EP2417606B1 - Method for decontaminating surfaces - Google Patents

Description

The invention relates to a method for surface decontamination of components of the coolant circuit of a pressurized water reactor. The core of the coolant circuit is a reactor pressure vessel in which nuclear fuel-containing fuel elements are arranged. At the reactor pressure vessel several cooling loops are usually connected, each with a coolant pump and a steam generator.

Under the conditions of the power operation of a pressurized water reactor with temperatures in the range of 288 ° C show even stainless austenitic FeCrNi steels, which, for example, the tube system of the cooling loops, Ni alloys, of which, for example, the exchanger tubes of steam generators and others about for Coolant pumps used, for example cobalt-containing components, a certain solubility in water. Metal ions liberated from the abovementioned alloys pass with the coolant flow to the reactor pressure vessel, where they are partially converted into radioactive nuclides by the neutron radiation prevailing there. The nuclides, in turn, are dispersed by the coolant flow throughout the coolant system and are stored in oxide layers that form on the surfaces of coolant system components during operation. As the operating time increases, the amount of deposited activated nuclides adds up so that the radioactivity or dose rate on the components of the coolant system increases. The oxide layers contain depending on the type of a component For example, as the main component, the alloy used is iron oxide with di- and trivalent iron and oxides of other metals, especially chromium and nickel, which are present as alloying constituents in the above-mentioned steels. Nickel is always present in divalent form (Ni ²⁺ ), chromium in trivalent (Cr ³⁺ ) form.

From the EP 1 422 724 For example, a decontamination process for chromium-nickel steels is known which comprises a reduction step, an oxidation step and a decontamination step. In this case, a mixture of oxalic acid and formic acid (more than 90%) is used. The oxidizing agent used is ozone, permanganic acid or permanganate. Before control, maintenance, repair and demolition measures can be carried out on the coolant system, it is necessary to reduce the radioactive radiation of the respective components in order to reduce the radiation exposure of the personnel. This happens because the oxide layer present on the surfaces of the components is removed as completely as possible by means of a decontamination process. In such a decontamination, either the entire coolant system or a part separated therefrom by valves is filled with an aqueous cleaning solution or individual components of the system are treated in a separate container containing the cleaning solution. The oxide layer is first treated oxidatively in the case of components containing chromium, for example in the case of a pressurized water reactor (oxidation step), and then the oxide layer is dissolved under acidic conditions in a so-called decontamination step with the aid of an acid, which is referred to below as decontamination or deconic acid. The metal ions which pass from the oxide layer into the solution can then be removed from the solution by passing it through an ion exchanger. Excess oxidizing agent of the oxidation step is neutralized or reduced in a reduction step by adding a reducing agent. The dissolution of the oxide layer or the dissolution of metal ions in the decontamination step thus takes place in the absence of an oxidizing agent. The reduction of excess Oxidizing agent may be an independent treatment step, wherein the cleaning solution is dosed a reducing agent serving for the purpose of reduction, for example ascorbic acid, citric acid or hydrogen peroxide for the reduction of permanganate ions and manganese dioxide. However, the reduction of excess oxidant can also take place in the context of the decontamination step, wherein in addition to the reducing agent, a resolution of the oxide layer causing decontamination or acid is used, which is capable of excess oxidant, such as the frequently used permanganate ion and the thereof to reduce the resulting brownstone. In the said case, an amount of decontamination acid sufficient to neutralize excess oxidant on the one hand and to cause oxide dissolution on the other hand is added to the solution. In general, the treatment sequence "oxidation step reduction step decontamination step" or "oxidation step decontamination step with simultaneous reduction" is applied several times to obtain a sufficient result. The same deconic acid or mixture of deconic acids is always used in the decontamination step.

The oxidative treatment of the oxide layer is necessary because chromium-III oxides and trivalent chromium-containing mixed oxides, especially of the spinel type, are difficult to dissolve in the acids that are suitable for decontamination. To increase the solubility, therefore, the oxide layer is first treated with an aqueous solution of an oxidizing agent such as Ce ⁴⁺ , HMnO ₄ , H ₂ S ₂ O ₈ , KMnO ₄ , KMnO ₄ with acid or alkali or O ₃ . The result of this treatment is that Cr-III is oxidized to Cr-VI, which goes into solution as CrO ₄ ^2- .

Due to the presence of a reducing agent in the decontamination step, the Cr-VI formed in the oxidation step, which is present as chromate in the cleaning solution, is reduced again to Cr-III. At the end of a decontamination step, the cleaning solution contains Cr-III, Fe-II, Fe-III, Ni-II and, in addition, radioactive isotopes, e.g. Co-60th These metal ions can be removed from the cleaning solution with an ion exchanger. Deconic acid, which is commonly used in the decontamination step, is oxalic acid because it dissolves the oxide layers to be removed from component surfaces.

