EP0073366B2 - Process for decontaminating steel surfaces and disposing of nuclear wastes - Google Patents

Description

The invention relates to a method for the decontamination of steel surfaces, in particular in reactor cooling circuits, by removing the contaminated surface layer with an acidic aqueous decontamination solution and preparing the decontamination solution containing the detached radioactive substances for disposal.

Aqueous solutions of mineral acids have often been used to decontaminate reactor cooling circuits. Mineral acids are aggressive substances for the metal of the cooling circuits, and it is therefore extremely difficult to let the acid concentration alone run the decontamination process in such a way that the contaminated surface layer is effectively removed in an acceptable time, but does not corrode the pure metal of the cooling circuit, because Corroded areas in the cooling system could lead to leaks, U. serious consequences must not arise.

More complicated decontamination processes have therefore been developed, one of the best known of which is the so-called "AP-Citrox" process ("Nuclear Energy", 11th year, 1968, pp. 285-290). In this two-stage process, the contaminated metal surface is prepared in the first process stage by treatment with an oxidizing alkaline permanganate solution for several hours for the dissolution in the second process stage with a reducing aqueous solution of dibasic ammonium citrate, which also takes several hours. A rinse with water follows at each stage.

A similar two-stage decontamination process is described in US-A-3,873,362. Aqueous solutions of alkali metal permanganates, nitric acid, sodium persulfate, sodium bromate and preferably hydrogen peroxide are used here to oxidize the contaminated steel surface layer in the first process step. For the reducing second process stage, aqueous solutions of mixtures of mineral acids, such as sulfuric acid and / or nitric acid, and complex-forming substances, such as oxalic acid, citric acid or formic acid, are given, to which corrosion inhibitors, e.g. Iron (III) sulfate, iron (III) nitrate, nitric acid, phenylthiourea, or the like can be added. The use of hydrogen peroxide in the first process stage has the particular advantage that it can easily be broken down into water and oxygen so that the subsequent rinsing with water can be dispensed with.

The dissolved metallic components together with the radioactive substances are then precipitated from the used decontamination solution of the second process stage. For precipitation, the sulfuric and oxalic acid contained in the decontamination solution can be neutralized with calcium hydroxide, so that calcium sulfates and calcium oxalates are formed, which contain a large proportion of the radioactive substances present and are separated from the liquid by filtration. Instead, potassium permanganate can first be added to the used decontamination solution to decompose the oxalic acid and obtain manganese dioxide and manganese sulfates, which can then be obtained by adjusting a pH of about 10 with e.g. Calcium hydroxide can be precipitated. Here too, the precipitate precipitates only a small, albeit large, portion of the radioactive substances, so that in both cases the filtrate is still contaminated and must be disposed of in a nuclear waste disposal facility.

Such two-stage decontamination processes can be carried out as a continuous process or as a batch process, a so-called batch process. However, apart from the large amount of time required, it is unsatisfactory that chemicals and water are required, and above all that liquid radioactive waste is obtained in addition to relatively large amounts of solid radioactive waste, which makes the disposal of used decontamination solutions difficult. With the known methods, the decontamination of reactor cooling circuit runs is complex and comparatively expensive, in particular if corrosion of the pure metal surfaces is excluded for the desired safety.

It was therefore an object of the present invention to provide a decontamination process for reactor cooling circuits which, for the decontamination of a steel surface of the same size, requires smaller amounts of chemicals and flushing water than the known two-stage process, which permits such a treatment of used decontamination solution that Solid radioactive waste is only generated in minimal amounts and the resulting liquid waste has at most a low radioactivity, which is as far as possible below the permitted limit value, and which enables easy control of the decontamination process and practically eliminates corrosion of the pure steel surfaces.

The achievement of the object according to the invention consists in the method characterized in claim 1.

