FR2599543A1 - PROCESS FOR DECONTAMINATING SOLID MATERIALS CONTAMINATED BY CONTAMINANT ELEMENTS, IN PARTICULAR BY RADIOACTIVE ELEMENTS SUCH AS RUTHENIUM - Google Patents

Abstract

THE INVENTION IS CONCERNED WITH THE DECONTAMINATION OF SOLID MATERIALS CONTAMINATED BY ELEMENTS SUCH AS METAL ELEMENTS OF THE PLATINUM MINE, FOR EXAMPLE RUTHENIUM -106. TO CARRY OUT THIS DECONTAMINATION, A REAGENT IS APPLIED TO THE MATERIAL CAPABLE OF SELECTIVELY DISSOLVING THE CONTAMINANT ELEMENT (S) WITHOUT APPRECIABLY ATTACKING THE SOLID MATERIAL TO BE DECONTAMINED. IN THE CASE OF RUTHENIUM, A REAGENT CONSTITUTED BY A SOLUTION OF POTASSIUM PEROXIDISULPHATE IN 8N SODA MAY BE USED. PREFERABLY, AFTER THIS APPLICATION, THE MATERIAL IS SUBJECTED TO RINSING, FOR EXAMPLE BY MEANS OF DILUTED SULFURIC ACID.

Description

PROCESS FOR DECONTAMINATION OF CONTAMINATED SOLID MATERIALS

  BY CONTAMINANT ELEMENTS! IN PARTICULAR BY

  RADIOACTIVE ELEMENTS AS QUELE-RUTHENIUM

  The present invention relates to a method for decontaminating contaminated solid materials. It applies in particular to the radioactive decontamination of metal parts bearing radioactive colloidal particle deposits, constituted for example by nodules of precious metals or precious metal alloys insoluble in nitric solution. In particular, metal parts contaminated with such deposits are found in irradiated nuclear fuel reprocessing plants. Indeed, in 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 therefore remain in the form of colloidal particles or insoluble sludge which can be deposited on certain parts of the installation. Also, it is necessary to have decontamination processes to remove in good conditions these deposits of radioactive elements which generally comprise

  metal elements of the platinum group.

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

  has many disadvantages.

  Indeed, it can take a lot of time. Furthermore, it requires the use of pickling reagents generally consisting of acid mixtures generally comprising a halogenated acid in substantial proportion, which are inconveniently used when liquid and which are difficult to incorporate into processes. pitiful matrices due to their aggressiveness. In addition, when these reagents are used to carry out decontaminations on an industrial scale, aggressive effluents are obtained at the end of the operation, the treatment of which causes additional problems while at the same time causing

significant costs.

  The present invention specifically relates to a

  method of decontaminating contaminated solid materials, which makes it possible to avoid this disadvantage.

  The method, according to the invention, of decontaminating solid materials contaminated with one or more radioactive contaminants of the platinum mine chemically unaffected by the pickling reagents, is characterized in that it consists in applying to said material a reagent capable of selectively dissolving said contaminant element (s) without appreciably attacking the solid material to be decontaminated.

  The reagent is selected according to the nature of the contaminant or elements to be removed.

  Generally, the insoluble contaminants are

  consisting of platinum group metals, platinum group metal alloys or platinum group metal compounds such as ruthenium, rhodium, palladium, osmium, iridium and platinum. .

  In this case, the reagent capable of selectively dissolving the contaminating element (s) is preferably

  consisting of a basic solution containing an oxidizing agent.

  A well-known potent oxidant is potassium permanganate. But it has the disadvantage of rapidly precipitating manganese dioxide. Another common oxidant, hydrogen peroxide, failed to obtain dissolution

  appreciable colloidal ruthenium.

  The oxidizing agents which may be used may in particular be peroxydisulphates, for example alkali metal peroxydisulphates, such as potassium peroxydisulphate. In this case, the solution is preferably a strongly basic solution and it may be constituted by a

  alkali metal hydroxide solution such as sodium hydroxide.

  The peroxydisulfate concentrations of the solution are chosen so as to obtain a good degree of dissolution of the noble metal by using relatively short application times. Generally, the concentration of alkali metal peroxydisulfate is 10 to 25 g / l, but 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 a strong base solution such as an alkali metal hydroxide, the concentration of which is preferably 6 to 8 N. By way of example, good results are obtained. when the reagent consists of 8N sodium hydroxide solution

  containing 25 g / l of potassium peroxydisulfate.

