U.S. patent application number 14/048984 was filed with the patent office on 2014-04-10 for control rod for nuclear reactor and method of manufacturing control rod.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. The applicant listed for this patent is Kabushiki Kaisha Toshiba. Invention is credited to Yoshiji Karino, Yoshinori Katayama, Mitsuharu Nakamura, Yuuji Saito, Satoko Tajima, Motoji Tsubota, Kosaku Tsumita, Makoto Ueda, Kenichi Yoshioka.
Application Number | 20140098925 14/048984 |
Document ID | / |
Family ID | 39715900 |
Filed Date | 2014-04-10 |
United States Patent
Application |
20140098925 |
Kind Code |
A1 |
Saito; Yuuji ; et
al. |
April 10, 2014 |
CONTROL ROD FOR NUCLEAR REACTOR AND METHOD OF MANUFACTURING CONTROL
ROD
Abstract
A control rod for nuclear reactors includes four wings including
neutron absorbers containing hafnium, a front end structural member
which has a cross shape in cross section and includes brackets
bonded to the leading ends of the wings, and a terminal end
structural member which has a cross shape in cross section and
includes brackets bonded to the tailing ends of the wings. The four
wings are bonded to a wing-bonding member including a cross-shaped
center shaft so as to form a cross shape. The front end structural
member and the wing-bonding member are made of a zirconium alloy.
The wings include neutron-absorbing plates having neutron-absorbing
portions and each have an outer surface which is opposed to a fuel
assembly and at which a hafnium-zircaloy composite member covered
with zircaloy is disposed. The neutron-absorbing plates are opposed
to each other with trap spaces disposed therebetween.
Inventors: |
Saito; Yuuji; (Yokohama-shi,
JP) ; Nakamura; Mitsuharu; (Yokohama-shi, JP)
; Ueda; Makoto; (Yokohama-shi, JP) ; Katayama;
Yoshinori; (Yokohama-shi, JP) ; Tsubota; Motoji;
(Fujisawa-shi, JP) ; Tajima; Satoko;
(Yokohama-shi, JP) ; Karino; Yoshiji;
(Kamakura-shi, JP) ; Yoshioka; Kenichi;
(Yokohama-shi, JP) ; Tsumita; Kosaku;
(Setagaya-ku, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba |
Minato-ku |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Toshiba
Minato-ku
JP
|
Family ID: |
39715900 |
Appl. No.: |
14/048984 |
Filed: |
October 8, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12034966 |
Feb 21, 2008 |
|
|
|
14048984 |
|
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|
|
Current U.S.
Class: |
376/327 |
Current CPC
Class: |
G21C 7/113 20130101;
G21C 21/18 20130101; Y02E 30/31 20130101; Y02E 30/30 20130101; Y02E
30/39 20130101 |
Class at
Publication: |
376/327 |
International
Class: |
G21C 7/113 20060101
G21C007/113 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2007 |
JP |
2007-042444 |
Dec 13, 2007 |
JP |
2007-321907 |
Claims
1. A control rod for nuclear reactors comprising: a neutron
absorber constituting a structural member of a control rod, the
neutron absorber being of a composite member including a hafnium
plate and at least one zirconium plate bonded to the hafnium
plate.
2. The control rod according to claim 1, wherein the neutron
absorber has a cross shape in horizontal cross section.
3. The control rod according to claim 1, wherein the zirconium
plate is disposed on a surface of the neutron absorber that is
configured to contact a reactor water.
4. The control rod according to claim 1, further comprising wings
fixed with fixing members in a thickness direction of each wing,
wherein the fixing members are disposed at positions in a vicinity
of bases of the wings and arranged in a longitudinal direction.
5. The control rod according to claim 1, further comprising wings
fixed with fixing members in a direction of a base of each wing,
wherein the fixing members are disposed at positions arranged in a
longitudinal direction and located at the bases of the wings which
are opposed to each other.
6. The control rod according to claim 4, further comprising a
terminal end structural member and a front end structural member
including a handle, wherein the wings are formed from the composite
member so as to provide a cross shape and the front end structural
member is fixed to upper portions of the wings with the fixing
members or the terminal end structural member is fixed to lower
portions of the wings with the fixing members.
7. The control rod according to claim 5, further comprising a
terminal end structural member and a front end structural member
including a handle, wherein the wings are formed from the composite
member so as to provide a cross shape and the front end structural
member is fixed to upper portions of the wings with the fixing
members or the terminal end structural member is fixed to lower
portions of the wings with the fixing members.
8. The control rod according to claim 6, wherein the front end
structural member is formed from the composite member.
9. The control rod according to claim 1, wherein the at least one
zirconium plate is bonded to the hafnium plate by hot rolling.
10. The control rod according to claim 1, wherein the at least one
zirconium plate is bonded to a first surface of the hafnium plate,
and the neutron absorber of the composite member includes another
different zirconium plate, that is bonded to a second surface of
the hafnium plate, the at least one zirconium plate and the another
different zirconium plate sandwiching the hafnium plate.
11. A control rod for nuclear reactors comprising: a neutron
absorber of a composite member including a hafnium plate, a first
zirconium plate, and a second zirconium plate, the first zirconium
plate being bonded to a top surface of the hafnium plate, and the
second zirconium plate being bonded to a bottom surface of the
hafnium plate, the top surface and the bottom surface of the
hafnium plate being covered by the first and second zirconium
plates, respectively, without side surfaces of the hafnium plate
being covered by the zirconium plates.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims the benefit
under 35 U.S.C. .sctn.120 from U.S. application Ser. No.
12/034,966, filed Feb. 21, 2008, which claims priority under 35
U.S.C. .sctn.119 from Japanese Patent Application No. 2007-042444,
filed Feb. 22, 2007 and Japanese Patent Application No.
2007-321907, filed Dec. 13, 2007, the entire contents of each of
which are hereby incorporated.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a control rod for nuclear
reactors such as boiling water reactors and also relates to a
method for manufacturing such a control rod.
[0004] 2. Related Art
[0005] One example of a conventional control rod having a long life
for boiling water reactor (BWR) is shown in FIGS. 30 to 32 with
reference numeral of 200 as disclosed in Japanese Unexamined Patent
Application Publication No. 63-8594 (hereinafter referred to as
Patent Document 1) or a publication by M. Ueda, T. Tanzawa, and R.
Yoshioka of "Critical Experiment on a Flux-Trap-Type Hafnium
Control Rod for BWR", Transaction of the American Nuclear Society,
vol. 55, p. 616 (1987) (hereinafter referred to as Non-patent
Document 1). Each control rod 200 includes four wings 207, each of
them including a sheath 201, made of stainless steel (SUS), having
a U-shape in cross section.
[0006] Namely, FIG. 30 shows a control rod 200 for a nuclear
reactor such as boiling water reactor (BWR). The control rod 200
includes, for example, a tie rod 202 connecting a front end
structural member 203 to a terminal end structural member 204, the
wings 207 radially extending from the tie rod 202, and a plurality
of neutron absorbers 210 arranged in parallel to the axis of the
tie rod 202. The front end structural member 203 includes guide
rollers 203a and a handle 211 located at an end of the front end
structural member 203. Each of the wings 207 includes a sheath 201
having an outer end portion with a U-shape in cross section and has
cooling holes 209. The neutron absorbers 8 are accommodated in the
sheaths 201.
[0007] The control rod 200 shown in FIGS. 31 and 32 is a
conventional flux trap-type hafnium control rod 200, which is known
to have a long life. The flux trap-type hafnium control rod 200
includes the wings 207 including sheaths 201, hafnium plates 205
accommodated in these sheaths 201, and a tie rod 202. The hafnium
plates 205 function as neutron absorbers, are made of hafnium or a
hafnium alloy, and are disposed in separated sections of each wing
207 that are arranged in parallel to the axis of the wing 207. Each
pair of the hafnium plates 205 are opposed to each other. The
respective hafnium plates 205 have different thicknesses depending
on the amount of neutrons absorbed by the separated sections. This
allows the hafnium plates 205 to have a uniform life.
[0008] With reference to FIGS. 31 and 32, the hafnium plates 205
are fixed to the inner surfaces of these sheaths 201, which form
shells of these wings 207, with fixing pieces 208 by welding. These
sheaths 201 are fixed to this tie rod 202 by means of spot
welding.
[0009] In the flux trap-type hafnium control rod 200, since the
fixing pieces 208 are fixed to these sheaths 201 by means of
welding, these sheaths 201 are slightly recessed toward the hafnium
plates 205 because of welding distortion. This can eliminate spaces
between the hafnium plates 205 and these sheaths 201 and can cause
these sheaths 201 and the hafnium plates 205 to be tightly fixed to
each other. In this case, any space for absorbing a corrosive
component is not present between the hafnium plate 205 and these
sheath 201 and a large stress may be applied to the sheath 201
because the hafnium plate 205 cannot be displaced from the sheath
201 although the thermal expansion and irradiation growth of the
hafnium plate 205 are different from that of the sheath 201. In the
flux trap-type hafnium control rod 200, the hafnium plate 205 is
fixed to the sheath 201 with the fixing piece 208 by welding as
described above. The welded portion receive relatively large load
such as scrum load during operation. The fixing of the fixing piece
208 by means of welding can develop residual tensile stress around
the welded portion to cause stress corrosion cracking in the sheath
201 located near the welded portion. This leads to a reduction in
the life of the flux trap-type hafnium control rod 200 and may
threaten the safety of nuclear reactor.
[0010] Japanese Unexamined Patent Application Publication No.
9-113664 (hereinafter referred to as Patent Document 2) also
discloses a control rod, manufactured by means of welding, for the
BWR. However, Patent Document 1 discloses no technique for reducing
residual stresses caused by welding.
