U.S. patent application number 12/264604 was filed with the patent office on 2009-03-05 for method, use and device concerning cladding tubes for nuclear fuel and a fuel assembly for a nuclear pressure water reactor.
This patent application is currently assigned to WESTINGHOUSE ELECTRIC SWEDEN AB. Invention is credited to Mats Dahlback, Lars Hallstadius.
Application Number | 20090060115 12/264604 |
Document ID | / |
Family ID | 20289408 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090060115 |
Kind Code |
A1 |
Dahlback; Mats ; et
al. |
March 5, 2009 |
METHOD, USE AND DEVICE CONCERNING CLADDING TUBES FOR NUCLEAR FUEL
AND A FUEL ASSEMBLY FOR A NUCLEAR PRESSURE WATER REACTOR
Abstract
A method of producing a cladding tube for nuclear fuel for a
nuclear pressure water reactor includes forming a tube which at
least principally consists of a cylindrical tube component of a
zirconium-based alloy, where the alloying element, except for
zirconium, which has the highest content in the alloy is niobium,
wherein the niobium content in weight percent is between about 0.5
and about 2.4 and wherein no alloying element, except for zirconium
and niobium, in the alloy, has a content which exceeds about 0.2
weight percent. The cladding tube is then annealed such that the
tube component is partly but not completely recrystallized. The
degree of recrystallization in the tube component is higher than
about 40% and lower than about 95%. A fuel assembly for a nuclear
pressure water reactor also has a plurality of such cladding
tubes.
Inventors: |
Dahlback; Mats; (Vasteras,
SE) ; Hallstadius; Lars; (Vasteras, SE) |
Correspondence
Address: |
MICHAUD-DUFFY GROUP LLP
306 INDUSTRIAL PARK ROAD, SUITE 206
MIDDLETOWN
CT
06457
US
|
Assignee: |
WESTINGHOUSE ELECTRIC SWEDEN
AB
Vasteras
SE
|
Family ID: |
20289408 |
Appl. No.: |
12/264604 |
Filed: |
November 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10533467 |
Apr 29, 2005 |
7473329 |
|
|
12264604 |
|
|
|
|
Current U.S.
Class: |
376/412 ;
29/890.053 |
Current CPC
Class: |
Y02E 30/40 20130101;
G21C 3/07 20130101; C22F 1/186 20130101; C22C 16/00 20130101; Y02E
30/30 20130101; Y10T 29/49391 20150115 |
Class at
Publication: |
376/412 ;
29/890.053 |
International
Class: |
G21C 3/06 20060101
G21C003/06; B23P 15/26 20060101 B23P015/26 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2002 |
SE |
SE0203198.7 |
Oct 30, 2003 |
SE |
PCT/SE2003/001685 |
Claims
1. A method of producing a cladding tube for nuclear fuel for a
nuclear pressure water reactor, which method comprises the
following steps: formation of a tube which at least principally
consists of a cylindrical tube component of a Zr-based alloy, where
the alloying element, except for Zr, which has the highest content
in the alloy is Nb, wherein the Nb content in weight percent is
between about 0.8 and about 1.2 and wherein no alloying element,
except for Zr and Nb, in said alloy, has a content which exceeds
about 0.2 weight percent, and carrying out a final anneal of the
cladding tube at a temperature and during a time such that said
tube component is partly recrystallized but not completely
recrystallized, and wherein said final anneal is carried out such
that the degree of recrystallization in said tube component is
higher than about 40% and lower than about 95%.
2. A method according to claim 1, wherein between said formation
step and said final annealing step, said method includes the steps
of rolling and heat treating said cladding tube.
3. A method according to claim 1, wherein the final anneal is
carried out at a temperature which is lower than 550.degree. C.
4. A method according to claim 1, wherein the final anneal is
carried out at a temperature which is between about 400.degree. C.
and about 540.degree. C.
5. A method according to claim 4, wherein the final anneal is
carried out at a temperature which is between about 450.degree. C.
and about 500.degree. C.
