Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Description
Method for decontaminating surfaces
The invention relates to a method for decontaminating surfaces of
components of the coolant circuit of a pressurized water reactor. The key
element of the coolant circuit is a reactor pressure vessel in which fuel
elements containing the reactor fuel are situated. Multiple cooling loops,
each having a coolant pump and a steam generator, are usually connected to
the reactor pressure vessel.
Under the conditions of on-load operation of a pressurized water reactor at
temperatures in the range of 288 C, even stainless austenitic FeCrNi steels,
of which the piping system of the cooling loops, for example, is composed,
Ni alloys, of which the exchanger tubes of steam generators, for example,
are composed, and other components containing cobalt, for example, for
such elements as coolant pumps, have a certain solubility in water. Metal
ions which leach from the referenced alloys are carried by the coolant flow
to the reactor pressure vessel, where they are partially converted into
radioactive nuclides as the result of neutron radiation which is present at
that location. The nuclides are then distributed through the entire coolant
system by the coolant flow, and are deposited in oxide layers which form on
the surfaces of components of the coolant system during operation. Over
extended operating periods the quantity of deposited activated nuclides
accumulates, resulting in an increase in the radioactivity, i.e., the dose
rate,
of the components of the coolant system. Depending on the type of alloy
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used for a component, the oxide layers contain as the primary component
iron oxide with bi- and trivalent iron and oxides of other metals, primarily
chromium and nickel, which are present as alloy components in the above-
mentioned steels. Nickel is always present in the bivalent form (Ni2+), and
chromium, in the trivalent form (Cr3+)
Before inspection, maintenance, repair, and dismantling procedures can be
carried out on the coolant system, it is necessary to reduce the radioactive
radiation of the components in question in order to decrease the level of
personal radiation exposure. This is achieved by removing as much as
possible of the oxide layer which is present on the surfaces of the
components, using a decontamination method. In such decontamination,
either the entire coolant system or a portion which is separated therefrom by
valves, for example, is filled with an aqueous cleaning solution, or
individual components of the system are treated in a separate container
which contains the cleaning solution. For chromium-containing
components, for example in the case of a pressurized water reactor, the
oxide layer is first oxidatively treated (oxidation step), and the oxide layer
is
subsequently dissolved under acidic conditions in a so-called
decontamination step using an acid, referred to below as decontamination
acid or decon acid. The metal ions which pass from the oxide layer into the
solution may then be removed from the solution by leading them through an
ion exchanger. Excess oxidizing agent from the oxidation step is
neutralized, i.e., reduced, in a reduction step by adding a reducing agent.
Thus, the dissolution of the oxide layer or the leaching of metal ions in the
decontamination step occurs in the absence of an oxidizing agent. The
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reduction of the excess oxidizing agent may be an independent treatment
step, whereby a reducing agent which is used only for the purpose of
reduction, for example ascorbic acid or citric acid, or hydrogen peroxide for
the reduction of permanganate ions and manganese dioxide, is added to the
cleaning solution. However, excess oxidizing agent may also be reduced
within the scope of the decontamination step, using, in addition to the
reducing agent, a decontamination acid which causes the oxide layer to
dissolve, or an acid which is able to reduce excess oxidizing agent, for
example the frequently used permanganate ion and the resulting manganese
dioxide. In the mentioned case, a quantity of decontamination acid which is
sufficient on the one hand to neutralize excess oxidizing agent and on the
other hand to dissolve the oxide is added to the solution. As a rule, the
treatment sequence "oxidation step-reduction step-decontamination step" or
"oxidation step-decontamination step with simultaneous reduction" is
applied multiple times to achieve the desired result. The same decon acid or
mixture of decon acids is always used in the decontamination step.
The oxidative treatment of the oxide layer is necessary due to the fact that
chromium(III) oxides and mixed oxides, primarily of the spinel type,
containing trivalent chromium are only sparingly soluble in the acids which
are suitable for decontamination. For this reason, to increase the solubility
the oxide layer is first treated with an aqueous solution of an oxidizing
agent such as Ce4+, HMnO4, H2S208, KMnO4, KMnO4 with acid or base, or
03. As a result of this treatment, Cr(III) is oxidized to Cr(VI), which goes
into solution as Cr042-.
