EP1968075B1 - Method for decontaminating an oxidised surface of a component or a system of a nuclear plant - Google Patents

Description

The invention relates to a method for decontamination of an oxide layer having surface of a component or a system of a nuclear facility. During operation of a light water reactor, an oxidation layer forms on system and component surfaces which must be removed in order, for example, to minimize the radiation exposure of the personnel in the case of revision work. Austenitic chromium-nickel steel, for example, with 72% iron, 18% chromium and 10% nickel comes into consideration as material for a system or a component. Oxidation on the surfaces forms oxide layers with spinel-like structures of the general formula AB ₂ O ₄ . The chromium is always present in trivalent, nickel always in divalent and iron in both the two- and trivalent form in the oxide structure. Such oxide layers are chemically almost insoluble. The removal or dissolution of an oxide layer in the context of a decontamination process thus always precedes an oxidation step in which the trivalent chromium is converted into hexavalent chromium. In the process, the compact spinel structure is destroyed and iron, chromium and nickel oxides are formed which are readily soluble in organic and mineral acids. Conventionally, therefore, an oxidation step is followed by treatment with an acid, in particular with a complexing acid, such as oxalic acid.

The mentioned pre-oxidation of the oxide layer is conventionally in acidic solution with potassium permanganate and nitric acid or in alkaline solution with potassium permanganate and sodium hydroxide. At one off EP 0 160 831 B1 known process is carried out in the acidic range and used instead of potassium permanganate permanganic acid. The said processes have the disadvantage that during the oxidation treatment manganese dioxide (MnO ₂ ) is formed, which deposits on the oxide layer to be treated and inhibits the passage of the oxidant (permanganate ion) into the oxide layer. In conventional methods, therefore, the oxide layer can not be completely oxidized in one step. Rather, acting as a diffusion barrier manganese dioxide layers must be removed by intermediate reduction treatments. Normally, three to five such reduction treatments are required, which is associated with a correspondingly high expenditure of time. Another disadvantage of the known methods is the large amount of secondary waste, which is mainly due to the removal of manganese by means of ion exchangers.

In addition to the permanganate oxidation in the literature, the oxidation by means of ozone in aqueous acidic solution with the addition of chromates, nitrates or cerium-IV salts is described. The oxidation with ozone under the conditions mentioned requires process temperatures in the range of 40-60 °. Under these conditions, however, the solubility and thermal stability of ozone is relatively low, so that it is almost impossible to produce ozone concentrations on an oxide layer which are sufficiently high to break up the spinel structure of the oxide layer in an acceptable time. In addition, the introduction of ozone in large volumes of water is technically complex. Therefore, despite its disadvantages, oxidation with permanganate or permanganic acid has become established worldwide.

Furthermore, from the WO 98/53462 a method for cleaning radioactively contaminated plastic material is known in which the contaminated plastic with a decontamination solution which consists of an aqueous nitric acid solution containing a No _x ^- generating reagent.

On this basis, it is the object of the invention to provide a method for the decontamination of an oxide layer having a surface of a component or a system of a nuclear technology To propose plant, which works effectively and is particularly feasible in one stage.

This object is achieved in a method according to claim 1, characterized in that the oxidation of the oxide layer with gaseous nitrogen oxide (NO _x ) is performed. Such a procedure initially achieves the advantage that the oxidizing agent can be applied to the oxide layer at a considerably higher concentration than is the case with an aqueous solution with its limited solubility for the oxidizing agent. In addition, nitric oxide is less stable in aqueous solution than in the gas phase. In addition, an oxidant in aqueous solution, such as the primary coolant of a light water reactor, usually finds a variety of reactants, so that a portion of the oxidizing agent is consumed on its way from the feed point to the oxide layer.

With a completely dry oxide layer, the required oxidation reactions, in particular the conversion of chromium-III to chromium-VI, would take place slowly. Therefore, it is advantageous if a water film is maintained on the oxide layer during the treatment. The nitrogen oxide (NO _x ) then finds in the oxide film covering the water film or in water-filled pores of the oxide layer, the aqueous conditions required for the course of the oxidative reactions. In the event that emptied a previously filled with water system and then the gas phase oxidation is carried out, the oxide layer is still moistened or moistened with water, so a water film already exists, so this may need to be maintained only during the gas phase oxidation. A water film is preferably generated or maintained by means of water vapor.

For the desired oxidation reactions to take place in economically justifiable periods, an elevated temperature can be achieved to be required. In a further preferred variant of the method it is therefore provided that heat is supplied to the surface of a system or a component or the oxide layer present on it, which takes place for example with the aid of an external heating device or preferably with the aid of superheated steam or hot air. In the former case, the desired water film is also formed on the oxide layer at the same time.

