Patent Number: 043427219
Section: summary

This invention relates to a fast nuclear reactor comprising at least one auxiliary heat exchanger which serves in particular to cool the liquid metal (such as sodium, for example) in the event of any operational incident resulting in stoppage of the primary pumps. It is known that, in a fast reactor and especially in the so-called "integrated" design, the reactor core is immersed beneath the level of a predetermined volume of liquid metal contained within a vessel having a vertical axis. Said vessel is suspended from a horizontal concrete slab of substantial thickness or reactor vault roof which is traversed by primary pump bodies and intermediate heat-exchanger shells. During reactor operation, the liquid sodium is circulated continuously and cooled by means of said primary pumps and intermediate heat exchangers, thus extracting the heat picked up by the sodium as it passes through the reactor core and as a result of nuclear fission. If an accident occurs within the reactor for any reason and in turn causes stoppage of the primary pumps, the fission reaction within the reactor core is immediately stopped as a result of the safety rod drop. It is readily apparent under such conditions that there still remains within the reactor core a high residual thermal output which must be removed in a reliable and efficient manner in order to prevent local core meltdown. The present invention relates to a reactor equipped with an auxiliary heat exchanger which conforms in particular to these requirements. It is also known that, as a general rule, the central portion of the reactor vault roof is occupied by a system of rotary shield plugs, with the result that the "stationary" portion of the vault roof is in fact limited to an annular ring. Moreover, in the case of integrated reactors, this ring is already largely occupied in particular by the primary pumps and by the main intermediate heat exchangers, namely the intermediate exchangers which normally serve to remove the power delivered by the reactor. As can therefore be understood, it is particularly advantageous to design auxiliary heat exchangers which take up only a small space in the reactor vault roof (roof penetration) in respect of a given thermal output. To this end, the fast reactor under consideration is of the type comprising a vessel having a vertical axis and containing the reactor core, a volume of liquid metal for cooling the core and at least one main heat exchanger, said reactor vessel being closed at the top by a reactor vault roof. In addition, the reactor essentially comprises at least one auxiliary heat exchanger constituted by a plurality of vertical heat-exchange modules each provided with a circuit for the flow of coolant fluid. Said reactor vault roof is pierced by a vertical cylindrical orifice for each auxiliary heat exchanger, the dimension of each orifice in horizontal cross-section being larger than that of a heat-exchange module but smaller than the overall dimension of the auxiliary heat exchanger in horizontal cross-section. Provision is made for shield plugs which are adapted to close said orifice or orifices after said modules have been placed in position, said modules being disposed around an axial extension of each shield plug. In a first embodiment, the fluid circuit of each module is constituted by a heat-transfer tube connected at the ends thereof respectively to an inlet tube and to an outlet tube for admission and discharge of the coolant fluid, the inlet and outlet tubes being adapted to extend in a direction parallel to the axis of the shield plug within a groove formed within this latter along one of its vertical generator-lines. In a second embodiment, the circuit of each module is constituted by a plurality of U-tubes connected to an inlet header and an outlet header, said headers being associated respectively with an inlet tube and with an outlet tube for admission and discharge of the coolant fluid, said inlet and outlet tubes being adapted to extend parallel to the axis of the shield plug within a groove formed in this latter along one of its vertical generator-lines. In a third embodiment, the 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 headers being in turn connected respectively to an inlet tube and to an outlet tube for admission and discharge of the coolant fluid, the inlet and outlet tubes being adapted to extend in a direction parallel to the axis of said shield plug within a groove formed in this latter along one of its vertical generator-lines. Finally and in accordance with another distinctive feature, each module is provided at the lower end with a centering pin adapted to engage in a bore formed in a support structure which is mounted within the reactor vessel and rigidly fixed to this latter. By virtue of these arrangements, each auxiliary heat-exchanger module can be separately disassembled; after mounting within the reactor vessel, all these modules are spaced at intervals on a circle having a diameter which is larger than that of the vertical passage provided in the reactor vault roof for the introduction of each module in succession.