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Due to the presence of a reducing agent in the decontamination step, the
Cr(VI) which is produced in the oxidation step and which is present as
chromate in the cleaning solution is reduced back to Cr(III). At the end of a
decontamination step, the cleaning solution contains Cr(III), Fe(II), Fe(III),
and Ni(II), in addition to radioactive isotopes such as Co-60. These metal
ions may be removed from the cleaning solution using an ion exchanger. A
frequently used decon acid in the decontamination step is oxalic acid due to
its ability to dissolve oxide layers to be removed from component surfaces.
However, it is disadvantageous that oxalic acid together with bivalent metal
ions such as Nit+, Fee+, Coe+, and Cu 2+ forms sparingly soluble oxalate
precipitates which become distributed through the entire coolant system and
deposit on the inner surfaces of pipes and of components, for example
steam generators. As a result, the precipitates complicate carrying out the
overall method. For this reason, organic constituents of a solution are often
converted to carbon dioxide and water by treatment with an oxidizing agent
and UV irradiation, and are thus removed from the solution. However, the
precipitates cause turbidity of the solution, which significantly reduces the
effectiveness of the UV irradiation. In addition, this results in co-
precipitation of radionuclides, and thus, recontamination of the component
surfaces. The risk of recontamination is particularly high for components
having a large surface-to-volume ratio. This is the case for steam generators
in particular, which have a very large number of small-diameter exchanger
tubes. Another disadvantage of the use of oxalic acid is that oxalate
precipitates may plug filter units, such as the filters and sieve plates
provided upstream from an ion exchanger, or the protective filters of
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circulation pumps. Lastly, a further disadvantage results when an above-
described treatment cycle composed of an oxidation step and a
decontamination step is repeated, i.e., when a decontamination step is
followed by another oxidation step. If oxalate precipitates were produced in
the preceding decontamination step, the corresponding metal ions, such as
Ni in the case of a nickel oxalate precipitate, cannot be removed from the
solution by using ion exchangers. As a result, in the subsequent oxidation
step the oxalate residue of the precipitates is oxidized to form carbon
dioxide and water, and therefore oxidizing agent is needlessly consumed.
On the other hand, if the oxalate is present in solution, i.e., not bound in
the
form of a precipitate, the oxalate may be easily and cost-effectively
decomposed by UV light, for example, i.e., converted to carbon dioxide and
water, for example before the cleaning solution is led into an ion exchanger.
Lastly, a further disadvantage is that turbidity caused by an oxalate
precipitate interferes with monitoring of the process, using photometry, for
example.
On this basis, the object of the invention is to provide a decontamination
method which is improved with regard to the described disadvantages.
This object is achieved by a decontamination method which is divided into
two process stages according to Claim 1.
In the first process stage at least one treatment cycle is performed,
comprising an oxidation step, a reduction step, and a subsequent first
decontamination step. Depending on the extent and type of oxide formation
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on the component surfaces, such a treatment cycle can be performed only
once, or also multiple times. In the oxidation step the component is treated
with an _aqueous cleaning solution which contains an oxidizing agent whose
oxidizing power is sufficient to convert the trivalent chromium contained in
the oxide layer to hexavalent chromium. As previously described, as a result
of this step the solubility of an oxide layer present on the component is
increased. In the reduction step the component is treated with a solution
containing a reducing agent in order to reduce excess oxidizing agent from
the oxidation step. In the first decontamination step the component is
treated with an aqueous solution which exclusively or predominantly (i.e.,
in a proportion greater than 50 mol-%) contains at least one
decontamination acid that forms no sparingly soluble precipitates with
metal ions present in the solution, in particular bivalent metal ions such as
Ni(II), Fe(II), Co(II), and Mn(II), as is the case for oxalic acid, for
example.
It is practical to use a decon acid which also forms no sparingly soluble
precipitates with tribasic and higher basic acids, which, however, is the case
for the acids typically used for decontamination of the present type, for
example formic acid and glyoxylic acid. The formation of sparingly soluble
nickel oxalate precipitates in particular is prevented in this manner. During
or at the end of the decontamination step, the solution for removal of metal
ions contained therein which originate from the oxide layer and/or the base
metal of the component is led through an ion exchanger.
The reduction step and the decontamination step may also be carried out
together, i.e., simultaneously, as described above.
