Patent Number: 048184748
Section: description

For a better understanding of the invention, it is merely pointed out that the assemblies constituting the core of pressurized water nuclear reactors are essentially constituted by a framework or structure which supports and positions a bundle or group of fuel rods. The structure generally comprises a lower end fitting, an upper end fitting and intermediate grids or gratings, said components being connected by guide tubes. The latter also serve to receive mobile rods belonging to grapnels fulfilling various functions, such as the checking of the fission reaction in the reactor core. The fuel rods supported by the structure have a can, in which is located a stack of nuclear fuel pellets, the can being sealed at its ends by plugs. Generally, the pressurized water reactor assemblies have a square section and are positioned in vertically juxtaposed manner in order to form the reactor core. In order to permit the realization of the inventive control or management process, said assemblies are dismantlable. More specifically, the upper end fitting can be separated from the remainder of the structure in order to ensure the replacement of the fuel rods. Such dismantlable assemblies are known. As stated hereinbefore, assemblies of the AFA type belong to this category. However, it is also possible to use other dismantlable assembly types. According to the invention, the assemblies constituting the core of a pressurized water reactor are of two types. A first type of assembly is constituted by rods containing uranium dioxide UO.sub.2 pellets. These assemblies are identical to those presently used in most pressurized water reactors and they e.g. form approximately two thirds of the reactor core. A second type of assembly e.g. forming the final third of the reactor core uses a fuel produced by the recycling of the plutonium produced in the uranium dioxide assemblies after irradiation. More specifically, the rods of these assemblies contain mixed uranium and plutonium oxide pellets UO.sub.2 --PuO.sub.2. In order to take account of the juxtapositioning of the assemblies of both types in the reactor core and in order to prevent the formation of hot points, the mixed uranium and plutonium oxide assemblies are produced in a special way, illustrated in FIG. 1a. In order to simplify figs. 1a and 1b, no account is taken of the construction of the previously described assemblies. The hatched areas correspond to fuel rod systems all constructed in the same way within each zone and the framework or structure is not shown. Thus, each of these mixed oxide assemblies, such as assembly 10 in FIG. 1a comprises, during the initial loading of the core, several concentric zones in which the plutonium concentration of teh mixed oxide pellets contained in the rods differs. More specifically, this concentration decreases from the centre of the assembly towards its peripherary, the concentration being uniform within each zone. In the embodiment shown in exemplified manner in FIG. 1a, assembly 10 comprises three concentric zones with different plutonium concentrations. Thus, from the centre of the assembly towards its peripherary, are provided a central zone 12, an intermediate zone 14 and a peripheral zone 16. The dimensions of zones 12, 14 and 16 are determined in such a way that each of them contains the same number of rods. During the initial loading of the reactor illustrated in FIG. 1a, the central zone 12 is filled with identical new rods C.sub.1 containing mixed UO.sub.2 --PuO.sub.2 oxide pellets, whereof the plutonium concentration is higher than in zones 14 and 16. In said zone 12, the initial plutonium concentration can e.g. be approximately 4%. In the same way, the plutonium concentration of the mixed UO.sub.2 --PuO.sub.2 oxide pellets located in rods C.sub.2 filling the intermediate zone 14 is higher than the plutonium concentration of the pellets located in the rods C.sub.3 filling the peripheral zone 16. In the aforementioned example, the initial concentrations in zones 14 and 16 can be respectively close to 3% and 2%. When an irradiation cycle is ended, e.g. after approximately one year following the first loading of the core, the plutonium concentration in each of the zones 12, 14 and 16 of the mixed uranium and plutonium oxide assemblies 10 has dropped. More specifically, the plutonium concentration of rods C.sub.1 located in central zone 12 is then very close to the initial concentration in the intermediate zone 14 (approximately 3% in the aforementioned example). In the same way, the plutonium concentration of the rods C.sub.2 in intermediate zone 14 has become close to the initially existing concentration in peripheral zone 16 (approximately 2% in the present example). Finally, the plutonium concentration of rods C.sub.3 in the peripheral zone 16 has also dropped to well below its initial