6. A method according to claim 1, wherein the final anneal is
carried out such that the degree of recrystallization in said tube
component is higher than about 60% and lower than about 90%.
7. A method according to claim 1, wherein the final anneal is
carried out during about 1 h to about 6 h.
8. A method according to claim 1, wherein before said final anneal,
the method comprises the following steps: forming a bar of said
Zr-based alloy; heating the bar to between about 900.degree. C. and
about 1300.degree. C. and then quenching the bar; extruding a
billet from the bar after heating to between about 500.degree. C.
and about 900.degree. C.; and cold rolling the billet into a tube
in at least two steps, with heat treatments between them at between
about 550.degree. C. and about 650.degree. C.
9. A method according to claim 1, wherein said alloy contains
between about 800 ppm and about 1700 ppm O.
10. A method according claim 1, wherein said alloy contains between
about 50 ppm and about 600 ppm Fe.
11. A method according claim 1, wherein said alloy in addition to
Zr contains about 0.8 weight percent to about 1.2 weight percent
Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm to about 1700
ppm O, less than about 250 ppm C, less than about 150 ppm Si, less
than about 1000 ppm S and in addition to that only impurities of a
content which does not exceed that which is normally accepted in Zr
or Zr alloys for applications in nuclear reactors.
12. A cladding tube for nuclear fuel for a nuclear pressure water
reactor, comprising a generally cylindrical tube component of a
Zr-based alloy, wherein the alloying element, except for Zr, having
the highest content in the alloy is Nb, wherein the Nb content in
weight percent is between about 0.8 and about 1.2 and wherein no
alloying element, except for Zr and Nb, in said alloy, has a
content which exceeds about 0.2 weight percent, wherein said tube
component has been finally annealed such that it has a structure
that is partly recrystallized but not completely recrystallized and
wherein the degree of recrystallization in said tube component is
higher than about 40% and lower than about 95%.
13. A cladding tube according to claim 12, wherein the degree of
recrystallization in said tube component is higher than about 60%
and lower than about 90%.
14. A cladding tube according to claim 12, wherein said alloy
contains between about 800 ppm and about 1700 ppm O.
15. A cladding tube according to claim 12, wherein said alloy
contains between about 50 ppm and about 600 ppm Fe.
16. A cladding tube according to claim 12, wherein said alloy in
addition to Zr contains about 0.8 weight percent to about 1.2
weight percent Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm
to about 1700 ppm O, less than about 250 ppm C, less than about 150
ppm Si, less than about 1000 ppm S and in addition to that only
impurities of a content which does not exceed that which is
normally accepted in Zr or Zr alloys for applications in nuclear
reactors.
17. A fuel assembly for a nuclear pressure water reactor,
comprising: a plurality of cladding tubes, each having a generally
cylindrical tube component of a Zr-based alloy, wherein the
alloying element, except for Zr, having the highest content in the
alloy is Nb, wherein the Nb content in weight percent is between
about 0.8 and about 1.2 and wherein no alloying element, except for
Zr and Nb, in said alloy, has a content which exceeds about 0.2
weight percent, wherein said tube component has been finally
annealed such that it has a structure that is partly recrystallized
but not completely recrystallized and wherein the degree of
recrystallization in said tube component is higher than about 40%
and lower than about 95%, and wherein each of said cladding tubes
is filled with nuclear fuel suitable for such cladding tubes for a
nuclear pressure water reactor; wherein the fuel assembly also
comprises: a top plate, a bottom plate, a plurality of guide tubes
for control rods, which guide tubes extend between the top plate
and the bottom plate, and a plurality of spacers arranged for
maintaining said cladding tubes in position in the fuel assembly
and at suitable distances from each other.
18. A fuel assembly according to claim 17, wherein the degree of
recrystallization in said tube component is higher than about 60%
and lower than about 90%.
19. A fuel assembly according to claim 17, wherein said alloy
contains between about 800 ppm and about 1700 ppm O.
20. A fuel assembly according to claim 17, wherein said alloy
contains between about 50 ppm and about 600 ppm Fe.
