U.S. patent application number 12/491737 was filed with the patent office on 2009-11-26 for method and apparatus for suppressing corrosion of carbon steel, method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant, and film formation apparatus.
Invention is credited to Hideyuki Hosokawa, IIchiro Kataoka, Satoshi Morisawa, Makoto Nagase, Motoaki Sakashita, Katsuo Yokota.
Application Number | 20090290675 12/491737 |
Document ID | / |
Family ID | 40160492 |
Filed Date | 2009-11-26 |
United States Patent
Application |
20090290675 |
Kind Code |
A1 |
Nagase; Makoto ; et
al. |
November 26, 2009 |
METHOD AND APPARATUS FOR SUPPRESSING CORROSION OF CARBON STEEL,
METHOD FOR SUPPRESSING DEPOSIT OF RADIONUCLIDE ONTO CARBON STEEL
MEMBERS COMPOSING A NUCLEAR POWER PLANT, AND FILM FORMATION
APPARATUS
Abstract
The present invention is a method for suppressing corrosion of
carbon steel members composing a nuclear power plant. That is, the
processing solution contains a chemical including iron (II) ions,
an oxidizing agent for oxidizing at least one part of the iron (II)
ions into iron (III) ion, and a pH adjustment agent for adjusting
pH. The pH of the processing solution is adjusted in the range of
5.5 to 9.0 by the pH adjustment agent. The processing solution is
introduced into a purifying system pipe having the carbon steel
members. The iron (II) ions are adsorbed on an inner surface of the
purifying system pipe, namely, a surface of the carbon steel
members. The ferrite film is formed on the surface of the carbon
steel members by oxidizing the absorbed iron (II) ions. Therefore,
corrosion of the carbon steel members is suppressed by the ferrite
film.
Inventors: |
Nagase; Makoto; (Mito,
JP) ; Hosokawa; Hideyuki; (Hitachinaka, JP) ;
Morisawa; Satoshi; (Hitachi, JP) ; Sakashita;
Motoaki; (Hitachi, JP) ; Yokota; Katsuo;
(Hitachi, JP) ; Kataoka; IIchiro; (Hitachi,
JP) |
Correspondence
Address: |
BRUNDIDGE & STANGER, P.C.
1700 DIAGONAL ROAD, SUITE 330
ALEXANDRIA
VA
22314
US
|
Family ID: |
40160492 |
Appl. No.: |
12/491737 |
Filed: |
June 25, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11769045 |
Jun 27, 2007 |
|
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12491737 |
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Current U.S.
Class: |
376/305 |
Current CPC
Class: |
C23C 22/68 20130101 |
Class at
Publication: |
376/305 |
International
Class: |
C23F 11/00 20060101
C23F011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 2006 |
JP |
2006-001256 |
Jan 19, 2006 |
JP |
2006-011227 |
Claims
1.-3. (canceled)
4. A method for suppressing corrosion of carbon steel, comprising
the steps of: preparing a solution having a first chemical
including iron (II) ions and nickel ions, a second chemical for
oxidizing at least one part of the iron (II) ions to iron (III)
ions, and a third chemical for adjusting the pH of a solution
including the first chemical and the second chemical to be 5.5 to
9.0; and contacting the solution with a surface of carbon steel
members composing a nuclear power plant, in a state of shutdown of
the nuclear power plant, sufficiently for a nickel ferrite film to
form on the surface in the state of the shutdown of the nuclear
power plant.
5. (canceled)
6. The method for suppressing corrosion of carbon steel according
to claim 4, wherein the first chemical is formic acid including the
iron (II) ions and the nickel ions.
7. The method for suppressing corrosion of carbon steel according
to claim 4, wherein the second chemical contains at least one of
hydrogen peroxide solution, oxygen, and ozone.
8. The method for suppressing corrosion of carbon steel according
to claim 4, wherein the third chemical is hydrazine.
9. The method for suppressing corrosion of carbon steel according
to claim 4, wherein the formation of the nickel ferrite film is
performed under a temperature condition from 60.degree. C. to
100.degree. C.
10. The method for suppressing corrosion of carbon steel according
to claim 4, wherein a gas portion of a tank for storing the first
chemical is purged with inert gas.
11. A method for suppressing corrosion of carbon steel, comprising
the steps of: preparing a second chemical that oxidizes at least
one part of iron (II) ions into iron (III) ions, and a third
chemical that contains nickel ions, preparing a solution having a
first chemical including iron (II) ions, the second and third
chemicals, and a fourth chemical for adjusting pH of a solution
including the first chemical, the second chemical and the third
chemical to be 5.5 to 9.0; and contacting the solution with a
surface of carbon steel members composing a nuclear power plant in
a state of shutdown of the nuclear power plant, sufficiently for a
nickel ferrite film to form on the surface in the state of shutdown
of the nuclear power plant.
12. The method for suppressing corrosion of carbon steel according
to claim 11, wherein the first chemical is formic acid including
the iron (II) ions.
13. The method for suppressing corrosion of carbon steel according
to claim 11, wherein the second chemical contains at least one of
hydrogen peroxide solution, oxygen, and ozone.
14. The method for suppressing corrosion of carbon steel according
to claim 11, wherein the fourth chemical is hydrazine.
15. The method for suppressing corrosion of carbon steel according
to claim 11, wherein the formation of the nickel ferrite film is
performed under a temperature condition from 60.degree. C. to
100.degree. C.
16. The method for suppressing corrosion of carbon steel according
to claim 11, wherein the gas portion of a tank for storing the
first chemical is purged with inert gas.
17.-19. (canceled)
20. A method for suppressing deposit of radionuclides onto carbon
steel members composing a nuclear power plant, comprising the steps
of: preparing a first chemical including iron (II) ions and nickel
ions, a second chemical for oxidizing the iron (II) ions into iron
(III) ions, and a third chemical for adjusting pH, in a state of
shutdown of the nuclear power plant; mixing the first chemical and
the second chemical under an ordinary temperature condition of
100.degree. C., in the state of shutdown of the nuclear power
plant; preparing a processing solution whose pH is adjusted in the
range of pH 5.5 to pH 9.0 by mixing the mixture including the first
chemical, the second chemical, and the third chemical in the state
of shutdown of the nuclear power plant; and forming a nickel
ferrite film on the surface of the carbon steel members by using
the processing solution, in the state of the shutdown of the
nuclear power plant.
21. The method for suppressing deposit of radionuclide onto carbon
steel members composing a nuclear power plant according to claim
20, further comprises a step of: removing contaminants including
oxide films from the surface of the carbon steel members before
forming the nickel ferrite film, in the state of the shutdown of
the nuclear power plant.
22. The method for suppressing deposit of radionuclides onto carbon
steel members composing a nuclear power plant according to claim
21, wherein removal of the contaminant is performed by a chemical
decontamination containing at least one reducing removal.
23. The method for suppressing deposit of radionuclides onto carbon
steel members composing a nuclear power plant according to claim
20, wherein the position to inject the third chemical into the
mixture is inside the reactor containment vessel.
24. The method for suppressing deposit of radionuclides onto carbon
steel members composing a nuclear power plant according to claim
20, wherein the first chemical is a formic acid solution including
the iron (II) ions and the nickel ions.
25. The method for suppressing deposit of radionuclides onto the
carbon steel members composing a nuclear power plant according to
claim 21, wherein the formation of the nickel ferrite film is
carried out after a period between the finish of removal of the
contaminants and the start of the operation of the nuclear power
plant.
26. A method for suppressing deposit of radionuclides onto carbon
steel members composing a nuclear power plant, comprising the steps
of preparing a first chemical including iron (II) ions and nickel
ions prepared by dissolving iron and nickel in organic acid, a
second chemical for oxidizing the iron (II) ions into iron (III)
ions, and a third chemical for adjusting pH, in a state of shutdown
of the nuclear power plant; mixing the first chemical and the
second chemical under an ordinary temperature condition of
100.degree. C., in the state of shutdown of the nuclear power
plant; preparing a processing solution whose pH is adjusted in the
range of pH 5.5 to pH 9.0 by mixing the mixture including the first
chemical, the second chemical, and the third chemical, in the state
of shutdown of the nuclear power plant; and forming a nickel
ferrite film on the surface of the carbon steel members by using
the processing solution, in the state of shutdown of the nuclear
power plant.
27. The method for suppressing deposit of radionuclide onto carbon
steel members composing a nuclear power plant according to claim
26, wherein the organic acid is formic acid.
28. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to methods and apparatus for
suppressing corrosion of carbon steel, method for suppressing
deposit of radionuclide onto carbon steel members composing a
nuclear power plant, and film formation apparatus.
