Patent Number: 047056622
Section: claims

1. A fast neutron nuclear reactor comprising: a primary circuit integrated into a reactor vessel filled with liquid metal and containing a reactor core of the reactor, means for circulating the liquid metal, means for transferring the heat carried by the liquid metal to water circulating in a water/steam circuit, the heat transferring means comprising 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 liquid metal of the primary circuit, the outer tubes respectively surrounding the inner tubes in order to define therewith an annular space under a pressurized neutral gas and connected to a secondary circuit with a low thermal power having means for circulating the pressurized neutral gas in said annular space and for ensuring heat exchange, said reactor vessel being sealed by a horizontal sealing slab tightly passing through the steam generator, the steam generator having above said slab an upper tube plate supporting the group of inner tubes, the inner tubes being open at inner ends thereof, 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 and being closed at lower ends thereof and into the lower ends of which issue the inner tubes, and a lower tube plate surmounted by a neutral gas discharge and supporting the group of outer tubes, said outer tubes surrounding the intermediate tubes, the steam generator also having neutral gas tubes connecting bottoms of the outer tubes to neutral gas inlet ports located between the lower tube plate and the sealing slab, said neutral gas tubes being provided below the bottoms of the outer tubes with a substantially horizontal portion serving to compensate for differential expansions. 2. A reactor according to claim 1, wherein the steam generator has, below the lower tube plate, a liquid metal confinement ferrule encircling the groups of fitted inner, intermediate and outer tubes, said ferrule having an intake for the liquid metal located above the normal level of the liquid metal in the reactor vessel and is open at its lower end to permit the outflow of liquid metal. 3. A reactor according to claim 2, wherein the ferrule also has, below the normal level of the liquid metal in the reactor vessel, a normally closed entry window, means controlling the opening of said window when the liquid metal circulating means are stopped, so as to permit the circulation of the liquid metal within the ferrule by natural convection and its cooling by heat exchange with the neutral gas circulating in the annular space. 4. A reactor according to claim 2, wherein the ferrule has, below the normal level of the liquid metal in the reactor vessel, calibrated openings in order to permit the circulation of the liquid metal within the ferrule by natural convection and its cooling by heat exchange with the neutral gas circulating in the said annular space, when the liquid metal circulating means are stopped. 5. A reactor according to claim 1, wherein the ferrule is generally shaped like a vertically axed cylinder of revolution and defines with the inner wall at least one zone extending over its entire height and into which descends the vertical parts of the neutral gas tubes. 6. A reactor according to claim 1, wherein each of the inner tubes has two coaxial walls defining between them an annular zone closed at one of its ends and filled with a static fluid forming a heat shield. 7. A reactor according to claim 6, wherein the upper tube plate is surmounted by a water intake chamber of the water/steam circuit. 8. A reactor according to claim 7, wherein the annular zone is closed at its upper end, open at its lower end and filled with stagnant steam/water. 9. A reactor according to claim 6, wherein the upper tube plate forms the upper base of the steam generator, the inner walls of the inner tube extending above the upper tube plate to issue into water inlets of the steam/water circuit. 10. A reactor according to claim 9 wherein the annular zone is closed at its lower end, open at its upper end and filled with a stagnant gas above the reactor slab. 11. A reactor according to claim 6 wherein 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. 12. A fast neutron nuclear reactor comprising: a primary circuit integrated into a reactor vessel filled with liquid metal and containing a reactor core of the reactor, means for circulating the liquid metal, means for transferring the heat carried by the liquid metal to water circulating in a water/steam circuit, the heat transferring means comprising 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 liquid metal of the primary circuit, the outer tubes respectively surrounding the inner tubes in order to define therewith an annular space under a pressurized neutral gas and connected to a secondary circuit with a low thermal power having means for circulating the pressurized neutral gas in said annular space and for ensuring heat exchange, said reactor vessel being sealed by a horizontal sealing slab tightly passing through the steam generator, the steam generator having above said slab an upper tube plate supporting the group of inner tubes, the inner tubes being open at inner ends thereof, 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 and being closed at lower ends thereof and into the lower ends of which issue the inner tubes, and a lower tube plate surmounted by a neutral gas discharge and supporting the group of outer tubes, said outer tubes surrounding the intermediate tubes, the steam generator also having neutral gas tubes connecting bottoms of the outer tubes to neutral gas inlet ports located between the lower tube plate and the sealing slab, said neutral gas tubes being provided below and bottoms of the outer tubes with a substantially horizontal portion serving to compensate for differential expansions, said steam generator having a dismantlable part comprising the upper tube plate, the lower tube plate, the intermediate tube plate, the steam discharge chamber and the group of intermediate tubes.