Patent Publication Number: US-2005129165-A1

Title: Nuclear power plant

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP03/03037, filed Mar. 24, 2003, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 102 13 608.4, filed Mar. 27, 2002; the prior applications are herewith incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      Field of the Invention  
      The invention relates to a nuclear power plant, in particular a boiling water reactor (BWR) with a reactor pressure vessel disposed in a containment unit.  
      Nuclear power plants of this type are known, for example, from German Patent DE 198 53 618 C1 or Published, Non-Prosecuted German Patent Application DE 195 38 009 A1. The interior of the containment unit of nuclear power plants of this type is divided into various subspaces by a plurality of inner walls and intermediate ceilings and has a charging cover that can be closed tightly. The reactor pressure vessel (RPV) is disposed in the central inner region and has a reactor core, in which the fuel assemblies are disposed, in its lower region and an opening, which can be tightly closed by a cover, at the top. The outside spaces of the containment serve as condensation chambers and flooding tanks for cooling the reactor pressure vessel and are connected to the latter via various lines.  
      To exchange the fuel assemblies, it is necessary for the reactor space to be flooded with demineralized water beyond the top edge of the reactor pressure vessel after the charging cover and the RPV cover have been removed. In the process, it must be ensured that there is a seal between the reactor pressure vessel and the flooding tank. For this purpose, each time fuel assemblies are changed, what is known as a flood compensator weighing a few tons is used in conventional nuclear power plants, and for the rest of the time the compensator has to be mounted outside the space which is to be flooded. The use of flood compensators of this type entails a number of drawbacks. For example, the changing of fuel assemblies is very time-consuming and requires a large staff, the flood compensator is expensive to produce, needs somewhere where it can be put down and also requires maintenance.  
     SUMMARY OF THE INVENTION  
      It is accordingly an object of the invention to provide a nuclear power plant that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which fuel assemblies can be changed without high levels of outlay in terms of time and operating staff and therefore at low cost.  
      According to the invention, the object is achieved, for a nuclear power plant having a containment unit, a reactor space formed in the containment unit, a reactor pressure vessel disposed in the reactor space, and a flooding tank disposed in the containment unit. The reactor pressure vessel has an opening that can be closed off by a cover, by virtue of the fact that a fixedly installed seal is provided between the reactor pressure vessel and the flooding tank.  
      If there is a fixedly installed seal between the reactor pressure vessel and the flooding tank, there is no need for the expensive deployment and removal of a removable flood compensator, which requires large numbers of people, when changing fuel assemblies. Moreover, there is no need for there to be anywhere to put down such a compensator outside the flood space, and the outlay on maintenance, cleaning and regular inspections can be minimized. Moreover, the fixedly installed seal is less complex to produce and therefore less expensive than the conventional removable flood compensator, including the required assembly devices. The reduced operating staff costs when changing fuel assemblies also reduces the exposure of the staff to radiation. Furthermore, decontamination of the fixedly installed seal is also relatively easy to carry out.  
      In a particularly advantageous configuration, the seal between the reactor pressure vessel and the flooding tank is affected in the form of a fixedly installed sealing membrane. This is expediently connected on one side to the top edge of the reactor pressure vessel and on the other side to a wall of the reactor space surrounding it and contains a plurality of segments which are tightly connected to one another, for example by welding.  
      The seal may have at least one emptying line for discharging residual liquid following a change of fuel assemblies.  
      To achieve a high thermal barrier action, as is required for starting up and running down the nuclear power plant, within the seal, it is expedient for a thermally insulating device to be fitted to the underside of the seal.  
      A preferred material for the seal is an austenite, in particular the austenite given the DIN designation X6CrNiTi 1810.  
      Other features which are considered as characteristic for the invention are set forth in the appended claims.  
      Although the invention is illustrated and described herein as embodied in a nuclear power plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
      The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagrammatic depiction of a containment unit in which there is fitted a reactor pressure vessel for a nuclear power plant according to the invention;  
       FIG. 2  is a partial sectional view of the reactor pressure vessel with a fixedly installed seal and taken along the line II-II shown in  FIG. 1 ; and  
       FIG. 3  is an enlarged sectional view showing the seal of the nuclear power plant in accordance with detail III shown in  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a sketch of a containment unit  10  of a boiling water reactor plant (BWR plant). An interior of the containment unit  10  is divided into various subspaces by an inner cylinder  12  and an intermediate ceiling  14 , with the structure of the containment unit  10  overall being substantially rotationally-symmetrical with respect to a longitudinal center axis  16 . A central opening  20 , which can be tightly closed off by a dome-shaped charging cover  22 , is provided in a ceiling  18 . All the walls and partitions of the containment unit  10  are preferably made from concrete.  