[0002] Well-known nuclear power generation plants are, for example,
boiling water type power generation plant (hereinafter referred to
as BWR plant) and pressurized water type power generation plant
(hereinafter referred to as PWR plant). The BWR plant supplies feed
water into with a reactor having a reactor pressure vessel in which
a plurality of fuel assemblies are loaded, through a feed water
system, and causes cooling water (coolant) to boil into steam.
Almost all of the generated steam is supplied to a steam turbine to
drive the steam turbine and a power generator connected to the
turbine in order to generate electric power. The steam exhausted
from the steam turbine is condensed into water by a condenser. The
condensed water is returned as feed water to the reactor by the
feed water system. The PWR plant introduces high-temperature
high-pressure cooling water heated in the reactor into a steam
generator and heats feed water being fed to the steam generator
into steam by the high-temperature high-pressure cooling water. The
steam is supplied to the steam turbine. The low-temperature cooling
water coming from the steam generator is returned to the reactor.
The PWR plant has a secondary system including a steam generator,
steam pipes, feed water pipes, and so on.
[0003] In nuclear power plants such as BWR plant and PWR plant,
wetted surface, which contact with the cooling water, of major
apparatus such as a reactor pressure vessel are usually made of
stainless steel and nickel-based alloy to protect the wetted
surface against corrosions. However, major structure members of the
feed water and condensate systems mainly use carbon steel members
to reduce the plant construction cost and avoid stress corrosion
cracking of stainless steel to be caused by high-temperature water
that flows through the feed water and condensate systems.
[0004] However, the carbon steel members that constitute the feed
water and condensate systems also contain surfaces that are wet
with the cooling water and cannot be free from being corroded when
wet. In the BWR plant arranging the carbon steel members in the
downstream of a purifying apparatus, corrosive products of the
carbon steel members may flow into the reactor and become activated
corrosive products there. Further, the corrosive products may cause
reduction in heat exchange efficiency of the secondary system of
the PWR plant.
[0005] To suppress corrosion of carbon steel members that
constitute the power plant, there have been proposed, for example,
a method for forming an oxide film on the surface of the carbon
steel members by feeding oxygen into the feed water system of the
plant and a method for keeping feed water alkaline (pH of 7 or
greater) by feeding ammonia, hydrazine, or other chemicals into the
feed water system of the plant (for example, Japanese Patent
Laid-open No. 2000-292589).
[0006] In addition to the above consideration, metallic impurities
that slightly generate in the reactor water (that is the cooling
water in the reactor pressure vessel) are also removed positively
by purifying part of the reactor water in a reactor water purifying
apparatus. However, in spite of the above-mentioned corrosion
suppressing measures, traces of metallic impurities inevitably
exist in the reactor water and part of the metallic impurities
deposit as metal oxide on the surfaces of fuel rods in fuel
assemblies. Metal elements on the surfaces of the fuel rods cause
nucleus reaction by irradiation of neutrons emitted from nuclear
fuel in the fuel rods and produce radionuclide such as cobalt 60,
cobalt 58, chromium 51, manganese 54, and so on. Almost all of
these radionuclide remain deposited as oxides on the surfaces of
the fuel rods. However, part of the radionuclide dissolves into the
reactor water under a specific solubility of respective oxides that
contains the radionuclide and finally become insoluble solids
called "crud" in the reactor water. While circulating in the
primary cooling system together with the reactor water, the
radionuclide in the reactor water deposit on the wetted surfaces of
structure members such as stainless steel and Inconel and carbon
steel members of the reactor water purifying system pipes. As the
result, persons who are working on periodic inspection of the
nuclear power generation plant are possibly exposed to radiations
from the surfaces of such carbon steel members. Particularly, an
advanced BWR plant has no re-circulation pipe. Therefore, the
carbon steel pipes of the reactor water purifying system and the
residual heat removal system, and the like greatly affects to
atmosphere dose in the reactor containment vessel. The exposure
dose during working is controlled to be under the specified value
for each worker. Lately, however, this specified value has been
reduced and the exposure dose of each person must be reduced as low
as economically possible.
[0007] So, various methods such as a method of reducing deposition
of radionuclide on the inner surface of carbon steel pipes and a
method of reducing the concentration of radionuclide in the reactor
water, etc. are studied. For example, Japanese Patent Laid-open No.
Sho 58(1983)-79196 discloses a method of suppressing to take
radionuclide such as cobalt 60 and cobalt 58, etc. into oxide films
by injecting metal ions such as zinc into the reactor water and
forming a close oxide film including zinc on the surface, on which
the reactor water is contacted, of the re-circulation system pipes.
Further, Japanese Patent Laid-open No. Hei 9(1997)-166694 discloses
a method of making the reactor water alkaline (pH of 7 or greater)
before radionuclide are dissolved or released into the cooling
water and forming oxide films on the inner surfaces of the
re-circulation system pipes and the reactor water purifying system
pipes through which the reactor water flows during the operation of
the plant.
SUMMARY OF THE INVENTION
[0008] However, a conventional method of injecting oxygen to feed
water of the power generation plant cannot suppress corrosion of
metals when oxygen injection is stopped and must keep on injecting
oxygen during the operation of the power generation plant. This
method runs contrary to the recent plant tendency that keeps the
in-reactor environment in the reducing status in order to suppress
stress corrosion cracking of the stainless steel structure
members.
[0009] Similarly, another conventional method (e.g. Japanese Patent
Laid-open No. 2000-292589) of adding chemicals to feed water of the
nuclear power plant to control pH of the reactor water to greater
than 7 is forced to keep on feeding chemicals during the operation
of the nuclear power plant. Further, since the added chemicals
increase the load of the condensate purifying apparatus, the
radioactive wastes from the condensate purifying apparatus may
increase. Accordingly, it is desired to suppress corrosion of
carbon steel members that constitute the nuclear power plant.
[0010] The method of Japanese Patent Laid-open No. Sho
58(1983)-79196 that injects the metal ions such as zinc into the
reactor water has problems that injection of zinc ions must be
always continued to the reactor water during the operation of the
nuclear power plant, that depleted zinc must be used to avoid zinc
itself being activated, and that these requirements push up the
power generation cost.
[0011] The method disclosed in Japanese Patent Laid-open No. Hei
9(1997)-166694 for forming oxide films forms oxide films on
surfaces of the structure members in the operating temperature
range (250.degree. C. to 300.degree. C.) of the BWR. This method
cannot be formed oxide films on low-temperature surfaces of the
reactor water purifying system and the carbon steel pipes of the
residual heat removal system except for the high-temperature
surface of the reactor water purifying system. Further, just after
the nuclear power plant starts to operate for example, after
chemical decontamination, radionuclide are inevitably contained
also in the reactor water being used for formation of oxide films.
Accordingly, it is desired to suppress deposit of radionuclide onto
the surface of carbon steel members.
[0012] A first object of the present invention is to provide a
method of suppressing corrosion of carbon steel members that
constitute the plant and a corrosion suppressing apparatus
thereof.
[0013] A second object of the present invention is to provide
method for suppressing deposit of radionuclide onto carbon steel
members composing a nuclear power plant (e.g., carbon steel members
of the reactor water purifying system and the residual heat removal
system) and film formation apparatus.
[0014] To attain the first object of the present invention,
inventors of the present invention made various studies and
researches and found that corrosion of the wetted surfaces of
carbon steel members can be effectively suppressed by forming a
fine film of ferrite (for example magnetite or nickel ferrite) of
low water-solubility on the surface of the carbon steel members.
The ferrite film functions as a protective film that prevents the
carbon steel member from being wet with water.
[0015] Referring to FIG. 1, the effect of suppressing corrosion of
carbon steel members by using the result of an experiment which
attains the first object of the present invention will be
explained. The vertical axis of FIG. 1 indicates the relative
weight reduction of samples A, B, and C. The sample A is a carbon
steel piece whose surface is mechanically polished. The sample B is
a carbon steel piece whose surface is covered with a magnetite
film. The sample C is a carbon steel piece whose surface is covered
with a nickel ferrite film. The inventors kept these samples (A, B,
and C) under pure water at ordinary temperature for ten days and
then measured their weight reduction.
[0016] As shown in FIG. 1, the sample B forming a magnetite film
and the sample C forming a nickel ferrite film produce less weight
reduction than the sample A. In other words, the samples B and C
are protected better than the sample A against corrosion. The
corrosion suppressing effect of the sample B is less than that of
the sample C because some parts of the film of the sample B are not
so resistant to corrosion and because corrosion starts from there
and destroys the nearby film.
