WO1994020962A1

WO1994020962A1 - Mixing spacer grid for pwr and bwr applications

Info

Publication number: WO1994020962A1
Application number: PCT/US1993/008680
Authority: WO
Inventors: Richard P. Broders
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1993-03-08
Filing date: 1993-09-15
Publication date: 1994-09-15
Anticipated expiration: 1995-09-08
Also published as: AU4921693A

Abstract

A spacer grid which supports nuclear reactor fuel rods, includes an intersecting pattern of strips combined with cylindrical members which are slotted and fit onto the strips in a manner which encircles the intersections defined between the strips. The tops of the cylindrical members can be shaped or formed with vanes to induce swirl within the flow channels defined between the fuel rods or deflect part of the flow in one channel into an adjacent one.

Description

MIXING SPACER GRID FOR P R AND B R APPLICATIONS

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to a nuclear reactor and more specifically to an improved spacer grid which increases the coolant flow mixing within the nuclear reactor fuel assembly.

Description of the Prior Art

Nuclear fuel assemblies are customarily of the rod type where the fuel rods are held in a square array by a spacer grid assembly. The spacer grid assemblies are typically made from relatively thin elongate strips arranged in an "egg crate" manner with the strips positioned at right angles to one another. Soft (springs) and hard (arches) support features are stamped into the strip and are used to position the fuel rods. The grids are often equipped with vanes or other devices to enhance the Critical Heat Flux (CHF) and general heat transfer performance of the fuel. Since the projection of the grid into the flow stream causes pressure losses, spacer grid design optimization requires that the necessary structural support and CHF performance be provided with minimum pressure losses. In addition, any CHF enhancement devices (e.g. vanes) on the grid must be strong enough and adequately supported for fabrication, performance of their design function while in reactor, and fuel reconstitution.

A so called "Split Vaned Zircaloy Grid" was developed by the entity to which the instant application is assigned for the purposes of achieving high thermal-hydraulic performance while still providing load-carrying strength and handling performance for PWR operating conditions. This spacer grid utilizes split vanes to improve CHF performance by mixing coolant between adjacent channels. and swirling the flow within sub-channels and around the rod. The split vanes are optimized with respect to size, shape, and bend angle to enhance CHF performance and the top weld nugget is recessed with no cut-out in the vane so the turbulent wake produced by the nugget has less impact on vane performance. This grid has been thoroughly tested to assess the mechanical, hydraulic and CHF performance. Results of these tests have shown that this type of split vane grid produces high CHF performance in the PWR operating space and that requirements of grid pressure drop, load-carrying strength, and handling performance are satisfied.

However, this type of split vane arrangement suffers from two undesirable features. It exhibits weak mechanical support which makes fuel reconstitution difficult from the top of the fuel assembly; and the enhancement of CHF performance tends to diminish relative to a no vane spacer grid at coolant qualities greater than 20%. Fortunately, PWR operating space is typically less than 20% quality.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a spacer grid which exhibits a suitable level of component strength and compatibility with fuel assembly reconstitution requirements while providing flow redistribution which minimizing pressure drop.

In brief, the above object is achieved by a structural arrangement wherein a spacer grid which supports nuclear reactor fuel rods, includes an intersecting pattern of strips combined with cylindrical members which are slotted and fit onto the strips in a manner which encircles the intersections defined between the strips. The tops of the cylindrical members can be shaped or formed with vanes to induce swirl within the flow channels defined between the fuel rods or deflect part of the flow in one channel into an adjacent one.

More specifically, a first important aspect of the present invention resides in a spacer grid which features: a plurality of spaced first strips which are arranged essentially parallel to one another; a plurality of space second strips which are arranged essentially parallel to one another and which intersect the first strips at a predetermined angle to form a plurality of intersections; and a plurality of cylindrical members which are slotted and which fit onto the first and second strips so that each cylindrical member encloses an intersection therein. An important feature of the above mentioned structure comes in that the top portion of at least one of the cylindrical members extends above the upper edges of the first and second strips and is provided with flow deflecting means which alters the direction of a flow of fluid which passes therepast.