  Preferably, the contaminant dissolving reagent is prepared only at the time of use because the potassium peroxydisulfate has moderate stability in the aqueous sodium hydroxide solution. Also, to maintain sufficient reagent activity, it is best to prepare it for

last moment.

  After applying the reagent to the room for

  selectively dissolving the contaminating element (s), the solid material is preferably subjected to washing. This washing is intended in particular to promote the total removal of the reagent containing the contaminants in solution, which could remain on the part after the first application.

  Indeed, when using potassium peroxydisulfate, which has a reduced stability in the basic solutions, one generally obtains, at the end of the operation, a veil of amorphous appearance still containing contaminating elements, which consists of a residue. sulfur from the decomposition of potassium peroxydisulfate. Also, the solution used for the washing must be capable of dissolving this decomposition residue. When the oxidizing agent is potassium peroxydisulfate, sulfuric acid is advantageously used.

  diluted, for example 1N sulfuric acid.

  To carry out the process of the invention, the reagent can be applied to the part by the conventional techniques normally used. Thus, the reagent can be applied to the surface of the workpiece by brush, but when

  This is to decontaminate small parts, they can be dipped in the reagent. This decontamination can also be accomplished 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 piece can be carried out by the same techniques. When the washing solution consists of dilute sulfuric acid, it can also be applied in admixture with a pasty matrix such as silica gel, which makes it possible to limit the volume of acid while increasing the concentration. of it to several times the amount contained

in a normal solution.

  The duration of application of the reagent on 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 room temperature to accelerate the dissolution process of the contaminating elements. The parts that can be 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 be optionally formed by a deposited coating.

on the piece.

  By way of example, these may be stainless steel parts, which is a material widely used in irradiated nuclear fuel reprocessing plants. Other features and advantages of the invention will become more apparent upon reading the following examples given well.

  heard for illustrative and not limiting.

  Examples 1 to 7 describe the results obtained with

  different reagents to dissolve ruthenium. Of these examples, only Examples 1, 5, 6 and 7 use the reagent of the invention.

Examples I to 4.

  In these examples, the properties of different reagents are tested to dissolve an aqueous colloidal suspension of ruthenium hydroxide Ru (OH) from the hydrolysis of ruthenium trichloride RuCL. This solution does not dissolve in a nitric solution of uranyl nitrate, nor at 30 C,

1020C.

  In Example 1, a reagent according to the invention consisting of 8N sodium hydroxide containing 25 g / l of potassium peroxydisulfate is used to carry out this dissolution.

  mixed only at the time of use. To carry out the dissolution, 48 mg of the Ru (OH) aqueous colloidal suspension is introduced into 200 cm of the reagent, and the reaction is carried out at 300 ° C.

  After one hour, the amount of Ru (OH) aqueous colloidal suspension which was solubilized in the solution was determined. The results obtained are given in the attached Table I. In Example 2, an aqueous solution of potassium peroxydisulfate at 25 g / L was used as dissolving reagent and the dissolution was carried out under the same conditions. The results obtained are also given in

Table I.

  In Example 3, a ferritic mixture of 6N hydrofluoric acid and 6N nitric acid was used and the dissolution was 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 view of the results of Examples 1 to 4, it can be seen that only the reagent of the invention makes it possible to obtain satisfactory dissolution of 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 the

  ruthenium in appreciable quantity.

  Exemxtel5. In this example, the same procedure is used as in Examples 1 to 4, but using 0.112 mg. 3 colloidal suspension of Ru (OH) in 10 cm of a 4 N nitric solution containing 314 g / l of uranyl nitrate at 30 ° C., and 0.2 l of 8N sodium hydroxide solution reagent containing 25 g / l 1 of potassium peroxydisulfate. In these circumstances, La

  dissolved fraction of ruthenium is 99%. ElSPL -6.

  In this example, the procedure of Example 5 is repeated, but the ruthenium comes from a suspension of Ru (OH) in the same solution of uranyl nitrate which has been

  brought to 102 C for 40h and then cooled. Under these conditions, the dissolved ruthenium fraction is 97.3%.