[0011] In the flux trap-type hafnium control rod 200, each pair of
the hafnium plates 205, which are opposed to each other, are
disposed in one of the sheaths 201 and a distance between each
hafnium plate 205 and the corresponding sheath 201 is maintained
with the fixing pieces 208. The welding of the sheaths 201 to upper
portions of the fixing pieces 208 causes thin portions of the
sheaths 201 to be recessed toward the hafnium plates 205 to develop
the residual tensile stress in the sheaths 201.
[0012] If the flux trap-type hafnium control rod 200 is used in
such a state, the residual tensile stress may cause stress
corrosion cracking in the sheaths 201 in cooperation with
high-temperature water. The distortion of the sheaths 201 due to
welding may eliminate spaces between the sheaths 201 and the
hafnium plates 205 to cause crevice corrosion. This leads to a
reduction in the reliability of the flux trap-type hafnium control
rod 200.
[0013] Furthermore, the sheath 201 has an aperture fitted over a
projecting portion of the narrow tie rod 202 having a cross shape
in cross section and also has an inner space containing the pair of
hafnium plates 205 that are neutron absorbers. Each of the wings
207 has a leading portion 211 bonded to a front end structural
member 203 and a tailing portion bonded to a terminal end
structural member 204.
[0014] In the control rod 200, the space between the hafnium plates
is filled with water in a nuclear reactor. The reactor water
moderates neutrons, which are therefore efficiently absorbed by the
hafnium plates 205. Therefore, the hafnium plates 205, which are
expensive and heavy, can be saved because of the presence of the
reactor water between the hafnium plates 205. The space
therebetween is called a trap or a trap space.
[0015] The hafnium plates 205 are spaced from each other in the
axial direction of the control rod 200, which is inserted into or
withdrawn or removed from the nuclear reactor, because the amount
of hafnium contained in the hafnium plates 205 located closer to
the entrance of the nuclear reactor may be small. The hafnium
plates 205 are fixed to the sheaths 201 with fixing pieces 208,
referred to as space/load-retaining members, disposed
therebetween.
[0016] No techniques for preventing stress corrosion cracking are
disclosed in conventional technical documents. The hafnium plates
205 are spaced from each other in the axial direction of the
control rod 200 and have different thicknesses. However, there are
problems in that an increase in the number of the hafnium plates
205 leads to an increase in manufacturing cost and the hafnium
plates 205 are nonuniform in mechanical strength in the axial
direction (that is, the hafnium plates 205 located at lower
positions have lower mechanical strength).
[0017] The sheaths 201 are located close to the hafnium plates 205
and therefore the control rod 200 is under corrosive conditions
because the stainless steel used to make the sheaths 201 has
electrochemical properties different from those of hafnium in the
hafnium plates 205. Furthermore, the control rod 200 suffers from
corrosion because the atmosphere in the nuclear reactor is
corrosive.
[0018] Japanese Unexamined Patent Application Publication No.
58-147687 (hereinafter referred to as Patent Document 3) discloses
a hafnium control rod including no sheath. The hafnium control rod
has a structure for solving a problem that hafnium and stainless
steel cannot be welded to each other. The hafnium control rod
includes a tie rod made of stainless steel. However, no measure
against corrosion or no measure against a problem, called blade
history, are disclosed in Patent Document 3.
[0019] A long-life control rod is mostly inserted in a nuclear
reactor in high-power operation. Therefore, portions of fuel
assemblies that are adjacent to neutron absorbers have a low
neutron flux level and therefore burn slowly. Hence, fissionable
content in the fuel assembly portions is relatively large. When the
long-life control rod is withdrawn from nuclear reactor, a large
amount of energy is generated. This influences on the health of the
fuel assemblies.
[0020] This problem may be called blade history. The prevention of
a reduction in neutron flux is effective in solving this problem
and usually reduces the reactivity worth of the long-life control
rod, thereby causing a shortage in reactivity worth.
[0021] Conventional control rods have been used in commercial
reactors to exhibit satisfactory irradiation resistance. However,
it has become clear that the conventional control rods are
susceptible to stress corrosion cracking and are electrochemically
activated. In order to use the conventional control rods in nuclear
reactors for a long time, problems caused by a difference in
irradiation growth or a difference in thermal expansion need to be
solved and the following problem also needs to be solved in such a
manner that a reduction in reactivity worth is suppressed, i.e., a
problem that fuel assemblies adjacent to the conventional control
rods generate a large amount of power when the conventional control
rods are removed from the nuclear reactors (that is, a problem that
blade history is serious).
[0022] Furthermore, it is desired that neutron-absorbing plates are
improved in manufacturability, have a uniform structure in the
axial direction thereof, and are reduced in manufacturing cost.
SUMMARY OF THE INVENTION
[0023] The present invention was conceived in consideration of the
circumstances encountered in the prior art mentioned above and an
object of the present invention is to provide a control rod, having
a long life, for nuclear reactors and also provide a method of
manufacturing such control rod.
[0024] The present invention is effective in preventing stress
corrosion cracking, effective in reducing electrochemical
activation, effective in reducing blade history, effective in
improving axial mechanical strength distribution, and effective in
enhancing manufacturability.
[0025] The above and other objects can be achieved according to the
present invention by providing, in one aspect, a control rod for a
nuclear reactor including a neutron absorber of a composite member
including a hafnium plate and at least one zirconium plate bonded
to the hafnium plate.
[0026] This aspect may include the following embodiments.
[0027] The neutron absorber may have a cross shape in horizontal
cross section.
[0028] The zirconium plate may be disposed on a surface of the
neutron absorber which contacts reactor water.
[0029] The control rod may further include wings fixed with fixing
members in a thickness direction of each wing, wherein the fixing
members are disposed at positions in a vicinity of bases of the
wings and arranged in a longitudinal direction.
[0030] The control rod may further include wings fixed with fixing
members in a direction of a base of each wing, wherein the fixing
members are disposed at positions arranged in a longitudinal
direction and located at the bases of the wings which are opposed
to each other.
[0031] The control rod may further includes a terminal end
structural member and a front end structural member including a
handle, wherein the wings are formed from the composite member so
as to provide a cross shape and the front end structural member is
fixed to upper portions of the wings with the fixing members or the
terminal end structural member is fixed to lower portions of the
wings with the fixing members. The front end structural member may
be formed from the composite member.
[0032] The above object can be also achieved by providing, in
another aspect, a method for manufacturing the control rod, which
includes a neutron absorber of a composite member including a
hafnium plate and at least one zirconium plate bonded to the
hafnium plate, the method including the steps of:
[0033] shaping the composite member such that the composite member
have a rectangular tubular shape and the zirconium plate is located
outside the hafnium plate;
[0034] forming a rectangular tube by welding both end portions of
the composite member to each other, the end portions being arranged
in a longitudinal direction of the composite member; and
[0035] shaping the rectangular tube such that the rectangular tube
has a cross shape in horizontal cross section.
[0036] This aspect may include the following preferred
embodiments.
[0037] The manufacturing method may further include a step of
placing fixing members for fixing in a thickness direction of each
wing of the neutron absorber at positions located in a vicinity of
bases of the wings and arranged in a longitudinal direction to
prevent distortion of a rectangular tube having a cross shape in
horizontal cross section.
[0038] The manufacturing method may further include a step of
placing fixing members for fixing in a direction of a base of each
wing at positions arranged in a longitudinal direction and located
at the bases of the wings which are opposed to each other to
prevent distortion of a rectangular tube having a cross shape in
horizontal cross section.
[0039] The above object can be achieved also by providing, in a
further aspect, a control rod for nuclear reactors including:
[0040] four wings including neutron absorbers containing
hafnium;
[0041] a front end structural member which has a cross shape in
cross section and includes brackets bonded to leading ends of the
wings; and
[0042] a terminal end structural member which has a cross shape in
cross section and includes brackets bonded to tailing ends of the
wings,
[0043] wherein the four wings are bonded to a wing bonding member
including a cross-shaped center shaft so as to form a cross shape
in such a manner that the wings are spaced from each other at
predetermined intervals in an axial direction, at least the front
end structural member and the wing bonding member are made of a
zirconium alloy containing hafnium of which the hafnium content is
greater than or equal to that of natural compositions, the wings
have principal portions including neutron absorbing plates having
neutron absorbing portions made of a hafnium-zirconium alloy
diluted with hafnium or zirconium and each have an outer surface
which is opposed to a fuel assembly and at which a hafnium-zircaloy
composite member covered with zircaloy is disposed, the
neutron-absorbing plates are opposed to each other in such a manner
that trap spaces in which reactor water is present are disposed
between the neutron absorbing plates, and a thickness of each
neutron absorbing plate is substantially uniform in a direction in
which the control rod inserted or withdrawn.
[0044] In this aspect, the following preferred embodiment may be
further provided.
[0045] The control rod may further include tie rods, disposed in
the wings, for connecting the front end structural member and the
terminal end structural member to each other, wherein the
neutron-absorbing plates are mounted in the wings so as to slide
from the leading ends toward the tailing ends of the wings or from
the tailing ends toward the leading ends of the wings. The tie rods
may be made of hafnium.
[0046] The control rod may further include wing end reinforcing
members which are disposed in the trap spaces between the neutron
absorbing plates and which slides in the axial direction of the
control rod. The wing end reinforcing members may be made of
hafnium.
[0047] Each of the neutron absorbing portions may have a first
portion extending from the leading end of the neutron-absorbing
portion and having a length equal to 1/24 to 2/24 of a length of
the neutron absorbing portion, a second portion extending from the
first portion and having a length equal to a difference obtained by
subtracting the length of the first portion from 1/4 to 1/2 of the
length of the neutron absorbing portion, and a third portion
extending from the tailing end of the neutron absorbing portion, in
which the second portion has a width greater than that of the third
portion, and an outer end of a leading portion of each wing is
aligned with that of a tailing portion of the wing. The first
portion may have a width less than that of the second portion.
