Patent Number: 044877420
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 diagrammatically shows a fast neutron nuclear reactor of the integrated type. The reactor core 10 is immersed in a volume 12 of cooling liquid metal (normally sodium) contained within a vertically axed vessel 14. The upper part of vessel 14 is sealed by a horizontal sealing slab 16, which rests by its periphery on a vessel shaft 18. The main vessel 40 is duplicated within the vessel shaft 18 by a safety vessel 12 which, like vessel 14 is suspended on slab 16. The components placed above slab 16 are disposed in a confinement enclosure 19, whereof only part is shown in FIG. 1. In reactors of the integrated type, like that diagrammatically shown in FIG. 1, slab 16 is traversed by a series of components necessary for the operation of the reactor. Thus, in its central part, the slab supports a system of rotary plugs 22 and in its peripheral part intermediate exchangers 24 and pre-vacuum pumps 26 regularly distributed around the core. An inner vessel 28 defines within vessel 14 a "hot" collector 30 containing the "hot" liquid metal issuing into the upper end of core 10 and a "cold" collector 32 in which is collected the liquid metal leaving exchangers 24. The liquid metal is then taken up by pumps 26 in order to be passed through the pipes 34 into a support 36 ensuring both the supply of liquid metal to core 10 and the supporting of the said core on the bottom of vessel 14 by means of flooring 38. Thus, in operation, the liquid metal permanently circulates through the core. Exchangers 24 ensure the extraction of the heat given off by the fission reaction. This heat is then transferred to a not shown secondary circuit before being used in the turbines of a water/vapour or steam circuit for generating electricity. According to the invention, in the interior of vessel 14 are also provided the evaporators 42 of the residual power removal devices 40. These devices 40, whereof only is shown in FIG. 1, pass through slab 16 and make it possible, in the case of a stoppage of the pre-vacuum pumps 26, to ensure an appropriate cooling of the hot liquid metal contained in collector 30, so as to remove the residual power of the reactor. FIG. 2 illustrates on a larger scale the constructional details of one of the devices 40. Each of the devices 40 comprises an evaporator 42 positioned below slab 16 and immersed in liquid metal 12, a condenser 44 positioned above slab 16 and within the reactor enclosure 19 (FIG. 1) and an adiabatic collector 46 passing through slab 16 to link evaporator 42 with condenser 44. As is more clearly shown in FIGS. 2 and 3, the evaporator 42 comprises a bundle of straight, vertical tubes 48 sealed at their lower end so as to have a glove finger-like configuration. Tubes 48 are entirely positioned below the free level 13 of liquid metal 12. In order to ensure, in the manner described hereinafter, the piping of the liquid phase of the heat transfer fluid contained in device 40, the inner wall of each of the tubes 48 is covered with a capillary structure 50. Each of the tubes 48 is fixed in the vicinity of its upper open end to a horizontal tube plate 52, which at the same time defines the lower end of adiabatic collector 46. Obviously, this fixture takes place in a tight manner, e.g. by welding. Evaporator 42 also comprises a ferrule 54 welded to the tube plate 52 and surrounding the bundle of tubes 48 . Ferrule 54 serves to pipe or channel the flow of liquid metal 12 around tubes 48. For this purpose, in its upper part in the vicinity of tube plate 52, ferrule 54 has inlet ports 56 and is open in its lower part. Thus, it establishes a flow of liquid metal 12 by a thermosiphon effect between ports 56 and the lower opening of said ferrule. In the same way, this configuration of ferrule 54 enables it to freely downwardly expand at the same time as tubes 48. It can be seen in FIG. 3 that the upper end of each of the tubes 48 projects above the tube plate 52 by a given height. This feature makes it possible to define a buffer reservoir 57 in the lower part of a vertical pipe 58, which extends ferrule 54 above tube plate 52 to define the adiabatic collector 44. The buffer reservoir 57 formed in this way above the tube plate 52 also constitutes the supply overflow for the capillary structure lining the interior of each of the tubes 48. As is illustrated in FIG. 3, the uniform distribution of the flow of heat transfer fluid in the liquid state in each of the tubes can be obtained by making sawtooth-like slits 60 at the upper end of each of the tubes and by making a row of holes 62 in the side wall of the tubes. Obviously, these two solutions can be separated from one another, i.e. the upper end of each of the tubes can be provided with slits like slits 60, or can be provided with holes like holes 62. On referring once again to FIG. 2, it can be seen that the existence of the adiabatic collector 46, constituted by pipe 58 is imposed by the distance separating the free level 13 of the liquid metal