Method for treating acidic aqueous solutions containing fluoride ions and radionuclides - Patent Application 20070122997

Abstract

The present invention relates to a method for treating an acidic aqueous solution A containing hydrofluoric acid, fluoride ions, and radionuclides, comprising at least the following steps: a) adjusting the pH of aqueous solution A to a value of 10 or higher; b) reacting the fluoride ions of aqueous solution A with calcium ions by mixing a calcium ion source with aqueous solution A in a proportion such that the calcium ions are in excess of the fluoride ions, while optionally maintaining the pH of the mixture at a value of 10 or higher; c) subjecting the mixture formed in step b) to solid-liquid separation, thereby obtaining a fluorine-containing precipitate and a fluoride-ion-depleted aqueous phase, and the radionuclides being distributed between the precipitate and the aqueous phase depending on their solubility in the aqueous medium at the pH of the mixture formed in step b), wherein step b) is carried out simultaneously with or after step a). Field of application: nuclear industry.

Description

[Technical Field]

[0001] The present invention relates to the field of treatment of acidic and radioactive aqueous solutions.

[0002] More particularly, the present invention relates to a method for treating an acidic aqueous solution containing both fluoride ions and radionuclides with the aim of optimizing the treatment of this solution and the management of waste resulting from such solutions.

[0003] The invention is applicable for the treatment of a solution obtained directly or after optional dilution from a process of etching with hydrofluoric acid, in particular with hydrofluoric acid alone or in a mixture with another inorganic acid, one or more radioactive solids produced in the nuclear industry during spent nuclear fuel handling and / or cleaning and / or decontamination operations of nuclear facilities in operation or during decommissioning. [Background technology]

[0004] At the Spent Nuclear Fuel Processing Plant in La Hague (France), laboratories tasked with characterizing radioactive solids produced during spent nuclear fuel processing or cleaning or decontamination operations in operating or dismantled nuclear facilities are required to solution treat these solids by subjecting them to etching with a concentrated mixture of hydrofluoric and nitric acid, the operating procedure of which is described in standard NF M60-323.

[0005] Thus, the following: - solution treatment (also called etching) of fission product (FP) precipitates with a mixture of 0.25N hydrofluoric acid and 14N nitric acid, followed by a solution obtained from a subsequent dilution with 0.5N nitric acid, in particular 4:5; - Solution treatment (also called etching) of dissolved residual particles using a mixture of 11N hydrofluoric acid and 7N nitric acid, followed by 0.5N nitric acid, in particular the solution obtained from a 4:5 dilution. is particularly generated.

[0006] These solutions, of extremely high acidity, are enriched in fluoride ions resulting from the dissociation of hydrofluoric acid in aqueous media, and in various radionuclides, essentially beta / gamma emitters of cesium-137 and cesium-134, or alpha emitters of americium-241 and curium-244. Furthermore, they may contain cations of the type molybdenum, zirconium, palladium, iron, etc., depending on the initial solution-processed solids (precipitates or particulates).

[0007] Only a small portion of these solutions is used for characterization, and the residue may be converted into solid waste without pretreatment by immobilization in a stable matrix.

[0008] With such immobilization, on the one hand, all the radiological activity of the residues is found in the solid waste thus generated, and on the other hand, the volume of this solid waste would be relatively large. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, for the purpose of optimizing nuclear waste management, it is desirable to have a method for treating residues from fluoronitric acid solution etching of radioactive solids, and generally any aqueous acidic solution containing fluoride ions as well as radionuclides, in order to reduce as much as possible both the radioactivity and the volume of solid waste resulting from such solutions.

