Patent Number: 046577254
Section: summary

FIELD OF THE INVENTION The invention relates to a core for a nuclear reactor cooled with water under pressure. Pressurized water nuclear reactors comprise a core consisting of prismatic assemblies arranged vertically and side by side and immersed in the cooling water which circulates inside the reactor vessel enclosing the core. Each of the assemblies consists of a cluster of parallel fuel rods arranged in the longitudinal direction of the assembly and spaced in the transverse direction to form a regular lattice in each of the cross sections. Each rod of the cluster consists of a cladding tube enclosing pellets of a fuel material which is generally uranium oxide enriched in fissile uranium. The core unit is made up of identical assemblies and the concentration of fissile material is sufficient to permit the emission of neutrons and the maintaining of the neutron reaction inside the core. The latter is then said to be critical. The space between the rods inside the core is such that a layer of cooling water of a certain thickness surrounds each rod during the operation of the reactor. This layer of water permits the moderation of the emitted neutrons and their energy spectrum remains in a region called the thermal region. The reactors which operate with a core formed as has just been described consume large quantities of natural uranium which captures a fraction of the emitted neutrons to form plutonium. Furthermore, in the pressurized water reactors known at present, only a small part of the natural uranium introduced into the core is applied. The plutonium which is formed is recovered during plant reprocessing of the irradiated fuel assemblies but this plutonium, which may be employed to form a part of the load in the nuclear reactors of the converter type remains largely unused at the present time since the reactors of the converter type have not yet reached an adequate stage of industrial development. PRIOR ART Thought has therefore been given to employing this plutonium to reduce the consumption of uranium in water cooled reactors. For this purpose plutonium in the form of oxide is incorporated in the uranium oxide powder in the manufacture of fuel pellets with which the cladding tubes of the assemblies are filled. It is impossible however to introduce large quantities of plutonium into the nuclear fuel and moreover this plutonium is rapidly degraded by preferential production of a non-fissile isotope through the action of low-energy thermal neutrons present in the reactor core. In fact, the plutonium produced by the reprocessing contains 70% of fissile material consisting of the odd-numbered isotopes Pu 239 and Pu 241 and 30% of fertile material consisting of the non-fissile, even-numbered isotopes Pu 240 and Pu 242. Now, the neutrons which are in the thermal range do not permit an optimum use of the fissile isotopes nor make it possible to obtain an appreciable conversion of the fertile isotopes. Consequently, plutonium cannot be employed in an economical manner in water-cooled nuclear reactors as designed at present, because of the relatively low energy of the neutrons in the core of these reactors. Plutonium can be recycled in these reactors only a very limited number of items because it is degraded by an increase in the proportion of the non-fissile even-numbered isotopes which cannot be converted into fertile material to a marked degree in the reactor. In order to improve the yield of nuclear fuel in the pressurized water nuclear reactors, it has been proposed to increase the neutron energy in the first part of the life of the reactor core by reducing the quantity of the moderating water inside the assemblies and by introducing a neutron-absorbing material in some guide tubes in the latter. This method which is the subject of French Patent specifications Nos. 2,535,509 and 2,535,508 makes it possible to obtain an increased production of fissile plutonium which takes part in maintaining the fission reaction and makes it possible to save a part of the uranium. However, the fissile plutonium produced is not employed in an optimum manner, particularly when the reactor is brought back into a low-energy neutron spectrum, during the second part of the cycle. SUMMARY OF THE INVENTION The aim of the invention is therefore to offer a core for a nuclear reactor cooled with water under pressure, consisting of prismatic assemblies arranged vertically and side by side, immersed in the cooling water and each formed by a cluster of parallel fuel rods spaced so as to permit the circulation of the cooling water and the formation around these rods of a layer of water for moderating the neutrons emitted, this core permitting the recycling plutonium to be employed under very good conditions, while being of a construction which does not require major modifications of the water-cooled nuclear reactors as constructed at present. To this end, the core comprises: a first group of zones extending over its entire height and over a fraction of its transverse section where the fuel rods contain mainly enriched uranium oxide so as to emit neutrons and to maintain the neutron reaction and are spaced at a distance permitting the formation of a layer of moderating water around these rods which is sufficient to bring the energy spectrum of the neutrons into the thermal region, and a second group of zones inserted between the zones of the first group where the fuel rods contain mainly plutonium and are spaced at a distance which is appreciably smaller than the distance separating the rods of the first group.