U.S. patent application number 10/715069 was filed with the patent office on 2005-06-23 for method for protecting components of a primary system of a boiling water reactor in particular from stress corrosion cracking.
Invention is credited to Ruhle, Wilfried, Stellwag, Bernhard.
Application Number | 20050135542 10/715069 |
Document ID | / |
Family ID | 7684916 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050135542 |
Kind Code |
A1 |
Stellwag, Bernhard ; et
al. |
June 23, 2005 |
Method for protecting components of a primary system of a boiling
water reactor in particular from stress corrosion cracking
Abstract
A method for protecting the components of the primary system of
a boiling water reactor, especially against stress cracking
corrosion, includes feeding a reducing agent into the primary
coolant in order to reduce the number of substances having a
oxidizing effect or in order to modify the electrochemical
potential of the component surfaces covered with an oxide layer to
negative values. An alcohol that can be oxidized in the conditions
of a reactor is fed in as a reducing agent, preferably in a liquid
form, into the primary coolant. The component surfaces are provided
with a blank coating or a single intrinsic oxide coating.
Inventors: |
Stellwag, Bernhard;
(Nurnberg, DE) ; Ruhle, Wilfried; (Eppelheim,
DE) |
Correspondence
Address: |
LERNER AND GREENBERG, PA
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Family ID: |
7684916 |
Appl. No.: |
10/715069 |
Filed: |
November 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10715069 |
Nov 17, 2003 |
|
|
|
PCT/EP02/05274 |
May 14, 2002 |
|
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Current U.S.
Class: |
376/277 ;
376/305 |
Current CPC
Class: |
G21C 15/28 20130101;
C23F 11/122 20130101; C23F 11/00 20130101; Y02E 30/30 20130101;
G21C 19/28 20130101; G21C 19/307 20130101; G21C 17/0225 20130101;
G21Y 2002/103 20130101; G21Y 2004/10 20130101; Y02E 30/31
20130101 |
Class at
Publication: |
376/277 ;
376/305 |
International
Class: |
G21C 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2001 |
DE |
101 23 690.5 |
Claims
1. A method for protecting components of a primary system of a
boiling water reactor having a pressure vessel and a feedwater line
opening out into the pressure vessel, the method which comprises:
providing an alcohol that is oxidizable under operating conditions
of the primary system; feeding the alcohol into a primary coolant
to establish an alcohol concentration of from 0.1 to less than 10
.mu.mol/kg (.apprxeq.0.0032 to less than 0.32 ppm for methanol) in
a downcomer, the downcomer extending downward at an opening of the
feedwater line, with surfaces of the components still being bright
or covered only by a native oxide layer.
2. (canceled)
3. The method according to claim 1, which comprises protecting the
components against stress corrosion cracking.
4. The method according to claim 1, which comprises feeding the
alcohol into a condensate or feedwater system and carrying the
alcohol into the primary system with the feedwater.
5. The method according to claim 1, wherein the alcohol is selected
from the group consisting of methanol, ethanol, and propanol.
6. The method according to claim 1, which comprises doping the
component surfaces with a precious metal.
7. The method according to claim 6, wherein the component surfaces
are doped with platinum.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuing application of copending International
Application No. PCT/EP02/05274, filed May 14, 2002, which
designated the United States and which was not published in
English.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] The invention lies in the boiling water reactor technology
field. More-specifically, the invention relates to a method for
protecting the components of the primary system of a boiling water
reactor in particular from stress corrosion. In a boiling water
reactor, the coolant which comes into contact with the reactor core
is known as primary coolant, and the lines and components which are
exposed to the primary coolant are known as the primary system. In
addition to the reactor pressure vessel, the primary system of a
boiling water reactor includes systems of lines as well as various
internal fittings and pumps. The components generally consist of
stainless steel, for example of a CrNi steel, or an Ni-base alloy,
such as Inconel.RTM. 600 (Inco Alloys International, Inc.).
