Patent Number: 043427219
Section: claims

1. A fast neutron nuclear reactor comprising a vesel having a vertical axis and containing a reactor core, a volume of liquid metal for cooling the core, at least one main heat exchanger and at least one auxiliary heat exchanger, said reactor vessel being closed at the top by a reactor vault roof, said auxiliary heat exchanger being constituted by a plurality of vertical heat-exchanger modules, each of said modules comprising a circuit for the flow of coolant fluid, said reactor vault roof being pierced by a single orifice in vertical alignment with the vertical axis of said auxiliary heat exchanger, the dimension of said orifice in horizontal cross-section being larger than that of each one of said heat-exchanger modules but smaller than the overall dimension of the auxiliary heat exchanger in horizontal cross-section, and a removable shield plug closing said orifice, said shield plug having a depending axial extension penetrating into said reactor vessel with said modules disposed around said extension, the fluid circuit of each module communicating with an inlet tube and with an outlet tube for admission and discharge of the coolant fluid, said inlet and oulet tubes of each module extending in a direction parallel to the shield plug and the axial extension thereof within a respective recess formed in the inner surface of one of said orifice and said shield plug, whereby any of said modules of said auxiliary heat exchanger can be individually introduced into and taken out of said reactor in any order through said orifice after removal of said shield plug. 2. A nuclear reactor according to claim 1, wherein the fluid circuit of each module is constituted by a heat-transfer tube connected at the ends thereof respectively to said inlet tube and to said outlet tube for admission and discharge of the coolant fluid. 3. A nuclear reactor according to claim 2, wherein said heat-transfer tube has the shape of a tube coil. 4. A nuclear reactor according to claim 2, wherein said heat-transfer tube has the shape of a series of reversed hairpins. 5. A nuclear reactor according to claim 1, wherein the fluid circuit of each module is constituted by a plurality of U-tubes connected to an inlet header and an outlet header, said headers communicating respectively with said inlet tube and with said outlet tube for admission and discharge of the coolant fluid. 6. A nuclear reactor according to claim 1, wherein said fluid circuit of each heat-exchange module is constituted by a plurality of straight tubes connected at the ends thereof to an inlet header and to an outlet header, said header communicating respectively with said inlet tube and with said outlet tube for admission and discharge of the coolant fluid. 7. A nuclear reactor according to claim 2, wherein the inlet tubes of the modules are connected to an annular inlet manifold and the outlet tubes of the modules are connected to an annular discharge manifold communicating with means for pumping the coolant fluid, said annular manifolds being located externally of the reactor vessel and above the reactor vault roof and said pumping means being located externally of the reactor vessel. 8. A nuclear reactor according to claim 7, wherein the coolant fluid is liquid sodium and wherein the pumping means are constituted by an electromagnetic pump. 9. A nuclear reactor according to claim 1, wherein the modules constituting said auxiliary heat exchanger are surrounded externally by a cylindrical guide shell provided with open portions. 10. A nuclear reactor according to claim 1, wherein each heat-exchange module is provided at the lower end with a centering pin engaged in a bore provided in a support structure which is mounted within the reactor vessel and rigidly fixed thereto.