[0010] It is also desirable that the method be easy to implement and not require any equipment for its implementation other than that conventionally provided in shielded lines and glove boxes in laboratories of nuclear fuel cycle facilities. [Means for solving the problem]

[0011] The present invention is more precisely a method for treating an acidic aqueous solution A containing hydrofluoric acid, fluoride ions resulting from the dissociation of hydrofluoric acid, and radionuclides, comprising at least the following steps: a) adjusting the pH of aqueous solution A to a value of 10 or greater; b) mixing, under stirring, a source of calcium ions with aqueous solution A in such a proportion that the calcium ions are in excess relative to the fluoride ions, and, if necessary, maintaining the pH of the mixture at a value greater than or equal to 10, thereby reacting the fluoride ions present in aqueous solution A with the calcium ions; c) subjecting the mixture formed in step b) to solid-liquid separation, whereby a fluorine-containing precipitate and a fluoride-depleted aqueous phase are obtained, the radionuclides being distributed between the precipitate and the aqueous phase according to their solubility in the aqueous medium at the pH of the mixture formed in step b); Including, It is an object of the present invention to provide a method in which step b) is carried out simultaneously with step a) or after step a).

[0012] Thus, according to the present invention, aqueous solution A is defluorinated by reacting fluoride ions with an excess of calcium ions, e.g., about 5% to 10%, thereby precipitating the fluoride ions present in the solution in the form of fluorine, CaF2. This reaction is:

[0013] [ka]

[0014] It is expressed as:

[0015] This reaction is carried out at a pH of at least 10, so that radionuclides present in aqueous solution A that are insoluble or sparingly soluble in aqueous medium at the pH of the mixture formed in step b) are completely or partially immobilized in the precipitate (as is the case, for example, of antimony-125, americium-241, europium-154, and cobalt-60, if these radionuclides are present in aqueous solution A), while those that are soluble in aqueous medium at the pH of the mixture formed in step b) are found mainly, indeed even exclusively, in the aqueous phase obtained from the solid-liquid separation (as is the case, for example, of cesium-137 and cesium-134, if these radionuclides are present in aqueous solution A).

[0016] As a result, by treating aqueous solution A with the method according to the invention: - solid waste (i.e. precipitate) in which the fluoride ions are stably immobilized in the form of fluorine, the radioactivity and volume of which are significantly reduced compared to the radioactivity and volume of the waste obtained by immobilizing the entire solution in the form of a solid material, and - on the other hand, an aqueous phase which has a low, in fact even extremely low, concentration of fluoride ions and can therefore be sent to a device for concentration (by evaporation), calcination and vitrification (i.e., immobilization of the radionuclides in a glass matrix) or to a wastewater treatment device before discharge into the environment in accordance with an administrative permit. can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0017] In the above, the expression "if necessary, maintaining the pH of the mixture at a value greater than or equal to 10" means that in step b), if the pH of the mixture tends to fall below 10 due to the reaction of fluoride ions with calcium ions, this pH is maintained at a value at least equal to 10.

[0018] According to the present invention, aqueous solution A is preferably a solution containing hydrofluoric acid together with an inorganic acid other than hydrofluoric acid, such as nitric acid, phosphoric acid, or sulfuric acid, preferably together with nitric acid.

[0019] Furthermore, in step a), the pH of aqueous solution A is advantageously adjusted to a value greater than or equal to 11, preferably greater than or equal to 12, and more preferably between 13 and 14.

[0020] The source of calcium ions is preferably an inorganic calcium salt, such as calcium hydroxide (or hydrated lime), calcium nitrate, calcium carbonate, calcium sulfate, or calcium phosphate, optionally in a hydrated form.

[0021] In particular, calcium hydroxide, calcium nitrate, and calcium carbonate are preferred, and considering that the fluoride ion content of aqueous solution A is 30 g / L or more, the use of calcium hydroxide is highly recommended.

[0022] According to the invention, the calcium ion source may be in the form of an aqueous suspension, in particular when the calcium ion source is calcium hydroxide. Indeed, preference is given to using calcium hydroxide in the form of milk of lime, obtained beforehand by dissolving slaked lime in water, preferably by supersaturating the water with slaked lime (for example up to 400 g of slaked lime per liter of water), to which aqueous solution A is added in portions, with stirring, and the pH is raised to above 10, preferably above 11, by adding sodium hydroxide as needed.

