Patent Number: 045086788
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a nuclear reactor cooled by a liquid metal of the type comprising a vessel filled with liquid metal, whose upper part is sealed by a rigid slab which supports the heat exchangers and the pumps of the primary circuit. More specifically the invention relates to an integrated fast neutron nuclear reactor, i.e. of the type in which the complete primary circuit of the reactor is housed in the vessel. In this type of reactor, the core thereof containing the fuel assemblies rests on the bottom of the vessel or on its periphery by means of a support for the supply of liquid metal (generally sodium) and a plate system. The liquid sodium is heated in the core by the fission reaction of the nuclear fuel before entering a hot collector placed above the core. It then circulates in heat exchangers in which it transmits a large part of its heat to the fluid (generally sodium) flowing in the secondary circuits. The cooled primary sodium leaving the lower part of the heat exchangers in a cold collector is sucked in by primary pumps, which reinject it into the support. In this type of reactor, the exchangers and primary pumps are generally suspended on the slab sealing the reactor vessel. The same applies with regards to a certain number of other members necessary for the operation or safety of the reactor, including the exchangers for cooling the reactor when it is shut down. In known manner the slab sealing the vessel is constituted by a welded sheet metal structure forming a group of concrete-filled cavities in order to constitute a neutron protection and contribute to the rigidity of the slab. The lower face of the slab is provided with a cooling circuit and a lower thermal insulation covering immersed in the neutral gas above the liquid metal at approximately 500.degree. C. This face can then be kept at a temperature which does not exceed e.g. 100.degree. C. The thickness of the slab, which is fixed in such a way that an adequate strength and displacements which are sufficiently reduced during the temperature variations of the lower face are obtained is approximately 2.50 m for large reactors with an electric power of 1000 to 1500 MW. The exchangers and primary pumps are installed in a group of orifices or vertical shafts passing through the slab and having a diameter which is sufficient to permit their vertical introduction. In their part corresponding to the thickness of the slab, these components have sealing, as well as thermal and neutron protection members ensuring a functional continuity with the slab. The heads of these components, which are specific to their operation (motors for the pumps, connections to the secondary circuits, etc.) are positioned above the slab, as well as the system of circuits. A slab formed in this way is considered to be thin because every effort is made to reduce the heights devoted to neutron insulation and supporting functions, both with respect to the slab in order to facilitate the overall design and on the components, whose total height considerably influences the cost and ease of handling. A structure formed in this way with a thin slab in stages and an upper area for the heads of the components suffers from several disadvantages. Firstly the group of equipment and heads of components above the slab are vulnerable to impacts and shocks taking place during the handling of heavy objects above the slab. In addition, the thus heightwise exposed heads of exchangers can only be protected from possible secondary sodium leaks leading to a fire by adding a doubling envelope, which is onerous and makes it more difficult to regularly inspect the main wall. Moreover, for a given strength, the thinness of the slab makes it necessary to use more steel than if freedom existed with regards to the thickness. Furthermore, to limit heightwise displacements linked with expansion of the lower plate, this thinness imposes severe constraints with regards to its temperature. Finally the cavities within the slab, which are entirely filled with concrete cannot be inspected. BRIEF SUMMARY OF THE INVENTION The present invention relates to a liquid metal-cooled nuclear reactor, which obviates the disadvantages referred to hereinbefore in connection with the hitherto known reactors with a relatively thin slab. More specifically the invention relates to the construction of a reactor in which the slab sealing the vessel substantially forms a flat floor preventing any risk of impact during handling operations above the slab, whilst ensuring an individual confinement of the heads of the exchangers permitting a direct inspection of the primary wall. To this end the present invention proposes a liquid metal-cooled nuclear reactor comprising a vessel filled with liquid metal and whose upper part is sealed by a rigid slab having a central opening in which is housed at least one rotary plug overhanging the reactor core and first peripheral openings arranged in the form of a ring around the central opening and by which are suspended heat exchangers, wherein at least one of said first peripheral openings has, in the thickness of the slab, a lower part by which is suspended one of the heat exchangers and a first upper housing by which is confined the exchanger head, said first housing being sealed by a cover above the exchanger head. It is clear that in a reactor constructed in this way due to the individual confinement of the exchanger heads a high degree of safety and reliability is obtained with respect to sodium leaks and impacts due to handling operations carried above the slab. In addition, these characteristics make it possible to simplify the heads of the exchangers by eliminating their double-walled fairing so that the primary wall can be directly inspected e.g. by a television camera. Moreover, the increase in the thickness of the slab compared with the prior art reactors, which increase is rendered necessary by the use of the above-indicated housings, makes it possible for a given mass of metal, to increase the strength and the rigidity of the slab, subject to maintenance of the necessary strength of the radial positions which exist between the housings on the whole height of the slab. This increase of the thickness of the slab, e.g. until 6 m, also makes it possible to much more easily perform the protective fillings because it makes the metal structures of the slab much more easily accessible for human intervention. Finally these characteristics make it possible to construct a flat floor around the rotary plug, which facilitates operation. The housings in which the exchanger heads are received can be extended radially for the passage of the pipes of the secondary cooling circuit, until a cylindrical wall surrounding the slab, said wall being traversed by the pipes through external tight packings. The exchanger heads and the pipes being thus confined within these housings, the double walls required in the prior art reactors can be suppressed. According to another feature of the invention, the slab having in its thickness second peripheral openings by which are suspended pumps, at least one of these second peripheral openings having, in the thickness of the slab, a lower part by which is suspended one of said pumps, and a second upper housing by which is confined the head of said pump, constituted by the going out of the pump shaft and by the required tightness, and at least a position of the driving members constituted by a drive shaft, possibly a flywheel, and a motor, this second housing being sealed by a cover above the pump head. It is favourable to locate the flywheel in the housing to limit the consequences of possible breaking. According to the particular case, the motor driving the rotating shaft can either be confined in the second housing, or can be placed outside said housing above the cover. The latter solution has the advantage of cooling the motor and of not excessively increasing the thickness of the slab due to the overall dimensions of such motors. According to another feature of the invention in the thickness of the slab there is a third housing for handling fuels, said third housing being connected to the interior of the vessel and to a fuel handling installation by two transfer devices. According to yet another feature of the invention it is also possible to cover the rotary plug of a detachable cover fixed to the slab, said cover being more particularly shaped like a dome.