Helically fluted tubular fuel rod support

A support grid for a nuclear fuel assembly, the fuel rod assembly having a generally cylindrical fuel rod with a diameter, wherein the support grid includes a frame assembly having a plurality of outer straps and a plurality of helical frame members. The helical frame members have a contact portion structured to contact an adjacent helical frame member and at least one helical fuel rod contact portion with a lesser diameter. The lesser diameter is generally equivalent to the fuel rod diameter such that a fuel rod disposed in the helical frame member would engage the inner helical frame member at helical fuel rod contact portion. The helical frame members are coupled to each other at the contact portions thereby forming a grid. The plurality of outer straps are disposed about the perimeter of the helical frame members.

CROSS REFERENCE TO RELATED APPLICATION

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nuclear reactor fuel assemblies and more particularly to an array for supporting fuel rods wherein the array, or support assembly, consists of a matrix of substantially flat members forming a grid-like frame assembly and a plurality of helically fluted tubular members.

2. Background Information

In a typical pressurized water reactor (PWR), the reactor core is comprised of a large number of generally vertically, elongated fuel assemblies. The fuel assemblies include a support grid structured to support a plurality of fuel rods. The fuel assembly includes a top nozzle, a bottom nozzle, a plurality of the support grids and intermediate flow mixing grids, and a plurality of thimble tubes. The support grids are attached to the plurality of elongated thimble tubes which extend vertically between the top and bottom nozzles. The thimble tubes typically receive control rods, plugging devices, or instrumentation therein. A fuel rod includes a nuclear fuel typically clad in a cylindrical metal tube. Generally, water enters the fuel assembly through the bottom nozzle and passes vertically upward through the fuel assembly. As the water passes over the fuel rods, the water is heated until the water exits the top nozzle at a very elevated temperature.

The support grids are used to position the fuel rods in the reactor core, resist fuel rod vibration, provide lateral support for the fuel rods and, to some extent, vertically restrain the fuel rods against longitudinal movement. One type of conventional support 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 cells that are sized the same as those that receive fuel rods therein, or can be received in relatively larger thimble cells defined in the interleaved straps.

The straps are generally flat, elongated members having a plurality of relatively compliant springs and relatively rigid dimples extending perpendicularly from either side of the flat member. Slots in the straps are utilized to effect an interlocking engagement with adjacent straps, thereby creating a grid of “vertical” and “horizontal” straps which form generally square cells. The location of the springs and dimples are configured such that each cell typically has a spring on each of two adjacent sides. On each of the sides of the cell opposite the springs there are, typically, two dimples. The springs must be disposed opposite the dimples so that the fuel rod is biased against the dimples by the springs. The springs and dimples of each cell engage the respective fuel rod extending through the cell thereby supporting the fuel rod at six points (two springs and four dimples) in each cell. Preferably, each spring and/or dimple includes an arcuate, concave platform having a radius generally the same as a fuel rod. This concave platform helps distribute the radial load on the sides of the fuel rods. The perimeter straps have either springs or dimples extending from one side and peripherally enclose the inner straps of the grid to impart strength and rigidity to the grid. During assembly, the straps must be assembled in a specific configuration to ensure that each cell has the springs and dimples in the proper position. As such, assembly of the prior art frame assembly is a time consuming process. It would be advantageous to have a support assembly that is more easily constructed.

The straps may include one or more mixing vanes formed thereon that facilitate mixing of the water within the reactor to promote convective heat exchange between the fuel rods and the water. This motion, along with the elevated temperatures, pressures, and other fluid velocities within the reactor core tend to cause vibrations between the grids and the fuel rods. As with the proper positioning of the straps, care must be used to ensure that the mixing vanes are disposed at the proper locations. Additionally, the action of the water flow impinging on the mixing vanes cause both a pressure drop in the pressure vessel and creates torque in the frame assembly, neither of which are desired.

Since the grids support the fuel rods within the fuel cell, such vibrations therebetween can result in fretting of the fuel rods. Such fretting, if sufficiently severe, can result in breach of the fuel rod cladding with resultant nuclear contamination of the water within the reactor.

It is desired to provide an improved grid designed to minimize fretting wear between the grids and the fuel rods while maintaining a mixed flow of water through the reactor core. It is also desired to have a support assembly that is easily assembled.

There is, therefore, a need to provide a support grid for a nuclear fuel assembly wherein the fuel rods are supported by a tubular member having a helical, fluted fuel rod contact portion.

There is a further need for a support assembly that is easily assembled.

There is a further need for a nuclear fuel assembly wherein a support grid includes a tubular member having a helical, fluted fuel rod contact portion for supporting fuel rods.

