Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2002-12-24
2004-10-19
Keith, Jack (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S462000, C376S439000, C376S432000
Reexamination Certificate
active
06807246
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates, in general, to spacer grids used for supporting fuel rods in a nuclear fuel assembly of a nuclear reactor and, more particularly, to a lips-type multi-purposed spacer grid used in such nuclear fuel assemblies and designed to support fuel rods by springs which are in contact with the fuel rods at equiangular curved contact surfaces having areas larger than those of springs of conventional spacer grids, thus enhancing the fuel rod support performance of spacer grids and accomplishing desired soundness of the spacer grids when the spacer grids support the fuel rods in a nuclear fuel assembly. The spacer grid according to the present invention also has a structure designed such that load applied to the springs is distributed to the entire structure of the spacer grid, and the coolant deflecting area of the spacer grid is increased by mixing blades provided at the upper part of the spacer grid, and the spacer grid has a structure capable of effectively filtering the debris moving along with the coolant.
Description of the Prior Art
FIG. 1
is a perspective view, showing the construction of a conventional nuclear fuel assembly.
In each spacer grid
7
of the nuclear fuel assembly
1
, springs and dimples support a plurality of elongated nuclear fuel rods
8
and a plurality of guide tubes
5
so as to maintain the arrangement of the nuclear fuel rods
8
placed at regular intervals in the nuclear fuel assembly
1
. That is, the springs and dimples maintain the regular intervals between the nuclear fuel rods
8
while preventing impact-induced deformation of the nuclear fuel rods
8
and the guide tubes
5
, thus reliably defining passages for coolant in the fuel assembly
1
and allowing the coolant to effectively cool the fuel rods
8
in a reactor core. A plurality of mixing blades are attached to the upper edges of intersecting strips of each spacer grid
7
so as to mix the thermally imbalanced coolant within the nuclear fuel assembly
1
. In addition, the spacer grids
7
may be designed to have a structure capable of effectively filtering debris from the coolant.
The coolant mixing function, the wear and the debris filtering function of the spacer grids
7
are recognized as important factors in development of nuclear fuel assemblies of high burn-up and zero defects. In order to develop the nuclear fuel assemblies of high burn-up and zero defects, it is necessary to enhance the thermal efficiency of the nuclear fuel assemblies. The thermal efficiencies of the nuclear fuel assemblies may be enhanced by improving the flow characteristics of coolant around the fuel rods.
The improvement in the flow characteristics of coolant around the fuel rods may be effectively accomplished by a change of the structure of a spacer grid. That is, the thermal mixing of coolant may be improved by attachment of mixing blades to the spacer grid, a change of the shape of the mixing blades and/or defining appropriately designed coolant channels in the spacer grid. However, the above-mentioned methods of enhancing the thermal efficiencies of nuclear fuel assemblies also generate turbulences in coolant flowing around the fuel rods, and the turbulences of the coolant undesirably cause to vibrate the elongated, parallel, closely spaced fuel rods within the nuclear fuel assembly. When the fuel rods are so vibrated over a lengthy period of time, the claddings of the fuel rods are repeatedly and frictionally abraded at their contact parts at which the fuel rods are brought into contact with the springs and dimples of the spacer grids. The claddings are thus reduced in their thickness so as to be finally perforated at the contact parts. Such an abrasion of the fuel rods is so-called “fretting wear of fuel rods” in the art.
When the spacer grids
7
are exposed to neutron irradiation in the reactor core for a lengthy period of time, the material characteristics of the springs of the spacer grids
7
are changed, and the springs gradually lose their elasticity. The springs in such a case fail to stably and steadily support the fuel rods
8
, and so allow the fuel rods
8
to vibrate. The claddings of the fuel rods
8
are thus abraded at the contact parts at which the fuel rods are brought into contact with the springs and dimples of each spacer grid
7
, so that the fretting wear of the fuel rods
8
occurs. There have been a lot of reports of leakage of radioactive materials from fuel rods due to perforation of the claddings of the fuel rods
8
caused by the fretting wear. In the art, it has been recognized that the fretting wear of the fuel rods is greatly affected by the design of the spacer grid, including the shapes of contact parts between the fuel rods and the spacer grids.
In the conventional spacer grid
7
, the fuel rods
8
are in contact with the springs and dimples in a manner of point contact or linear contact mostly. Of the two contact manners, the linear contact manner confers higher resistance to flow-induced vibration and abrasion of fuel rods, in comparison with the point contact manner, so that the linear contact manner more effectively protects the fuel rods
8
from fretting wear. That is, when the contact surface areas between the fuel rods
8
and the springs and dimples of the spacer grid
7
are increased under the condition that the spring force is not changed, the peak value of contact pressure at the contact parts is reduced such that the fretting wear of the fuel rods
8
is diminished.
In addition, during a process of designing the springs of spacer grids, it is necessary to preset the spring force of each spacer grid while considering expected irradiation-induced changes in the material characteristics of the springs. When the spring force is too low, the springs may fail to accomplish desired soundness of a spacer grid when the spacer grid supports fuel rods. When the spring force is too high, the springs may impose excessive frictional force on the claddings of fuel rods, thus scratching the claddings to damage the claddings during a process of installing the fuel rods in the spacer grid while producing a nuclear fuel assembly. In addition, the springs having such excessively high spring force may cause the fuel rods to be undesirably bent when the fuel rods are elongated due to irradiation-induced growth during an operation of a nuclear reactor. A desired fuel rod support soundness of a spacer grid
7
is accomplished when the springs and dimples of the spacer grid
7
have appropriate spring force.
The power distribution of the fuel rods
8
in the reactor core are unevenly distributed, so that the temperature of coolant flowing around higher power fuel rods
8
is relatively high. When the coolant temperature reaches close to the saturation temperature, an excessive amount of bubbles may be generated at a certain area of the cladding of a fuel rod
8
as much as to cover the cladding, so that the bubble crowding remarkably reduce heat transfer from fuel rod to coolant. In such a case, the temperature of the local area of the cladding covered with the bubbles is quickly increased, so that the cladding or the uranium pellets in the fuel rod
8
may be over-heated up to a melting temperature.
In an effort to suppress the possibility of the above-mentioned problem, the spacer grid
7
is designed to forcibly mix the coolant flowing around the fuel rods
8
of the nuclear fuel assembly
1
, thus creating a uniform temperature distribution of the coolant in the nuclear fuel assembly
1
as for as possible and thus enhancing the heat transfer rate of the claddings of the fuel rods
8
. The spacer grid
7
thus restricts critical nucleate boiling of the coolant and allows the nuclear reactor to be safely operated for a desired lengthy period of time. In order to accomplish the above-mentioned function, mixing blades as a means for increasing the heat transfer efficiency between the fuel rods
8
and the coolant are attached to the spacer grid
7
, as a fuel rod support means.
In the art, coolant mi
Chun Tae Hyun
In Wang Kee
Jung Youn Ho
Kang Heung Seok
Kim Dae Ho
Bachman & LaPointe P.C.
Keith Jack
Korea Atomic Energy Research Institute
Palabrica R
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