Patent Number: 040173577
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates generally to nuclear reactor and more particularly to a nuclear core inlet flow arrangement of a liquid metal-cooled fast breeder nuclear reactor. 2. Description of the Prior Art: A nuclear core of a liquid metal-cooled fast breeder reactor is comprised of a plurality of fuel assemblies. The fuel assemblies contain nuclear fuel, such as plutonium 239 and uranium 238, which undergo nuclear fission and produce heat. The heat produced by the fission process is removed by flowing a reactor coolant, for example liquid sodium, through the nuclear core. This heat is eventually converted into commercial electrical energy by means of conventional steam turbine-generator apparatus. In the type of nuclear reactor considered herein the fuel assemblies are made up of fuel rods arranged on a triangular pitch. A plurality of fuel rods thus arranged are contained within a hexagonal enclosure which serves as a flow barrier. The hexagonal enclosure or flow barrier prevents the reactor coolant flow, which is internal to the fuel assemblies, from flowing between different fuel assemblies. Nuclear cores using this reactor coolant flow arrangement are designated as closed cores. By way of contrast, most pressurized water reactors use an open core arrangement which does not allow reactor coolant to flow between the different fuel assemblies. A number of advantages are gained from the use of a closed core in a nuclear reactor. First, a significant improvement in reactor performance is realized. Second, it enhances the structural integrity of the nuclear core by reducing the probability of failure propagation between fuel assemblies. Unfortunately, a number of disadvantages also result from the use of a closed core. One major disadvantage is that the enclosed fuel assembly is susceptible to overheating failures caused by a loss of reactor coolant flow due to debris blockage. In the prior art, guard structures, similar to grids, are provided at the flow inlets of the fuel assemblies to prevent loss of coolant failures. The guard structures limit the size of foreign objects which can enter the fuel assembly such that the subsequent flow blockage within the fuel assembly does not cause flow starvation. That is, that even with the blockage, an adequate quantity of reactor coolant flows through the fuel assembly to sufficiently cool the fuel assembly. However, the guard structures themselves may become blocked by specially shaped, resilient or flexible foreign objects and cause flow starvation. The present invention overcomes the possibility of core damage due to the susceptibility to flow blockage of the prior art by providing an inlet flow system which is virtually insensitive to flow blockage. SUMMARY OF THE INVENTION In this invention, a hollow, cylindrically shaped inlet nozzle is attached to a fuel assembly. The unattached end of the inlet nozzle is open to allow reactor coolant to enter the fuel assembly. As in the prior art, this main reactor coolant inlet is protected by a grid type of guard structure. The end of the inlet nozzle containing the guard structure is immersed in a pool of reactor coolant comprising a main flow plenum which supplies the fuel assembly with reactor coolant. The inlet nozzle of the invention is provided with an alternate flow inlet consisting of a plurality of holes in the cylindrical surface of the nozzle. The alternate flow inlet is opened to a second pool of reactor coolant comprising an alternate reactor coolant flow plenum. The main and alternate flow plenums are physically separated by a plate which acts as a sealing member. The sealing member is perforated by a plurality of small diameter holes which permit reactor coolant flow communication between the two plenums. The holes in the sealing plate are the size similar to the size of the openings in the guard structure. Thus, if the main reactor coolant inlet becomes clogged by a large foreign object, the fuel assembly received an adequate supply of reactor coolant through the flow path comprising the alternate inlet and the alternate flow plenum. Further, since the sealing plate prevents passage of large foreign objects, the fuel assembly is still provided with adequate protection against internal flow blockage. In this manner, the present invention protects the fuel assembly from damage caused by flow starvation resulting from either internal flow blockage or inlet flow blockage.