EP0247933B1 - Process for decontaminating solid materials contaminated by contaminating elements, especially by radioactive elements such as ruthenium - Google Patents

Description

The present invention relates to a method for decontaminating contaminated solid materials.

It applies in particular to the radioactive decontamination of metallic parts carrying deposits of radioactive colloidal particles, constituted for example by nodules of precious metals or of alloys of precious metals insoluble in nitric solution. Metal parts contaminated by such deposits are found in particular in facilities for reprocessing irradiated nuclear fuels.

Indeed, at the first stage of the reprocessing of irradiated nuclear fuels, the usual practice is to dissolve these fuels in a solution of nitric acid, but certain fission products present in the fuels, such as ruthenium 106 in metallic form or in the form of alloys are not soluble in such solutions and they therefore remain in the form of colloidal particles or insoluble sludge which can deposit on certain parts of the installation.

Also, it is necessary to have methods of decontamination making it possible to eliminate under good conditions these deposits of radioactive elements which generally comprise metallic elements of the platinum group.

This poses problems because these deposits are insoluble in most of the reagent solutions used for decontamination. Also, until now, decontamination of solid materials contaminated by deposits of this type has been carried out using reagents which are sufficiently aggressive to strip the material to be decontaminated and thus simultaneously remove radioactive particles deposited on its surface. This method of decontamination, which involves stripping the contaminated part, has many drawbacks.

Indeed, it can take a long time. Furthermore, it requires the use of pickling reagents generally consisting of mixtures of acids generally comprising a halogenated acid in significant proportion, which are of awkward use when they are liquid and which are difficult to incorporate into matrices. pasty due to their aggressiveness. In addition, when these reagents are used to carry out decontamination on an industrial scale, aggressive effluents are obtained at the end of the operation, the treatment of which poses additional problems while entailing significant costs.

The present invention specifically relates to a process for decontaminating contaminated solid materials, which precisely avoids this drawback.

The process according to the invention for decontaminating solid materials contaminated with one or more radioactive elements contaminating the platinum mine is characterized in that it consists in applying to said material a reagent consisting of a metal hydroxide solution alkali metal and alkali metal peroxydisulfate, so as to selectively dissolve said contaminating element (s) without appreciably attacking the solid material to be decontaminated.

In the process of the invention, a basic solution containing an oxidizing agent is thus used to directly dissolve the contaminating elements of the platinum mine.

Generally, these contaminating elements are formed by platinum group metals, by alloys of a platinum group metal or by compounds of platinum group metals. The platinum group metals are, for example, ruthenium, rhodium, palladium, osmium, irridium and platinum, in particular ruthenium 106.

The choice, in the process of the invention, of an oxidizing agent constituted by an alkali metal peroxydisulfate, makes it possible to directly obtain the dissolution of the contaminating element in the basic solution.

On the other hand, if other powerful oxidizing agents were used, this result could not be achieved. A well-known strong oxidant is potassium permanganate. However, it is practically impossible to obtain a significant dissolution of ruthenium in a soda solution containing potassium permanganate. Indeed, if one operates at a temperature of 30 ° C, the amount of dissolved ruthenium is less than 0.01%, that is to say insignificant. Furthermore, potassium permanganate has the disadvantage of rapidly precipitating manganese dioxide. Similarly, another common oxidant, hydrogen peroxide, did not make it possible to obtain appreciable dissolution of the colloidal ruthenium.

European patent EP-A 0 164 988 and Japanese patent JP-A- 76/92 736 disclose methods for removing sparingly soluble deposits, in which an oxidizing agent such as hypohalogenite is first used or an ammonium persulfate in order to oxidize the element to be dissolved in order to then dissolve it in a second reagent consisting of an acid solution. The mechanisms used in these latter processes are therefore different from those of the invention since, in this case, direct contamination of the contaminating elements in the basic solution of the oxidizing agent is not obtained.

Also known from the document Chemical Abstracts, vol. 85, 1976, abstract ^No. 50,701th, and US-A- 2,476,823 are methods using an aqueous solution of ammonium hydroxide and ammonium persulfate to remove deposits on surfaces of steel or copper steam generators or surface impurities, including lead and bismuth compounds, present on metal sheets coated with polonium, but in these two cases, they are not contaminants of the mine of the platinum.

In the invention, on the other hand, direct dissolution of the sparingly soluble elements of the platinum mine is obtained by the choice of the oxidizing agent.