In the method according to the invention, the decontamination solution contains formic acid and / or acetic acid and a reducing agent containing formaldehyde and / or acetaldehyde. These chemicals are not only very cheap but also relatively non-toxic, so that no special precautions are required when handling this decontamination solution. Upon contact with the steel surfaces to be decontaminated, Fe2 + ions dissolve. The decontamination process according to the invention is accordingly a one-step process which ensures a saving in time and effort compared to a two-step process. The reducing agent contained in the decontamination solution keeps the Fe ²⁺ stable in the solution. The Liquid is then slightly greenish in color, but clearly transparent without clouding, and its composition can be monitored relatively easily during the treatment of the steel surface. It has been shown that such a decontamination solution removes iron oxides 10 to 50 times faster than the pure base material, and this allows the decontamination process to be carried out without great difficulty in such a way that the pure steel surface is attacked by the pure steel surface and leads to harmful corrosion Decontamination solution is practically excluded. Iron compounds are precipitated from the decontamination solution for disposal. Since the used decontamination solution only contains Fe2 + ions, there are no problems with the precipitation. The precipitates formed have the property of adsorbing the radioactive substances contained in the solution, so that very high precipitation decontamination factors can be achieved by separating the precipitates. The separated solid precipitate then contains practically all radioactive substances from the decontamination solution, while the liquid has at most only an insignificant residual activity, which can be below the tolerance limit, and thus regenerates the liquid for reuse or simple chemical disposal by decomposing the dissolved substances in gaseous products and water, NaOH, possibly Na ₂ C0 ₃ , can be supplied. The chemical composition of the decontamination solution provided according to the invention makes it possible to precipitate the Fe ²⁺ ions in the form of iron compounds, the density of which corresponds approximately to the density of iron oxide or which can easily be converted into such iron compounds. The radioactive waste obtained in a decontamination process is then approximately equal to the material removed from the contaminated surface and is therefore a minimum.

Advantageous developments of the invention are specified in the dependent claims.

The invention is explained in more detail below, for example.

For example, a reactor cooling circuit made of low-alloy or stainless steel can be decontaminated in a continuous process. The size of the inner surface and the capacity of the cooling circuit are known.

According to the invention, an aqueous solution of formic acid and / or acetic acid and of at least one reducing agent containing formaldehyde and / or acetaldehyde is to be used as the decontamination solution.

With the contaminated surface, radioactive substances are adsorbed in a layer made of a mixture of iron oxides, and the thickness and composition of the surface layer to be removed can be determined through previous sampling (EP-A-36541). Based on the data available or determined and the options available, such as in particular the time available for decontamination, heating or cooling devices, etc., the appropriate composition, the required amount and also the process flow for the decontamination solution the main features.

From the decontamination solution introduced into the cooling circuit, the oxides of the contaminated steel surface are dissolved directly and / or reductively and converted into soluble iron (11) formates and / or iron (11) acetates, which are caused by the decontamination solution, especially by the solution contained reducing agents are stabilized created reducing conditions and in particular an oxidation to noticeable iron (III) compounds does not take place. Used decontamination solution is therefore slightly green in color, but clearly transparent and without clouding and at most contains solid particles of the oxide layer that occur during the solution process, which do not interfere with the decontamination itself or with the treatment of the used decontamination solution for disposal.

A decontamination solution according to the invention, which generally leads to satisfactory results, needs e.g. contain only formic acid and formaldehyde, the liter of decontamination solution containing, for example, 7-22 ml formic acid and 12-36 ml formaldehyde.

• für das Reduktionsmittel Ameisensäure
  
  und für das Reduktionsmittel Formaldehyd
  
• Für die Auflösung der kontaminierten Oberflächenschicht gilt:
  
  
  
  

Such a decontamination solution is characterized by the following formulas in the presence of O ² ions:

• for the reducing agent formic acid
  
  and for the reducing agent formaldehyde
  
• The following applies to the dissolution of the contaminated surface layer:
  
  
  
  

One mole of iron reacts with two moles of formic acid and, since the molecular weights of the substances used for the decontamination solution (HCOOH: MG = 46.03, HCOH: MG = 30.03) are low, and, as has been shown experimentally, 1 I decontamination solution can take up to 30 g of iron in the form of Fe ²⁺ , the decontamination results in a relatively low consumption of chemicals, and the costs for formic acid and formaldehyde are also low, so that the method according to the invention with such a decontamination solution is particularly economical. This also applies if, instead of or in addition to formic acid and formaldehyde, acetic acid and acetaldehyde are used for the decontamination solution, so that the decontamination solution according to the invention compared to known decontamination solutions is generally characterized by a low chemical consumption and low costs as well as a high absorption capacity for Iron.