SUMMARY OF THE INVENTION

These needs, and others, are met by the present invention which provides a support grid for a nuclear fuel assembly, wherein the fuel rod is a generally cylindrical fuel rod with a diameter, and the support grid includes a frame assembly having a plurality of generally uniform cells, each cell having at least one sidewall and a width, and at least one generally cylindrical tubular member. The tubular member has a cell contact portion with a greater diameter and at least one fluted helical fuel rod contact portion with a lesser diameter. As used herein, a “fuel rod contact portion” is typically, but is not limited to, an arcuate line extending at least partly around the cylinder that is a fuel rod. The cell contact portion and the fuel rod contact portion are joined by a transition portion. The greater diameter is generally equivalent to the cell width, and the lesser diameter is generally equivalent to the fuel rod diameter. In this configuration, a fuel rod disposed in the tubular member would engage the inner diameter. The tubular member is disposed in one cell of the plurality of generally square cells so that the cell contact portion engages the at least one cell sidewall. In this manner, the fuel rod is held by the helical fuel rod contact portion and the tubular member is held by the frame assembly.

In a preferred embodiment, the tubular member has a wall of uniform thickness so that the helical fuel rod contact portion defines a passage with a helical shape on both the side adjacent to the fuel rod and the side adjacent to the cell wall. These helical shaped passages act to mix the water so that mixing vanes are not required. There are at least two advantages to using the helical shaped passages; first, the water flow does not impinge on the shaped passage, so there is a minimal pressure drop created by the mixing structure. Second, by reversing the direction of the helical passage in selected cells, the amount of torque exerted on the frame assembly may be controlled.

The helical fuel rod contact portion may be formed in various configurations. For example, there may be a single (or multiple) helical fuel rod contact portion having an angular displacement of 360 degrees, that is, extending 360 degrees around the tubular member. However, given the relatively short height of a typical cell, the pitch (radial distance/height) of the helical fuel rod contact portion may be too great thereby restricting the flow of water through the helical portion of the passage. Alternatively, there may be at least two helical fuel rod contact portions each extending 180 degrees around the tubular member. However, in a preferred embodiment, there are four helical fuel rod contact portions each extending 90 degrees around the tubular member. While these examples have used a number (N) of helical fuel rod contact portions and an angular displacement (A) that equals 360 (N*A=360), this is not required. That is, virtually any number of helical fuel rod contact portion(s) may be used with any angular displacement. It is further noted that, while a symmetrical helical contact portion is preferred, a helical contact portion may be an asymmetrical helix; that is the pitch may be variable along the tubular member.

The tubular members, preferably, have a smooth transition between the cell contact portion and the helical fuel rod contact portion. Where there are four helical fuel rod contact portions, the shape of the tubular member is similar to the perimeter of a flower with four petals. Alternatively, the tubular member may include extended platform sections structured to engage either the wall of the frame assembly and/or the fuel rod. Where there is a platform, the transition section will typically be a sharp curve. In another embodiment, the greater portion of the length of the transition portion is generally flat and the ends are sharply angled.

The frame assembly includes a plurality of cells typically structured to contain a nuclear fuel rod. As noted above, some cells are adapted to enclose a thimble rod or other device. However, the non-fuel rod cells are not relevant to this invention and, while noted, will not be discussed hereinafter. In the preferred embodiment, the frame assembly is made from a plurality of substantially flat, elongated strap members disposed in two interlocked sets, a “vertical” set and a “horizontal” set. The vertical set of strap members is disposed generally perpendicular to the horizontal strap members. Also, the strap members in each set are generally evenly spaced. In this configuration, the cells are generally square. In an alternate embodiment, the frame assembly is made from tubular members that have been welded together, preferably at 90 degree intervals.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, directional terms, such as, but not limited to, “upper” and “lower” relate to the components as shown in the Figures and are not limiting upon the claims.

As shown inFIG. 1, there is a fuel assembly20for a nuclear reactor. The fuel assembly20is disposed in a water vessel (not shown) having an inlet at the bottom and an outlet at the top. The fuel assembly20comprises a lower end structure or bottom nozzle22for supporting the fuel assembly20on the lower core plate (not shown) in the core region of a reactor (not shown); a number of longitudinally extending control rod guide tubes, or thimbles24, projecting upwardly from the bottom nozzle22; a plurality of transverse support grids26axially spaced along the guide thimbles24; an organized array of elongated fuel rods28transversely spaced and supported by the grids26; an instrumentation tube30located in the center of the assembly; and an upper end structure or top nozzle32attached to the upper ends of the guide thimbles24, in a conventional manner, to form an integral assembly capable of being conventionally handled without damaging the assembly components. The bottom nozzle22and the top nozzle32have end plates (not shown) with flow openings (not shown) for the upward longitudinal flow of a fluid coolant, such as water, to pass up and along the various fuel rods28to receive the thermal energy therefrom. To promote mixing of the coolant among the fuel rods28, a mixing vane grid structure, generally designated by the numeral34, is disposed between a pair of support grids26and mounted on the guide thimbles24.

