Spacer grid with mixing vanes and nuclear fuel assembly employing the same

A spacer grid having tunable mixing vanes for optimizing coolant flow is provided for a nuclear fuel assembly. The mixing vanes include an upper piece and a lower piece, which are coupled to a middle ligament that is coupled to the top or downstream portion of one or more of the grid straps forming the spacer grid. The upper piece, lower piece, and middle ligament are bent, rotated, and/or twisted into various positions to more effectively mix the coolant and affect the coolant's flow as it moves upwards through the fuel assembly. For example, in one embodiment, the mixing vane has an “S” shape wherein the middle ligament is positioned parallel to the longitudinal axes of the fuel rods of the fuel assembly, the upper piece bends towards one of the fuel rods, and the lower piece bends away from the fuel rod. In this manner, optimized even and sustained mixing of the coolant is provided.

FIELD OF THE INVENTION

The present invention relates generally to nuclear reactors and, more particularly, to spacer grids for a nuclear fuel assembly, including mixing vanes to effectively mix coolant as it flows through the fuel assembly.

BACKGROUND INFORMATION

Nuclear fuel assemblies for light water nuclear reactors, such as, for example, pressurized water reactors (PWRs) and boiling water reactors (BWRs), generally include a plurality of fuel rods with circular cross-sections that are arranged parallel to one another at regularly or irregularly spaced intervals. Each fuel rod comprises a stack of fuel pellets (e.g., uranium oxide pellets) surrounded with cladding that is made from zirconium alloy or other suitable material. The fuel rods are held at the spaced intervals with respect to one another by one or more spacer grids. Each spacer grid includes a plurality of interlocking grid straps that are welded together to form an array of four-walled cells in an “egg-crate” configuration. A fuel rod may be contained within each of the four-walled cells. The entire fuel assembly typically has a square cross-section with a 14×14, 15×15, 16×16, or 17×17 array of fuel rods. One or more outer straps may encircle the periphery of each spacer grid.

FIG. 1presents a partial perspective view of a conventional spacer grid100for a fuel assembly102. The spacer grid100includes two sets of perpendicularly placed grid straps112,114. Each grid strap112,114includes a series of regularly spaced notches (not shown) that allow for the assembly and interlocking of the grid straps112,114to form an array of four-walled cells116. Each four-walled cell116contains four intersections118. The grid straps112,114may be welded together at these intersections118. The purpose of the four-walled cell116is to support a single fuel rod130(FIG. 2) in the square array of the fuel assembly102. The periphery of the grid straps112,114may be encircled with one or more outer straps120. A plurality of springs122and a plurality of dimples124are integrally formed on, or attached to, the grid straps112,114and extend inwardly within each four-walled cell116. The springs122and dimples124provide support structures for contacting the fuel rod cladding and holding it within the four-walled cell116.

FIG. 2presents a cross-sectional top plan view of one of the aforementioned four-walled cells116, with a fuel rod130contained therein. As shown, the springs122and dimples124extend inwardly within the four-walled cell116to engage and provide support for the fuel rod130, as previously discussed.

When the PWR or BWR is in use, a coolant, such as for example, water, flows from the bottom of the fuel assembly upwards through the spaces between the fuel rods. The temperature of the coolant varies as it travels upwards, absorbing thermal energy from the fuel rods. At locations adjacent to the fuel rods, the coolant may be partially overheated, which can adversely affect the thermal performance of the fuel assembly and reduce the output power of the fuel rods. One way of alleviating these partially overheated regions is to design the spacer grids to more effectively deflect and mix the coolant as it flows upwards through the fuel assembly, thereby promoting a more uniform distribution of coolant temperature. Such a design can be accomplished by attaching “mixing vanes” to the top, downstream portion of the grid straps that comprise the spacer grid, as shown. The mixing vanes are intended to promote the flow of coolant in a lateral direction as well as a longitudinal direction along the fuel rod axes. This flow pattern allows the coolant to more effectively move between the fuel rods, and between the lower temperature regions and the partially overheated regions of the fuel assembly.

