Patent Number: 047056622
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fast neutron nuclear reactor of the type comprising a primary circuit integrated into a liquid metal-filled vessel and containing the reactor core, as well as primary pumps for bringing about the circulation of said liquid metal and means for transferring the heat carried by the liquid metal to the water circulating in a water/steam circuit having means for converting said heat into electricity. In existing reactors of this type, such as the French Phenix and Super-Phenix reactors, the heat carried by the liquid metal of the primary circuit (generally sodium) is transferred to the water of a water/steam circuit by an intermediate circuit, called the secondary circuit. A liquid metal (generally sodium) circulating in the secondary circuit removes the heat carried by the primary liquid metal in heat exchangers integrated into the reactor vessel and transfers it to the water of the water/steam circuit by steam generators outside the vessel. In these reactors, the liquid metal secondary circuit is provided so that there is no risk of contact taking place between the sodium of the primary circuit and the water of the water/steam circuit within the actual reactor vessel. Thus, it is known that a violent reaction is brought about by contact between the sodium and the water. In a fast neutron nuclear reactor, such a contact can occur at the steam generators. In order to prevent the shock wave resulting from a sodium - water reaction having repercussions up to the reactor core, it has been decided to position the steam generators outside the reactor vessel. In parallel, the existing reactors such as the French Phenix and Super-Phenix reactors are of the integrated type, in which the complete primary circuit is confined within the reactor vessel. This option has been chosen to take account of the fact that it is always preferable to confine a radioactive primary circuit both for radiation protection and security reasons. Bearing in mind these two opposing requirements, the existing integrated fast neutron reactors consequently comprise an intermediate liquid metal circuit, so that costs are significantly increased. Therefore, consideration has been given to the elimination of the intermediate liquid metal circuit by removing the primary circuit from the reactor vessel. However, this solution is not completely satisfactory, due to the high radioactivity of the fission products entrained by the primary liquid metal. SUMMARY OF THE INVENTION The object of the present invention is a fast neutron nuclear reactor maintaining the integration of the primary circuit within the reactor vessel, whilst making it possible to eliminate the conventional intermediate liquid metal circuit through the use of a novel type of steam generator, which can be placed within the reactor vessel without a sodium - water reaction occurring at this level. The present invention therefore proposes a fast neutron nuclear reactor of the type comprising a primary circuit integrated into a liquid metal-filled vessel and containing the reactor core, as well as means for circulating the said liquid metal and means for transferring the heat carried by the liquid metal to the water circulating in a water/steam circuit, wherein the heat transfer means comprise at least one steam generator located in the reactor vessel and having at least one group of inner tubes in which circulates the water of the water/steam circuit, at least one group of outer tubes immersed in the primary liquid metal, the outer tube surrounding each of the inner tubes in order to define therewith an annular space under a neutral gas pressure, connected to a secondary circuit with a low thermal power having means for circulating this pressurized neutral gas, such as helium in said annular space and ensure the heat exchange. As a result of these characteristics, liquid metal - neutral gas exchangers, rather than steam generators in the normal sense of the term, are integrated into the interior of the reactor vessel. Thus, the security requirements are satisfied and the costs for such a reactor are significantly reduced. According to a preferred embodiment of the invention, the reactor vessel is sealed by a horizontal sealing slab tightly passing through the steam generator, whereby the latter is provided above said slab with an upper tube plate supporting the group of inner tubes and open at their inner end, an intermediate tube plate surmounted by a steam discharge chamber of the water/steam circuit and supporting a group of intermediate tubes surrounding the inner tubes, closed at their lower ends and into the base of which issue the inner tubes, and a lower tube plate surmounted by a neutral gas discharge chamber and supporting the group of outer tubes surrounding the intermediate tubes, the steam generator also having neutral gas tubes connecting the bottom of the outer tubes to neutral gas inlet ports located between the lower tube plate and the sealing slab. In such a structure, it should be noted that all the tubes are suspended independently of one another on corresponding tube plates and are free to expand downwards. The mechanical reliability of the tubes and the steam generator is consequently improved. In the same way, although the lower ends of the outer tubes are connected to neutral gas tubes, the expansion of said outer tubes is also made possible by the fact that each of the neutral gas tubes preferably has a substantially horizontal part with an adequate length and reduced diameter and consequently a high expansion compensation capacity, positioned in the lower part of the apparatus. According to the invention, it is also necessary to note that the conventional jacket or casing of steam generators is eliminated and is replaced by a liquid metal confinement ferrule which has a limited thickness. This very simply designed ferrule is placed below the lower tube plate and encircles the groups of inner, intermediate and outer fitted tubes. It has an inlet for the liquid metal positioned above the normal level of said liquid metal in the reactor vessel and is open at its lower end to permit the discharge of the liquid metal. Thus, a satisfactory exchange height can be obtained without increasing the reactor vessel height. Moreover, the integration of steam generators having an intermediate neutral gas outflow within the reactor vessel makes it possible to eliminate the cooling exchangers of the reactor on shutdown. Thus, this reactor cooling function during the stoppage of the primary pumps can be fulfilled by the liquid metal - neutral gas exchanger part of each of the steam generators. In order that a circulating of the liquid metal by natural convection can then take place within the ferrule in the case of a stoppage of the primary pumps, the ferrule also has, below the normal liquid metal level in the reactor vessel, entry windows which are either normally sealed, to control means controlling the automatic opening of these windows during the stoppage of the primary pumps, or calibrated in such a way that under normal or rated operating conditions they only permit the passage of a reduced primary sodium flow. Preferably, the steam generator has a dismantlable part comprising the upper tube plate, the inner tube group, the intermediate tube plate, the steam discharge chamber and the intermediate tube group. This arrangement facilitates the manufacture in the factory, transportation, in site installation and in particular in service inspections, as well as subsequent checks or repairs to vertical outer tubes constituting the main tubes of the steam generator. In order to improve the thermal efficiency of the steam generators, each of the inner tubes preferably has two coaxial walls defining between them an annular zone sealed at one of its ends and filled with a fluid forming a heat shield. According to a first constructional variant according to the invention, the upper tube plate is surmounted by a water intake chamber of the water/steam circuit. The annular zone formed between the walls of each of the inner tubes is then sealed at its upper end, open at its lower end and filled with stagnant steam/ water, with a low heat exchange. According to a second constructional variant of the invention, the upper tube plate forms the upper base of the steam generator, the inner walls of the inner tubes are extended above the upper tube plate to issue into the water intake of the water/steam circuit. The annular zone formed between the walls of each of the inner tubes is then closed at its lower end, open at its upper end and filled with a stagnant gas, such as air above the reactor slab. In order to reduce the speed of the water at the bottom of each inner tube before it rises into the corresponding intermediate space, the lower end of the inner wall of each of the inner tubes is positioned above the lower end of the outer wall of each of these tubes. The water/steam circuit in conventional manner comprises a steam turbine, the secondary circuit preferably comprises a resuperheater ensuring a transfer of the heat carried by the neutral gas of said circuit to the steam to be resuperheated, before the latter is admitted into the corresponding part of the turbine. As a variant, consideration can be given to a secondary neutral gas circuit confined within the reactor building. In this case, the quantity of heat carried away out of the main steam generator can e.g. be used for additional live steam production and for additional reheating of the feed water.