Patent Application
20100020916
1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assembly grids and, more particularly, is concerned with the connection between the fuel assembly grid straps and the control rod guide thimbles.
2. Description of the Related Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies, each of which is composed of top and bottom nozzles with a plurality of elongated, transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles, and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission, and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during operation, and at shut down is achieved by varying the neutron flux. Generally this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux, and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
The guide thimbles are rigidly connected at each end respectively to the top nozzle and bottom nozzle and the grids are fixably attached to the guide thimbles at the cell locations through which the guide thimbles pass. The top nozzle, bottom nozzle, guide thimbles and grids thus form the structural elements of the fuel assembly also known as the fuel assembly skeleton.
The grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Grids are made of materials with low neutron absorption cross-sections such as stainless steel, inconel, and alloys of zirconium, such as zircaloy, to minimize grid deformation and the loss of structural integrity during a radiation. Conventional designs of grids for nuclear fuel assemblies include a multiplicity of interleaved interior grid straps formed in an egg-crate configuration defining cells which accept the fuel rods and the guide thimbles. The ends of each of the interior grid straps are interlocked with an outer grid strap, forming the peripheral cells of the grid. Each cell through which the fuel rods pass provide support to one fuel rod at a given axial location through the use of relatively resilient springs of various forms. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length. In a pressurized water reactor, typically each grid is held in place along the fuel assembly by its attachment to the control rod guide thimbles.
The interior straps of the grids that are interlocked in an egg-crate pattern are generally held in place by a welded or braised joint at their intersecting locations. The ends of the lattice straps are similarly affixed to the perimeter straps that surrounds them by welds or braises. If the straps are made of zircaloy or stainless steel, they can generally be welded. If inconel or nickel plated inconel are employed, they generally have to be braised. Various means of attachment are used to position and secure the spacer grid assemblies to the guide thimble tubes. These means of attachment include welding of the grids to the tubes, braising, bulging of the tubing into sleeves that are attached to the grids, and welding split rings 40 to the guide thimbles 18 directly above and below the grid straps 42 as shown in
Designers are constantly seeking to improve the means of manufacture of the grids and fuel assembly skeletons. Areas of interest include mechanisms for reducing the manufacturing effort, and meeting the stringent design envelope, or tolerances, on dimensional parameters of the grid. Furthermore, considerations include retaining the structural rigidity of the fuel assembly skeleton. More particularly, the need specifically exists for an improved connection between the grid straps and the guide thimbles that will accommodate the use of dissimilar materials for the grid straps and the guide thimbles. While zircaloy has a lower neutron capture cross section than inconel, inconel has a greater stiffness and a lower relaxation rate than zircaloy and thus is more desirable for use as a grid strap material.
This invention achieves an improvement in the manufacture of nuclear fuel assemblies by providing an improved connection between the grid straps surrounding the cells through which the guide thimbles pass and the guide thimbles. The improvement comprises a through grid split sleeve that extends from above the grid straps to a distance below the grid straps. The uncompressed diameter of the sleeve is equal to or larger than the diameter of the cell through which the guide thimble extends. Preferably, the sleeve is made of a resilient material that is the same as or similar to that of the guide thimble. The sleeve is compressed and in its compressed condition it is inserted through the corresponding cell of the grid with a portion of the sleeve extending above and below the grid straps. The sleeve is then allowed to expand to its uncompressed condition, thereby securing the sleeve within the guide thimble cell. In the case where the materials of the sleeve grid straps are compatible the sleeve may be welded to the grid strap. When the sleeve is secured within the cell, the guide thimble can be inserted and welded or braised at either end of the sleeve or at both ends of the sleeve.
In the preferred embodiment, the split sleeve of this invention has windows stamped in the sleeve with an opening height equal to the height of the grid straps. When the split sleeve is inserted into the corresponding guide thimble cell and allowed to expand, the upper and lower horizontal ledges of the windows in the sleeve rest upon the upper and lower edges of the grid strap locking the sleeve into its axial position. The guide thimbles may then be inserted and welded into position after the axial location of the grid is fixed.
