Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2001-05-22
2003-11-18
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S438000, C376S440000, C376S441000, C376S442000, C376S443000, C376S449000, C376S454000, C376S462000
Reexamination Certificate
active
06650723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to spacer grids used for placing and supporting fuel rods in nuclear reactor fuel assemblies and, more particularly, to a double strip mixing grid used in such nuclear reactor fuel assemblies and designed to effectively deflect and mix coolants together so as to improve the heat transferring effect between the fuel rods and the coolants, the mixing grid also designed to improve its fuel rod support performance so as to effectively protect the fuel rods from vibration and fretting failure of the fuel rods, and improve to effectively resist laterally directed forces acting thereon.
2. Description of the Prior Art
In typical nuclear reactors, a plurality of elongated nuclear fuel rods
125
are regularly and parallelly arranged in a fuel assembly
101
having a square cross-section. In such a case, 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 order to place and support the fuel rods
125
within the nuclear fuel assembly
101
, a plurality of spacer grids
110
are used. Each of such grids
110
is produced by intersecting a plurality of inner strips at right angles to form an egg-crate pattern, and welding the intersected strips at their intersections prior to encircling the periphery of the grid
110
with four perimeter strips. The top and bottom of the fuel assembly
101
are, thereafter, covered with top and bottom pallets
111
and
112
. Therefore, the nuclear fuel assembly
101
is protected from any external loads acting on the top and bottom thereof. In the assembly, the spacer grids
110
and the pallets
111
and
112
are integrated into a single structure using a plurality of guide tubes
113
. The elongated fuel rods
125
, placed and supported within the fuel assembly
101
by the grids
110
, are typically fabricated such that a fissionable fuel material, such as uranium core
114
, is contained in a hermetically sealed elongated zircaloy tube, known as the cladding.
The above spacer grids
110
are each fabricated as follows. As best seen in
FIG. 2
, two sets of inner strips
115
and
116
, individually having a plurality of notches at regularly spaced portions, 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. Each of the cells has four intersections
117
. The assembled strips
115
and
116
are, thereafter, welded together at the intersections
117
prior to being encircled with the perimeter strips
118
. A desired spacer grid
110
with such four-walled cells is thus fabricated.
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 inner strips
115
and
116
. In such a case, the springs
119
and the dimples
120
extend inwardly with respect to each of the four-walled cells. 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 such a typical nuclear fuel assembly
101
, a plurality of spacer grids
110
having the above-mentioned construction are regularly and perpendicularly arranged along the axes of the fuel rods
125
at right angles, thus placing and supporting the fuel rods
125
within the assembly
101
at multiple points. That is, the spacer grids
110
form a multi-point support means for placing and supporting the fuel rods
125
within a nuclear fuel assembly
101
.
In the typical nuclear fuel assembly
101
, the positioning springs
119
elastically and lightly force the fuel rods
119
against the dimples
120
such that the fuel rods
125
are slidable on the support points of both the springs
119
and the dimples
120
when the fuel rods
125
are elongated due to thermal expansion or irradiation-induced growth of the fuel rods
125
within the fuel assembly
101
.
When the fuel rods
125
are fixed to the spacer grids
110
at the support points of the grids
110
, the fuel rods
125
may be bent at portions between the support points, thus undesirably reducing the intervals between the fuel rods
125
of the assembly
101
as shown in FIG.
4
.
In some typical nuclear reactors using water as coolant, such as in the case of the nuclear reactors recently used in Korea, water receives thermal energy from the fuel rods
125
prior to converting the thermal energy into desired electric energy through a plurality of processes.
During an operation of a 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 assembly
101
, the coolant flows upwardly through the passages, defined between the fuel rods
125
, and receives thermal energy from the hot fuel rods
125
.
The sectioned configuration of the coolant passages formed in the fuel assembly
101
is shown in FIG.
4
.
In a conventional nuclear reactor, the amounts of thermal energy, generated from different portions of a nuclear fuel assembly
101
, are not equal to each other. Since the fuel assembly
101
has a rectangular configuration, with a plurality of elongated, parallel fuel rods
125
closely set within the assembly
101
while being spaced apart from each other at irregular intervals, the temperature of coolant flowing around the fuel rods
125
is variable in accordance with positions of coolant currents relative to the rods
125
.
That is, the amount of thermal energy received by water flowing around the corners
123
of each four-walled cell-is less than that received by water flowing around the fuel rods
125
. The coolant passages of typical fuel assemblies
101
thus undesirably have low temperature regions.
Such low temperature regions reduce the thermal efficiency of the nuclear reactor. The coolant passages of the fuel assemblies
101
may also have partially overheated regions at positions adjacent to the fuel rods
125
having a high temperature. Such partially overheated regions deteriorate soundness of the assemblies
101
.
In order to prevent such partially overheated regions from existing in a nuclear fuel assembly, it is necessary to design the spacer grid such that a uniform temperature distribution is formed in the fuel assembly. The grid must be also designed to effectively deflect and mix the coolant within the fuel assembly. Such effectively mixed coolant makes uniform the increase in enthalpy and maximizes the core output.
Typical examples of such designed spacer grids are disclosed in Korean Patent Publication Nos. 91-1978 and 917921.
In the spacer grids disclosed in the above-mentioned Korean patents, so-called “mixing blades” or “vanes” are attached along the upper edges of the intersecting strips of each grid, and are used for mixing coolants within- the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally, in addition to normally flowing in an axial direction, as shown in
FIG. 3
, and so the coolants are effectively, mixed with each other between the coolant channels and between the lower temperature regions and the partially overheated regions of the fuel-assembly.
In the prior art, the techniques for mixing the coolants with each other between the coolant channels and between the lower temperature regions and the partially overheated regions of the fuel assembly using such mixing blades or vanes are classified into two types: the first technique using large-scaled mixing blades for creating a lateral flow of coolant and the second technique using vanes provided at the intersections for creating a swirling flow of coolant. In the first technique, the coolants, axiall
Chun Taehyun
In Wangkee
Jung Yeonho
Kang Heungseok
Kim Hyungkyu
Bachman & LaPointe P.C.
Carone Michael J.
Korea Atomic Energy Research Institute
Richardson John
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