Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1996-07-02
2001-11-20
Wasil, Daniel (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S462000
Reexamination Certificate
active
06320924
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a spacer for nuclear fuel rods and particularly to a unique, substantially I-shaped spring and associated ferrule assembly for the fuel rod spacer.
BACKGROUND
In a nuclear reactor, for example a boiling water reactor, nuclear fuel rods are grouped together in an open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. In each assembly or bundle, fuel rods are supported between upper and lower tie plates in side-by-side parallel arrays. Spacers are employed at predetermined elevations along each fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation, and to protect the fuel rod assembly during possible loading events, such as handling and shipping.
Typical spacers often include a plurality of ferrules arranged in side-by-side relation and secured by, for example, welding to one another to form the support matrix of the spacer. A single fuel rod passes through each generally cylindrically shaped ferrule. The ferrules include circumferentially spaced, axially extending interior protuberances (or hard stops) and spring assemblies seated in openings formed in opposite sides of the ferrule from the protuberances, for centering and biasing the fuel rods against the hard stops, thereby maintaining the fuel rods in fixed relation one to the other across the spacer. The spacer itself constitutes an obstacle to bundle performance in that its cross-section interferes with the flow of water/moderator through the bundle. An ideal spacer would have minimal impact on bundle performance (thermal hydraulics, critical power), while still restraining the rods in their intended positions and protecting them. Consequently, an optimum fuel bundle spacer should have as little cross-section as possible, use a minimum amount of material and simultaneously meet structural requirements for positioning and protecting the fuel rods.
In developing new spacer spring designs for denser bundle matrices (for example, 8×8, 9×9, and 10×10), one challenge is to design the spring so that it will be sufficiently flexible to maintain historical preload limits as the space between the fuel rods becomes smaller (i.e., the spring deflection increases as the space decreases). Since it has been determined that the accelerated dead weight of the fuel rods at the. spacer locations damages the springs, a second challenge is to design the spring such that assembled fuel bundles can be shipped without the aid of plastic inserts to carry the weight of the rods as they travel on trucks. The damage mentioned above occurs because current spacer designs involve one spring being shared between two adjacent fuel rods, and this type of arrangement means that some fuel rods are sitting on the springs when the bundle is laid horizontally on the truck bed, causing the rod's own dead weight to be accelerated directly into the spring underneath it. Some prior spacer designs which include ferrules with one spring per two adjacent fuel rods are disclosed in commonly owned U.S. Pat. Nos. 5,173,252 and 5,078,961.
In commonly owned application Ser. Nos. 08/380,591 filed Jan. 30, 1995 and 08/516,203 filed Aug. 17, 1995, spring designs are disclosed which are based on a one spring per fuel rod criteria, with the ability to ship without conventional plastic inserts. However, a disadvantage of these spring designs is that too much spring material protrudes into the subchannel regions between the ferrules, and their respective geometries are thought to be susceptible to self-vibration due to coolant water flow across the springs.
DISCLOSURE OF THE INVENTION
The spring design for ferrule spacers in accordance with this invention do not require plastic insert supports for the spacers during shipping. The individual springs and ferrules are simply aligned within the fixture (before welding), with the associated springs all oriented in a single chosen direction, such that the shipping loads, which occur during bundle shipment to customers, can be taken up by the hard stops on the ferrules and not by the springs themselves.
In addition, the springs in accordance with this invention have the required flexibility but with minimal projection into the subchannels. More specifically, the spring is given generally the shape of an “I”, so that only the lateral ends or ears of the spring protrude into the subchannel, with the majority of the spring geometry captured in a cutout formed in the ferrule.
In the exemplary embodiment, the ferrule is substantially cylindrical, but with a pair of axially extending grooves or indents which provide a pair of hard stops on the interior of the ferrule when the fuel rod is inserted within the spacer. In the arcuate wail of the ferrule, opposite the pair of hard stops, there is a generally “I” shaped cutout or opening, also extending axially of the ferrule. Thus, the opening has a pair of relatively wider cutout portions connected by an axially extending narrower portion.
The I-shaped spring in accordance with an exemplary embodiment of the invention also includes upper and lower wider portions, or flanges connected by a stem. Each flange is formed with a centrally located outward (away from the ferrule) projection, while the center portion of the narrower stem which connects the flanges is formed with an inward (toward the ferrule) projection. This inwardly directed projection is also provided with an inwardly extending dimple which is adapted to engage a fuel rod placed within the ferrule. The spring in accordance with the exemplary embodiment of the invention is also form ed with a pair of “T” shaped cutouts, one upright and one inverted, located on either side of the inward projection on the spring stem.
When the spring is aligned within the “I” shaped opening of the ferrule, only the upper and lower flanges of the spring protrude from the ferrule and extend into the subchannel regions. The remaining portion of the spring including the entire stem portion is substantially located within the ferrule cut-out. This new design concept significantly reduces the amount of spring material that protrudes into the coolant flow, and the geometry is such that the spring is captured more securely within the ferrule cutout to thereby guard against spring movement due to flow-induced vibration. This is in sharp contrast to previous designs which have significant areas of the spring geometry within the subchannel flow, making the springs susceptible to movement from the force of the water impacting on the spring.
Thus, in accordance with the exemplary embodiment of the invention, there is provided a sub-assembly for a spacer useful in a nuclear fuel bundle for maintaining a matrix of a plurality of nuclear fuel rods passing through the spacer in spaced-apart relation, comprising at least first and second ferrules lying adjacent one another for receiving respective nuclear fuel rods, each ferrule having fuel rod contacting points along one side of the ferrule for abutting a fuel rod within the ferrule and a substantially I-shaped opening along a side of the ferrule opposite the one side; a substantially I-shaped spring including a spring body lying in a plane and having opposite horizontal end portions connected by a vertical stem portion, a central portion of each horizontal end portions projecting away from the substantially I-shaped opening to one side of the plane and a center portion of the vertical stem projecting into the substantially I-shaped opening to an opposite side of the plane; the spring being disposed between said adjacent ferrules with the vertical stem seated in the opening of the first ferrule with the center portion of the stem adapted to bear against the fuel rod within the first ferrule and maintaining the fuel rod against the fuel rod contacting points of the first ferrule, the horizontal end portions lying substantially outsi
General Electric Company
Nixon & Vanderhye P.C.
Wasil Daniel
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