Patent Number: 040627241
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nuclear reactor shown in FIG. 1 is of the liquid sodium cooled fast breeder kind which comprises a mass of nuclear fuel 1 constituting the reactor core submerged in a pool 2 of liquid sodium contained by a vessel 3. The vessel 3 is suspended within a concrete vault 4 from the roof 5 and the core 1 is supported on a diagrid 6 also suspended from the roof. The core 1 is enclosed by a shroud 7 which is connected to a heat exchanger inlet 8, the heat exchanger outlet 9 discharging into the pool. A sodium pump 10 which draws sodium from the pool 2 is connected to the diagrid 6 so that, in use of the nuclear reactor, sodium coolant is pumped from the pool to the diagrid upwardly through the core 1 thence through the shroud 7 to the heat exchanger inlet 8 and back to the pool. A collecting tray for molten nuclear fuel according to the invention is designated 11 and comprises a base plate 12 forming a plane tube sheet in which one end of each tube of a complex of cooling tubes 13 is received. A peripheral wall 14 of the tray forms an annular tube sheet in which the other open end of each tube of the complex is received. The tube connections with the plane tube sheet are arranged on concentric pitch circles and the tube connections with the annular tube sheet are arranged in layers thereby to effect a regular distribution of cooling tubes throughout the tray so that in the event of a melt down of the core the molten mass is cooled substantially uniformly by sodium flowing through the tubes by natural convection. The perimeter of the tray extends almost to the wall of the vessel which carries an internal skirt 15 arranged to overlap the perimeter of the collecting tray so that fuel debris carried upwardly by coolant flow through the shroud thence to the outer regions of the pool is directed into the tray instead of on the floor of the vessel. Thermal syphoning pipes 16 penetrate the skirt 15 and extend to the upper regions of the pool of liquid sodium to enable coolant below the skirt and tray to flow into upper regions of the pool coolant. In a second construction of nuclear reactor generally similar to the first described construction the collecting tray 21 is as shown in FIG. 2 and provides for increased coolant flow natural head and increases the local heat transfer coefficient. The tray comprises an annular base plate 22 forming a plane tube sheet in which one open end of each tube 23 of a complex of cooling tubes is received. A peripheral vertical wall 24 forms an annular tube sheet in which the other open end of each tube of the complex is received. The tube connections with the plane tube sheet are arranged on concentric pitch circles and the tube connections with the annular tube sheet are arranged in layers thereby to effect a regular distribution of cooling tubes throughout the tray. The base plate 22 has an inner, annular, vertical wall 25 surmounted by a conical portion 26 having apex disposed on the vertical axis of the reactor core. The wall of the conical portion 26 and an upper wall extension 27 of the wall 24 are inclined at an angle greater than the angle of repose of the fuel (for example, uranium oxide) in sodium so that debris from the core is directed into the wall formed by the inner and peripheral walls of the tray. A basic feature of the second construction is that a layer of low temperature solid fuel (which could be uranium dioxide) is formed on the base plate to a height determined by the redundancy requirements of the cooling tubes and then a limitless amount of molten fuel can be allowed to collect on top of this solid layer without affecting the temperature gradient through the base plate. The described constructions of nuclear reactors have advantage in that, in the event of an emergency, the devices for collecting molten fuel debris are completely passive, circulation of coolant being by natural convection. By interrupting the mass of fuel debris by the cooling pipes nuclear criticality of the mass is also reduced. Except for a small quantity of finely divided fuel debris which can be supported by the low velocities inherent with natural convection circulation, all the debris borne by the coolant is diverted into the collecting tray.