Apparatus for controlling coolant level in a liquid-metal-cooled nuclear reactor

In a liquid-metal-cooled fast-breeder reactor incorporating a core submerged in liquid metal coolant within a pressure vessel, there being an inlet coolant plenum below the core and an outlet coolant plenum above the core, a thermal liner extending from the bottom of the outlet plenum to above the normal operating coolant level in the outlet plenum spaced from the pressure vessel to define a bypass annulus therebetween, and means for establishing a bypass flow of coolant through the bypass annulus to insulate the pressure vessel from the hot coolant in the outlet plenum, said means for establishing a bypass flow of coolant including means for bypassing a portion of the coolant from the inlet coolant plenum around the core and into the bypass annulus, the improvement wherein the thermal liner has a plurality of control ports therein located just below the normal operating coolant level in the pressure vessel and wherein the improvement includes an overflow nozzle located in the pressure vessel below the control ports connected to an overflow line leading to an exterior overflow vessel, said overflow line including a portion at an elevation such that overflow coolant flow is established therethrough when the coolant level in the outlet plenum is above the top of the control ports.