Patent Application
20110200160
1. Field of the Invention
The present invention relates generally to nuclear reactors, and more particularly, to nuclear reactors having fuel assemblies that employ grids.
2. Description of the Related Art
In most water cooled nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. In pressurized water nuclear reactors (PWR), these fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles are on opposite ends of the fuel assembly and are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods and induce mixing of the coolant. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in the cells that are sized the same as those that receive fuel rods therein, or in relatively larger thimble cells defined in the interleaved straps. The interleaved straps provide attachment points to the thimble tubes, thus enabling their positioning at spaced locations along the length of the fuel assembly.
Conventionally, the interleaved straps are configured such that the cells through which the fuel rods pass each include one or more relatively compliant springs and a plurality of relatively rigid dimples. The springs and dimples may be formed in the metal of the interleaved straps and protrude outwardly therefrom into the cells through which the fuel rods pass. The springs and dimples of each fuel rod cell then contact the corresponding fuel rod extending through the cell. Outer straps of the grid are attached together and peripherally enclose the inner straps of the grid to impart strength and rigidity to the grid and to define individual fuel rod cells around the perimeter of the grid. The inner straps are typically welded or braised at each intersection and the inner straps are also welded or braised to the peripheral or outer straps defining the outer perimeter of the assembly.
Conventionally, at the individual cell level, the fuel rod support is normally provided by the combination of rigid support dimples and flexible springs as mentioned above. There are many variations to the spring-dimple support geometry that have been used, including diagonal springs, “I” shaped springs, cantilevered springs, horizontal and vertical dimples, etc. The number of springs per cell also varies. The typical arrangement is two springs and four dimples per cell. The geometry of the dimples and springs needs to be carefully determined to provide adequate rod support through the life of the assembly.
During irradiation, the initial spring force relaxes more or less rapidly, depending on the spring material and irradiation environment. The cladding diameter also changes as a result of the very high coolant pressure and operating temperatures and the pellets inside the rod also change their diameter by densification and swelling. The outside cladding diameter also increases, due to the formation of an oxide layer. As a result of these dimensional and material property changes, maintaining adequate rod support through the life of a fuel assembly is very challenging.
Under the effect of axial flow and cross flow induced by thermal and pressure gradients within the reactor and other flow disturbances, such as standing waves and eddies, the fuel rods, which are slender bodies, are continuously vibrating with relatively small amplitudes. If the rod is not properly supported, this very small vibration amplitude may lead to relative motion between the support points and the cladding. If the pressure exerted by the sliding rod on the relatively small dimple and grid support surfaces is high enough, a small corrosion layer on the surface of the cladding can be removed by abrasion, exposing the base metal to the coolant. As a new corrosion layer is formed on the exposed fresh cladding surface, it is also removed by abrasion until ultimately the wall of the rod is perforated. This phenomenon is known as grid-to-rod fretting and in 2006 it was the leading cause of fuel failures in PWR reactors.
Support grids also provide another important function in the assembly, that of coolant mixing to decrease the maximum cladding temperature. Since heat generated by each rod is not uniform, there are thermal gradients in the coolant. One important parameter in the design of the fuel assembly is to maintain efficient heat transfer from the rods to the coolant. The higher the amount of heat removed per unit time, the higher the power being generated. At high enough coolant temperatures, the rate of heat that can be removed per unit of cladding area in a given time decreases abruptly in a significant way. This phenomenon is known as deviation from nucleate boiling or DNB. If within the parameters of reactor operation, the coolant temperature would reach the point of DNB, the cladding surface temperature would increase rapidly in order to evacuate the heat generated inside the rod and rapid cladding oxidation would lead to cladding failure. It is clear that DNB needs to be avoided to prevent fuel failures. Since DNB, if it occurs, takes place at the point where the coolant is at its maximum temperature, it follows that decreasing the maximum coolant temperature by coolant mixing within the assembly permits the generation of larger amounts of power without reaching DNB conditions. Normally, the improved mixing is achieved by using mixing vanes in the down flow side of the grid structure. The effectiveness of mixing is dependent upon the shape, size, and location of the mixing vanes relative to the fuel rod.
