Patent Number: 044877420
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a fast neutron nuclear reactor equipped with at least one residual power removal device. It is known that in a fast neutron reactor, the reactor core is immersed in a predetermined volume of liquid metal (generally sodium) contained in a vertically axed vessel, sealed in its upper part by a horizontal sealing slab. In normal operation, the power given off by the fision reaction in the reactor core is absorbed by the circulation of the liquid metal in a primary circuit incorporating the pre-vacuum pumps and the intermediate exchangers respectively ensuring in operation the continuous circulation and cooling of the liquid metal. In integrated reactors the complete primary circuit is located in the reactor vessel, whereas it passes out of the vessel in the case of a loop-type reactor. In general, the heat extracted from the reactor core by the liquid metal of the primary circuit is transferred to a liquid metal (generally sodium) circulating in a secondary circuit comprising steam generators which, in turn, transfer the heat to a water/steam circuit operating the turbines of an electricity generating plant. It is obvious that in the case of an operational accident leading to the stoppage of the pre-vacuum pumps, the core in which the fission reactor is immediately stopped due to the dropping of the scram rods, still gives off a large amount of residual calorific power, which should be reliably and effectively eliminated in order to prevent local melting of the core. For this purpose, it is conventional practice to provide loops or circuits for cooling the reactor when shut down and they comprise heat exchangers directly immersed into the liquid metal contained in the vessel and pumps ensuring the circulation of the liquid metal (generally sodium) circulating in said loops or circuits in order to remove the residual power from the core by means of liquid metal/air exchangers. Although such shutdown reactor cooling loops or circuits have quite satisfactory operating characteristics, the numerous components (exchangers, expansion vessel, electromagnetic pumps, purification system, sodium storage tanks) located on these loops, as well as the level difference (approximately 18 m) between the exchanger immersed in the sodium and the air exchanger (level difference which is added to the height of the chimney or flue ensuring the cooling of the sodium-air exchanger) make their construction relatively complex and therefore costly. Moreover, the operation of these loops necessitates an external mechanical energy supply at the pumps ensuring the circulation of the liquid metal. From the safety standpoint, this feature is obviously not satisfactory, because it renders these loops ineffective in the case of failure of the electric power supply circuit. Finally, the quantity of heat removed by the shutdown reactor cooling loops must be adjusted as a function of the temperature of the liquid metal in the vessel. BRIEF SUMMARY OF THE INVENTION The object of the present invention is a fast neutron nuclear reactor incorporating a novel residual heat removal system and having a capacity comparable to that of the prior art systems, whilst having a simpler construction and therefore involving lower initial costs. The invention also relates to a reactor in which the residual heat removal system operates entirely in natural convection without any external mechanical energy supply, whilst benefiting from a self-regulating effect making it possible to remove an energy quantity proportional to the temperature of the liquid metal in the vessel. Therefore, the present invention proposes a fast neutron nuclear reactor comprising a vertically axed vessel containing the reactor core and a volume of liquid metal for cooling the same, a horizontal sealing slab sealing the upper part of the vessel, at least one pre-vacuum pump and at least one heat exchanger respectively ensuring in operation the circulation of the liquid metal in the core and its cooling, as well as at least one device for removing the residual power ensuring the cooling of the liquid metal in the case of a stoppage of the pre-vacuum pumps, wherein the residual power removal device comprises an evaporator incorporating a bundle of tubes in glove finger-like form immersed in the liquid metal, so as to pass into the vapour phase a heat transfer fluid initially in the liquid phase, an adiabatic collector of said vapour phase incorporating a pipe traversing the reactor slab and a condenser in which the heat transfer fluid in the vapour phase exchanges its heat with an external cooling fluid and condenses in the liquid phase before dropping again into the evaporator by the adiabatic collector. As a result of these features, a novel residual heat removal system is obtained, which is based on a thermodynamic evaporation/condensation cycle of a heat transfer fluid such as mercury. This system makes it possible to ensure an axial energy transfer between the evaporator in the liquid metal contained in the vessel and the condenser, cooled e.g. by air which is circulated by means of a chimney or flue, without any external mechanical energy supply, the system operating entirely with natural convection. Furthermore, as a result of a pipe constituting the adiabatic collector connecting the evaporator and the condenser passing through the reactor slab makes it possible to benefit from the reduced diametral dimensions in connection with the slab and a reduced pressure drop between the evaporator and the condenser. According to a secondary feature of the invention, the wall of the pipe constituting the collector, as well as the wall of each of the tubes of the evaporator internally have a capillary structure piping the heat transfer fluid in the liquid phase, so as to prevent the entrainment of the latter by the fluid in the vapour phase, whilst regularizing its flow towards the evaporator. Preferably, the upper end of the bundle of tubes is installed on a tube plate constituting the lower end of the adiabatic collector and the upper end of each of the tubes projects over a given height above the tube plate in order to define a buffer reservoir for the heat transfer fluid in the liquid phase. In order to ensure the uniform distribution of the flow of the heat transfer fluid in the liquid phase in each tube, the upper end of the latter projecting above the tube plate can have at least one row of slits and/or holes made on the side wall of the tubes. The circulation of the liquid metal contained in the vessel around the bundle of tubes of the evaporator can be piped by means of a ferrule surrounding the bundle of tubes, having at least one inlet port at its upper end and open at its lower end to permit the removal of the liquid metal and to permit the free downward expansion of the ferrule and the tube. According to a first embodiment of the invention, the condenser is supported by the reactor slab and is located within the reactor enclosure. It can then comprise a caisson or box in which are located a toroidal supply or feeding collector connected to the upper end of the pipe constituting the adiabatic collector, a toroidal collector for receiving the condensate positioned below the supply collector and an annular bundle of fin tubes connecting the supply and condensate receiving collectors, the external cooling fluid being atmospheric air which enters the box by a lateral pipe and leaves it by a chimney or flue positioned above the supply collector. According to a second embodiment of the invention, the condenser is positioned outside the reactor enclosure. It can then comprise a caisson or box in which are located at least one substantially horizontal and rectilinear supply collector connected to the upper end of the pipe constituting the adiabatic collector, two condensate receiving collectors positioned below the supply collector and on either side of the vertical plane passing through the latter and two planar bundles of fin tubes connecting the supply collector to the condensate receiving collector in order to define a dihedron with substantially horizontal edges, the cooling fluid being atmospheric air which enters the box via the interior of the dihedron and leaves it by a chimney or flue positioned above the bundle of tubes.