The top nozzle32includes a transversely extending adapter plate (not shown) having upstanding sidewalls secured to the peripheral edges thereof in defining an enclosure or housing. An annular flange (not shown) is secured to the top of the sidewalls. Suitably clamped to this flange are leaf springs36(only one of which being shown inFIG. 1) which cooperate with the upper core plate (not shown) in a conventional manner to prevent hydraulic lifting of the fuel assembly caused by upward coolant flow while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Disposed within the opening defined by the sidewalls of the top nozzle32is a conventional rod cluster control assembly38having radially extending flukes, being connected to the upper end of the control rods, for vertically moving the control rods in the control rod guide thimbles24in a well known manner. To form the fuel assembly20, support grids26and a mixing vane grid structure34are attached to the longitudinally extending guide thimbles24at predetermined axially spaced locations. The bottom nozzle22is suitably attached to the lower ends of the guide thimbles24and then the top nozzle32is attached to the upper ends of guide thimbles24. Fuel rods28are then inserted through the grids26and grid structure34. The fuel rods28are generally elongated cylinders having a diameter. For a more detailed description of the fuel assembly20, reference should be made to U.S. Pat. No. 4,061,536.

The fuel assembly20depicted in the drawings is of the type having a square array of fuel rods28with the control rod guide thimbles24being strategically arranged within the fuel rod array. Further, the bottom nozzle22, the top nozzle32, and likewise the support grids26are generally square in cross section. In that the specific fuel assembly20represented in the drawings is for illustrational purposes only, it is to be understood that neither the shape of the nozzles or the grids, or the number and configuration of the fuel rods28and guide thimbles24are to be limiting, and the invention is equally applicable to different shapes, configurations, and arrangements than the ones specifically shown.

For example, as shown inFIGS. 2 and 4, the support grid26includes a frame assembly40and at least one generally cylindrical tubular member50. The frame assembly40includes a plurality of cells42defined by cell walls43. Each cell42has a width as indicated by the letter “w.” In one embodiment, the cells42and cell walls43are formed from a plurality of substantially flat, elongated strap members44disposed in two interlocked sets, a vertical set46and a horizontal set48. The strap members44in the vertical and horizontal sets48of strap members44are generally perpendicular to each other. Additionally, the strap members44in each set are generally evenly spaced. In this configuration, the strap members44form generally square cells42A. Thus, each cell42A has two diagonal axes “d1” and “d2,” which are perpendicular to each other and extend through the corners of the cell42A, as well as two normal axes “n1” and “n2,” which are perpendicular to each other and extend through the center of the cell42A and which intersect perpendicularly with the cell walls43. The points on the cell wall43that the two normal axes pass through are the closest point, “cp,” between the cell wall43and the center of the cell42. As shown inFIG. 3, the frame assembly40also has a height, indicated by the letter “h,” wherein the height is substantially less than the width or length of the frame assembly40. Further, the frame assembly40has a top side47and a bottom side49. It is notable that the strap members44of the present invention do not include protuberances, such as springs and dimples, as did strap members of the prior art. The lack of additional support structures make the construction of the frame assembly40very easy.

The tubular member50of the support grid26is shown inFIGS. 4 and 5. The tubular member50includes at least one helical fluted portion or fuel rod contact portion52, a cell contact portion54, and a transition portion56disposed therebetween. As shown inFIGS. 4-6, the tubular member50has four fuel rod contact portions52, which is the preferred embodiment. Other configurations are discussed below. The cell contact portion54has a greater diameter being generally equivalent to said cell width and is structured to snugly engage the cell42. The fuel rod contact portion52has a lesser diameter, being generally equivalent to said fuel rod28diameter. Thus, the tubular member50may be disposed in a cell42and a fuel rod28may be disposed in the tubular member50. In a preferred embodiment, the tubular member50is made from a material having a uniform thickness. Thus, the helical fuel rod contact portion52defines an outer passage60between the outer side of the tubular member50and the cell wall43. Additionally, the cell contact portion54, which is spaced from the fuel rod28, defines an inner passage62. Water which flows through either the outer or inner passages60,62is influenced by the shape of the helical fuel rod contact portion52resulting in the water being mixed.

The tubular member50may be constructed with any number of helical fuel rod contact portions52which may have any degree of pitch. For example, as shown inFIG. 7, a tubular member50has a single helical fuel rod contact portion52that extends 360 degrees about the tubular member50. As shown inFIG. 8, a tubular member50has a two helical fuel rod contact portions52that each extend 180 degrees about the tubular member50. As shown inFIG. 9, a tubular member50has a two helical fuel rod contact portions52that each extend 360 degrees about the tubular member50. As noted above,FIG. 5shows a tubular member50having a four helical fuel rod contact portions52that each extend 90 degrees about the tubular member50. Preferably, the helical fuel rod contact portions52are spaced evenly about the tubular member50, but this is not required.