[0048] The control rod may further include a hafnium-zircaloy
composite material and short narrow hafnium rods, wherein the
hafnium-zircaloy composite material is repeatedly mount-folded and
valley-folded so as to provide mount-folded and valley-folded
portions which are arranged at equal intervals and which extend in
parallel to each other, the valley-folded folded portions are
brought close to each other so that the folded hafnium-zircaloy
composite material has a cross shape in horizontal cross section,
and the hafnium rods are arranged in end portions of the wings in
form of spacers. The control rod may further include a tie cross
made of zircaloy, wherein the valley-folded portions partially have
longitudinal holes regularly and intermittently arranged in the
axial direction and portions of the tie cross are arranged above
and below the longitudinal holes so as to maintain the cross shape
and improve mechanical strength.
[0049] The control rod may further include short narrow hafnium
rods functioning as spacers, wherein the four hafnium-zircaloy
composite members are bent so as to provide an L-shape, bent
portions of the hafnium-zircaloy composite members are brought
close to each other so as to be directed to a center of a cross
shape, and the hafnium rods are attached to end portions of the
bent hafnium-zircaloy composite members. The control rod may
further include a tie cross made of zircaloy, wherein the bent
portions partially have longitudinal holes regularly and
intermittently arranged in the axial direction and portions of the
tie cross are arranged above and below the longitudinal holes so as
to maintain the cross shape and improve mechanical strength.
[0050] Each of the wings may be formed so that two of the
hafnium-zircaloy composite members are opposed to each other with a
space therebetween and spacers for keeping spaces are fixed to both
ends of the hafnium-zircaloy composite members in an inserting or
withdrawing direction and a perpendicular direction, and the four
wings are bonded to a tie cross including a cross-shaped center
shaft so as to form a cross shape in such a manner that the wings
are spaced from each other at predetermined intervals in the axial
direction.
[0051] Each of the wings may be formed so that one of the
hafnium-zircaloy composite members is bent so as to provide a
U-shape with a space, and a plurality of short spacers are fixed to
end portions of the bent hafnium-zircaloy composite member located
on the side close to a cross-shaped center shaft included in a tie
cross, the tie cross is spaced from the wing at a predetermined
distance in the axial direction, and the four wings are bonded to
each other so as to form a cross shape.
[0052] Each of the wings may be formed so that one of the
hafnium-zircaloy composite members is bent so as to provide a
cylindrical shape, both end portions of the bent hafnium-zircaloy
composite member are bonded to each other to form a cylinder, which
is then pressed into a flattened tube, and a plurality of short
spacers are fixed to outer end portions and inner portions of the
flattened tube, the inner portions being located on the side close
to a cross-shaped center shaft, which is included in a tie cross,
and the four wings are bonded to form a cross shape so that the tie
cross is spaced from the wings at a predetermined distance in the
axial direction.
[0053] The wings may be fixed with members, located in a vicinity
of end portions of the cross-shaped center shaft for preventing the
wings from being opened.
[0054] The spacers, made of hafnium, disposed in the outer end
portions of the wings may be short rods and center portions of the
short rods are fixed to the hafnium-zircaloy composite members.
[0055] According to the present invention of the characters
mentioned above, the control rod is effective in suppressing stress
corrosion cracking and/or electrochemical activation. When the
control rod is used in a nuclear reactor, the inconveniences,
encountered in the prior art, caused by a difference in irradiation
growth or a difference in thermal expansion can be solved and the
reduction in reactivity worth is suppressed, which can solve a
problem that a fuel assembly adjacent to the control rod generate a
large amount of power when the control rod is removed from the
nuclear reactor (i.e. a problem of serious blade history). The
neutron-absorbing plates can be processed so as to have a uniform
thickness in the axial direction, thereby improving
manufacturability, reducing manufacturing cost, and enhancing
mechanical health.
[0056] The nature and further characteristic features of the
present invention will be made clearer from the following
descriptions made with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] In the accompanying drawings:
[0058] FIG. 1 is a partial sectional view of a neutron absorber
included in a control rod according to a first embodiment of the
present invention;
[0059] FIG. 2 is a partial sectional view of a neutron absorber
included in a control rod according to a second embodiment of the
present invention;
[0060] FIG. 3 is a horizontal sectional view of a composite member
included in the control rod according to the first embodiment, the
composite member being molded so as to have a rectangular shape,
and end portions of the composite member being welded to each
other;
[0061] FIG. 4 is a horizontal sectional view of the composite
member according to the first embodiment, the composite member
being molded so as to have a rectangular shape and being further
molded so as to have a cross shape;
[0062] FIG. 5 is a horizontal sectional view of a control rod
according to a fifth embodiment of the present invention;
[0063] FIG. 6 is a horizontal sectional view of a control rod
according to a sixth embodiment of the present invention;
[0064] FIG. 7 is a partial cutaway perspective view of a control
rod according to a seventh embodiment of the present invention;
[0065] FIG. 8 is a plan view of the inside of a nuclear reactor
used for experiments;
[0066] FIG. 9 is an enlarged view of a portion represented by B in
FIG. 1;
[0067] FIGS. 10A to 10C are graphs showing the results obtained
from the above experiments;
[0068] FIG. 11 is a partial sectional side view of a control rod
according to a tenth embodiment of the present invention;
[0069] FIGS. 12A, 12B and 12C are sectional views of the control
rod taken along the line A1-A1, the line B1-B1 and the line C1-C1,
respectively, of FIG. 11;
[0070] FIGS. 13A and 13B are plan views of a control rod according
to an eleventh embodiment of the present invention, FIG. 13C is a
sectional view of the control rod taken along the line C21-C21 of
FIG. 13A, and FIG. 13D is a side view of one of neutron-absorbing
plates each used to form one wing;
[0071] FIGS. 14A, 14B and 14C are sectional views of the
neutron-absorbing plate taken along the line A2-A2, the line B2-B2
and the line C22-C22, respectively, of FIG. 13C;
[0072] FIG. 15 is a vertical sectional view of one of wings
included in a control rod according to a twelfth embodiment of the
present invention;
[0073] FIGS. 16A, 16B, and 16C are sectional views of the wing
taken along the line A3-A3, the line B3-B3 and the line C3-C3,
respectively, of FIG. 15;
[0074] FIG. 17 is a vertical sectional view of one of wings
included in a control rod according to a thirteenth embodiment of
the present invention;
[0075] FIGS. 18A, 18B and 18C are sectional views of the wing taken
along the line A4-A4, the line B4-B4 and the line C4-C4,
respectively, of FIG. 17;
[0076] FIG. 19 is a vertical sectional view of one of wings
included in a control rod according to a fourteenth embodiment of
the present invention;
[0077] FIGS. 20A, 20B and 20C are sectional views of the wing taken
along the line A5-A5, the line B5-B5 and the line C5-C5,
respectively, of FIG. 19;
[0078] FIG. 21 is a developed view of a hafnium sheet included in
the control rod according to the fourteenth embodiment;
[0079] FIG. 22 is an illustration of a principal portion of the
hafnium sheet shown in FIG. 21;
[0080] FIG. 23 is an enlarged view of a principal portion of the
hafnium sheet shown in FIG. 21;
[0081] FIG. 24 is a sectional view of one of wings included in a
control rod according to a fifteenth embodiment of the present
invention;
[0082] FIGS. 25A, 25B and 25C are sectional views of the wing taken
along the line A6-A6, the line B6-B6 and the line C6-C6,
respectively, of FIG. 24;
[0083] FIG. 26 is a sectional view of one of wings included in a
control rod according to a sixteenth embodiment of the present
invention;
[0084] FIGS. 27A, 27B and 27C are sectional views of the wing taken
along the line A7-A7, the line B7-B7 and the line C7-C7,
respectively, of FIG. 26;
[0085] FIG. 28A is a vertical sectional view of a leading portion
of one of wings included in a control rod according to a
seventeenth embodiment of the present invention;
[0086] FIG. 28B is a sectional view of the wing taken along the
line B8-B8 of FIG. 28A;
[0087] FIG. 29A is a vertical sectional view of a tailing portion
of the wing shown in FIG. 28B, and FIG. 29B is a sectional view of
the wing taken along the line B9-B9 of FIG. 29A;
[0088] FIG. 30 is a perspective view of a conventional control
rod;
[0089] FIG. 31 is a horizontal sectional view of a conventional
control rod; and
[0090] FIG. 32 is a vertical sectional view of a conventional
control rod.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0091] Control rods, according to embodiments of the present
invention, for nuclear reactors will now be described with
reference to the accompanying drawings.
[0092] Further, it is to be noted that terms "upper", "lower",
"right", "left" and like terms are used herein with reference to
the illustrations of the drawings or in an actual charged state of
a control rod.
First Embodiment
[0093] FIG. 1 is a partial sectional view of a neutron absorber 16
included in a control rod 15 according to a first embodiment of the
present invention. The control rod 15 itself has a general
structure such as shown in FIG. 30. The neutron absorber 16
includes a hafnium plate 20 bonded to a zirconium plate 21 by means
of hot rolling or the like. The hafnium plate 20 and the zirconium
plate 21 form a composite member 22.
[0094] A conventional control rod, of the structure mentioned with
reference to FIGS. 30 to 32, for example, includes U-shaped sheaths
having fitting portions, hafnium plates, and supporting pieces
having projecting portions bonded to the fitting portions by TIG
welding. If the conventional control rod is exposed to
high-temperature water in a nuclear reactor in such a state that
the U-shaped sheaths have high residual tensile stress due to
welding, stress corrosion cracking occurs in the U-shaped sheaths
to deteriorate the performance of the conventional control rod.
[0095] However, according to this embodiment, the control rod 15
includes the composite member 22, which includes the hafnium plate
20 and the zirconium plate 21 bonded to each other. Since the
zirconium plate 21 functions as a fuel cover and has good
irradiation properties, the control rod 15 has high corrosion
resistance. Therefore, stress corrosion cracking can be prevented
from occurring in the control rod 15 though the control rod 15
contacts high-temperature water.
