Patent Number: 050135210
Section: claims

1. A fast neutron nuclear reactor which comprises a main vessel (1) enclosing liquid metal for cooling the reactor, an upper free level of said liquid metal being capable of moving inside an internal shell, during normal operation of the reactor, between two defined positions respectively referred to as high level (14a) and low level (14b), and, inside the main vessel (1), the internal shell (6) comprising at least one cylindrical sleeve (10) with a vertical axis, a top part of which is located above said high level (14a) of the liquid metal inside the main vessel, and an annular enclosure (20) on the internal periphery of the cylindrical sleeve (10) of the internal shell (6), open upwards at its top part and delimited by a secondary sleeve (21) arranged substantially coaxially and inside the cylindrical sleeve (10), and by an annular base (22) fixed to the bottom end of the cylindrical sleeve (21) and onto the internal surface of the cylindrical sleeve (10) of the internal shell, below the low level (14b) of the liquid metal, wherein the top end of the secondary sleeve (21) is located below the high level (14a) of the liquid metal. 2. Nuclear reactor according to claim 1, wherein the annular base (22), which is solid, forms a reinforcement for the cylindrical sleeve (10) of the internal shell (6) to limit the effects of seismic stresses. 3. Nuclear reactor according to claim 1, wherein the annular enclosure (20) has a width, in the radial direction of the cylindrical sleeve (10) of the internal shell (6), up to 20 cm, and a height, in the axial direction of the cylindrical sleeve (10), up to 1.5 m. 4. Nuclear reactor according to claim 3, wherein the top end of the secondary sleeve (21) of the annular enclosure (20) is arranged at a level situated substantially at 10 cm below the high level (14a) of the liquid metal. 5. Nuclear reactor according to claim 3, wherein the secondary sleeve (21) of the annular enclosure (20) is made from a sheet having a thickness of about 1.5 cm.