The used decontamination solution emerging from the cooling circuit is monitored during the detachment process, the Fe ^{2 +} , acid and aldehyde concentrations being continuously checked. Such a check is analytically simple and permits reliable control of the entire decontamination process, which reliably rules out inadmissible corrosion of the pure metal surface.

The iron compounds contained in the decontamination solution emerging from the cooling circuit are precipitated, and the used and thus cleaned decontamination solution is used for reuse, i. H. regenerated for reintroduction into the cooling circuit. The iron compounds are preferably precipitated electrolytically by passing the used decontamination solution through an electrolysis stage which contains an iron cathode and a graphite anode.

zu Ameisensäure oder zu CO₂ und Wasser oxydiert und an der Kathode Fe^2+-lonen gemäss

zu metallischen Eisen reduziert, an das zumindest ein erheblicher Anteil der in der Dekontaminationslösung enthaltenen radioaktiven Stoffe adsorbiert ist. Die aus der Elektrolysestufe austretende Dekontaminationslösung wird gegebenenfalls nach Ergänzung des Gehaltes an Ameisensäure und/oder Formaldehyd erneut in den Kühlkreislauf eingespeist. Anstelle der elektrolytischen kann auch eine chemische Ausfällung von Fe²⁺ vorgesehen sein, wobei dann allerdings darauf zu achten ist, dass durch den Ausfällungsprozess keine schädlichen Stoffe, vor allem keine S-Ionen, eingeschleppt werden. Im allgemeinen wird daher eine elektrolytische Ausfällung bevorzugt.At the anode are COOH'-ions according to.

oxidized to formic acid or to CO ₂ and water and at the cathode Fe ^{2 +} ions according to

reduced to metallic iron, to which at least a significant proportion of the radioactive substances contained in the decontamination solution is adsorbed. The decontamination solution emerging from the electrolysis stage is fed back into the cooling circuit, if necessary after adding the formic acid and / or formaldehyde content. Instead of electrolytic, chemical precipitation of Fe ²⁺ can also be provided, but care must then be taken to ensure that no harmful substances, especially no S ions, are introduced by the precipitation process. Electrolytic precipitation is therefore generally preferred.

Another advantage of the decontamination method according to the invention is that the reactions when the contaminated surface layer is detached are irreversible and therefore no carryover of radioactive substances to surface areas that are not or no longer contaminated is to be expected.

After removing the intended layer thickness, the decontamination solution is drained from the cooling circuit. In any case, after draining, any residues will remain in the cooling circuit. According to the composition of the decontamination solution, only those residues are present in the decontamination process according to the invention which, by simple heat treatment at 175-300 ° C., are thermal in iron oxide and in gaseous decomposition products, in particular CO, CO ₂ and H ₂ 0, ie in the cooling circuit Decomposition products, are decomposed and do not have any harmful influence on the operation. This thermal decomposition of the residues can be carried out by introducing hot air or hot water, but is generally not necessary since the cooling circuit warms up to the required temperature in a short time when it is started up again.

A cooling circuit which still has residual activities after decontamination can be rinsed in a conventional manner by ion exchange in a nuclear clean manner. However, such a flushing will only be necessary in exceptional cases, since residual activities can be easily excluded by means of a corresponding layer thickness to be removed.

The used decontamination solution that has been drained off is treated further for disposal. In the decontamination solution according to the invention, the transporter for the removed radioactive substances is the iron itself which has gone into solution and not some other additional substance, so that if the iron precipitates out of the decontamination solution, practically all the activity will be contained in the precipitate and the separated liquid at best will only have permitted radioactivity.