[0096] According to this embodiment, in the control rod 15, the
composite member 22 is shaped into a cross-shaped structure, and
hence, portions of the control rod 15 have low residual stress, low
distortion and a long life. Therefore, the control rod 15 has high
reliability and quality.
Second Embodiment
[0097] FIG. 2 is a partial sectional view of a neutron absorber 16
included in a control rod 15 according to a second embodiment of
the present invention. In this embodiment, the neutron absorber 16
includes a composite member 23 including a hafnium plate 20 and
zirconium plates 21 bonded to both surfaces of the hafnium plate 20
by means of hot rolling or the like as shown in FIG. 2.
[0098] The hafnium plate 20 is protected from high-temperature,
high-pressure water that is a moderator, and hence, the corrosion
of the neutron absorber 16 can be prevented and the creation of
oxides in the control rod 15 can be suppressed. This allows the
control rod 15 to have a long life.
Third Embodiment
[0099] A third embodiment of the present invention provides a
method of manufacturing the control rod 15 according to the first
embodiment. The control rod 15 includes the composite member 22,
which includes the hafnium plate 20 and the zirconium plate 21.
[0100] As shown in FIG. 3, after the composite member 22, which has
been prepared by bonding the hafnium plate 20 and the zirconium
plate 21 together by means of rolling, for example, is shaped so as
to have such a box shape that the zirconium plate 21 is located
outside the hafnium plate 20, end portions of the composite member
22 are bonded to each other with a welding member 24 or the like,
whereby a rectangular tube A is prepared.
[0101] As shown in FIG. 4, the rectangular tube A is molded into a
cross-shaped tube B such that the zirconium plate 21 is located
outside the hafnium plate 20. The cross-shaped tube B has an inner
space 25 which has a cross shape in cross section and through which
water used as a moderator can flow. As shown in FIG. 30, a front
end structural member 203 including a handle 211 is welded to an
upper portion of the cross-shaped tube B and a terminal end
structural member is welded to a lower portion thereof, whereby the
control rod 15 is obtained.
[0102] This allows cooling water used as a moderator to smoothly
flow through the control rod 15. The inner space 25 contains no
obstacle, and hence, no corrosive product is accumulated in the
inner space 25. This allows the control rod 15 to have a long
life.
Fourth Embodiment
[0103] A fourth embodiment of the present invention provides a
method of manufacturing the control rod 15 according to the second
embodiment. The control rod 15 includes the composite member 23,
which includes the hafnium plate 20 and the zirconium plates 21
bonded to both surfaces of the hafnium plate 20. The method of this
embodiment is similar to that of the third embodiment. In the
control rod 15, the hafnium plate 20 is covered with the zirconium
plates 21 and therefore can be prevented from being corroded. This
allows the control rod 15 to have a long life.
Fifth Embodiment
[0104] FIG. 5 shows a control rod 15 according to a fifth
embodiment of the present invention. The control rod 15 of this
embodiment has a cross shape in cross section and includes a
plurality of (four, in this embodiment) wings 2 fixed with rivets
26 that are fixing members for preventing distortion. The rivets 26
are disposed at positions which are arranged in the longitudinal
direction and which are located near the base portions of the wings
2. Therefore, if the control rod 15 is used for a long time, the
thickness direction of each wing 2 can be fixed by the presence of
the rivets 26. This prevents the distortion of the control rod 15.
If bolts are used instead of the rivets 26, the distortion of the
control rod 15 can be prevented. This allows the control rod 15 to
have a long life.
Sixth Embodiment
[0105] A sixth embodiment of the present invention provides a
method of preventing the distortion of the control rod 15 described
in the third or fourth embodiment. The control rod 15 has a cross
shape in cross section. With reference to FIG. 6, the control rod
15 includes wings 2 fixed with rivets 27 that are as fixing members
for preventing distortion. The rivets 27 are disposed at positions
which are arranged in the longitudinal direction of the control rod
15 and which are located near the base portions of the wings 2.
Each rivet 27 and the longitudinal axis of each wing 2 form an
angle of 45 degrees. The distortion of the control rod 15 can be
prevented if the control rod 15 is used for a long time. If bolts
are used instead of the rivets 27, the distortion of the control
rod 15 can be prevented. This allows the control rod 15 to have a
long life.
Seventh Embodiment
[0106] A control rod 15 according to a seventh embodiment of the
present invention is similar to that described in the third or
fourth embodiment. With reference to FIG. 7, the control rod 15
includes a front end structural member 4 fixed with rivets 28 that
are fixing members. Since the welding is not performed to fix the
front end structural member 4, the front end structural member 4
has no residual welding stress and can be prevented from being
distorted. If bolts are used instead of the rivets 28, the
distortion of the control rod 15 can be prevented. This allows the
control rod 15 to have a long life.
Eighth Embodiment
[0107] A control rod 15 according to an eighth embodiment of the
present invention includes a front end structural member integrally
molded from the hafnium-zirconium composite member 22 or 23
described in the first or second embodiment. Therefore, the control
rod 15 is stable and uniform and can have a long life.
Ninth Embodiment
[0108] A control rod 15 according to a ninth embodiment of the
present invention is similar to that described in the third or
fourth embodiment. The control rod 15 includes a terminal end
structural member 5 fixed with rivets that are fixing members.
Since the welding is not performed to fix the terminal end
structural member 5, the terminal end structural member 5 has no
residual welding stress and can be prevented from being distorted.
If bolts are used instead of the rivets, the distortion of the
control rod 15 can be prevented. This allows the control rod 15 to
have a long life.
[0109] As described above, the present invention of the first to
ninth embodiments provides a control rod for a nuclear reactor and
a method of manufacturing the same. The control rod includes a
composite member including a hafnium plate functioning as a neutron
absorber and a zirconium plate bonded to the hafnium plate.
Therefore, the control rod can be prevented from being deteriorated
and can be prevented from being corroded by high-temperature water.
This allows the control rod to have high reliability and
quality.
[0110] The followings are further embodiments of the control rods
according to the present invention.
[0111] Beforehand the description of the further preferred
embodiments, critical experiments performed for the embodiments
will be described with reference to FIGS. 8 to 10.
[0112] FIGS. 8, 9 and 10A to 10C are illustrations showing critical
experiments performed to evaluate the arrangement of neutron
absorbers according to the present invention. In particular, FIG. 8
is an illustrated plan view showing an inside of a nuclear reactor
used for the experiments, FIG. 9 is an enlarged view of a portion
represented by B in FIG. 8, and FIGS. 10A to 10C are graphs showing
the results obtained from the experiments.
[0113] In the experiments, a cross-shaped control rod 111 having
the same cross section as that of an existing control rod is placed
at the center of a core tank 110 of a nuclear critical assembly
(NCA), and four fuel assemblies 112 different from channel boxes
are arranged around the control rod 111 as shown in FIGS. 8 and 9.
Furthermore, fuel rods 113 are symmetrically arranged outside the
fuel assemblies 112 so as to form a square in horizontal cross
section until the core of the NCA reaches a critical point.
[0114] All the fuel rods 113 have an enrichment of 2%. The control
rod 111 includes neutron-absorbing rods prepared by packing
podiatry boron carbide (B.sub.4C) in stainless steel (SUS) tubes
having an outer diameter of 4.8 mm and an inner diameter of 3.5 mm
at a theoretical density of about 70% and also includes hafnium
(Hf) rods having substantially the same outer diameter and
reactivity worth as those of the neutron-absorbing rods.
[0115] A control rod 111a located in the first row in FIG. 10A
includes B.sub.4C-filled SUS tubes 114 and water-filled SUS tubes
115 (this configuration is hereinafter referred to as Configuration
"a"), the water-filled SUS tubes 115 being marked with Symbol X. A
control rod 111b located in the second row in FIG. 10A included Hf
rods 116 and the water-filled SUS tubes 115 (this configuration is
hereinafter referred to as Configuration "b"). A control rod 111c
located in the third row in FIG. 10A includes acrylic rectangular
rods 117 and the B.sub.4C-filled SUS tubes 114 (this configuration
is hereinafter referred to as Configuration "c"), the acrylic
rectangular rods 117 being marked with Symbol X. A control rod 111d
located in the fourth row in FIG. 10A includes the B.sub.4C-filled
SUS tubes 114 only (this configuration is hereinafter referred to
as Configuration "d"). The control rods 111a to 111d includes
sheaths 118, made of stainless steel, having a thickness of about
1.4 mm and a U-shape in horizontal cross section.
[0116] The control rod 111 includes a center member (tie rod)
located at the center thereof. In the Configuration "d", a tie rod
is present. In the Configuration "a", three absorbing rods which
are arranged in each wing and which located on the side close to
the side surface of a tie rod are replaced with three of the
water-filled SUS tubes 115. In the Configuration "b", the
water-filled SUS tubes 115 and the Hf rods 116 are alternately
arranged in each wing so as to be located on the side close to a
tie rod such that the water-filled SUS tubes 115 and the Hf rods
116 occupy two thirds of this wing. In the Configuration "c", a tie
rod is removed such that a region occupied by this tie rod is
filled with water. In the experiments, four types of control rods
having any one of the Configurations "a" to "d" are used to measure
the activation of copper foil to determine the neutron flux
distribution of the surfaces of the control rods as shown in FIGS.
10B and 10C.
[0117] Strips of the copper foil are tightly attached to the
sheaths 118, the core tank 110 is supplied with water, the core is
made critical; and the copper foil strips are irradiated with
neutrons, removed from the core tank 110, and then cut into pieces.
Beta rays emitted from each piece are measured, whereby the induced
radioactivity of the piece is determined.
[0118] FIG. 10C shows radioactivity intensity distribution
normalized with a point (a normalization point in this figure) that
is hardly affected by the variation of the configuration of each
control rod. FIG. 10B shows the ratio of the radioactivity
intensity distribution of each configuration to that of the
Configuration "a".
