Patent Number: 048030449
Section: summary

The above application discloses a fuel assembly for boiling water nuclear reactors (BWR). It is characterized by a "water rod," i.e. a tube extending vertically through the fuel assembly parallel to the fuel rods. It is open at its lower end to receive cooling and moderating water, but is closed at its upper end. A plurality of holes are formed in the tube to permit egress of water. An inner tube, open at its upper and lower ends, is arranged concentrically in the water tube. The annulus formed by the inner tube and the water tube is closed at the lower end of the former. Openings are provided in the water tube above its juncture with the inner tube. These holes are located in the critical heat flux zone of the assembly. There are also openings below this juncture, some of which are located at the lower end of the critical heat flux zone. There may also be other openings farther down the tube. In operation, water enters the lower end of the water tube and flows upwardly through it. This adds moderation to the assembly and improves its performance, and introduction of water into and just below the critical heat flux zone improves the cooling and permits operation of the assembly at higher power. The desirability of a flow of non-boiling water through portions of the assembly has been recognized for some time as desirable, and arrangements other than the typical "water rods" have been designed for this purpose. The object of this invention is to obtain in these alternate designs the advantages which are secured by the design of the above-identified application. SUMMARY OF THE INVENTION According to this invention, an inner flow channel having rectilinear sides is disposed within the fuel assembly, extending vertically therethrough and arranged to receive cooling water at or below the bottom of the assembly. The channel is of such a size as to span several cooling rods in each direction. A closure is provided in the channel within of the critical heat flux zone of the assembly. At least one inner tube extends through the closure. The channel is closed at its upper end, and the inner tube is open at its upper end, which is located near the upper end of the channel. Egress openings are provided in the walls of the channel just above the closure. These openings are so positioned relative to the fuel tubes that the jets of water emitted from them do not impinge directly on the fuel tubes, but rather enter the flow spaces between them. In one embodiment, the channel is a flat sided, box-like structure, positioned centrally of the assembly. In another, it is of cruciform shape. Still other arrangements are possible, such as narrow rectangular boxes extending inwardly from the walls of the outer flow channel which forms the boundary of the assembly.