In alternate embodiments, the axial slit in the sleeve may be formed between windows, at the edge of one window or fully within one window depending on the amount of diameter reduction required to insert the sleeve into the grid's guide thimble cell opening. Desirably, in the embodiments employing windows to axially secure the sleeve with respect to the grid strap, the expanded sleeve exerts an approximately zero lateral force on the grid strap walls within which it is secured.
A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
For simplicity, this invention will be described with reference to a pressurized water reactor, though it should be appreciated that this invention may be used with other reactor designs that employ similar guide tubes within a support cell structure. Accordingly, reference to a pressurized water reactor is not meant to be limiting upon the scope of the invention.
Directional phrases used herein, such as, for example, upper, lower, top, bottom, left, right, and derivatives thereof for the most part relate to the orientation of the elements shown in the drawings and are not meant to be limiting upon the claims, unless expressly recited therein. As employed herein, the statement that two or more parts are “coupled” together shall mean that the parts are joined together, either directly or joined through one or more intermediate parts. In addition, as employed herein, the term “number” shall refer to one and more than one, i.e., a plurality.
Referring now to the drawings, and particularly to
The fuel assembly 10 further includes a plurality of transverse grids 20 axially spaced along and mounted to the guide thimble tubes 18 and an organized array of elongated fuel rods 22 transversely spaced and supported by the grids 20. The assembly 10 also has an instrumentation tube 24 located in the center thereof and extending between and mounted to the bottom and top nozzles 12 and 16. In view of the foregoing arrangement of parts, it should be understood that the fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
As previously discussed, the array of fuel rods 22 in the fuel assembly 10 are held in spaced relationship with one another by the grids 20 which are spaced along the fuel assembly length. Each fuel rod 22 includes nuclear fuel pellets 26 and is closed at its opposite ends by upper and lower end plugs 28 and 30. The pellets 26 are maintained in a stack by a plenum spring 32 disposed between the upper end plug 28 and the top of the pellet stack. The fuel pellets 26, composed of fissile material, are responsible for creating the reactive power of the reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through a plurality of How openings in the lower core plate 14 to the fuel assembly. The bottom nozzle 12 of the fuel assembly 10 passes the coolant upwardly through the guide tubes 18 and along the fuel rods 22 of the assembly, in order to extract heat generated therein for the production of useful work. To control the fissile process, a number of control rods 34 are reciprocally movable in the guide tubes 18 located at pre-determined positions in the fuel assembly 10. A spider assembly 39 positioned above the top nozzle 16 supports the control rods 34.
As previously mentioned, the interior lattice work of conventional designs of grids 20 for nuclear reactor fuel assemblies 10 include a multiplicity of interleaved, inner straps 44 and 46 forming an egg-crate configuration defining cells which accept fuel rods 22. The interleaved design is enabled by vertical cut opposing slots in the inner straps 44 and 46 at the intersecting locations, which interlock to form the egg crate configuration, as is commonly known in the art. The ends of each of the inner grid straps 44 and 46 are connected to an outer grid strap 50 to form the peripheral cells of the grid 20. Most of the individual cells of the grid 20 provide support for one fuel rod 22 at a given axial location through the use of the combination of relatively resilient springs 52 and dimple 54 of various forms. The outer grid strap 50 encloses the inner grid straps 44 and 46 to impart strengthen rigidity to the grid 20. The cells 48 through which the guide thimble extend can be identified in
In accordance with this invention, the control rod guide thimbles are attached to the grid straps 44 and 46 surrounding the cells 48 employing a through grid split sleeve 56, one embodiment of which is illustrated in
The preferred embodiment of the through-grid split sleeve 56 of this invention provides a number of advantages over the current split ring design 40 shown in
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only, without limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.