  L__W2Le_7. In this example, we repeat the procedure of

  Example 1, but using 8.9 mg of ruthenium metal powder. Under these conditions, the dissolved ruthenium fraction is 99.92%.

  The results of Examples 1, 6 and 7 thus show that only the reagent of the invention makes it possible to obtain a solution

important ruthenium.

Ex3 ples 8 and 9.

  In these examples, the method of the invention is used to decontaminate ruthenium-contaminated 316L stainless steel sheets by using a 25 g / l solution of potassium peroxydisulfate in 8N sodium hydroxide solution and for the dissolution of ruthenium. performing two successive applications of this reagent on the plates for a period of 1 hour. In end of

  treatment, rinsing is carried out with 1N sulfuric acid.

  In Example 8, the sheets are rolled 316L steel sheets, whereas in Example 9 they are steel sheets.

316L pickled and passivated.

  At the end of the treatment, the decontamination factor F is determined in ruthenium which corresponds to the ratio of D

  Surface activities of radioactive ruthenium before and after The decontamination treatment and 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

  produce a noticeable stripping of the surface of the pieces.

TABLE I

  Origin of Ruthenium x Fraction Reagent Dissolved Dissolution Colloidal Suspension K2S208 at 25 grams of Ru (OH) i in NaOH, 8N 99, 93%

  from hyrolysis 1 hour at 30C.

trichloride.

  2 K2S208 at 25 g.1-1 in water 1 hour 1.02%

at 30 C.

  HF fluonitric mixture, 6N + HNO3.6N 0.74% 1 hour at 30 C NaOH, 8N 1 hour at 30 C 0.77% Colloidal suspension K2S208 at 25 g of Ru (OH) 3 in NaOH, 8N 99 3 in NaOH, 8N 99%

  nitric solution of 1 hour at 300C.

uranyl nitrate at 30 C.

  Suspension of Ru (OH) 3 K2S208 at 25 g.1 in a 2 2 nitric solution of nitrate in NaOH, 8N 97.3%

  uranyl having been 1 hour at 30 C.

  brought to 102 C for 40 hours and cooled.

  Metallic Ruthenium K2S208 at 25 g.1 powdered in NaOH, 8N 99.92% 1 hour at 30 ° C

TABLE II

  Ex. Nature and quality Decontamination factor Loss of thick material (M) Steel sheet 316 L 233 + 14 0.044 + 0.006 rolling mill Steel plate 316 L 177 + 26 0.048 + 0.006 9 pickled and passivated

Claims (12)

  1. A method for decontaminating a solid material contaminated with one or more radioactive contaminants of the platinum mine chemically unaffected by the pickling reagents, characterized in that it consists in applying to said material a reagent capable of selectively dissolving the said contaminants or elements without attacking so   appreciable the solid material to decontaminate.   2. Process according to claim 1, characterized in that the contaminating element is a platinum group metal, an alloy of a platinum group metal or a platinum group metal compound, the reagent capable of to dissolve or Contaminants is = a basic solution containing an oxidizing agent.   3. Method according to claim 2, characterized in that   that the oxidizing agent is an alkali metal peroxydisulfate.   4. Method according to any one of claims 2   and 3, characterized in that the basic solution is a solution of alkali metal hydroxide.   5. Method according to claim 4, characterized in that   that the alkali metal hydroxide is sodium hydroxide.   6. Process according to claim 2, characterized in that the reagent consists of a soda solution containing potassium peroxydisulfate.   7. Method according to any one of claims 3   to 6, characterized in that the concentration of peroxydisulphate   alkali metal solution is 10 to 25 g / l.   8. Method according to any one of claims 3   at 7, characterized in that the concentration of alkali metal hydroxide of said solution is from 6 to 8 N.   9. A method as claimed in any one of claims 2   8, characterized in that the solid material is then washed with a washing solution capable of   dissolving the decomposition residues of said oxidizing agent.   10. Process according to claim 9, characterized in that the washing solution is diluted sulfuric acid.   11. Method according to any one of claims 1   at 10, characterized in that the contaminating element comprises ruthenium -106, a ruthenium alloy -106 or a compound of ruthenium -106.   12. Process according to any one of claims 1   at 11, characterized in that the solid material is a stainless steel.