WO2024134076A1 - Method for treating an acidic aqueous solution comprising fluoride ions and radionuclides - Google Patents

Abstract

The invention relates to a method for treating an acidic aqueous solution A comprising hydrofluoric acid, fluoride ions and radionuclides, which comprises at least the following steps: a) adjusting the pH of the aqueous solution A to a value greater than or equal to 10; b) reacting the fluoride ions of the aqueous solution A with calcium ions by mixing a source of calcium ions with the aqueous solution A in proportions such that the calcium ions are in excess relative to the fluoride ions, while maintaining, if necessary, the pH of the mixture at a value greater than or equal to 10; c) subjecting the mixture formed in step b) to a solid-liquid separation, whereby a precipitate comprising fluorine and an aqueous phase depleted in fluoride ions are obtained, the radionuclides being distributed between the precipitate and the aqueous phase according to their solubility in aqueous medium at the pH of the mixture formed in step b); and in which step b) is carried out simultaneously with step a) or after step a). Field of application: nuclear industry.

Description

METHOD FOR TREATING AN ACIDIC AQUEOUS SOLUTION COMPRISING FLUORIDE IONS AND RADIONUCLEIDS

DESCRIPTION

TECHNICAL AREA

The invention relates to the field of treatment of acidic and radioactive aqueous solutions.

More specifically, it relates to a process for treating an acidic aqueous solution comprising both fluoride ions and radionuclides, with a view to optimizing the treatment of this solution and the management of waste produced from this type of solution.

The invention finds application in the nuclear industry, in particular for the treatment of solutions resulting, directly or after possible dilution, from an attack by hydrofluoric acid, alone or mixed with another inorganic acid, one or more radioactive solids produced during spent nuclear fuel processing operations and/or flushing and/or sanitation operations of a nuclear installation in operation or dismantling.

STATE OF PRIOR ART

In the spent nuclear fuel processing plant in La Hague (France), the laboratories, responsible for characterizing the radioactive solids produced during this treatment or during the rinsing or sanitation operations of nuclear installations in operation or in dismantling, are required to put these solids into solution by subjecting them to attack with concentrated mixtures of hydrofluoric acid and nitric acid, the procedures for which are described in standard NF M60-323.

The following are produced in particular:

- solutions resulting from a dissolution (otherwise called attack) of precipitates of fission products (FP) with a mixture of hydrofluoric acid 0.25 N and 14 N nitric acid, followed by a dilution, in particular ^4/5 with 0.5 N nitric acid, and

- solutions resulting from a dissolution (otherwise called attack) of dissolution fines with a mixture of 11 N hydrofluoric acid and 7 N nitric acid, also followed by a dilution, in particular ^4/5 by 0.5 N nitric acid.

These solutions of very high acidity are rich in fluoride ions, which result from a dissociation of hydrofluoric acid in an aqueous medium, and in various radionuclides, essentially P/y emitters including cesium 137 and cesium 134, or a emitters. including americium 241 and curium 244. In addition, they may include cations of the molybdenum, zirconium, palladium, iron, etc. type, depending on the initial solid put into solution (precipitated or fines).

It turns out that only a small fraction of these solutions is used for characterizations and the remainders are likely to be transformed, without prior treatment, into solid waste by immobilization in a stable matrix.

With such immobilization, on the one hand, all the radiological activity of the remainders would be found in the solid waste thus produced and, on the other hand, the volume of this solid waste would be relatively large.

From the perspective of optimizing the management of nuclear waste, it would therefore be desirable to have a process which makes it possible to treat the remainders of fluoronitric attack solutions for radioactive solids and, in general, any aqueous solution. acid which includes fluoride ions together with radionuclides, so as to minimize both the radiological activity and the volume of solid waste produced from this type of solution.