Radiolysis of the primary coolant causes, inter alia, the reaction
products hydrogen peroxide, oxygen, and hydrogen to form in the
boiling water reactor. The oxidizing conditions which result from
the excess of oxidizing agents promote corrosion, in particular
stress corrosion cracking, of the components. To remedy this, it is
known to admix hydrogen with the primary coolant. This bonds
oxidizing agents contained in the primary coolant and shifts the
electrochemical potential of the component surfaces toward negative
values. A drawback of the conventional method is that relatively
large quantities of hydrogen are required to ensure sufficient
protection against corrosion. The high demand for hydrogen, which
entails corresponding costs, is attributable not least to the fact
that the electrochemical oxidation of the hydrogen on the component
surfaces which are covered with an oxide layer is subject to
considerable reaction inhibition, and this has to be compensated
for by increased hydrogen concentrations. A further drawback is the
outlay on apparatus for metering the gaseous hydrogen.
[0003] European patent disclosure EP 0736878 describes a method in
which the oxide layer of the component surfaces in the primary
system is doped with precious metal, which makes it possible to use
smaller quantities of hydrogen. German published patent application
DE 100 30 726 A1 describes a method in which the quantities of
hydrogen and precious metal are supposed to be reduced by coating
the component surfaces with a film which includes a substance with
a photocatalytic action. The substances with photocatalytic action
that are used--preferably TiO.sub.2 and ZrO.sub.2--are N-type
semiconductors which are excited by the Cherenkov radiation which
is present in the reactor, shifting the corrosion potential of the
component surfaces toward negative values.
[0004] Soviet Union patent disclosure SU 653953 describes a system
having to do with what is referred to in the document as "boiling
nuclear reactors." There, an alcohol is added into the primary
coolant instead of hydrazine. While relatively little information
is presented in the document concerning the operational setup of
the reactor, certain statements strongly suggest that the boiling
reactor of the prior art publication is not a boiling water reactor
according to the Western understanding. One such hint is that the
publication states that, in its prior art, hydrazine had been
introduced in such boiling nuclear reactors during the reactor
operation for the purpose of providing corrosion protection. The
addition of hydrazine, however, during the operation of a boiling
water reactor would be entirely prohibited.
[0005] The Soviet document discloses corrosion protection measures
by way of the addition of alcohol in the coolant/moderator. The
specific concentration disclosed is approximately 10 to 10.sup.5
.mu.mol/kg in order to completely prevent oxygen formation during
the radiolysis of the coolant. In order to ensure this, the
disclosed alcohol concentration must necessarily be present at
those locations at which the radiolysis processes are the
strongest, that is, at the fuel rods in the reactor core.
[0006] A problem associated with very high alcohol concentration is
that a relatively large portion of the alcohol remains unused,
i.e., it is not oxidized by radiolysis oxygen or decomposed by the
radiolysis, it subsequently passes through a phase change into the
vapor phase and then reaches the steam turbine and the condenser
downstream of the steam turbine. There, the alcohol is cooled to
about 40.degree. C. At this temperature, only a small proportion of
the alcohol is dissolved in the liquefied condensate which is fed
back into the reactor pressure vessel in the form feedwater. The by
far largest proportion is contained in the vapor phase. The latter
is not simply let go into the environment but it is transported via
an off-gas path within which a catalytic recombination of hydrogen
and oxygen to water is effected. An alcohol component in the vapor
phase could, on the one hand, disturb the recombination. On the
other hand, additional functional elements and processing steps
would have to be provided in order to hold back the alcohol or to
convert the same into a non-damaging form.
[0007] High alcohol contents, furthermore, lead to radiolysis in
the reactor due to the high radiation density, which results in
products such as CO.sub.2, formaldehyde, and formic acid. These
products, of course, are undesirable in the reactor pressure vessel
itself and in the downstream vapor carrying systems such as the
condenser. Besides an increase in the conductivity of the primary
coolant, they can lead to a decrease in the pH which has a negative
effect on the component corrosion. Yet, it is exactly the component
corrosion which is to be avoided or reduced with the addition of
alcohol.