Other important functions of the grid include the ability to sustain handling and normal operation at anticipated accident loads without losing function and to avoid “hot spots” on the fuel rods due to the formation of steam pockets between the fuel rods and the support points, which may result when not enough coolant is locally available to evacuate the heat generated in the rod. Steam pockets cause overheating of the fuel rod to the point of failure by rapid localized corrosion of the cladding.
Maintaining a substantially balanced cooling flow through the fuel assemblies across the core is a desirable objective to maintain substantially uniform heat transfer. Any changes in fuel assembly design can alter the pressure drop and effect the relative balance in flow resistance through the core among the various types of fuel assemblies. Changes in grid design that reduce pressure drop are desirable because such changes enable a fuel assembly designer to introduce other improvements that will restore the pressure drop equilibrium among fuel assemblies.
As previously mentioned, grid strap dimples and springs protrude into a grid cell location to position a nuclear fuel rod in the lattice array. The taller the dimple and the more it protrudes into the grid cell, the stiffer the dimple. This increased stiffness can result in the fuel rod being scratched or galled during rod loading. A stiffer dimple also increases the risk of dimple-to-rod fretting due to higher fuel rod contact stresses. Therefore, the dimple designer needs to provide adequate stiffness to position the fuel rod, but minimize stiffness to reduce scratches, galling and the potential for fretting.
It is thus desired to provide an improved grid that exhibits effective heat transfer and improved fuel rod support with less potential for scratching or galling the fuel rods when they are loaded into the assembly. It is a further object of this invention to provide such an improved grid that has a number of manufacturing advantages.
The foregoing objectives are achieved by an improved nuclear fuel assembly grid having a first plurality of spaced, parallel, elongated straps and a second plurality of spaced, parallel, elongated straps positioned orthogonal to the first plurality of spaced, parallel, elongated straps. The first and second plurality of spaced, parallel, elongated straps are aligned in a regular lattice pattern so that the intersection of each set of four adjacent straps defines a cell, some of which support fuel rods, with an extent of each of the first and second straps that border each cell forming a wall of the cell. Preferably, the straps are interleaved at their intersections in an egg-crate pattern. A bordering strap surrounds the exterior perimeter of the first and second plurality of spaced, parallel, elongated straps and is affixed at the intersections with the interior strap. At least one wall of the cells that support fuel rods has a vertically elongated spring that extends into the cell. The vertically elongated spring has a vertical slit that separates the most extended portion of the spring into two vertically oriented ligaments with the most extended portion of the ligaments being rounded in two directions to provide two, co-planar, point contacts of support for a fuel rods.
In one embodiment, each wall of the cell that supports the fuel rods includes the vertically elongated spring at the same elevation. The springs form eight co-planar point contacts of support for the fuel rods.
In still another embodiment, the elongated spring is formed separate from the strap from which it extends. The spring is attached to the wall at an upper and lower vertical end portion of the spring.
In a third embodiment, the vertically elongated spring is formed separate from the strap from which it extends. The strap has an upper and lower opening in the wall through which the spring is attached to a second spring extending in an opposite direction into an adjacent cell that supports another fuel rod. Preferably, the upper and lower openings in the wall are either vertical slots or notches in the top and bottom of the strap.
Preferably, each of the ligaments has a leg that laterally extends towards each other with each leg stopping short of contact with each other. The opposing ends of the legs extends slightly further into the cell so that the point of contact with the fuel rod passing through the corresponding cell is made on the legs.
In still another embodiment, a lower portion of the spring that extends into the cell is connected to the strap at approximately mid height of the wall. The portion of the spring that extends below the mid height of the wall then extends in the opposite direction into an adjacent cell. Preferably, the vertical slit on the spring does not extend to the top or bottom of the spring. In still one further embodiment, an opposing wall of the cell that supports fuel rods, in which a spring extends, has at least one dimple that extends into the cell to oppose the spring.