[0119] The activation of copper is caused by neutrons with low
thermal energy, and therefore, can be assumed to be thermal neutron
flux distribution. Neutron flux distribution sharply increases at
an about 15-mm outer end portion of a wing.
[0120] The neutron flux of a region near the tie rod of the
Configuration "d" is slightly high. The neutron flux of the
Configuration "c" is very high because a region containing no tie
rod occupies by water. The neutron flux of one of the fuel rods
that is located near the center axis of the control rod of the
Configuration "a" is very high.
[0121] The neutron flux of the Configuration "b" is high over a
wide range. The power output from the fuel rods located near the
control rod is not sharply varied as compared with the neutron flux
distribution but similar variation is caused.
[0122] It is therefore an object of the present invention to
increase a neutron flux over a wide range without greatly reducing
the reactivity worth of a control rod.
[0123] As is clear from the measurement results, in the
Configuration "c" having a preferable neutron flux distribution,
the reactivity worth is lowest and the reduction in reactivity
worth is about 8%, which is allowable. However, it is not
preferable that the reactivity worth of the control rod be reduced
by 8%, and hence, this configuration is used only in a necessary
area. In the design of an ordinary control rod, it is unallowable
that the reactivity worth of this control rod be reduced by greater
than 10%.
[0124] In the Configuration "a", the reduction in reactivity worth
is about 3.5%. In the Configuration "c", the reactivity worth is
increased. The life and reactivity worth of the control rod can be
enhanced by arranging a large number of the neutron absorbers in an
end portion of each wing because the wing end portion has
particularly a high neutron flux.
[0125] In an actual control rod, the end portions of the neutron
absorbers arranged in each wing are irradiated with a high dose of
neutrons. Therefore, when a long-life control rod is designed,
long-life neutron absorbers are arranged. When a control rod with a
high reactivity worth is designed, neutron absorbers with high
neutron-absorbing effect are arranged. Conditions for selecting
neutron absorbers arranged in a center portion of each wing are
relatively easy.
[0126] Hereunder, embodiments of preferable control rods will be
described on the basis of the above measurements with reference to
the accompanying drawings of FIGS. 11 to 29.
Tenth Embodiment
[0127] FIG. 11 shows a control rod 111 according to a tenth
embodiment of the present invention. The right half of this figure
is a sectional view of a part of the control rod 111. FIG. 12A is a
sectional view of the control rod 111 taken along the line A1-A1 of
FIG. 11, FIG. 12B is a sectional view of the control rod 111 taken
along the line B1-B1 of FIG. 11, and FIG. 12C is a sectional view
of the control rod 111 taken along the line C1-C1 of FIG. 11.
[0128] With reference to FIG. 11, the control rod 111 includes a
front end structural member 121, located on the control rod
insertion side (the upper side in this figure), having a cross
shape in horizontal cross section and a terminal end structural
member 122, located on the tailing side that is the control rod
withdrawal (removal) side (the lower side in this figure), having a
cross shape in horizontal cross section.
[0129] The front end structural member 121 and the terminal end
structural member 122 are connected to each other with a long tie
cross 123 serving as a wing-bonding member. The tie cross 123
includes a center shaft 23a and has a cross shape in horizontal
cross section. At least the front end structural member 121 and the
tie cross 123 are made of a zirconium alloy containing hafnium. The
hafnium content of the zirconium alloy may be greater than or equal
to that of natural compositions.
[0130] Four wings 124 are connected to the tie cross 123 so as to
form a cross shape in horizontal cross section. Upper end portions
of the wings 124 are engaged with a lower portion of the front end
structural member 121 and fixed thereto through welding portions
125. Each wing 124 includes a pair of plates opposed to each other.
The plates sandwich each bracket portion of the tie cross 123. The
wing 124 has a principal portion including neutron absorbing plates
having neutron absorbing portions made of a hafnium-zirconium alloy
diluted with hafnium or zirconium. The wing 124 is narrow and
tabular and has an edge section, opposed to the tie cross 123,
having a narrow lower portion.
[0131] The wing 124 has a lower end portion which is engaged with
an upper end portion of the terminal end structural member 122 with
a gap 130, located therebetween, having a predetermined size and
which is supported with the upper end portion thereof so as to be
horizontally slidable. This allows the wing 124 to be expanded or
shrunk due to irradiation growth or the like during fuel burning.
The front end structural member 121 and the terminal end structural
member 122 each include four brackets connected to the wings
124.
[0132] A plurality of short hafnium rods 128, functioning as wing
end reinforcing members, are vertically arranged in a side end
portion of each wing 124 with spaces, located therebetween, for
absorbing thermal expansion. The hafnium rods 128 are fixed to the
wing 124 with pins 129 and can vertically slide together with the
wing 124 when the wing 124 is expanded or shrunk.
[0133] The wing 124 has end portions located on the tie cross side.
The end portions of the wing 124 sandwich tabular portions 131,
vertically arranged, extending from each bracket of the tie cross
123 and are fixed to the tabular portions 131 with pins 132. FIG.
11 shows one welding line 141 formed by welding an upper portion
and lower portion of the wing 124 together (an actual control rod
has a plurality of welding lines).
[0134] FIG. 12A shows an upper portion of the control rod 111 in
cross section taken along the line A1-A1 of FIG. 11. FIG. 12B shows
a lower portion of the control rod 111 in cross section taken along
the line B 1-B 1 of FIG. 11. The two opposed plates included in the
wing 124 are neutron absorbing plates 135 each including a
composite member including a zircaloy sheet 133 and a hafnium sheet
134 bonded to the zircaloy sheet 133 by means of hot rolling or the
like. The zircaloy sheet 133 is located outside the hafnium sheet
134. The neutron absorbing plates 135 are opposed to each other
with a trap space 136 therebetween and have substantially a uniform
thickness in the axial direction of the control rod 111.
[0135] A tie rod 137 extends in the wing 124, which includes the
neutron absorbing plates 135. The tie rod 137 functions as a
connecting rod for connecting the front end structural member 121
to the terminal end structural member 122 and has welding portions
138 and 139 bonded to the front end structural member 121 and the
terminal end structural member 122.
[0136] With reference to FIG. 10C, one of the pins 132 is bonded to
the zircaloy sheets 133 and the hafnium sheets 134 with welding
portions located therebetween.
[0137] The neutron absorbing plates 135 are mounted in the wing 124
so as to be slidable from the leading end toward the tailing end of
the wing 124 or from the tailing end toward the leading end of the
wing 124. The hafnium rods 128 are arranged in the trap space 136
between the neutron absorbing plates 135 in the axial direction of
the control rod 111. The front end structural member 121 and the
terminal end structural member 122 are fixed to each other with the
tie rod 137. That is, in this embodiment, the front end structural
member 121 and the terminal end structural member 122 are not fixed
to each other with an intersection (center shaft) of the four wings
124 but are fixed to each other using the trap spaces 136 in the
wings 124 without using a conventional tie rod (center member).
[0138] A primary function of the tie rod 137, as well as that of
the conventional tie rod, is to maintain the mechanical strength.
The tie rod 137 is located at a position different from that of the
conventional tie rod. This is because the a configuration similar
to the Configuration "c" shown in FIG. 10C is obtained such that
the burning of fuel rods located near the control rod 111 is
prevented from being delayed during the insertion of the control
rod 111 by preventing the reduction of a thermal neutron flux near
the center shaft 123a.
[0139] Since the tie rods 137 are disposed in the wings 124, water
is removed from a zone occupied by each tie rod 137, and hence, the
neutrons are moderated, and therefore, the absorption of the
neutrons is reduced. This provides a neutron absorber removal
effect similar to that obtained from the Configuration "b". That
is, an advantage due to the Configuration "c" can be obtained, as
well as an advantage due to the Configuration "b". The
Configurations "b" and "c" are effective in preventing the burning
of the fuel rods located near the control rod 111 from being
delayed. This effect depends on design conditions such as the size
and positions of the tie rods 137 in the wings 124.
[0140] In this embodiment, each wing 124 includes the neutron
absorbing plates 135, which are opposed to each other and which
include the composite members including the zircaloy sheets 133 and
the hafnium sheets 134. The zircaloy sheets 133 have a thickness of
about 0.2 to 0.5 mm and are each located at an outer surface (a
fuel assembly-side surface) of one of the wings 124. The composite
members have a thickness of about 2 to 2.5 mm. The wings 124 are
retained with the hafnium rods 128, the tie cross 123, and the pins
132. The hafnium rods 128 have a wing end spacer function, a
reinforcing function, and a neutron-absorbing function and function
as wing end-reinforcing members. The tie cross 123 is a member for
bonding portions of the wings 124 located on the center shaft side.
The pins 132 are members for preventing the wings 24 from being
opened. Principal portions of the wings 124 are the composite
members.
[0141] The tie cross 123 retains the front end structural member
121 and the terminal end structural member 122 and also retains the
four wings 124 such that the wings 124 form a cross shape. The pins
132 are located at positions where the tie cross 123 is not present
so as to prevent the wings 124 from being opened.
[0142] In this embodiment, the thickness of each neutron absorbing
plate 135 is uniform in the axial direction of the control rod 111.
It is known that the bending resistance of a plate is proportional
to the cube of the thickness of the plate and proportional to the
square of the width thereof, the leading half of a
neutron-absorbing plate preferably has a high ability to absorb
neutrons, and the tailing half thereof preferably has a low ability
to absorb neutrons.
[0143] Conventional neutron-absorbing plates have thin tailing
portions, which have low strength. In this embodiment, however, the
neutron absorbing plates 135 have a uniform thickness as described
above and the neutron-absorbing ability of the neutron absorbing
plates 135 is adjusted by varying the width of tailing portions of
the neutron absorbing plates 135. The insertion or withdrawal of
the control rod 111 is interrupted if the side end of a leading
portion of each wing 124 is not aligned with that of a tailing
portion of the wing 124. Hence, the portions located on the center
shaft side are removed from the neutron absorbing plates 135, which
provides the effect of preventing the reduction of a thermal
neutron flux as described with reference to FIG. 10. Therefore, the
sharp increase of the output can be prevented during the removal of
the control rod 111, thereby improving fuel health (improving the
blade history phenomenon).