It would also be desirable for this process to be simple to implement and for its implementation to not require equipment other than that which is conventionally equipped in armored chains and glove boxes in laboratories of nuclear fuel cycle installations. STATEMENT OF THE INVENTION

The invention aims precisely to propose a process for treating an acidic aqueous solution A, comprising hydrofluoric acid, fluoride ions resulting from a dissociation of hydrofluoric acid and radionuclides, which process comprises at least the steps following: a) adjust the pH of aqueous solution A to a value greater than or equal to 10; b) react the fluoride ions present in the aqueous solution A with calcium ions by mixing, with stirring, a source of calcium ions with the aqueous solution A in proportions such that the calcium ions are in excess compared to the fluoride ions, while maintaining, if necessary, the pH of the mixture at a value greater than or equal to 10; c) subjecting the mixture formed in step b) to a solid-liquid separation, whereby a precipitate is obtained comprising fluorine and an aqueous phase poor in fluoride ions, the radionuclides having been distributed between the precipitate and the phase aqueous depending on their solubility in an aqueous medium at the pH of the mixture formed in step b); and in which step b) is carried out simultaneously with step a) or after step a).

Thus, according to the invention, a defluorination of the aqueous solution A is carried out by precipitating the fluoride ions present in this solution in the form of fluorine, CaF2, by reaction of said fluoride ions with an excess of calcium ions, for example the order of 5% to 10%, reaction which is written:

This reaction being carried out at a pH at least equal to 10, the radionuclides present in aqueous solution A, which are insoluble or poorly soluble in an aqueous medium at the pH of the mixture formed in step b), are in whole or in part immobilized in the precipitate (which will be, for example, the case of antimony 125, americium 241, europium 154 and cobalt 60 if, of course, these radionuclides are present in aqueous solution A) while those which are soluble in an aqueous medium at the pH of the mixture formed in step b) are found mainly or even exclusively in the aqueous phase resulting from the solid-liquid separation (which will, for example, be the case for cesium 137 and cesium 134 if, of course, these radionuclides are present in aqueous solution A).

As a result, the treatment of aqueous solution A by the process of the invention makes it possible to obtain:

- on the one hand, a solid waste (namely the precipitate) in which the fluoride ions are stably immobilized in the form of fluorine and whose radiological activity and volume are significantly reduced compared to those which a waste would present obtained by immobilization of the entirety of this solution in the form of a solid material, and

- on the other hand, an aqueous phase which, because it has a low, or even very low, concentration of fluoride ions, can be directed towards a concentration unit (by evaporation), calcination and vitrification (i.e. i.e. immobilization of radionuclides in a vitreous matrix) or towards an effluent treatment unit before discharge into the environment in compliance with administrative authorizations.

In the above, the expression "while maintaining, if necessary, the pH of the mixture at a value greater than or equal to 10" means that in step b), the pH of the mixture is maintained at a value at least equal to 10 if it turns out that this pH tends to fall below 10 due to the reaction of fluoride ions with calcium ions.

According to the invention, the aqueous solution A is, preferably, a solution which comprises hydrofluoric acid together with an inorganic acid other than hydrofluoric acid, for example nitric acid, phosphoric acid or sulfuric acid, preference being given to nitric acid.

Furthermore, in step a), the pH of the aqueous solution A is advantageously adjusted to a value equal to or greater than 11, preferably equal to or greater than 12 and, better still, between 13 and 14.

The source of calcium ions is preferably an inorganic calcium salt such as calcium hydroxide (or slaked lime), calcium nitrate, calcium carbonate, calcium sulfate or calcium phosphate, this salt may or may not be in hydrated form. Among these, all preference is given to calcium hydroxide, calcium nitrate and calcium carbonate knowing that, if the fluoride ion content of aqueous solution A is greater than or equal to 30 g/L, then the use of calcium hydroxide is highly recommended.

According to the invention, the source of calcium ions can be in the form of an aqueous suspension, which will particularly be the case if the source of calcium ions is calcium hydroxide. In fact, it is preferred to use calcium hydroxide in the form of a milk of lime previously obtained by dissolving slaked lime in water, preferably by supersaturating the water with slaked lime (for example, at the level of 400 g of slaked lime per liter of water), and to which the aqueous solution A is added in portions and with stirring, raising the pH above 10 and, preferably, 11 by adding sodium hydroxide if necessary .

