U.S. patent application number 16/489693 was filed with the patent office on 2020-04-16 for multi-layered nuclear fuel cladding and method for manufacturing multi-layered nuclear fuel cladding.
The applicant listed for this patent is GACHON UNIVERSITY OF INDUSTRY-ACADEMIC COOPERATION FOUDATION. Invention is credited to In Yea KIM, Kang Soo LEE, Sung Pil WOO, Young Soo YOON.
Application Number | 20200118697 16/489693 |
Document ID | / |
Family ID | 63857136 |
Filed Date | 2020-04-16 |
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United States Patent
Application |
20200118697 |
Kind Code |
A1 |
YOON; Young Soo ; et
al. |
April 16, 2020 |
MULTI-LAYERED NUCLEAR FUEL CLADDING AND METHOD FOR MANUFACTURING
MULTI-LAYERED NUCLEAR FUEL CLADDING
Abstract
Multi-layered nuclear fuel cladding, according to the present
invention, comprises: an inner tube of zirconium alloy, of which
both ends are open for providing an accommodation space into which
a sintered nuclear fuel pellet is inserted; and an outer tube,
disposed coaxially with the inner tube, having a greater diameter
than the inner tube so as to surround the outer surface of the
inner tube, wherein the outer tube and the inner tube are fixed to
closely contact each other, and may be formed from metals different
from each other.
Inventors: |
YOON; Young Soo; (Seoul,
KR) ; LEE; Kang Soo; (Seongnam-Si, Gyeonggi-Do,
KR) ; WOO; Sung Pil; (Ulsan, KR) ; KIM; In
Yea; (Guri-Si, Gyeonggi-Do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GACHON UNIVERSITY OF INDUSTRY-ACADEMIC COOPERATION
FOUDATION |
Seongnam-Si, Gyeonggi-do |
|
KR |
|
|
Family ID: |
63857136 |
Appl. No.: |
16/489693 |
Filed: |
April 17, 2018 |
PCT Filed: |
April 17, 2018 |
PCT NO: |
PCT/KR2018/004435 |
371 Date: |
August 28, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 30/40 20130101;
G21C 3/20 20130101; C22C 16/00 20130101; G21C 3/07 20130101 |
International
Class: |
G21C 3/20 20060101
G21C003/20; C22C 16/00 20060101 C22C016/00; G21C 3/07 20060101
G21C003/07 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 18, 2017 |
KR |
10-2017-0049971 |
Claims
1. A multi-layered nuclear fuel cladding comprising: an inner tube
of zirconium alloy, of which both ends are open for providing an
accommodation space into which a sintered body of nuclear fuel is
inserted; and an outer tube, disposed coaxially with the inner
tube, having a greater diameter than the inner tube so as to
surround an outer surface of the inner tube, wherein the outer tube
and the inner tube are fixed to closely contact each other, and are
formed from metals different from each other.
2. The multi-layered nuclear fuel cladding of claim 1, wherein the
metal configured to form the outer tube has a thermal expansion
coefficient of 1 ppm/K to 40 ppm/K.
3. The multi-layered nuclear fuel cladding of claim 1, wherein the
metal configured to form the outer tube has a thermal neutron
absorption cross-sectional area of 0.0045 barn to 440 barn.
4. The multi-layered nuclear fuel cladding of claim 1, wherein the
outer tube has greater ductility than the inner tube.
5. The multi-layered nuclear fuel cladding of claim 1, wherein the
outer tube comprises a protective layer provided on an outer
surface of the outer tube.
6. The multi-layered nuclear fuel cladding of claim 5, wherein the
protective layer is a metal oxide or a metal nitride of the metal
configured to form the outer tube.
7. A method for manufacturing a multi-layered nuclear fuel cladding
comprising: forming a preliminary cladding by inserting a
preliminary inner tube of zirconium alloy provided with an
accommodation space into which a sintered body of nuclear fuel is
inserted, into a preliminary outer tube having a greater diameter
than the preliminary inner tube and filling the preliminary inner
tube with a filler; closing both ends which are open in the
preliminary cladding, with shielding members; and applying a
pressure from an outside toward an inside of the preliminary
cladding to reduce a diameter of the preliminary cladding, wherein
the preliminary outer tube and the preliminary inner tube are
formed from metals different from each other.
8. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, wherein a metal configured to form the
preliminary outer tube has a thermal neutron absorption
cross-sectional area of 0.0045 barn to 440 barn.
9. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, wherein the metal configured to form the
preliminary outer tube has a thermal expansion coefficient of 1
ppm/K to 40 ppm/K.
10. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, wherein the preliminary outer tube has greater
ductility than the preliminary inner tube.
11. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, further comprising dissolving the filler
inside the preliminary inner tube, after the applying of the
pressure.
12. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 11, further comprising forming a protective layer
on an outer surface of an outer tube manufactured by closely
contacting and fixing the preliminary outer tube and the
preliminary inner tube, after the dissolving of the filler.
13. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 12, wherein the protective layer is formed by
oxidation or nitrification of a metal configured to form the outer
tube by plasma surface treatment.
14. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, wherein in the applying of the pressure, the
pressure applied to the inside of the preliminary cladding
gradually increases, by moving the preliminary cladding between a
plurality of pairs of rolls which are arranged to gradually
decrease distances between rolls of each of the pairs, each of the
plurality of pairs of rolls are respectively comprised in each of a
plurality of roll units which are arranged to be spaced apart from
each other in a lengthwise direction of the preliminary
cladding.
15. The method for manufacturing a multi-layered nuclear fuel
cladding of claim 7, wherein in the applying of the pressure, the
preliminary outer tube is mutually closely contacted and fixed, to
the preliminary inner tube, by being more contracted than the
preliminary inner tube.
Description
TECHNICAL FIELD
[0001] The present disclosure herein relates to a multi-layered
nuclear fuel cladding and a method for manufacturing the
multi-layered nuclear fuel cladding, and more particularly, to a
multi-layered nuclear fuel cladding and a method for manufacturing
the multi-layered nuclear fuel cladding, in which an inner tube and
an outer tube are formed from metals different from each other.
BACKGROUND ART
[0002] In general, nuclear power plants, where steam is generated
by heat generated by nuclear fission, and electrical energy is
produced by operating a turbine using the force of generated steam,
are each constituted by several folds of protective walls in order
to prevent leakage of radioactive materials and protect the safety
of the nuclear power plants. Among these, a nuclear fuel cladding,
which is a second protective wall, encloses a sintered body of
nuclear fuel and functions to isolate first-system cooling water
circulating a nuclear reactor from nuclear fuel causing nuclear
fission, prevent nuclear fission products generated during a
nuclear fission reaction from moving to the first-system cooling
water, and effectively transfer the heat generated by nuclear
fission to the first-system cooling water.
[0003] A great amount of energy is generated by a nuclear fission
chain reaction of nuclear fuel at the reactor core of the nuclear
reactor equipped with nuclear fuel claddings therein, and thus, the
first-system cooling water circulating through the nuclear reactor
receives thermal energy and not only functions to vaporize water in
a second system, but also should cool down the heat generated by
nuclear fission reaction at the reactor core of the nuclear reactor
while circulating inside the nuclear reactor.