However, it is disadvantageous that oxalic acid with divalent metal ions, such as Ni ²⁺ , Fe ²⁺ , CO ²⁺ , Cu ^{2+ forms} sparingly soluble oxalate precipitates which are distributed throughout the coolant system and spread on the inner surfaces of pipelines and of components, for example of steam generators. deposit. In addition, the rainfall complicates the entire process implementation. Thus, organic components of a solution are often converted by treatment with an oxidizing agent and UV irradiation to carbon dioxide and water and thus removed from the solution. However, precipitation makes the solution cloudy, which significantly reduces the effectiveness of UV radiation. It also comes to coprecipitation of radionuclides and thus to a recontamination of the component surfaces. The risk of recontamination is particularly high for components with a large surface to volume ratio. This is especially the case with steam generators which have a very large number of small diameter exchanger tubes. Another disadvantage of the use of oxalic acid is that oxalate precipitates filter devices, such as the upstream of an ion exchanger filter and sieve plates or the Clogging filter from circulation pumps can clog. Finally, a further disadvantage arises when a treatment cycle consisting of oxidation step and decontamination step described above is repeated, that is to say when a renewed oxidation step follows a deconstruction step. If oxalate precipitates have formed in the previous deconvolution step, the corresponding metal ions, such as Ni in the case of a nickel oxalate precipitate, can not be removed from the cleaning solution with the aid of ion exchangers. The consequence is that in the subsequent oxidation step, the oxalate residue of the precipitates is oxidized to carbon dioxide and water, thereby consuming oxidizing agent uselessly. In contrast, if the oxalate is in solution, that is not bound in the form of a precipitate, the oxalate in a simple manner, such as before the cleaning solution is passed into an ion exchanger, destroyed in a simple and cost-effective manner, for example by means of UV light, ie converted to carbon dioxide and water. Finally, it is disadvantageous that turbidity caused by an oxalate precipitate interferes with the monitoring of the process, for example with photometry.

On this basis, it is the object of the invention to propose a decontamination method, which is improved in terms of the described disadvantages.

This object is achieved by a decontamination process according to claim 1, which is subdivided into two process stages.

In the first process stage, at least one treatment cycle is carried out, which comprises an oxidation step, a reduction step, and a subsequent first decontamination step includes. Depending on the extent and type of oxide formation on the component surfaces, such a treatment cycle can be carried out only once or even several times. In the oxidation step, the component is treated with an aqueous cleaning solution containing an oxidizing agent whose oxidizing power is sufficient to convert trivalent chromium contained in the oxide layer to hexavalent chromium. As already explained above, this step increases the solubility of an oxide layer present on the component. In the reduction step, the component is treated with a solution containing a reducing agent to reduce excess oxidizing agent from the oxidation step. In the first decontamination step, the component is treated with an aqueous solution which contains exclusively or predominantly, ie more than 50 mol%, at least one decontamination acid which is mixed with metal ions contained in the solution, in particular bivalent metal ions such as Ni-II, Fe. II, Co-II and Mn-II forms no sparingly soluble precipitates, as is the case with oxalic acid. Conveniently, a deconic acid is used which does not form sparingly soluble precipitates even with trihydric and higher valency acids, but this is the case with the acids commonly used for decontamination of the present type, for example in the case of formic acid and glyoxylic acid. In this way, especially the formation of sparingly soluble nickel oxalate precipitates is prevented. Already during or at the end of the deconcentration step, the solution is led over an ion exchanger to remove metal ions contained in it and originating from the oxide layer and / or the base metal of the component.

The reduction step and the decontamination step can also be carried out together, as already explained above.

In the first process stage can thus be removed in the proposed manner, a significant portion of the critical with respect to the formation of poorly soluble precipitates metal ions, so especially Ni-II, Fe-II and Co-II from the cleaning solution and thus from the component surface to be decontaminated without the risk of the formation of sparingly soluble precipitation. It is now possible to carry out a second decontamination step in a second process stage, in which the highly effective oxalic acid can now be used without problems especially for dissolving Fe-III present in the oxide layer and also Fe-II, since the critical divalent ions, Above all, Ni-II, no longer or in a no longer leading to precipitation concentration in the cleaning solution are present. In the method according to the invention thus two different Dekontaminationsvarianten be applied, with the first variant or the first decontamination step sparingly soluble Oxalatniederschläge forming ions and then remaining ions such as Fe-III and Fe-II can be brought into solution with respect to oxide dissolution highly effective oxalic acid , It is irrelevant in this case whether or not the "uncritical" decontamination acid used in the first process stage with respect to the dissolution of Fe-II or Fe-III from the oxide layer is effective, since this is done effectively with oxalic acid in the second process stage can.

