Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-11-12
2002-05-21
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S438000, C376S442000, C376S462000
Reexamination Certificate
active
06393087
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 duct-type spacer grid consisting of a plurality of duct-shaped grid elements individually having an octagonal cross-section. The grid is also designed to have a plurality of swirl flow vanes at the top of each grid element.
2. Description of the Prior Art
As shown in
FIG. 1
, a conventional nuclear fuel assembly
100
typically comprises a plurality of spacer grids
110
, a bottom nozzle
101
, a top nozzle
102
, a plurality of guide tubes
103
, and a plurality of elongated fuel rods
106
.
In the above fuel assembly
100
, the elongated fuel rods
106
are regularly and arranged in parallel to form a structure having a square cross-section while being placed and supported by the spacer grids
110
. Each of the grids
110
is fabricated by assembling a plurality of intersecting inner strips into an egg-crate pattern. The intersecting inner strips are also welded together at their intersections.
As best seen in
FIGS. 2 and 3
, a plurality of inner strips
113
intersect each other to form a plurality of four-walled cells
108
having a square cross-section. In each of the four-walled cells
108
, two springs
114
are provided on the interior of two neighboring walls, while two dimples
115
are provided on the interior of the opposite two walls. The strength of the two dimples
115
is higher than the two springs
114
. Both the two springs
114
and the two dimples
115
are used for supporting an elongated fuel rod
106
within each cell
108
.
In the above fuel assembly
100
, it is necessary for the springs
114
and the dimples
115
to effectively support the fuel rods
106
while restricting undesirable movement of the rods
106
even when the assembly
100
is impacted by any external force applied in an axial direction A, a radial direction B and/or a rotating direction C. Such an external force, applied to the assembly
100
, may be caused by the coolant flow, an earthquake or any unexpected external impact. In addition, the grid structure
111
, consisting of the intersecting inner strips
113
, has to maintain the originally designed configuration of the cells
108
even when a lateral impact is applied to a sidewall of the grid
110
. In the fuel assembly
100
, the spring force of the grid
110
may be gradually reduced due to neutron irradiation of the assembly
110
. The spacer grid
110
has to be designed to maintain an effective spring force capable of continuing elastic contact of the springs
114
with a fuel rod
106
until the existing rod
106
is changed with a new one.
In the above fuel assembly
100
, the fuel rods
106
may be grown in an axial direction A due to the neutron irradiation, and so the grids
110
have to be designed to appropriately support the rods
106
while allowing such an axial growth of the rods
106
. However, when the spring force of the grids
110
undesirably exceeds a reference level, the fuel rods
106
may be prevented from being grown in the axial direction A. This sometimes results in a bending of the fuel rods
106
. When the fuel rods
106
are undesirably bent as described above, it is difficult to secure a subchannel
107
within the fuel assembly
100
. This deteriorates the cooling performance of the assembly
100
.
FIG. 4
shows a subchannel
107
, formed by four fuel rods
106
. On the other hand, when the spring force of the grid
110
is less than the reference level, the grids
110
may fail to effectively place or support the fuel rods
106
within the assembly
100
. This finally results in vibration or fretting wear of the fuel rods
106
, thus severely damaging the rods
106
.
As well known to those skilled in the art, the power output from a nuclear reactor is partially used as an energy source for causing the coolant to effectively flow within the reactor core. The amount of power, required to cause the coolant flow within the core, is determined by a hydraulic resistance in the flow paths. In a conventional nuclear fuel assembly
100
, the flow paths comprise a main flow path and a sub-flow path. When the flow paths are designed having a shape which disturbs the coolant flow, a large amount of power has to be consumed to cause the coolant flow. On the other hand, when the flow paths are designed having a streamline shape, a small amount of power is needed to cause the coolant flow. It is necessary to make the passages effectively cause the coolant flow using a small amount of power by reducing the hydraulic resistance.
A typical spacer grid for nuclear fuel assemblies, used in light water reactors, may be referred to U.S. Pat. No. 3,395,077. Another conventional spacer grid, having a specifically designed inner strip and a fuel rod support spring, may be referred to U.S. Pat. Nos. 4,426,355, 4,726,926, 4,803,043 or 4,888,152.
In the spacer grid of U.S. Pat. No. 4,426,355, the inner strips are corrugated to form a plurality of wavy dimples at regularly spaced positions. In the spacer grid of U.S. Pat. No. 4,726,926, a plurality of thin and narrow inner strips intersect each other prior to being welded together at their intersections, thus forming a grid structure. After the grid structure is formed by the intersecting inner strips, the strips are appropriately deformed to form a plurality of flow paths, springs and dimples. In the spacer grid of U.S. Pat. No. 4,803,043, the springs of the inner strips are positioned to be diagonally opposite to each other, thus having an increased effective spring length. In the above-mentioned spacer grids, each grid consists of a plurality of intersecting inner strips. In such a spacer grid having the intersecting inner strips, the inner strips pass across the subchannel having a high flow rate. This type of spacer grid is thus problematic in that it undesirably results in an increase in pressure loss.
On the other hand, U.S. Pat. No. 4,888,152 discloses a ring-type spacer grid that comprises a plurality of duct-shaped grid elements individually having a square cross-section. In order to form a spacer grid, the grid elements are slitted at appropriate portions and are intersected to each other in a way such that the grid elements form a grid structure arranged in pararell. Such a ring-type spacer grid does not pass across the subchannel different from the grids having the inner strips. However, this ring-type grid is problematic in that the fuel rods are placed and supported by rigid corners of the grid elements, thus being apt to be severely damaged when the fuel rods have vibrated.
As well known to those skilled in the art, there is a difference between the output powers of the fuel rods within a reactor core due to a nonuniform distribution of neutron flux. Therefore, a subchannel, adjacent to a fuel rod having a high thermal power output, may be highly increased in enthalpy comparing with the other neighboring subchannels. In accordance with an increase in the power output of the fuel rods, coolant in the subchannel having the high enthalpy rise, may be boiled prior to cooling within the other subchannels. There primarily occurs a nucleate boiling and secondarily a film boiling of water within the subchannel having the high enthalpy rise. When a film boiling occurs, a bubble film is formed on a fuel rod surface. Such a bubble film decreases heat transfer from the fuel rod surface to the coolant, thus increasing the temperature of the cladding surface of the fuel rod. Such an increased temperature of the cladding surface results in a partial thermal stress on the cladding. When the temperature of the cladding is further increased, both the cladding may be melted. It is thus necessary to limitedly operate the reactor core in a way such that any film boiling does not occur in the subchannels. Such an undesirable phenomenon, caused by film boiling in the subchannel, is a so-called “Departure from Nucleate Boiling(DNB)” in the f
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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