21. A fuel assembly according to claim 17, wherein said alloy in
addition to Zr contains about 0.8 weight percent to about 1.2
weight percent Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm
to about 1700 ppm O, less than about 250 ppm C, less than about 150
ppm Si, less than about 1000 ppm S and in addition to that only
impurities of a content which does not exceed that which is
normally accepted in Zr or Zr alloys for applications in nuclear
reactors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/533,467, filed on Apr. 29, 2005, the
contents of which are incorporated herein by reference in their
entirety. This application is also entitled to the benefit of and
incorporates by reference essential subject matter disclosed in
International Application No. PCT/SE2003/001685 filed on Oct. 30,
2003 and Swedish Patent Application No. 0203198.7 filed on Oct. 30,
2002.
FIELD OF THE INVENTION
[0002] The present invention concerns cladding tubes for nuclear
fuel for a nuclear pressure water reactor. More precisely, the
invention concerns such cladding tubes which are formed of a
Zr-based alloy which contains Nb. The invention concerns, inter
alia, a method. According to the method, a tube is formed which at
least principally consists of a cylindrical tube component of a
Zr-based alloy, where the alloying element, except for Zr, which
has the highest content in the alloy is Nb, wherein the Nb content
in weight percent is between 0.5 and 2.4.
[0003] The invention also concerns a cladding tube as such, a use
of a cladding tube and a fuel assembly for a nuclear pressure water
reactor comprising such a cladding tube.
BACKGROUND OF THE INVENTION
[0004] Methods of the kind which is described above are known. With
such methods, cladding tubes of Zr-based alloys which contain Nb
are thus produced. For example, U.S. Pat. No. 5,648,995 describes
such a method and a cladding tube of this kind.
[0005] When a cladding tube is used in a nuclear reactor, it
contains nuclear fuel, usually in the form of pellets comprising
enriched uranium, usually in the form of UO.sub.2. The cladding
tube with its content thus constitutes a fuel rod. Because of the
very particular environment in which cladding tubes are used,
different requirements must be fulfilled.
[0006] There are mainly two kinds of modern light water reactors:
boiling water reactors (BWR) and pressure water reactors (PWR). In
these kinds of reactors different conditions prevail which call for
different requirements on the parts which form part of the
reactors. In a PWR, the fuel rods are cooled mainly by water that
is in a liquid phase under high pressure. In a BWR, the pressure is
lower and the water which cools the fuel rods is evaporated such
that the fuel rods are surrounded both by water in a liquid phase
and in a steam phase. Furthermore, the fuel assemblies have
different construction in a BWR and a PWR. In one kind of BWR, the
fuel rods in a fuel assembly extend all the way between a top plate
and a bottom plate which keep the fuel assembly together. In a PWR,
on the other hand, the fuel rods are usually held in position with
the help of spacers and do not reach all the way to the top plate
and the bottom plate.
[0007] A fuel rod which is used in a nuclear reactor is exposed to
high temperatures and pressures. Over time thereby creep phenomena
occur. Such a creep should as far as possible be avoided since it
can have negative effects. For example, a creep of the fuel rods
may have as a consequence that they will press against the fuel
pellets which are located therein. The neutron radiation to which a
fuel rod is exposed when it is used may also have as a consequence
that the fuel rod tends to grow with time. Also such a growth
caused by neutron radiation may have undesired effects. It should
therefore be avoided that the cladding tube grows to a larger
extent. Modern fuel rods which are produced in suitable zirconium
alloys and which undergo special heat treatments during the
production often have a relatively low tendency to grow when they
are exposed to neutron radiation. The tendency to grow may be
reduced, inter alia in that the cladding tube during the production
undergoes a final recrystallization anneal.
[0008] Through a suitable choice of the material for the cladding
tube and a suitable method of production, the cladding tube can
obtain suitable properties concerning for example hardness and
ductility.
