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.