Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within...
Reexamination Certificate
2000-03-15
2003-11-11
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
C376S327000, C376S333000
Reexamination Certificate
active
06647082
ABSTRACT:
BACKGROUND OF THE INVENTION
This application is based on Japanese Patent Application No. 11-69908 filed Mar. 16, 1999, and No. 2000-51779 filed Feb. 28, 2000, the contents of which are cited herein by reference.
The present invention relates to a method and apparatus for manufacturing a control rod for a nuclear reactor, the parts of the control rod are adhered to one another by laser welding or TIG welding.
FIG. 1
schematically shows an example of the internal structure of a reactor pressure vessel (RPV) of a boiling water reactor (BWR).
Referring to
FIG. 1
, water as a coolant is heated and boiled, while flowing upward from a bottom portion of a reactor core through a gap between fuel rods
1
containing fuel. Power of the reactor varies in accordance with withdrawal or insertion of a control rod
2
. The reactivity is increased when the control rod
2
is withdrawn. The reactivity is decreased when the control rod
2
is inserted.
An example of the structure of the control rod
2
will be described with reference to a schematic perspective view of FIG.
2
A. As shown in
FIG. 2A
, the control rod
2
mainly comprises blades
3
, a handle
4
attached to the upper ends of the blades
3
and lower-blades
5
attached to the lower ends of the blades
3
. More specifically, the blade
3
comprises neutron absorbers
6
, a sheath
7
housing them and insertion pieces
8
inserted for the purpose of securing a portion to allow water to flow between the neutron absorbers
6
in the sheath
7
.
Four of the blades
3
make a set, and the adhesion ends of the respective blades are connected to a tie rod so as to form a cross shape as a whole.
Conventionally, the control rod
2
of the reactor is manufactured by the following method.
FIGS. 3A and 3B
show examples of processes for manufacturing the control rod
2
.
First, a process for manufacturing the control rod
2
using boron carbide (B
4
C) as the neutron absorbers
6
will be described with reference to the flowchart of FIG.
3
A.
In this case, first, the periphery of the sheath
7
is cut (S
1
), and weep holes are formed by cutting (S
2
). Thereafter, the sheath
7
is bent like a C-shape (S
3
). After the ends of the C-shaped sheath
7
are aligned, a tube containing boron carbide as the neutron absorbers
6
is inserted in the sheath
7
(S
4
). Thus, the blade
3
is assembled.
The blades
3
are fixed to the tie rod
9
, with the result that the control rod
2
is assembled (S
5
). Thereafter, welding of the tie rod
9
(S
6
) and welding of the handles
4
and the lower-blades
5
(S
7
) are carried out. Finally, the control rod
2
is smoothed by finishing with a wire brush or the like (S
8
).
No insertion piece
8
is employed in the control rod
2
which uses B
4
C as the neutron absorbers
6
.
In the above case, B
4
C is used as the neutron absorbers
6
. Next, the control rod
2
using hafnium (Hf) as the neutron absorbers
6
will be described.
In this case, as shown in
FIG. 3B
, the periphery of the sheath
7
is cut first (S
11
), and weep holes are formed by cutting (S
12
). Thereafter, the sheath
7
is bent like a C-shape (S
13
). After the ends of the C-shaped sheath
7
are aligned, piece-inserting holes through which the insertion pieces
8
are inserted inside the C shape is formed by cutting (S
14
). Thereafter, an Hf plate as the neutron absorbers
6
, and the insertion pieces
8
are inserted in the sheath
7
, thereby assembling the blade
3
(S
15
). The sheath
7
and the insertion pieces
8
are welded together (S
16
) and finishing with a wire brush is carried out (S
17
), thus producing the blade
3
.
Further, the tie rod
9
, the handles
4
and the lower-blades
5
are combined with the blades
3
produced by the above method, with the result that the control rod
2
is assembled (S
18
). Thereafter, welding of the handles
4
and the lower-blades
5
(S
20
) are carried out. Finally, the control rod
2
is finished with a wire brush (S
21
).
In the case where the control rod
2
of the reactor is manufactured as described above, first, the sheath
7
is cut and bent, and thereafter the blade
3
is assembled. Then, the frames of the tie rod
9
, the handles
4
and the lower-blades
5
are assembled. Further, welding of the blades
3
, the tie rod
9
, the handles
4
and the lower-blades
5
and welding of the sheath
7
and the insertion pieces
8
are carried out.
Conventionally, welding of these parts is performed by manual TIG welding. The TIG welding is a method in which arc is generated between a nonconsumable tungsten electrode and a base material in an inert gas atmosphere for the purpose of melting.
According to the manual TIG welding, since heat input is great, deformation after the process is great, and in particular, welding deformation is considerable. Therefore, welding is frequently performed while the deformation is corrected. In addition, since the process speed of welding is low, the productivity is low.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for manufacturing a control rod for a nuclear reactor, by reducing heat input to suppress deformation after the process and increasing welding speed to improve the productivity.
The above object is obtained by the invention as described below.
(1) In a method for manufacturing a control rod for a nuclear reactor comprising blades including neutron absorbers, insertion pieces and a sheath, weep holes and piece-inserting holes are formed in the sheath by cut working and the periphery is cut. Thereafter, the neutron absorbers and the insertion pieces are inserted in a recess portion formed by bending the sheath. The piece-inserting holes and the insertion pieces are welded together by a laser beam, thereby forming an integral blade. The blades and handles are welded by a laser beam, and subsequently, the blades and lower-blades are welded by a laser beam. Then, the welded portions are surface-finished by a wire brush or the like.
(2) In a method for manufacturing a control rod for a nuclear reactor comprising blades including neutron absorbers and a sheath, weep holes are formed in the sheath by cut working. After the periphery is cut, the neutron absorbers are inserted in a recess portion formed by bending the sheath, thereby forming an integral blade. The blades and handles are welded by a laser beam, and subsequently, the blades and lower-blades are welded by a laser beam. Then, the welded portions are surface-finished by a wire brush or the like.
With the method for manufacturing a control rod described above, the heat input in the welding time is kept low and the deformation after the process is suppressed. Moreover, since the process speed of welding is increased, the productivity can be improved.
(3) In the welding time, to reliably melt a groove line and secure the penetration bead on the rear surface of the sheath, a YAG laser beam or a CO2 laser beam is irradiated on a position located immediately above a groove position or a position deviated in parallel from the groove position within a range of 2 mm. The laser beam may be irradiated several times so as to lap over one another, shifted within a range of 0.1 to 2.0 mm from a first pass or a first pass target position. Alternatively, the laser beam is irradiated while traveling forward with circular motion in a diameter within 2 mm. To produce circular motion of the laser beam, a prism is inserted before a condenser lens, and while the prism is rotating on an axis of the laser beam, the laser beam is irradiated on the rotating prism. As a result, the laser beam is incident on the condenser lens, while rotating.
With the above method, in any case, the groove line is reliably melted and the penetration bead on the rear surface is secured.
(4) As another method, in the step for welding the blade to the tie rod, the blade to the handle or the blade to the lower-blade, two slits are provided in the tie rod, the handle or the lower-blade, and projections of the blade is inserted in the slits. The laser beam is irradiat
Araya Masahirou
Kimura Seiichiro
Suenaga Toshihiro
Tamura Masataka
Tongu Yuichi
Carone Michael J.
Kabushiki Kaisha Toshiba
Matz Daniel
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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