Patent Number: 046577254
Section: description

DETAILED DESCRIPTION FIG. 1 shows a core of a pressurized water nuclear reactor incorporating prismatic assemblies with hexagonal cross-sections placed side by side and occupying the entire height of the core. In contrast to the core of pressurized water nuclear reactors of the prior art, these assemblies are not all identical. Referring to FIGS. 1 and 2, assemblies 1 and 2 consist of clusters of rods 5 containing enriched uranium oxide and arranged according to a lattice providing interstices between the rods for the formation of layers of water of a sufficient thickness for moderating the neutrons as far as the thermal region. Some rods of the assembly lattice are replaced with guide tubes and into these guide tubes can be inserted rods of depleted uranium as described in French Pat. No. 2,535,508. The insertion of rods of depleted uranium into the guide tubes 4 of the assemblies of the type 1 and 2 during the first part of the fuel cycle makes it possible to displace water from these guide tubes, to harden the neutron spectrum and thus to increase the production of plutonium in the fuel. An additional hardening derives from the fact that the rods of depleted uranium absorb the low-energy neutrons. The production of plutonium is also increased by the fact that a part of uranium 238 present in the depleted uranium rods is converted into plutonium. In the assemblies of type 1, only the uranium rods which permit a spectral shift to be obtained are inserted in the guide tubes 4, while in the assemblies of type 2, some guide tubes are reserved for the movement of the rods controlling the reactivity of the reactor core during its operation. The guide tubes of these assemblies 2 therefore receive, on the one hand, the rods of depleted uranium of the device for spectral shift control and, on the other hand, the reactor control rods. The assemblies 1 and 2 form a group of zones which extend over the entire height of the reactor core where the uranium oxide fuel rods have interstices of a relatively large size. Between the zones formed by the assemblies 1 and 2, the core comprises assemblies 3 of a composition and structure which are completely different from the assemblies 1 and 2. The assemblies 3 are arranged as a checkboard between the assemblies 1 and 2. The assemblies 3 consist of rods 6 comprising mainly recovered plutonium containing 70% of fissile isotopes and 30% of non-fissile isotopes. These rods are arranged in a regular lattice whose interstices have a size which is much less than the size of the interstices in the assemblies 1 and 2. The interstices have an average size which is approximately three times less than the size of the interstices of assemblies 1 and 2. This small interstice between the plutonium rods can be obtained by winding spacer wires in a spiral on these tubes as in the assemblies of the reactors of the undermoderated type whose spectrum is intermediate between a thermal neutron spectrum and a fast neutron spectrum. In the assemblies 3, no control rods nor spectral shift control rods are inserted and the whole lattice consists of rods of recovered plutonium, independently of the components of the assembly required to ensure its rigidity. The whole of the core which can be seen in FIG. 1 therefore consists of a juxtaposition of zones each of which consists of an assembly of the type 1 or 2 or of type 3. A checkboard arrangement such as shown has the advantage that each of the assemblies 3 is surrounded by assemblies of the type 1 and 2 which produce the neutrons required to maintain the nuclear reactions. The assemblies 3 may be produced in the form of subcritical assemblies, that is to say whose neutron activity would be insufficient to maintain the neutron reaction. Inside the assemblies 3, the neutrons produce the fission of some of the nuclei of the odd-numbered isotopes of plutonium, which produces neutrons which are only very slightly moderated by the thin layer of water present between the rods of the assembly 3. These high-energy neutrons convert a part of the non-fissile plutonium into fissile plutonium, with the result that the latter is not degraded during the use of the reactor. A recycling of this plutonium can therefore be envisaged as for the assemblies containing uranium oxide. The core shown in FIG. 1 comprises 236 assemblies of types 1 and 2, namely 163 assemblies of the spectral shift type receiving only rods of enriched uranium and 73 assemblies receiving both rods of enriched uranium and reactor control clusters. This core comprises, inserted among these 236 assemblies 1 and 2, 90 assemblies of the type 3 which are undermoderated and contain plutonium. These 90 assemblies 3 contain an insufficient quantity of fissile material to produce neutron criticality by themselves, and they are therefore called sub-critical. The assemblies 1 and 2 which act as the neutron source for the assemblies 3 do not require a high initial enrichment since the spectral shift control rods permit the reactor to be operated with neutrons of an increased energy during the first part of the life of the reactor core. The core of a pressurized water nuclear reactor such as shown in FIG. 1 makes it possible to obtain a saving of fissile material of 30% relative to a fissile load containing only uranium, by virtue of the 90 assemblies containing rods of recycled plutonium. The fact that the assemblies containing plutonium are arranged checkboard fashion between the zones formed by the spectral shift control assemblies permits an additional hardening of the neutron spectrum relative to that obtained solely by the undermoderation in the assemblies 3 and therefore an increased production of fissile material which permits an additional gain of the order of 20%. However, the invention is not limited to the embodiment which has been described; on the contrary, it comprises all the alternative forms. Thus, the assemblies forming the first group of the core zones which are produced in a heterogeneous form could consist of conventional assemblies of a pressurized water nuclear reactor which are not intended to receive spectral shift control rods. However, in this case it is necessary to employ assemblies having a high initial enrichment so that they can fulfill their function as a source for the assemblies containing plutonium. This presents disadvantages if it is intended to use the fuel with high burn-up ratios. The assemblies forming the core can have a cross-section which is different from a hexagonal section, for example a square section, as is current practise for the assemblies forming the cores in pressurized water nuclear reactors. The zones of the first group containing uranium can consist of a single assembly, of several assemblies or even of a part of an assembly containing both uranium oxide rods and plutonium rods. In all these cases, however, the zones of the second group containing plutonium must be distributed between the zones of the first group containing uranium oxide, to permit a satisfactory neutron operation of the core. These zones must also have transverse dimensions which are sufficiently small to ensure a good neutron operation. Finally, the invention applies to all the watercooled nuclear reactors whose core consists of a juxtaposition of clusters of parallel fuel elements.