[0144] According to this embodiment, the control rod 111 has
increased mechanical strength, and therefore, the fuel health can
be improved.
[0145] In this embodiment, the neutron absorbing plates 135 are
uniform in thickness, and therefore, the type of the neutron
absorbing plates 135 is single. Hence, the neutron absorbing plates
135 can be manufactured at low cost. The tie rod 137 is also
uniform in thickness, and hence, the tie rod 137 has good sliding
properties and can be manufactured at low cost. The sliding
performance of the tie rod 137 relates to the absorption of a
difference in the thermal expansion and a difference in the
irradiation growth.
[0146] Each neutron absorbing plate 135 may be manufactured from a
single material and a portion located on the center shaft 123a may
be then removed from the tailing portion of the neutron absorbing
plate 135. Alternatively, the leading and tailing portions of the
neutron absorbing plate 135 may be separately manufactured and then
welded to each other. In the case where the leading and tailing
portions thereof are welded to each other, the health of the
welding portion can be improved in such a manner that the welding
portion is set a position shifted from the center of the neutron
absorbing plate 135 toward the tailing end thereof such that the
neutron irradiation dose of the welding portion is reduced, because
the neutron irradiation dose of a portion located below the center
of the neutron absorbing plate 135 is significantly less than that
of a center portion of the neutron absorbing plate 135.
[0147] In the neutron absorbing plate 135, the outer surface of the
hafnium sheet 134 is covered with the zircaloy sheet 133 and the
inner surface thereof is polished so as to provide less
irregularity and a reduced area. In view of manufacture, both
surfaces of the hafnium sheet 134 are preferably covered in some
cases. This, however, reduces the trap space 136 in each wing 124
and causes the following disadvantages of a reduction in the
reactivity worth of the control rod 111, a reduction in the
diameter of the tie cross 123, and the like.
[0148] The purpose of covering both surfaces of the hafnium sheet
134 and reducing the surface area of the hafnium sheets 134 is to
suppress or prevent the hafnium sheet 134 from corroding during the
long-term use of the control rod 111 in a nuclear reactor. Products
of the corrosion of the hafnium sheet 134 are radioactive and
therefore need to be suppressed from being generated. On the other
hand, products of the corrosion of the zircaloy sheet 133 are very
slightly radioactive.
[0149] Although the hafnium sheet 134 has high corrosion
resistance, corrosion products are generated on the hafnium sheet
134 while the hafnium sheet 134 is being used in high-temperature
water for a long time. It has been known that the corrosion
products fall away from the hafnium sheet 134 because of some
causes. The corrosion products are radioactive. A principal nuclide
in the corrosion products is Hf.sup.181, which has a half-life of
43 days and emits gamma-rays with relatively low energy (482, 346,
or 133 keV). A slight amount of Ta.sup.182, which has a half-life
of 111 days and emits a 1.2 MeV gamma-ray, is produced.
[0150] The water quality of the current BWR is greatly improved as
compared to that of the conventional BWR. Since the radioactivity
of the water in the current BWR is extremely low, the low
radioactivity of Hf.sup.181 can be measured. Although the
environmental damage caused by Hf.sup.181 has not been confirmed
because the half-life thereof is relatively short, it has become
clear that the radioactivity in nuclear reactor buildings needs to
be reduced. Therefore, in this embodiment, the outer surface of the
hafnium sheet 134 is covered with the zircaloy sheet 133 and the
inner surface thereof is polished so as to provide less
irregularity.
[0151] Outer surfaces of the control rod 111 are rubbed with
zircaloy channel boxes of the fuel assemblies opposed to the
control rod 111 because of the movement of the control rod 111, so
that the corrosion products may fall away from the outer surfaces
thereof. Therefore, the zircaloy sheets 133 are located at the
outer surfaces thereof. The corrosion products present on the inner
surfaces of the hafnium sheets 134 may fall away due to the impact
caused by scrum, earthquake or the like to contaminate the cooling
water in a nuclear reactor through water channels. Hence, the inner
surfaces thereof are polished.
[0152] The zircaloy sheets 133 and the hafnium sheets 134 are
manufactured by processes different from those of manufacturing the
neutron absorbing plates 135, the tie rods 137 and the hafnium rods
128 because of characteristics of crystal grains in the zircaloy
sheets 133 and the hafnium sheets 134, and therefore, are different
in irradiation growth from the neutron absorbing plates 135, the
tie rods 137 and the hafnium rods 128. An increase in irradiation
dose may exert a negative influence on the health of the control
rod 111. In this embodiment, various measures are taken against the
negative influence.
[0153] In particular, the wings 124 and the tie rods 137 are
slidable. The leading portions of wings 124 are fixed to the front
end structural member 121 by welding (another technique such as
pinning may be used) and the tailing portions thereof slidably
sandwich the thin portions of the terminal end structural member
122. The hafnium rods 128 are short and center portions thereof are
pinned with the pins 129. The hafnium rods 128 are fixed to the
neutron absorbing plates 135 with the pins 129 and the upper and
lower end portions of the hafnium rods 128 can be freely expanded
or shrunk.
[0154] The tie cross 123 and the pins 132 located on the center
shaft 23a side are short, and therefore, have no problem due to the
expansion or shrinkage of the wings. If slight differences are
caused by the expansion or shrinkage of the wings 124, a problem
caused by the slight differences can be solved in such a way that
small clearances are formed or rotatability is employed.
[0155] The wings 124 and the tie rods 137 may slide from the
terminal end structural member 122 toward the front end structural
member 121 or may longitudinally expand or contract. The wings 124
may include U-shaped composite absorbing plates instead of the
hafnium rods 128.
Eleventh Embodiment
[0156] FIG. 13A is a plan view of a control rod 111 according to an
eleventh embodiment of the present invention. FIG. 13B is another
plan view of the control rod 111. FIG. 13C is a sectional view of
the control rod 111 taken along the line C21-C21 of FIG. 13A. FIG.
13D is a side view of one of neutron absorbing plates 135 each used
to form one wing. FIG. 14A is a sectional view of the neutron
absorbing plate 135 taken along the line A2-A2 of FIG. 13C. FIG.
14B a sectional view of the neutron absorbing plate 135 taken along
the line B2-B2 of FIG. 13C. FIG. 14C is a sectional view of the
neutron absorbing plate 135 taken along the line C22-C22 of FIG.
13C.
[0157] In this embodiment, the neutron absorbing plate (composite
absorbing plate) 135 includes a first zircaloy sheet 133a, a second
zircaloy sheet 133b, and a hafnium sheet 134 sandwiched
therebetween. The same components as those described in the tenth
embodiment will not be described herein in detail.
[0158] With reference to FIGS. 13A and 13B, the control rod 111 has
a cross shape in a plan view and includes wings 124. With reference
to FIG. 13C, a first tie rod 137a and a second tie rod 137b extend
through each wing 124.
[0159] The leading end of the wing 124 is fixed to a front end
structural member 121. The wing 124 has a small clearance located
near the leading end of the neutron absorbing plate 135 and pinned
with pins 132. A tailing portion of the first tie rod 137a and a
tailing portion of the second tie rod 137b are fitted in a first
recess 142a and second recess 142b, respectively, disposed in a
thin portion of a terminal end structural member 122, the thin
portion being sandwiched between portions of the wing 124. The
tailing portions of the first and second tie rods 137a and 137b can
slide such that differences in the irradiation growth between the
wing 124 and the first and second tie rods 137a and 137b can be
absorbed.
[0160] The first tie rod 137a is located on the wing tip side and
is made of hafnium so as to have high neutron absorbing properties.
The second tie rod 137b is located near the center axis and is made
of zirconium such that the neutron-absorbing effect due to the
removal of water is slightly suppressed.
[0161] As shown in FIG. 13D, the neutron absorbing plate 135 is
bent along dashed line 0 so as to have substantially an L-shape in
a plan view. The bent neutron absorbing plate 135 forms surfaces of
the two adjacent wings 124 as indicated by imaginary line (D) in
FIG. 13B (that is, four of the neutron absorbing plates 135 form
outer surfaces of the control rod 111).
[0162] One of the wings 124 made from the neutron absorbing plates
135 has a closed configuration in which end portions 135C of two of
the neutron absorbing plates 135 are bent so as to oppose to each
other and fixed to each other with a welding portion 150 as shown
in FIGS. 14A, 14B, and 14C.
[0163] As shown in FIG. 13C, each neutron absorbing plate 135 has a
first portion 143, a second portion 144, a third portion 145 and a
fourth portion 146. The first portion 143 extends from the leading
end of the neutron absorbing plate 135 and has a length equal to
one 24th ( 1/24) of the length of the neutron absorbing plate 135
(it is known that no problem occurs if the reactivity worth is
reduced to a certain extent, because the length of the first
portion 143 is about 15 to 16 cm). The second portion 144 extends
from the tailing end of the neutron absorbing plate 135, has a
length equal to one half (1/2) of the length of the neutron
absorbing plate 135 and is relatively greatly recessed from the
center axis to have a small width. The third portion 145 extends
from the first portion 143, has a length equal to the difference
obtained by subtracting the length of the first portion from one
fourth (1/4) of the length of the neutron absorbing plate 135 and
is not recessed because the third portion 145 is the most important
in reactivity worth. The fourth portion 146 extends from the fourth
portion 145 to the second portion 144 and is slightly recessed
because the fourth portion 146 needs to have a good balance between
reactivity worth and measures against blade history.