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Thus, in the proposed manner, in the first process stage a significant portion
of the metal ions, primarily Ni(II), Fe(II), and Co(II), which are critical
with
regard to the formation of sparingly soluble precipitates may be removed
from the cleaning solution, and thus, from the component surface to be
decontaminated, without the risk of forming sparingly soluble precipitates.
In a second process stage there is the option of carrying out a second
decontamination step in which highly effective oxalic acid may be easily
used, primarily to leach out Fe(III) and Fe(II) present in the oxide layer,
since the critical bivalent ions, primarily Ni(II), are no longer present or
are
present in a concentration in the cleaning solution which no longer results in
precipitates. Thus, in the method according to the invention two different
decontamination variants are used, whereby in the first variant or the first
decontamination step, ions which form sparingly soluble oxalate
precipitates are removed, and remaining ions such as Fe(III) and Fe(II) may
subsequently be brought into solution using oxalic acid, which is highly
effective with regard to oxide dissolution. It is irrelevant per se whether
the
dissolution of Fe(II) or Fe(III) from the oxide layer, brought about by the
"noncritical" decontamination acid used in the first process stage, is
effective, since this may be effectively carried out in the second process
stage using oxalic acid.
Preferably only oxalic acid is used in the second decontamination step.
However, a mixture of one or more other decon acids in which oxalic acid
predominates, i.e., is present in a proportion greater than 50 mol-%, is also
conceivable.
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In summary, a method according to the invention provides the option of
preventing or at least greatly reducing the formation of sparingly soluble
precipitates without decreasing the effectiveness of the decontamination.
The method may be carried out in such a way that in the first process stage,
at least one treatment cycle is first carried out, and in the subsequent
second
process stage the component surface is treated without a preceding
oxidation in the second decontamination step; i.e., the oxide layer of the
component is treated with oxalic acid. However, it is also conceivable that
in the second process stage the oxide layer is first treated, for example
using
the above-mentioned oxidizing agents, and only then is the oxide layer
dissolution with oxalic acid carried out. In this case, of course, a reduction
step as described above is also necessary.
An organic acid is preferably used in the first decontamination step, since
its organic component-provided that the acid consists of carbon, hydrogen,
and oxygen-may be converted to carbon dioxide and water and thus
removed with practically no residue, since the carbon dioxide escapes from
the solution as a gas. The organic constituents are removed in a manner
known per se by irradiating the solution, to which an oxidizing agent such
as hydrogen peroxide is added, with UV light. Acids are preferably used
which consist exclusively of carbon, hydrogen, and oxygen, so that no
residues, resulting from elements such as nitrogen, remain behind which are
removable only with the aid of ion exchangers, and which therefore result in
the generation of secondary waste (additional exchanger material which
must be disposed of).
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In some countries such as Japan, the loading of ion exchangers with
complex-forming acids or complexes of such acids in the course of
decontamination measures of the present type is not allowed. Therefore, in
these cases it is advantageous to use acids which do not form complexes
with metal ions.
An acid containing a maximum of two carbon atoms is preferably used in
the first decontamination step. The decomposition of such an acid to form
carbon dioxide and water takes place more rapidly than the decomposition
of acids containing three or more carbon atoms, so that time, energy, and
oxidizing agent, and ultimately also costs, may be saved.
Examples of acids which are suitable for the decontamination step in the
first process stage include inorganic acids such as HNO3, HBF4, and H2SO4,
noncomplex-forming monocarboxylic acids such as formic acid, acetic acid,
monohydroxyacetic acid, and dihydroxyacetic acid, and complex-forming
acids such as EDTA, nitrilotriacetic acid, and tartronic acid. Formic acid
and glyoxylic acid have proven to be suitable for waste prevention, the best
decontamination factors being achieved when only glyoxylic acid is used in
the first process stage. These acids form a soluble salt with the metal ions,
in particular with the nickel of the oxide layer. When such a salt-containing
solution is led through a cation exchanger, the metal ion is retained and the
acid anions remain in solution, and, as described above, may be
subsequently decomposed by oxidation in a residue-free manner. This is not
the case for glycine, for example, which contains a nitrogen atom, or for the
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inorganic acids.
