Patent Number: 042499950
Section: description

In the example shown in FIG. 1, the reference numeral 1 generally designates the lower portion of a fast nuclear reactor which is cooled by a liquid metal. Said reactor comprises in particular a main vessel 2 which is open at the top portion (not shown) and constituted by a lateral cylindrical shell 3 which terminates in a substantially hemispherical bottom wall 4. The main vessel 2 is surrounded externally in known manner by a second vessel 5 having a parallel wall or so-called safety vessel. The reactor core 6 is placed within the main vessel 2 beneath the level (not shown) of the liquid metal contained within this latter and rests on a core support diagrid 7 which is applied against the bottom wall 4 of the vessel 2 by means of a support structure 8. The reactor core and diagrid support structure 8 are completely immersed in the liquid metal contained in the main vessel 2 and usually consisting of sodium. This volume of liquid metal is also supplied through holes 9 formed in the base of the diagrid support structure 8 to a narrow annular space 10 formed first between the bottom wall 4 of the main vessel 2 and a sheet metal member 11 which is parallel to this latter, then extends opposite to the lateral cylindrical shell 3 by means of two parallel walls 12 and 13 which define two spaces 14 and 15. During reactor operation, the main vessel 2 is cooled by the flow of liquid metal which is circulated upwards within the space 14, then downwards within the space 15. At the bottom of the space 15, this coolant flow is discharged through holes 16 formed in the wall 13 in order to return to the volume contained within the main vessel 2. In accordance with an arrangement which is also conventional, the reactor core 6 is placed within an inner vessel, the lateral wall 17 of which has a substantially conical contour in the example of construction under consideration in order to be joined tangentially to a portion of torus 18 which extends annularly around the axis of the reactor core and of the main vessel. This portion of torus 18 which forms a skew wall is extended by a second conical portion 19 which is bent downwards and joined to the wall 13 of the cooling structure which forms an internal jacket for the lateral cylindrical shell 3 of the main vessel. Under these conditions, the skew wall 18 and its conical extensions 17 and 19 separate the volume of liquid metal within the main vessel 2 into two regions 20 and 21 respectively which are located in one case above said skew wall and in the other case below this latter. The nuclear reactor shown in FIG. 1 corresponds to a general arrangement known in the technique as an integrated design. Provision is accordingly made for a series of heat exchangers 22 and circulating pumps 23 placed within the interior of the main vessel 2 and disposed at suitable intervals around the reactor core 6 in such a manner that the bodies of said heat exchangers and of said pumps extend vertically through the skew wall 18 which forms a separation between said regions 20 and 21. Each heat-exchanger body 22 is provided with inlet ports or windows 24 located in the region 20 above the skew wall 18 and outlet windows 25 provided beneath said skew wall in the region 21 between the inner vessel 17 and the main vessel 2. The skew wall 18 is traversed by each body of the heat exchangers 22 or pumps 23 through wells each constituted by a cylindrical sleeve 26 which surrounds the heat-exchanger or pump body and is welded to the skew wall. In the case of the heat exchanger, said sleeve is in turn covered by a bell-cap 27 connected to the heat exchanger and forming a space 28 in which is trapped a suitable quantity of neutral blanket gas. The levels of liquid metal inside and outside the sleeve 26 are in communication respectively with the regions 20 and 21 and are represented in the drawing by the references 29 and 30. In accordance with the invention, the baffle 18 together with its conical extensions 17 and 19 towards the inner vessel and the main vessel is associated with a baffle 31 designed in the form of a single and substantially horizontal sheet metal plate 32 as shown in the example of construction of FIG. 1. Said baffle is provided with sliding contacts or shoes 33 which rest on bearing members of the L-section type, for example, these latter being rigidly fixed either to a support bracket 35 extending from the outer surface of the cylindrical sleeve 26 or provided at the top of the lateral neutron shield 36 which surrounds the reactor core 6 within the inner vessel. The plate 32 is provided at its periphery with a bent-back edge 37 which leaves a small clearance space with respect to the wall 13. Finally, the plate 32 is advantageously provided with circumferential ribs 38 for absorbing thermal shocks and especially for reducing stresses within the baffle at the time of variations in operating regime. During reactor operation, the liquid metal which has passed upwards through the reactor core 6 is collected within the region 20 within the inner vessel above the skew wall 18, then penetrates into the heat-exchanger bodies 22 through their inlet windows 24. After cooling, said liquid metal is discharged from said heat exchangers through the windows 25 and collected within the region 21 beneath the skew wall 18, between the inner vessel and the main vessel. In this region, the cooled liquid metal is recirculated by the pumps 23. After suction through the diffusers 39 of said pumps which are