Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2002-10-25
2004-04-13
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S442000, C376S438000, C376S462000
Reexamination Certificate
active
06721384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to spacer grids used for supporting fuel rods in nuclear fuel assemblies and, more particularly, to a spacer grid provided with side weld supports and flow mixing vanes at the intersections of its inner straps for creating a lateral flow of coolant within coolant channels, in addition to improving the structural strength of the spacer grid and the integrity of the nuclear fuel assembles.
2. Description of the Prior Art
As shown in
FIG. 1
, a conventional nuclear fuel assembly
500
of a nuclear reactor comprises a plurality of fuel rods
502
, each fabricated such that a fissionable fuel material, such as a uranium pellet, is contained in a hermetically sealed elongated tube, known as the cladding. The fuel rods
502
are supported within the fuel assembly by a plurality of spacer grids
501
, which also generate a strong turbulent flow of coolant within the fuel assembly. The bottom of the fuel assembly is covered with a bottom end piece
503
, which stably connects the fuel rods
502
to the lower structure of a reactor core. The top of the fuel assembly is covered with a top end piece
504
, which stably connects the fuel rods
502
to the upper structure of the reactor core. In the fuel assembly, the spacer grids
501
and the two end pieces
503
and
504
are integrated into a single structure using a plurality of guide tubes
505
. The guide tubes
505
also form a plurality of passages for receiving a variety of monitoring tubes used for measuring the operational conditions of the reactor.
As shown in
FIG. 2
, each spacer grid
501
is produced by intersecting a plurality of thin metal inner straps at right angles to form an egg-crate pattern, and spot-welding the interlaced straps at the top and bottom of their intersections, and so a desired integrity of the spacer grid
501
is accomplished. A plurality of flow mixing vanes
540
are provided on the top of each intersection of the spacer grid
501
for changing a portion of axial flow coolant
510
to cross flow
520
or swirl flow
530
. The spacer grid
501
, fabricated by the interlaced inner straps, defines a plurality of four-walled cells for receiving and holding the fuel rods
502
therein as shown in FIG.
3
. In each of the cells, a plurality of grid springs
560
and a plurality of strong dimples
570
are formed on the inner straps such that the springs
560
and dimples
570
face each other. The springs
560
and dimples
570
support the fuel rods
502
in the spacer grids
501
.
In the fuel assembly, the fuel rods
502
are axially set in the cells of the spacer grids
501
such that four fuel rods
502
inside four adjacent cells of each spacer grid
501
form a coolant channel
550
′ as shown in
FIG. 4
, and so the coolant axially flows along the channel
550
′. The coolant channel
550
′ is open at each side thereof by a gap formed between two fuel rods
502
, and has an intersection of the inner straps at the center thereof. The fuel rods of a nuclear fuel assembly typically have different thermal outputs due to an imbalance in the neutron flux distribution, and so the coolant flowing through some coolant channels surrounded by fuel rods having high thermal outputs is highly increased in its temperature in comparison with the coolant flowing through other coolant channels surrounded by fuel rods having low thermal outputs. In order to improve thermal efficiency of a reactor core, it is necessary to accomplish a uniform coolant temperature distribution within the coolant channels. The above objective may be accomplished by a provision of flow mixing vanes on the top of the spacer grid. That is, the flow mixing vanes accomplish a uniform coolant temperature distribution and prevent a part of the coolant from being overheated during an operation of a nuclear reactor.
When the coolant passes through the spacer grids, frictional and form pressure losses are presence due to the friction between the coolant and surface of the grid and the momentum exchange as area change of the flow path, respectively.
Such pressure losses result in an energy loss, which requires high capacity of main coolant pump to compensate the losses. Therefore, it is preferred to design the nuclear reactor system with low pressure losses.
The nuclear fuel assembly is fabricated by mounting the upper and lower end pieces to the bundle formed by inserting the fuel rods in the spacer grids. On inserting in the spacer grids, the fuel rods may contact with the vanes and cause the vane deformation, which may scratch the external surfaces of the fuel rods. Therefore, in the prior art, the flow mixing vanes of the spacer grid have been designed such that they do not interfere with the fuel rods.
Conventional flow mixing vanes for spacer grids may be referred to U.S. Pat. No. 4,692,302 (Inventors: Edmund E. Demario et al., Applicant: Westinghouse Co. Ltd.) and U.S. Pat. No. 5,440,599 (Inventors: Thomas Rodack et al., Applicant: Combustion Engineering Co. Ltd.).
In a flow mixing device disclosed in U.S. Pat. No. 4,692,302, two flow mixing vanes are formed at each intersection of the inner straps of a spacer grid such that the two vanes are formed along the top edge of one of two inner straps crossing each other at the intersection and are deflected in opposite directions. This flow mixing device changes a portion of coolant along the central axis of each channel to a cross flow guided to the gaps between fuel rods.
In the above flow mixing device, a weld window
580
and a weld tap
590
are provided at the gap between two flow mixing vanes
540
as shown in
FIGS. 5 and 6
.
The interlaced straps are welded at the upper and lower intersection and then the welding taps
590
are changed into weld beads
590
′. The weld beads
590
′ increase hydraulic resistance by generating flow separation downstream of the beads thereof, thus reducing the flow mixing efficiency of the vanes
540
. In addition, the weld window
580
positioned between two vanes
540
makes the edges of the two vanes become closer to the rod, thus may allowing an interference of the vanes
540
with the fuel rods.
In a flow mixing device disclosed in U.S. Pat. No. 5,440,599, a triangular vane support extends upward from the top edge of one of two inner straps crossing each other at each intersection, with two mixing vanes formed on opposite sides of the triangular vane support and deflected in opposite directions.
This flow mixing device changes a portion of coolant along the central axis of the channel to the gaps between the fuel rods. However, this flow mixing device is problematic in that a weld tap is provided under the vane support and becomes a weld bead when welding the interlaced inner straps together.
The weld beads
590
′ increase hydraulic resistance by generating flow separation downstream of the beads thereof, thus reducing the flow mixing efficiency of the vanes
540
.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an objective of the present invention is to provide a spacer grid for nuclear fuel assemblies, which has a plurality of side weld supports, that is, main supports, upper sub-supports and lower sub-supports, on its interlaced inner straps, with two flow mixing vanes integrally extending upward from each of the main supports, and which is fabricated by seam-welding the interlaced inner straps to each other along the upper axial junction lines of the crossing main and upper sub-supports at the top of the intersections, and along the lower axial junction lines of the crossing lower sub-supports at the bottom of the intersections, and which prevents the flow mixing vanes from interfering with fuel rods during a fuel rod installation process, accomplishes a desired integrity, improves the coolant mixing efficiency of the flow mixing vanes, and reduce form pressure loss caused by weld beads.
In order to accomplish the above object
Chun Taehyun
In Wangkee
Jung Younho
Kang Heungseok
Kim Hyungkyu
Bachman & LaPointe P.C.
Carone Michael J.
Korea Atomic Energy Research Institute
Palabrica Rick
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