Exemplary Embodiments:
To verify the effectiveness of the proposed method, tests were conducted
using samples from the primary circuit of a pressurized water reactor (see
Table 1). The samples were immersed in a 1-liter container containing a
cleaning solution at a temperature of approximately 90 C. As described
above, in a decontamination method the metal ions leached from an oxide
layer are removed from the cleaning solution using an ion exchanger. For
simplicity, ion exchange was not performed in the tests; instead, the
particular cleaning solution was discarded at the end of a treatment cycle
(oxidation step and decontamination step) and replaced with a new cleaning
solution. All of the tests described below were conducted in the acidic range
of approximately pH 2.
Three different method variants, each composed of three treatment cycles,
were carried out using the samples according to Tables 1 through 3. Each
treatment cycle included an oxidation step and a decontamination step. For
oxidation of the oxide layer, the container containing the samples was filled
with an HMnO4 solution (concentration = 240 ppm). The exposure time was
16 hours in each case. In the first two cycles, formic acid and/or glyoxylic
acid, not oxalic acid, was used for the decontamination step (see Tables
1-3). After each oxidation step, excess oxidizing agent (HMnO4) was
neutralized by adding an appropriate amount of reducing agent, followed by
addition of the particular acid used in the decontamination step. The time of
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exposure to the acid in the decontamination step was 5 hours in each case.
Table 1
Variant 1 using sample TA-03-2
Oxide layer dissolution
(decontamination step)
Method stage 1 Cycle 1 50 mmol/L formic acid
Cycle 2 25 mmoUL glyoxylic acid
Method stage 2 Cycle 3 2000 ppm oxalic acid
Table 2
Variant 2 using sample TA-03-3
Oxide layer dissolution
(decontamination step)
Method stage 1 Cycle 1 25 mmol/L glyoxylic acid
Cycle 2 25 mmol/L glyoxylic acid
Method stage 2 Cycle 3 2000 ppm oxalic acid
Table 3
Variant 3 using sample TA-03-1
Oxide layer dissolution
(decontamination step)
Method stage 1 Cycle 1 50 mmol/L formic acid
Cycle 2 50 mmol/L formic acid
Method stage 2 Cycle 3 2000 ppm oxalic acid
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For the samples, in each case the Co-60 gamma activity (Becqerel or Bq)
present initially and after a decontamination step was measured, and the
overall decontamination factor (DF), i.e., the ratio of the initial activity
to
the activity of a sample present after a cycle, was determined. The results
are summarized in Table 4.
Table 4
Co-60 activity in Bq/sample before/after treatment, and decontamination
factors
Sample TA-03-2 TA-03-3 TA-03-1
Bq / DF Bq / DF Bq / DF
Untreated 5.40E+4 4.48E+4 5.08E+4
Cycle 1 1.32E+4 / 4.1 1.01E+4 / 4.4 9.15E+3 / 5.6
Cycle 2 4.67E+3 / 11.6 1.61E+3 / 27.8 1.65E+2 / 72
Cycle 3 1.38E+2 / 391 5.78E+1 / 776 3.07E+1 / 1654
In the evaluation of the results, it is noted that a decontamination factor of
approximately 10 is generally sufficient. Such a factor is already achieved
after the second cycle. It is further noted that glyoxylic acid is most
effective for the decontamination, i.e., dissolution of the oxide layer, in
particular when this acid is used in multiple, preferably all, decontamination
cycles in the first process stage.
In the above-described tests which simulate the method according to the
invention, the organic acids glyoxylic acid and formic acid were used as
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examples. However, inorganic acids are also suitable for the
decontamination steps of the first process stage. To demonstrate their
effectiveness, a test was conducted in which a sample from the primary
circuit of a pressurized water reactor having a size corresponding to the
above-mentioned samples was subjected to a cycle composed of an
oxidation step and a decontamination step. For a cleaning solution volume
of 600 mL and a temperature of approximately 95 C, oxidation was first
carried out on the oxide layer present on the sample, using HMnO4
(300 pm), for a period of 20 hours. Residual oxidizing agent present after
this step was neutralized with a mixture of hydrogen peroxide and nitric
acid, the first component being necessary to dissolve the manganese dioxide
(Mn02) formed from HMnO4 in the oxidation step. This was followed by a
5-hour decontamination step in which the nitric acid (HNO3) already
contained in solution acted as decon acid, i.e., for dissolving the oxide
layer
present on the sample. The gamma activity of the sample dropped to a value
of 2.18E+4 Bq after the decontamination step. Compared to the initial
activity of 6.88E+4 Bq of the sample, this corresponds to a decontamination
factor of 3.16.
