Patent Number: 048184748
Section: summary

The invention relates to a novel process making it possible to control or manage the core of a pressurized water nuclear reactor, when said core is formed both from uranium dioxide UO.sub.2 assemblies and assemblies of mixed uranium and plutonium oxide UO.sub.2 --PuO.sub.2. In pressurized water nuclear reactors, the reactor core is presently formed from assemblies having rods containing uranium dioxide pellets. A 900 MW reactor has 157 assemblies, each containing 264 rods. The presently most widely adopted solution for ensuring the management or control of the core of such a reactor consists of replacing one third of the assemblies and at the same time rearranging the remaining assemblies at the end of each irradiation cycle, whose duration is e.g. approximately one year. Thus, each of the assemblies undergoes three irradiation cycles before being discharged from the core. Thus, the management of the core takes place as from an elementary quantity which is the assembly containing e.g. 264 rods in the case of a 900 MW reactor. In this presently used solution, each of the assemblies is formed from rods containing uranium dioxide UO.sub.2 pellets which, at the time of manufacture, all have the same uranium 235 content (e.g. 3.25%). Moreover, it is known that nuclear fission reactions occurring in the reactor transform part of the uranium 238 into plutonium. When they are discharged from the core, the assemblies consequently contain a large amount of plutonium, which can in turn be used as nuclear fuel following reprocessing. It is therefore envisaged to recycle the plutonium formed in the irradiated uranium dioxide UO.sub.2 assemblies in pressurized water reactors in order to produce new assemblies, whereof the rods contain a mixed oxide of uranium and plutonium UO.sub.2 --PuO.sub.2. Such mixed oxide UO.sub.2 --PuO.sub.2 assemblies would e.g. constitute approximately one third of the core of a pressurized water reactor, whilst the other two thirds would be formed from conventional uranium dioxide assemblies. For reasons linked with the different physical properties of uranium and plutonium, hot points could occur in the core if the mixed oxide assemblies UO.sub.2 --PuO.sub.2 had a uniform plutonium concentration. This new solution based on the recycling of plutonium consequently presupposes that the mixed uranium and plutonium assemblies are subdivided, from their centre towards their peripherary, into several zones of different Pu concentrations. These assemblies can e.g. be constituted by three concentric zones formed from rods containing mixed oxide UO.sub.2 --PuO.sub.2 pellets, whose plutonium concentration decreases from the central zone towards the peripheral zone. It is therefore envisaged to equip approximately one third of the core of pressurized water nuclear reactors with such assemblies formed from three types of rods containing mixed UO.sub.2 --PuO.sub.2 oxide pellets having a different plutonium concentration. Like the other assemblies, only one third of these assemblies would be replaced during each cycle, so that their total irradiation time would also be three cycles. Thus, as with the present management of the core, the elementary quantity involved is here again the assembly considered as a whole. This novel solution, which has the essential advantage of permitting the recycling of plutonium, however, suffers from disadvantages. Thus, at the end of each irradiation cycle, new mixed oxide UO.sub.2 --PuO.sub.2 assemblies formed from rods of different types (e.g. three) must be introduced into the core. Thus, for each of these new assemblies, this presupposes the manufacture of rods having different plutonium concentrations and consequently the manufacturing costs for such assemblies would be high. Moreover, in order to ensure that a can fracture in one of the rods of an assembly does not require the discharge of said assembly from the reactor core and also to facilitate assembly decanning operations, over the past few years dismantlable assemblies have been developed, in which the rods can be replaced without destroying the assembly structure or framework. Reference is e.g. made to assemblies of the type called AFA (Advanced French Assembly), described on pp 546 to 549 of the article "Les reacteurs nucleaires a eau ordinaire" edited under the direction of Guy DREVON in the Commissariat a l'Energie Atomique collection and published by Eyrolles, 1983. The present invention therefore relates to a novel process for controlling or managing the core of a pressurized water nuclear reactor, partly constituted by mixed oxide UO.sub.2 --PuO.sub.2 assemblies formed from several concentric zones with different enrichments, said process permitting, other than during the first loading of the core, to manufacture a single type of rod containing mixed UO.sub.2 --PuO.sub.2 oxide pellets with a single enrichment, by using dismantlable assemblies. According to the invention this objective is achieved as a result of a process for the control of the core of a pressurized water reactor, formed from dismantlable assemblies, each having a group of rods containing fissile material pellets, characterized in that it comprises initially placing in the core at least one first type of assembly, whereof the rods contain uranium oxide pellets and a second type of assembly. whose rods contain mixed uranium and plutonium oxide pellets, the rods of the assemblies of said second type being distributed in accordance with at least two concentric zones containing mixed oxide pellets having different plutonium concentrations, said concentrations decreasing towards the outside of the assemblies from a central zone towards a peripheral zone, making these assemblies undergo successive irradiation cycles and periodically transferring, following each of these irradiation cycles, the rods of each zone of the assemblies of the second type into the adjacent zone towards the outside of said assemblies, the rods located in the peripheral zone being discharged and new rods containing the mixed oxide pellets with a plutonium concentration equal to the plutonium concentration of the pellets contained in the rods initially placed in the central zone being loaded into said central zone. Obviously, this process is superimposed on the standard control or management process for a pressurized water reactor core. Thus, the internal control of each of the mixed oxide assemblies taking place at the end of each cycle, according to the invention, is accompanied by an overall control of all the core assemblies. More specifically, part of the uranium dioxide assemblies is replaced during each cycle and the other assemblies, no matter whether they are of uranium dioxide or mixed uranium and plutonium oxide, are rearranged in the core in order to obtain an optimum homogenous power distritubion. This overall management of the assemblies is of a conventional nature and does not form part of the invention. The inventive process making it possible to manage the distribution of the rods in the different zones of the mixed oxide assemblies UO.sub.2 --PuO.sub.2 has significant advantages. Firstly, during the life of the reactor, a single type of rods containing mixed UO.sub.2 --PuO.sub.2 oxide pellets with a single enrichment has to be manufactured, so that manufacturing costs are greatly reduced. Moreover, all the rods containing mixed UO.sub.2 --PuO.sub.2 oxide pellets pass successively into different zones of the assemblies which they constitute. Thus, their irradiation is very similar, so that the nuclear material can be better used. The average properties of these assemblies evolve little throughout their life, so that the overall control of the core is improved. In addition, this process makes it possible to independently control the mixed oxide pellets and the skeleton of the corresponding assemblies. Finally, bearing in mind the non-linear character of the evolution of the multiplication factor of the neutrons as a function of the plutonium content (said factor remaining substantially constant beyond a certain concentration), the plutonium quantity present in the reactor is reduced.