[0164] The whole of the neutron absorbing plate 135 is shown in
FIG. 13D. The neutron absorbing plate 135 has an upper notch 151,
located at the leading end thereof (the upper end of FIG. 13D),
having a large width and also has a lower notch 151a. The upper
notch 151 corresponds to the first portion 143 shown in FIG. 13C
and the lower notch 151a corresponds to the second, third and
fourth portions 144, 145 and 146 shown in FIG. 13C. This
configuration is used for measures against the blade history
because no reactivity worth is required. A tie cross, which is not
shown, is fixed to portions of the wings 124 located close to the
center axis of the control rod 111.
[0165] Pins 132 functioning as members for preventing the wings 124
from being opened may be arranged as required. End portions 135c of
the neutron absorbing plates 135, as well as those described in the
first embodiment, are fixed to the welding portions 150 in such a
state that the end portions 135c thereof are bent so that the
insertion or withdrawal of the control rod 111 is not prevented.
The leading ends of the wings 124 are fixed to the front end
structural member 121 and the tailing ends thereof are fixed to the
terminal end structural member 122.
[0166] The first tie rod 137a is located on the wing tip side and
is made of hafnium so as to have high neutron-absorbing properties.
The second tie rod 137b is located close to the center axis and is
made of zirconium so that the neutron-absorbing effect due to the
removal of water is slightly suppressed.
[0167] In this embodiment, the same advantages as those described
in the tenth embodiment are obtainable. The first and second
zircaloy sheets 133a and 133b have a uniform width and may have a
nonuniform width depending on design conditions. This may be
applied to following embodiments.
Twelfth Embodiment
[0168] FIG. 15 is a vertical sectional view of one of wings 124
included in a control rod according to a twelfth embodiment of the
present invention. FIG. 16A is a sectional view of the wing 124
taken along the line A3-A3 of FIG. 15. FIG. 16B is a sectional view
of the wing 124 taken along the line B3-B3 of FIG. 15. FIG. 16C is
a sectional view of the wing 124 taken along the line C3-C3 of FIG.
15.
[0169] The control rod of this embodiment has a configuration
similar to that of the eleventh embodiment have a similar
configuration, but both the control rods are different from each
other in that a single slidable tie rod 137 is located near the
side end of the wing 124, and the wing 124 has a configuration
different from that of the wings 124 in the control rod 111 of the
eleventh embodiment.
[0170] With reference to FIG. 15, the wing 124 includes a neutron
absorbing plate 135 as absorbing plate. The neutron absorbing plate
135 has a first portion 143, a second portion 144 and a third
portion. The first portion 143 extends from the leading end of the
neutron absorbing plate 135 and has a length equal to one 24th (
1/24) of the length of the neutron absorbing plate 135. The second
portion 144 extends from the tailing end of the neutron absorbing
plate 135 and has a length equal to one half (1/2) of the length of
the neutron absorbing plate 135. The first portion 143 and the
second portion 144 are configured of two sheets opposed to each
other. The third portion extends from the first portion 143 and has
a length equal to the difference obtained by subtracting the length
of the first portion from one half (1/2) of the length of the
neutron absorbing plate 135. The neutron absorbing plate 135, as
well as that described in the eleventh embodiment, is bent to an
L-shape to form the wing 124. The outer surface of the neutron
absorbing plate 135 is covered with zircaloy and the inner surface
thereof is polished so as to have a reduced effective area.
Therefore, the corrosion area of the neutron absorbing plate 135
can be reduced.
[0171] In this embodiment, no zircaloy coating which eliminates
trap water is present on the inner surface of the neutron absorbing
plate 135. This configuration is suitable for thin control rods
that need to include thin wings. A plurality of short spacers 152
are located near the center axis of the control rod. Center
portions of the spacers 152 are fixed to a tie cross 123 by means
of pins 129.
[0172] With reference to FIGS. 16A, 16B, and 16C, the wing 124 has
an outer end portion 153 bent at a relatively large angle. The
outer end portion 153 is fixed to the neutron absorbing plate 135
through a welding portion 139 disposed therebetween. A portion of
the wing 124 that is located near the center axis of the control
rod is fixed by means of the pins 129.
[0173] The spacers 152 are made of hafnium so as to have a small
weight when the reactivity worth is primarily desired. The spacers
152 are made of zircaloy when measures against blade history are
primarily desired. The spacers 152 are alternately arranged in the
axial direction of the control rod so as to partially overlap with
each other. This is because the spacers 152 are short and the
bending strength of boundaries between the spacers 152 is prevented
from being reduced. In this embodiment, other components and
advantages are substantially the same as those described in the
tenth or eleventh embodiment.
Thirteenth Embodiment
[0174] FIG. 17 is a vertical sectional view of one of wings 124
included in a control rod according to a thirteenth embodiment of
the present invention. FIG. 18A is a sectional view of the wing 124
taken along the line A4-A4 of FIG. 17. FIG. 18B is a sectional view
of the wing 124 taken along the line B4-B4 of FIG. 17. FIG. 18C is
a sectional view of the wing 124 taken along the line C4-C4 of FIG.
17.
[0175] The control rod of this embodiment has a configuration
similar to that of the control rod of the twelfth embodiment. The
outer surface of the hafnium sheet included in the neutron
absorbing plate 135 of the twelfth embodiment is covered with the
zircaloy sheet. Each neutron absorbing plate 135 which is a
composite absorbing plate and which is included in the control rod
of this thirteenth embodiment includes zircaloy sheets 133 and a
hafnium sheet 134 sandwiched therebetween.
[0176] The configuration of this embodiment is suitable for control
rods in which trap water regions between the absorbing plates 135
can be kept within a desired range and is particularly suitable for
thick control rods. Other components and advantages are
substantially the same as those described in the twelfth
embodiment, and therefore, will not be described.
Fourteenth Embodiment
[0177] FIG. 19 is a vertical sectional view of one of wings 124
included in a control rod 111 according to a fourteenth embodiment
of the present invention. FIG. 20A is a sectional view of the wing
124 taken along the line A5-A5 of FIG. 19. FIG. 20B is a sectional
view of the wing 124 taken along the line B5-B5 of FIG. 19. FIG.
20C is a sectional view of the wing 124 taken along the line C5-C5
of FIG. 19.
[0178] With reference to FIG. 19, a lower end portion of the wing
124 has substantially the same size and configuration of an end
portion thereof. The upper and lower end portions of the wing 124
need not have the same size and configuration and may have
different sizes and configurations. The optimum sizes of the upper
and lower end portions of the wing 124 may be designed.
[0179] With reference to FIGS. 19, 20A, 20B and 20C, the control
rod 111 has a cross shape in cross section and there is no
significant difference in configuration between the control rod 111
of this embodiment and those of the above-mentioned embodiments.
Therefore, FIGS. 19, 20A, 20B and 20C will not be described herein
in detail.
[0180] This embodiment focuses on a method of manufacturing the
control rod 111 and measures against the blade history in view of
the transverse cross-sectional structure of the control rod
111.
[0181] FIG. 21 is a developed view of a hafnium sheet included in
the control rod 111 and shows the configuration of a material 161,
having four parts, for manufacturing the control rod 111. The
material 161 is a composite absorbing plate and includes the
hafnium sheet covered with zircaloy. With reference to FIG. 21, the
material 161 has not been bent to a cross shape and has been
punched.
[0182] The material 161 includes four composite absorbing plate
elements 162a to 162d having the same shape as that of the
neutron-absorbing plates 135 which are shown in FIG. 13D described
with reference to the tenth embodiment and which are composite
absorbing plates. The composite absorbing plate elements 162a to
162d have notches 164 and 165 having the same shape as that of
notches present in the neutron absorbing plate 135 indicated by the
broken line in FIG. 19. The composite absorbing plate elements 162a
to 162d have folding lines 171a to 171h. The material 161 is
valley-folded at a right angle along the folding lines 171a, 171c,
171e and 171g passing through the notches 164 and 165 and is
mountain-folded at a right angle along the folding lines 171b,
171d, 171f and 171h passing through regions between the notches 164
and 165.
[0183] This allows the material 161 to have a wavy shape with
mountain portions and valley portions. The resulting material 161
is further folded, whereby the control rod 111 having a cross shape
in cross section can be formed.
[0184] With reference to FIG. 21, the leftmost and rightmost
composite absorbing plate elements 162a and 162d, respectively,
have different widths. This allows welding portions of the finally
assembled material 161 not to be located at an end portion of one
of the wings 124 but to be located at a flat portion thereof. The
welding portions are welded to each other, whereby workability and
strength properties can be enhanced.
[0185] FIG. 22 shows a principal portion of the material 161 shown
in FIG. 21. With reference to FIG. 22, the material 161, which is
used to manufacture the control rod 111, has a length of about 3.6
m and a width of about 1 m. If the size of the material 161 is too
large to manufacture the control rod 111, the composite absorbing
plates 135 prepared by cutting the material 161 along the line
connecting the center of the line .alpha.-.alpha. and that of the
line .beta.-.beta. in FIG. 21 may be welded into one piece.
[0186] The reason why the material 161 is cut along the line
connecting the center of the line .alpha.-.alpha. and that of the
line .beta.-.beta. is to avoid the welding portions from being
located at mountain- or valley-folded portions of the control rod
111 having a cross shape. The line .alpha.-.beta. is determined on
the basis of the same concept as described above.
[0187] The tailing end of a front end structural member is located
at a position represented by the dotted line present in a leading
end portion of the material 161 and the leading end of a terminal
end structural member is located at a position represented by the
dotted line present in a tailing end portion of the material 161.
The material 161 has a first portion "c", a second portion "d", and
a third portion "e". The first portion "c" extends from the leading
end of the material 161 and has a length equal to about one 24th (
1/24) of the length of the material 161. The second portion "d"
extends from the first portion "c" and has a length equal to the
difference obtained by subtracting the length of the first portion
"c" from one fourth (1/4) of the length of the material 161. The
third portion "e" extends from the tailing end of the material 161.