SUMMARY OF THE INVENTION
[0008] It is accordingly an object of the invention to provide a
method for protecting the components of the primary system of a
boiling water reactor which overcomes the above-mentioned
disadvantages of the heretofore-known devices and methods of this
general type and which ensures efficient protection against
corrosion with little outlay on materials and time.
[0009] With the foregoing and other objects in view there is
provided, in accordance with the invention, a method for protecting
components of a primary system of a boiling water reactor having a
pressure vessel and a feedwater line opening out into the pressure
vessel. The novel method, which is particularly suitable to protect
the components against stress corrosion cracking, comprises the
following steps:
[0010] providing an alcohol that is oxidizable under operating
conditions of the primary system;
[0011] feeding the alcohol into a primary coolant to establish an
alcohol concentration of from 0.1 to 300 .mu.mol/kg in a downcomer,
the downcomer extending downward at an opening of the feedwater
line, with surfaces of the components still being bright or covered
only by a native oxide layer.
[0012] In accordance with an added feature of the invention, the
alcohol concentration is adjusted to less than 10 .mu.mol per
kg.
[0013] In other words, the objects of the invention are achieved by
a method in which an alcohol that can be oxidized under the
conditions prevailing in the reactor system, preferably in liquid
phase, is fed into the primary coolant instead of hydrogen, with
the component surfaces being bright or being covered only by a
native oxide layer. In this context, a native oxide layer is to be
understood as meaning an oxide layer which forms as a result of
corrosion to the component material, if appropriate with the
intercalation of foreign metals or foreign metal oxides, during
reactor operation or during an oxidizing pretreatment. It has been
found that the metering-in of an alcohol of the above type as the
only measure is sufficient to reduce the corrosion potential of the
component surfaces to values of lower than -230 mV, and it is
possible to dispense with complex coatings in particular comprising
substances with a photocatalytic action.
[0014] The advantage of an alcohol over hydrogen as reducing agent
is firstly that it can be metered in in liquid form or as a
solution. A liquid is more easy to feed into the primary coolant
than a gaseous substance in terms of the apparatus required.
Furthermore, the compounds mentioned offer advantages in terms of
handling and storage. Finally, they are less expensive than
hydrogen, with the result that the plant operating costs can also
be reduced.
[0015] In accordance with a concomitant feature of the invention,
the component surfaces are doped with precious metal, for example
with Pt, with the result that a lower concentration of alcohol is
required in the primary coolant.
[0016] The alcohol concentration is maintained at between 0.1 and
300 .mu.mol per kg of the primary coolant and, in a preferred
embodiment, it is maintained at less than 10 .mu.mol/kg. It is
expedient for the alcohol to be fed into the condensate or
feedwater system. The quantity which is metered in is in this case
such that the abovementioned concentration is established in the
downcomer of the boiling water reactor. The downcomer is the area
in the reactor pressure vessel which extends downward from the
opening points of the feed tubes. It is preferable to use methanol,
ethanol and propanol. However, formic acid, formaldehyde, and
acetaldehyde are also eminently suitable.
[0017] As noted above, the metering-in of alcohol may lead to
several disadvantageous results. That is, it is in effect a
balancing act between the positive and the negative effects
thereof. The instantly claimed invention provides a successful
compromise with highly improved corrosion protection while the
negative effects of the alcohol are virtually unnoticeable. This is
particularly so when the alcohol concentration is maintained at
below 10 .mu.mol/kg.
[0018] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0019] Although the invention is illustrated and described herein
as embodied in a method for protecting the components of the
primary system of a boiling water reactor in particular from stress
corrosion cracking, it is nevertheless not intended to be limited
to the details shown, since various modifications and structural
changes may be made therein without departing from the spirit of
the invention and within the scope and range of equivalents of the
claims.