Conventional mixing vanes, however, tend to be restricted in the ways in which they deflect the flow of coolant as it moves upwards through the fuel assembly. They do not provide a robust means for adjusting or tuning the vanes in order to optimize the flow pattern that is formed. As a result, conventional mixing vanes cannot effectively achieve the most desirable type of coolant flow—even and sustained mixing—for the particular application at hand. Thus, there exists a need for a new type of mixing vane that guides the coolant in a desired flow pattern to more effectively mix the coolant as it moves upward through the fuel assembly.

SUMMARY OF THE INVENTION

The present invention provides a spacer grid for a nuclear fuel assembly having a plurality of elongated fuel rods. The spacer grid employs novel mixing vanes which include an upper piece and a lower piece, which are connected to a middle ligament that is coupled to the top or downstream portion of a grid strap forming the spacer grid. Although the present invention is not limited to any particular number of mixing vanes, in a preferred embodiment, each four-walled cell of the spacer grid may contain four mixing vanes—one positioned at the top, downstream portion of each wall. The upper piece, lower piece, and middle ligament may be bent, rotated, and/or twisted into various positions to more effectively mix the coolant and affect the coolant's flow as it moves upwards through the fuel assembly. In a preferred embodiment, the mixing vane has an “S” shape when viewed from an end elevational vantage point. More specifically, the middle ligament is positioned parallel to the longitudinal axes of the fuel rods, the upper piece is bent towards the fuel rod, and the lower piece is bent away from the fuel rod, thereby defining the general S-shape. In another preferred embodiment, the mixing vane has a parabolic shape when viewed from the end elevational vantage point. A nuclear fuel assembly employing such mixing vanes, is also disclosed.

An object of the present invention is to provide a mixing vane that is tunable in order to affect (i.e., control or adjust) and thus optimize for the particular application at hand the coolant flow pattern that is formed, rather than merely deflecting the coolant flow.

Another object of the present invention is to provide a mixing vane that promotes optimized even and sustained interchannel mixing.

Yet another object of the present invention is to provide a mixing vane that more effectively mixes coolant as it moves upwards through the fuel assembly, thereby mitigating regions of varying coolant temperature.

A further object of the present invention is to provide mixing vane that is easily manufactured using conventional die technology.

Another object of the present invention is to provide a mixing vane with an upper piece, a lower piece, and a middle ligament, that are easily tuned or adjusted by bending, rotating, and twisting, in order to optimize the coolant flow.

These and other objects of the present invention will become more readily apparent from the following detailed description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description contained herein primarily refers to the use of a mixing vane having a generally rectangular shape. It will, however, be appreciated the mixing vanes may have any suitable shape, size, or dimensions. It will also be appreciated that although each mixing vane is contemplated as being formed from the grid strap material and connected to the grid strap as an integral component, that the mixing vanes could also be connected by welding, brazing, or mechanically securing them to the grid straps. The mixing vanes can also be made from any known or suitable material (e.g., without limitation zirconium alloy or nickel-steel alloy). It will further be appreciated that the Figures provided herein are provided for simplicity of illustration of only certain examples or variations of mixing vanes in accordance with the invention, and are not meant to be limiting upon the scope of the invention. It will also be appreciated that the Figures and, in particular, certain features depicted therein, are not drawn to scale.

Directional phrases used herein, such as, for example, clockwise, counterclockwise, top, bottom, upper, lower and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein.

As used herein, the terms “bend” and “bend angle” are used when referring to a bend in the upper or lower piece of the mixing vane. The terms “rotate” and “rotation angle” are used when referring to a rotation of the middle ligament. The terms “twist” and “degree of twist” are used when referring to a twist of the upper piece, lower piece, or middle ligament.