A second important aspect of the invention resides in a spacer grid for supporting a plurality fuel rods in a nuclear reactor, which features: plurality of spaced first strips which are arranged essentially parallel to one another; a plurality of spaced second strips which are arranged essentially parallel to one another and which intersect the first strips at a predetermined angle to form a plurality of intersections; a plurality of cylindrical members which are slotted and which fit onto the first and second strips so that each cylindrical member encloses an intersection therein, the plurality of fuel rods being disposed between the plurality of cylindrical members so as to establish an essentially parallel relationship with one another and with the cylindrical members, the fuel rods defining a plurality of flow channels therebetween; and means defined at the upper portions of the plurality of cylindrical members for deflecting a flow of fluid passing between the fuel rods in a manner which induces a swirl in the flow within the flow channels and/or cause a portion of a flow in one flow channel to be deflected into an adjacent flow channel.

A further important aspect of the present invention resides in a structure for supporting a plurality of rods in a liquid which is characterized by first cross members which are arranged essentially parallel to one another; second cross members which are arranged essentially parallel to one another and which intersect the first cross members at a predetermined angle to form a grid; a plurality of tubular members which are slotted and which fit onto the first and second cross members so that each cylindrical member encloses a junction at which a first cross member and a second cross member intersect, the plurality of rods being disposed in the grid between the plurality of cylindrical members so as to establish an essentially parallel relationship with one another and with the cylindrical members; a plurality of flow channels defined between the plurality of rods; and means defined at the upper portions of the plurality of tubular members for deflecting a flow of fluid passing between the rods in a manner which induces a swirl in the flow within the flow channels and/or cause a portion of a flow in one flow channel to be deflected into an adjacent flow channel. The design of the spacer grid offers advantages over existing designs because of its effective flow redistribution technique which minimizes pressure drop losses while maintaining component strength and compatibility with fuel assembly reconstitution. Since the flow redistribution technique should continue to work well at high coolant qualities, the spacer grid design is applicable to BWR applications as well as PWR applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention will become more clearly understood as a detailed description of a preferred embodiment is made with reference to the appended drawings in which:

Fig. 1 is a plan view showing a first embodiment of the invention;

Figs. 2 and 3 show second and third embodiments of the invention, respectively; and Fig. 4 is a schematic drawing depicting axial velocity characteristics obtained with the second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a basic embodiments of the invention. A distinctive feature of the basic spacer grid design is that, in addition to the thin strips that form the "egg crate", there is a thin-walled tube at each strip intersection that fits within the channel formed by four fuel rods. The tube is slotted on the bottom to fit over the straight strips and then is welded thereto.

In accordance with the present invention, the tops of the tubes which extends above the straight strips and can be shaped to redistribute the coolant flow in a variety of ways. Figure 2 shows a second embodiment wherein the tops are flared in a manner which forms a "figure eight" type of configuration. This causes swirl around the fuel rods by moving flow from one channel to another. Fig. 3 shows another configuration which causes swirl within the coolant flow channel itself. It will be understood that the present invention is not limited to the illustrated arrangement and that a different number of lobes or vanes can be used, as well as asymmetric lobe or vane geometries, without departing from the spirit of the invention.

The spacer grid is designed to enhance the CHF performance of the fuel without significantly increasing the pressure drop of the fuel assembly. This is done by using the overall pressure drop of the spacer grid to accomplish the flow mixing by channeling the flow captured within the tube into the desired patterns of swirl or redistribution. In this way the performance at high qualities can be expected and maintained. In addition, the channeled flow can be mixed more efficiently with shallower deflection angles, thus minimizing the pressure drop associated with a given increase in CHF performance.

The design lends itself to be used for the main structural spacer grid that supports the fuel rods, or as an intermediate (non-structural) grid positioned between the structural grids. Any rod support features (arches and springs) can be formed in the slotted portion of the tubes. Since the outer diameter of the tubes are in close proximity to the fuel rod cladding, the projection of these support features into the coolant flow is minimal, thus minimizing any pressure drop increase associated with the features. The spring supports are axially oriented. In addition to minimizing the grid's pressure drop by not having any leading edges, the axial spring can stay in closer proximity to the fuel rod cladding during operation due to differential growth between the spring and the tube.

For a given strip thickness, the welding of the slotted tubes to the interlocking strips significantly reduces the column lengths of the spacer grid when loaded laterally. The reduced lengths can be expected to significantly increase the buckling capability of the spacer grid. In addition, the strips have no cutouts for springs and arches since they are formed into tubes. Therefore, the height and thickness of the strips can be reduced by while maintaining the required grid strength. Both of these height and thickness features help decrease the pressure drop of the spacer grid.