The oxidizing agents used in the invention are peroxydisulfates of alkali metals such as potassium peroxydisulfate. In this case, the solution is preferably a strongly basic solution and it can be constituted by a sodium hydroxide solution.

The peroxydisulfate concentrations of the solution are chosen so as to obtain a good rate of dissolution of the noble metal using relatively short application times. Generally, the concentration of alkali metal peroxydisulfate is 10 to 25 g / l, but amounts less than 10 g / l can be used. On the other hand, concentrations higher than 25 g / l would involve excessive consumption of reagent. Likewise, it is preferred to use a strongly basic solution, for example an alkali metal hydroxide solution, the concentration of which is preferably 6 to 8 N.

For example, good results are obtained when the reagent consists of an 8N sodium hydroxide solution containing 25 g / l of potassium peroxydisulfate.

Preferably, the reagent for dissolving contaminating elements is prepared only at the time of use since the potassium peroxydisulfate exhibits moderate stability in the aqueous sodium hydroxide solution. Also, to maintain sufficient activity of the reagent, it is preferable to prepare it at the last moment.

After applying the reagent to the part to selectively dissolve the contaminating element (s), the solid material is preferably subjected to washing. The purpose of this washing is in particular to promote the total removal of the reagent containing the contaminating elements in solution, which could remain on the part after the first application.

Indeed, when potassium peroxydisulfate is used, which has a reduced stability in basic solutions, one generally obtains, at the end of the operation, a veil of amorphous appearance still containing contaminating elements, which is constituted by a sulfur residue. from the decomposition of potassium peroxydisulfate. Also, the solution used for washing must be able to dissolve this decomposition residue.

When the oxidizing agent is potassium peroxydisulfate, sulfuric acid is advantageously used diluted, for example 1N sulfuric acid.

To implement the process of the invention, the reagent can be applied to the part by the conventional techniques usually used. Thus, the reagent can be applied to the surface of the part with a brush, but when it is a question of decontaminating small parts, they can be soaked in the reagent. This decontamination can also be carried out by spraying the reagent on the parts by appropriate means to promote sufficient contact time and these operations can be repeated several times.

The washing of the part can be carried out by the same techniques. When the washing solution consists of dilute sulfuric acid, it can also be applied as a mixture with a pasty matrix such as silica gel, which makes it possible to limit the volume of acid while increasing the concentration of this several times the amount contained in a normal solution.

The duration of application of the reagent to the part is chosen according to the nature of the parts to be treated and their contamination. Generally, contact times of one hour are sufficient at room temperature. It is also possible to operate at temperatures above ambient temperature to accelerate the process of dissolution of the contaminating elements.

The parts capable of being decontaminated by the process of the invention can be made of different materials and the surface to be decontaminated can be made of this material or possibly be formed by a coating deposited on the part.

For example, these may be parts made of stainless steel which is a material widely used in installations for the reprocessing of irradiated nuclear fuels.

Other characteristics and advantages of the invention will appear better on reading the following examples, given clearly heard by way of illustration and not limitation.

Examples 1 to 7 describe the results obtained with various reagents for dissolving ruthenium. Among these examples, only examples 1, 5, 6 and 7 use the reagent of the invention.

Examples 1 to 5 .

In these examples, the properties of various reagents are tested to dissolve an aqueous colloidal suspension of ruthenium hydroxide Ru (OH) ₃ originating from the hydrolysis of ruthenium trichloride RuCl₃. This colloid does not dissolve in a nitric solution of uranyl nitrate, neither at 30 ° C, nor at 102 ° C.

In Example 1, a reagent according to the invention is used to carry out this dissolution, which consists of 8N sodium hydroxide containing 25 g / l of potassium peroxydisulfate mixed only at the time of use. To carry out the dissolution, 4.8 mg of the aqueous colloidal suspension of Ru (OH) introduit are introduced into 200 cm³ of the reagent, and the operation is carried out at 30 ° C. After one hour, the quantity of aqueous colloidal suspension of Ru (OH) ₃ which has been dissolved in the solution is determined. The results obtained are given in table I appended.

In Example 2, an aqueous solution of potassium peroxydisulfate at 25 g / l is used as dissolution reagent and the dissolution is carried out under the same conditions. The results obtained are also given in Table I.