[0009] In the environment where the cladding tubes are used they
may be subject to different corrosive attacks. These attacks may
come from the outside or from the inside. The attacks from the
inside often have their basis in an influence from the nuclear fuel
material that is located there, so-called pellet-cladding
interaction (PCI). If a crack is formed through the cladding tube
(a so-called primary damage), water may penetrate in through the
crack and spread along the inside of the tube. This may lead to new
corrosive attacks from the inside of the tube, so-called secondary
damages. A cladding tube of zirconium or zirconium-based alloys may
also react with hydrogen such that hydrides are formed in the
cladding tube. These hydrides may be formed from the inside of the
tube, particularly if a crack has been formed such that water has
penetrated into the tube. These hydrides make the tube more fragile
and the probability for the formation of cracks increases.
Particularly hydrides that extend in a radial direction through the
tube constitute an increased risk for crack formation. Such radial
hydrides may therefore speed up possible secondary damages and
crack formations.
[0010] The complicated chemical, mechanical and metallurgical
conditions that are the case in a nuclear reactor have lead to the
fact that a very large number of suggestions have been proposed for
the selection of materials and for the methods of production of
cladding tubes. Even small changes in the composition of alloys or
production parameters may have a large importance for the
properties of the cladding tube.
SUMMARY OF THE INVENTION
[0011] Cladding tubes produced of a Zr-based alloy which contains
Nb has appeared to have good properties in many respects. By
suitable alloying contents (for example such as described in the
above mentioned U.S. Pat. No. 5,648,995) and by a suitable choice
of parameters of production, a cladding tube can be obtained which
has good chemical, mechanical and metallurgical properties. It has
however become clear that also for tubes of this kind there is a
risk of damages.
[0012] An object of the present invention is therefore to achieve a
method of producing a cladding tube, of a Zr-based alloy which
includes between 0.5 weight percent and 2.4 weight percent Nb and
which has an improved resistance against damages than prior
cladding tubes of this kind of alloys.
[0013] These objects are achieved by a method characterized in that
after that the cladding tube has been formed according to the above
and after possible rolling steps with heat treatments between them,
the cladding tube is finally annealed at a temperature and during a
time such that said tube component is partly recrystallized but not
completely recrystallized. The tube component is thus partially
recrystallized (pRXA).
[0014] A cladding tube produced according to this method has
appeared to have a good resistance against damages caused by PCI at
the same time as the risk for the formation of radial hydrides is
low. Thereby, the risk for cracks is reduced. The cladding tube has
at the same time also a high ductility, a low creep rate and a low
tendency to growth caused by neutron radiation. Further objects and
advantages of the invention will become clear from the
following.
[0015] Since the tube component is pRXA (and not completely
recrystallized), it has become clear that hydrides which are formed
tend to extend in mainly a tangential direction while the risk for
radial hydrides is low. Thereby, an improved resistance against
crack formation is obtained. The reason why radial hydrides are
avoided is probably that certain tensions which originate from the
production of the tube are maintained since the tube component is
not completely recrystallized. These tensions have a consequence
that the tendency for radial hydrides is reduced.
[0016] It can be noted that previously known cladding tubes of this
kind of alloys have undergone a final anneal such that the cladding
tube has become completely recrystallized (see for example the
above mentioned U.S. Pat. No. 5,648,995). Such an RXA is
advantageous in certain respects (for acting against creep and
growth caused by neutron radiation and for achieving resistance
against PCI damages). However, the inventors of the present
invention have found that these advantages to a large extent can be
obtained also if the cladding tube is only finally annealed for
achieving pRXA. It has thus thereby become clear that an improved
resistance against damages may be obtained through this final
anneal.
[0017] It should be noted that the final anneal is normally the
last heat treatment step in the method of production. Possibly, a
certain after treatment of the cladding tube may be carried out,
but such an after treatment should be such that the structure which
is obtained through the final anneal is not essentially
destroyed.
[0018] It should also be noted that according to a preferred
embodiment, the cladding tube consists only of said tube component.
There are thus no further layers. The composition of the outer
surface and the inner surface of the tube may however differ from
the composition in the inner of the tube, for example due to the
substances that the tube has come into contact with. The tube may
for example be oxidised through the fact that it has been kept in
an environment of air. According to an alternative embodiment, it
is however feasible that the tube comprises one or more further
protective layers on its inside or its outside. In this case, the
tube thus consists of several components. It is however always the
case that said tube component constitutes the main component of the
tube, for example that this tube component constitutes more than
60% of the thickness of the tube. As has been pointed out above, it
is however preferred that the whole thickness of the tube is made
up of said tube component.