In this regard, we recall that slaked lime is the product of the reaction of quicklime with water, quicklime being itself the product of thermal decomposition of limestone (for example, by calcination ) and being, as such, essentially made up of calcium oxide, CaO. By reaction with water, the calcium oxide in quicklime is transformed into calcium hydroxide, Ca(OH)2, which is therefore the main constituent of slaked lime.

Alternatively, the source of calcium ions can also be in a particulate form, typically a powder, which will particularly be the case if the source of calcium ions is calcium nitrate, calcium carbonate, calcium phosphate or calcium sulfate.

As previously indicated, step b) can be carried out simultaneously with step a) or after step a). In fact, everything depends on the source of calcium ions used to bring, when mixed with aqueous solution A, the pH of the latter to a value at least equal to 10.

Thus, if the source of calcium ions is a milk of lime, then mixing this milk of lime with the aqueous solution A can make it possible to carry out steps a) and b) simultaneously. On the other hand, if the source of calcium ions is calcium nitrate, calcium carbonate, calcium sulfate or calcium phosphate, then it is preferable to carry out steps a) and b) one after the other. other.

In which case, step a) comprises an adjustment of the pH of the aqueous solution A by mixing with an aqueous solution of a strong base, typically sodium hydroxide or potassium hydroxide, preferably in the form of a very concentrated solution, for example 18 N or 20 N.

In accordance with the invention, the process can comprise between steps b) and c) a step consisting of leaving the mixture formed in step b) to rest (also called ripening), in which case the sizes of the fluorite seeds formed go increase until you obtain grains which can be more easily separated by solid-liquid separation during step c) while depleting the solution in fluoride ions.

Step c) can be carried out by any technique making it possible to separate a precipitate from the aqueous medium in which it is immersed, for example by filtration, in particular under vacuum or under pressure, by decantation, by centrifugation or even by decantation followed by a centrifugation, the latter technique being preferably used in shielded chains or glove boxes in laboratories of nuclear fuel cycle installations.

Furthermore, step c) can be followed by one or more washings of the precipitate with water or with a weakly concentrated sodium hydroxide solution, for example 0.2 N, each washing itself being supplemented by a solid-liquid separation.

In accordance with the invention, the aqueous solution A may be any solution resulting, directly or after possible dilution, from an attack by hydrofluoric acid, alone or mixed with another inorganic acid such as nitric acid. , one or more radioactive solids produced during spent nuclear fuel processing operations and/or flushing and/or sanitation operations of a nuclear installation in operation or dismantling.

Thus, the aqueous solution A can in particular be:

- a solution resulting from an attack on one or more precipitates produced during the processing of spent nuclear fuel and, in particular, a solution resulting an attack on one or more PF precipitates with a mixture of hydrofluoric acid, for example 0.25 N, and nitric acid, for example 14 N, or

- a solution resulting from an attack on dissolution fines with a mixture of hydrofluoric acid, for example 11 N, and nitric acid, for example 7 N, or else

- a mixture of these.

Alternatively, the aqueous solution A can also be a solution resulting from an attack, in particular fluoronitric, of one or more radioactive solids resulting from rinsing and/or sanitation operations of a spent nuclear fuel processing plant. or a nuclear reactor in operation or being dismantled.

By PF precipitates is meant the residues based on zirconium molybdate and/or cesium phosphomolybdate, which are obtained by concentration, typically by evaporation, of the aqueous solutions resulting from the extraction, separation and purification operations of the Uranium and plutonium typically used in the processing of spent nuclear fuel.

By dissolution fines, we mean all the solids of small particle size remaining after dissolution of the spent nuclear fuel in nitric acid and separated by clarification of the solution thus obtained by means of pendulum centrifugal decanters.