The first portion "c" has wide notches located at the valley-folded
portions. The second portion "d" has no notch. The third portion
"e" has wide notches located at the valley-folded portions and also
has narrow notches extending from a position apart from the leading
end of the material 161 at a distance equal to one fourth (1/4) of
the length of the material 161 to a position apart from the leading
end of the material 161 at a distance equal to two fourth ( 2/4) of
the length of the material 161.
[0188] The above notches are eliminated from portions that need to
have high reactivity worth but are arranged in portions that may
have slightly low reactivity worth so that the reactivity worth and
measures against blade history are balanced. This concept is
consistent herein.
[0189] The need of the first portion "c" is low in view of
reactivity worth during the shutdown of a nuclear reactor. The
first portion "c" is not provided in the control rod 111 of the
tenth embodiment because the first portion "c" is supposed to
influence scrum properties at the moment of the insertion of the
control rod 111 of the first embodiment when the insertion rate of
the control rod 111 of the first embodiment is not large. Short
rod-shaped portions 166, horizontally extending, surrounded by
dotted lines show a wing-bonding member, that is, a tie cross 123
for keeping a cross shape. The wing-bonding member is not attached
to the material 161 when the material 161 is plate-shaped but is
attached to the material 161 after the material 161 is folded to a
cross shape.
[0190] The method of this embodiment will now be described.
[0191] In a first step, the hafnium sheet covered with zircaloy is
processed in advance such that the material 161 is formed as shown
in FIGS. 21 to 23, the material 161 is mountain-folded at a right
angle along the folding lines 171b, 171d, 171f and 171h passing
through the regions between the notches 164 and 165, and both ends
of the material 161 represented by the line .alpha.-.beta. are
welded to each other, whereby an object, not shown, having a square
shape in cross section is obtained.
[0192] In a second step, the object is valley-folded at a right
angle along the folding lines 171a, 171c, 171e and 171g passing
through the notches 164 and 165. A welding line corresponding to
the line .alpha.-.beta. is located between one of mountain-folded
portions and one of valley-folded portions. This is probably
because metal crystals in the welding portion are reformed by the
welding and the presence of the welding line at the mountain- or
valley-folded portion may cause the deterioration of health due to
irradiation.
[0193] In the second step, the mountain-folded portions are further
folded at an angle of 180 degrees to for in outer end portions of
the wings 124 and the valley-folded portions are further folded at
an angle of 90 degrees to form portions of the wings 124 that are
located near the center axis of the control rod 111. With reference
to FIG. 23, outer rods, that is, short hafnium rods 128 that are
wing end-reinforcing members are pinned to the mountain-folded
portions. The tie crosses 123 are pinned using pairs of holes 174
located between window-shaped holes (longitudinal holes) that are
intermittently arranged in the valley-folded portions in the axial
direction of the control rod 111. The pins used are made of
zircaloy or hafnium. The control rod 111 shown in FIGS. 19 and 20
is obtained.
[0194] The distance between the line .alpha.-.alpha. and line
.beta.-.beta. shown in FIG. 21 need to be 3 m or more, and the line
.alpha.-.alpha. usually have a length of about 1 m. Therefore, the
control rod 111 may be manufactured so as to have a length
exceeding 3 m so that a plurality of neutron absorbing plates 135
are prepared, processed as shown in FIG. 21, and then welded to
each other in the axial direction thereof. In this case, the same
nucleic properties (blade history-improving properties) as
described in the twelfth or thirteenth embodiment can be obtained
in such a manner that holes disposed in valley-folded portions are
varied in size in the axial direction of the control rod 111 (the
holes located closer to the tailing end of the control rod 111 have
larger sizes in the wing end direction). The hafnium sheet may be
replaced with a hafnium-zirconium alloy sheet.
Fifteenth Embodiment
[0195] FIG. 24 is a sectional view of one of wings 124 included in
a control rod according to a fifteenth embodiment of the present
invention. FIG. 25A is a sectional view of the wing 124 taken along
the line A6-A6 of FIG. 24. FIG. 25B is a sectional view of the wing
124 taken along the line B6-B6 of FIG. 24. FIG. 25C is a sectional
view of the wing 124 taken along the line C6-C6 of FIG. 24.
[0196] This embodiment focuses on a method of manufacturing the
control rod and measures against the blade history in view of the
transverse cross-sectional structure of the control rod. The method
of this embodiment is simpler than that described with reference to
FIGS. 19 to 23.
[0197] FIGS. 24 and 25A to 25C are substantially the same as FIGS.
19 and 20A to 20C and therefore will not be described herein.
[0198] In this embodiment, longitudinal holes are intermittently
provided in folded portions, pairs of small holes for fixing a tie
cross 123 are provided between the longitudinal holes, and the
folded portions are valley-folded. Mountain-folded portions are to
be finally formed into the ends of the wings 124. Short hafnium
rods 128 are attached to the mountain-folded portions with pins
129, made of zircaloy or hafnium, inserted in the fitting
holes.
[0199] In this embodiment, the wings 124 usually have a width of 50
cm or less and therefore may be separately manufactured. The wings
124 can be manufactured from a single material if the wings 124
have a length exceeding 3 m. Holes, as well as those described in
the above embodiments, located closer to the tailing end of the
control rod preferably expand toward the wing ends.
Sixteenth Embodiment
[0200] FIG. 26 is a sectional view of one of wings 124 included in
a control rod, according to a sixteenth embodiment of the present
invention, for nuclear reactors. FIG. 27A is a sectional view of
the wing 124 taken along the line A7-A7 of FIG. 26. FIG. 27B is a
sectional view of the wing 124 taken along the line B7-B7 of FIG.
26. FIG. 27C is a sectional view of the wing 124 taken along the
line C7-C7 of FIG. 26.
[0201] This control rod has substantially the same configuration as
that of the control rod of the fifteenth embodiment. In this
embodiment, as shown in FIG. 26, a zircaloy coating 133a is
provided on a surface of each hafnium sheet 134, another surface of
the hafnium sheet 134 opposing to the zircaloy coating 133a is
polished so as to have a reduced effective area, the hafnium sheet
134 is rolled into a cylindrical shape so that the zircaloy coating
133a is located outside, and end portions of the rolled hafnium
sheet 134 are then welded to each other, whereby a cylinder is
formed. The cylinder is pressed into a flattened tube 181 as shown
in this figure.
[0202] Three types of cylinders having different diameters are
prepared and then formed into the flattened tubes 181 as shown in
FIG. 27A, 27B or 27C. A notch fitting over a tie cross 123 is
provided in a portion of the flattened tube 181 located near the
center axis of this control rod. The center axis-side portion of
the flattened tube 181 is fixed to short spacers 152, which is
fixed to the composite absorbing plate.
[0203] The distance from the center axis thereof to a first portion
of each flattened tube 181 is large as shown in FIG. 27A, the first
portion extending from the leading end of the flattened tube 181
and having a length equal to one 24th ( 1/24) of the length of the
flattened tube 181. The distance from the center axis thereof to a
second portion of the flattened tube 181 is also large, the second
portion extending from the tailing end of the flattened tube 181
and having a length equal to one half (1/2) of the length of the
flattened tube 181. The distance from the center axis thereof to a
third portion of the flattened tube 181 is the least as shown in
FIG. 27B, the third portion extending from the first portion and
having a length equal to one fourth (1/4) of the length of the
flattened tube 181. As shown in FIG. 27C, the distance from the
center axis thereof to a fourth portion of the flattened tube 181
is between that shown in FIG. 27A and that shown in FIG. 27B, the
fourth portion extending from the third portion.
[0204] This configuration is determined on the basis of the same
concept as that described in the above embodiments.
Seventeenth Embodiment
[0205] FIG. 28A is a vertical sectional view of a leading portion
(an upper portion) of one of wings 124 included in a control rod,
according to a seventeenth embodiment of the present invention, for
nuclear reactors. FIG. 28B is a sectional view of the wing 124
taken along the line B8-B8 of FIG. 28A. FIG. 29A is a vertical
sectional view of a tailing portion (a lower portion) of the wing
124. FIG. 29B is a sectional view of the wing 124 taken along the
line B9-B9 of FIG. 29A.
[0206] The control rod of this embodiment has substantially the
same configuration as that described in the sixteenth embodiment
except that the distance from the center axis of the control rod to
a leading portion of the composite absorbing plate 135 is different
from that from the center axis thereof to a leading portion
thereof.
[0207] With reference to FIGS. 28A and 28B, in the upper half of
the wing 124, short hafnium rods 137 are fixed to an outer end
portion of the wing 124 by means of the pins 132 located near the
centers of the hafnium rods 137. Short spacers 191 made of hafnium
are fixed to a portion of the wing 124 with the pins 132, the
portion being located near the center axis of the control rod. Each
portion of the tie cross 123 is fixed between the spacers 191
adjacent to each other in the axial direction of the control rod
with one or three of the pins 132.
[0208] In order to solve a problem caused by the difference in the
irradiation growth, one of the pins 132 may be preferably used in
some cases. Three of the pins 132 may be used to solve this problem
if an appropriate clearance is formed.
[0209] With reference to FIGS. 29A and 29B, the control rod has
substantially the same configuration as that described in the
seventh embodiment except that the distance from the center axis of
the control rod to the leading portion of the composite absorbing
plate 135 is different from that from the center axis thereof to
the leading portion thereof. In the lower half of the wing 124, the
short hafnium rods 137 are fixed to an outer end portion of the
wing 124 with the pins 132 located near the centers of the hafnium
rods 137. Short zircaloy spacers 191 are fixed to a portion of the
wing 124 with the pins 132, the portion being located near the
center axis of the control rod. Each portion of the tie cross 123
is fixed between the zircaloy spacers 191 adjacent to each other in
the axial direction of the control rod with one or three of the
pins 132.
[0210] In order to solve a problem caused by the difference in the
irradiation growth, one of the pins 132 may be preferably used in
some cases. Three of the pins 132 can be used to solve this problem
if an appropriate clearance is formed.
* * * * *