Patent Number: 047012994
Section: description

DETAILED DESCRIPTION FIGS. 1 and 2 show the core containment 1 of a pressurized water nuclear reactor, welded at its lower part to the core support plate 2 on which the assemblies forming the reactor core 3 rest. The core containment 1 is fixed at its upper part to the collar of the reactor vessel and arranged co-axially with the vessel. The core containment 1 and the support plate 2 form part of the lower internal equipment of the reactor. FIG. 1 shows that the outer surface of the core 3 bearing against the modular lining 5 has a complex shape comprising numerous steps corresponding to the peripheral assemblies 4 of the reactor. The modular lining 5, which occupies virtually all the volume of the annular space between the outer surface of the core and the inner surface of the containment 1, consists of three types of modular element 5a, 5b and 5c of different shapes. All these modular blocks have the same height and the modular lining consists of successive layers of modular elements 5a, 5b and 5c arranged noncontiguously on top of one another over the height of the core. FIG. 2 shows that the modular lining as a whole consists of nine layers of modular elements such as shown in FIG. 1, arranged on top of one another. FIG. 1 also shows that the elements 5a are not all identical to one another and that the same applies to the elements 5b and 5c, which are designed to match the external shape of the core. The elements 5a, 5b and 5c are arranged along the inner surface of the containment in a non-contiguous manner relative to this containment, packing pieces 7 being arranged between the modular elements and the inner surface of the containment 1. These packing pieces 7 make it possible to create a space 6 for the circulation of the reactor cooling water between the modular lining 5 and the containment 1. Arranged between the modular elements 5a and 5b and between the elements 5b and 5c, there are also packing pieces 8 making it possible to maintain, under hot conditions, a certain distance between the modular blocks for the circulation of reactor cooling water. In the cold, there is a small clearance between the packing pieces 8 which are firmly fixed to one of the modular elements (for example 5a), and the adjacent face of the other modular element (for example 5b), which enables the elements 5a and 5b to expand freely within the limit allowed by the clearance, and this reduces the thermal stresses. This also produces correct cooling of the modular blocks without the need to machine cooling channels and thus with the introduction of only a small amount of water into the modualr lining, which does not lower the reflectance. Finally, the small size of the modular blocks limits their thermal deformation, which makes it possible to overcome the problems relating to the geometry of the modular lining and aviod the need to provide an excessive clearance between the blocks and the core, which would detract from holding the core in the event of an earthquake and would be liable to result in starving the peripheral assemblies of cooling fluid. The modular blocks 5a, 5b and 5c are joined to the core containment 1 by means of fixing devices 10, which will be described in detail with reference to FIGS. 3 and 4. The blocks 5a are joined to the core containment by four devices 10, two of which are arranged at the upper part of the block and two at the lower part. The fixing devices are located in the region of the packing pieces 7. The central block 5a, however, is fixed to the core containment by only two devices 10, one of which is located at its upper part and the other at its lower part. The modular elements 5b, of profile shape, are joined to the core containment 1 by four screws arranged in pairs on either side of the block 5a, the block 5b which surrounds the block 5a towards the inside bearing against the core containment on either side of this block 5a. The blocks 5c are also fixed to the core containment by a set of four devices 10. FIG. 3 shows the core containment 1 through which a bore 12 passes, the said bore comprising a part 12a, of large diameter, towards the outside of the core containment, ending in a bearing surface 12b, and a part 12c of small diameter. The modular lining element 5 joined to the core containment 1 bears against the latter via the packing piece 7 possessing a groove 14 for the passage of water, in communication with the space 6, permitting the circulation of the reactor cooling water between the core containment and the modular blocks. This modular block 5 has a blind hole 15 tapped over a part 15a of its inner surface. A threaded sleeve 16 is screwed inside the blind hole 15 and then locked against rotation relative to the element 5 by a circular weld 17. Before the sleeve is fixed in the blind hole in the modular block 5, it is equipped with a screw 18 whose head 20, possessing two flat parts 19 visible in FIG. 4, is inserted in a housing 21 machined in the end of the sleeve 16. The length of the sleeve 16 is less than that of the blind hole 15, with the result that a space is created between the end of the sleeve in which the housing 21 is machined and the bottom 15b of the blind hole 15. The end of the sleeve 16 is hollowed out over a width corresponding to the width of the screw head 20 between the two flat parts 19, in order to create the housing 21. The screw head 20 can thus be locked in rotation when it is inserted in the housing 21 (FIG. 4). The diameter of the central bore 24 of the sleeve 16 is substantially equal to the diameter of the part 12c of the bore passing through the core containment 1, and when the modular block is placed in the fixing position along the core containment, the bore 12 and the sleeve 16, or its central bore 24, have a common axis 25. The diameter of the bore 24 and the diameter of the bore 12c are slightly greater than that of the non-threaded part of the screw 18, so that the latter is caused to bend and not to shear, which very substantially improves its fatigue characteristics. For a given screw size, this design makes it possible to make a maximum reduction in the length of the thread and consequently to maximize its bending length, leading to good fatigue characteristics, which are essential since the screws must take up the movements of the blocks resulting from the temperature transitions. The sleeve 16 has a radial hole 26 passing right through it. Between this hole 26 and that end of the sleeve which is located towards the bottom of the blind hole 15b, the diameter of the sleeve is less than the diameter of the blind hole 15, with the result that an annular space brings the entrance of the radial hole 26 on the outer surface of the sleeve into communication with the space created between the bottom of the blind hole 15b and that end of the sleeve which possesses the housing 21. The screw 18 has an axial hole 28 over the whole of its length, emerging, when the screw is in place in the sleeve, in the terminal space in the blind hole 15. At its other end, the central hole 28 emerges on the outside of the core containment 15, the screw being inserted in the bore 12 passing through the core containment, so that its end opposite the head 20, having a thread 29, is inside the large diameter 12a of the bore in communication with the outer part of the containment. A nut 30, bearing against the outer surface of the containment, in the region of the bearing surface 12b, is fitted to this threaded part 29. Between its threaded part 29 and its head 20, the screw comprises a tubular body 31 whose diameter is less than the internal diameter of the part 12c of the bore 12 and of the central bore 24 of the sleeve 16. An annular channel 32 is therefore created around the screw over the whole of its passage through the sleeve 16 inside the block 5 and the containment 1. To be put in place, the block 5 is brought into a position opposite the packing piece 7 fixed to the inner surface of the core containment, this block 5 being equipped with the sleeve 16 and with the screw 18, which is then inserted in the bore 12 passing through the containment, until the part 29 of this screw reaches the outside of the containment 1. The nut 30 is then screwed on to the part 29 in order to tighten and fix the block 5 against the packing piece 7. This tightening can be carried out since the screw 18 is locked against rotation by its head 20, comprising flat parts 19, in the housing 21. The same procedure is adopted for the four screws belonging to the four devices 10 for fixing the block 5. When the tightening has been carried out, the nut is locked against rotation by means of a weld 33. When the nuclear reactor is operating, there is a pressure difference between the cooling water located outside the containment and the cooling water located inside the containment. In fact, the water cooled by the steam generators, which is returned to the base of the core through the space existing between the core containment 1 and the vessel, undergoes a pressure drop on passing through the core, which is itself substantially in pressure equilibrium at a given height with the space 6 for the circulation of the cooling water between the core containment and the blocks 5. There is hence a pressure difference between the outside of the core containment and the groove 14 for the passage of water in communication with the water circulation space 32. A circulation of cooling water is therefore established between the outside and the inside of the core containment through the hole 28 in the screw, the terminal space in the blind hole 15, the annular space between the end of the sleeve 16 and the bottom 15b of the blind hole, the radial hole 26, the annular channel 32 and, finally, the groove 14 for