Patent Number: 042499950
Section: summary

This invention relates to a nuclear reactor, especially a liquid-metal cooled fast reactor, and is primarily concerned with a particular arrangement of a first vessel or so-called inner vessel which contains the reactor core. Said inner vessel has a lateral skew wall which forms a separation between the volume of hot liquid metal discharged from the reactor core and collected within said inner vessel and the liquid metal which is cooled as it passes through heat exchangers arranged in spaced relation around the core. After cooling, said liquid metal is returned into the space located between said inner vessel and the wall of a second vessel or so-called main vessel which surrounds the inner vessel and contains all the internal reactor components. The general structural characteristics of a liquid-metal cooled fast reactor corresponding to an integrated reactor-block arrangement are well known in the technique. In this type of reactor, which includes the liquid metal fast breeder reactor of French design known as Super Phenix, the reactor core is formed by an array of fuel assemblies maintained in the vertical position by engagement of their bottom end-fittings in a core support diagrid. An inner vessel which contains the reactor core is surrounded by an open-topped main vessel and this latter is suspended from a reactor vault roof of substantial thickness. The cavity or reactor vault which is closed by the roof is formed within a shield structure which is usually of concrete and forms an outer containment for the installation. The main vessel contains a suitable volume of liquid metal which usually consists of sodium and is circulated upwards through the reactor core within the inner vessel and in contact with the fuel assemblies. The hot sodium which has absorbed the heat produced by nuclear fission within the fuel assemblies is collected at the top of said inner vessel, then directed towards heat-exchanger inlet ports. The heat exchangers are suspended vertically from the reactor vault roof in such a manner as to extend downwards to a point of immersion below the level of liquid metal. After passing through the heat exchangers, the cooled sodium is discharged from the lower ends of these latter into the space formed between the inner vessel and the main vessel, then distributed between said vessels and recycled by circulating pumps which are also suspended from the reactor vault roof and spaced at intervals around the reactor core between the heat exchangers. Said pumps then return the cold sodium into the core support diagrid at a sufficient pressure to permit a further passage through the reactor core and thus to produce a continuous circulation. In accordance with an arrangement which is conventional in this type of integrated reactor, the volumes of hot sodium within the inner vessel and of cold sodium between said inner vessel and the main vessel are separated by a transverse skew wall constituting an extension of the lateral wall of the inner vessel, said skew wall being traversed in leak-tight manner by the bodies of components such as the pumps and heat exchangers in particular. In the case of these latter, the hot sodium inlet ports are located above the skew wall and the cold sodium outlet ports are located beneath this latter. In accordance with French Pat. No. 2,220,847, the peripheral edge of the skew wall is bent-back towards the bottom of the main vessel and joined to this latter or to a structure which extends in a direction parallel to the main vessel wall, thus forming a total separation between the cold sodium region and the hot sodium region. The shape which is thus adopted for the edge of the skew wall makes it possible in particular to eliminate the presence of zones of stagnant sodium beneath the skew wall while avoiding the creation of harmful stresses and at the same time offering a high degree of mechanical strength. It is not possible, however, to eliminate the effects of thermal shocks produced during variations in operating regime, especially at the time of reactor shutdown. This invention relates to a liquid-metal cooled nuclear reactor comprising an open-topped main vessel having a vertical axis and containing the liquid metal, an inner vessel mounted within the main vessel, and an inner vessel extension in the form of a transverse skew wall provided with a downwardly bent edge joined to the main vessel or to a structure connected to said main vessel. The shape and arrangement of the skew wall are such as to achieve enhanced mechanical strength and also to ensure continuous thermal protection of said skew wall by producing an appreciable drop in temperature. This accordingly permits of a considerable reduction in the stresses developed in the skew wall at the different operating regimes. To this end, the reactor under consideration is distinguished by the fact that the skew wall is associated with a baffle which extends above said skew wall and delimits with this latter a space containing a practically static volume which forms a thermal screen between the hot liquid metal located within the inner vessel above the baffle and the cold liquid metal located between the inner vessel and the main vessel beneath the skew wall. As an advantageous feature, the skew wall has the shape of a portion of a torus of revolution about the axis of the main vessel and is joined by means of conical walls on the one hand to the inner vessel and on the other hand to the main vessel or to the structure which is connected to said main vessel. In a first embodiment of the invention, the baffle which is placed above the skew wall is horizontal and rests on stationary bearing members. In a first alternative form of this embodiment, the baffle consists of a single unit which is supported on stationary bearing members by means of sliding contacts. In another alternative form, the baffle is constituted by adjacent sectors in juxtaposed relation and provided successively with overlapping edges for ensuring continuity of the baffle, each sector being joined to one of the cylindrical sleeves through which a pump or heat exchanger is intended to pass. Preferably, the baffle is provided with circumferential ribs for facilitating the absorption of thermal stresses during operation. In accordance with a second embodiment, the baffle is self-supporting and inclined towards the axis of the main vessel, said baffle being provided with an extension in the form of a lateral and vertical bearing shell placed within the inner vessel. Depending on requirements, the sloping side portions of the baffle are bent downwards or raised in a direction parallel to the axis of the main vessel in order to ensure confinement of the space in which the liquid metal is conveyed between the skew wall and the baffle as well as insulation of said space with respect to the hot liquid metal within the inner vessel. Finally, in another alternative form, the baffle has a horizontal surface which rests freely on stationary bearing members and is provided on its internal and external peripheries as well as at the point of penetration by each cylindrical sleeve with a downwardly-extending side portion which is immersed in the liquid metal and traps a blanket layer of neutral gas beneath the horizontal surface.