The spacer grid design is completely compatible with fuel assembly reconstitution from the top of the bundle. The feature of the tubes that mix the flow are either no susceptible to damage from axial loads imposed by the fuel rod being reinserted (design shown in Fig. 2) or, if damaged would deflect away from the rod into a harmless position toward the center of tube (Design shown in Fig.

3).

The embodiment shown in Fig. 2 which features flaring at the top of the tubes was fabricated, assembled and tested in a cold water flow loop. Laser doppler velocimeter (LDV) measurements were taken. The test setup was able to measure axial velocities and lateral velocities at right angles to the laser path. Unfortunately, the mixing between sub-channels for this grid design is done in the lateral plane and could not be measured using the LDV tests. Nevertheless, an evaluation of the axial velocity data, as depicted in Fig. 4, verified that the separation angle of the high velocity lobes correspond to the sloped angle of the flared tube. This suggests that the flared tube design of the second embodiment was such as to redistribute the flow in the sub-channels.

Claims

WHAT IS CLAIMED IS

1. A spacer grid comprising: plurality of spaced first strips which are arranged essentially parallel to one another; a plurality of spaced second strips which are arranged essentially parallel to one another and which intersect said first strips at a predetermined angle to form a plurality of intersections; and a plurality of cylindrical members which are slotted and which fit onto said first and second strips so that each cylindrical member encloses an intersection therein.

2. A spacer grid as set forth in claim 1, wherein said plurality of cylinders are arranged with respect to one another so that a plurality of fuel rods can be disposed therebetween, said fuel rods defining flow channels therebetween.

3. A spacer grid as set forth in claim 1, wherein the top portion of at least one of said cylindrical members extends above the upper edges of said first and second strips and is provided with flow deflecting means which alters the direction of a flow of fluid which passes therepast.

4. A spacer grid as set forth in claim 3, wherein the top portion of said tube is flared so as to figure eight-like shaped orifice.

5. A spacer grid as set forth in claim 4, wherein each of the figure eight-like shaped orifices has a waist portion and two lobe portions and wherein said plurality of cylindrical members are arranged with respect to one another so that a lobe of one figure-eight orifice is oriented toward the waist portion of an adjacent figure-eight orifice.

SUBSTITUTESHEET

6. A spacer grid as set forth in claim 3, wherein the top portion of said tube is formed with vanes which induce a swirling motion in the fluid which flows through the cylindrical member and which induces a swirling motion in the flow in a coolant flow channel which is defined above the

7. A spacer grid for supporting plurality fuel rods in a nuclear reactor, comprising: plurality of spaced first strips which are arranged essentially parallel to one another; a plurality of spaced second strips which are arranged essentially parallel to one another and which intersect said first strips at a predetermined angle to form a plurality of intersections; a plurality of cylindrical members which are slotted and which fit onto said first and second strips so that each cylindrical member encloses an intersection therein, said plurality of fuel rods being disposed between said plurality of cylindrical members so as to establish an essentially parallel relationship with one another and with said cylindrical members, said fuel rods defining a plurality of flow channels therebetween; and means defined at the upper portions of said plurality of cylindrical members for deflecting a flow of fluid passing between said fuel rods in a manner which induces a swirl in the flow within the flow channels and/or cause a portion of a flow in one flow channel to be deflected into an adjacent flow channel.

8. A structure for supporting a plurality of rods in a liquid comprising: first cross members which are arranged essentially parallel to one another; second cross members which are arranged essentially parallel to one another and which intersect said first cross members at a predetermined angle to form a grid;

SUBSTITUTESHEET a plurality of tubular members which are slotted and which fit onto said first and second cross members so that each cylindrical member encloses a junction at which said a first cross member and a second cross member intersect, said plurality of fuel rods being disposed in said grid between said plurality of cylindrical members so as to establish an essentially parallel relationship with one another and with said cylindrical members; a plurality of flow channels defined between said plurality of rods; and means defined at the upper portions of said plurality of tubular members for deflecting a flow of fluid passing between said rods in a manner which induces a swirl in the flow within the flow channels and/or cause a portion of a flow in one flow channel to be deflected into an adjacent flow channel.

SUBSTITUTESHEET