[0004] However, as the Fukushima nuclear power plant was exploded
due to a stop in cooling water circulation caused by cutoff of
power supply, when cooling water is not smoothly supplied due to
reasons such as failure of cooling system which should cool down
huge heat generated by nuclear fission, there is a problem in that
not only the internal temperature of a nuclear reactor pressure
vessel accommodating the reactor core of the nuclear reactor
rapidly rises, but also a rapid oxidation reaction occurs while the
material constituting a nuclear fuel cladding reacts with
high-temperature steam.
[0005] When the material constituting the nuclear fuel cladding
reacts with high-temperature steam (H.sub.2O) and is oxidized due
to loss in a cooling function, a large amount of hydrogen is
generated, and the generated hydrogen is concentrated in an upper
portion of the nuclear reactor pressure vessel, and when the
hydrogen is accumulated to raise the hydrogen concentration in the
upper portion of the nuclear reactor pressure vessel, a big problem
occurs in which the hydrogen reacts with oxygen and hydrogen
explosion is caused. In the Fukushima nuclear disaster, hydrogen
was generated in nuclear fuel claddings due to a stop in cooling
water circulation, and the hydrogen concentrated in an upper
portion of a nuclear reactor building reacts with air and causes
hydrogen explosion.
[0006] Moreover, radioactive materials, which are nuclear fission
products, may leak to the outside due to high-temperature oxidation
and corrosion, and the replacement term of the nuclear fuel
cladding is reduced and may thereby cause economic loss.
Furthermore, embrittlement phenomena of the nuclear fuel cladding
caused by rapid oxidation reactions may degrade the ductility of
materials and threaten the safety of a nuclear fuel assembly.
[0007] (Patent document 1) Korean Patent No. 10-0963472
Technical Problem
[0008] The present disclosure provides a multi-layered nuclear fuel
cladding manufactured from metals different from each other and a
method for manufacturing the multi-layered nuclear fuel
cladding.
Technical Solution
[0009] In accordance with an exemplary embodiment, a multi-layered
nuclear fuel cladding includes: an inner tube of zirconium alloy,
of which both ends are open for providing an accommodation space
into which a sintered body of nuclear fuel is inserted; and an
outer tube, disposed coaxially with the inner tube, having a
greater diameter than the inner tube so as to surround an outer
surface of the inner tube, wherein the outer tube and the inner
tube are fixed to closely contact each other, and may be formed
from metals different from each other.
[0010] The metal that forms the outer tube may have a thermal
expansion coefficient of 1 ppm/K to 40 ppm/K.
[0011] The metal that forms the outer tube may have a thermal
neutron absorption cross-sectional area of 0.0045 barn to 440
barn.
[0012] The outer tube may have greater ductility than the inner
tube.
[0013] The outer tube may include a protective layer provided on an
outer surface of the outer tube.
[0014] The protective layer may be a metal oxide or a metal nitride
of the metal configured to form the outer tuber.
[0015] In accordance with another exemplary embodiment, a method
for manufacturing a multi-layered nuclear fuel cladding includes:
forming a preliminary cladding by inserting a preliminary inner
tube of zirconium alloy provided with an accommodation space into
which a sintered body of nuclear fuel is inserted, into a
preliminary outer tube having a greater diameter than the
preliminary inner tube and filling the preliminary inner tube with
a filler; closing both ends which are open in the preliminary
cladding, with shielding members; and applying a pressure from an
outside toward an inside of the preliminary cladding to reduce a
diameter of the preliminary cladding, wherein the preliminary outer
tube and the preliminary inner tube are formed from metals
different from each other.
[0016] The metal that forms the preliminary outer tube may have a
thermal neutron absorption cross-sectional area of 0.0045 barn to
440 barn.
[0017] The metal that forms the preliminary outer tube may have a
thermal expansion coefficient of 1 ppm/K to 40 ppm/K.
[0018] The preliminary outer tube may have greater ductility than
the preliminary inner tube.
[0019] The method for manufacturing a multi-layered nuclear fuel
cladding may further include dissolving the filler inside the
preliminary inner tube, after the applying of the pressure.
[0020] The method for manufacturing a multi-layered nuclear fuel
cladding may further include forming a protective layer on an outer
surface of the outer tube manufactured by closely and contacting
and fixing the preliminary outer tube and the preliminary inner
tube, after the dissolving of the filler.
[0021] The protective layer may be plasma surface treated to be
formed by oxidizing or nitrifying a metal configured to form the
outer tube.
[0022] In the applying of the pressure, the pressure applied to the
inside of the preliminary cladding may gradually increase, by
moving the preliminary cladding between a plurality of pairs of
rolls which are arranged to gradually decrease distances between
rolls of each of the pairs, each of the plurality of pairs of rolls
are respectively comprised in each of a plurality of roll units
which are arranged to be spaced apart from each other in a
lengthwise direction of the preliminary cladding.
[0023] In the applying of the pressure, the preliminary outer tube
may be mutually closely contacted and fixed, to the preliminary
inner tube, by being more contracted than the preliminary inner
tube.
Advantageous Effects
[0024] According to an exemplary embodiment, the mechanical
strength of a multi-layered nuclear fuel cladding may be improved
by providing an inner tube made of a zirconium alloy having
superior mechanical property, and the risk of hydrogen explosion
accident may be reduced by providing, on an outside, an outer tube
made of metal different from the zirconium alloy to prevent
oxidation of zirconium even when an accident occurs in which
cooling water cannot be supplied to a nuclear reactor due to a
failure of a cooling system or the like.
[0025] In addition, the outer tube may have further superior
oxidation resistance by forming a protective layer on an outer
surface of the outer tube, and since the progress of the surface
oxidation may be prevented even a temperature rapidly increases
under a nuclear power accident atmosphere, it is possible to
effectively prevent water molecules from being dispersed up to the
inner tube and oxidized. In addition, a stable operation of nuclear
power may be enabled by enhancing the stability of nuclear fuel by
preventing leak of nuclear fuel caused by a high-temperature
oxidation reaction, and economical loss may be prevented by
extending the replacement period of a nuclear cladding.
[0026] Furthermore, a clearance created between the inner tube and
the outer tube under a high-temperature atmosphere may not be
formed by forming the outer tube using a metal having a similar
thermal expansion coefficient to the thermal expansion coefficient
of the inner tube made of a zirconium alloy, and thus, thermal
energy generated from nuclear fuel may further smoothly be
transferred to a first system cooling water, and thus may lower the
temperature of the nuclear fuel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a perspective view illustrating a multi-layered
nuclear fuel cladding in accordance with an exemplary
embodiment.
[0028] FIG. 2 is a perspective view illustrating a multi-layered
nuclear fuel cladding having a protective layer formed thereon in
accordance with an exemplary embodiment.
[0029] FIG. 3 is a flowchart illustrating a method for
manufacturing a multi-layered nuclear fuel cladding in accordance
with another exemplary embodiment.