[0020] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagrammatic longitudinal section taken through
a boiling water reactor; and
[0022] FIG. 2 is a diagrammatic chart showing the potentials of
CrNi steel and Pt in the presence of CH.sub.3OH and hydrogen as
reducing agents.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Referring now to the figures of the drawing in detail and
first, particularly, to FIG. 1 thereof, there is shown a highly
simplified illustration of a boiling water reactor. A pressure
vessel 1 of the reactor houses fuel assemblies 2 or fuel elements.
An alcohol of the above-mentioned type, preferably methanol, is
injected into a feedline 3, which continues inside the pressure
vessel in the form of an annular distributor line, to protect
against corrosion and in particular against stress corrosion
cracking (IGSCC). The reactor is in an operating state in which the
components of the reactor, i.e. for example the pressure vessel 1
and the non-illustrated core grid, which usually consist of CrNi
steel or an Ni-base alloy, are bright or are covered only with a
native oxide layer. The former case occurs, for example, if an
oxide layer has been removed from the component surfaces during
maintenance work. The quantity injected into the feedline 3 is such
that a concentration of from 0.1 to 300 .mu.mol/kg of alcohol, in
particular methanol, is established in the downcomer 4 which
adjoins the feedline 3 at the bottom. The optimum concentration of
alcohol is dependent on various factors, such as the component
material, the presence of precious metal doping, etc., and is
therefore to be determined on a case-by-case basis for each
individual reactor. In a specific embodiment, the concentration is
set to less than 10 .mu.mol/kg which, in a given context, provides
for an acceptable compromise with regard to good corrosion
protection and virtually negligible disadvantages otherwise
associated with the alcohol.
[0024] Tests using Pt and CrNi steel electrodes were carried out in
order to test the theoretical effectiveness of the proposed method.
The electrodes made from CrNi steel were subjected to preliminary
oxidation for 500 hours at 280.degree. C. with a water chemistry
that corresponds to the conditions of use in the reactor. The CrNi
steel electrodes which have been pretreated in this way and the Pt
electrodes were arranged in an autoclave through which hot water at
a temperature of 280.degree. C. was flowing. The chemistry of the
circulated water was set so as to correspond to the conditions in a
boiling water reactor. The oxygen content was kept at between 0.2
and 2 ppm. The reducing agents used were methanol and, for
comparison purposes, hydrogen. The potentials of the electrodes
were determined as a function of the methanol or hydrogen content
and are plotted in the diagram shown in FIG. 2 against the
methanol/oxygen or hydrogen/oxygen molar ratio. In FIG. 2, the
indication "CrNi" indicates CrNi steel. It can be seen that
metering of methanol results in a protective action which is
comparable to that achieved by metering hydrogen. In both cases,
the Pt potential is reduced to below the protection potential of
-230 mV. In the case of the undoped CrNi steel electrode, it is
likewise possible to observe similar electrochemical activities
with both methanol and hydrogen. However, in order in this case to
reduce the potential to below the protection potential, it is
necessary to establish significantly higher molar ratios.
Therefore, it is necessary to operate with a lower oxygen content
or with an excess of reducing agent. A potential of -500 mV was
measured for an oxygen content of less than 10 ppb and a methanol
content of 2 ppm (.apprxeq.62.5 .mu.mol/kg).
[0025] Although hydrogen and methanol or other alcohols, such as in
particular ethanol or propanol, have similar electrochemical
activities to hydrogen, their reactivity with respect to the
strongly oxidizing OH radicals formed during the radiolysis of
water is greater. A further advantage of the proposed method
results from the significantly lower volatility of the alcohols in
question. Whereas a large proportion of the hydrogen which is
metered in is converted into the vapor phase, discharged with this
phase and has to be catalytically oxidized as a non-condensable gas
in the off-gas system of the reactor by adding stoichiometric
quantities of oxygen, the proportion of alcohols which is converted
into the vapor phase is lower. Moreover, the proportion of alcohol
discharged with the vapor phase can be virtually completely
condensed and thereby recycled to the reactor. Consequently, the
outlay on chemicals, apparatus and control engineering measures is
reduced compared to conventional methods.
* * * * *