A further understanding of the invention can be gained from the following description of the preferred embodiment when read in conjunction with accompanying drawings in which:
Nuclear fuel spacer grids are used in fuel assemblies to position nuclear fuel rods. Accurately positioning nuclear fuel rods is critical to assure proper nuclear and thermal hydraulic performance of a nuclear core of a reactor. Many conventional spacer grids are composed of straight grid straps that are interleaved together to form an egg-crate lattice having a plurality of roughly square cells many of which support fuel rods. An example of such a conventional fuel grid 10 can be found in
Referring to
As mentioned above, the fuel rods 36 and the array thereof in the fuel assembly 40 are held in spaced relationship with one another by the grids 10 spaced along the fuel assembly length. Each fuel rod 36 includes nuclear fuel pellets 50 and the opposite ends of the rods 36 are enclosed by upper and lower end plugs 52 and 54, to hermetically seal the rods. Commonly, a plenum spring 56 is disposed between the upper end plug 52 and the pellets 50 to maintain the pellets in a tight, stacked relationship within the rod 36. The fuel pellets 50 composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/coolant, such as water or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work.
To control the fission process, a number of control rods 58 are reciprocally movable in the guide thimbles 44 located at predetermined positions in the fuel assembly 40. Specifically, the top nozzle 48 has associated therewith a rod cluster control mechanism 60, having an internally threaded cylindrical hub member 62 with a plurality of radially extending flukes or arms 64 such that the control mechanism 60 is operable to move the controls rods 58 vertically in the guide thimbles 44 to thereby control the fission process in the fuel assembly 40, all in a well known manner.
As previously mentioned, the design of the contact area between the side walls of the grid cells that support the fuel rods and the cladding surface of the rods is critical to assure that there is enough force to secure the rods against harmful vibrations without scarring the cladding. Traditionally, support of the fuel rods within the spacer grid is accomplished using a configuration of two vertically spaced dimples and one opposing spring in each of two directions, thus resulting in a six-point contact support structure. Despite many variations on this concept, it has proven difficult to find a design that is consistently immune to fretting induced rod failures. Recent flow loop testing has indicated that co-planar supports (i.e., supports at the same axial elevation within a given grid cell) will provide a more fretting resistant rod support system. Additionally, it has been theorized that more than six contact points in a given cell will provide additional margin to fretting. This invention, various embodiments of which are illustrated in
The concepts of this invention employ two main components, namely, the base strap 12/14 which forms the walls 28 of the grid cells and the split springs 26 which may either be separate, but attached or integral to the base strap 12-14. Each of the embodiments of this invention utilizes the same basic split spring 26 as shown in
The fuel rod support design of this invention provides a number of benefits over the traditional “two dimples and a spring” rod support systems. For example, the design of this invention provides a pivot point for the rod, which allows the rod to rotate during flow induced vibration instead of being rotationally constrained by the traditional “two dimple and a spring” configuration. This allows the rod to dissipate its energy, instead of the vibrational energy being resisted by the rotational resistance of the “two dimple and spring” configuration. Furthermore, the split down the middle of the cell effectively provides for two springs to support the rod on each side of the cell. This results in eight points of support of the rod in a given cell. This eight point contact rod support system will help to limit the effective grid cell size by limiting the amount that the rod can move in the orthogonal directions in the plane of the grid, which will provide for additional fretting margin. Further, the radius of curvature on the spring along the axis of the rod will preclude the fuel rod from impacting a hard edge, such as the edge of a conventional spring or dimple, thus increasing the fretting resistance of the design. Having two points of contact as well as a curved surface on the spring will result in fewer gall balls and lighter scratches on the fuel rod during rod loading. Dimple cocking has been an issue for some designs, and commonly wear scars indicate the dimple surface is not entirely flat. With the curved nature of the rod support feature of this invention, any worries of the support structure not be perpendicular to the rod are eliminated.
Using the base strap with the springs attached significantly simplifies the manufacturing process. Many of the conventional straps have very complex forming features. Straps with such a high percentage of the strap being formed tend to be difficult to maintain dimensional stability. With the design of this invention, both the base strap and the separate springs are simple formed features that can be easily manufactured and inspected. The manufacturing process can be even further simplified by a progressive dye stamping process where both the base strip material and the spring material are fed into the dye simultaneously and subsequently punched and welded as one continuous process. Use of the base strap provides additional crush strength of the grid for seismic/LOCA considerations. The thickness of the material for the separate springs can be optimized in order to achieve the desired spring stiffness. Furthermore, the concepts illustrated 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 and not limiting as to the scope of the invention which is to be given the breath of the appended claims and any and all equivalents thereof.