[0017] A well-known technology to form ferrite film on magnetic
recording media (see JP63-15990B) can be used to form ferrite films
on the surfaces of major components of the power plant. However,
since this technology (JP63-15990B) uses chlorine to form ferrite
films, chlorine must be avoided when this technology is applied to
the nuclear power plant to form ferrite films for assurance of the
soundness (e.g., resistance to corrosion) of structure members of
the nuclear power plant. Therefore, the technology of the present
invention is different from the technology disclosed in
JP63-15990B.
[0018] To attain the above first object, the present invention is
characterized by a method of using non-chlorine chemicals,
adsorbing iron (II) ions on the surface of carbon steel members
composing a nuclear power plant, oxidizing the adsorbed iron (II)
ions to form a ferrite film under a temperature condition from
ordinary temperature to 200.degree. C., preferably from ordinary
temperature to 100.degree. C., more preferably from 60 to
100.degree. C., and thus protecting the carbon steel members by the
ferrite film against corrosion.
[0019] In detail, this method adds a chemical containing iron (II)
ion in organic acid or carbonic acid, an oxidizing agent that
oxidizes iron (II) ions into iron (III) ion, and a pH adjustment
agent that adjusts pH of the solution in the range of 5.5 to 9.0 to
the processing solution. When this processing solution touches the
surface of carbon steel members, a ferrite film is formed on the
surface.
[0020] Particularly, chemicals including iron (II) ions should
preferably be organic acid that can be easily decomposed after film
formation since chemicals used in the nuclear power plant may
become radioactive wastes. After film formation, organic acid is
decomposed into carbon dioxide and water. Representative organic
acids that can be decomposed easily are formic acid, malonic acid,
diglycolic acid, and oxalic acid. The samples B and C of FIG. 1 use
formic acid to form ferrite films. For example, the sample B uses
formic acid solution that contains only iron (II) ions to form
ferrite film. The sample C uses formic acid solution in which the
concentration of nickel ions is half of the concentration of iron
ions so that the iron-nickel ratio of the solution may be equal to
the chemical composition ratio of nickel-ferrite.
[0021] FIG. 2 shows the result of analysis of a ferrite film formed
on the surface of the sample C by the Laser Raman method. As seen
in FIG. 2, the Raman peak of the ferrite film of the sample C is a
little shifted from the standard nickel-ferrite Raman peak, but it
can be assumed that the analyzed film is a nickel-ferrite film
judging from the composition. It is assumed that this positional
deviation of the peak is caused by non-stoichiometric effect of
nickel ferrite, which contains a little magnetite.
[0022] The method of suppressing corrosion of carbon steel members
in accordance with the present invention is preferably applicable
to carbon steel members composing the condensate system or feed
water system of a power plant but is not limited thereto. For
example, the present invention can be applied to a method of
suppressing corrosion of wetted carbon steel members in the
auxiliary equipment cooling system and the cooling water system
that uses a cooling tower. In short, the present invention can be
applied when carbon steel members are wet with water.
[0023] A ferrite film formation apparatus for the method of
suppressing corrosion of carbon steel members in accordance with
the present invention comprises
[0024] a surge tank for storing processing solution,
[0025] a circulation pump for sucking the processing solution from
the surge tank,
[0026] a processing solution supply pipe for introducing the
processing solution pressurized by the pump to pipings forming
film,
[0027] a first chemical tank for storing iron (II) ions to be added
to the processing solution that flows through the processing
solution supply pipe,
[0028] a second chemical tank for storing oxidizing agent to be
added to the processing solution that flows through the processing
solution supply pipe,
[0029] a third chemical tank for storing a pH adjustment agent to
adjust pH of the processing solution in the range of 5.5 to
9.0,
[0030] a processing solution return pipe for introducing the
processing solution from the pipes forming film to the surge tank,
and
[0031] a heating apparatus for heating the processing solution to a
temperature of 60.degree. C. to 100.degree. C.
[0032] To attain the second object of the present invention,
inventors of the present invention made various studies and
researches and found that fine magnetite film can be suppressed
taking in of cobalt of radionuclide. The fine magnetite film is
formed under a thermal condition at which dissolved oxygen diffuses
slower into base metal, for example, under a thermal condition of
100.degree. C. or lower. Below will be explained the result of this
consideration.
[0033] The samples D and E are prepared. The sample D is a carbon
steel piece whose surface is mechanically polished. The sample E is
a carbon steel piece whose surface is covered with a ferrite film
including magnetite as the main component. This ferrite film was
formed at a temperature of 100.degree. C. or lower. These samples D
and E are immersed in high-temperature water under the BWR
operating condition and then a deposit amount of Co-60 is examined.
FIG. 3 shows the examined result. In FIG. 3, the ordinate indicates
the relative values of the deposit amount of Co-60 of the samples D
and E. FIG. 3 shows the deposit amount of Co-60 to the sample E is
suppressed in comparison with the sample D. The sample D is a
polished metal piece and had no rust on it before the corrosion
test. However, in the actual power plant, the carbon steel members
already have rust on their surfaces before the operation of the
nuclear power plant is started and are ready to store radionuclide.
Consequently, in the carbon steel members of the actual power
plant, the deposit amount of the radionuclide on the carbon steel
members increase after the operation of the nuclear power plant.
Therefore, it is assumed that a carbon steel member without a
ferrite film takes in Co-60 more easily than a carbon steel member
covered with a ferrite film. A method for forming a ferrite film
including the magnetite as the main component is disclosed in above
JP63-15990B.
[0034] As one of methods of forming ferrite films on the surface of
carbon steel members composing a nuclear power plant by using a
chemical that does not contain chlorine, the inventors invented a
method of suppressing deposit of radionuclide on the carbon steel
members. This method is characterized by preparing a first chemical
including iron (II) ions, a second chemical for oxidizing the iron
(II) ions to iron (III) ion, and a third chemical for adjusting pH
of processing solution, mixing the first and second chemicals under
a temperature condition of ordinary temperature to 100.degree. C.,
preparing a processing solution whose pH is adjusted in the range
of pH 5.5 to pH 9.0 by mixing the resulting mixture and the third
chemical, and forming a ferrite film on the surface of carbon steel
members by using this processing solution.
[0035] The first chemical including iron (II) ions is prepared by
dissolving iron into organic aid or carbonic acid. Since used
chemicals become radioactive wastes in the nuclear power plant, the
first chemical should preferably be an organic acid that can be
easily decomposed into carbon dioxide and water. Representative
organic acids that can be decomposed easily are formic acid,
malonic acid, diglycolic acid, oxalic acid and the like. As the
result of the film formation test shown in FIG. 4, the inventors
found that these organic acids can form the magnetite film on the
surface of carbon steel members, and more particularly that formic
acid has the greater effect on film formation speed and
uniformity.
[0036] Immediately after the chemicals are mixed up, the processing
solution starts forming fine magnetite particles in the solution
even when there is no object to be film-formed. Therefore, the
chemicals must be mixed up just before the film formation starts.
To determine the order of mixing up the chemicals to effectively
form magnetite films, the inventors prepared two mixing orders;
adding first an oxidizing agent and then a pH adjustment agent to
the processing solution including the iron (II) ions, and adding
first a pH adjustment agent and then an oxidizing agent to this
processing solution, and checked the resulting magnetite films.
FIG. 5 shows the result of film formation. When the oxidizing agent
is added after the pH adjustment agent, the resulting ferrite film
contains greater magnetite particles, and is dispersed unevenly.
Contrarily, when the pH adjustment agent is added after the
oxidizing agent, the resulting ferrite film closely contains
uniform magnetite particles and the rate of film formation is
great.
[0037] The chemicals of iron (II) ions, oxidizing agent, and pH
adjustment agent must be added into the circulation system in that
order from the upstream side of system. Particularly, the adding
point of pH adjustment agent should be placed in the downstream of
the circulation pump and just in the upstream of the object forming
the film. This is to avoid formation of unwanted ferrite film in
the temporary piping.
[0038] To perform both chemical decontamination and ferrite film
formation, it is possible to connect an oxidizing agent tank and a
reducing agent tank for chemical decontamination respectively to
the solution supplying pipe that leads to the piping that the film
is to be formed.
[0039] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant should
preferably remove contaminants from the surface of the carbon steel
member before forming ferrite films, which is a pre-treating
process of the ferrite formation.
[0040] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant should
preferably perform chemical decontamination which contains at least
one reducing removal step to remove contaminants.
[0041] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant should
place preferably a position to add the third chemical in the
reactor containment vessel.
[0042] Another invention suppressing deposit of radionuclide onto
carbon steel members composing a nuclear power plant should
preferably use a solution in which iron (II) ion in formic acid as
the first chemical is dissolved.
[0043] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant should
preferably perform ferrite film formation in a time period between
the end of decontamination removal and the start of the operation
of the nuclear power plant.