contained in the vessel and below which must be positioned evaporator 42 from the entrance into condenser 44 positioned above slab 16. In the said collector, the flow of the heat transfer fluid in the vapour phase undergoes an inevitable pressure drop, which reduces the axial flow. In order to reduce this pressure drop to the greatest possible extent and reduce the diameter of the passage through slab 16 by collector 46, it has been chosen in the manner indicated hereinbefore to connect the tubes 48 of evaporator 42 to a single condenser 44 via a single vertical pipe 58 ensuring the collection of the vapour or steam reduced in tubes 48. This solution significantly reduces the pressure drops in the adiabatic collector in a minimum proportion of 20% compared with other possible solutions, such as that consisting of having a group of individual heat pipes, each having an adiabatic zone and a condenser. Preferably, the inner wall of the vertical pipe 58 is covered on its inner face by a capillary structure 64 which, like capillary structure 50 of each of the tubes 48, makes it possible to prevent entrainments of the heat transfer fluid in the liquid phase by the gaseous phase leaving tubes 48 and regularizes the liquid film returning to evaporator 42 via buffer reservoir 57. According to another, not shown, constructional variant, capillary structure 64 can be replaced by at least one small diameter pipe for returning the liquid to the buffer reservoir 57. In the embodiment of the invention shown in FIG. 2, the condenser 44 comprises a caisson or box 66 resting via a cylindrical skirt 68 on the reactor slab 16. The vertical pipe 58 defining the adiabatic collector 46 is extended upwards within box 66 to issue into four bends 70, which are positioned at 90.degree. from one another. Bends 70 drop down again towards a toroidal supply collector 72 of the actual condenser. The latter comprises fin tubes 74 distributed e.g. over 10 circular and concentric levels or layers so as to form an annular bundle. The lower ends of tubes 74 issue into a toroidal condensate-receiving collector 76, similar to collector 72 and positioned below the latter. More specifically, the ends of the tubes 74 are sealingly fixed to the collectors 72 and 76 e.g. by welding. Collectors 72 and 76, as well as the bundle of tubes 74, are arranged coaxially with respect to the vertical pipe 58. The heat transfer fluid in the liquid phase forming in collector 76 is recycled towards the adiabatic collector 46 by bent pipes 78, which issue into pipe 58 at the upper end of the capillary structure 64 formed within the latter. FIG. 2 shows that the pipes 78 are bent in siphon-like manner, so as to form a hydraulic seal, whose developed height corresponds to the difference of the vapour pressures. In the embodiment shown in FIG. 2, condenser 44 is cooled by means of atmospheric air. The air is sucked into box 66 by a lateral pipe 80 under the effect of the pressure reduction created in the box by a vertical chimney or flue 82 positioned above the latter. FIG. 4 shows that the box 66 is shaped like a centrifugal fan helix level with the bundle of tubes 74, which makes it possible for the cooling air to circulate relatively homogeneously with the bundle. To permit the putting into operation or out of operation of the device shown in FIG. 2, vents 84 are positioned in the lateral pipe 80 at the entrance to box 66. Finally, a biological shield 86 is provided within the supporting skirt 68. According to a not shown constructional variant, condenser 44 can be connected to the assembly constituted by evaporator 40 and adiabatic collector 46 by a flange joint, in such a way that the disassembly of said joint enables the evaporator to be repaired. Preferably, the heat transfer fluid in device 40 is mercury. Preference is given to this product because of the wide temperature range in which it can be used (170.degree. to 600.degree. C.) its low vapour pressure (2 to 11 bars) in the considered temperature range (390.degree. to 530.degree. C.), the high axial heat transfer level which it permits, its dissolving in sodium in the case of a leak, as well as its radiation behaviour, when compared with other products which can be used such as potassium, sodium and sulphur. As is illustrated by the arrows in FIGS. 2 to 4, the present device operates in the following way. Mercury in the filled state at the bottom of tubes 48 of evaporator 42 is heated by the liquid metal 12 contained in the reactor vessel and which circulates in natural convection between inlet ports 58 and the outlet opening formed at the lower end of ferrule 54. Thus, the mercury is vaporized and rises in vertical pipe 58 in order to enter the supply collector 72 via bent pipe 70. When the device is put into operation by opening vents 84, the pressure drop created in box 66 as a result of the suction of air through chimney or flue 82, makes air circulate through the bundle of fin tubes 74, which has the effect of condensing the mercury, whose liquid phase is collected