In a further particularly preferred process variant, ozone is used as the oxidizing agent. In the case of the redox reactions taking place in or on the oxide layer, ozone is converted to oxygen, which can be supplied to the exhaust air system of a nuclear installation without further aftertreatment. Ozone is also much more stable in the gas phase than in the aqueous phase. Solubility problems as in the aqueous phase, especially at higher temperatures, do not occur. The ozone gas can thus be brought in high doses to a water-wetted oxide layer, so that the oxidation of the oxide layer, in particular the oxidation of chromium-III to chromium-VI proceeds faster, especially when working at higher temperatures.

Not only ozone, but also other oxidants have a higher oxidation potential in acidic solution than in alkaline solution. Ozone, for example, has an oxidation potential of 2.08 V in an acid solution, but only 1.25 V in a basic solution. In a further preferred process variant, acidic conditions are created in the water film wetting the oxide layer, which occurs in particular due to the metered addition of nitrogen oxides can. In particular, in the case of ozone as an oxidizing agent, a pH of 1 to 2 is maintained. The acidification of the water film is preferably carried out with the aid of gaseous acid anhydrides. These form acids under water accumulation in the water film.

If the acid anhydrides have an oxidizing effect, they can simultaneously be used as the oxidizing agent, as is the case in a preferred process variant described below.

As already mentioned, the running oxidation reactions can be accelerated by using elevated temperatures. In the case of oxidation with ozone, a temperature range of 40-70 ° C has been found to be particularly advantageous. From 40 ° C, the oxidation reactions take place in the oxide layer at an acceptable rate. However, a temperature increase is only useful up to about 70 ° C, since at higher temperatures, the decomposition of ozone in the gas phase increases significantly. The duration for the oxidation treatment of the oxide layer can be influenced not only by the temperature but also by the concentration of the oxidizing agent. In the case of ozone, acceptable conversion rates, optimum ratios at concentrations of 100 to 120 g / Nm ³ , are achieved within the abovementioned temperature range only from about 5 g / Nm ³ .

In a further preferred process variant, mixtures of various nitrogen oxides such as NO, NO ₂ , N ₂ O and N ₂ O ₄ are used for the oxidation. Even when using nitrogen oxides, the oxidation effect can be increased by using elevated temperatures, with such an increase from about 80 ° C is noticeable. The best effectiveness is achieved when working in a temperature range of about 110 ° C to about 180 ° C. The oxidation effect can also, as in the case of ozone, be influenced by the concentration of nitrogen oxides. An NO _x concentration of less than 0.5 g / Nm ³ is hardly effective. Preferably, work is carried out at NO _x concentrations of 10 to 50 g / Nm ³ .

Before a dissolution of the oxide layer present on a component surface is initiated after completion of the oxidation treatment, a rinse is that of the above-described Way treated oxide layer, for example, with deionized appropriate. In a preferred variant of the method, however, an oxide layer is subjected to steam after the oxidation treatment, wherein a condensation of the water vapor takes place at the oxide layer. In order for water vapor to condense, it may be necessary to cool the component surfaces or an oxide layer on them to a temperature below 100 ° C. It has surprisingly been found that by this treatment in or adheres to the oxide layers or component surfaces adhering activity, such as in particulate form or in dissolved or colloidal form in the condensate and is removed with this from the surfaces. This effect is clearly noticeable at water vapor temperatures above 100 ° C. Another advantage of this approach is the comparatively small amount of accumulating condensate.

Excess water vapor, that is, which has not been condensed on the treated surfaces, is removed from the system to be cleaned or a container in which an oxidative treatment has been carried out and condensed. Together with the condensate draining from a component surface, it is passed over a cation exchanger. In this way, the condensate is released from the activity and can be disposed of easily. However, a further treatment may be expedient in advance, especially if nitrate ions are contained which originate from the oxidative treatment of an oxide layer or an acidification of a water film with nitrogen oxides. The nitrates are preferably removed from the condensate by reacting with a reducing agent, in particular with hydrazine, to form gaseous nitrogen. It is expedient to set a molar ratio of nitrate to hydrazine of 1: 0.5 to 2: 5.