Preferably, only oxalic acid is used in the second decontamination step. However, it is also conceivable to mix with one or more other deconic acids, but oxalic acid predominates, i. with more than 50 mole% is present.

In summary, a method according to the invention thus offers the possibility of preventing or at least greatly reducing the formation of sparingly soluble precipitates without thereby impairing the effectiveness of decontamination.

The method can be carried out in such a way that at least one treatment cycle is carried out in the first process stage, and in the subsequent second process stage, the component surface is carried out without a preceding oxidation of the second decontamination step, ie the oxide layer of the component is treated with oxalic acid. However, it is also conceivable that in the second process stage, first the oxide layer is treated approximately with the oxidizing agents mentioned above, and only then is the oxide layer dissolution carried out with oxalic acid. In this case, of course, a reduction step, as described above, is required.

Preferably, in the first decontamination step, an organic acid is used because its organic constituent, insofar as it consists of carbon, hydrogen and oxygen, converts into carbon dioxide and water and thus can be removed virtually without residue, since the carbon dioxide escapes as gas from the solution. The removal of the organic constituents takes place in a manner known per se by irradiating the solution, which has been mixed with an oxidizing agent such as hydrogen peroxide, with UV light. Preference is given to using acids arising exclusively from carbon, hydrogen and oxygen, so that even elements such as nitrogen leaves no residues in the solution, which can be removed only with the help of ion exchangers and thus with the formation of secondary waste (to be disposed of additional exchanger material).

In some countries, such as Japan, it is not allowed to load ion exchangers with complexing acids or with complexes of such acids in the course of decontamination measures of the present type. Therefore, it is useful in these cases to use acids that do not form complex compounds with metal ions.

Preferably, in the first decontamination step, an acid with max. used two carbon atoms. The decomposition of such an acid to carbon dioxide and water is faster than the decomposition of three and more carbon atoms containing acids, so that time, energy and oxidizing agent, ultimately cost can be saved.

For example, inorganic acids such as HNO3, HBF4 and H2SO4, non-complexing monocarboxylic acids formic acid, acetic acid, monohydroxyacetic acid and dihydroxyacetic acid, and complexing acids such as EDTA, nitrilotriacetic acid and tartronic acid are suitable for the decontamination step in the 1st process stage. Formic acid and glyoxylic acid have proven to be suitable with regard to waste prevention, with 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. Will be a solution containing such a salt guided over a cation exchanger, the metal ion is retained, wherein the acid anions remain in the solution and later, as described above, can be decomposed oxidatively without residue. For example, glycine containing a nitrogen atom or inorganic acids is not.

EXAMPLES

In order to test the effectiveness of the proposed method, experiments are carried out with samples from the primary circuit of a pressurized water reactor (see Table 1). The samples are immersed in a container in a cleaning solution having a volume of 1 liter and a temperature of about 90 ° C. As explained above, in a decontamination process, the metal ions dissolved out of an oxide layer are removed from the cleaning solution with an ion exchanger. For reasons of simplification, no ion exchange is carried out in the experiments, but the respective cleaning solution is discarded at the end of a treatment cycle (oxidation and decontamination step) and replaced by a new cleaning solution. In all experiments described below, the acidic range, pH about 2, worked.

With the samples according to Tables 1 to 3, 3 different process variants are carried out, each with 3 treatment cycles. Each treatment cycle includes an oxidation step and a decontamination step. For oxidizing the oxide layer, the container containing the samples is filled with an HMnO4 solution (concentration = 240 ppm). The exposure time is 16 hours. In the first two cycles, the decontamination step is not oxalic acid, but formic acid and / or glyoxylic acid (see Tables 1-3). After each oxidation step, excess oxidizing agent (HMnO4) is neutralized by adding an appropriate amount of reducing agent and then adding the acid used in each decontamination step. The reaction time of the acid in the decontamination step is in each case 5 hours. <u> Table 1 </ u> Variant 1 with sample TA-03-2 Oxide layer resolution (decontamination step) Process stage 1 Cycle 1 50 mmol / l formic acid Cycle 2 25 mmol / l glyoxylic acid Process stage 2 Cycle 3 2000 ppm oxalic acid Variant 2 with sample TA-03-3 Oxide layer resolution (decontamination step) Process stage 1 Cycle 1 25 mmol / l glyoxylic acid Cycle 2 25 mmol / l glyoxylic acid Process stage 2 Cycle 3 2000 ppm oxalic acid Variant 3 with sample TA-03-1 Oxide layer resolution (decontamination step) Process stage 1 Cycle 1 50 mmol / l formic acid Cycle 2 50 mmol / l formic acid Process stage 2 Cycle 3 2000 ppm oxalic acid