FIGS. 4a,4b,5,6a,6band7illustrate a preferred embodiment of the present invention in which the grid straps of a spacer grid include generally S-shaped mixing vanes. While the present invention is not limited to any particular number of mixing vanes, four mixing vanes are positioned around each fuel rod and four-walled cell in the preferred embodiment (best shown inFIG. 7).

FIGS. 4aand4bare partial elevational views of two types of perpendicularly positioned, interlocking grid straps160,162. To form the spacer grid200(FIG. 7), a plurality of these grid straps160,162are placed perpendicular to each other and connected together such that the downwardly positioned notches166on the first type of grid strap160, shown inFIG. 4a, engage the upwardly positioned notches168on the second type of grid strap162, shown inFIG. 4b. The location where the notches166,168are connected is known as a grid strap intersection170(FIGS. 6aand6b). Each grid strap160,162includes a plurality of mixing vanes172, which are shown in a flattened state in the unassembled views ofFIGS. 4aand4b.

Each mixing vane includes an upper piece174and a lower piece176that are connected to a middle ligament178. The middle ligament178is connected to the grid strap160,162. While the present invention is not limited to any particular number of mixing vanes172, each cell wall180(two cell walls180are shown in cross-hatch inFIGS. 4aand4b) of each grid strap160,162preferably contains a single mixing vane172. The mixing vane172may be connected to the grid strap160,162at its top or “downstream” portion182. The mixing vane172is preferably connected to the grid strap160,162so that the middle ligament extends outward from the grid strap160,162in a direction that is substantially orthogonal with respect to the longitudinal axis184of the cell wall180. For simplicity of illustration, the grid straps160,162in the examples ofFIGS. 4aand4bare shown without springs and dimples in the cell walls180. It will, however, be appreciated that any suitable number and configuration of dimples and springs could be employed (see, for example,FIG. 7). It will also be appreciated that although the mixing vanes172ofFIGS. 4aand4bare shown in a flattened position, prior to being assembled, the mixing vanes172are preferably bent and/or twisted into various positions to positively affect and thus optimize the flow of coolant as it moves through the fuel assembly. More specifically, the vanes172may be adjusted or fine tuned in order to optimize the coolant flow, as desired, for the particular application in which they are being employed.

FIG. 5is an elevational view of a single cell wall180and mixing vane172that has been bent in accordance with a preferred embodiment of the invention. Specifically, the upper piece174of the vane172has been bent at its connection with the middle ligament178, and placed at a “bend angle” of approximately +45 degrees from the vertical. The vertical direction is defined by the longitudinal axis184of the cell wall180. The lower piece176of the vane172has also been bent at its connection with the middle ligament178, and placed at a “bend angle” of approximately −45 degrees from the vertical. The middle ligament178has been bent at its connection with the grid strap160and placed at a “rotation angle” of approximately −45 degrees with respect to the plane of the grid strap160. A negative rotation angle for the middle ligament178indicates a counterclockwise rotation when viewed from the top of the fuel assembly, and a positive rotation angle indicates a clockwise rotation. Despite its rotation, the middle ligament178, in the example shown, remains vertically aligned and parallel to the longitudinal axis184of the cell wall180. Thus, the mixing vane172appears to have an S-shape when it is viewed directly from the side (i.e., from the left end with respect toFIG. 5). As noted previously with respect toFIGS. 4aand4b, although the grid strap160ofFIG. 5does not show springs or dimples in cell wall180, it will be appreciated that any suitable number and configuration of dimples and springs could be employed (see, for example,FIG. 7).