[0044] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant can
comprise the steps of:
[0045] preparing a first chemical that contains iron (II) ions, a
second chemical that contains iron (III) ion, and a third chemical
that adjusts pH of processing solution;
[0046] obtaining a processing solution whose pH is adjusted in the
range of pH 5.5 to pH 9.0 by mixing the first and second chemicals
under a temperature condition of ordinary temperature to
100.degree. C., and mixing the resulting mixture and the third
chemical; and
[0047] forming ferrite films on the surface of carbon steel members
by using the processing solution.
[0048] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant can
preferably use formic acid as the organic acid.
[0049] A ferrite film formation apparatus for forming ferrite films
on the surface of carbon steel members composing a nuclear power
plant to suppress corrosion of carbon steel members is
characterized by comprising
[0050] a surge tank for storing processing solution,
[0051] a circulation pump for sucking the processing solution from
the surge tank,
[0052] an outward pipe for supplying the processing solution
exhausted from the circulation pump to piping to form film
therein,
[0053] a first chemical tank for storing iron ions to be added to
the processing solution,
[0054] a second chemical tank for storing oxidizing agent to be
added to the processing solution,
[0055] a third chemical tank for storing a pH adjustment agent to
adjust pH of the processing solution that is a mixture of chemicals
from the first and second chemical tanks in the range of 5.5 to
9.0,
[0056] a homeward pipe for introducing the processing solution from
the piping being formed the film into the surge tank, and
[0057] a heating apparatus for heating the processing solution to a
temperature of 60.degree. C. to 100.degree. C.,
[0058] wherein the position of adding the third chemical is
provided in the reactor containment vessel.
[0059] Another invention of suppressing deposit of radionuclide
onto carbon steel members composing a nuclear power plant is
preferably applied to members composing the reactor water purifying
system or residual heat removal system of the BWR plant. However,
the deposit suppressing method is not limited thereto. For example,
the method is also applicable as a method of suppressing deposit of
radionuclide on the carbon steel members that are in contact with
reactor water in a CANDU (Canada Deuterium Uranium) type
heavy-water reactor
[0060] In accordance with the present invention that attains the
first object, it is possible to suppress corrosion of carbon steel
members composing the nuclear power plant.
[0061] In accordance with the present invention that attains the
second object, it is possible to effectively suppress deposit of
radionuclide on carbon steel members composing a nuclear power
plant and to protect persons who are working on periodic inspection
of the nuclear power plant against exposure to radiations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] FIG. 1 is an explanatory drawing showing the result of an
experiment that tests the effect of suppressing corrosion of carbon
steel members.
[0063] FIG. 2 is an explanatory drawing showing the result of a
laser raman analysis of the nickel-ferrite films.
[0064] FIG. 3 is an explanatory drawing showing the result of an
experiment that places stainless steel samples covered with a
ferrite film containing magnetite as the main component in
hot-temperature water under the BWR operating condition and
measures their deposit amount of Co-60.
[0065] FIG. 4 is an explanatory drawing showing the relationship of
kinds of organic acids and formation rate of ferrite films.
[0066] FIG. 5 is an explanatory drawing showing the relationship of
the adding order of chemicals and rate of film formation.
[0067] FIG. 6 is a flow chart showing a method for suppressing
corrosion of carbon steel of an embodiment 1 which is one preferred
embodiment of the present invention.
[0068] FIG. 7 is an explanatory drawing showing the connection of a
film formation apparatus to the feed water pipe of the BWR plant
when the method of suppressing corrosion of carbon steel members of
FIG. 6 is applied to the feed water of the BWR plant.
[0069] FIG. 8 is a detailed structural diagram showing the film
formation apparatus of FIG. 7.
[0070] FIG. 9 is a flow chart showing a method for suppressing
corrosion of carbon steel of embodiment 2 which is another
embodiment of the present invention.
[0071] FIG. 10 is a detailed structural diagram showing the film
formation apparatus of FIG. 9 which is used for the method of
suppressing corrosion of carbon steel members.
[0072] FIG. 11 is an explanatory drawing showing the connection of
a film formation apparatus to the feed water pipe in the secondary
system of the PWR plant when the method of suppressing corrosion of
carbon steel members of embodiment 3 which is another embodiment of
the present invention is applied.
[0073] FIG. 12 is an explanatory drawing showing the connection of
a film formation apparatus to the feed water pipe of a thermal
power plant when the method of suppressing corrosion of carbon
steel members of embodiment 4 which is still another embodiment of
the present invention is applied.
[0074] FIG. 13 is a detailed schematic system diagram showing
another film formation apparatus to which the present invention is
applied.
[0075] FIG. 14 is a flow chart showing a method for suppressing
deposit of radionuclide of embodiment 6 which is another embodiment
of the present invention.
[0076] FIG. 15 is an explanatory drawing showing the connection of
a film formation apparatus to the pipe of a reactor water purifying
system of a nuclear power plant when the deposit suppressing method
of FIG. 14 is applied to the pipe of the reactor water purifying
system of the plant.
[0077] FIG. 16 is an explanatory drawing showing the connection of
a film formation apparatus to the pipe of a residual heat removal
system of a nuclear power plant when the deposit suppressing method
of FIG. 14 is applied to the pipe of the residual heat removal
system of the plant.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0078] Below will be explained some embodiments of the present
invention concerning to the method of suppressing corrosion of
carbon steel members that constitute a nuclear power plant.
Embodiment 1
[0079] FIG. 6 is a flow chart showing a method for suppressing
corrosion of carbon steel of an embodiment 1 which is one preferred
embodiment of the present invention. FIG. 7 is an explanatory
drawing showing the connection of a film formation apparatus to the
feed water pipe of the BWR plant when the method of suppressing
corrosion of carbon steel members of FIG. 6 is applied to the feed
water of the BWR plant. FIG. 8 is a detailed structural diagram
showing the film formation apparatus of FIG. 7.
[0080] As shown in FIG. 7, the BWR plant comprises a reactor 1
having a reactor pressure vessel loading a plurality of fuel
assemblies filled with nuclear fuel materials which generates
nuclear fission, a main steam pipe 2 connected to the reactor 1, a
steam turbine 3 connected to the main steam pipe 2, and a condenser
4 connected to the steam outlet of steam turbine 3. Steam generated
in the reactor is supplied to the steam turbine 3. The steam
exhausted from the steam turbine 3 is condensed by condenser 4. The
condensate which is water is pressurized up by a condensate pump 5
and a feed water pump 7 and supplied as feed water to the reactor 1
through a feed water pipe 10 of feed water system. The feed water
pipe 10 connected to the condenser 4 connects the condensate pump
5, a condensate demineralizer 6 to purify condensate, the feed
water pump 7, low pressure feed water heaters 8, and high pressure
feed water heaters 9 in that order. The low pressure feed water
heater 8 and the high pressure feed water heater 9 use steam
extracted from steam turbine 3 as their heat sources.
[0081] The BWR plant provides with a plurality of re-circulation
systems supplied cooling water (reactor water) to the core in which
a plurality of fuel assemblies are load. The re-circulation systems
respectively contain a re-circulation pipe 22 with re-circulation
pump 21. The reactor water surrounding the core in reactor 1 is
pressurized by re-circulation pump 21 and jetted in a jet pump (not
shown) arranged in the reactor 1 through the re-circulation pipe
22. The reactor water exhausted from the jet pump jets is
introduced into the core.
[0082] The reactor water purifying system to purify reactor water
in the reactor 1 is provided with a purifier pipe 20 one end of
which is connected to the re-circulation pipe 22 and the other end
is connected to the feed water pipe 10. This reactor water
purifying system, further, is equipped with a purifying system pump
24, a regenerative heat exchanger 25, non-regenerative heat
exchanger 26, and reactor water purifying apparatus 27 which are
provided with the purifier pipe 20. Part of the reactor water
flowing through the re-circulation pipe 22 is introduced into the
purifier pipe 20 by the purifying system pump 24. The reactor water
is cooled by the regenerative heat exchanger 25 and the
non-regenerative heat exchanger 26, purified by the reactor water
purifying apparatus 27, and heated by the regenerative heat
exchanger 25. The reactor water exhausted from the regenerative
heat exchanger 25 is supplied to the reactor 1 through the feed
water pipe 10 in the downstream side of the high-pressure feed
water heater 9.