      A reactor pressure vessel (RPV)  26  is disposed in a central reactor space  24 , surrounded by the inner cylinder  12 , and the reactor pressure vessel  26  is supported on the inner cylinder  12  via a strut  28 . The reactor pressure vessel  26 , in its lower region, has a reactor core  30 , in which the non-illustrated fuel assemblies are disposed. To increase the power, control rods are introduced into a reactor core  30  by a control rod drive  32 , which is disposed at a lower end outside the reactor pressure vessel  26 . Control rod guide tubes  34  extend from the control rod drive  32  through a wall of the reactor pressure vessel  26  into the reactor core  30 .  
      At its upper end, the reactor pressure vessel  26  has an opening  36  which can be closed off tightly by a cover  38 . The reactor pressure vessel  26  and its cover  38  are completely surrounded by an insulation canning  40   a  and  40   b , respectively. The insulation canning  40   a  surrounding the reactor pressure vessel  26  is secured to the inner cylinder  12  by a large number of spacers  42  and is thereby spaced apart from the reactor pressure vessel  26  so as to form an intermediate space  44 , so that the reactor pressure vessel  26  is externally accessible for maintenance purposes. The insulation cannings  40   a ,  40   b  serve to thermally insulate the reactor pressure vessel  26 , so that the temperature in the intermediate space  44  is approximately 275° C. when the reactor is operating, and can therefore be kept in the region of the operating temperature inside the reactor pressure vessel  26 . Outside the insulation canning  40   a ,  40   b , the temperature is typically only approximately 50° C., for which purpose a cooling air stream L is provided from below between the insulation canning  40   a  and the inner cylinder  12 .  
      Furthermore, the inner cylinder  12  in the interior of the containment unit  10 , forms an annular outer space, which is divided into an upper outer space and a lower outer space by the intermediate ceiling  14 . A lower annular outer space forms a condensation chamber  46 , and the upper annular outer space forms a flooding tank  48 , both of which contain a cooling liquid F, in particular cooling water. The flooding tank  48  and the condensation chamber  46  serve to cool the reactor pressure vessel  26  if a critical pressure is exceeded in the latter or in the reactor space  24 . For this purpose, moreover, a plurality of cooling lines and non-illustrated cooling devices are present between the reactor pressure vessel  26  and the flooding tank  48  or the condensation chamber  46 .  
      To cool the reactor pressure vessel  26 , there is, inter alia, provision for external cooling or external flooding of the reactor pressure vessel  26 , during which the cooling liquid F from the flooding tank  48  flows into the intermediate space  44 , for example through a flooding line  50 , so that the cooling liquid F comes into contact with the outer wall of the reactor pressure vessel  26 . In the case of external flooding, the cooling liquid F is heated by the hot reactor pressure vessel  26 , with the result that steam is formed in the intermediate space  44 , and the steam can pass out of the intermediate space  44  into the upper region of the flooding tank  48  via a flow path that is not illustrated. In the upper region of the flooding tank  48  there is a condenser  52 , at which the steam condenses, with the result that the pressure in the containment unit  10  can be reduced.  
      When changing fuel assemblies, it is necessary, inter alia, for the reactor space  24  to be flooded with demineralized water beyond the top edge of the reactor pressure vessel  26  after removal or opening of the charging cover  22  of the containment unit  10  and the cover  38  of the reactor pressure vessel  26 . For this reason, it must be ensured that a seal is provided between the reactor pressure vessel  26  and the flooding tank  48  at least during this time. According to the invention, for this purpose a seal  54 , as illustrated in  FIG. 1 , is provided between the reactor pressure vessel  26  and the wall of the reactor space  24 , i.e. the inner cylinder  12  of the containment unit  10 . As is illustrated in the sectional plan view shown in  FIG. 2 , the reactor space  24  above the reactor pressure vessel  26  is completely sealed off with respect to the wall  12  of the reactor space  24  by the seal  54  via the insulation canning  40   a.    
      The seal  54  is fixedly installed, i.e. does not need to be fitted prior to a fuel assembly change or removed following a fuel assembly change. For this reason, less time and fewer operating staff are required for a fuel assembly change, which leads to considerable cost savings when operating the nuclear power plant. Moreover, there is no need for a separate space for the seal  54  to be provided outside the reactor space  24 , as was the case with the conventional removable flood compensators. Furthermore, the permanently installed seal  54  makes it possible to ensure a continuously good functionality, since the sealing action is not dependent on the way in which the seal  54  is fitted prior to a fuel assembly change.  
      One criterion when configuring the fixedly installed seal  54  according to the invention is that the seal  54  should be able to absorb the thermal expansions that occur during operation, in particular when the reactor is being started up and shut down. In the exemplary embodiment shown in  FIG. 1 , the reactor pressure vessel  26  is accommodated in the upper region of the containment unit  10 , so that in this case a lower axial thermal expansion needs to be taken into account compared to plants with reactor pressure vessels in the lower region of the containment unit  10 , and consequently the solution proposed according to the invention of the fixedly installed seal  54  can be realized with an acceptable level of outlay. The required absorption of axial expansion which is to be taken into account here for the seal  54  is in the range of approximately 20 to 30 mm, whereas its radial expansion absorption is in the range from approximately 8 to 15 mm, with the temperature range during operation of the nuclear power plant extending from approximately room temperature (wall of the reactor space) up to approximately 290° C. (reactor pressure vessel). Moreover, the seal  54  must, of course, also be able to withstand the compressive load applied by the water column above it when the reactor space  24  is flooded.  
