Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2001-12-07
2004-03-16
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S441000, C376S462000
Reexamination Certificate
active
06707872
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 spacer grid with grid springs designed to have an optimal shape of their vertical support parts, thus reducing fretting wear of the fuel rods caused by contact of the fuel rods with the grid springs, the grid springs also designed to optimize the distribution and to minimize the intensity of contact stress caused by the contact between the springs and the fuel rods, thus being less likely to cause fretting wear of the fuel rods and stably placing and supporting the fuel rods in the assembly during the effective life of the fuel rods expires, and thereby improving the soundness of the assembly.
2. Description of the Prior Art
As well known to those skilled in the art, spacer grids are elements of a nuclear reactor fuel assembly, and each has a plurality of grid springs and dimples in their fuel rod cells for placing and supporting a plurality of fuel rods within the spacer grids of the fuel assembly.
FIG. 1
is an exploded perspective view, showing the construction of a typical nuclear reactor fuel assembly.
FIG. 2
a
is a plan view, showing a conventional spacer grid used in the fuel assembly of FIG.
1
.
FIG. 2
b
is a side view of the spacer grid of
FIG. 2
a
, with one fuel rod placed and supported by grid springs and dimples within a fuel rod cell of the spacer grid of
FIG. 2
a
.
FIG. 3
is a sectional view, showing a deformation of a conventional grid spring due to a force applied from a fuel rod set in the cell of the spacer grid of
FIG. 2
a.
FIG. 4
a
is a perspective view of a conventional grid spring used in the prior art spacer grid.
FIG. 4
b
is a perspective view of a conventional dimple used in the prior art spacer grid.
In a typical nuclear reactor fuel assembly
2
having a plurality of spacer grids
110
, a plurality of guide tubes
113
are vertically arranged between top and bottom support pallets
111
and
112
.
The spacer grids
110
for placing and supporting the fuel rods
125
in the assembly
2
are arranged along the guide tubes
113
at regular intervals in a vertical direction, and fixed to the tubes
113
through a welding process.
Each of the spacer grids
110
is made of a zircaloy or Inconel, and defines both a plurality of fuel rod cells
123
for supporting the fuel rods
125
and a plurality of guide tube cells
124
for supporting the guide tubes
113
. Each of the fuel rod cells
123
has two grid springs
118
and four dimples
119
, and so the fuel rod
125
inside each fuel rod cell
123
is supported at six positions through point contact-type support and surface contact-type support. The four dimples
119
are arranged in each fuel rod cell
123
such that two dimples
119
are formed at positions above and under each grid spring
118
.
Since the grid springs
118
of the spacer grid
110
are brought into direct contact with the fuel rods
125
, the springs
118
may be deformed by the fuel rods
125
. That is, the vertical support parts
121
or the root parts of a grid spring
118
may be depressed due to the force applied from a fuel rod
125
to the grid spring
118
as shown in FIG.
3
. In such a case, as the strength of the vertical support parts
121
of each grid spring
118
is almost equal to that of the central curved support part
122
of the spring
118
at which the spring
118
comes into direct contact with the fuel rod
125
, the vertical support parts
121
of the spring
118
are deformed by both a bending moment
131
and a twisting moment
132
caused by the force applied from the fuel rod
125
to the spring
118
at the same time. Such simultaneous application of the bending moment
131
and twisting moment
132
to the vertical support parts
121
of the springs
118
prevents the springs
118
from stably supporting the fuel rods
125
, in addition to making the springs
118
fail to effectively resist fatigue due to stress. In addition, the external surface of the fuel rod
125
may slide slightly on the central curved support part
122
of the spring
118
in the case of the application of the bending moment and twisting moment to the spring
118
. In such a case, a fretting wear of the fuel rods
125
inside the spacer grid
110
may be easily caused by sliding movement.
In the spacer grid
110
, the grid springs
118
and the dimples
119
of each fuel rod cell
123
have the same radius of curvature as that of the fuel rods
125
to accomplish conformal contact of the springs
118
and dimples
119
with the fuel rod
125
. However, the strength of the vertical support parts
121
of each spring
118
is almost equal to that of the central curved support part
122
of the spring
118
as described above, and so the two parts
121
and
122
are deformed at the same time when a force is applied from the rods
125
to the spring
118
.
Therefore, the external surface of the fuel rod
125
slides slightly on the central curved support part
122
of the spring
118
, while the vertical support parts
121
of the spring
118
is deformed by the bending moment and twisting moment applied from the fuel rod
125
to the spring
118
. Therefore, the vertical support parts
121
are bent in a loaded direction in addition to being distorted.
When the grid springs
118
and dimples
119
of the conventional spacer grids
110
have insufficient spring force, it is almost impossible for them to stably place and support the fuel rods
125
at desired positions inside the assembly
2
, thus reducing the soundness of the fuel assemblies. On the contrary, when the grid springs
118
and dimples
119
of the conventional spacer grid
110
are too high in their spring forces, there may be formed excessive frictional force between the fuel rods
125
and the spacer grids
110
during an insertion of the fuel rods
125
into the cells
123
of the grids
110
. Such an excessive frictional force may cause damages, such as scratches, on the external surface of the fuel rods
125
, and fail to appropriately support the fuel rods
125
in the case of lengthwise growth of the fuel rods due to neutron radiation during the operation of a nuclear reactor. In such a case, the fuel rods
125
may be undesirably bent.
When the fuel rods
125
bend as described above, the rods
125
become closer to each other to be sometimes brought into undesired contact with each other, thus making the coolant channels between the fuel rods
125
of the fuel assembly
2
become narrower or closed. In such a case, it could impede the effective heat transfer from the fuel rods
125
to the coolant, and so the fuel rods
125
may be partially overheated, thus sometimes causing a DNB (Departure from Nucleated Boiling) and reducing the output power of the nuclear fuel.
The recent trend of development in the nuclear reactor fuel assemblies aims at the provision of high burn-up and defect-free nuclear fuel.
Particularly, in order to provide desired high burn-up nuclear fuel, it is necessary to improve the heat transfer efficiency between the fuel rods and the coolant in the nuclear reactor fuel assembly. The above object may be accomplished by designing the nuclear reactor fuel assembly to allow the coolant to enhance the mixing flow around the fuel rods in the assembly.
In an effort to accomplish such mixing flow of coolant within the nuclear reactor fuel assembly, several types of spacer grids having new structures have been proposed. For example, the mixing flow of the coolant may be accomplished by attaching specifically designed mixing vanes in the fuel assembly or providing effective coolant channels in the assembly.
The conventional techniques for mixing flow of coolant in the nuclear reactor fuel assemblies are based on the formation of more active turbulent flow of coolant with high Reynolds number around the fuel rods of the assembly. Therefore, the conventional techniques undesirably induce the fuel rod vibration in the assem
Chun Tae Hyun
In Wang Kee
Jung Youn Ho
Kang Heung Seok
Kim Hyung Kyu
Bachman & LaPointe P.C.
Carone Michael J.
Korea Atomic Energy Research Institute
Palabrica R
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