The attached figure shows a flow chart for a decontamination process. The system 1 to be decontaminated, for example the primary circuit of a pressurized water system, is first emptied. In the decontamination of a component, such as a primary system pipeline, this is arranged in a container. Such a container would correspond in the flow chart to the system 1. To the system 1 and the container, a decontamination circuit 2 is connected. This is gas-tight. Before commissioning, the decontamination circuit 2 and the system are checked for leaks, for example by evacuation. As a next step, the entire system, ie system 1 and decontamination circuit 2, is heated up. For this purpose, a feed station 3 for hot air and / or superheated steam is arranged in the decontamination circuit 2. The supply of air or steam via a supply line 4. In the decontamination circuit 2, a pump 5 is further provided to fill the system 1 with the appropriate gaseous medium and this, as long as necessary, to circulate in the entire system. With the help of hot air or superheated steam, the system is brought to the intended process temperature, in the case of ozone to 50-70 ° C. To generate a water film on the oxide layer of the system 1 or a system component present in a container, steam is added via the feed station 3. Separating or condensing water is separated at the system outlet 6 by means of a liquid separator 7 and removed from the decontamination circuit 2 with the aid of a condensate line 8. To accelerate the CrIII / CrVI oxidation, the water film wetting the oxide layer to be oxidized is acidified. For this purpose, 2 gaseous nitrogen oxides or finely atomized nitric acid are added at a feed station 9 of the decontamination cycle. The nitrogen oxides dissolve in the water to form the corresponding acids, such as to form nitric or nitrous acid. The metered amounts of NO _x or nitric acid / nitrous acid are chosen so that in the water film a pH of about 1 to 2 sets. As soon as the required process parameters, ie the desired temperature of the system or of an oxide film present on a surface, presence of a water film and acidity of the water film, are reached, the system 1 is supplied with ozone at a concentration of preferably 100 to 120 g via a feed stadium 10 / Nm ³ continuously supplied with in-service pump 5. If necessary, there is a continuous feed of NO _x (or HNO ₃ ) to maintain the acidic conditions in the water film and hot air or superheated steam to maintain the set temperature parallel to the ozone feed. At the system outlet 6, part of the gas / vapor mixture present in the decontamination cycle 2 is discharged, so that fresh ozone gas and possibly other auxiliary substances such as NOx can be metered in, the discharged quantity corresponding to the metered amount of gas. The discharge takes place via a scrubber for the separation of NO _x / HNO ₃ / HNO ₂ and then via a catalyst 12, in which a conversion of ozone to oxygen takes place. The ozone-free, optionally still containing water vapor oxygen-air mixture is fed to the exhaust system of the power plant. During the oxidation treatment, the ozone concentration is measured at the system return 13 by means of measuring probes (not shown). A temperature monitoring is carried out with appropriate, arranged in the area of the system 1 sensors. The amount of metered NO _x is a function of the amount of water vapor supplied. Per Nm ^{3 of} water vapor is supplied at least 0.1 g of NO _x, thereby ensuring a pH of the water film of <2.

When the Cr-III is present in an oxide layer converted to Cr VI, at least to a substantial extent, ozone, NO _x will be - hot-air feed is stopped and started by a rinsing step. Preferably, the oxide layer is acted upon by steam and ensured that the component surfaces or an oxide layer located thereon a Temperature of below 100 ° C, so that the water vapor can condense it. As already mentioned above, activity present in or on the oxide layer is removed by this treatment. In addition, the respective surfaces of acid residues, mainly so rinsed by nitrates. These have arisen during the oxidative treatment of an oxide film or during the acidification of an oxide film present on an oxide film from the nitrogen oxides used for this purpose by reaction with water. After the rinsing step carried out with water vapor, there is thus an aqueous solution containing nitrate and radioactive cations. First of all, the nitrate is converted to gaseous nitrogen with the aid of a reducing agent, the best results of which were achieved with hydrazine, and thus removed from the condensate solution. In order to completely remove the nitrate, a stoichiometric amount of hydrazine is preferably used, ie a molar ratio of nitrate to hydrazine of 2: 5 is set. Next, the active cations are removed by passing the solution through a cation exchanger.

Of course, the rinsing of an oxidatively treated oxide layer can also be done by filling the system 1 with deionized water. When filling the displaced gas is passed over the catalyst 12 while the residual ozone therein is reduced to O ₂ and, as already mentioned above, fed to the exhaust system of the nuclear power plant. The nitrate ions present on the surface of the components to be decontaminated or of the oxide layer still present there, which have been formed by metering in nitric acid or by oxidation of NO _x , are taken up by the deionate and remain during the subsequent treatment to dissolve the oxide coating the decontamination solution. This is for the purpose mentioned, an organic complexing acid, preferably oxalic acid, approximately corresponding to one in EP 0 160 831 B1 described method at a temperature of for example 95 ° C added. It will circulated the decontamination solution by means of the pump 5 in the decontamination circuit 2, wherein via a shunt (not shown), a part of the solution passed through ion exchange resins and cations dissolved out of the oxide layer are bound to the exchange resins. Finally, at the end of decontamination, an oxidative decomposition of the organic acid by means of UV irradiation to carbon dioxide and water, approximately corresponding to that in the EP patent 0 753 196 B1 described method.

