Patent Number: 047708409
Section: summary

FIELD OF THE INVENTION The invention relates to light water nuclear reactors and, more particularly, to a method for operating such a reactor for operation with different neutron energy spectra. BACKGROUND OF THE INVENTION The light water nuclear reactors typically comprise a pressure containment vessel containing a core constituted of vertical fuel assemblies located in side-by-side relation, each having fuel elements distributed at the nodes of a regular network, each fuel element containing fissile material and possibly fertile material. The fuel elements are substituted at some of the nodes of the network with guide tubes slidably receiving movable rods of control clusters. The fuel assemblies are at least partially substituted with other fuel assemblies after each burn-up cycle of the reactor. In each fuel assembly, the fuel elements are separated by a gap which flows cooling and moderating water. A moderation ratio VM/VU is defined as a ratio of a moderator module VM to the fissile material volume VU in the core. Conventional light water reactors now in operation have a moderation ratio such that the energy spectrum of the neutrons is thermal. Two directions have been explored for improving the light water reactors and for a better use of the fuel material. Both directions imply that the moderation ratio is at least temporarily decreased. The first approach consists of varying the neutron energy spectrum as the fuel burns up during a cycle. The natural uranium consumption and the initial degree of enrichment may be decreased for a predetermined burn-up rate. A spectral shift reactor which appears of particular interest is described in French Patent Application No. 82 18011 (FR-A-2,535,509). That reactor is comparable in structure to the conventional PWRs but includes a mechanical device for shifting the neutron spectrum, comprising clusters of rods containing fertile material, such as natural or depleted uranium oxide. The clusters are movable for insertion into the core or removal from the core during operation of the reactor. When fertile rods are introduced in guide tubes of the assemblies, they force moderating water out of the guide tubes and decrease the moderator volume VM in the core. As a consequence, during an operating cycle of the core, the neutron energy spectrum may be shifted. During a first part of the cycle, the clusters of fertile rods are maintained in the core. They shift the energy spectrum toward higher energies and increase the conversion rate of fertile material (uranium 238) into fissile material (plutonium). During a second part of the cycle, the clusters dedicated to spectral shift are progressively removed. The fissile material formed during the first part of the cycle is then partially burnt. The conversion rate is increased by about 10% with respect to a conventional thermal neutron PWR due to conversion rate increase. The other approach consists of under-moderating the reactor at all times. Then it is possible to use a mixed fuel comprising natural uranium and plutonium with a "breeding" rate of plutonium of about 1. However, the necessary decrease of the moderation rate VM/VU typically requires that the fuel elements be located in the fuel assemblies in a triangular rather than square array. Most under-moderated reactors comprise two types of fuel assemblies. Some, called fissile assemblies, contain principally fissile material; the others, called fertile assemblies, contain a material capable of being converted to fissile material under the effect of neutron bombardment. The fertile assemblies are generally disposed at the periphery of the core where they collect neutrons produced by the fissile assemblies. Proposals have been made for combining the advantages of spectral shift (particularly the gain on uranium consumption) and of under-moderation (plutonium breeding rate possibly greater than 1, possible use of depleted uranium, increased cycle duration). French Patent Application No. 83 15591 discloses a reactor having a heterogeneous core in which fuel assemblies of a type suitable for use in under-moderated reactors and fuel asssemblies for spectral shift reactors are associated for best utilization of plutonium previously produced in the fuel assemblies of conventional thermal neutron PWRs. That plutonium is recovered during reprocessing of spent fuel assemblies. However, that approach requires that a utility which has a plurality of nuclear power plants should dedicate at least one power plant to use of recovered plutonium. A first consideration suggests that the difficulty cannot be overcome since that type of fuel assembly which is required for under-moderated reactors does not lend to use in association with the internals and the control system of a reactor for operation with thermal neutrons, and conversely. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved method of operation of a spectral shift reactor. It is a more specific object to overcome the drawback consisting of the need for operation of a plurality of different types of reactors each of specific design. It is still an other object to provide a flexible process of operation of a PWR having a core consisting of fuel elements all having the same geometry. For that purpose, there is provided a method of operating a light water nuclear reactor wherein the internals of the reactor are so designed that it may receive either fuel assemblies of a first type for under-moderated operation with a high breeding rate or fuel assemblies of a second type having guide tubes authorizing spectral shift from the thermal range to the intermediary range. Part at least of the assemblies of one type are substituted with the assemblies of the other type after an operating cycle of the core. If the assemblies for under-moderated operation are so designed that they also authorize spectral shift and if the guide tubes for fertile rods are located at the same places in all fuel assemblies, it will be possible, without any modification of the higher internals of the reactor and of the cluster control mechanisms, to operate the reactor within a very broad moderation range, from thermal neutron reactors up to plutonium producing under-moderated reactors, during operating cycles. For achieving the required compatibility of the different fuel assemblies, all fuel assemblies will include fuel elements distributed according to the same, typically triangular, array. The reactor will typically be operated during the first cycle with regular spectral shift, the fuel assemblies including U235 enriched uranium as fissile material. During the second, under-moderated, operating cycle, the fissile material will be natural or depleted uranium with plutonium, as oxides UO.sub.2 -PUO.sub.2. As an example only, the moderation ratio ranges may be as follows: ______________________________________ SSR UMR ______________________________________ VM/VU Lifted fertile clusters 2 1,4 Inserted fertile clusters 1,5 1,1 ______________________________________ The abbreviations SS and UM respectively correspond to operation with spectral shift and under-moderation. The present day PWRs generally operate with a moderation rate of about 1.9. For carrying out the invention, it will typically be necessary to provide the pressure vessel with a control rod operating mechanism for each fuel assembly location. About two thirds of the mechanisms will be for spectral shift (fertile clusters) while one third will be for the usual fine control regulation and safety purposes (absorbing clusters). The actuating mechanisms may be conventional. The absorbing clusters will typically be associated with ratchet type electromechanical mechanisms, while the fertile clusters will be associated with hydraulic mechanisms. According to an other aspect of the invention, there is provided a reactor comprising, in a pressure containment vessel, a core consisting of adjacent vertical fuel assemblies each comprising fuel elements and guide tubes distributed at the nodes of a triangular array. The vessel is provided with mechanisms for vertical movement of control clusters. Upper internals in the vessel are used for guiding the clusters. Some of the clusters consist of fertile rods for shifting the neutron spectrum during a cycle. The other control clusters include absorbing material. The reactor has an actuating mechanism above each fuel assembly location and the fuel element array in each assembly is such that the core is under-moderated when all nodes are occupied by a rod or fuel element. Some at least of the assemblies comprise water filled tubes at some of the nodes of the network. The number and the distribution of such tubes are such that they authorize thermal operation of the reactor. The invention will be better understood from the following description of a particular embodiment, given by way of example only. The description refers to the accompanying drawings.