[0023] In this regard, it is recalled that slaked lime is the reaction product of quicklime with water, which itself is the product of the thermal decomposition of limestone (e.g., by calcination) and therefore essentially consists of calcium oxide, CaO. By reacting with water, the calcium oxide from the quicklime is converted into calcium hydroxide, Ca(OH)2, which is therefore the main component of slaked lime.

[0024] Alternatively, the calcium ion source may also be in particulate form, typically a powder, particularly where the calcium ion source is calcium nitrate, calcium carbonate, calcium phosphate, or calcium sulfate.

[0025] As mentioned above, step b) can be carried out simultaneously with step a) or after step a), in fact it all depends on the calcium ion source used, which, when mixed with aqueous solution A, brings the pH of the mixture to a value at least equal to 10.

[0026] Therefore, when the calcium ion source is milk of lime, mixing this milk of lime with aqueous solution A makes it possible to carry out step a) and step b) simultaneously.

[0027] Alternatively, when the calcium ion source is calcium nitrate, calcium carbonate, calcium sulfate, or calcium phosphate, it is preferred to carry out steps a) and b) in sequence.

[0028] In that case, step a) preferably comprises adjusting the pH of aqueous solution A by mixing it with an aqueous solution of a strong base, typically sodium hydroxide or potassium hydroxide, in the form of a highly concentrated solution, for example 18N or 20N.

[0029] According to the present invention, the method may comprise, between steps b) and c), a step of allowing the mixture formed in step b) to stand (also called aging), in which case the size of the formed fluoride nuclei will increase with depletion of the fluoride ion solution until granules are obtained that can be more easily separated by solid-liquid separation in step c).

[0030] Step c) can be carried out by any technique making it possible to separate the immersed precipitate from the aqueous medium, for example by filtration, in particular in vacuum or under reduced pressure, by sedimentation, by centrifugation or by centrifugation after sedimentation, the latter technique preferably being used in a shielded line or glove box in the laboratory of a nuclear fuel cycle facility.

[0031] Furthermore, step c) may be followed by one or more washing steps of the precipitate with water or with low concentration sodium hydroxide solution, for example 0.2 N, each washing step being followed by its own solid-liquid separation.

[0032] According to the invention, aqueous solution A may be any solution obtained directly or after optional dilution from a process of etching, with hydrofluoric acid alone or in a mixture with another inorganic acid, for example nitric acid, one or more radioactive solids produced during spent nuclear fuel processing operations and / or cleaning operations and / or decontamination operations of an operating or dismantled nuclear facility.

[0033] Thus, aqueous solution A is, in particular: - solutions obtained from the process of etching one or more precipitates formed during the processing of spent nuclear fuel, and in particular from the process of etching one or more FP precipitates using a mixture of, for example, 0.25N hydrofluoric acid and, for example, 14N nitric acid, or a solution obtained from a step of etching dissolved residual particles using a mixture of, for example, 11N hydrofluoric acid and, for example, 7N nitric acid, or - A mixture of may be.

[0034] Alternatively, aqueous solution A may also be a solution obtained from a process for etching, in particular a fluoronitric acid etching process, of one or more radioactive solids resulting from cleaning and / or decontamination operations of a spent nuclear fuel processing plant or a nuclear reactor, in operation or during decommissioning.

[0035] FP precipitate refers to residues based on zirconium molybdate and / or cesium phosphomolybdate, obtained by concentrating, typically by evaporation, aqueous solutions resulting from uranium and plutonium extraction, separation, and purification operations commonly performed in spent nuclear fuel processing.

[0036] Dissolved residue particulates refer to all small particle solids remaining after dissolving spent nuclear fuel in nitric acid and separated by clarification from the solution, thus obtained by pendulum centrifugal decanting.

[0037] Other features and advantages of the present invention will become apparent from the additional description that follows.