In the case of precipitation for disposal, the aim is that all radioactive substances contained in the used decontamination solution are adsorbed onto the smallest possible amount of precipitate, that the precipitate is easy to dispose of and that the separated liquid does not cause any environmental pollution. In contrast to the precipitation provided for the regeneration of used decontamination solution, any substances, such as S-compounds, can be used for the disposal precipitation, provided that the precipitation results obtained are satisfactory in an economical manner.

• a) Fällung von Fe²⁺ als FeS mit (NH₄)₂S gemäss
  
  wobei Ammoniumformiat erhalten wird, das sich thermisch und/oder katalytisch in CO, CO₂, H₂0 und NH₃ zersetzen lässt, und als und Wasser unlösliches Eisen(II)-sulfid (D. 4,6) ausfällt, das ein verhältnismässig kleines Molekulargewicht (MG 87,9) hat, sehr gut filtrierbar ist und z.B. gegenüber Eisenhydroxid den Vorteil geringen Wassergehaltes im Filterkuchen hat, das aber bei der Entsorgung schwieriger ist, da es sich z.B. nur schwer einzementieren lässt. Zudem ist diese Ausfällung wegen des Schwefels besser nur dann anzuwenden, wenn die abgetrennte Flüssigkeit chemisch entsorgt und nicht zur Wiederverwendung als Dekontaminationslösung aufbereitet wird.
• b) Fällung von Fe³⁺ und Fe²⁺ als Hydroxide gemäss
  
  
  wobei als Fällungsreagenz z.B. NaOH verwendet werden kann.

The precipitation processes that can be used here are very well described in the literature (eg L. Hardinger “Taschenburch der Abwasserverarbeitung” Part I and Part 11, Karl Hanser-Verlag, 1977), so that there is no need to go into details. In summary, only the following are mentioned:

• a) Precipitation of Fe ²⁺ as FeS with (NH ₄ ) ₂ S according to
  
  whereby ammonium formate is obtained, which can be thermally and / or catalytically decomposed into CO, CO ₂ , H ₂ 0 and NH ₃ , and as and water-insoluble iron (II) sulfide (D. 4,6), which is a relatively has a small molecular weight (MW 87.9), is very easy to filter and has the advantage of low water content in the filter cake, for example compared to iron hydroxide, but is more difficult to dispose of because it is difficult to cement in, for example. In addition, because of the sulfur, this precipitation is only better to be used if the separated liquid is chemically disposed of and not prepared for reuse as a decontamination solution.
• b) Precipitation of Fe ³⁺ and Fe ²⁺ as hydroxides according to
  
  
  where NaOH can be used as the precipitation reagent.

wobei das Eisen(lll)-formiat als Formiat der Hexaformiatotrieisen(III) base (Fe₃(HCO₂)₆(OH)₂HCO₂)·4H₂O in der Struktur

vorliegt und ein Verhältnis Fe:(HC0₂) = 3:7 zu beachten ist. Das erhaltene Eisen(lll)-hydroxid lässt sich leichter als Eisen(II)-hydroxid von der Flüssigkeit z.B. durch Filtrieren abtrennen, zum Fällen wird jedoch mehr Fällungsmittel als bei Eisen(II)-hydroxid benötigt.Precipitation as iron (II) hydroxide has the advantage that less NaOH is used, but the disadvantage that the precipitate is somewhat more difficult to filter than iron (III) hydroxide. If this is undesirable, the used decontamination solution first oxidizes Fe (II) formate to Fe (III) formate, for example with hydrogen peroxide

where the iron (III) formate as formate of the hexaformiatotrieisen (III) base (Fe ₃ (HCO ₂ ) ₆ (OH) ₂ HCO ₂ ) · 4H ₂ O in the structure

is present and a ratio Fe: (HC0 ₂ ) = 3: 7 must be observed. The iron (III) hydroxide obtained can be separated from the liquid more easily than iron (II) hydroxide, for example by filtration, but more precipitant is required for precipitation than in the case of iron (II) hydroxide.