supported by beams 40, the liquid metal is returned into the diagrid 7 through large-section ducts 41, then undergoes a further passage through the reactor core 6, thus maintaining a continuous circulation. By positioning the baffle 31 above the skew wall 18, there is thus defined with this latter an internal region 42 which is capable of constituting an effective thermal screen during operation by virtue of the quantity of liquid metal which is contained within this region and remains practically static. Furthermore, the use of sliding bearing members permits of free expansion of the baffle whilst the ribs 38 formed in this latter ensure a reduction of thermal stresses. Finally, the solution which is contemplated offers great simplicity of construction and is of very limited overall size. FIG. 2 illustrates an alternative form of the embodiment described in the foregoing in which the baffle 31 is no longer designed as a single unit as in the previous embodiment but is constituted by adjacent sectors 31a, 31b, 31c . . . , each sector being joined to a cylindrical sleeve 26 in which a pump body or heat-exchanger body passes through the baffle and the skew wall. Preferably, these sectors are provided with edges 31'a, 31'b, 31'c, . . . , which overlap successively in order to ensure continuity of the baffle. In this alternative form, there are again shown the circumferential ribs 38 in the form of circular undulations which are intended to endow the baffle with the necessary degree of flexibility by virtue of the inherent elasticity of said ribs. In the first alternative form of a second embodiment shown in FIG. 3, the elements which were already illustrated in FIG. 1 are again shown partially. In this variant, the baffle 51 is self-supporting and has a flat portion 52 which is inclined towards the axis of the main vessel and rests on a lateral cylindrical shell 53 which is mounted within the inner vessel and the lower end of which in turn rests on the diagrid support structure 8. At the opposite end which is directed towards the periphery, the baffle 51 has a downwardly-extending side portion 54 which leaves a narrow clearance space with respect to the wall 13. In order to confine the volume within the region 42, the baffle 51 is also provided with a flange 55 opposite to each of the skirts 26 through which the bodies of the heat exchangers or pumps 22 and 23 traverse the skew wall 18. Both the flange 55 and the downwardly-extending side portion 54 extend to the bottom level of the baffle in order to prevent circulation of liquid metal by natural convection. In this embodiment as in the previous form of construction, the liquid metal contained between the skew wall and the baffle remains practically stagnant during operation. In a second alternative form of the second embodiment shown in FIG. 4, the baffle 51 is again self-supporting as in the alternative embodiment shown in FIG. 3 and also has a portion 52 which is inclined towards the axis of the main vessel. In this variant, the baffle is arranged as indicated hereinafter with a view to ensuring leak-tightness between the regions 42 and 20. The baffle 51 is provided at its periphery with an upwardly-directed side portion 56 which extends parallel to the wall 13 to the neutral gas atmosphere 60 located above the free level 58 of liquid metal. The baffle 51 is also provided with a side portion which is similar to the side portion 56 around each of the penetrations (not shown) provided in the skew wall 18 for the cylindrical sleeves 26 which surround the pump bodies. At the point of penetration of the skew wall 18 by the heat-exchanger bodies 22, the baffle 51 is provided with a side wall 57 which extends upwards and terminates in the neutral gas space 28. The foregoing arrangements make it possible to prevent any circulation between the region 42 located between the skew wall and the baffle and the region 20 containing the hot sodium. Equalizing of pressures between the region 20 and the confined region 42 is obtained by means of orifices 59 formed in the lower portion of the cylindrical shell 53 which supports the baffle 51. FIG. 5 illustrates another alternative embodiment in which the baffle associated with the skew wall is mounted in a floating arrangement. In this alternative embodiment, the baffle 62 has a flat surface 63 which extends horizontally above the skew wall 18. This surface rests on supports such as those designated by the reference 64 and formed in the wall 13 on the side nearest the main vessel or in a cylindrical shell 65 on the side nearest the reactor core, or alternatively in the external surface of the cylindrical sleeves 26 through which the pump and heat-exchanger bodies traverse the skew wall. The surface 63 of the baffle is provided with downwardly-bent side portions 66 extending beneath the level of the liquid metal which is trapped within the supports 64, thus confining beneath the baffle a blanket layer 67 of suitable neutral gas such as argon or helium. By virtue of these arrangements, total leak-tightness is accordingly obtained between the volume of hot liquid metal above the baffle and the volume of colder liquid metal located beneath this latter, thus permitting an appreciable reduction in friction forces at the time of differential radial expansions of the baffle. Finally, the presence of the gas blanket ensures more efficient thermal insulation and serves to lower the temperature of the practically static volume of liquid metal between the baffle and the skew wall. 9n