Patent Application
20060222140
1. Field of the Invention
The present invention relates to nuclear reactor fuel assemblies and more particularly to a nuclear reactor fuel assembly wherein the support grids have dimples with an increased height that causes the fuel rod to assume a non-linear shape.
2. Description of the Prior Art
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 frame assembly structured to support a plurality of fuel rods. The fuel assembly includes a top nozzle, a bottom nozzle, a plurality of support grids, 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. The support grids are commonly positioned approximately 20 inches apart in the axial direction to provide mechanical support to the rods. A common problem in the PWR fuel industry is vibration of the fuel rod/support system as a natural response to coolant flow. This phenomenon is manifested as grid-to-rod fretting, which can ultimately lead to a breach in the fuel rod cladding and leakage of nuclear fuel material into 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 generally 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 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 arches 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. Generally, the spring and dimple combination are sized to allow a linear fuel rod to extend through the center of the cell. That is, the fuel rod is generally straight from the bottom to the top of the fuel assembly. 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. Typically, the springs apply a force of 5-15 lbs. as built. This amount of force/pressure creates a significant stress in the spring structure.
The disadvantage to this type of structure is that irradiation and thermal effects alter the shape of the support grid allowing for a reduction in the lateral force applied to the fuel rod. That is, operational effects cause the springs to relax, the support grid to grow, and the fuel rod to shrink. Eventually, there are small clearances between the fuel rod and the springs and/or dimples. This space allows for vibration and more rapid degradation of the fuel rod due to fretting. The increased fretting reduces the operational life of the fuel rod.
There is, therefore, a need for an improved structure for securing fuel rods in a support grid.
There is a further need for a structure for securing fuel rods in a support grid that maintains the lateral pressure on a fuel rod for an extended operational period.
These needs, and others, are met by the present invention which provides a support grid structured to hold a fuel rod in a non-linear shape. That is, the support grid is structured to alter the shape of a linear fuel rod into a shape preferably similar, but not limited, to a sine wave or a spiral. The support grid is structured in a manner similar to the prior art support grids except that the dimples have a greater height relative to the strap, i.e., the dimples protrude a greater distance into the cell. With the support grids in this configuration, the fuel rods must be laterally deformed in order to pass through the cells. By having the extended dimples disposed in a selected pattern, varying from support grid to support grid, the overall shape of the fuel rod may be controlled. For example, in a series of three support grids, the upper and lower grids could have extended dimples on a first side while the middle support grid had the extended dimples on the opposite side. In this configuration, the lower support grid extended dimples would tend to deform the fuel rod away from the first side. The middle support grid extended dimples would tend to deform the fuel rod toward the first side. The upper support grid extended dimples would tend to deform the fuel rod away from the first side. Thus, if the support grids are evenly spaced, the fuel rod would have, generally, a shape similar to a sine wave. Likewise, by placing the extended dimples on selected sides of the cells in adjacent support grids, the shape of the fuel rod may be changed to various non-linear shapes. Additionally, each fuel rod may be structured to have a shape that is different from an adjacent fuel rod. In this manner, the flowpath of the coolant may be affected to further control heat transfer and vibration.
A non-linear fuel rod applies a lateral force against the dimples, extended dimples and/or springs. The lateral force may be estimated according to the following formula. P=(48*E*I*Delta)/(2*L)3 wherein; E is the Young's modulus for the fuel rod cladding material, I=π*(D4-d4)/64, D is the fuel rod cladding outer diameter, d is the fuel rod cladding inner diameter, L is the distance between support grids, and Delta is the eccentricity magnitude. The preferred eccentricity is 10-60 mils. In a typical fuel rod, it is expected to have about 0.2 pounds of lateral force for each ten mils of eccentricity. That is, fuel rod stress would be about 500 psi for each ten mils of eccentricity. This is approximately 100 times less than the typical spring stress.
In addition to a lower stress, the design of the present invention is less susceptible to operational effects. That is, effects such as irradiation creep, which is dependent upon the amount of stress, has a reduced effect due to the reduced stress. Accordingly, the time period before a space develops between the support grid and the fuel rod is extended, possibly indefinitely. That is, a gap between the fuel rod and the support structure may never occur. Thus, the wear due to fretting is reduced and the useful life of the fuel rod is extended.
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:
As shown in
The top nozzle 32 includes 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 and suitably clamped to this flange are leaf springs 36 (only one of which being shown in
The fuel assembly 20 depicted in the drawings is of the type having a square array of fuel rods 28 with the control rod guide thimbles 24 being strategically arranged within the fuel rod array. Further, the bottom nozzle 22, the top nozzle 32, and likewise the support grids 26 are generally square in cross section. In that the specific fuel assembly 20 represented 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 rods 28 and guide thimbles 24 are to be limiting, and the invention is equally applicable to different shapes, configurations, and arrangements than the ones specifically shown.
