas-water separation of steam Generated from the core, a fixing pedestal disposed above the separator means and provided with a steam flow hole through which steam separated by the separator means passes, a control rod driving mechanism mounted on the fixing pedestal and adapted to drive the control rods, a drier means arranged along an upper inner wall surface of the reactor pressure vessel and adapted to dry the steam passing through the steam flow hole of the fixing pedestal, and a jet pump means disposed in a space between an outer surface of the shroud and an inner wall surface of the reactor pressure vessel. In preferred embodiments, the drier means comprises a plurality of drier elements arranged annularly along the upper inner wall surface of the reactor pressure vessel, drain receiving vessels disposed at lower end portions of the drier elements and drain tubes extending downward from the drain receiving vessels. The drier element is composed of a metal plate formed with a number of punched holes. The control rod driving mechanism is provided with a drive means composed of electromagnetic coils or an electromagnetically driven motor. The separator means comprises a plurality of tube like cylindrical bodies standing upward from the shroud head, a plurality of cross-shaped control rod guide tubes to be inserted into a space between adjacent fuel assemblies and a support plate for supporting the cylindrical bodies in vertical attitude. Each of the control rods has a vertical length substantially equal to a vertical length of the core and a vertically moving length of a control rod driving shaft is substantially equal to the vertical length of the core. The jet pump means comprises a plurality of jet pumps arranged with equal spaces from each other and annularly along the inner wall surface of the reactor pressure vessel. The jet pump comprises first and second stage nozzles, first and second stage throats and a diffuser connected to the second stage throat. The jet pump means is driven by a plurality of jet pump driving pumps or a plurality of steam injectors. The jet pump means is operated so that a flow rate ratio of a driving water with respect to a driven water is at least more than 6. According to the present invention of the structures described above, since the control rod driving mechanism is disposed at a portion above the reactor core and the control rods are handled from the upper side of the core, any piping means or machineries for the control rod driving are not positioned below the core. Accordingly, the operators or workers can be made free from the working at a portion below the reactor pressure vessel and the entire vertical height of the reactor containment vessel, i.e. reactor building, can be reduced. Furthermore, the core is positioned at a lower portion in comparison with a conventional arrangement, so that a space in which relatively long separators or other means are arranged can be ensured above the core. The location of the long separator can ensure wide allowance with respect to a water level change in the reactor pressure vessel. Still furthermore, since the control rod driving mechanism is disposed at the upper portion of the reactor pressure vessel, so that the control rod driving mechanism and associated members or elements can be entirely taken out from the reactor pressure vessel at a periodical inspection time, thus enabling easy maintenance.