[0038] However, it should be understood that this additional description is provided merely as an explanation of the inventive subject matter and should not be construed as limiting the inventive subject matter. [Example]

[0039] Example 1: Treatment of FP Precipitate Etching Solution Mixture of HF and HNO with Calcium Nitrate The following example concerns tests carried out with 100 mL of a solution (hereinafter referred to as "solution S1") previously obtained by combining four residues of 25 mL each, all four residues having been obtained from a process of etching fission product precipitates with 40 mL of a mixture of 0.25 N hydrofluoric acid and 14 N nitric acid, the volume of which had been adjusted to 50 mL by adding 0.5 N nitric acid.

[0040] Solution S1 contains approximately 3.8 g / L of fluoride ions, as well as cesium-137, cesium-134, antimony-125, europium-154, ruthenium-106, rhenium-106, cerium-144, and praseodymium-144, as shown in Table 1 below, with the radioactivity associated with cesium-137 predominating.

[0041] The radioactivity of each of these radionuclides in solution S1, measured by gamma spectrometry, is shown in Table 1.

[0042] For the treatment of solution S1 using the method according to the invention, the procedure is as follows: 1. A volume of 80 mL of 20 N sodium hydroxide is placed into the canister; 2. A volume of 100 mL of solution S1 is gradually poured into the canister while stirring, so that the pH of the mixture of solution S1 and NaOH is greater than 13 (measured with pH paper); 3. 32 g of calcium nitrate tetrahydrate Ca(NO3)2.4H2O is gradually added to solution S1 at the adjusted pH; 4. Allow the mixture obtained in item 2 to settle for at least 48 hours to obtain a precipitate large enough for the next step, solid / liquid separation, by aging; 5. At the end of the aging period, the mixture obtained is transferred in multiple passes into two centrifuge tubes, and after each transfer, it is centrifuged (at 3200 rpm or 3600 rpm), after which the supernatant is recovered; 6. The precipitate obtained in item 4 and separated by centrifugation in the two tubes in item 5 is subjected to two successive washing steps, in which each washing step is carried out by adding 10 mL of distilled water to each centrifuge tube, and each tube is centrifuged (3200 rpm or 3600 rpm) to recover the supernatant.

[0043] The supernatants obtained in items 5 and 6 were combined in the same container to form a solution (volume: 180 mL), which for simplicity will be referred to below as treated solution S1.

[0044] A first sample of this solution is subjected to fluoride ion assay by ion chromatography, while a second sample is subjected to gamma spectrometry analysis to determine the radioactivity of the various radionuclides contained.

[0045] According to the fluoride ion assay, the concentration of fluoride ions in the solution S1 after treatment is less than 10 mg / L, which makes it possible to envisage the option of sending this solution to a device for concentration by evaporation, calcination and vitrification.

[0046] The radioactivities of the radionuclides are reported in Table 1 below.

[0047] [Table 1]

[0048] According to this table, more than 99% of the radioactivity of Cs-137 and Cs-134 in solution S1 before treatment is present in solution S1 after treatment. Therefore, no significant immobilization of Cs-137 and Cs-134 occurred in the precipitate formed during this treatment.

[0049] The radioactivity of cesium-137 and cesium-134 alone was found to represent approximately 93% of the βγ radioactivity of S1 before treatment, and therefore the majority of the βγ radioactivity of solution S1 is present in this solution after treatment.

[0050] On the other hand, Table 1 shows a significant decrease in the radioactivity of antimony-125, europium-154, cerium-144, and praseodymium-144 in solution S1 after treatment compared with that before treatment, which means that these radionuclides were partially or almost completely immobilized in the precipitate.

[0051] In this example, it should be noted that the activity of Ruthenium-106, and therefore the activity of Rhenium-106, is higher in solution S1 after treatment than in solution S1 before treatment, which is due to the surrounding environmental contamination from the shielded line where the test was performed.