These examples have used a number (N) of helical fuel rod contact portions52and an angular displacement (A) that equals 360 degrees or a multiple of 360 degrees. This configuration is especially adapted for use in a square cell42A. That is, the cell contact portion54will only contact the cell wall43at the closest point on the cell wall43. At other points, e.g., the corner of the cell42A, the tubular member50greater diameter, that is the cell contact portion54, will not contact a cell wall43. Thus, as shown best inFIG. 6, where there are four evenly spaced, helical fuel rod contact portions52that each extend 90 degrees about the tubular member50, there are four corresponding cell contact portions54, each disposed between a helical fuel rod contact portions52. To ensure the greatest amount of surface area contact between the tubular member50and the cell wall43, the tubular member50is disposed with each helical fuel rod contact portion52generally aligned with a diagonal axis at the top side47of the cell and aligned with a different diagonal axis at the bottom side49of the cell. In this orientation, the cell contact portion54is aligned with a cell wall43closest point at the top side47and at the bottom side49. A similar configuration may be made with cells42of any shape. That is, the number (N) of helical fuel rod contact portions52is preferably equal to the number of sides (S) to the cell42, and the angular displacement (A) is preferably 360 degrees/S. Thus, the tubular member may be positioned with each helical fuel rod contact portion52generally aligned with an axis passing through the corner of the cell42at the top side47of the cell and aligned with a different axis passing through the corner of the cell42at the bottom side49of the cell. Thus, the cell contact portion54is aligned with the cell wall43closest point at the top side47and at the bottom side49.

In another embodiment, the frame assembly40includes a plurality of cylindrical cells42B defined by a plurality of connected tubular frame members70. As shown inFIG. 10, the frame assembly40may have a plurality of densely packed tubular frame members70, however, as shown inFIG. 11, a pattern of aligned tubular frame members70is preferred. That is, the tubular frame members70are coupled to each other at 90 degree intervals about the perimeter of each tubular frame member70. The tubular member50is disposed within the cylindrical cells42B. As shown inFIG. 12, the combination of the tubular member50and the cylindrical cell42B again creates an inner passage62between the fuel rod28and the tubular member50and an outer passage60between the tubular member50and the tubular frame member70. The cylindrical cell42B of the tubular frame member70has the additional advantage that the entire cell contact portion54abuts the cell wall43. That is, the diameter of the cylindrical cell42B is the same as the cell width, which is also the same as the closest point, and, as such, the cell contact portion54will engage the cell wall43along the entire height of the cell wall43. This is unlike a square cell42A wherein the cell contact portion54does not contact the cell wall43at the corners.

In another embodiment, shown inFIG. 13, the functions of the tubular member50and the tubular frame member70have been combined in a helical frame member80. That is, the frame assembly40includes a plurality of helical frame members81disposed in a matrix pattern. The helical frame member80, like the tubular member50, includes at least one helical fuel rod contact portion52, however, instead of a cell contact portion54, the outer side of the helical frame member80is a contact portion55structured to be directly coupled to the contact portion55of an adjacent helical frame member80. As with the tubular frame member70embodiment of the frame assembly40, the helical frame members80are coupled to each other at 90 degree intervals about the perimeter of each helical frame member80. Additionally, in this embodiment the frame assembly40preferably includes a plurality of outer straps82structured to extend about the perimeter of the plurality of helical frame members81. The outer straps82are coupled to the contact portion55of the helical frame members80disposed at the outer edge of the plurality of helical frame members81. A fuel rod28is disposed through at least one helical frame member80.

As shown best inFIG. 12, as viewed as a cross-section, the tubular member50components, i.e., the helical fuel rod contact portion52, the cell contact portion54, and the transition portion56, preferably, are shaped as smooth curves. This configuration gives the tubular member50a compressible, spring-like quality. However, as shown inFIG. 14, the cell contact portion54may include an extended planar length or platform90. The platform90is structured to provide a greater surface area which engages the cell wall43. The greater length of the platform90will necessitate the transition portion56having a sharp curve. Similarly, as shown inFIG. 15, the helical fuel rod contact portion52may include a concave platform92adapted to extend radially about the fuel rod28. As before, greater length of the concave platform92will necessitate the transition portion56having a sharp curve. A tubular member50may also include both a platform90at the cell contact portion54and a concave platform92at the helical fuel rod contact portion52. Finally, the tubular member50may also be constructed with a generally flat transition portion56with angled ends94. As shown inFIG. 16, in this embodiment the transition portion56is generally planar in a cross-sectional top view. It is understood that, due to the helical nature of the fuel rod contact portion52, the transition portion56is not flat in the direction of the height of the frame assembly40.