[0019] As used herein % or ppm are used in connection with contents
of different substances, it is, if nothing else is said, referred
to weight percent of the respective substances.
[0020] According to a preferred manner of carrying out the method
according to the invention, the final anneal is carried out such
that the degree of recrystallization in the tube component is
higher than 5% and lower than 95%, preferably higher than 40%, for
example between 60% and 90%. It has become clear that such degrees
of recrystallization are particularly suitable for achieving the
described advantages.
[0021] The temperature and the time that are needed in order to
achieve such a degree of recrystallization depend on the contents
of the alloying elements. The temperature for the final anneal is
preferably lower than 550.degree. C., for example between
400.degree. C. and 540.degree. C., and often most preferred between
450.degree. C. and 500.degree. C. The final anneal may suitably be
carried out during 1 h to 6 h, preferably during 1 to 3 hours.
[0022] According to a preferred manner, the method comprises,
before said final anneal, the following steps: [0023] a bar of said
Zr-based alloy is formed; [0024] this bar is heated to between
900.degree. C. and 1300.degree. C. and is thereafter quenched,
preferably in water; [0025] a billet is extruded from the bar after
heating to between 500.degree. C. and 900.degree. C.; [0026] the
billet is cold rolled to a tube in at least two steps, with heat
treatments between them at between 550.degree. C. and 650.degree.
C.
[0027] Such a method of production is suitable in order to obtain
favourable properties of the cladding tube. It should be noted that
the method of production of course may comprise further steps (for
example further heat treatments or cold rolls) in addition to those
mentioned above.
[0028] According to a preferred manner, the Nb-content in said
alloy is between 0.8 weight percent and 1.2 weight percent.
Preferably, no alloying element, except for Zr and Nb, in said
alloy has a content which exceeds 0.3 weight percent, and
preferably not above 0.2 weight percent.
[0029] The alloy may suitably contain between 800 ppm and 1700 ppm
O. Such a selection of the content of O leads to the fact that the
cladding tube has good creep properties.
[0030] According to an advantageous embodiment, the alloy contains
between 50 ppm and 600 ppm Fe. By keeping the content of Fe low,
the creep properties are further improved. The Fe-content may for
example be lower than 250 ppm. It should be noted that these low
Fe-contents are only preferred embodiments of the invention.
According to another embodiment, also a higher Fe-content may be
permitted. The alloy may also contain a certain amount of S, for
example between 20 ppm 5 and 600 ppm S, or between 100 ppm 5 and
600 ppm S. Such an amount of S can improve the corrosion resistance
of the alloy and the creep properties.
[0031] According to a preferred embodiment, said alloy contains, in
addition to Zr, 0.8 weight percent to 1.2 weight percent Nb, 50 ppm
to 600 ppm Fe, 800 ppm to 1700 ppm O, less than 250 ppm C, less
than 150 ppm Si, less than 1000 ppm S and in addition to that only
impurities of a content which does not exceed that which is
normally accepted in Zr or Zr alloys for applications in nuclear
reactors.
[0032] Examples of what is considered as acceptable impurities in
this context are mentioned for example in the patent document EP 0
674 800 B1, column 5.
[0033] The present invention also resides in a cladding tube
produced according to the method described in any of the preceding
embodiments is used in a fuel assembly for a nuclear pressure water
reactor. Thereby the above described advantages with such a
cladding tube are achieved.
[0034] The invention also concerns a cladding tube as such,
suitable to contain nuclear fuel for a nuclear pressure water
reactor, which cladding tube at least principally consists of a
cylindrical tube component of a Zr-based alloy, where the alloying
element which, except for Zr, has the highest content in the alloy
is Nb, wherein the Nb content in weight percent is between 0.5 and
2.4, wherein said tube component has been finally annealed such
that it has a structure such that it is partly recrystallized but
not completely recrystallized. The degree of recrystallization in
the tube component is higher than 5% and lower than 95%, preferably
higher than 40%, for example between 60% and 90%.