FIG. 6ais a partial perspective view of a grid strap intersection170of the spacer grid, andFIG. 6bis a top plan view of such intersection170. As noted previously, the grid strap intersection170occurs where two grid straps160,162have been perpendicularly positioned and interlocked together using their upwardly and downwardly positioned notches166,168(FIGS. 4aand4b). Because each grid strap cell wall180contains a single mixing vane172, the grid strap intersection170is surrounded with a total of four mixing vanes172, two per grid strap160,162. The upper piece174of each vane172has been bent at its connection with the middle ligament178, and placed at a bend angle of approximately +45 degrees from the vertical. The lower piece176of each vane172has been bent at its connection with the middle ligament178, and angled approximately −45 degrees from the vertical. A positive angle for the upper or lower piece174,176indicates that the piece has been bent away from the grid strap intersection170, and towards an adjacent fuel rod (not shown). A negative angle for the upper or lower piece174,176indicates that the piece has been bent towards the grid strap intersection170, and away from an adjacent fuel rod (not shown). This may be further appreciated and understood with reference to the top plan view ofFIG. 6b. The middle ligament178of each vane172has been bent at its connection with the grid strap160,162, and placed at a rotation angle of approximately +30 degrees from the plane of each grid strap160,162. A positive rotation angle for the middle ligament178indicates a clockwise rotation when viewed from a top plan view perspective (see, for example,FIG. 6b), and a negative rotation angle indicates a counterclockwise rotation.

The top plan view ofFIG. 6bfurther shows the interaction of the mixing vanes172with fuel rods210of the fuel assembly. One complete fuel rod210(shown in simplified form in phantom line drawing) and portions of the three adjacent fuel rods210, are shown.

FIG. 7is a partial perspective view of a spacer grid200in accordance with the invention. For simplicity of illustration, only one complete four-walled cell202of the spacer grid200is shown. It will be appreciated, however, that the spacer grid200in accordance with the invention forms any known or suitable array of a plurality of four-walled cells202, similar for example, to the spacer grid100shown inFIG. 1. It will also be appreciated thatFIG. 7and, in particular mixing vanes172, are not drawn to scale. The four-walled cell202occurs where four grid straps160,162,164,165interlock in a generally perpendicular configuration to create an opening for supporting a fuel rod210(shown in simplified form in phantom line drawing) therein. As shown, each grid strap160,162,164,165includes a plurality of springs186and dimples188of any known or suitable orientation, for securing the fuel rod210. Since each cell wall180includes a single mixing vane172, a total of four mixing vanes172surround the fuel rod210and four-walled cell202. The upper piece174of each vane172has a bend angle of approximately +45 degrees from the vertical184. The positive angle indicates that the piece has been bent towards the fuel rod210. The lower piece176of each vane172has a bend angle of approximately −45 degrees from the vertical. The negative angle indicates that the piece has been bent away from the fuel rod210. The middle ligament178of each vane172has a rotation angle of approximately +45 degrees from the plane of each grid strap160,162,164,165, with the positive angle indicating a clockwise direction of rotation when viewed from a top plan view perspective.

Accordingly, the spacer grid200includes a plurality of perpendicular, interlocking grid straps160,162,164,165that define the aforementioned grid strap intersections170(FIGS. 6aand6b) and the four-walled cells202. When viewed from an end elevational vantage point, each mixing vane172appears to have an S-shape. It is in this manner (i.e., the upper piece174bending toward fuel rod210, the lower piece176bending toward the fuel rod210, and the middle ligament178rotating toward the fuel rod) that the exemplary mixing vanes172assist in mixing coolant to create optimized even and sustained mixing (e.g., without limitation, a strong swirl and interchannel mixing) of the coolant as it moves upwards through the fuel assembly.

The present invention is not, however, limited to the embodiment shown hereinbefore. It will be appreciated that a wide variety of other mixing vane configurations are within the scope of the invention. For example, the upper and lower pieces of the mixing vane can have any suitable positive or negative bend angle220,222with respect to the vertical (i.e., the longitudinal axis184of the cell wall180), ranging from about 0 to about +90 degrees or about 0 to about −90 degrees.