In a laboratory experiment, a gas phase oxidation was carried out on a pipe section of a primary system pipeline. For this purpose, a test setup corresponding to the attached flow chart was used. The pipeline originated from a pressurized water system with more than 25 years of service operation and was provided with an inner cladding made of austenitic Fe-Cr-Ni steel (DIN 1.4551). Accordingly, dense and difficult to dissolve was the oxide formation present on the pipe inner surface. In a second laboratory experiment, the oxide layer of Inconel 600 steam generator pipes, which had been in power operation for 22 years, was preoxidized with ozone in the gas phase. Comparative tests with permanganate as the oxidizing agent were carried out in each case for the first and second laboratory tests. In further experiments, original samples from a pressurized water plant, which were in power operation for 3 years, were subjected exclusively to NO _x gas phase oxidation. The results are summarized in Tables 1, 2 and 3 below. The term "cycle" given in the tables refers to 1 pre-oxidation and 1 decontamination step. Table 1: Decontamination of austenitic Fe / Cr / Ni steel plating (DIN 1.4551) from a primary pipeline of a pressurized water reactor decontamination procedures Pre-oxidation step Sum of treatment time [h] Decontamination step Sum of treatment time [h] DF Decontamination based on permanganate + oxalic acid
3 cycles, temp. 90-95 ° C 40-60 20 10-17 Decontamination based on ozone / NO _x gas phase
1 cycle, temp. 50-55 ° C 12 6 300-400 decontamination procedures Pre-oxidation step Sum of treatment time [h] Decontamination step Sum of treatment time [h] DF Decontamination based on permanganate + oxalic acid
3 cycles, temp. 90-95 ° C 40-60 20 3-8 Decontamination based on ozone / NO _x gas phase
1 cycle, temp. 50-55 ° C 6 6 30-60 decontamination procedures Total treatment time DF Decontamination based on permanganate + oxalic acid
3 cycles, temp. 90-95 ° C 36 hours 20-35 NO _x treatment
1 cycle, temp. 150-160 ° C 12 hours 100-280

It can be seen that a much lower treatment time was required for the gas phase oxidation with ozone than with a pre-oxidation with permanganate. Surprisingly, it has also been found that the decontamination phase subsequent to the preoxidation, in which the pretreated oxide layer was thus removed with the aid of oxalic acid, could likewise be carried out in a much shorter time. As a further surprising result, it was found that in a procedure according to the invention significantly higher decontamination factors (DF) can be achieved. Since the aftertreatment was the same in the experiments and their corresponding comparative experiments, this result can only be interpreted as an effect of the pre-oxidation in the gas phase. This apparently includes an oxide film in a manner that significantly favors subsequent dissolution of the oxide layer with oxalic or other complexing organic acid.

Comparable results (see Table 3) were achieved with pre-oxidation using only NO _x as the oxidant.

Claims (20)

1. Method of decontaminating an oxide layer-comprising surface of a component or a system of a nuclear facility, wherein the oxide layer is treated with gaseous nitrogen oxide (NO_x) as oxidant.
2. Method according to Claim 1, characterized in that a film of water is maintained on the oxide layer during the treatment and a water-soluble oxidant is used.
3. Method according to Claim 2, characterized in that the film of water is produced by means of steam.
4. Method according to any of the preceding claims, characterized in that heat is supplied to the surface or the oxide layer present thereon.
5. Method according to Claim 4, characterized in that the heat is supplied by means of hot steam or hot air.
6. Method according to Claim 4, characterized in that the heat is supplied by means of an external heating device.
7. Method according to any of the preceding claims, characterized in that the surface to be treated is heated to a temperature of at least 80°C.
8. Method according to Claim 7, characterized by a temperature of from 110°C to 180°C.
9. Method according to any of the preceding claims, characterized in that an NO_x concentration of at least 1g/Nm³ is maintained during the treatment.
10. Method according to Claim 9, characterized by an NO_x concentration of from 10 to 50 g/Nm³.
11. Method according to any of the preceding claims, characterized in that the treated surfaces are treated with steam after the oxidative treatment, with condensation of the steam occurring on the surfaces.
12. Method according to Claim 11, characterized by a temperature of the steam of greater than 100°C.
13. Method according to Claim 12, characterized in that excess steam is condensed.
14. Method according to Claim 12 or 13, characterized in that the condensate is passed over a cation exchanger.
15. Method according to Claim 12, 13 or 14, characterized in that the condensate is treated with a reducing agent to remove nitrate present therein.
16. Method according to Claim 15, characterized in that hydrazine is used as reducing agent.
17. Method according to Claim 16, characterized by a molar ratio of nitrate to hydrazine of at least 1 to 0.5.
18. Method according to Claim 17, characterized by a molar ratio of nitrate to hydrazine of from 1:0.5 to 2:5.
19. Method according to any of the preceding claims, characterized in that the oxide layer is treated with an aqueous solution of an organic acid after the oxidative treatment.
20. Method according to Claim 19, characterized by the use of oxalic acid.

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