Patent Number: 046845046
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending application dealing with related subject matter and assigned to the assignee of the present invention: "Nuclear Reactor" by Harry M. Ferrari et al, assigned U.S. Ser. No. 732,220 and filed May 9, 1985. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a bow resistant fuel assembly structure for non-control rod locations of the reactor core. 2. Description of the Prior Art The cores of nuclear reactors conventionally include a plurality of fuel assemblies. In a typical pressurized water nuclear reactor (PWR), all fuel assemblies are geometrically alike. Each fuel assembly includes a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles of the fuel assembly are secured to opposite ends of the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. The guide thimbles together with the top and bottom nozzles rigidly attached thereto compose the structural skeleton of the fuel assembly. To control the fission process created by nuclear fuel contained in the fuel rods, typically a number of control rods are reciprocally positioned for movement in the guide thimbles of the fuel assembly. However, not all of the fuel assembly locations of a reactor core use control rods. Only about one-third of the fuel assemblies are in control rod locations. But since heretofore all PWR fuel assemblies have been constructed to be alike geometrically, this means that the fuel assemblies for control rod locations have been the same as those for non-control rod locations. A departure from this prior practice of constructing all PWR fuel assemblies alike has been proposed recently. As described and illustrated in the patent application cross-referenced above, a separate fuel assembly design for non-control rod locations includes a bottom nozzle, a number of longitudinally extending structural members which contain a burnable poison and a top nozzle. It also includes a number of grids which are axially spaced and attached to the longitudinal structural members and support an array of fuel rods. The top and bottom nozzles are attached to the longitudinal structural members by screw thread connections or other suitable rigid attaching means. An instrumentation tube is located in the center of the assembly and is supported by the top and bottom nozzles and by the grids. One important difference in this non-control rod fuel assembly over the conventional control rod fuel assembly lies in the design of the longitudinal structural members which interconnect the top and bottom nozzles to form the structural skeleton of the assembly. In the conventional PWR assembly, the structural members are the hollow guide thimble tubes which are open at the top and closed at the bottom (except for small holes for coolant flow). These tubes are positioned within the fuel assembly to align with the control rods. During reactor operation, the control rods move reciprocally in the tubes. On the other hand, in the non-control rod fuel assembly intended for use in non-control rod core locations, the structural member also in the form of tubes do not receive control rods. Therefore, different functional as well as structural use can be made of the tubes. Functionally, this non-control rod structural member contains burnable absorber material. Burnable absorbers, such as a suitable compound of boron, are used in modern reactors to provide an additional means for controlling reactivity especially at the beginning of life of the nuclear fuel. Structurally, the elongated tube of the structural member is closed at each end by end plugs which are welded to the tube. The tube and end plug material is preferably Zircaloy-4. A spring holds the absorber material in place in the tube and provides a plenum for accumulation of helium gas which is released when a neutron interacts with a boron atom. To assemble the non-control rod structural members into the fuel assembly, the tubes must be empty and open at one end. After the grids are bulge fitted to the tubes, the absorber material and spring are loaded into the tubes and the remaining one end plugs welded in place. The fuel rods are then loaded and the top and bottom nozzles are bolted on. In the non-control rod fuel assembly, there are eight absorber structural members whereas the conventional control rod fuel assembly has twenty-four guide thimbles. Thus, there are sixteen more fuel rods per non-control rod fuel assembly which has the benefits described in the above cross-referenced application. The use of non-control rod fuel assemblies in PWRs having the design described above has created an opportunity to possibly overcome an important problem which has been present for a long time and affects the overall performance of PWR fuel assemblies: fuel assembly bow. There appears to be a definite relationship between the magnitude of fuel assembly bow and compressive stresses in the guide thimbles. Unfortunately, there is no readily apparent method of appreciably reducing the compressive stresses in the guide thimbles of control rod fuel assemblies which are in control rod core locations. However, for fuel assemblies in non-control rod core locations and designed as described above, an opportunity would appear to exist to find a way of greatly reducing the compressive stresses in the longitudinal structural members. SUMMARY OF THE INVENTION The present invention provides an improved longitudinal structural member for the non-control rod fuel assembly designed to satisfy the aforementioned needs. In the non-control rod fuel assembly described above and more completely disclosed in the cross-referenced patent application, axial force from the top nozle hold-down spring is transmitted from the top nozzle adapter plate to the top end plug of the longitudinal structural member, through its cladding tube to the bottom end plug and then to the bottom nozzle adapter plate. The cladding tube is thus placed in a state of compression which will result in permanent fuel assembly bow. The present invention improves the design of the longitudinal structural member so as to greatly reduce or counteract the deleterious effects of compressive stresses on its cladding tube. Basically, the solution involves preloading the cladding tube of the structural member in tension. Preloading the tube of a free standing structure of this type in tension means that the central part of it must be loaded in compression. Also, the material loaded in compression must not be subject to thermal or irradiation induced creep or the structure will creep to a permanently bowed position. Thus, the center portion must be made of a creep resistant material. Ceramic materials which are very creep resistant can be used. Therefore, the improvement of the present invention envisions a unique arrangement for applying a compressive load on the ceramic material, such preferably being in a stacked pellet form, so that the cladding tube can be preloaded in tension. Accordingly, the present invention is set forth in a fuel assembly for use at non-control rod locations of a nuclear reactor core. The fuel assembly includes top and bottom nozzles and a plurality of longitudinal structural members extending between and attached to the nozzles for forming the assembly into an integral unitary structure. At least certain of the structural members includes an elongated hollow cladding tube extending between the top and bottom nozzles and means secured to opposite ends of the tube for hermetically sealing the tube and attaching it to the top and bottom nozzles. The present invention relates to the improvement which comprises: (a) a quantity of irradiation-induced creep resistant material disposed within the tube; and (b) pretensioning means positioned within the tube for applying a predetermined compressive load to the creep resistant material therein and reacting the load so as to preload the tube in a state of pretension having a magnitude sufficient to substantially counteract an axial load typically transmitted through the unitary structure of the fuel assembly and thereby greatly reduce the compressive stress in the tube of the structural member. More particularly, the creep resistant material is a ceramic material, such as zirc oxide, in pellet form. The ceramic pellets are coated with a burnable absorber material. Also, the pretensioning means can be either of two embodiments. In one embodiment, the pretensioning means is an elongated bellows type device positioned within the tube between the stack of creep resistant pellets and one of the tube ends. The interior of the bellows type device is pressurized to create a predetermined axial force therein which places the creep resistant pellets in compression and the tube in the state of pretension. Additionally, the remainder of the tube can be pressurized. In an alternative embodiment, the pretensioning means is an arrangement of belleville springs positioned within the tube between the stack of creep resistant pellets and one of the tube ends so as to create the predetermined axial force therein which places the creep resistant material in compression and the tube in the state of pretension. The belleville springs in the arrangement thereof are both stacked in parallel and in series. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.