Patent Number: 047012994
Section: summary

FIELD ON THE INVENTION The invention relates to a moldular lining for adapting the containment to the core of a pressurized water nuclear reactor. BACKGROUND OF THE INVENTION In pressurized water nuclear reactors, the core generally consists of prismatic assemblies arranged vertically and side by side. The outer surface of the core has a complex shape possessing numerous steps. Arranged around the core, inside the nuclear reactor vessel, there is a core containment consisting of a cylindrical shell with a vertical axis. Between the outer surface of the core and the inner surface of the core containment, there is an annular region of varying thickness. To hold the reactor core laterally and to channel the greater part of the cooling water flow into the core itself, a modular lining is arranged in this annular region, the said lining consisting of elements joined to one another, some of which can be fixed to the core containment. This modular lining generally consists of metal plates screwed to one another, some of them being arranged horizontally over the whole width of the annular space, and the others being arranged vertically, in contact with the outer surface of the core. The horizontal plates, called shape adaptors, are in contact via their outer edge with the inner surface of the core containment and via their inner edge with the outer surface of the core. These shape adaptors are arranged at different levels over the height of the core. The shape adaptors can be fixed to the core containment by screws inserted in a horizontal bore passing through the core containment and bearing against the core containment with their heads, the shape adaptor possessing a bore in the extension of the bore of the core containment, which is partially tapped to enable screwing the threaded part of the screw. The temperature of the reactor cooling water is higher inside the core than outside the core containment, because this water heats up in contact with the assemblies in which it circulates from bottom to top, and then, after cooling in the steam generators, returns to the base of the core through the peripheral region of the vessel around the core containment. The core containment is therefore in contact via its outer surface with cooling water at a temperature below the temperature of the water inside the core, and more particularly of the water in the upper part of the core. The shape adaptors which are in contact both with the core containment and with the vertical plates forming the shield in contact with the outer surface of the core are therefore subjected to substantial thermal gradients. Furthermore, the shape adaptors which are subjected to the radiation emitted by the core are subjected to heating within their bulk. The same applies to the fixing screws. It is therefore necessary to avoid any confined space between the screws and their housing which could lead to vaporization of the water contained in this confined space and to heating and corrosion of the screw. It is therefore necessary to cool the fixing screws of the shape adaptors and especially to make their temperature uniform. PRIOR ART It has been proposed, for example in French Patent No. 2,333,325, to establish a reactor cooling water circuit around the screw. To do this, the bore is provided with sufficiently large dimensions to create a water circulation space around the screw in the shape adaptor, and water inlet and outlet holes are provided in the shape adaptor, communicating with the bore of the screw. In a modified embodiment, a screw is used which possesses an axial hole over part of its length, communicating, via a lateral hole, with the space created around the screw in the bore of the shape adaptor. Thus, water circulation can be established through the interior of the screw between the outside and the inside of the core containment. However, this screw cooling process requires special machining of the shape adaptors and can result in mechanical weakening of the fixing screws. French Patent Application No. 82/18,012, filed by the assignee of the present application, discloses a nuclear reactor of improved efficiency comprising, in particular, a solid modular lining occupying virtually all the volume of the annular space between the core and the core containment. A solid modular lining of this type, consisting, for example, of stainless steel blocks, can act as a reflector for the fast neutrons in addition to performing the usual functions of modular linings consisting of metal plates joined together. Furthermore, this solid modular lining makes it possible to protect the vessel from excessively intense neutron bombardment and very substantially to reduce the amount of water in the space between the core and the core containment. This improves the resistance of the modular lining in the event of an accident resulting in a loss of pressurized cooling water. Moreover, French Patent No. 2,499,752, discloses a solid modular lining consisting of modular blocks stacked on top of one another over the height of the core and firmly fixed to one another by long, vertically arranged ties. The modular blocks are cooled by vertical channels passing through each of the blocks and located in their mutual extension. Keys inserted in grooves on the lateral faces of contacting blocks enable the blocks to be joined together before completing the fixing by means of the ties, to which nuts are fixed at the top part of the core. This modular lining is not therefore fixed to the core containment, giving rise to disadvantages associated with the fact that the position of the modular lining is not perfectly defined relative to the containment. Furthermore, it is necessary to make a large number of cooling channels in order to limit the stresses and the deformations of thermal origin, which greatly reduces the reflectance of the modular lining. In fact, on the one hand, if the cooling is inadequate, the stack of modular blocks will suffer overall thermal deformation, making it necessary to provide a large clearance between the blocks and the core, which results in starving the peripheral assemblies and maintaining the core less effectively, and on the other hand, in the event of an earthquake, the long ties are subjected to high stresses during the operation of the reactor and are liable to break and separate from the modular lining. The whole construction is furthermore relatively complex and requires the machining of solid blocks, which must be matched perfectly to one another. Finally, it is not possible, for solid blocks, to use the method of fixing to the core containment recommended for shape adaptors consisting of relatively thin plates. In fact, for cooling the screws, these devices require holes to be provided in order to bring the bore for the passage of the screws in the shape adaptors into communication with part of the annular space filled with cooling water. SUMMARY OF THE INVENTION The object of the invention is therefore to propose a modular lining for adapting the containment to the core of a pressurized water nuclear reactor, which consists of assemblies of prismatic shape, arranged side by side inside a cylindrical core containment with a vertical axis, so as to create an annular space surrounding the core, in which the modular lining is arranged, the said lining consisting of modular metal elements juxtaposed so that the modular lining occupies virtually all the volume of the annular space around the core, inside the core containment, this lining allowing the expansion and the cooling of the modular elements and allowing effective cooling of the screws without it being necessary to provide holes for the passage of water in the modular lining elements, the solid modular lining thus having a high reflectance towards fast neutrons. For this purpose, arranged between the adjacent faces of the modular elements, there are thin packing pieces maintaining a certain gap between the modular elements consisting of undrilled solid blocks, for their expansion and their cooling, these blocks being fixed non-contiguously to the core containment by screw devices cooled by water circulation.