Induced nuclear reactions: processes – systems – and elements – Grids
Reexamination Certificate
2000-04-26
2001-08-21
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Grids
C376S439000, C376S442000
Reexamination Certificate
active
06278759
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a spacer grid used for placing and supporting a plurality of fuel rods within a nuclear fuel assembly of a light water reactor and, more particularly, to a nuclear fuel spacer grid having both a plurality of arc-shaped main springs and a plurality of bow-shaped sub-springs on its grid strips. The sub-springs having the same radius of curvature as that of the external surface of each fuel rod and effectively supporting the fuel rods when the main springs fail to support the fuel rods, thus accomplishing desired soundness of the fuel rods within the fuel assembly during an expected life span of the rods, the spacer grid also having a plurality of dimple vanes on the grid strips, the dimple vanes being oppositely embossed in an axial direction of the fuel assembly so as to guide the coolant from one cell to neighboring cells of the spacer grid, thus accomplishing a desired coolant mixing effect without creating an excessive pressure drop within the fuel assembly.
2. Description of the Prior Art
In a conventional light water reactor, a plurality of elongated nuclear fuel rods
9
are regularly and parallelly arranged in an assembly
1
having a square cross-section in a way such that, for example, fourteen, fifteen, sixteen or seventeen fuel rods
9
arc regularly arranged along each side of the square cross-section, thus forming a 14×14, 15×15, 16×16, or 17×17 array as shown in FIG.
1
. In such a nuclear fuel assembly
1
, the elongated fuel rods
9
are placed and supported by a plurality of spacer grids
7
and
8
. Each of the fuel rods
9
is typically fabricated by containing a fissionable fuel material, or a uranium core
12
, within a hermetically sealed elongated zircaloy tube known as the cladding. The above spacer grids
7
and
8
are individually produced by welding a plurality of intersecting grid strips to each other into an egg-crate pattern prior to encircling the periphery of each grid
7
,
8
by four perimeter strips. The top and bottom of the fuel assembly
1
are, thereafter, covered with pallets
2
and
3
, respectively, and so the assembly
1
is protected from any external loads acting on its top and bottom. The spacer grids
7
and
8
and the pallets
2
and
3
are also integrated into a single structure using a plurality of guide tubes
4
. A framework of the fuel assembly
1
is thus fabricated.
In order to provide a high performance nuclear fuel assembly, it is necessary to effectively remove heat from the fuel rods within the assembly. In the prior art, such a removal of heat from the fuel rods is accomplished by making the temperature of coolant flowing around the fuel rods uniformed by mixing the coolant within the fuel assembly. This finally allows the fuel assembly to be free from a partial overheat. In order to mix the coolant within the fuel assembly in the prior art, two structures have been used as follows. In a first structure, a plurality of coolant mixing vanes are attached to the upper portion of each spacer grid, thus creating a turbulent flow or a swirling flow in the coolant passing through each spacer grid and finally mixing the coolant within the fuel assembly. In a second structure, a plurality of coolant mixing ducts are formed within each spacer grid so as to allow the coolant to be mixed together between neighboring cells while flowing upwardly within each spacer grid.
The coolant mixing vanes of the first structure arc inclined relative to the coolant flowing direction at a predetermined angle so as to maximize the coolant mixing effect in addition to reducing pressure drop. However, the conventional coolant mixing vanes increase the turbulent flow, thus undesirably resulting in an increase in pressure drop. In addition, the above coolant mixing vanes allow the fuel rods to be vibrated within the spacer grid, thus undesirably abrading the external surfaces of the fuel rods.
On the other hand, the coolant mixing ducts of the second structure are formed by integrating two grid strips into a single body, with each strip having a coolant mixing channel. In this structure, each duct is arrayed in an axial direction of the fuel assembly at its inlet end, but is bent at its outlet end so as to be inclined relative to the axial direction of the fuel assembly. Therefore, the ducts guide the coolant from one cell into neighboring cells and accomplish a coolant mixing effect within the fuel assembly. However, this structure uses two strips, thus reducing the coolant flowing area and increasing the pressure drop within each spacer grid. In addition, the ratio of the coolant flowing area to the coolant contact length within each duct is higher than that of the coolant flowing passage outside the duct, and so the duct creates a high resistance to the coolant flow. This structure thus finally reduces the coolant flowing velocity within the ducts, and so the structure may fail to accomplish a desired coolant mixing effect.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a spacer grid for nuclear fuel assemblies, which has a plurality of dimple vanes on its grid strips, with the dimple vanes being designed to guide the coolant from one cell to neighboring cells, thus accomplishing a desired coolant mixing effect and increasing the thermal allowance of the fuel rods and accomplishing a high performance fuel assembly, and which has a multi-spring structure, consisting of one arc-shaped main spring and two bow-shaped sub-springs, with the sub-springs having the same radius of curvature as that of the external surface of each fuel rod and effectively supporting the fuel rod when the main spring fails to support the fuel rod, thus accomplishing desired soundness of the fuel rods within the fuel assembly during an expected life span of the fuel rods.
In order to accomplish the above objects, the present invention provides a nuclear fuel spacer grid fabricated by intersecting a plurality of grid strips and used for placing and supporting a plurality of elongated fuel rods at multi-points within a nuclear fuel assembly, comprising: two dimple vanes formed on each of the grid strips at a position corresponding to each sidewall of a unit cell of the spacer grid, the two dimple vanes being oppositely embossed in an axial direction of the fuel assembly so as to accomplish a coolant mixing effect within the spacer grid; two main springs provided on each of the grid strips at respective positions above and under the two dimple vanes, the main springs having an arcuate shape suitable for elastically supporting an elongated fuel rod within the unit cell; and two sub-springs provided on each of the grid strips at positions above and under each of the main springs, the sub-springs having a bow shape suitable for supporting the fuel rod while reducing a maximum load applied to the fuel rod within the unit cell.
In the above spacer grid, each of the two dimple vanes has a streamline profile suitable for diverging the coolant flowing within the spacer grid in the axial direction, thus allowing the coolant to be mix together while flowing within the spacer grid. The two dimple vanes, formed on a sidewall of the unit cell, are opposite in their embossed directions to dimple vanes formed on neighboring sidewalls of the unit cell, thus allowing the coolant to be mixed together and to swirl within the spacer grid. The dimple vanes are also positioned on a reference surface at which the grid strip is cut at a predetermined angle so as to reduce a pressure drop.
In addition, the main springs and the sub-springs form a multi-spring structure, of which the main springs, having the arcuate shape, directly support the fuel rod, and the sub-springs, having the bow shape, support the fuel rod when the main springs fail to support the fuel rod. The main springs and the sub-springs have a limited number of angled po
Chun Taehyun
In Wangkee
Jung Youn Ho
Kang Heungseok
Oh Dongseok
Bachman & LaPointe P.C.
Behrend Harvey E.
Korea Atomic Energy Research Institute
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