As is shown in
As shown schematically in
The present invention provides for a series of support grids 26 structured to support at least one fuel rod 28 in a non-linear configuration. At least two support grids 26 are required wherein at least one set of aligned cells 56, that is a cell 56 at a certain location on one support grid 26 and the cell 56 at a similar location on another support grid 26, include at least one extended dimple 80 extending from one cell side 70, 72, 74, 76 on the first grid 26 and at least one extended dimple 80 on the second grid 26 extending from a cell side 70, 72, 74, 76 other than cell side 70, 72, 74, 76 having the at least one extended dimple 80 on the one support grid 26.
In a typical fuel assembly 20 there will be more that two support grids 26. Three support grids 26 are shown in the FIGS., however, it is understood that a fuel assembly 20 may have more that three support grids 26. In a series of three support grids 26A, 26B, 26C (discussed below), at least one set of vertically aligned cells 56 may include cells 56 on the first and second support grids 26A, 26B as described above. That is a cell 56 at a certain location on the first support grid 26A and the cell 56 at a similar location on the second support grid 26A, include at least one extended dimple 80 extending from one cell side 70, 72, 74, 76 on the first support grid 26A and at least one extended dimple 80 on the second support grid 26B extending from a cell side 70, 72, 74, 76 other than cell side 70, 72, 74, 76 having the at least one extended dimple 80 on the first support grid 26. The third support grid 26C, however, may have the at least one extended dimple 80 extending from a cell side 70, 72, 74, 76 the same as the cell side 70, 72, 74, 76 having an extended dimple 80 on the first support grid 26A. In this configuration, and as shown in
The present invention further provides for having different fuel rods 28 in adjacent cells 56 of the support grids 26 to have various shapes. That is, the fuel rod 28 in an adjacent cell 56 may have the identical shape to the fuel rod 28 described above as being shaped similar to a sine wave. Alternatively, the fuel rod 28 in the adjacent cell may be deformed in a mirror image shape, or with the deflections ninety degrees to the fuel rod 28 in an adjacent cell 56. The fuel rod 28 in an adjacent cell 56 may also be generally linear as in the prior art. The shape of the fuel rods 28 will effect the flow path of the coolant and may be configured in a defined pattern to control the heat transfer properties of the coolant flow.
Where there are two cell sides 70, 72, 74, 76 having extended dimples 80, the extended dimples 80 in vertically aligned cells 56 on different support grids 26 may be disposed in various configurations. For example, one support grid 26 may have extended dimples 80 extending from any two cell sides 70, 72, 74, 76. Another support grid also has an extended dimple 80 extending from any two cell sides 70, 72, 74, 76 wherein at least one cell side 70, 72, 74, 76 is a cell side 70, 72, 74, 76 other than one of the cell sides 70, 72, 74, 76 having an extended dimple 80 on the one support grid 26. As before, where there are three or more support grids 26A, 26B, 26C (described below), each having a set of vertically aligned cells 56 with extended dimples 80 on any two sides 70, 72, 74, 76, the third support grid 26C may have the extended dimples 80 extending from two cell sides 70, 72, 74, 76 and wherein the two cell sides 70, 72, 74, 76 are the same two cell sides 70, 72, 74, 76 as the cell sides 70, 72, 74, 76 having an extended dimple 80 on the first support grid 26A. Alternatively, the third support grid 26C may have the extended dimples 80 extending from two cell sides 70, 72, 74, 76 and wherein at least one extended dimple 80 extends from a cell side 70, 72, 74, 76 that is different from the two cell sides 70, 72, 74, 76 having an extended dimple 80 on the first support grid 26A.
An example of the present invention is shown schematically in
As shown in
The deformation of the fuel rods 28 creates a lateral force that is applied to the protuberances 53. The amount of force may be estimated by the formula: P=(48*E*I*Delta)/(2*L)3 wherein; E is the Young's modulus for the fuel rod cladding material, I=π*(D4−d4)/64, D is the fuel rod cladding outer diameter, d is the fuel rod cladding inner diameter, L is the distance between support grids, and Delta is the eccentricity magnitude. The preferred eccentricity is 10-60 mils. The eccentricity is determined as the distance between the center of the fuel rod 28 adjacent to one set of protuberances 53 as compared to a line between the center of the fuel rod 28 at two adjacent sets of protuberances 53 as shown on
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 arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.