[0030] FIG. 4 is a perspective view illustrating a step for
manufacturing a preliminary cladding for performing a tube diameter
reduction process in accordance with another exemplary
embodiment.
[0031] FIG. 5 is a cross-sectional view illustrating the tube
diameter reduction process and a change in an inner diameter of the
preliminary cladding in accordance with another exemplary
embodiment.
[0032] FIG. 6 is a perspective view illustrating a step for
dissolving a filler after the tube diameter reduction process in
accordance with another exemplary embodiment.
DETAILED DESCRIPTION
[0033] Hereinafter, exemplary embodiments will be described in
detail with reference to the accompanying drawings. The present
disclosure may, however, be embodied in different forms and should
not be construed as limited to the embodiments set forth herein.
Rather, these embodiments are provided so that this disclosure will
be thorough and complete, and will fully convey the scope of the
present disclosure to those skilled in the art. The drawings may be
exaggerated to describe the present disclosure in detail, and like
numeral references refer to like elements in the drawings.
[0034] FIG. 1 is a perspective view illustrating a multi-layered
nuclear fuel cladding in accordance with an exemplary embodiment,
and FIG. 2 is a perspective view illustrating a multi-layered
nuclear fuel cladding having a protective layer formed thereon in
accordance with the exemplary embodiment. Here, (a) of FIG. 1 is an
exploded perspective view of a multi-layered nuclear fuel cladding,
and (b) of FIG. 1 is a perspective view of a multi-layered nuclear
fuel cladding.
[0035] Referring to FIGS. 1 and 2, a multi-layered nuclear fuel
cladding 100 in accordance with the exemplary embodiment may
include: an inner tube 110 of zirconium alloy, of which both ends
are open for providing an accommodation space 115 into which a
sintered body of nuclear fuel is inserted; and an outer tube 120
which is coaxially arranged with the inner tube 110 and has a
greater diameter than the inner tube 110 so as to surround an outer
surface of the inner tube 110, wherein the outer tube 120 and the
inner tube 110 may be fixed to be closely contact each other, and
be formed from metals different from each other.
[0036] The inner tube 110 of zirconium alloy may be arranged so as
to be inserted into an accommodation space 125 of the outer tube
120 to be described later, and have a hollow cylindrical shape
which has two ends penetrated in one direction and has the
accommodation space 115 (or hollow section) so that a sintered body
of nuclear fuel is inserted thereinto.
[0037] Since the inner tube 110 is inserted into the accommodation
space 125 of the outer tube 120 and arranged to be in close contact
thereto, the outer diameter of the inner tube 110 may be formed to
be smaller than the inner diameter of the outer tube 120. That is,
the outer diameter of the inner tube 110 may be smaller than the
inner diameter of the outer tube 120 so that the inner tube 110 is
inserted into the accommodation space 125 of the outer tube 120,
and the inner tube 110 of zirconium alloy inserted into the
accommodation space 125 of the outer tube 120 may be arranged
innermost in the multi-layered nuclear fuel cladding 100.
[0038] The sintered body of nuclear fuel to be inserted into the
accommodation space 115 of the inner tube 110 is a cylindrical
nuclear fuel pellet made of ceramic by compressing and sintering
nuclear fuel materials such as uranium dioxide. A material which
causes nuclear fission and radiates energy is referred to as a
nuclear fuel. The nuclear fuel may be composed of elements such as
uranium (atomic number 235 and 238) and plutonium (atomic number
239) which absorbs thermal neutrons or high-speed neutrons and
generates energy during nuclear fission.
[0039] Here, in order to use uranium present in nature as a nuclear
fuel charged in a nuclear reactor, the uranium should be processed
into a sintered body having proper shape and characteristic, and
therefore the uranium may be converted into uranium dioxide powder,
which is a final chemical form to be used as a nuclear fuel,
through a concentration process and a chemical process, and the
converted powder-shaped uranium dioxide may be formed in a
cylindrical shape, processed at high temperature, and be
manufactured into a ceramic sintered body of nuclear fuel.
[0040] The cylindrical sintered body of nuclear fuel manufactured
as such may be inserted into a nuclear fuel cladding, that is, into
the accommodation space 115 of the inner tube 110 in accordance
with the exemplary embodiment, and both ends of the nuclear fuel
cladding may be welded and sealed with a cylindrical plug to make a
nuclear fuel rod. Since the final shape of the nuclear fuel charged
in the nuclear reactor is a bundle-shaped nuclear fuel assembly,
several above-mentioned nuclear fuel rods may be bundled and formed
into the bundle-shaped nuclear fuel assembly. Finally, nuclear
fission starts when the nuclear fuel assembly is charged into
cooling water of the nuclear reactor, and thermal energy is
radiated. Here, a great amount of thermal energy radiated by
nuclear fission chain reactions is transferred to cooling water
outside the nuclear fuel cladding.
[0041] As such, the nuclear fuel cladding is a very important
component which functions as a protective wall so as to protect
nuclear fuel to safely generate a nuclear fission reaction and to
prevent radioactive materials from leaking to the outside, and
should have superior mechanical characteristics and a small thermal
neutron absorption cross-sectional area in order to be used for
long time.
[0042] In the exemplary embodiment, since the zirconium
constituting the inner tube 110 has a very small thermal neutron
absorption cross-sectional area of 0.18 barn, the probability that
thermal neutrons which cause nuclear fission are absorbed to a
nuclear fuel cladding is reduced. Thus, a nuclear fission reaction
may effectively be generated, and thermal energy may be generated
due to nuclear fission, and thus, the output of nuclear fuel may be
increased. In addition, zirconium has not only a small thermal
neutron absorption cross-sectional area, but also superior
mechanical strength, and therefore may also be suitable for being
used as a material for a nuclear fuel cladding. At this point, when
manufacturing the inner tube 110 with a zirconium alloy obtained by
appropriately adding a small amount of elements such as tin,
niobium, iron, nickel, chromium, copper, or the like, to the
zirconium the mechanical strength thereof may further be increased,
so that stable operation of nuclear power may become possible.
[0043] However, since a nuclear fuel cladding formed of a zirconium
alloy is in a state of being in contact with water (H.sub.2O) or
steam at a high temperature or a high pressure for a long time, a
severe corrosion occurs in the nuclear fuel cladding, and there is
a problem of being very weak in safety when an accident of losing
cooling function occurs as in Fukushima nuclear disaster in Japan.
That is, when an accident occurs in which it is impossible to
supply cooling water to a nuclear reactor due to a failure of a
cooling system or the like, there is a problem in that oxidation of
zirconium rapidly progresses and the integrity of claddings is
degraded, and that the risk of hydrogen explosion accident
increases due to hydrogen gas generated while zirconium is oxidated
and corroded. Thus, it is impossible to guarantee the safety of
nuclear fuel any more.
[0044] The oxidation reaction of zirconium is expressed by the
following formula.
Zr+2H.sub.2O->ZrO.sub.2+2H.sub.2 <Reaction formula 1>
[0045] Thus, in an accidental situation, a nuclear fuel cladding
formed of zirconium may not only discharge nuclear fuel and the
like to the outside by a high-temperature oxidation reaction, but
also generate hydrogen explosion. Therefore, in order to solve the
problems of the nuclear fuel cladding formed of zirconium alloy and
remarkably enhance the safety of nuclear fuel, the outer tube 120,
formed by using a metal different from the inner tube 110, may be
formed to enclose the inner tube 110 of zirconium alloy.