[0083] As shown in FIG. 7, a film formation apparatus 30 being used
in the method for suppressing corrosion of carbon steel members of
the present embodiment is connected to the feed water pipe 10 so as
to bypass the feed water pump 7, the low pressure feed water heater
8, and the high pressure feed water heater 9. In other words, when
reactor 1 was shut down, the heaters 8 and 9 are bypassed for
example by opening the bonnet of valve 23 which is provided at the
exit of condensate demineralizer 6, closing the opening of feed
water pipe 10 in the demineralizer side, connecting one end of a
valve 34 in a processing solution pipe 35 of the film formation
apparatus 30 to the opening end of the feed water pipe 10 in the
feed water pump side with the flange of the valve 23, disconnecting
the feed water pipe 10 (for example, drain pipe or sampling pipe)
in the downstream side of the high pressure feed water heater 9,
and connecting the valve 47 of the processing solution pipe 35 to
the disconnected opening. Although film formation apparatus 30 is
connected to the feed water pipe in this embodiment, the connection
is not limited thereto. Film formation apparatus 30 can be
connected to any wetted portion of the carbon steel member
composing the nuclear power plant such as those in the condensate
system, auxiliary equipment cooling water system, and cooling water
system that uses a cooling tower.
[0084] The film formation apparatus 30 is so constructed as to work
also for chemical decontamination. For example, the film formation
apparatus 30 are equipped with a surge tank 31 which stores water
for film formation, and a circulation pump 32 to pressurize water
in the surge tank 31 as shown in FIG. 8. The circulation pump 32
supplies the water from the surge tank 31 to one end of the feed
water pipe 10 through the processing solution pipe 35 with which
the valves 33 and 34 are provided. A chemical tank 40 is connected
to the processing solution pipe 35 through a valve 38 and an
injection pump 39. The chemical tank 40 stores hydrazine as a
chemical to adjust pH of the processing solution. Further, a flow
path is provided from the discharge side of the circulation pump 32
to the surge tank 31 through a valve 36 and an ejector 37. The
ejector 37 is equipped with a hopper that supplies permanganic acid
to oxidize and dissolve contaminants in pipes or oxalic acid to
reduce and dissolve contaminants in pipes.
[0085] A chemical tank 45 is connected to the processing solution
pipe 35 via an injection pump 43 and a valve 41. A chemical tank 46
is connected to the processing solution pipe 35 via an injection
pump 44 and a valve 42. The chemical tanks 45 and 46 store
chemicals for formation of ferrite films. For example, the chemical
tank 45 stores an iron (II) ion solution prepared by dissolving
iron in formic acid. The chemicals capable of dissolving iron are
not limited to formic acid and can be organic acid or carbonic acid
that can be pair anions of iron (II) ions. The chemical tank 46
stores hydrogen peroxide as an oxidizing agent used for film
formation.
[0086] Meanwhile, the processing solution supplied from the
circulation pump 32 to one end of feed water pipe 10, flows through
the feed water pipe 10 (the feed water pump 7, the low pressure
feed water heater 8, and the high pressure feed water heater 9),
and is returned to the valve 47 from the other end of the feed
water pipe 10. Then, the processing solution is returned to the
surge tank 31 through the processing solution pipe 35 having a
circulation pump 48, a valve 49, a heater 53, and valves 55, 56,
and 57. A valve 50 and a filter 51 are connected to the processing
solution pipe 35 so as to bypass the valve 49. A cooler 58 and a
valve 59 are connected to the processing solution pipe 35 in
parallel with the heater 53 and the valve 55. A cation exchange
resin column 60 (filled with, for example, cation exchange resin)
and a valve 61 are connected to the processing solution pipe 35 in
parallel with the valve 56. A set of mixed-bed resin column 62 is
connected to the processing solution pipe 35 in parallel with the
valve 56. A decomposition apparatus 64 and valve 65 are connected
to the processing solution pipe 35 in parallel with the valve
57.
[0087] The decomposition apparatus 64 is connected to the discharge
side of injection pump 44 being connected to chemical tank 46
through a valve 54. In this configuration, hydrogen peroxide
solution in the chemical tank 46 can be introduced to the
decomposition apparatus 64. The present embodiment uses hydrogen
peroxide as oxidizing agent for both the formation of the ferrite
films and the decomposition of the processing solution. Therefore,
the chemical tank 46 and injection pump 44 can be used for both the
formation of the ferrite films and the decomposition of the
processing solution. Thus, the present embodiment can simplify the
facility. However, if pipes are lengthened because of insufficient
installation spaces, the decomposition apparatus for the formation
of the ferrite films and the decomposition apparatus for the
decomposition of the processing solution can be provided
separately.
[0088] The valve 42 to inject the oxidizing agent is connected to
the processing solution pipe 35 in the downstream side of the valve
41 injecting iron (II) ions. Further, the valve 42 is connected to
the processing solution pipe 35 in the upstream side of the valve
38 injecting pH adjustment agent. It is preferable that the valve
38 is connected to the processing solution pipe 35 as close to an
object that the ferrite film is to be formed as possible in the
downstream side of the valve 42. It is preferable that the film
formation apparatus 30 is constitute so as to feed the processing
solution from the circulation pump 48 to filter 51 (see FIG. 10)
after the ferrite films have been formed.
[0089] Inert gas such as nitrogen or argon should preferably be
spouted out in the aqueous solution in the chemical tank 45 and the
surge tank 31 for storing the chemicals including the iron (II) ion
to remove oxygen being included the aqueous solution. Decomposition
apparatus 64 can decompose organic acid being used as the pair
anions of iron (II) ions and hydrazine being used as a pH
adjustment agent. In short, chemicals used as pair anions of iron
(II) ions can be organic acids that can be decomposed into water
and carbon dioxide to reduce the waste amount or carbonic acid that
can be exhausted as gas and does not increase the waste amount. To
suppress the amount of chemicals being used, it is preferable to
separate and remove excessive reaction products, and to recover and
re-use unreacted chemicals being left in the processing
solution.
[0090] Referring to the flow chart of FIG. 6, below will be
explained a procedure to form the ferrite films by using film
formation apparatus 30. First, the film formation apparatus 30 is
connected to a piping system including the carbon steel members
that is the object that the ferrite film is to be formed on the
surface thereof (step S1). For example, the film formation
apparatus 30 is connected to the feed water pipe 10 that is made of
carbon steel member as shown FIGS. 7 and 8.
[0091] Corrosive products such as oxide films on the internal
surface of the feed water pipe 10 is chemically decontaminated and
removed by film formation apparatus 30 (step S2). It is preferable
to perform chemical decontamination in the method of suppressing
corrosion of carbon steel members in accordance with the present
embodiment. However, this chemical decontamination can be omitted
if the surface of carbon steel members on which a ferrite film is
formed is exposed before the ferrite film is formed. Mechanical
decontamination such as grinding can be applied to remove
contaminants instead of the chemical decontamination.
[0092] The chemical decontamination of Step S2 is a well-known
method that is applied to chemically decontaminate in the
recirculation system, however, it will be briefly explained below.
First, the valves 33, 34, 47, 49, 55, 56, and 57 are opened, under
the condition that the other valves are closed. The circulation
pumps 32 and 48 are started to circulate water from the surge tank
31 through the feed water pipe 10 that is target of the chemical
decontamination. Further, the temperature of the circulating water
is raised to about 90.degree. C. by heater 53. After the heating,
the valve 36 is opened to supply potassium permanganate (oxidizing
agent) of a predetermined amount from a hopper connected to ejector
37 into the surge tank 31. The potassium permanganate is dissolved
in the water in the surge tank 31. The chemical decontamination
solution (oxidizing decontamination solution) including the
dissolved potassium permanganate is introduced into the feed water
pipe 10 through the processing solution pipe 35 and oxidizes and
dissolves part of the corrosive products such as oxide films from
the feed water pipe.
[0093] After the above chemical decontamination using the oxidizing
decontamination solution is completed, oxalic acid is supply from
the above hopper into the surge tank 31 to decompose permanganate
ions that is left in the oxidizing decontamination solution. Step
S2 can be omitted for the feed water system since the feed water
system does not contain so much oxide such as chromium to be
oxidized and dissolved. Oxalic acid (reducing agent) is further
added to the chemical decontamination solution, in which the
permanganate ions were removed, to reduce and dissolve the
corrosive products on inner surface of the feed water pipe 10. To
adjust pH of the chemical decontamination solution (reducing
decontamination solution) including the oxalic acid, the valve 38
is opened and the injection pump 39 is started to inject the
hydrazine from the chemical tank 40 into the reducing
decontamination solution flowing in the processing solution pipe
35. After the oxalic acid and the hydrazine are injected in the
reducing decontamination solution, the valve 61 is opened and the
degree of the opening of the valve 56 is adjusted. Therefore, part
of the reducing decontamination solution is introduced to the ion
exchange column 60. Metal cations (ex. Iron (II) ions) eluted from
the feed water pipe 10 into the reducing decontamination solution
are adsorbed by cation exchange resin in the ion exchange column 60
and removed from the reducing decontamination solution. The
aforesaid oxidizing decontamination solution is also one of
chemical decontaminators.