[0052] Example 2: Treatment of a Dissolved Residual Particle Etching Solution Mixture of HF and HNO with Milk of Lime The following example relates to tests carried out using 80 mL of a solution (hereinafter referred to as "Solution S2") corresponding to a mixture of various residues of a solution obtained from etching dissolved residue particles using a mixture of 11 N hydrofluoric acid and 7 N nitric acid.

[0053] Solution S2 contained 170 g / L of fluoride ions and 1.0 g / L of β / γ activity, primarily associated with ruthenium-106. 10 Bq / L (45%), other radionuclides are rhodium-106, antimony-125, and cesium-137.

[0054] For the treatment of this solution S2 with milk of lime, the procedure is as follows: 1. Prepare 275 mL of milk of lime in a pot with an initial pH equal to 13; 2. Add 80 mL of solution S2 into the pot gradually in 10 mL portions, checking (with pH paper) that the pH of the mixture is greater than 11 after each addition; if not, adjust the pH to 13 by adding 20 M sodium hydroxide; 3. Allow the mixture to settle for 24 hours, thereby obtaining a suspended precipitate and some clear liquid supernatant; 4. Check that the pH of the clear liquid supernatant is greater than 11, if not, readjust the pH to 13 by adding sodium hydroxide, then centrifuge (2200 rpm) this liquid together with a portion of the suspension, and then recover the centrifugation supernatant (47 mL); 5. The suspension remaining in the pot in item 4 is subjected to two successive washing steps with 0.2 N sodium hydroxide, followed by settling, centrifuging the liquid obtained from the settling step together with a portion of the suspension, and then recovering the centrifugation supernatant (30 mL and 74 mL for washing step 1 and washing step 2, respectively).

[0055] The centrifugation supernatants obtained in items 4 and 5 were combined in the same container to form a solution (volume: 151 mL), which for simplicity will be referred to below as treated solution S2.

[0056] A first sample of this solution is subjected to fluoride ion assay by ion chromatography, while a second sample is subjected to gamma spectrometry analysis to determine the radioactivity of the various radionuclides contained.

[0057] According to the fluoride ion assay, the concentration of fluoride ions in the treated solution S2 was 10.6 mg / L, i.e., 99.99% of the fluoride ions were immobilized in the precipitate.

[0058] The radioactivities of the radionuclides are reported in Table 2 below.

[0059] [Table 2]

[0060] Example 3: Treatment of HF and HNO3 Etching Solution Mixtures with Various Calcium Ion Sources The following examples are: - Various residues (40 mL) of the solution obtained from the etching of sediments or other solid samples from various locations of the La Hague Spent Nuclear Fuel Processing Plant with a mixture of 0.25 N hydrofluoric acid and 14 N nitric acid, the volume of which was adjusted to 50 mL by adding 0.5 N nitric acid; and - Various residues (40 mL) of solutions obtained from the etching of sediments or other solid samples from various locations of the La Hague spent nuclear fuel processing plant with a mixture of 11 N hydrofluoric acid and 7 N nitric acid, the volume of which was also adjusted to 50 mL by adding 0.5 N nitric acid. The various tests were carried out on a solution (hereinafter referred to as "solution S3") with a volume of 100 mL or 120 mL depending on the test, corresponding to the mixture of

[0061] Solution S3 contained 18.7 g / L of fluoride ions. Solution S3 had a β / γ activity of 2.085 g / L, primarily (98%) associated with cesium-137.7 Bq / L (other beta / gamma emitting radionuclides detected by gamma spectrometry are europium-154 and cobalt-60), and alpha activity associated with americium-241 1.27.10 7 It has Bq / L.

[0062] Tests are carried out with different calcium ion sources: milk of lime (Test 1), calcium nitrate (Test 2), and calcium carbonate (Test 3).

[0063] For the treatment with calcium nitrate and calcium carbonate, the procedure was as described in Example 1 above, except that there was no step of washing the resulting precipitate.

[0064] For treatment with milk of lime, the procedure was as described in Example 2 above, again without the washing step.

[0065] The characteristics of these studies are summarized in Table 3 below.

