Patent Number: 040627241
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and to collecting trays for molten nuclear fuel debris. In one form of nuclear reactor a nuclear fuel mass constituting a reactor core is submerged within a pool of liquid coolant which serves as a vehicle for the transfer of heat from the reactor core to steam generating equipment. Examples of reactors of this form are the liquid metal cooled fast breeder reactor, boiling and pressurised water reactors. In the event of loss of coolant from the core, or restricted flow of coolant through the core, the nuclear fuel could melt down and fall to the floor of the coolant containing vessel. Because of the high heat content of the fuel debris supplemented by fission produce decay heat and, possibly, by criticality of the nuclear mass, serious damage could be caused to the floor of the vessel and its environment. For a liquid metal cooled fast breeder reactor several proposals have been made for retaining molten fuel debris before it reaches the floor of the vessel one such proposal including the provision of a plurality of layers of collecting trays disposed beneath the core, trays in the upper layers being arranged to overlap trays in the lower layers. However, the trays would be subject to such high heat flux that serious distortion could occur. SUMMARY OF THE INVENTION According to the invention, in a nuclear reactor construction comprising a reactor core submerged in a pool of coolant in a containing vessel and a collecting tray for core debris submerged in the pool of coolant below the core and spaced from the floor of the containing vessel, the wall of the vessel has an internal skirt arranged to overlap a peripheral wall of the tray to direct core debris into the tray, and the collecting tray has a plurality of cooling tubes extending between a base plate and the peripheral wall and through which coolant from the lower regions of the pool can be circulated upwardly. In use, coolant is circulated through the cooling tubes so that the heat flux into the base of the tray is maintained at a tolerable level. Preferably, circulation of coolant through the tubes is by natural convection to provide a completely passive system but, alternatively, the coolant could be pumped through the tubes. The tubes of the complex are arranged to be evenly distributed within the tray so that, in the event of an emergency resulting in melt down of fuel, cooling is substantially uniform throughout the mass of fuel debris. Bends in the tubes provide adequate flexibility to accommodate thermal linear expansion. The invention also resides in a collecting tray for a nuclear reactor construction according to the preceding paragraph, the collecting tray comprising a base plate in which one open end of each tube of a complex of cooling tubes is received and a peripheral wall in which the other open end of each tube of the complex of cooling tubes is received.