und

With NaOH as the precipitant, the reactions are as follows:

and

At least a very large proportion of the radioactive substances contained in the decontamination solution is adsorbed on the precipitated iron hydroxide, and the liquid separated from the precipitate, in the present case an aqueous solution of sodium formate with any remains of formaldehyde, will be only very little or gas not radioactive. The sodium formate can now be decomposed oxidatively to NaOH, Na ₂ C0 ₃ , C0 ₂ and H ₂ 0.

An advantage of this precipitation is that the separated precipitate corresponds in weight to the material removed during decontamination, i.e. practically no weight gain has taken place and that the precipitate can be easily disposed of by mixing with cement without further treatment, expediently producing products similar to ferrocement that ensure a particularly low amount of contaminated material to be disposed of.

Another advantage of this iron hydroxide precipitation is the decomposability of the sodium formate obtained. Instead of subjecting the entire amount of used decontamination solution to the precipitation process at once during the decontamination of a cooling circuit, the decontamination solution is expediently divided into several batches. The first batch is then, if necessary after treatment with hydrogen peroxide, the required small amount of precipitant, e.g. NaOH, and after separating the precipitate, the sodium formate obtained is oxidatively, electrolytically or pyrolytically decomposed as indicated above. The liquid product obtained is then used to precipitate the second batch of decontamination solution, etc. It is therefore possible to use a significantly smaller amount of precipitant, and the disposal of the used decontamination solution can be carried out as a cycle or as such can be incorporated into a continuous decontamination process. Such a procedure is particularly expedient if the liquid separated off after the precipitation still has certain residual activities, since a corresponding dilution of the activity is thereby achieved. Which precipitation method is to be used in the individual case depends on the facilities, design options and in particular also on the capacity of the cooling circuit and the amount of material to be decontaminated.

The precipitation and liquid can be separated by simple filtration. For easy filtering, the used decontamination solution can contain flocculants, e.g. Polyacrylamide can be added, through which the precipitated particles are combined to form larger particles. The precipitate from an earlier precipitation process is used as the preferred flocculant.

und die so gewonnene Ameisensäure wird zusammen mit der vorhandenen Ameisensäure durch ein Oxydationsmittel in H₂0 und CO₂ zersetzt

und ebenso werden Salze der Ameisensäure entsorgt.As mentioned, the separated liquid is either prepared for reuse as a decontamination solution or disposed of “chemically”. For chemical disposal, formaldehyde in particular is oxidized to formic acid

and the formic acid thus obtained is decomposed together with the existing formic acid by an oxidizing agent in H ₂ 0 and CO ₂

and salts of formic acid are also disposed of.

The waste materials obtained in this way are the most environmentally friendly and do not cause any problems in disposal. Any oxidizing agent can be used, the selection being based essentially only on economy, i. H. low costs, and care must be taken to ensure that the advantageous chemical disposal is not impaired by the oxidizing agent.

Above, the invention has been treated in detail using a simple decontamination solution with formic acid and formaldehyde; however, it is readily understood that these statements apply to any other composition of the decontamination solution according to the invention.

The decontamination method according to the invention can be carried out as a continuously running process with circulated decontamination solution as well as a batch process, the advantages achieved being the same.

It has been shown in particular that surfaces decontaminated from low-alloy steel and from stainless steel can be decontaminated effectively with the decontamination method according to the invention. For example, a sample of stainless steel, whose main Magnetite-bearing surface has an activity of 8 μCi / cm ² , by means of the decontamination process according to the invention the activity is reduced to 0.025 pCi / cm ² , which results in a high decontamination factor of 330 with a material removal of approximately 10 mg / cm ² .

