Patent Number: 041347897
Section: summary

This invention relates to a method of refuelling of a nuclear reactor and to a device for carrying out said method. In more exact terms, the invention relates to an improvement in the refuelling of a nuclear reactor which permits more rapid handling operations. It is known that a nuclear reactor core which serves as an energy source is constituted by fuel assemblies which are in turn constituted by an array of fuel elements of fissile material. During reactor operation, the fraction of uranium-235 contained in said fuel decreases. It is therefore necessary at intervals to replace the partly spent fuel by fresh fuel. Refuelling of the reactor core therefore consists on the one hand in removing the spent fuel and on the other hand in introducing fresh fuel into the reactor or in displacing a fuel element from one region of the reactor core to another. In order to obtain a better understanding of the problems which arise at the moment of re-charging of a reactor with nuclear fuel, reference can be made to the accompanying FIG. 1 in which a pressurized water reactor (PWR) is shown diagrammatically in sectional elevation. The following description relates to the case of a PWR but it is readily apparent that the invention could apply just as readily to any type of reactor. The pressurized water reactor essentially comprises a pressure vessel 2 which is closed at the top by a closure head assembly 4. The pressure vessel is pierced by apertures such as the nozzle 6 which are connected to ducts for circulating the coolant liquid within the pressure vessel 2. Provision is made within the vessel for a lower structure consisting essentially of a core barrel 8, the top portion of which is applied against an internal annular shoulder or support ledge 16 of the pressure vessel 2. By means of the lower core support plate 10, the bottom portion of the core barrel 8 supports the fuel assemblies such as those designated by the reference 12. Provision is also made within the pressure vessel 2 for a top internal structure which is constituted by the upper support plate 14 and the upper core plate 18, said plates being connected by the spacer members formed by the control rod guide tubes 20, the complete assembly being positioned by the upper plate 14 which rests on the support ledge 16 of the pressure vessel. The upper core plate 18 is essentially intended to prevent "levitation" of the fuel assemblies 12 under the action of the coolant which flows upwards through the reactor core. To this end, the upper portion of each fuel assembly 12 can be provided with an elastic system compressed by the upper core plate 18. Reactivity control is accomplished by means of neutron absorbers or control rods. A control rod is constituted by a drive shaft 22 slidably fitted within the control rod guide tubes 20 and adapted to carry absorber rods such as the rod 24 which are inserted into hollow tubes formed within the fuel assemblies 12. The upper end of each control rod drive shaft 22 traverses the closure head assembly 4 of the reactor through leak-tight thermal sleeves 26 and is connected to a control rod drive mechanism 28. It is clear from this intentionally brief outline that, in order to refuel a nuclear reactor core or in other words to withdraw and replace the fuel assemblies in an unloading and reloading operation, it is necessary to remove the reactor closure head assembly 4, then the upper internal structure which is essentially constituted by the upper support plate 14 and the upper core plate 18 as well as the control rods and drive shafts. All these operations are complex and time-consuming and obviously entail the need for reactor shutdown. It is therefore essential to simplify refuelling operations and to reduce the time required. This becomes a particularly important consideration if refuelling is to be performed several times a year. French Pat. No. 71,24817 of July 7th, 1971 describes a method of refuelling a nuclear reactor which consists in removing in a single unit both the pressure vessel lid, the upper internal structure and the control rods, and in storing the complete assembly next to the reactor vessel. This system is attended by certain drawbacks in that means have to be provided for locking the control rod drive mechanisms in the top position and that both the upper internal structure and the control rod absorbers have to be stored with the reactor vessel lid or closure head. These two elements are radioactive, which does not facilitate inspection of the closure head assembly and control rod drive mechanisms. The present invention is precisely directed to a method for refuelling a nuclear reactor and to a device for carrying out said method which overcomes the disadvantages mentioned in the foregoing. In particular, the method according to the invention permits separate removal of the reactor vessel closure head and the upper internal structure as well as the control rods. The method of refuelling essentially comprises the following steps: the reactor closure head is removed after having disengaged this latter from the control rods which are left in the bottom position and said closure head is then stored; the control rods are brought to the top position and secured to the upper internal structure; the assembly constituted by the upper internal structure and the control rods is removed and stored; the spent fuel is replaced and the upper internal structure and the closure head are put back in position. In an alternative mode of execution, the method comprises the following steps: the control rods are brought to the top position and secured to the upper internal structure; the reactor closure head is removed; the assembly constituted by the control rods and the upper internal structure is removed and stored; the spent fuel is replaced and the upper internal structure and the closure head are put back in position. The invention further relates to an unloading device which essentially comprises a frame provided in the lower portion thereof with members for locking said frame on the upper internal structure and a moving platform which is capable of translational motion in a vertical direction with respect to said frame, said moving platform being provided with means for securing the upper extremities of the control rod drive shafts to said moving platform, said frame being provided with means for producing said movement of translation and guiding of said platform, the travel of the moving platform being substantially equal to the travel of the control rods within the reactor core. In a preferred form of construction, the frame is provided at the lower end with a circular flange having an internal diameter at least equal to that of the circle which is circumscribed about all the control rod position locations, vertical columns which serve to guide the moving platform and are attached to said flange at the lower ends, said columns being attached at their upper ends to a second flange comprising means for displacing said platform in translational motion, said platform being provided with an aperture opposite to each control rod drive shaft, the end of each shaft being capable of penetrating into the corresponding aperture, a member for locking each drive shaft being associated with each aperture. The invention is also concerned with a device for carrying out the alternative form of the method, said device being distinguished by the fact that the rigid structure comprises radial ribs fixed on an open-topped cylindrical canister, the wall of said canister being provided with bored recesses for accommodating a first series of balls which are capable of projecting from said canister and penetrating to a partial extent into notches formed in the so-called upper plate, that said drive shaft is hollow and provided at the lower end which penetrates into said canister with bored recesses for accommodating a second series of balls which are capable of projecting from said drive shaft so as to penetrate to a partial extent into notches formed in the internal wall of said canister, and that said device comprises movable means for causing alternate penetration of the balls of the first series into the corresponding notches so as to secure the absorber rods to said upper internal structure and the balls of the second series into the corresponding notches so as to secure said drive shaft to said canister. Said movable means are preferably constituted by a sleeve whose lower portion is capable of sliding within the interior of said canister and whose upper portion is capable of sliding within the interior of the lower end of the drive shaft, and means for displacing said sleeve in vertical motion, the external profile of the upper portion and the lower portion of said sleeve being such that said portions perform a cam function which permits the alternate motion of said balls.