Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-11-12
2002-07-16
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S438000, C376S442000, C376S443000, C376S462000
Reexamination Certificate
active
06421407
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 nuclear fuel rods within a nuclear fuel assembly and, more particularly, to a nuclear fuel spacer grid with dipper vanes designed to maintain a desired space between the elongated fuel rods and to appropriately support the fuel rods and to prevent the vibrating or fretting corrosion of the fuel rods while allowing coolant to smoothly flow within the fuel assembly and resulting in a high coolant mixing effect and retaining a desired structural strength of the fuel assembly even in case of the occurrence of an emergency.
2. Description of the Prior Art
In conventional light water reactors, a plurality of elongated nuclear fuel rods
125
are regularly and parallelly arranged in an assembly
101
having a square cross-section in a way such that, for example, fourteen, fifteen, sixteen or seventeen fuel rods
125
are 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
101
, the elongated fuel rods
125
, fabricated by containing a fissile material within a hermetically sealed elongated zircaloy tube
114
known as the cladding, are placed and supported by a plurality of spacer grids
110
. Each of such spacer grids
110
is 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
110
by four perimeter strips. The top and bottom of the fuel assembly
101
are, thereafter, covered with pallets
111
and
112
, respectively. The fuel assembly
101
is thus protected from any external loads acting on the top and bottom thereof. The spacer grids
110
and the pallets
111
and
112
are also integrated into a single structure using a plurality of guide tubes
113
. A framework of the fuel assembly
101
is thus fabricated.
Each of the above spacer grids
110
is fabricated as follows. As best seen in
FIG. 2
, two sets of grid strips
115
and
116
, individually having a plurality of notches at regularly spaced positions, are assembled with each other by intersecting the two sets of strips
115
and
116
at the notches, thus forming a plurality of four-walled cells individually having four intersections
117
. The assembled strips
115
and
116
are, thereafter, welded together at the intersections
117
prior to being encircled by the perimeter strips
118
, thus forming a spacer grid
110
with such four-walled cells. As shown in
FIG. 3
, a plurality of positioning springs
119
and a plurality of positioning dimples
120
are integrally formed on or attached to the rid strips
115
and
116
in a way such that the springs
119
and the dimples
120
extend inwardly within each of the four-walled cells. In such a case, the dimples
120
are more rigid than the springs
119
. In each of the four-walled cells, the positioning springs
119
force a fuel rod
125
against associated dimples
120
, thus elastically positioning and supporting the fuel rod
125
at four points within each of the cells. In the above nuclear fuel assembly
101
, a plurality of grids
110
are regularly and perpendicularly arranged along the axes of the fuel rods
125
at right angles, thus placing and supporting the fuel rods
125
at multiple points. The grids
110
thus function as a multi-point support means for placing and supporting the fuel rods
125
within the fuel assembly
101
.
In conventional light water reactors of Korea, water is used as coolant. In such a light water reactor, water receives thermal energy from the fuel rods
125
prior to converting the thermal energy into electric energy. During an operation of the nuclear fuel assembly
101
of such a reactor, water or liquid coolant is primarily introduced into the assembly
101
through an opening formed on the core supporting lower plate of the reactor. In the above assembly
101
, coolant flows upwardly through the passages, defined between the fuel rods
125
, and receives thermal energy from the fuel rods
125
. The sectioned configuration of the coolant passages, provided within the fuel assembly
101
, is shown in FIG.
4
.
Since the elongated, parallel fuel rods, having a circular cross-section, are closely arranged within a fuel assembly, having a rectangular configuration, while being spaced apart from each other at irregular intervals, the temperature of coolant flowing around the fuel rods is variable in accordance with positions. Therefore, it is almost impossible to obtain a uniform temperature distribution within such a conventional fuel assembly. Thus, the coolant passages of the fuel assembly may be partially overheated at positions adjacent to the fuel rods having a high temperature. Such partially overheated regions of the coolant passages deteriorate soundness of the assembly and reduce the output power of the fuel rods. In order to remove such partially overheated regions from a nuclear fuel assembly, it is necessary to design the spacer grids in a way such that the grids allow a uniform temperature distribution of the coolant within the fuel assembly while effectively deflecting and mixing the coolant within the assembly. A conventional example of such designed grids is disclosed in Korean Patent Publication No. 91-7921. In the grid disclosed in the above Korean patent, so-called “mixing blades” or “vanes” are attached to the upper portion of each grid and are used for mixing coolant within the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally in addition to normally longitudinally, and so the coolants are effectively mixed with each other between the passages and between the lower temperature regions and the partially overheated regions of the fuel assembly.
The important factors necessary to consider while designing the grids
110
for use in nuclear fuel assemblies are improvement in both the fuel rod supporting function of the grids
110
and the buckling strength resisting a laterally directed force acting on the grids
110
. During an operation of a nuclear reactor, the fuel assembly
101
may be vibrated laterally due to a load acting on the assembly and this causes an interference between the fuel rods within the assembly. Therefore, the grids of the fuel assembly may be impacted due to such an interference between the fuel rods as disclosed in U.S. Pat. No. 4,058,436. In the prior art, the grid's buckling strength, resisting a lateral load acting on the grid, is reduced since the grid strips have to be partially cut away through, for example, a stamping process at a plurality of portions so as to form positioning springs
119
and dimples
120
within a fuel assembly. Such cut-away portions (or windows) reduce the effective cross-sectional area of the grid
110
capable of resisting impact, thus reducing the buckling strength of the grid
110
. Therefore, it is almost impossible to avoid a reduction in the buckling strength of the grid
110
in a conventional design of grids wherein the grid strips have to be partially cut away to form such positioning springs
119
and the dimples
120
.
In the conventional nuclear fuel assembly
101
, the fuel rods
125
are placed and supported by positioning springs
119
and the positioning dimples
120
within the grids
110
. However, the lateral flow or mixing of coolant regrettably vibrates the elongated, parallel, closely spaced fuel rods
125
within the assembly, and so the fuel rods
125
easily and periodically interfere with the intersecting strips of the grids
110
. When the fuel rods
125
are so vibrated for a lengthy period of time, the claddings of the fuel rods
125
are repeatedly and frictionally abraded at their contact parts at which the fuel rods
125
are brought into contact with the springs and dimples of the grids
110
. The claddings are thus reduced in their thicknesses so as to be finally perforated at the gri
Chun Tae-Hyun
In Wang-Kee
Jung Youn-Ho
Kang Heung-Seok
Kim Hyung-Kyu
Bachman & LaPointe P.C.
Carone Michael J.
Keith Jack
Korea Atomic Energy Research Institute
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