Other characteristics and advantages of the invention will emerge from the additional description which follows.

It goes without saying, however, that this additional description is given only by way of illustration of the object of the invention and should in no way be interpreted as a limitation of this object.

DETAILED DESCRIPTION OF SPECIFIC IMPLEMENTATION MODES

Example I: Treatment with calcium nitrate of a mixture of solutions for attacking a PF precipitate with HF and HNO3:

The example which follows relates to a test carried out with 100 mL of a solution, hereinafter called "SI solution", previously obtained by bringing together four remainders, of 25 mL each, of solutions all four resulting from the attack of a precipitate of products fission with 40 mL of a mixture of 0.25 N hydrofluoric acid and 14 N nitric acid, the volume of which was adjusted to 50 mL by the addition of 0.5 N nitric acid.

The SI solution comprises approximately 3.8 g/L of fluoride ions and, as visible in Table 1 below, cesium 137, cesium 134, antimony 125, europium 154, ruthenium 106 , rhenium 106, cerium 144 and praseodymium 144, the activity linked to cesium 137 being largely in the majority.

The activity of each of these radionuclides in the SI solution, as measured by γ spectrometry, is shown in Table 1.

For the treatment of the SI solution by the method of the invention, the procedure is as follows:

1. a volume of 80 mL of 20 N sodium hydroxide is introduced into a can;

2. a volume of 100 mL of the SI solution is gradually introduced with stirring into the can so that the pH of the mixture between the SI solution and NaOH is greater than a value of 13 (measured with pH paper);

3. 32 g of calcium nitrate tetrahydrate, Ca(NO3)2.4H2O, are gradually added to the SI solution at the adjusted pH;

4. the mixture obtained at point 2 is allowed to decant for at least 48 hours to obtain by maturation a precipitate of sufficient size for the solid/liquid separation of the following step;

5. the mixture obtained at the end of ripening is transferred several times into two centrifuge tubes, each transfer being followed by centrifugation (3,200 or 3,600 rpm) then by sampling of the supernatant;

6. the precipitate obtained at point 4, separated by centrifugation in the two tubes at point 5, is subjected to two successive washes, each wash being carried out by introducing 10 mL of desilicated water into each centrifuge tube, subjecting each tube to centrifugation (3,200 or 3,600 rpm) and removing the supernatant.

The supernatants obtained in points 5 and 6 above are combined in the same container to form a solution (volume: 180 mL) which, for the sake of simplicity, will be considered in the following as being the SI solution after treatment. A first sample of this solution is subjected to a determination of fluoride ions by ion chromatography while a second sample is subjected to an analysis by spectrometry to measure the activity of the different radionuclides it contains.

The dosage of fluoride ions shows that their concentration in the SI solution after treatment is less than 10 mg/L, which makes it possible to consider the possibility of directing this solution to a concentration unit by evaporation, calcination and vitrification.

As for the activity of radionuclides, it is reported in Table 1 below.

Table 1

* Volume of supernatants obtained at points 5 and 6 combined

This table shows that more than 99% of the activity of cesium 137 and 134 in the SI solution before treatment is found in the SI solution after treatment. There was therefore no significant immobilization of cesium 137 and 134 in the precipitate formed during this treatment.

Knowing that the activity of cesium 137 and 134 alone represented approximately 93% of the P/y activity of the SI solution before treatment, the majority of the P/y activity of the SI solution is therefore found in this solution after treatment.

Table 1 shows, on the other hand, a strong decrease in the activity of antimony 125, europium 154, cerium 144 and praseodymium 144 in the SI solution after treatment compared to what it is before treatment. , which means that these radionuclides have been partially or almost completely immobilized in the precipitate. In this example, it should be noted that the activity of ruthenium 106 and, consequently, of rhenium 106 is higher in the SI solution after treatment than that in the SI solution before treatment, which is attributed to pollution by the atmosphere of the armored chain in which the test was carried out.