[0046] The outer tube 120 may have a hollow cylindrical shape,
which has two ends penetrated in one direction and the
accommodation space (or hollow section) 125, and the inner tube 110
may be inserted into the accommodation space 125 formed inside the
outer tube 120.
[0047] The outer tube 120 may be arranged coaxially with the inner
tube 110 so as to prevent leakage of nuclear fuel and hydrogen
explosion due to oxidation of the inner tube 110 caused by loss of
cooling function, and may enclose the outer surface of the inner
tube 110, and since the inner tube 110 is inserted and arranged
into the accommodation space 125 of the outer tube 120, the inner
diameter of the outer tube 120 may be larger than the outer
diameter of the inner tube 110.
[0048] More specifically, the outer tube 120 formed of a metal
different from the inner tube 110 of zirconium alloy, such as a
metal with superior corrosion-resistance, may prevent the
generation of hydrogen caused by a reaction of the inner tube 110
and high-temperature steam under high-temperature and high-pressure
atmosphere due to loss of cooling function, and may protect a
sintered body of nuclear fuel charged in the accommodation space
115 of the inner tube 110 from high-temperature oxidation, and thus
may enhance nuclear power operation safety. In other words, a
hydrogen explosion accident occurring in a nuclear power plant has
a very close relation with the oxidation characteristic of
zirconium. Therefore, when the outer tube 120 is formed by using a
metal different from the inner tube 110 of zirconium alloy so as to
enclose the inner tube 110 from the outside, the outer tube 120 may
improve the oxidation resistance against a loss of cooling water,
and thus may also prevent the inner tube 110 from oxidation and
corrosion.
[0049] Thus, the dual-structure nuclear fuel cladding of the
exemplary embodiment, which takes only strong points of the inner
tube 110 of zirconium alloy and the outer tube 120, may improve
mechanical strength, effectively reduce the accident risk due to
high-temperature oxidation, and protect the inner tube 110 and the
nuclear fuel accommodated in the inner tube 110 from a nuclear
reactor accident, and thus, safe operation of nuclear power is
enabled.
[0050] The outer tube 120 and the inner tube 110 inserted into the
accommodation space 125 of the outer tube 120, which have the above
characteristics, may be closely contacted and fixed each other, and
in the method for manufacturing a nuclear fuel cladding 100 to be
described later, the outer tube 120 and the inner tube 110 may be
closely contacted and fixed each other, by a pressure applied from
the outside toward the inside of a preliminary outer tube 220 and
by a supporting force of a filler 230 which fills the accommodation
space 115 of a preliminary inner tube 210 against external
pressure. That is, the outer tube 120 may have a greater ductility
than the inner tube 110 so that the outer tube 120 may completely
be in close contact with the outer surface of the inner tube 110
during the tube reduction process. Accordingly, the outer tube 120
and the inner tube 110 may have a minimized clearance therebetween
during the tube diameter reduction process and be closely contacted
and fixed each other. This close contact method for outer tube 120
and the inner tube 110 will be described in detail through a method
for manufacturing a multi-layered nuclear fuel cladding to be
described later.
[0051] The metal that forms the outer tube 120 may have a thermal
expansion coefficient of 1 ppm/K to 40 ppm/K.
[0052] The thermal expansion coefficient of zirconium that forms
the inner tube 110 is approximately 3.3 ppm/K to 4.0 ppm/K, and the
thermal expansion coefficient of a metal forming the outer tube 120
that is closely contacted and fixed with the inner tube 110, may be
1 ppm/K to 40 ppm/K similar to that of zirconium.
[0053] When the thermal expansion coefficients of the inner tube
110 and the outer tube 120 have large difference from each other in
a nuclear fuel cladding in which a great amount of thermal energy
is radiated due to a nuclear fission reaction, volume expansion is
caused by the mutually different thermal expansion coefficients and
a separation space or a clearance may be created between the inner
tube 110 and the outer tube 120. That is, when the thermal
expansion coefficient of the outer tube 120 is, for example,
greater than 40 ppm/K which is substantially greater than the
thermal expansion coefficient of approximately 3.3 ppm/K to 4.0
ppm/K of the inner tube 110 of zirconium alloy, the outer tube 120
further expands than the inner tube 110 of zirconium alloy under a
high-temperature atmosphere, and a clearance is created between the
outer surface of the inner tube 110 and the inner surface of the
outer tube 120 due to the difference in the thermal expansion
coefficients of the inner tube 110 and the outer tube 120 which are
closely contacted and fixed each other.
[0054] When a clearance is created between the outer surface of the
inner tube 110 and the inner surface of the outer tube 120, thermal
conductivity is lowered, and therefore there are problems in that
not only the heat generated by nuclear fission may not be quickly
transferred to cooling water through the nuclear fuel cladding, but
also the nuclear fuel accommodated in the accommodation space 115
of the inner tube 110 has a higher temperature than the cooling
water, and thus, safety against accidents of melting of nuclear
fuel and nuclear reactor may not be secured. In addition, there
occurs a problem in that when the operation of a nuclear reactor is
stopped, cracks may occur in the inner tube or the outer tube due
to a temperature change, and a fracture is thereby caused and
radioactive materials included in the nuclear fuel may leak to the
outside.
[0055] Thus, the outer tube 120 is formed by using a metal having a
thermal expansion coefficient of 1 ppm/K to 40 ppm/K that is
similar to the thermal expansion coefficient of the inner tube 110
of zirconium alloy, so as to prevent clearance from being created
between the inner tube 110 and the outer tube 120. Thus, thermal
energy generated in the sintered bodies of nuclear fuel
accommodated in the nuclear fuel cladding, more specifically, in
the accommodation space 115 of the inner tube 110 may further
smoothly be transferred to the outer cooling water of the nuclear
fuel cladding, and accordingly, the output of the nuclear fuel may
be improved and the temperature of the sintered bodies of nuclear
fuel may be lowered.
[0056] The metal that forms the outer tube 120 may have a thermal
neutron absorption cross-sectional area of 0.0045 barn to 440
barn.
[0057] The outer tube 120 may be formed by using a metal having a
thermal neutron absorption cross-sectional area of 0.0045 barn to
440 barn, and the thermal neutron absorption cross-sectional area
means a cross-sectional area with respect to the neutron
absorption. That is, neutrons such as thermal neutrons or
high-speed neutrons do not have an electrical charge and easily
approaches an atomic nucleus and causes various mutual actions.
Among these, a cross-sectional area with respect to an absorption
reaction toward a target atomic nucleus is referred to as a neutron
absorption cross-sectional area. 10.sup.-28 m.sup.2 or barn is used
as a unit of the neutron absorption cross-sectional area.
