Patent Number: 044938136
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an upper neutron protection device for a nuclear reactor assembly and more particularly for fast neutron reactors. In such reactors, it is necessary to limit the activation of the secondary sodium passing through the heat exchangers and to reduce damage to the various primary components, particularly the reactor vessels. The standard method consists of confining the neutrons in the central area of the core, by placing on the periphery thereof assemblies only containing fertile material in order to absorb the neutrons and also by placing fertile material in the upper and lower parts of the fuel assemblies, on either side of the fissile material. The protection is completed by so-called "upper neutron protection" sleeves, positioned in the upper part of the assemblies. In most existing systems, each assembly comprises in the upper part a head in the form of a sleeve and in the lower part a foot, which are disposed on either side of the assembly. The sleeve has a greater thickness than the assembly and the space provided in its central part makes it possible to circulate the cooling fluid, e.g. liquid sodium, from bottom to top through the assembly. However, it is known that the assemblies have to be replaced in the core and this takes place for each loading campaign of the latter, as a function of the irradiation undergone. Each spent assembly must be removed from the core and is then passed to a reprocessing installation, where the head, assembly and foot are separated from the needles or rods. As a result the sleeve is lost on each occasion, because it is fixed to the body of the assembly. Devices have also been proposed which have a removable sleeve, but the latter have a considerable wall thickness and often the fixing devices are complex, which increases the cost of the equipment and complicates disassembly. BRIEF SUMMARY OF THE INVENTION The present invention relates to a device obviating the aforementioned disadvantages, whilst being simple to construct and easy to disassemble, during the dismantling of the irradiated assembly. According to the main feature of the upper neutron protection device according to the invention, the latter comprises a container, at least partly filled with a neutron-absorbing product, held in place with the aid of at least one spacing plate, the assembly constituted by the container and the spacing plates being located within a wall positioned in the upper part of the assembly body. According to another feature of the invention, each of the aforementioned spacing plates has an outer edge, whereof at least part is substantially rectilinear and an inner edge, whereof at least part matches the shape of the container. Two types of spacing plates are used in the device according to the invention. In the first type, called male spacing plates, the upper part projects beyond the top of the assembly body, has an outer edge which slopes from top to bottom towards the outside of the wall surrounding the device and has a slot opening towards the outside of the wall and permitting the attachment of a handling means. In the other type, called female spacing plates, they are level with the top of the assembly body and have at least one portion of the upper part of their inner edge which slopes from top to bottom towards the inside of the space defined by the aforementioned wall and, still in the upper part of their inner edge, a slot opening towards the inside of said space and which also makes it possible to attach a handling means. These two types of plates prevent the inadvertent blocking of an assembly by another assembly during handling. Finally, according to a special embodiment of the invention, the device has a plurality of containers, at least partly filled with a neutron-absorbing product, each of the containers being held in place by a single spacing plate.