[0066] [Table 3]

[0067] The radioactivities of the radionuclides are reported in Table 4 below.

[0068] [Table 4]

[0069] Americium-241, europium-154, and cobalt-60 were immobilized in the precipitate, since they were not detected in solution S3 after treatment, regardless of the calcium ion source used.

[0070] The described embodiment can be used as a basis for other applications, and the solution residues referred to in this embodiment can result from etching deposits and / or other solid samples removed from equipment during operational stages, during cleaning or decontamination operations, or during dismantling stages, both in fuel cycle plants and nuclear facilities of the type of reactor or laboratory.

Claims

1. 1. A method for treating an acidic aqueous solution A containing hydrofluoric acid, fluoride ions resulting from the dissociation of hydrofluoric acid, and radionuclides, comprising at least the following steps: a) adjusting the pH of said aqueous solution A to a value of 10 or more; b) reacting the fluoride ions present in aqueous solution A with the calcium ions by mixing a calcium ion source with aqueous solution A in a proportion such that the calcium ions are in excess of the fluoride ions, while optionally maintaining the pH of the mixture at a value of 10 or greater; c) subjecting the mixture formed in step b) to solid-liquid separation, whereby a fluorine-containing precipitate and a fluoride-depleted aqueous phase are obtained, the radionuclides being distributed between the precipitate and the aqueous phase according to their solubility in the aqueous medium at the pH of the mixture formed in step b); Including, A method wherein step b) is performed simultaneously with step a) or after step a).

2. 2. The method according to claim 1, wherein the aqueous solution A is a solution containing hydrofluoric acid and an inorganic acid other than hydrofluoric acid, preferably nitric acid, phosphoric acid, or sulfuric acid, more preferably nitric acid.

3. 3. The method according to claim 1 or claim 2, wherein the pH of said aqueous solution A is adjusted to a value of 11 or more, preferably to a value of 12 or more, more preferably to a value of 13 to 14.

4. 4. The method of claim 1, wherein the source of calcium ions is an inorganic calcium salt, preferably calcium hydroxide, calcium nitrate, calcium carbonate, calcium sulfate, or calcium phosphate, and the inorganic calcium salt may be in a hydrated form.

5. 5. The method according to claim 4, wherein the calcium ion source is calcium hydroxide used in the form of milk of lime previously obtained by dissolving slaked lime in water, to which aqueous solution A is added.

6. the calcium ion source is milk of lime; 6. The method of claim 4 or claim 5, wherein step b) is carried out simultaneously with step a).

7. the calcium ion source is calcium nitrate, calcium carbonate, calcium sulfate, or calcium phosphate; the pH of said aqueous solution A is adjusted in step a) by mixing said aqueous solution A with a strong base, preferably sodium hydroxide or potassium hydroxide; The method of claim 4, wherein step b) is performed after step a).

8. 8. The method according to claim 1, further comprising the step of allowing the mixture formed in step b) to stand between steps b) and c).

9. 9. The method according to any one of claims 1 to 8, wherein the aqueous solution A is a solution obtained directly or after optional dilution from a process of etching, with hydrofluoric acid, alone or in a mixture with another inorganic acid, one or more radioactive solids produced during spent nuclear fuel processing operations and / or cleaning operations and / or decontamination operations of an operating or dismantled nuclear facility.

10. 10. The method of claim 9, wherein the aqueous solution A is a solution obtained from a process for etching one or more precipitates formed during spent nuclear fuel processing.

11. 10. The method of claim 9, wherein the aqueous solution A is a solution obtained from a process of etching one or more fission product precipitates with a mixture of hydrofluoric acid and nitric acid; a solution obtained from a process of etching dissolved residue particulates with a mixture of hydrofluoric acid and nitric acid; or a mixture thereof.

12. 10. The method of claim 9, wherein the aqueous solution A is a solution obtained from a process for etching one or more radioactive solids resulting from cleaning and / or decontamination operations of a spent nuclear fuel processing plant or a nuclear reactor in operation or during decommissioning.

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