[0094] After the above chemical decontamination by reducing
dissolution is completed, the valve 65 is opened and the degree of
the opening of the valve 65 is adjusted. At the same time, the
degree of the opening of the valve 57 is adjusted to reduce its
opening. With this, part of the reducing decontamination solution
is supplied to the decomposition apparatus 64. The valve 54 is
opened and the injection pump 44 is rotated at the same time. The
hydrogen peroxide in the chemical tank 46 is added to the reducing
decontamination solution being introduced into the decomposition
apparatus 64 and decomposes oxalic acid and hydrazine in the
reducing decontaminator.
[0095] To remove impurities from the chemical decontamination
solution after the oxalic acid and hydrazine are decomposed, heater
52 is turned off and valve 55 is closed. At the same time, the
valve 59 of cooler 58 is opened to supply the decontamination
solution to cooler 58 and the temperature of the decontaminator is
reduced. After the decontamination solution is cooled down enough
to be supplied to the mixed bed resin column 62, the valve 61 is
closed and the valve 63 is opened to supply the whole flow rate of
the decontamination solution to the mixed bed resin column 62.
Therefore, residual impurities are removed from the decontamination
solution.
[0096] A series of the operations that are oxidizing dissolution,
decomposition of the oxidizing agent, reducing dissolution,
decomposition of the reducing agent, and purification can dissolve
and remove the corrosive products including the oxidized films from
the carbon steel members being the target to be decontaminated.
[0097] In this way, after corrosive products including oxidized
films of the carbon steel members are removed, a ferrite film
forming processing in accordance with the present embodiment is
performed. After finishing of the purification operation of Step
S2, the valve 50 is opened and the valve 49 is closed to introduce
the processing solution to filter 51. The processing solution
passing through filter 51 is heated to a set temperature by heater
53 (Step S3). If the processing solution contains fine solids,
ferrite films are also formed on the surface of the solids while
the ferrite film forming processing is performed. Therefore, it is
possible to prevent waste of chemicals by supplying the processing
solution to filter 51 and removing such solids. However, this
supply of processing solution to filter 51 is not appropriate
because hydroxide (ex. iron hydroxide) of high concentration is
removed by filter 51 and because the pressure loss of the filter 51
increases. The hydroxide is formed based on iron (II) ion, which is
generated by the dissolution of the corrosive products, of high
concentration in the processing solution. The valve 56 is opened
and the valve 63 is closed to stop supply of the processing
solution for purification to the mixed bed resin column 62.
[0098] Here, the temperature of the processing solution is
preferably about 75.degree. C. but not limited thereto. Here, the
most important thing is that the temperature is high enough to form
ferrite films having so fine and strong structure (including
crystal structures) to protect carbon steel members against
corrosion during the operation of the reactor. Therefore, the
preferable temperature condition should be a temperature of lower
than the maximum operating temperature of the feed water system or
a temperature of at least 200.degree. C. or lower. Further, the low
temperature limit can be ordinary temperature, but preferably be
60.degree. C. or higher at which the practical film formation speed
is obtained. When the temperature is 100.degree. C. or higher, the
facility must be pressurized to suppress boiling of the processing
solution. Therefore, the temporary facility is required resistance
of pressure and increases the construction cost. Thus, the
temperature to form the fine ferrite films should preferably be
100.degree. C. or lower.
[0099] To form the ferrite films, iron (II) ions must be adsorbed
to the surface of an object that the ferrite film is to be formed.
However, if the processing solution contains dissolved oxygen, iron
(II) ions are oxidized to iron (III) ions by dissolved oxygen
according to a reaction expressed by Formula (1). Since the iron
(III) ions are lower in solubility than the iron (II) ions, the
iron (III) ions are precipitated out of the processing solution as
iron hydroxide according to a reaction expressed by Formula (2), so
that the formation of the ferrite films is blocked by the
precipitation of iron hydroxide. Therefore, it is preferable to
spout out the inert gas in the processing solution or vacuum
deaeration to remove dissolved oxygen from the processing
solution.
4Fe.sup.2++O.sub.2+2H.sub.2O.fwdarw.4Fe.sup.3++4OH.sup.- (1)
Fe.sup.3++3OH.sup.-.fwdarw.Fe(OH).sub.3 (2)
[0100] A chemical including the iron (II) ions from the chemical
tank 45 is injected into the processing solution by opening the
valve 41 and rotating the injection pump 43 when the temperature of
the processing solution reaches the set temperature (step S4). The
iron (II) ions in the chemical are adsorbed to the surface to the
carbon steel members that the ferrite film is to be formed. The
chemical contains the iron (II) ion that was prepared by dissolving
iron in formic acid. Then, in order to form the ferrite films by
oxidizing the iron (II) ions adsorbed on the surface of the carbon
steel members, the oxidizing agent from chemical tank is injected
into the processing solution by opening the valve 42 and rotating
the injection pump 44 into the processing solution (step S5).
Hydrogen peroxide used to decompose the chemical decontamination
solution is also used as the oxidizing agent. However, the
oxidizing agent can use a solution dissolving ozone or oxygen.
Finally, the pH adjustment agent from chemical tank 40 is injected
into the processing solution by opening the valve 38 and rotating
the injection pump 39 (Step S6). The pH adjustment agent is for
example hydrazine. In treatment of Step S6, pH of the processing
solution is adjusted to 5.5 to 9.0 which is the condition of
starting reaction to form the ferrite films. Consequently, the
reaction to form the ferrite films advances. Thus, Steps S4 to S5
form oxidize films being the ferrite films (hereinafter called
magnetite films) including magnetite as the main component on the
wetted surface of the carbon steel members.
[0101] Steps S4, S5, and S6 should preferably be conducted
continuously. More specifically, it is preferable to start
injection of the oxidizing agent when the processing solution
reaches the oxidizing agent injection point after the iron (II)
ions are injected into the processing solution. Further, it is
preferable to start injection of the pH adjustment agent
immediately when the processing solution including the iron (II)
ions and the oxidizing agent reaches the pH adjustment agent
injection point. If the processing solution including only the iron
(II) ions is circulated through the processing solution pipe 35,
the iron (II) ions may be oxidized by dissolved oxygen being left
in the processing solution. This may cause losses of the chemicals
due to unwanted reaction and inhibition of the regular
reaction.
[0102] When the oxidizing agent is added to the processing solution
including the iron (II) ions, the oxidative reaction of the iron
(II) ions starts and the ratio of the iron (II) ions to iron (III)
ions in the processing solution becomes fit for the film formation
reaction. However, the film formation reaction does not advance
because the processing solution is acidic in this status. By adding
the pH adjustment agent to the processing solution, the film
formation reactions start. Therefore, to prevent formation of
unwanted films on the inner surface of the temporary pipe, the
point of injecting the pH adjustment agent should preferably be
close to object that the ferrite film is to be formed and a point
at which the temporary facility is connected to the structure
members of the BWR plant.
[0103] As already explained for Steps S4 to S6, chemicals are
injected in the order of the iron (II) ions, the oxidizing agent,
and the pH adjustment agent. The order of the oxidizing agent, the
iron (II) ions and the pH adjustment agent can be reversed. If
hydrogen peroxide (as the oxidizing agent) is added first, however,
part of hydrogen peroxide may be used in vain because hydrogen
peroxide may be easily decomposed on the surface of the hot metal.
Further, when chemicals are injected in the order of the iron (II)
ions, the pH adjustment agent and the oxidizing agent, the ferrite
films can be formed but magnetite particles in the formed ferrite
film may be comparatively greater. In summary, when the chemicals
are injected in the order that is described in Steps S4, S5, and
S6, the chemicals are used effectively and fine magnetite films can
be formed.
[0104] After the film formation process is finished, a waste
solution treatment process is performed (Steps S7 and S8).
Contrarily, when the film formation process is not finished, the
processes of the Step S4 to S7 are performed again by adding
continuously the chemical to the processing solution to form a
magnetite film of a desired thickness.
[0105] When the film formation process is finished, the processing
solution used to form the magnetite films still contains formic
acid and hydrazine. So the waste solution treatment process of Step
S8 is performed. That is, the formic acid and hydrazine must be
removed from the processing solution before the processing solution
is exhausted from the film formation apparatus 30. If the
remainders in the processing solution are treated by cation
exchange resin in the cation exchange resin column 60, wastes (used
cation exchange resins) from the cation exchange resin column 60
will increase. To avoid this, the waste solution treatment process
of Step S8 should preferably decompose formic acid in the
processing solution into carbon dioxide and water and hydrazine
into nitrogen and water by the decomposition apparatus 64 that was
used to decompose the decontamination solution. This can reduce the
load of the cation exchange resin column 60 and the amount of waste
from the cation exchange resin column 60.