[0035] Such a cladding tube can be produced according to the
above-described method. Advantageous embodiments, for example
concerning included alloying elements and alloying contents, are
clear from the examples above in connection with the method
according to the invention. With these embodiments of the cladding
tube, the above-described advantages are achieved.
[0036] Finally, the invention also concerns a fuel assembly for a
nuclear pressure water reactor. The fuel assembly comprises a
plurality of cladding tubes according to the invention filled with
nuclear fuel suitable for such cladding tubes for a nuclear
pressure water reactor.
SHORT DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows schematically a fuel assembly for a nuclear
pressure water reactor.
[0038] FIG. 2 shows schematically a cross-section through a
cladding tube according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0039] FIG. 1 shows schematically a fuel assembly for a PWR. The
fuel assembly comprises a top plate 4 and a bottom plate 5. Between
the top plate 4 and the bottom plate 5 a plurality of guide tubes 3
for control rods extend. Furthermore, the fuel assembly comprises a
plurality of cladding tubes 1. These cladding tubes 1 thus contain
a nuclear fuel material and are thereby called fuel rods. In this
kind of fuel assembly for PWR, the fuel rods do not reach all the
way to the top plate 4 and to the bottom plate 5. The fuel rods are
kept in position in the fuel assembly with the help of spacers
2.
[0040] FIG. 2 shows schematically a cross-section through a
cladding tube according to the invention. The cross-section shows
the cladding tube strongly enlarged. In reality, the cladding tube
is of a dimension and of a length which are suitable for use in a
PWR. The cladding tube comprises a cylindrical tube component 1. In
the shown case, the cylindrical tube component 1 constitutes the
whole cladding tube. This is the preferred embodiment. As has been
mentioned above, it is however possible that this tube component 1
has one or more protective layers on its inside or outside. The
tube component 1 consists of a Zr-based alloy. This means that the
tube component to the largest extent, always more than 95%,
consists of Zr. According to an embodiment, the tube component 1
contains the following alloying elements: 1% Nb, 1200 ppm O, 200
ppm Fe, less than 200 ppm C, less than 150 ppm Si, less than 1000
ppm S and in addition to that only impurities of a content which
does not exceed that which is normally accepted in Zr or Zr alloys
for applications in nuclear reactors. The cladding tube has been
finally annealed such that the tube component 1 has a structure
such that it is partly recrystallized but not completely
recrystallized. The degree of recrystallization may for example be
about 85%.
[0041] The invention also concerns a method of producing a cladding
tube for nuclear fuel for a nuclear pressure water reactor. The
method according to the invention may be carried out in the
following manner.
[0042] A bar of for example the above-mentioned alloy is formed.
This bar is heated to between 900.degree. C. and 1300.degree. C.
and is thereafter quenched, preferably in water. A billet is
extruded from the bar after heating to between 500.degree. C. and
900.degree. C. The billet is cold rolled to a tube in at least two
steps (for example in three steps), with heat treatments between
them at between 550.degree. C. and 650.degree. C. The tube is final
annealed at a temperature and during a time such that the tube
component is partly recrystallized but not completely
recrystallized. The final anneal may for example be carried out at
a temperature of about 490.degree. C. during about two hours. The
final anneal is carried out such that a suitable degree of
recrystallization is obtained in the tube. This degree of
recrystallization ought to be higher than 5% and lower than 95%. A
degree of recrystallization of above 40%, for example between 60%
and 90% can be suitable, for example a degree of recrystallization
of about 85%.
[0043] A cladding tube produced according to the method may
suitably be used in a fuel assembly in a nuclear PWR.
[0044] When a fuel assembly of for example the above-described kind
is supplied with a plurality of cladding tubes according to the
invention, a fuel assembly according to the invention is thus
obtained.
[0045] The invention is not limited to the above given examples but
may be varied within the scope of the following claims.
* * * * *