in collector 76. This liquid phase is then transferred by pipes 78 and capillary structure 64 into buffer reservoir 57, from where it drops again into each of the tubes 48 via capillary structures 50. Thus, cooling is brought about on reactor shutdown without any external mechanical energy supply, because the device functions entirely in natural convection. Furthermore, within the maximum power limits which can be removed with condenser 44 as a function of the mercury vapour temperature, it should be noted that an increase in the vaporization temperature of the mercury and consequently the axial power transferred corresponds to any increase in the temperature of the liquid metal 12 within the vessel. Thus, the device is self-regulating. In the embodiment described with reference to FIGS. 2 to 4, reactor enclosure 19 (FIG. 1) must be traversed by large-size air ducts in the form of supply pipe 80 and flue 82. Moreover, the dimensions of condenser 44 above slab 16 are relatively large as a result of the presence of other components. To obviate these disadvantages, FIGS. 5 and 6 show a second embodiment of the invention, which differs from the embodiment of FIGS. 2 to 4 through the different design of the condenser and through the arrangement of said condenser outside the reactor enclosure. For simplification purposes, the same reference numerals, increased by 100, are used for designating the same elements as in the first embodiment. FIG. 5 shows the right-hand part of the reactor vessel shown in FIG. 1 and it is possible to see the main vessel 114 containing liquid metal 112, the safety vessel 120 duplicating the main vessel 114, slab 116 which supports vessels 114 and 120 and whose peripheral edge rests on the vessel shaft 118, internal vessel 128 separating the hot collector 130 from the cold collector 132, as well as a heat exchanger 124. FIG. 5 also shows a residual power removal device 140 comprising an evaporator 142 immersed in the liquid metal 112, a condenser 144 positioned above slab 116 and outside reactor building 119 in this embodiment, as well as the adiabatic collector 146. Evaporator 142 and adiabatic collector 146 are identical to those described relative to the first embodiment. However, apart from its positioning outside enclosure 119, condenser 114 is of a different design to that of the preceding embodiment. Thus, although this condenser also has a box 166 resting via a ferrule 168 on the structure of the vessel shaft 118 and although the cooling air is supplied and removed by means of a lateral pipe 180 and a flue 182, it can be seen that the actual condenser is formed by two planar bundles of fin tubes 174 (FIG. 6), arranged in accordance with a dihedron having a horizontal edge or an edge which is inclined slightly relative to the horizontal. The edge of the thus formed dihedron is materialised by the vapour supply pipe 188, which extends the pipe 158 constituting the adiabatic collector 146 and traverses enclosure 119 in order to enter box 166. This supply pipe 188 is linked with two supply collectors 172 into which issues the upper end of each of the bundles of tubes 174. The lower end of these bundles issues into the condensate-receiving collectors 176. Like pipe 188, collectors 172 and 176 are rectilinear and substantially horizontal. More specifically, it can be seen in FIG. 6 that collectors 176 are positioned ebelow the supply collectors 172 and on either side of the vertical plane passing through pipe 188. Moreover, the collectors 172, 176, as well as the tube bundles 174 are arranged substantially symmetrically with respect to said plane. Obviously, as in the first embodiment, collectors 176 are connected to pipe 158 within the capillary structure of the latter by siphon-like bent pipes 178. As is shown in FIGS. 5 and 6, the air supply by pipe 180, after opening vents 184, takes place by means of the dihedron formed by tubes 174, so as to aid natural convection. Thus, the air passes through the tube bundles before escaping through flue 182. Obviously, the invention is not limited to the embodiments described in exemplified manner hereinbefore and in fact covers all variants thereof. Thus, it is readily apparent that the fluid cooling condenser 144 is not limited to atmospheric air or even to gases and could optionally be water, although the safety function which must be fulfilled by the devices for cooling the reactor on shutdown make it preferable to carry out cooling by atmospheric air. In the same way, it is obvious that the arrangements of the condensors described with reference to FIGS. 2 to 4 and FIGS. 5 to 6 can optionally be reversed. Thus, it would be possible to place within the reactor enclosure a condenser of the type described with reference to FIGS. 5 and 6. Conversely, it would also be possible to place outside the enclosure a condenser of the type described with reference to FIGS. 2 to 4. Finally, it is obvious that this residual power removal device can be used both in a loop-type reactor and in an integrated reactor.