Patent Number: 047708409
Section: description

DETAILED DESCRIPTION OF AN EMBODIMENT Referring to FIGS. 1a and 1b, a core 1 comprises fuel assemblies within an envelope 2 received in the pressure containment vessel of the reactor (not shown). The fuel assemblies have a prismatic shape with a hexagonal cross-section. They are standing vertically and in mutually adjacent position. A heavy neutron reflector 3 is located between the fuel assemblies and the core envelope 2. That reflector may consist of stainless steel blocks so shaped as to provide a transition between the generally hexagonal shape of the core and the circular shape of the envelope 2. The reflector also constitutes a baffle around the fuel assemblies and may be secured to the core envelope 2. The core of FIGS. 1a and 1b is for under-moderated operation. It comprises fuel assemblies which may be distributed into three groups, depending on their own structure or the structure of the clusters cooperating with them. Fuel assemblies 4 are fissile assemblies. Their fuel elements contain U235 or Pu enriched uranium oxide. They are so located that their guide tubes receive clusters of depleted uranium oxide. When the rods are actually in the core, they decrease the volume of moderator in the core and cause the reactor to operate as described in French Patent Application No. 84 02329. Assemblies 5 are also fissile assemblies whose guide tubes receive absorbing rod clusters which conventionally allow the power of the reactor to be adjusted; such rods include material having a high neutron capture cross-section. Assemblies 6 are fertile assemblies formed by elements containing natural uranium and allow production of fissile material. Referring to FIG. 1b, a fissile assembly 4 comprises a set of fissile elements 7 containing a column of pellets formed by a mixture of uranium oxide and plutonium oxide whose fissile material mainly consists of uranium 235 and plutonium 239. The fissile elements 7 are disposed in a regular triangular lattice. Each of assemblies 4 with hexagonal section is surrounded, in the central part of the core, by six other fissile assemblies 4 or 5. At the periphery, the fissile assemblies are adjacent to three fissile assemblies 4 or 5 and three fertile assemblies 6. The set of fuel elements of the assemblies forms a regular network with triangular lattice in the core cross-sections. Each fuel element 1 comprises in each of its end portions a column of natural uranium oxide pellets forming an axial blanket which, in the same way as the heavy reflector 3, limits the leak of neutrons towards the equipments of the reactor. In the network of fuel elements 7 forming assembly 4, forty nodes of the lattice are occupied by guide tubes 8 of the same diameter as the fuel elements 7. As described in French Patent Application 84 02329 the guide tubes 8 increase the spacing between elements 7 and thus increase the moderation ratio when they are filled with water. Their presence ensures compatibility with the requirements of mechanical resistance of the fuel assembly and thermohydraulic behavior. To counteract the increase of the moderation ratio and allow under-moderated operation, the guide tubes 8 are arranged to receive clusters of fertile elements which, when they are inserted, reduce the volume of moderator in the core thus equipped, thus allowing a high conversion rate of the uranium 238 contained in the fertile clusters and in the fissile elements into plutonium 239. The guide tubes 8 of the fuel assemblies 5 are arranged to receive clusters of absorbing elements for controlling the nuclear reactor. Referring to FIG. 2a, an arrangement of reactor core 9 has been shown formed from fuel assemblies which, in accordance with the present invention, are positioned in the same way as assemblies 4, 5 and 6 of the core previously described in the heavy reflector 3 fixed to the core casing 2. Two types only of prismatic shaped assemblies with hexagonal section having the same dimension as assemblies 4, 5 and 6 are disposed inside the baffle plating. Each of the assemblies 10 and 11 is equipped, as in the previously described case and at the same positions in the lattice of fuel elements 12 (identical to that of elements 7), with guide tubes 13 for receiving either fertile element clusters for assemblies 11 or absorbing element clusters for assemblies 10. Introducing fertile clusters into the guide tubes 13 of assemblies 11 reduces the volume of the moderator in the core and thus allows operation of the reactor identical to that described in French Patent Application No. 81 18011 (FR-A-2,535,509) thus causing local "hardening" of the neutron spectrum. Referring to FIG. 2b, such local hardening is made necessary because assemblies 11 are for use in the spectrum of the thermal neutrons when the fertile clusters are removed from guide tubes 13. In accordance with the present invention, so that the moderation ratio is compatible with a conventional operating mode in the spectrum of the thermal neutrons, assemblies 10 and 11 comprise, in the lattice of the fuel element network, positions 14 devoid with fissile elements 12, such positions being evenly distributed. By thus omitting fissile elements, fuel assemblies 10 and 11 are defined, of the same dimension as assemblies 4, 5 and 6 of the preceding arrangement, but whose moderation ratio has been increased thus causing fission reaction to occur in the thermal neutrons energy spectrum. Positions 14 devoid of fissile elements may however be provided with sheaths or tubes, identical to the sheaths of fuel elements 12, containing water so that the total amount of water in assembly 11 is identical to the amount of water in assembly 4; the two types of assemblies are consequently identical in so far as the head loss of the coolant flowing therethrough is concerned. Sheaths full of water may similarly be provided which do not comprise closure plugs at their ends but which are in the form of open tubes allowing filling thereof with the reactor coolant. Referring to FIG. 3, the invention may be applied to a reactor having a general arrangement similar to that of the conventional PWRs. A pressure vessel 15 having a cover 16 contains the core envelope 2. The heavy reflector 3 is located inside the envelope and contains the core 1 consisting of adjacent fuel assemblies. Only a fuel assembly 17 for receiving a cluster of absorbing rods and a fuel assembly 18 for receiving a cluster of fertile rods have been illustrated. The upper internals of the reactor comprise a cluster guide 19 above each fuel assembly location. An actuating mechanism 20 is located above each such location. It will not be described here, since it may be fully conventional in nature. As a rule, hydraulic actuating mechanisms will be used for fertile clusters and electromechanical actuating mechanisms for absorbing clusters Referring to FIG. 4, the fertile rod clusters 21 are about twice as numerous as the absorbing rod clusters 22 indicated by hatching). It will be appreciated that it becomes possible to operate a same nuclear reactor according to anyone of a plurality of operating modes. For instance, for reducing the amount of plutonium required for the first load of an under-moderated reactor, one or more operating cycles may be carried out using a conventional spectral shift core with fertile clusters for spectral shift and for local hardening of the neutron flow during part of the cyles. Thus, and as an example, the method of the present invention allows an operator to select a working sequence of a nuclear reactor in which the components forming the core are either identical or compatible as regards dimensions by effecting the following steps: A core 9 formed from fuel assemblies 10 and 11 is loaded. After closure of the vessel lid, the fertile and absorbing clusters are fully inserted into the guide tubes 13. Then the fission reaction is initiated by extracting absorbing clusters. During an eight month duration, the reactor then operates with a spectrum which is locally hardened. The fertile material contained in the clusters as well as in the fissile elements 12 are transformed into fissile material. The fertile clusters are removed at the end of this time and for a period of two months the reactor operates in a thermal neutron energy spectrum using part of the fissile material produced. After shut-down of the reactor at the end of the cycle, it is possible: either to carry out again the same cycle by replacing part of assemblies 10 and 11 by new assemblies 10 and 11 or to select a different operating mode. Thus, either for increasing production of fissile material or for using fuel assemblies with low fissile material enrichment, or finally for economizing on the cycle of the raw material formed by the uranium, core 1 may be loaded while replacing all or part of assemblies 10 and 11 by assemblies 4, 5 and 6 allowing, after closure of the vessel and initiation of the fission reaction, operation with high conversion rate.