Example II: Treatment with milk of lime of a mixture of solutions for attacking fines dissolved by HF and HNO3:

The example which follows relates to a test carried out with 80 mL of a solution, hereinafter called "solution S2", corresponding to the mixture of different residues of solutions resulting from the attack of dissolution fines by a mixture of 11 N hydrofluoric acid and 7 N nitric acid.

Solution S2 includes 170 g/L of fluoride ions and has a P/y activity of 1.0 .10 ¹⁰ Bq/L, mainly linked to ruthenium 106 (45%), the other radionuclides being rhodium 106, antimony 125 and cesium 137.

For the treatment of this S2 solution with milk of lime, the procedure is as follows:

1. 275 mL of milk of lime with an initial pH equal to 13 is prepared in a pitcher;

2. a volume of 80 mL of solution S2 is introduced into the pitcher gradually, 10 mL by 10 mL, with, after each addition, a check (using pH paper) that the pH of the mixture is greater than 11 and, failing that, , the pH is adjusted to 13 by adding 20 M sodium hydroxide;

3. the mixture is left to settle for 24 hours, whereby a precipitate in suspension and a little clear supernatant liquid are obtained;

4. we check that the pH of the clear supernatant liquid is greater than 11 and, if this is not the case, this pH is readjusted to 13 by adding sodium hydroxide and then subjected to centrifugation (2,200 rpm). min) this liquid with a little of the suspension, after which the centrifugation supernatant (47 mL) is taken;

5. the suspension remaining in the jug at point 4 is subjected to two successive washes with 0.2 N sodium hydroxide followed by decantation and centrifugation of the liquid resulting from this decantation with a little suspension, then sampling of the centrifugation supernatants (30 mL and 74 mL respectively for washes 1 and 2). The centrifugation supernatants obtained in points 4 and 5 above are combined in the same container to form a solution (volume: 151 mL) which, for the sake of simplicity, will be considered in the following as solution S2 after treatment.

A first sample of this solution is subjected to a determination of fluoride ions by ion chromatography while a second sample is subjected to an analysis by spectrometry to measure the activity of the different radionuclides it contains.

The determination of fluoride ions shows that their concentration in solution S2 after treatment is 10.6 mg/L, or 99.99% of fluoride ions immobilized in the precipitates.

As for the activity of radionuclides, it is reported in Table 2 below.

Table 2

* Volume of supernatants obtained at points 4 and 5 combined

Example III: Treatment with different sources of calcium ions of a mixture of attack solutions by HF and HNO3:

The following example relates to different tests carried out on volumes of 100 mL or 120 mL depending on the tests, of a solution, hereinafter called "solution S3", corresponding to a mixture between:

- various remaining solutions (40 mL) resulting from attacks on precipitates or other solid samples coming from different locations in the La Hague spent nuclear fuel processing plant with mixtures of 0.25 N hydrofluoric acid and 14 N nitric acid and whose volumes were adjusted to 50 mL by adding 0.5 N nitric acid; And

- various remaining solutions (40 mL) resulting from attacks on precipitates or other solid samples coming from different locations in the La Hague spent nuclear fuel processing plant with mixtures of 11 N hydrofluoric acid and nitric acid 7 N and whose volumes were also adjusted to 50 mL by adding 0.5 N nitric acid.

Solution S3 includes 18.7 g/L of fluoride ions. It has a P/y activity of 2,085.10 ⁷ Bq/L, mainly linked to cesium 137 (98%) - the other P/y emitting radionuclides detected by spectrometry being europium 154 and cobalt 60 - as well as an activity a of l,27.10 ⁷ Bq/L linked to americium 241.

Tests are carried out with different sources of calcium ions, namely: milk of lime (test 1), calcium nitrate (test 2) and calcium carbonate (test 3).

For the treatments with calcium nitrate and calcium carbonate, the procedure is as described in Example I above except that there is no washing of the precipitates obtained.

For the treatment with milk of lime, the procedure is as described in Example II above except that there is no washing operation.

The characteristics of these tests are summarized in Table 3 below.