[0058] First, nuclear fission occurs such that when a neutron is
emitted to a heavy atomic nucleus such as uranium or plutonium, the
atomic nucleus is broken into two pieces while absorbing the neuron
and nuclear fission occurs, and a great amount of thermal energy
and two to three neutrons are output at the time when the atomic
nucleus is broken into two pieces.
[0059] The nuclear fission is expressed by the flowing reaction
formula 2.
atomic nucleus+neutron->nuclear fission products (radioactive
materials)+2-3 neutrons+thermal energy <Reaction formula
2>
[0060] In other words, nuclear fission occurs while the atomic
nucleus and the neutron react, and the 2-3 neutrons radiated during
one nuclear fission lose energy by a moderator and are decelerated.
The neutron (thermal neutron) radiated from the atomic nucleus and
decelerated causes another nuclear fission while colliding with
another atomic nucleus, and a repetitive nuclear fission reaction
(nuclear fission chain reaction) occurs due to such thermal
neutrons, and a great amount of thermal energy is generated in this
process.
[0061] As such, a decelerated thermal neutron should be absorbed to
an atomic nucleus in order to cause a nuclear fission chain
reaction for producing a great amount of energy. When a metal
forming the outer tube 120 has a thermal neutron absorption
cross-sectional area greater than 440 barn, the probability
increases that the thermal neutron which causes nuclear fission is
absorbed to the nuclear fuel cladding and therefore the probability
of occurrence of a nuclear fission reaction decreases, and
accordingly, thermal energy generated by nuclear fission is not
generated, and thus, there is a problem in that the output of
nuclear fuel is lowered. Conversely, when a metal forming the outer
tube 120 has a thermal neutron absorption cross-sectional area
smaller than 0.0045 barn, it may be effective to a nuclear fission
reaction, but there is a problem in that it is difficult to
actually form and manufacture the outer tube 120.
[0062] Thus, the metal that forms the outer tube 120 in accordance
with the exemplary embodiment may have a thermal neutron absorption
cross-sectional area of 0.0045 barn to 440 barn, and metals having
a thermal neutron absorption cross-sectional area of 0.0045 barn to
440 barn may include, for example, Mg (0.059 barn), Si (0.13 barn),
Pb (0.17 barn), Al (0.23 barn), Zn (1.1 barn), Nb (1.1 barn), Ba
(1.2 barn), Sr (1.2 barn), Ge (2.3 barn), Fe (2.4 barn), Mo (2.4
barn), Cr (2.9 barn), Ti (3.3 barn), Cu (3.6 barn), Ni (4.5 barn),
Te (4.5 barn), Sb (6.7 barn), Mn (13 barn), W (19 barn), Ta (21
barn), Co (35 barn), Hf (115 barn), Ir (440 barn), or the like. The
metal that forms the outer tube 120 are not limited to the above
metal, but a metal may form the outer tube 120 as long as the metal
has a thermal neutron absorption cross-sectional area of 0.0045
barn to 440 barn.
[0063] When the outer tube 120 in accordance with the exemplary
embodiment satisfies the thermal expansion coefficient of 1 ppm/K
to 40 ppm/K, the thermal neutron absorption cross-sectional area of
0.0045 barn to 440 barn, and the higher ductility than zirconium, a
very superior effect may be exhibited as a nuclear fuel cladding,
but required characteristics may selectively be applied to the
outer tube 120 if necessary even without satisfying the three
conditions.
[0064] In another exemplary embodiment, the outer tube 120 may
include a protective layer 121 provided to the outer surface of the
outer tube 120 in order to have further superior oxidation
resistance, and the protective layer 121 may be a metal oxide or a
metal nitride of a metal that forms the outer tube 120.
[0065] The protective layer 121 may be a metal that forms the outer
tube 120, for example, an aluminum oxide that is a metal oxide of
aluminum or an aluminum nitride that is a metal nitride of
aluminum, and the protective layer formed to be dense and thin on
the outer surface of the outer tube 120 may further improve the
oxidation resistance of the outer tube 120 and cause the outer tube
to have very superior corrosion resistance.
[0066] At this point, in order to prevent water molecules under an
accident atmosphere or a high-temperature steam (H.sub.2O)
atmosphere from passing through the outer tube 120, being
introduced up to the outer surface of the inner tube 110, and
reacting with zirconium of the inner tube 110, the protective layer
121 formed of a metal oxide may be more effective, but the
protective layer 121 formed of a metal nitride may also be densely
formed on the outer surface of the outer tube 120 and have a
superior high-hardness characteristic, and thus, stable operation
of nuclear power may be enabled.
[0067] In another exemplary embodiment, an aluminum oxide
protective layer 121 formed by oxidation of aluminum that forms the
outer tube 120 is formed in a thin and dense form on the outer
surface of the outer tube formed of aluminum. Therefore, it is
possible to limit the progress of oxidation to the surface of the
outer tube 120 and prevent water molecules from passing through the
outer tube 120 and being inserted up to the outer surface of the
inner tube 110, and accordingly, it is possible to suppress an
oxidation reaction in which zirconium in the inner tube 110 reacts
with water molecules and generates hydrogen, and protects the
sintered bodies of nuclear fuel contained in the accommodation
space 115 of the inner tube 110 from oxidation and corrosion, so as
to enhance the safety of nuclear power operation.
[0068] Thus, the mechanical strength of the nuclear fuel cladding
may be improved by providing the inner tube 110 made of a zirconium
alloy having superior mechanical strength, and may protect the
inner tube 110 and the nuclear fuel by providing the outer tube 120
formed of a metal and a metal oxide different from the zirconium
alloy because the protective layer 121 of a metal oxide film is not
oxidized any more even when an accident such as loss of cooling
materials in a nuclear reactor.
[0069] Hereinafter, the above-mentioned multi-layered nuclear fuel
cladding 100 is defined again, and then a method for manufacturing
the multi-layered nuclear fuel cladding 100 in accordance with the
exemplary embodiment will be described.
[0070] As described above, the multi-layered nuclear fuel cladding
100 may improve the safety of nuclear power operation not only in a
normal operation but also in an occurrence of accident because the
inner tube 110 of zirconium alloy is inserted into the outer tube
120, the inner tube 110 of zirconium alloy may serve a first
protective function of nuclear fuel and improve the mechanical
strength of the multi-layered nuclear fuel cladding 100, and the
outer tube 120 formed of a metal different from the inner tube 110
of zirconium alloy may prevent corrosion of the inner tube 110 from
an accident such as a failure of cooling system.
[0071] FIG. 3 is a flowchart illustrating a method for
manufacturing the multi-layered nuclear fuel cladding in accordance
with another exemplary embodiment, and FIG. 4 is a perspective view
illustrating a step for manufacturing a preliminary cladding for
performing a tube diameter reduction process in accordance with
another exemplary embodiment. In addition, FIG. 5 is a
cross-sectional view illustrating the tube diameter reduction
process and a change in an inner diameter of the preliminary
cladding in accordance with another exemplary embodiment, and FIG.
6 is a perspective view illustrating a step for dissolving a filler
after the tube diameter reduction process in accordance with
another exemplary embodiment.