[0106] The decomposition process of formic acid and hydrazine is
similar to decomposition of oxalic acid. The decomposition process
comprises steps of adjusting the degree of the openings of valves
65 and 57 to supply part of the processing solution to
decomposition apparatus 64 and supplying hydrogen peroxide from the
chemical tank 46 into the processing solution being introduced into
decomposition apparatus 64. Formic acid and hydrazine are
decomposed in the decomposition apparatus 64.
[0107] In accordance with the present embodiment, the magnetite
film is formed on the surface of the object (carbon steel members)
the ferrite film is to be formed while suppressing the amount of
waste from the cation exchange resin column 60. The magnetite film
can protect the wetted surfaces against corrosion during the
operation of the nuclear power plant. Consequently, the present
embodiment can omit addition of oxygen to feed water that has been
performed to suppress corrosion. Further, the present embodiment
will not spoil the soundness (for example, corrosion resistance) of
the structural materials composing the nuclear power plant because
chemical containing chlorine is not used for the film
formation.
Embodiment 2
[0108] Embodiment 2 is different from Embodiment 1 in that
Embodiment 2 forms nickel ferrite films and Embodiment 1 forms the
magnetite films.
[0109] Aforesaid Embodiment 1 describes the procedure and the
apparatus to form the magnetite film on the inner surface of the
carbon steel pipes of the feed water system. In the result (see
FIG. 1) of a test of dipping samples in pure water at ordinary
temperature, both magnetite and nickel ferrite films can obtain
almost the same corrosion suppressing effect. However, in the
actual service environment of the feed water system which uses pure
water at high temperature, the nickel ferrite films have longer
corrosion suppressing effect than the magnetite films because the
solubility of the nickel ferrite is less than that of the
magnetite. The embodiment 2 forms the nickel ferrite film on the
surface of wetted surfaces of the carbon steel members composing
the nuclear power plant.
[0110] FIG. 9 shows a method of suppressing corrosion of carbon
steel members which is another embodiment of the present invention,
and specifically a flow chart of Embodiment 2 for forming nickel
ferrite films. FIG. 10 shows a detailed structural diagram of the
film formation apparatus being used the formation of the nickel
ferrite films.
[0111] As seen from FIG. 9, Embodiment 2 is different from
Embodiment 1 in that Step S4' of injecting a chemical including
nickel ions to the processing solution is added to the procedure of
forming the ferrite films. The process of Step S4' is carried out
after Step S4 of adding the chemicals including the iron (II) ions
to the processing solution and before Step S5 of adding hydrogen
peroxide to the processing solution. To accomplish Step S4', the
film formation apparatus 30A used by the present embodiment is
additionally equipped with a chemical tank 68, an injection pump
67, and a valve 66. The other procedure and apparatus of Embodiment
2 are basically the same as those of Embodiment 1.
[0112] In Embodiment 2, the chemical tank 68 for storing the nickel
ions is provided separately from the chemical tank 45 for storing
the iron (II) ions. This is to avoid the formation of the
precipitation in the solution including the iron (II) ion. When the
solution including the iron (II) ion contains formic acid and
nickel ions, the precipitation is formed in this solution.
Therefore, it is preferable to use a nickel carbonate solution as a
solution including the nickel ions. If a solution that can dissolve
both iron (II) ions and nickel ions is used, chemical tank for
storing the solution including the iron (II) ions and chemical tank
for storing the solution including the nickel ion as in Embodiment
2 need not be separated. Embodiment 2 can form the nickel ferrite
films by using the same procedure and apparatus as those of
Embodiment 1 by adding the nickel ions to the processing solution
including the iron (II) ions.
[0113] In accordance with the present embodiment, the corrosion of
the carbon steel members composing the nuclear power plant can be
suppressed for a comparative long period because the nickel ferrite
films formed on the surface of the carbon steel members are less
soluble to water than the magnetite films.
Embodiment 3
[0114] Embodiment 3 is different from Embodiment 1 in that
Embodiment 3 connects the film formation apparatus 30 to the feed
water pipe 10 of the secondary system of a PWR plant but Embodiment
1 connects the film formation apparatus 30 to the feed water pipe
10 of the BWR plant. FIG. 11 is a structural diagram showing
Embodiment 3 that connects the film formation apparatus 30 to the
feed water pipe in the secondary system of the PWR plant. FIG. 11
shows only the secondary system including a steam generator 69 but
does not contain the configuration of a primary system of the PWR
plant.
[0115] Embodiment 3 is different from Embodiment 1 (for example,
FIG. 7) in that a deaerator 70 is provided with the feed water pipe
10 between the low pressure feed water heater 8 and the high
pressure feed water heater 9. The deaerator 70 is used to remove
gas components from the feed water. Embodiment 3 is similar to
Embodiments 1 and 2 as for a method of connecting the film
formation apparatus 30 to the feed water pipe 10 and the film
formation procedure. The secondary system of the PWR plant
generally uses ammonia, hydrazine, or other chemical to suppress
corrosion of carbon steel members. In Embodiment 3, however, since
the film formation apparatus 30 performs the film formation process
shown in FIG. 1, the treatment of such chemicals as ammonia, and
the like is not required. Therefore, the present embodiment can
accomplish environment-friendly plant operations and reduce the
running cost of apparatus.
Embodiment 4
[0116] Embodiment 4 is different from Embodiment 3 in that
Embodiment 4 connects the film formation apparatus 30 to the feed
water pipe 10 of a thermal power plant but Embodiment 3 connects
the film formation apparatus 30 to the feed water pipe of the
secondary system of the PWR plant. FIG. 12 shows a thermal power
plant in which a film formation apparatus is connected to the feed
water pipe of thereof. As shown in FIG. 12, Embodiment 4 is
different from Embodiment 3 in that a boiler 71 is used in place of
the steam generator 69.
[0117] Embodiment 4 is similar to Embodiments 1 and 3 as for a
method of connecting the film formation apparatus 30 to the feed
water pipe 10 and the ferrite film formation process. Like the
secondary system of a PWR plant, the feed water pipe 10 of the
thermal power plant generally uses ammonia, hydrazine, or other
chemical to suppress corrosion of the carbon steel members. In
Embodiment 4, however, since the ferrite film formation is
performed by the film formation apparatus 30, the treatment of such
chemicals as ammonia, and the like is not required. Therefore, the
present embodiment can accomplish environment-friendly plant
operations and reduce the running cost of apparatus.
Embodiment 5
[0118] Embodiment 5 describes another example of film formation
apparatus. FIG. 13 shows a detailed structural diagram of another
film formation apparatus to which the present invention is applied.
Film formation apparatus 30B shown in FIG. 13 is different from the
film formation apparatus 30 of Embodiment 1 (for example, FIG. 8)
in that a nitrogen bubbling apparatus 72 is connected to the surge
tank 31 and that another nitrogen purging apparatus 73 is connected
to the chemical tank 45.
[0119] In accordance with the present embodiment, the inside of the
surge tank 31 is purged with nitrogen gas to exhaust the dissolved
oxygen from the solution in the surge tank 31. This purging can
make the processing solution in the surge tank 31 substantially
free from oxygen and further suppress oxidization of iron (II) ions
in chemical tank 45. Therefore, the production of the iron (III)
ion that do not contribute to the formation of the magnetite films
in the solution can be suppressed. As the result, the reaction to
produce the magnetite films is activated and good magnetite films
can be formed.
Embodiment 6
[0120] Below will be explained a method for suppressing deposit of
radionuclide to carbon steel members composing a nuclear power
plant in accordance with Embodiment 6 which is another embodiment
of the present invention.
[0121] FIG. 14 shows a flow chart of Embodiment 6 which is another
preferred embodiment of the present invention to suppress deposit
of radionuclide. FIG. 15 is an explanatory drawing to indicate the
connection of a film formation apparatus to the pipe of a reactor
water purifying system of a nuclear power plant when the deposit
suppressing method of FIG. 14 is applied to the pipe of the reactor
water purifying system of the plant.
[0122] As shown in FIG. 15, Embodiment 6 connects the film
formation apparatus 30 to the purifying system pipe 20. In other
words, when the reactor 1 was shut down, the bonnet of valve 18 on
the purifying system pipe 20 is opened and the re-circulation pipe
22 side of the purifying system pipe 20 is closed. One end of the
processing solution pipe 35 (as a temporary pipe) of the film
formation apparatus 30 in the valve 34 side is connected to the
purifying system pipe 20 in the upstream side of the purifying
system pump 24. The bonnet of valve 19 in the upstream side of the
regenerative heat exchanger 25 is opened and the purifying system
pipe 20 is closed in the side of the regenerative heat exchanger
25. One end of the valve 47 side of processing solution pipe 35 is
connected to the purifying system pipe 20 in the downstream side of
the purifying system pump 24 by using the flange of the valve 19.
