Patent Number: 046769455
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to the field of nuclear power reactors which are generally of the pressurized water type. Such nuclear power reactors typically have a plurality of individual elongated and generally vertical fuel bundle assemblies which comprise the reactor core. During refueling of such a reactor core, it is typical to replace one-quarter to one-third of the total number of fuel bundle assemblies at a given time. Typically, such re-fueling of a reactor core is carried out by removing the spent fuel bundles that are to be withdrawn from the reactor core and thereafter repositioning some of the existing fuel bundles to new locations within the core and finally inserting the new fuel bundles into their desired initial position. Typically, such fuel repositioning and management is effected while the reactor core fuel assemblies are sumberged in the cooling fluid within the reactor vessel and various means for accomplishing such repositioning are employed at the top of the reactor vessel. Such means may take the shape of a traveling crane mechanism such as shown in U.S. Pat. No. 3,603,634 or U.S. Pat. No. 4,385,028. In any event, occasionally some difficulty will be experienced in inserting a fuel bundle assembly into certain reactor core locations due to inadequate physical space to receive the fuel assembly being inserted. This space problem is generally due to a slight elastic bowing in one of the fuel assemblies bordering the core location being loaded. In some cases, the resulting rectangular opening is too small to readily receive the lower end of the fuel bundle assembly being inserted, in spite of there being small chamfers on the lower bottom tie plate of the fuel bundle assembly. When such is the case, a delay of one to two hours is typical to overcome such a problem over the time otherwise required to insert a fuel bundle wherein the empty core position is of the proper dimension to receive the fuel assembly being inserted. Such unnecessary time delay in carrying out the loading and reshuffling of position of the fuel bundle assemblies in the reactor core results in unnecessary out-of-service time for the utility reactor and delay in its subsequent startup. Such delay is at considerable economic cost to the operating utility, since it unnecessarily increases the out-of-service time of the generating facility. The problem of physical interference upon insertion of a fuel bundle assembly into a reactor core has been addressed in typical prior art patents, such as the aforementioned U.S. Pat. No. 3,603,634. In this patent, the fuel bundle assembly being inserted is lowered down through a tubular centering device which includes a plurality of enlarged lower edge portions which are adapted to physically displace the fuel bundles adjacent the core position being loaded away from the empty core position. While the solution presented by this prior art patent may be applicable to fuel loading operations where the fuel assembly is lowered down through a centering mast or device, it is unsuitable for the environment of applicant's invention wherein a fuel bundle is merely lowered directly into an empty core position. SUMMARY OF THE INVENTION In accordance with the present invention, use is made of the adjacent vertically arranged fuel bundles which surround the empty core position to be filled with the fuel bundle to be inserted. Such use of the adjacent bundles involves the placement upon the top tie plate of each bundle of a separate and individual centering device, each centering device being of a generally box-like cap configuration and engaging the upper tie plate of each fuel bundle. Preferably, each such centering device includes a pair of tapered dowel pins which engage aperatures formed in the upper surface of the fuel assembly tie plate so that lateral forces imposed upon the centering device will be transmitted to the upper end of the fuel bundle assembly that may be bowed into the empty core position. Each centering device also preferably includes an elongated handling pole uniquely connected to the top wall of the centering device so that each centering device may be lowered individually onto the upper tie plate of its respective associated fuel bundle. This elongated handling pole engages the top wall of the centering device through a novel ball and socket arrangement which permits the handling pole to swing well clear of the fuel insertion operating area after the centering device has been placed on its respective fuel bundle. The unique ball and socket arrangement also allows the centering device to self-align itself on the handling pole when the handling pole is lifted vertically or, alternatively, being lowered down on top of the fuel bundle during installation thereon. Each of the centering devices in accordance with the invention includes top and side wall means which cooperate with an inclined rear plate and an inclined front wall portion in a manner to provide a hollow, box-like cap on top of its associated fuel bundle assembly. When each centering device is in engagement to its respective fuel bundle assembly, the front inclined wall of the centering device will be positioned so as to cooperate with the lower end of the fuel bundle assembly being inserted into the empty core position so that contact with said fuel bundle assembly will be effective to laterally deflect the bowed fuel bundle to its proper position and allow easy insertion of the fuel assembly into the empty core position. Another aspect of applicant's novel overall design for each individual centering device is their ability, even though engaged to the upper tie plate of its respective fuel bundle, to permit reactor core coolant to circulate up through the fuel bundle assembly during installation of the new fuel. This is accomplished by providing each of the side walls and the front and rear plates of the centering device with cut-out portions to provide flow passages for any reactor coolant fluid to exit from the centering device. While ordinarily the vertical flow due to convection of the core fluid will not be great during this shutdown state of the nuclear reactor, there will, nevertheless, be thermal gradients within the reactor core fluid which create vertical convection currents which might otherwise displace the centering device from the tops of their respective fuel bundle assemblies. Accordingly, it is a principal object of the invention to provide a new and novel centering device arrangement to assist in the loading of fuel bundle assemblies into a reactor core. A further object of the invention is to provide a fuel bundle centering device design which is simple to construct, rugged in performance, and economical to manufacture. A still further object of the invention is to provide a new and novel system for the insertion of fuel bundle assemblies into an empty core position of a nuclear reactor employing a plurality of individual and separate centering devices which physically engage the upper surface of the adjacent fuel bundles adjacent the core position to be loaded. These and other objects and advantages of the invention will become apparent, and the invention will be fully understood from the following description and drawings in which: