Patent Number: 045086788
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is assumed that the general design of an integrated sodium-cooled, fast neutron nuclear reactor is known and a detailed description thereof is provided in the Journal "Nuclear Engineering International", vol. 23, No. 272, June 1978. The complete reactor below the slab is not shown, it only being possible to see its main attachments to the slab. It is assumed that the reactor is contained in a hot vessel attached beneath the slab. However, the description can easily also relate to other types of support. According to the prior art the complete primary circuit is confined in a vertically axed main vessel 10 with a hot wall connected to the slab 12 kept cool by a ferrule 10a having a thermal gradient. Vessel 10 is externally provided with a thermal insulation layer 14 and it is surrounded by a normally cold, safety vessel 16 which is also attached to slab 12. A cylindrical steel skirt 18 surrounds safety vessel 16 and supports the slab 12 as from the not shown bed or floor of the installation. Skirt 18 is cooled by a not shown device and it is surrounded by a concrete neutron protection cylinder 20 rising up round slab 12. Slab 12 is perforated by a group of openings or vertical shafts used for the fixing of large components. The central opening 22 overhanging the core of the reactor (not shown) serves to carry a large rotary plug 24, which itself carries the small rotary plug 26, traversed by the control plug 28 equipped with not shown control rods and by at least one vertical fuel handling apparatus 30. As a result of the combined rotations of the two rotary plugs 24 and 26, apparatus 30 makes it possible to transfer fuel elements from the reactor between a not shown fixed base located to the side of the core and any desired location in the latter. The drawing shows one of the shafts or peripheral openings 32 in slab 12 fixing an intermediate exchanger 34 supplied with tepid secondary sodium by means of pipe 36 and externally supplying hot secondary sodium by means of pipe 38. It is also possible to see one of the shafts or peripheral openings 40 in slab 12 fixing the primary pump 42, which is driven by electric motor 44, a flywheel 46 being fixed to the shaft in order to ensure a slow decrease of pumping in the case of an electric failure. The slab is constructed from welded steel sheets, constituting recesses or internal spaces and each recess is provided in its floor with a lining of neutron-absorbing material. The internal spaces other than the shafts are at least partly lined with neutron-absorbing materials, such as concrete 48. The lower face 12a protected by a thermal insulation covering 50 is cooled by a not shown cooling circuit. All the above arrangements are in accordance with the prior art. According to the present invention the thickness of slab 12 is increased compared with the prior art. It can be close to 6 m instead of 2.50 m in the known reactors of this type. As illustrated by FIG. 1, this feature makes it possible to provide in the slab thickness above each of the shafts 32 in which are fixed the exchangers 34, a housing 32a for the corresponding exchanger head, and above each of the shafts 40 in which are fixed the primary pumps 42, a housing 40a for the head of the corresponding pump and its flywheel 46. The plane shape of each housing is conditioned by the mechanical conception of the rigid slab, the metallic partitions connecting the lower face 12a to the upper face 12b, said partitions being for example purely radial and thus delimiting adjacent housings surrounding the central opening for the rotary plugs. Such plane partitions are indicated on FIG. 1 for the housings 32a and 40a. Each housing 32a is sealed by a detachable cover 32b flush with the upper face 12b of the slab. For each of the exchangers 34, the secondary sodium pipes 36, 38 move radially away from the reactor, whilst traversing the lateral metal wall 12c of the slab by orifices 52 and 54 equipped with metal sealing bellows between the wall and the pipes in order to ensure the seal with respect to the outside of housing 32a and so as to be able to fill it with inert gas. Each housing 40a comprises an upper portion of revolution, the wall of which portion being only at a limited distance from the corresponding flywheel 46 in order to reduce impact in the case of any fracture of the latter. Housings 40a are subdivided into two in the heightwise direction by a sealing floor 56 integral with the pump head and positioned below flywheel 46, so as to support the shattered flywheel during its deceleration. Moreover, each housing 40a is closed in its upper part by a cover 40b integral with motor 44 in the represented variant. Obviously motor 44 can be placed above the cover, as shown in FIG. 1, or in housing 40a if the dimensions of the latter permit. It is possible to visit housing 40a as a result of manhole 58 and gallery 60. The central opening 22 for the rotary plugs can be provided with a detachable dome-shaped cover 62 in order to provide a supplementary confinement if this is considered useful. All the other vertical penetrations existing in the relatively thin slabs according to the prior art can be reproduced with the present thick slab by modifications which are obvious to the Expert. In particular in an integrated reactor under construction, there are transfer ramps for the fuel elements leading to a swinging or tilting system placed above the slab. FIG. 2a diagrammatically shows a main reactor vessel 10 surmounted by a thick slab 12 according to the invention, which is laterally hollowed out to receive a swinging system 70 identical to that of the prior art. However, preference may be given to an arrangement according to FIG. 2b, which instead has a laterally extended fixing slab for defining a housing cavity 72 for the swinging device. Obviously the thick slab can be adapted so as to receive other fuel element transfer devices and ingenuity can be used for reducing the neutron shielding constraints of such apparatus by reducing their height and concealing them completely beneath the flat upper floor 12b of slab 12. In particular the following advantages result from the slab of the reactor according to the invention: The individual protection of the exchanger heads, each in a cavity sealed by a cover, means that they are not exposed to impacts which could result from handling errors in connection with heavy objects. The individual confinement of the exchanger heads, each of which is positioned in its own cavity, can easily be effected in neutral gas to eliminate any fire risk, without it being necessary to add a special fairing and whilst permitting continuous inspection by a television camera. The heads of the pumps and their flywheels are also not exposed to accidental impacts and only the pump motors could be exposed. The embedding of the pump flywheels in robust cavities provides security against their possible shattering. The requisite heights in each large component for ensuring an appropriate neutron protection can be reduced because said component is surrounded and surmounted by additional protection. The increased thickness of the slab 12 makes it possible, bearing in mind the relatively reduced dimensions of the housings for the component heads, to increase its strength and rigidity for a given steel mass and to reduce the sag linked with the heating of the underface. This increased thickness facilitates the construction and inspection of the metal part, as well as the introduction of the concrete filling. By separating the problem of the strength of the slab from that of the construction of the components, a better evolution of the design of projects is possible.