Of the samples in each case the initial and after a decompression step present Co60 gamma activity (Becqerel or Bq) and the total decontamination factors (DF), ie the ratio of the initial activity to the activity of a sample present after one cycle. The results are summarized in Table 4. <u> Table 4 </ u> Co60 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 1st cycle 1,32E + 4 / 4,1 1.01E + 4 / 4.4 9,15E + 3 / 5,6 2nd cycle 4,67E + 3 / 11,6 1.61E + 3 / 27.8 1,65E + 2/72 3rd cycle 1,38E + 2/391 5.78E + 1/776 3.07E + 1/1654

When evaluating the results, it should be borne in mind that a decontact factor of about 10 is usually sufficient. Such a factor is already reached after the second cycle. Furthermore, it can be stated that glyoxylic acid is most effective for the decontamination or dissolution of the oxide layer, in particular if this acid is used in several, preferably in all decontamination cycles of the first process stage.

In the above-described experiments simulating the process according to the invention, glyoxylic acid and formic acid were used as examples of organic acids. However, inorganic acids are also suitable for the decontamination steps of the first process stage. To prove their effectiveness, an experiment is carried out in which a sample from the primary circuit of a pressurized water reactor having a size corresponding to the above-mentioned samples has undergone a cycle consisting of oxidation step and decontamination step. At a Volume of the cleaning solution of 600 ml and a temperature of about 95 ° C was first carried out an oxidation of the existing on the sample oxide layer with HMnO4 (300 ppm) in a period of 20 hours. After this step, the residual oxidizing agent is neutralized with a mixture of hydrogen peroxide and nitric acid, the former being necessary to dissolve the manganese dioxide (MnO2) formed from HMnO4 in the oxidation step. This is followed by a 5-hour decontamination step in which the nitric acid (HNO3) already contained in solution acts as deconic acid, ie to dissolve the oxide layer present on the sample. After the decontamination step, the gamma activity of the sample drops to a value of 2.18E + 4Bq. Compared to the initial activity of the sample of 6.88E + 4Bq, this means a decontamination factor of 3.16.

Claims (10)

1. Method for chemically decontaminating the surface of a metallic component of the primary circuit of a pressurised water reactor, having an oxide layer, said method being divided into two method steps and being designed as follows:

   - at least one treatment cycle is carried out in the first method step, which comprises an oxidation stage, a reduction stage and a first subsequent decontamination stage, wherein

   -- in the oxidation stage, the component is treated with an aqueous solution containing an oxidising agent, which converts trivalent chromium contained in the oxide layer into hexavalent chromium,

   -- in the reduction stage, the component is treated with an aqueous solution containing a reducing agent for reducing excess oxidising agent from the oxidation stage,

   -- in the first decontamination stage, the component is treated with an aqueous solution containing exclusively or predominantly at least one decontamination acid, which forms no sparingly soluble deposits with metal ions contained in the solution, in particular bivalent metal ions, and

   -- the solution is lead through an ion exchanger to remove the metal ions contained therein originating from the oxide layer and/or the base metal of the component,

   - at least one treatment cycle is carried out in the second method step, which comprises a second decontamination stage wherein the component is treated with an aqueous solution containing exclusively or predominantly oxalic acid as a decontamination acid.
2. Method according to claim 1,
   characterised in that,
   a treatment cycle of the second method step comprises an oxidation stage prior to the second decontamination stage.
3. Method according to claim 1 or 2,
   characterised in that,
   an organic acid is used in the first decontamination stage,
4. Method according to claim 3,
   characterised in that,
   a decontamination acid consisting exclusively of carbon, oxygen and hydrogen is used.
5. Method according to one of the preceding claims,
   characterised in that,
   an organic acid which does not form a complex compound with metal ions is used in the first decontamination stage.
6. Method according to one of the preceding claims,
   characterised in that,
   at least one decontamination acid with a maximum of two carbon atoms in the molecule are used in the first decontamination stage.
7. Method according to claim 6,
   characterised by
   using formic acid and/or glyoxylic acid.
8. Method according to claim 7,
   characterised in that,
   glyoxylic acid is used in each first decontamination stage.
9. Method according to one of the preceding claims,
   characterised in that,
   a residue of the oxidising agent present in the purification solution at the end of an oxidation step is neutralised with a reducing agent which is added to the solution, and the thus-treated solution is used in the subsequent decontamination stage.
10. Method according to claim 9,
    characterised in that,
    the decontamination acid used in the decontamination stage serves as the reducing agent.