Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2003-05-30
2004-06-01
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S442000
Reexamination Certificate
active
06744843
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to spacer grids used for receiving and supporting fuel rods in nuclear fuel assemblies and, more particularly, to a side-slotted nozzle type double sheet spacer grid used in such fuel assemblies and designed to effectively deflect and mix coolants together so as to improve the heat transfer effect between the fuel rods and the coolants, and also designed to enhance its fuel rod support performance so as to effectively protect the fuel rods from vibration and fretting wear, and enhance its strength to effectively resist laterally directed forces acting thereon, and remarkably improve the spring performance of its support parts directly supporting the fuel rods, thus accomplishing desired soundness of the fuel assemblies.
2. Description of the Prior Art
In typical nuclear reactors, a plurality of elongated nuclear fuel rods
125
are parallelly arranged at regular intervals 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 arranged along each side of the square cross-section at regular intervals, thus forming a 14×14, 15×15, 16×16, or 17×17 array as shown in FIG.
1
.
In order to receive and support the fuel rods
125
within the nuclear fuel assembly
101
, a plurality of single sheet spacer grids
110
are used. In order to produce each single sheet spacer grid
110
, a plurality of inner strips intersect together at right angles to form an egg-crate pattern, prior to being welded at their intersections. Thereafter, the periphery of the spacer grid
110
is encircled 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 fuel assembly
101
, 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
, received and supported within the fuel assembly
101
by the spacer 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 single sheet inner strips
115
and
116
, each 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. 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 single sheet spacer grids
110
having the above-mentioned structure are regularly arranged along the axes of the fuel rods
125
, as shown in
FIG. 1
, thus receiving and supporting the fuel rods
125
within the fuel assembly
101
at multiple points. That is, the spacer grids
110
act as a multi-point support means for receiving 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 slightly 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 of the grids
110
due to the thermal expansion or the irradiation-induced growth, so that the fuel rods
125
may fail to maintain the designed intervals between them. In such a case, the cross-sectional areas of coolant passages, defined between the fuel rods
125
to allow coolant to flow through them as shown in
FIG. 4
, are partly increased or reduced.
In some typical nuclear reactors using water as coolant, such as in the case of the nuclear reactors currently 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 fuel assembly
101
through an opening formed on the core supporting lower plate of the reactor. In the fuel assembly
101
, the coolant flows upwardly through the coolant 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 cross-section, with a plurality of elongated, parallel fuel rods
125
having a circular cross-section closely set within the assembly
101
while being spaced apart from each other at irregular radial 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 fuel 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 generated in the fuel assembly. The spacer grid must also be designed to effectively deflect and mix the coolant within the fuel assembly. Such effectively mixed coolant ensures a uniform increase in enthalpy, and maximizes the core output.
Typical examples of such designed spacer grids are disclosed in U.S. Pat. No. 4,728,489 (corresponding to Korean Patent Publication No. 91-1978) and U.S. Pat. No. 4,692,302 (corresponding to Korean Patent Publication No. 91-7921).
In the spacer grids disclosed in the above-mentioned patents, so-called “mixing blades” or “vanes” are attached along the upper edges of the intersecting strips of each spacer 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 the normal flow in an axial direction, as shown in
FIG. 3
, and so the coolants are effective
Chun Tae Hyun
In Wang Kee
Jung Yeon Ho
Kang Heung Seok
Kim Hyung Kyu
Bachman & LaPointe P.C.
Behrend Harvey E.
Korea Atomic Energy Research Institute
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