[0072] Referring to FIGS. 3 to 6, a method for manufacturing the
multi-layered nuclear fuel cladding 100 in accordance with another
exemplary embodiment may include: inserting a preliminary inner
tube 210 of zirconium alloy provided with an accommodation space
into which a sintered body of nuclear fuel is inserted, into a
preliminary outer tube 220 having a greater diameter than the
preliminary inner tube 210, and filling the filler 230 into the
preliminary inner tube 210 to form a preliminary cladding 200;
closing both ends which are open in the preliminary cladding 200,
with shielding members 240; and applying a pressure from the
outside toward the inside of the preliminary cladding 200 and
reducing the diameter of the preliminary cladding 200, wherein the
preliminary outer tube 220 and the preliminary inner tube 210 may
be formed from metals different from each other. In addition,
dissolving the filler 230 inside the preliminary inner tube 210
after the applying a pressure may further be provided.
[0073] First, the preliminary inner tube 210 provided with the
accommodation space, into which a sintered body of nuclear fuel is
inserted, and manufactured by using a zirconium alloy having
superior mechanical strength is prepared, and the preliminary outer
tube 220 capable of improving the corrosion resistance of the
nuclear fuel cladding is prepared (S100). In the exemplary
embodiment, a zirconium alloy is used for the preliminary inner
tube 210, and a metal having a thermal neutron absorption
cross-sectional area of 0.0045 barn to 440 barn and a thermal
expansion coefficient of 1 ppm/K to 40 ppm/K may be used for the
preliminary outer tube 220. Here, the preliminary outer tube 220
may have greater ductility than the preliminary inner tube 210.
[0074] More specifically, a metal that forms the preliminary outer
tube 220 may have a thermal neutron adsorption cross-sectional area
of approximately 0.0045 barn to approximately 440 barn, and the
thermal neutron adsorption cross-sectional area is a
cross-sectional area with respect to the absorption reaction in
which a thermal neutron is absorbed to a target atomic nucleus.
When the sintered body of nuclear fuel is inserted into the
accommodation space of the preliminary inner tube 210, and a
neutron is emitted to an atomic nucleus of nuclear fuel, for
example, uranium-235 included in the sintered body of nuclear fuel,
nuclear fission occurs in which the atomic nucleus is broken into
two pieces while absorbing the neutron, and 2-3 neutrons are output
together with a great amount of energy while nuclear fission
occurs.
[0075] In other words, nuclear fission occurs while the atomic
nucleus and the neutron react, and the 2-3 neutrons radiated during
one nuclear fission lose energy by a moderator and are decelerated.
The decelerated neutrons (thermal neutrons) cause nuclear fission
again while colliding with another atomic nucleus, and a nuclear
fission chain reaction occurs due to such thermal neutrons.
[0076] As such, a decelerated thermal neutron should be absorbed to
an atomic nucleus in order to cause a nuclear fission chain
reaction for producing a great amount of energy. When a metal
forming the preliminary outer tube 220 has a thermal neutron
absorption cross-sectional area greater than 440 barn, the
probability increases that the thermal neutron which causes nuclear
fission is absorbed to the nuclear fuel cladding and therefore the
probability of occurrence of a nuclear fission reaction decreases,
and accordingly, thermal energy generated by nuclear fission is not
generated, and thus, there is a problem in that the output of
nuclear fuel is lowered. Conversely, when the metal forming the
preliminary outer tube 220 has a thermal neutron absorption
cross-sectional area smaller than 0.0045 barn, it may be effective
to a nuclear fission reaction, but there is a problem in that it is
difficult to actually form and manufacture the preliminary outer
tube 220.
[0077] In addition, the metal that forms the preliminary outer tube
220 may have a thermal expansion coefficient of 1 ppm/K to 40
ppm/K, and the thermal expansion coefficient of the zirconium that
forms the preliminary inner tube 210 is approximately 3.3 ppm/K to
4.0 ppm/K, and the thermal expansion coefficient of the metal that
forms the preliminary outer tube 220 which is closely contacted and
fixed, with the preliminary inner tube 210, through the tube
diameter reduction process to be described later may be 1 ppm/K to
40 ppm/K similar to zirconium.
[0078] When the thermal expansion coefficients of the preliminary
inner tube 210 and the preliminary outer tube 220 are different
from each other in a nuclear fuel cladding in which a great amount
of thermal energy is radiated due to a nuclear fission reaction,
volume expansion is caused by the mutually different thermal
expansion coefficients and a clearance may be created between the
preliminary inner tube 110 and the preliminary outer tube 220.
[0079] When a clearance is created between the outer surface of the
preliminary inner tube 210 and the inner surface of the preliminary
outer tube 220, thermal conductivity is lowered, and therefore
there is a problem in that not only the heat generated by nuclear
fission may not be quickly transferred to cooling water through the
nuclear fuel cladding, but also the sintered body of nuclear fuel
accommodated in the accommodation space of the preliminary inner
tube 210 has a higher temperature than the cooling water, and thus,
safety against an accident of nuclear reactor may not be
secured.
[0080] Thus, creation of clearance between the preliminary inner
tube 210 and the preliminary outer tube 220 is prevented in advance
even under a high-temperature atmosphere caused by nuclear fission
by providing the preliminary outer tube 220 formed of a metal
having a thermal expansion coefficient of 1 ppm/K to 40 ppm/K
similar to the thermal expansion coefficient of the preliminary
inner tube 210 formed of a zirconium alloy, and thus, the output of
nuclear fuel may be improved by transferring, to cooling water, the
thermal energy generated from the sintered body of nuclear fuel
accommodated in the accommodation space of the preliminary inner
tube 210.
[0081] After preparing the preliminary outer tube 220 and the
preliminary inner tube 210 which have the above characteristics,
inserting the preliminary inner tube 210 of zirconium alloy
provided with an accommodation space, into which the sintered body
of nuclear fuel is inserted, into the preliminary outer tube 220
having a greater diameter than the preliminary inner tube 210
(S200), and the preliminary cladding 200 is formed, by filling the
preliminary inner tube 210 with the filler 230 (S300).
[0082] As shown in (a) of FIG. 3, the preliminary inner tube 210
may be inserted into the accommodation space of the preliminary
outer tube 220, and at this point, a predetermined separation gap
is present between the preliminary outer tube 220 and the
preliminary inner tube 210 inserted into the preliminary outer tube
220. That is, since the outer diameter of the preliminary inner
tube 210 should have a diameter of a degree that may be inserted
into the preliminary outer tube 220, the outer diameter of the
preliminary inner tube 210 may be smaller than the diameter of the
accommodation space of the preliminary outer tube 220.
[0083] In prior arts, in order to mitigate the separation between
the plurality of tubes, a method was used in which a mandrel was
inserted, then a pressure was applied from the outside while
drawing to increase close contact force between the plurality of
multiple tubes or to reduce the diameter and stretch the tubes.
However, such a method was performed in a state in which the
mandrel is inserted into the preliminary inner tube 210, and a
problem was caused in which the preliminary inner tube 210 (that
is, the inner tube 110) was damaged or failure is caused due to
friction between the preliminary inner tube 210 and the
mandrel.