Claims (16)

1. A process for the decontamination of steel surfaces, particularly in nuclear reactor coolant circuits, by the removal of the contaminated surface layer by means of an acid-containing aqueous decontaminating solution and for the preparation of the used decontaminating solution containing dissolved radioactive materials for waste disposal, characterised in that the contaminated steel surfaces are treated with an aqueous solution serving as the contaminating solution made from formic acid and/or acetic acid for dissolving of iron oxides and iron, and from at least one reducing agent containing formaldehyde and/or acetaldehyde for stabilization of the Fe²⁺ ions in the solution while monitoring during the treatment the composition of the solution, and that all dissolved iron is precipitated from the used aqueous decontaminating solution as an iron compound or iron compounds while the radioactive materials are adsorbed by the iron compound(s), and the contaminated precipitate is passed to nuclear waste disposal while the solution free of radioactivity is regenerated for re-use or is passed to chemical waste disposal.

2. A process according to claim 1 characterised in that the aqueous decontaminating solution is recirculated in a loop for the treatment of the contaminated steel surfaces wherein the dissolved iron is separated out and the composition of the decontaminating solution is adjusted for a new dissolution cycle.

3. A process according to claim 2 characterised in that the dissolved iron is separated from the used decontaminating solution by electrolysis wherein the Fe²⁺ ions are preferably reduced to metal at an iron cathode.

4. A process according to claim 1 characterised in that after removal of the contaminated surface layer the steel surface is subjected to heat treatment at a temperature from 175°-300°C wherein the residue is decomposed thermally into iron oxide as one of the own materials of the steel surface and into gaseous decomposition products, particularly CO, CO₂, H₂0.

5. A process according to claim 1 characterised in that the dissolved iron is precipitated from the used decontaminating solution as a sulphide.

6. A process according to claim 1 characterised in that the dissolved iron is precipitated from the used decontaminating liquid as iron-(11)-hydroxide.

7. A process according to-claim 1 characterised in that before precipitation of the iron in the used decontaminating solution, the dissolved iron-(II)-compounds are oxidized to iron-(III)-compounds by the addition of an oxidizing agent, particularly hydrogen peroxide and are precipitated as water-insoluble iron-(III)-compounds.

8. A process according to claim 6 or 7 characterised in that, to precipitate iron-(II)-hydroxide or iron-(III)-compounds from the used decontaminating liquid alkali metal hydroxide or carbonate, particularly NaOH, is added and after separation of the precipitate from the liquid the formic and/or acetic alkali metal salt present therein is oxidatively decomposed into alkali metal hydroxide, alkali metal carbonate, carbon dioxide and water.

9. A process according to claim 8 characterised in that the precipitation of water-soluble iron compounds from the used decontaminating solution is carried out in a batch process wherein after the precipitation of a first batch of decontaminating solution and the oxidizing treatment of the separated liquid the thus treated liquid is used for precipitation of the iron compounds from a second batch of decontaminating liquid and the process is repeated until all the iron is precipitated from the whole of the decontaminating solution.

10. A process according to one of claims 5 to 9, characterised in that the precipitated iron compounds are separated from the liquid by filtering.

11. A process according to claim 10 characterised in that before filtering a flocculating agent is added to the used decontaminating solution.

12. A process according to claim 11 characterised in that as flocculating agent the precipitate of a preceding precipitation process is added to the used decontaminating solution.

13. A process according to one of claims 6 to 12 characterised in that the precipitated iron compounds are decomposed thermally and/or catalytically into iron oxide-containing radioactive materials and radioactivity-free gaseous decomposition products, particularly CO, C0₂, H₂0 and the iron oxides are passed to nuclear waste disposal.

14. A process according to one of claims 5 to 13 characterised in that the precipitation containing radioactive materials are subjected to nuclear waste disposal by mixing with cement.

15. A process according to claim 14 characterised in that by mixing the precipitate with cement a ferrocement-like product is produced.

16. A process according to claim 1 characterised in that for chemical waste disposal the radioactivity-free solution is oxidized with an oxidising agent and is decomposed to water and particularly CO, C0₂, Na₂C0₃ and, in given cases, NaOH.