In Example 3, a fluonitric mixture of 6N hydrofluoric acid and 6N nitric acid is used and the dissolution is carried out under the same conditions. The results obtained are given in Table I.

In Example 4, 8N sodium hydroxide is used and the dissolution is carried out under the same conditions. The results are also given in Table I.

In Example 5, a solution of 0.1% potassium permanganate in 0.01N sodium hydroxide and the dissolution is carried out under the same conditions.

In view of the results of Examples 1 to 5, it can be seen that only the reagent of the invention makes it possible to obtain satisfactory dissolution of the ruthenium. On the other hand, each of the constituents of the reagent of the invention, taken in isolation, has practically no action on the dissolution of ruthenium. Similarly, the fluonitric mixture, although very aggressive, does not dissolve ruthenium in any appreciable quantity. It is the same with MnO₄K in NaOH.

Example 6 .

In this example, the same procedure is repeated as in Examples 1 to 5, but using 0.112 mg of colloidal suspension of Ru (OH) ₃ in 10 cm³ of a 4 N nitric solution containing 314 g / l of nitrate d uranyl at 30 ° C, and 0.2 l of reagent consisting of an 8N sodium hydroxide solution containing 25 g / l of potassium peroxydisulfate. Under these conditions, the dissolved fraction of ruthenium is 99%.

Example 7 .

In this example, the procedure of example 6 is repeated, but the ruthenium comes from a suspension of Ru (OH) ₃ in the same uranyl nitrate solution which was brought to 102 ° C. for 40 h and then cooled. . Under these conditions, the fraction of dissolved ruthenium is 97.3%.

Example 8 .

In this example, the procedure of Example 1 is repeated, but using 8.9 mg of powdered ruthenium metal. Under these conditions, the fraction of dissolved ruthenium is 99.92%.

The results of Examples 1, 6 and 7 thus show that only the reagent of the invention allows significant dissolution of the ruthenium to be obtained.

Examples 9 and 10 .

In these examples, the method of the invention is used to decontaminate contaminated 316L stainless steel sheets. with ruthenium using for the dissolution of ruthenium a 25 g / l solution of potassium peroxydisulfate in 8N sodium hydroxide and by carrying out two successive applications of this reagent on the sheets for a period of 1 hour. At the end of the treatment, rinsing is carried out with 1N sulfuric acid. In Example 9, the sheets are rough rolled 316L steel sheets, while in Example 10, they are pickled and passivated 316L steel sheets.

At the end of treatment, the decontamination factor F _D in ruthenium is determined which corresponds to the ratio of the surface activities of the radioactive ruthenium before and after the decontamination treatment as well as the loss of thickness of the sheets in micrometers. The results obtained are given in Table II.

In view of these results, it can be seen that a good ruthenium decontamination factor is obtained without any noticeable pickling of the surface of the parts.

Claims (9)

1. Process for decontaminating a solid material contaminated with one or more contaminating radioactive elements chosen from the metals of the platinum group, alloys comprising a metal of the platinum group and compounds of metals of the platinum group, the said contaminating elements being insoluble in nitric acid, characterised in that it consists in applying to the said material a reactant consisting of a solution of alkali metal hydroxide and of alkali metal peroxydisulphate, so as to dissolve selectively the said contaminating element(s) without appreciably attacking the solid material to be decontaminated.
2. Process according to Claim 1, characterised in that the alkali metal hydroxide is sodium hydroxide.
3. Process according to either of Claims 1 and 2, characterised in that the alkali metal peroxydisulphate is potassium peroxydisulphate.
4. Process according to any one of Claims 1 to 3, characterised in that the concentration of alkali metal peroxydisulphate in the solution is from 10 to 25 g/l.
5. Process according to any one of Claims 1 to 4, characterised in that the concentration of alkali metal hydroxide in the said solution is from 6 to 8 N.
6. Process according to any one of Claims 1 to 5, characterised in that the solid material is next subjected to washing by means of a washing solution capable of dissolving the residues from decomposition of the alkali metal peroxydisulphate.
7. Process according to Claim 6, characterised in that the washing solution is dilute sulphuric acid.
8. Process according to any one of Claims 1 to 7, characterised in that the contaminating element comprises ruthenium-106, an alloy of ruthenium-106 or a compound of ruthenium-106.
9. Process according to any one of Claims 1 to 8, characterised in that the solid material is a stainless steel.