The configuration of the BWR plant is the same as that of FIG. 7.
The configuration of the film formation apparatus 30 used by the
present embodiment is the same as that of FIG. 8.
[0123] Referring to FIG. 14, below will be explained a method for
suppressing deposit of radionuclide by using the film formation
apparatus 30 in accordance with the present embodiment. The film
formation apparatus 30 is connected to a piping including the
carbon steel members being the object that the ferrite film is to
be formed (Step 11). The film formation apparatus 30 is connected
to the purifying system pipe 20 as explained above.
[0124] Contaminants such as oxide films (that contains
radionuclide) formed on the surface of carbon steel members that
contacts with water are chemically decontaminated by using film
formation apparatus 30 (Step S12). The chemical decontamination of
Step S12 is not described in detail here because it is the same as
the chemical decontamination of Step S2.
[0125] After the contaminants including the oxide films formed the
surface of the carbon steel members are removed, the formation
process of the ferrite films is performed on inner surface of the
carbon steel members composing the purifying system pipe 20. First,
the temperature of the processing solution is adjusted to a set
temperature by using heater 53 (Step S13). In this case, the valve
50 is opened and the valve 49 is closed to introduce the processing
solution to filter 51. Filter 51 removes fine solids from the
processing solution. The purpose of the removal of the solids is
the same as that of Step S3 of Embodiment 1.
[0126] The set temperature is preferably about 100.degree. C. but
not limited thereto. The point is, the formed ferrite film must be
so fine as not to take in the radionuclide from the reactor water
during the operation of the reactor. Therefore, it is preferable
that the temperature of the processing solution is at least the
designed temperature of the system (for example, the purifying
system pipe 20) or lower. The low temperature limit of the
processing solution can be ordinary temperature, but preferably be
60.degree. C. or higher at which the practical ferrite film
formation speed is obtained. When the set temperature is
100.degree. C. or higher, the facility must be pressurized to
suppress boiling of the processing solution. Therefore, the
temporary facility must be resistant to the pressure.
[0127] To form the ferrite film on the surface of the object
(carbon steel members) that the ferrite film is to be formed, it is
necessary to let the iron (II) ions be adsorbed to the surface. To
reduce the dissolved oxygen in the processing solution to
effectively use the iron (II) ions in the processing solution for
the formation of the ferrite film, it is preferable to spout out
the inert gas in the processing solution or vacuum deaeration to
remove dissolved oxygen from the processing solution. This is
because, when the processing solution contains too much the
dissolved oxygen, the iron (II) ions will be precipitated as iron
hydroxide by the reactions of Formulae (1) and (2).
[0128] When the temperature of the circulated processing solution
reaches the set temperature, the chemical including the iron (II)
ions is injected to the processing solution (Step S14). The valve
41 is opened and the injection pump 43 is rotated. Therefore, the
chemical prepared by dissolving iron in formic acid, and including
the iron (II) ion is injected from the chemical tank 45 into the
processing solution. Hydrogen peroxide solution as the oxidizing
agent) is injected into the processing solution (Step S15). To form
the ferrite films by oxidizing the iron (II) ions adsorbed on the
surface of the carbon steel members, the valve 42 is opened, the
injection pump 44 is rotated, and the hydrogen peroxide solution is
injected from the chemical tank 46 into the processing solution.
Then, injects hydrazine being the pH adjustment agent is injected
into the processing solution (Step S16). To adjust pH of the
processing solution to 5.5 to 9.0 which is the condition of
starting reaction to form the ferrite films, the valve 38 is opened
and the injection pump 39 is rotated to inject hydrazine to the
processing solution from the chemical tank 40. Thus, the ferrite
film including magnetite as the main component is formed on the
inner surface of the carbon steel members of the purifying system
pipe 20.
[0129] When it is judged that the formation of the ferrite film
including the magnetite as the main component completed in Step
S17, the waste solution treatment of Step S18 is carried out. When
it is judged that the formation of the ferrite film is not
completed in Step S17, the processes of the Step S14 to S17 are
performed again by adding continuously the chemical to the
processing solution to form a magnetite film of a desired thickness
that contains the magnetite as the main component.
[0130] When the formation of the ferrite film containing the
magnetite as the main component completed, the processing solution
contains formic acid and hydrazine. Therefore, before the
processing solution is exhausted from the film formation apparatus
30, the waste solution treatment of Step S18 must be performed to
remove such impurities. The waste solution treatment of Step S18 is
the same as that of Step S8 of Embodiment 1. In this way, it is
possible to form the ferrite film including the magnetite as the
main component on the surface of the carbon steel members while
suppressing the amount of ion exchange resin waste that is, the
amount of radioactive waste. Thus, the deposit of radionuclide, for
example, radioactive cobalt ions onto the surface of the carbon
steel members as seen in FIG. 3 is suppressed. As the result, the
present embodiment can suppress the dose rate of pipes of the
reactor water purifying system and reduce the exposure dose of the
persons who are working on periodic inspection of the nuclear power
generation plant.
[0131] Further, the present embodiment will not spoil the soundness
of the structural materials composing the nuclear power plant
because chemical containing chlorine is not used for the film
formation.
Embodiment 7
[0132] FIG. 16 is an explanatory drawing showing the connection of
the film formation apparatus to the pipe of a residual heat removal
system of the nuclear power plant when the deposit suppressing
method shown in FIG. 14 is applied to the pipe of the residual heat
removal system of the plant. The present embodiment is a method for
suppressing the deposit of the radionuclide onto the carbon steel
members composing the nuclear power plant by connecting the film
formation apparatus 30 to the pipe 16 of the residual heat removal
system, which is another embodiment of the present invention. The
residual heat removal system pipe 16 connected to the
re-circulation pipe 22 is equipped with a circulation pump 14 and a
heat exchanger 15. When reactor 1 was shut down, the bonnet of the
valve 12 provided with the residual heat removal system pipe 16 is
opened and the opening of re-circulation pipe 22 side of the pipe
16 is closed. One end of the valve 34 side of the processing
solution pipe 35 of the film formation apparatus 30 is connected to
the residual heat removal system pipe 16 in the upstream side of
the circulation pump 14 by using the flange of the valve 12. The
bonnet of the valve 13 provided with the residual heat removal
system pipe 16 in the downstream side of the heat exchanger 15 is
opened. The opening of the re-circulation pipe 22 side of the pipe
16 is closed. One end of the valve 47 side of the processing
solution pipe 35 is connected to the residual heat removal system
pipe 16 by using the flange of the valve 13.
[0133] The chemical decontamination and the ferrite film formation
of the present embodiment are also carried out based on the
procedure shown in FIG. 14. The corrosive product (part of which is
oxide film) formed on the inner surface of the residual heat
removal system pipe 16 scarcely contains chromium because the
temperature of the processing solution is lower than the reactor
water temperature, that is, usually 150.degree. C. or lower and the
base metal of the residual heat removal system pipe 16 is made of
carbon steel. Therefore, in the residual heat removal system pipe
16, the oxidation process is not required in the chemical
decontamination process. Only a single reducing decontamination
process is sufficient. The others of the present embodiment are the
same as those of the above-described embodiments.
[0134] Since the present embodiment forms the ferrite film on the
inner surface of the residual heat removal system pipe 16, the
present embodiment suppresses corrosion of the carbon steel members
of the pipe 16 during the standby period of the residual heat
removal system. Further, the deposit of radionuclide onto the
surface of the carbon steel member is suppressed when the cooling
water is introduced through the pipe 16. In other words, the
present embodiment can keep the dose rate of the residual heat
removal system pipe 16 low and reduce the exposure dose of persons
who are working on maintenance of the residual heat removal
system.
Embodiment 8
[0135] Although Embodiments 6 and 7 use the chemical including the
iron (II) ions to form the ferrite film, it is possible to use
formic acid that does not include the iron (II) ions to form the
ferrite film on the surface of the carbon steel members. In other
words, when formic acid is supplied to the system and circulated in
this system, the base metal of the carbon steel members of the
system dissolves and the iron (II) ions dissolve into the solution.
Therefore, the iron (II) ions can be used for the formation of the
ferrite film.
[0136] In accordance with the present embodiment, iron formate
including iron (II) ions that can be easily oxidized need not be
added from the outside although it takes a long time to prepare the
iron (II) ions.
* * * * *