[0084] Meanwhile, in the tube diameter reduction process in
accordance with the exemplary embodiment, even when a gap is
present between the preliminary inner tube 210 and the preliminary
outer tube 220, occurrence of defects in the preliminary inner tube
210 is suppressed or prevented, and the preliminary outer tube 220
and the preliminary inner tube 210 may be brought into close
contact with each other. That is, a conventional configuration of
the mandrel is not required, and there is no friction between the
preliminary inner tube 210 and components, so that the preliminary
inner tube 210 may be prevented or suppressed from being torn or
damaged by friction and being defected.
[0085] Accordingly, as shown in (b) and (c) of FIG. 4, the
preliminary cladding 200 may be formed by filling the filler 230
into the preliminary inner tube 210 in order to increase the close
contact force between the preliminary inner tube 210 and the
preliminary outer tube 220, to reduce the diameter, or suppress
deformation and defect occurrence due to a pressure applied for
drawing in the tube reduction process.
[0086] The filler 230 fills the preliminary inner tube 210 in order
to form the preliminary cladding 200, and more specifically, the
filler may function to increase the close contact force between the
preliminary outer tube 220 and the preliminary inner tube 210,
change the diameter of the multi-layered nuclear fuel cladding 100
to be manufactured, and buffer the pressure applied to the
preliminary cladding 200 when performing the tube diameter
reduction process for drawing. That is, the filler 230, as shown in
(b) and (c) of FIG. 4, may fill the accommodation space of the
preliminary inner tube 210, and in the tube reduction process, the
filler may provide a resistive force against the pressure applied
to the inside of the preliminary cladding 200 from the outside of
the preliminary cladding 200.
[0087] Fine powder which has water solubility and dissolvability to
be easily dissolvable in a chemical solution may be used as the
filler 230 for reducing the occurrence of defect in the preliminary
inner tube 210 compared to a solid of larger lump when filling the
accommodation space of the preliminary inner tube 210. The fine
powder may enable a dissolving solution, such as an aqueous
solution or a chemical solution, to evenly penetrate into the
filler 230 inside the preliminary inner tube 210 when dissolving
the filler 230 of the preliminary cladding 200 after completing the
tube reduction process.
[0088] As the sequence of FIG. 5, when the preliminary cladding 200
is formed, the tube diameter reduction process for manufacturing
the multi-layered nuclear fuel cladding 100 may be started so as to
increase the close contact force between the preliminary outer tube
220 and the preliminary inner tube 210 of the preliminary cladding
200, and have a desired shape, diameter, and length (S400).
[0089] At this point, for the tube reduction process, both ends
which are open in the preliminary cladding 200 may be closed by
shielding members 240 (S410).
[0090] Closing of both ends of the preliminary cladding 200 is
performed by shielding exposed portions of both ends of the
preliminary outer tube 220 and the preliminary inner tube 210 by
the shielding members 240, and in this step, after inserting the
preliminary inner tube 210 into the preliminary outer tube 220, the
exposed portion of one end among both open ends of the preliminary
outer tube 220 and the preliminary inner tube 210 is first covered
with the shielding member 240, then the preliminary inner tube 210
is filled with the filler 230, and the exposed portion of the other
end may be closed by being covered with the shielding member
240.
[0091] As shown in FIG. 5, the shielding members 240 may close both
ends of the preliminary cladding 200 by covering the exposed
portions of both the end portions thereof, and by shielding, with
the shielding members 240, the exposed portions of the preliminary
outer tube 220 and the preliminary inner tube 210 of the
preliminary cladding 200, the filler 230 inside the preliminary
inner tube 210 does not leak to the outside, and a stable tube
diameter reduction process may be performed. Various members
capable of covering the exposed portions of both ends may be used
as the shielding member 240.
[0092] After closing both open ends of the preliminary claddings
200 with the shielding members 240, the diameter of the preliminary
cladding 200 may be reduced through the tube diameter reduction
process for applying a pressure from the outside toward the inside
of the preliminary cladding 200, and in the applying of the
pressure, the preliminary outer tube 220 may be mutually closely
contacted and fixed, to the preliminary inner tube 210, by being
more contracted than the preliminary inner tube 210 (S420).
[0093] In addition, in the applying the pressure, the pressure
applied to the inside of the preliminary cladding 200 may gradually
be increased, by moving the preliminary cladding 200 between a
plurality of pairs of rolls which have gradually reduced separation
distances between rolls of each of pairs. Here, the respective
plurality of pairs of rolls are respectively comprised in each of a
plurality of roll units 310a, 310b, and 310c that are disposed to
be spaced apart from each other in the lengthwise direction of the
preliminary cladding 200.
[0094] In the preliminary cladding 200, the preliminary outer tube
220 and the preliminary inner tube 210 are closely contacted and
fixed each other, by reducing the separation distance therebetween
through a tube diameter reducing device 300, and the diameter of
the preliminary cladding 200 may be reduced and the length thereof
may be increased through compression and drawing. Here, the tube
diameter reduction process will be described referring to FIG.
5.
[0095] The tube diameter reducing device 300 may cause the length
of the preliminary cladding 200 to be increased and the diameter
thereof to be reduced by compressing the preliminary cladding 200
with rollers from the outside, and may cause the preliminary outer
tube 220 and the preliminary inner tube 210 of the preliminary
cladding 200 to be formed in close contact with each other by
applying a pressure from the outside. At this point, the tube
diameter reducing device 300 may include at least one of roll units
310a, 310b, and 310c which are arranged to be spaced apart from
each other in the lengthwise direction (that is, one direction) of
the preliminary cladding 200. That is, in the exemplary embodiment,
a first roll unit 310a, a second roll unit 310b, and a third roll
unit 310c may be provided so as to be spaced apart from each other
in the lengthwise direction of the preliminary cladding 200. At
this point, each of the roll units 310a, 310b, and 310c is provided
with first rolls 311a, 311b, and 311c and second rolls 312a, 312b,
and 312c which make contact with the outer surface of the
preliminary cladding 200 interposed therebetween, and the
preliminary cladding 200 advances between the first rolls 311a,
311b, and 311c and the second rolls 312a, 312b, and 312c, and thus,
a pressure may be applied from the outside toward the inside of the
preliminary cladding 200. That is, the first rolls 311a, 311b, and
311c and the second rolls 312a, 312b, and 312c may be provided so
as to be in contact with and to face each other on the outer
surface of the preliminary cladding 200 on the basis of the
preliminary cladding 200. When the preliminary cladding 200 passes
between the first rolls 311a, 311b, and 311c and the second rolls
312a, 312b, and 312c, the diameter of the preliminary cladding 200
may be reduced and the preliminary outer tube 220 and the
preliminary inner tube 210 may be brought into close contact with
each other by the pressure applied thereto by coming into contact
with the first rolls 311a, 311b, and 311c and the second rolls
312a, 312b, and 312c.