Table 3

Regarding the activity of radionuclides, it is reported in Table 4 below.

Table 4

Americium 241, europium 154 and cobalt 60 were immobilized in the precipitate since they are not detected in the S3 solution after treatment, regardless of the source of calcium ions used.

The example which has just been described can be used as a basis for other applications, the remaining solutions referred to in this example may also result from attacks of precipitates and/or other solid samples taken from the equipment in the processing phase. operation, during rinsing or sanitation operations or during dismantling phases, of both fuel cycle plants and nuclear installations such as reactors or laboratories.

Claims

1. Process for treating an acidic aqueous solution A, comprising hydrofluoric acid, fluoride ions resulting from a dissociation of hydrofluoric acid and radionuclides, which process comprises at least the following steps: a) adjusting the pH of aqueous solution A at a value greater than or equal to 10; b) reacting the fluoride ions present in the aqueous solution A with calcium ions by mixing a source of calcium ions with the aqueous solution A in proportions such that the calcium ions are in excess relative to the fluoride ions, while by maintaining, if necessary, the pH of the mixture at a value greater than or equal to 10; c) subjecting the mixture formed in step b) to a solid-liquid separation, whereby a precipitate is obtained comprising fluorine and an aqueous phase poor in fluoride ions, the radionuclides having been distributed between the precipitate and the phase aqueous depending on their solubility in an aqueous medium at the pH of the mixture formed in step b); and in which step b) is carried out simultaneously with step a) or after step a). 2. Method according to claim 1, in which the aqueous solution A is a solution which comprises hydrofluoric acid and an inorganic acid other than hydrofluoric acid, preferably nitric acid, phosphoric acid or acid sulfuric and, better yet, nitric acid. 3. Method according to claim 1 or claim 2, in which the pH of the aqueous solution A is brought to a value equal to or greater than 11, preferably equal to or greater than 12 and, better still, between 13 and 14. 4. Method according to any one of claims 1 to 3, in which the source of calcium ions is an inorganic calcium salt, preferably calcium hydroxide, calcium nitrate, calcium carbonate, calcium sulfate, calcium or calcium phosphate, the salt being able to be in hydrated form. 5. Method according to claim 4, in which the source of calcium ions is calcium hydroxide which is used in the form of a milk of lime previously obtained by dissolving slaked lime in water and to which the aqueous solution A is added. 6. Method according to claim 4 or claim 5, in which the source of calcium ions is a milk of lime and step b) is carried out simultaneously with step a). 7. Method according to claim 4, in which the source of calcium ions being calcium nitrate, calcium carbonate, calcium sulfate or calcium phosphate, the pH of the aqueous solution A is adjusted in step a) by mixing said aqueous solution A with a strong base, preferably sodium or potassium hydroxide, and step b) is carried out after step a). 8. Method according to any one of claims 1 to 7, which further comprises, between steps b) and c), a step consisting of leaving the mixture formed in step b) to stand. 9. Method according to any one of claims 1 to 8, in which the aqueous solution A is a solution resulting, directly or after possible dilution, from an attack by hydrofluoric acid, alone or mixed with another inorganic acid, one or more radioactive solids produced during spent nuclear fuel processing operations and/or rinsing and/or sanitation operations of a nuclear installation in operation or dismantling. 10. Method according to claim 9, in which the aqueous solution A is a solution resulting from an attack of one or more precipitates produced during the treatment of spent nuclear fuel. 11. Method according to claim 9, in which the aqueous solution A is a solution resulting from an attack of one or more precipitates of fission products with a mixture of hydrofluoric acid and nitric acid, a solution resulting from an attack on dissolution fines by a mixture of hydrofluoric acid and nitric acid, or a mixture thereof. 12. Method according to claim 9, in which the aqueous solution A is a solution resulting from an attack on one or more radioactive solids resulting from rinsing and/or sanitation operations of a nuclear fuel processing plant spent or a nuclear reactor in operation or being dismantled.