value. According to the invention and as is very diagrammatically illustrated in FIG. 1b, each of the mixed uranium-plutonium oxide assemblies 10 is then dismantled and the following operations are performed: discharge of rods C.sub.3 located in peripheral zone 16 (arrow F.sub.1 in FIG. 1b), PA1 transfer of rods C.sub.2 from intermediate zone 14 into peripheral zone 16 (arrow F.sub.2), PA1 transfer of rods C.sub.1 from central zone 12 into intermediate zone 14 (arrow F.sub.3), and PA1 loading of new rods C.sub.4 into central zone 12 (arrow F.sub.4). The new rods C.sub.4 introduced into the central zone 12 of assemblies 10 are all identical and contain mixed UO.sub.2 --PuO.sub.2 oxide pellets with a uniform plutonium concentration. which is identical to the plutonium concentration which initially existed in central zone 12. In the aforementioned example, said concentration is approximately 4%. It should be noted that the concentration is below the mean value of the plutonium concentrations in the rods of a mixed oxide assembly managed solely in a conventional way. Thus, in the present example, this mean value would be approximately 4.5%, whereas the plutonium concentration of rods C.sub.1 and then C.sub.4 placed in central zone 12 is approximately 4%. At the end of each irradiation cycle, the aforementioned operations are repeated. Thus, according to the invention, there is a management or control of the position of the rods within the mixed oxide assemblies 10. This is carried out in such a way that, apart from the initial loading of the reactor, a single type of rod containing mixed UO.sub.2 --PuO.sub.2 oxide pellets has to be introduced into the assemblies. Thus, there is a considerable reduction in manufacturing costs compared with a traditional management, in which there would be a complete replacement of assembly 10. Thus, throughout the life of the reactor, it would be necessary to manufacture several rod types corresponding to each of the zones of assemblies 10. Obviously, the invention is not limited to the embodiment described, in which each of the assemblies 10 is subdivided into three concentric zones. Thus, the number of zones is equal to the number of irradiation cycles undergone by the uranium dioxide assemblies. Thus, the mixed uranium-plutonium oxide assemblies can also be formed from two, or at least four concentric zones, as a function of the number of cycles. The principle of the internal management or control of these assemblies still remains the same. Thus, at the end of each cycle, the rods contained in the peripheral zone are discharged and the rods contained in the other zones are transferred towards the outside of the assembly into the zone adjacent to that which they previously occupied. The central zone, which is consequently freed, is then filled with new rods having a single plutonium enrichment. As has already been stated, the inventive management of the distribution of the rods within mixed UO.sub.2 --PuO.sub.2 oxide assemblies is added to the overall management of the assemblies within the reactor core. The principle of this overall management remains unchanged compared with that normally used in pressurized water nuclear reactors. It is merely pointed out that the invention tends to simplify this overall management, because the average properties of the mixed UO.sub.2 --PuO.sub.2 oxide assemblies constituting e.g. approximately one third of the core assemblies, evolves very little over a period of time. Thus, these assemblies are to a certain extent restored to a new state at the end of each cycle, whereas conventional uranium dioxide assemblies are only replaced after three successive irradiation cycles. The process for the management of rods within mixed oxide assemblies according to the invention also makes it possible to successively pass each of the rods into the different zones of said assemblies. Therefore, when these rods are discharged, they have a very similar irradiation. This feature makes it possible to envisage a better use of the fuel compared with the mixed oxide assemblies which would remain in the core for three successive cycles. Thus, in view of the fact that the neutron multiplication factor as a function of the plutonium content increases ever more slowly when said content rises, the plutonium concentration in the rods introduced during each cycle into the central zone of the mixed oxide assemblies is below the mean value of the concentration in a mixed oxide assembly which would remain in the core for several successive cycles. For example, if said mean value is approximately 4.5%, a concentration of close to 4% would be adequate. Finally, it should be noted that the invention makes it possible to reuse the framework of the mixed oxide assemblies for a number of cycles which is only dependent on the ageing of said framework. This feature also helps to reduce manufacturing costs.