Patent Number: 048184730
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and has particular relationship to fuel bundles or fuel assemblies. To aid those skilled in the art in practicing this invention by the description of this invention in this application of a concrete structure, this application deals in detail with a fuel assembly of a boiling-water reactor (BWR). To the extent that this invention may be embodied in reactors of other types, such as pressurized-water reactors (PWR), it is understood that such embodiments are within the scope of equivalents of this invention. A fuel bundle includes a top plate and a bottom plate between which fuel rods are mounted. The fuel rods are held together by axially spaced straps or spacers. In accordance with teachings of the prior art, each bundle is provided with tie rods which are screwed into, or bolted to, the bottom plate. Each prior art bundle also has spacer-capture rods which are secured to the bottom plate by pins to suppress rotation about their axes and have axially spaced tabs to prevent axial displacement of the spacers. Removal of the tie rods during reconstitution, particularly complete reconstruction, of a fuel bundle presents formidable difficulties because it demands access to the bottom plate, usually in a radioactive environment, to unscrew or unbolt the tie rods and to disengage the spacer-capture rods. It is an object of this invention to overcome the above-described drawbacks and disadvantages and to provide a fuel bundle which may be readily completely reconstituted by removal of tie rods without access to the bottom plate. It is another object of this invention to provide a fuel bundle in which the spacer-capture rods shall be dispensed with. SUMMARY OF THE INVENTION In accordance with this invention there is provided a fuel bundle having one or more tie rods each secured to the bottom plate by a key by means of which the tie rod can be locked to, and unlocked from, the bottom plate without direct access to the bottom plate. Each tie rod is also secured to the top plate by means for preventing rotation of the tie rod. Specifically, the tie rod has at the bottom a flat key which passes through a slot in the bottom plate. The tie rod may be locked to the bottom plate by turning the rod so that the key is at an angle to the slot under the bottom plate. At the top the tie rod has a flattened plug whose transverser cross-section resembles a race track. The plug has a thread on its rounded sides. A nut is threaded onto the thread on the plug and engages the top plate. The plug passes through a slot in the top plate. The nut has a crimping lip which is crimped to the plug. To disassemble the bundle the crimp for each tie rod is broken by turning the nut. The nut and top plate can then be removed and the tie rod turned to unlock the key and removed by access to the top of the bundle. The plug has a flat tip which may be engaged to prevent rotation of a tie rod when the crimp is broken and the nut unscrewed by turning the nut. Since the tie rods of this invention are not rotatable they may carry the tabs which prevent displacement of the straps and the spacer-capture rods may be dispensed with.