[0096] Here, as shown in (a) of FIG. 5, points of the preliminary
cladding 200 passing through the plurality of roll units 310a, 310b
and 310c are classified into A, B, and C, and regarding the
diameter and the thickness thereof, as shown in (b) of FIG. 5, it
may be understood that as the number of the roll units 310a, 310b
and 310c through which the preliminary cladding 200 passes
increases, the diameter of the preliminary cladding 200, that is,
the diameter of the accommodation space of the preliminary inner
tube 210 gradually decreases from D1 to D3. As the respective
separation distances between the first rolls 311a, 311b and 311c
and the second rolls 312a, 312b and 312c of each of the plurality
of roll units 310a, 310b and, 310c decrease, the pressure applied
to the preliminary cladding 200 gradually increases, and thus, the
preliminary cladding 200 is deformed.
[0097] In the exemplary embodiment, it has been described that
three roll units 310a, 310b and 310c are provided, and each of the
roll units 310a, 310b, and 310c is configured by two rolls of the
first rolls 311a, 311b, and 311c and the second rolls 312a, 312b,
and 312c, but the number of the rolls and the roll units 310a, 310b
and 310c is not limited thereto, but may be changed into various
numbers. In addition, in (b) of FIG. 5 in the exemplary embodiment,
the diameter of the preliminary cladding 200 is illustrated to be
largely reduced, but this is merely an illustration for explaining
the change in diameter to help understanding the difference.
[0098] Meanwhile, more specifically, referring to (a) of FIG. 5, in
a method for fixing the preliminary outer tube 220 and the
preliminary inner tube 210 to be in close contact with each other
by increasing the close contact force therebetween through the tube
reduction process, the preliminary outer tube 220 move between the
first rolls 311a, 311b, and 311c and the second rolls 312a, 312b,
and 312c, so that a pressure is applied to the inside of the
preliminary cladding 200 by the pressure applied by the rolls. At
this point, the filler 230 present inside the preliminary inner
tube 210 of the preliminary cladding 200 supports the preliminary
inner tube 210 from the pressure applied toward the inside because
the filler 230 fully fills the accommodation space of the
preliminary inner tube 210. That is, explaining relatively, since
the filler 230 has strength that supports the preliminary inner
tube 210 against the pressure applied from the outside, and since
the preliminary inner tube 210 is at a certain position and the
preliminary outer tube 220 is further contracted compared to the
preliminary inner tube 210 due to the external pressure and to be
pushed toward the preliminary inner tube 210, the preliminary outer
tube 220 and the preliminary inner tube 210 may be closely
contacted and fixed each other. That is, the preliminary outer tube
220 may have greater ductility than the preliminary inner tube 210,
that is, the preliminary outer tube 220 may have greater ductility
than the preliminary inner tube 210 so as to be in completely close
contact with the outer surface of the preliminary inner tube 210
while a pressure is applied to the preliminary cladding 200, and
since the preliminary outer tube 220 has sufficient ductility
compared to the preliminary inner tube 210, the gap between the
preliminary outer tube 220 and the preliminary inner tube 210 may
be minimized and the preliminary outer tube 220 may be closely
contacted and fixed, with the preliminary inner tube 210.
[0099] When completing the tube diameter reduction process with the
abovementioned method (S500), as shown in (a) of FIG. 6, both
closed ends of the preliminary cladding 200 is opened (S510), so
that the filler 230 inside the preliminary inner tube 210 may be
dissolved and removed in a state in which both ends of the
preliminary cladding 200 are exposed to the outside (S520). That
is, both ends of the preliminary cladding 200 is opened by removing
the shielding members 240 provided on both ends of the preliminary
cladding 200, and as shown in (b) of FIG. 6, the filler 230 may be
dissolved by being immersed in a solution S accommodated in a
container L.
[0100] Aforementioned solutions (water-soluble solution and
chemical solution) may be used as the solution for dissolving the
filler 230, and in the exemplary embodiment, the filler 230 is
removed by immersing the preliminary cladding 200 in a solution,
but the exemplary embodiment is not limited thereto, and the filler
230 may also be dissolved by pouring the solution into the
accommodation space of the preliminary inner tube 210 and causing
the solution to penetrate between the filler 230.
[0101] A cladding may be obtained in which the close contact force
between the inner tube 110 and the outer tube 120 may be increased,
and the inner tube 110 and the outer tube 120 are closely contacted
and fixed each other, by dissolving and removing the filler 230 in
the preliminary cladding 200 in which preliminary inner tube 210
and the preliminary outer tube 220 are closely contacted and fixed
each other through the tube reduction process.
[0102] After the dissolving of the filler 230, forming the
protective layer 121 on the outer surface of the outer tube 120
manufactured by closely contacting and fixing the preliminary outer
tube 220 and the preliminary inner tube 210 may further be provided
(S600), and the protective layer 121 may be formed by the oxidation
or nitrification of the metal constituting the outer tube 120
through plasma surface treatment.
[0103] Referring again to FIG. 2, after dissolving the filler 230,
the protective layer 121 may be formed on the outer surface of the
cladding manufactured by closely contacting and fixing the
preliminary outer tube 220 and the preliminary inner tube 210 to
each other, and more specifically, on the outer surface of the
outer tube 120. The protective layer 121 may be a metal oxide film
or a metal nitride film, and a general surface treatment method for
forming such the metal oxide film or the metal nitride film may be
performed in a high-temperature process of approximately
600.degree. C. to 1,000.degree. C. However, when the surface
treatment of the outer tube 120 is performed at a high temperature,
a problem is caused such degradation of mechanical characteristics
or chemical characteristics of the cladding, and therefore a
low-temperature process is required which is capable of minimizing
a change in the mechanical property and the crystal structure of
the cladding.
[0104] Thus, in the exemplary embodiment, a plasma surface
treatment method of a low-temperature process was used in order to
form a metal oxide film or a metal nitride film. Since in the
plasma surface treatment, high energy of plasma may effectively
lower the heat treatment temperature of the cladding by 200.degree.
C. to 500.degree. C., the surface treatment of the outer tube 120
may be performed even at a low temperature, and thus, changes in
the mechanical property and the crystal structure of the cladding
may be minimized.
[0105] Accordingly, the protective layer 121 of the exemplary
embodiment may be formed by the plasma surface treatment, and the
metal that forms the outer surface of the outer tube 120 may be
changed into a metal oxide by causing the gas particles (for
example, O.sub.2) under a plasma atmosphere and a metal that forms
the outer tube 120 to react with each other. In other words, the
protective layer 121 made of a thin and dense metal oxide is formed
on the outer surface of a metal that forms the outer tube 120 by
plasma surface treatment of a low-temperature process, and thus,
the mechanical property of the cladding may be maintained and a
more superior corrosion resistance property may be provided to the
outer tube 120 by preventing the contact between the metal that
forms the outer tube 120 and oxygen or steam. Detailed effects
exhibited by forming the protective layer 121 are the same as the
aforementioned effects of the protective layer 121, and thus,
repetitive description (or effect) will not be provided.
[0106] So far, in the detailed description of the present
disclosure, specific exemplary embodiments have been described, but
various modifications can be made thereto without departing from
the spirit and scope of the present disclosure. Therefore, the
scope of the invention is defined not by the detailed description
of the invention but by the appended claims, and all differences
within the scope will be construed as being included in the present
invention.
* * * * *