the passage of water. This continuous circulation of water inside and around the screw makes it possible to avoid thermal gradients in the screw and cools the latter to a certain extent. This circulation is obtained without it being necessary to make holes in the modular blocks of the lining or to make holes in the screw in the radial direction. FIG. 5 shows a second type of solid modular lining fixed to the core containment 40 by fixing devices 50 identical to the fixing device shown in FIGS. 3 and 4. This solid modular lining, comprising modular blocks 45, can be used in the case of a pressurized water nuclear reactor of the undermoderated type in which the assemblies 41 have a hexagonal cross-section (instead of a square cross-section as for the reactor shown in FIG. 1). The various modular blocks 45 are arranged next to one another on the periphery of the core, without overlapping. Some of these blocks have a few vertical cooling holes 42 over their entire height. The number of cooling holes is small in this case, because undermoderated reactors are surrounded by a covering of fertile material arranged between the core and the modular lining, which reduces the heating due to the effect of the radiation on the modular blocks. As in the case of the modular lining shown in Figures 1 and 2, the modular lining as a whole is made up by the juxtaposition of layers of blocks, as shown in FIG. 5, arranged non-contiguously above one another. Arranged between two successive blocks 45 and between any one block 45 and the core containment 40, there are packing pieces 48 making it possible to create a cooling water circulation space 46 or 47. FIGS. 6 and 7 show a third type of modular lining which can be used in the case of a core consisting of assemblies of square cross-section, as shown in FIG. 1. This modular lining comprises modular blocks 55a and 55b fixed to the core containment 51 by fixing devices 60 identical to the devices described with reference to FIGS. 3 and 4. The modular blocks 55b are arranged so as to overlap with the blocks 55a, and the blocks 55a and 55b are fixed along the containment 51 in a non-contiguous manner by means of packing pieces 52. Keys 56 are fixed to some of the modular blocks 55a or 55b and are welded to the corresponding modular block and pass right through it. These keys 56 make it possible to fix a third type of modular block 55c to the modular blocks 55a or 55b by means of pins 57. The modular blocks 55c are not joined directly to the core containment 51, but only via the modular blocks 55a or 55b. Located between the blocks 55a, 55b and 55c, there are packing pieces 58 creating a space for the cooling and the relative movement of the blocks under the effect of expansion. It is seen that the main advantages of the lining according to the invention are that it permits cooling and relative movement of the blocks under the effect of expansion while avoiding the use of a large number of cooling channels, which reduce the mass of metal in the modular blocks and make them unsuitable as neutron reflectors. The particular type of fixing device described makes it possible to cool the screws for fixing the modular lining elements to the core containment and to make their temperature uniform, to a high degree of efficiency, without reducing the mechanical strength of the screws. This fixing device can be used in the case of solid modular blocks forming a modular lining occupying virtually all the volume of the annular space between the core and the inner surface of the core containment. The massive modular linings thus obtained are very firmly fixed to the core containment and have perfectly defined positions. The arrangement of these modular blocks for forming the modular lining makes it possible to reserve free spaces for the passage of the cooling water, without it being necessary to machine the blocks by drilling. The invention is not limited to the embodiments which have been described; on the contrary, it includes all the variants thereof. Thus, solid blocks having shapes different from those which have been described can be used. The end housing in the sleeve for locking the screw in rotation can have any non-cylindnrical shape in order to make it possible to lock a screw head of the corresponding shape. The sleeve can be fixed in any manner inside the blind hole created in the modular lining element. The fixing device described applies not only in the case of a solid reflecting modular lining according to the invention, comprising thick modular blocks, but also in the case of a conventional modular lining consisting of relatively thin metal plates. In this case, the fixing device according to the invention can advantageously be used for fixing the shape adaptors to the core containment. The invention applies to any pressurized water nuclear reactor for which an improvement in the neutron balance is desired.