      Only the following work needs to be carried out for a fuel assembly change in a nuclear power plant as illustrated in  FIG. 1 . First, the cooling water F is released from the flooding tank  48 , and then the charging cover  22  of the containment unit  10  is removed or opened. Then, the threaded bolts at the flange of the cover  38  of the reactor pressure vessel  26  are removed in the usual way. Next, the threaded holes for these threaded bolts have to be closed off by sealing plates, as is generally known, so that the demineralized water cannot come into contact with the ferritic threaded holes. Finally, before the cover  38  of the reactor pressure vessel  26  is opened, it is also necessary to seal off all the openings, such as manholes, ventilation flaps and the like, in the reactor space  24 . The reactor space  24  can then be flooded with demineralized water for a fuel assembly change and the fuel assembly change can be carried out.  
      A preferred exemplary embodiment of a fixedly installed seal  54  according to the invention will now be explained in more detail with reference to  FIG. 3 , which corresponds to an enlarged illustration of detail III shown in  FIG. 1 .  
      The seal  54  illustrated in  FIG. 3  is configured in the form of a sealing membrane  54 . The sealing membrane  54  is configured in the form of a circular ring in order to surround the entire top edge of the reactor pressure vessel  26  and, by way of example, has the semicircular cross-sectional form shown in  FIG. 3 . The sealing membrane  54  may preferably be composed of, for example, six segments that are very carefully welded together on site during installation. The structure of the sealing membrane  54  containing a plurality of segments can be seen in the plan view presented in  FIG. 2 .  
      The sealing membrane  54  of the preferred exemplary embodiment is formed of an austenitic material, for example a material with the DIN designation X6CrNiTi 1810. The sealing membrane  54  is uniformly approximately 2 to 3 mm, preferably approximately 2.5 mm, thick, and the semicircular shape of the cross section has a radius of approximately 150 to 250 mm, preferably approximately 200 mm, so that a distance of approximately 300 to 500 mm, in the preferred case of approximately 400 mm, between the reactor pressure vessel  26  and the wall  12  of the reactor space  24  or the insulation canning  40   a  provided inside the reactor space  24  can be sealed off over the intermediate space  44 .  
      To enable residual water which remains to be completely emptied out of the reactor space  24  again after the fuel assembly change has been completed, the sealing membrane  54  has, at its lowest point, at least one emptying line  56 , which is of course tightly closed in the normal state.  
      To achieve the maximum possible thermal barrier action on the part of the sealing membrane  54 , as is required in particular for starting up and shutting down the nuclear power plant, the sealing membrane  54  is provided with a thermal insulation  58  on its underside. Excessively rapid cooling of the sealing membrane  54  can be prevented by the thermal insulation  58 . The thermal insulation  58  is formed, for example, of a chloride-free mineral wool and is approximately 15 to 60 mm thick; this thickness may increase from the inside outward over the arc of the sealing membrane  54 , as illustrated in  FIG. 3 .  
      On its inner side, the sealing membrane  54  is welded to an austenitic plating  60  of a flange  62 , which surrounds the opening  36  of the reactor pressure vessel  26 , of the reactor pressure vessel  26 . By contrast, the outer side of the sealing membrane  54  is welded to the wall  12  of the reactor space  24  or the insulation canning  40   a  disposed inside the reactor space, i.e. in other words is directly or indirectly joined to the wall  12  of the reactor space  24 . The attachment points of the sealing membrane should satisfy not only the leaktightness requirement but also that of good heat conduction.  
      Furthermore, as illustrated in  FIG. 3 , an encircling grating  64  which can be walked upon and is intended to simplify maintenance of the sealing membrane  54 , which is in any case only minor, is provided above the sealing membrane  54  in the reactor space  24 . The distance between the sealing membrane  54  and the grating  64  is, for example, approximately 100 mm. Manholes for access to the grating  64  which can be walked upon must of course be sealed before the reactor space  24  is flooded in order for a fuel assembly change to be carried out.  
      Although the fixedly installed seal  54  according to the invention has been described above in the form of a sealing membrane, which forms the basis of a preferred embodiment, it is, of course, also possible to provide other designs of seals, provided that they ensure a suitable sealing action and are also able to withstand the thermal stresses which occur during operation of the nuclear power plant.  
      By way of example, it is also conceivable to provide a fixedly installed flooding space compensator in the reactor space. This would entail the same advantages in terms of time and operating staff when carrying out a fuel assembly change but would be more complex to produce and install.