FIG. 8ais a side view of an S-shaped mixing vane172which shows how bend angles220,222of the upper piece174and lower piece176are measured from the vertical or longitudinal axis184of the grid strap160.FIG. 8billustrates example alternative bends224,226for the upper piece174and lower piece176of the mixing vane172(with the alternatives shown in dashed line drawing). In the example ofFIG. 8b, the upper piece174and/or lower piece176have bends224,226ranging from about 0 to +45 degrees or about 0 to −45 degrees. It will also be appreciated, however, that the upper and lower pieces174,176could alternatively both be bent towards an adjacent fuel rod, in order to form a parabolic-shaped mixing vane (not shown). It will also be appreciated that, rather than being bent, the upper and/or lower pieces174,176could remain unbent (i.e., vertical at 0 degrees). It will still further be appreciated that while the upper and lower pieces174,176are preferably bent at their connection with the middle ligament178, a bend could occur at any location along the upper and lower piece174,176, and/or each piece174,176can contain multiple bends (not shown).

FIG. 9is a side view of an S-shaped mixing vane172which shows how the rotation angle228, as previously defined herein, for the middle ligament178is measured from the vertical or longitudinal axis184of the grid strap (i.e.,160). It will be appreciated that the middle ligament178may have any positive or negative rotation angle228with respect to the vertical, ranging from about 0 to +90 degrees or from about 0 to −90 degrees. For example, in one embodiment, shown in dashed line drawing, the middle ligament178is rotated from about 0 to +45 degrees (towards a fuel rod) or from about 0 to −45 degrees (away from a fuel rod). However, as shown in solid line drawing, rather than being rotated, the middle ligament178may remain at 0 degrees, parallel to the vertical. Additionally, while the example ofFIG. 9shows a pivot point230that is located approximately at the center of the middle ligament178, the pivot point may occur at any location on the middle ligament178. A middle ligament178may also contain multiple pivot points (not shown) and therefore multiple rotation angles (not shown).

In addition to being bent or rotated, the upper piece174, lower piece176, and middle ligament178may be “twisted” (i.e., subjected to torsion) in a clockwise or counterclockwise direction, as shown inFIGS. 10aand10b.FIG. 10ais a side view of two mixing vanes172, wherein the upper piece174of each vane172is twisted. The “degree of twist” may range from about 0 to +90 degrees or from 0 to about −90 degrees (either clockwise or counterclockwise) with respect to axis173. Although only the upper pieces174are twisted in the example ofFIG. 10a, it will be appreciated that any suitable combination of twisting of one or both of the upper and lower pieces174,176could be employed.

FIG. 10bis a partial perspective view of four mixing vanes172surrounding a grid strap intersection170. In the example ofFIG. 10b, the upper pieces174of two opposing mixing vanes172are twisted, while the upper pieces of the other two mixing vanes172are not. The degree of twist ranges from about 0 to +90 degrees or from about 0 to −90 degrees, in either a clockwise or counterclockwise direction. It will be appreciated that, in accordance with the invention, a single upper piece174, lower piece176, or middle ligament178may include multiple twists, and that the various mixing vanes172and components thereof, of a particular intersection170may have any suitable combination of bend, twist, and/or rotation.

Although the description contained hereinabove primarily refers to the use of four mixing vanes around a single fuel rod and four-walled cell, the present invention is not limited to any particular number of mixing vanes or placement of mixing vanes within the spacer grid. A spacer grid may contain fuel rods that are surrounded with mixing vanes and fuel rods that are not. Not every four-walled cell of a spacer grid is required to include a mixing vane. A fuel rod may be surrounded with anywhere from zero to four mixing vanes. A fuel rod is preferably surrounded with two or four mixing vanes. It is also preferable for the upper piece of each mixing vane to bend towards the fuel rod, and for the lower piece of each mixing vane to bend away from the fuel rod, although the mixing vanes in a given spacer grid may vary with respect to their bends, rotations, and twists. At least one upper piece may be twisted and/or bent with respect to the longitudinal axis of the cell walls, at least one lower piece may be twisted and/or bent with respect to the longitudinal axis of the cell walls, an d/or at least one middle ligament may be twisted and/or bent with respect to a plane on which the grid strap resides.