METHOD FOR TREATING AN ACID AQUEOUS SOLUTION COMPRISING FLUORIDE IONS AND RADIONUCLIDES

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 poor in fluoride ions is obtained, the radionuclides having been distributed between the precipitate and the aqueous phase according to 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). Field of applications: nuclear industry.;

Description

Title of the invention: METHOD FOR TREATING AN ACID AQUEOUS SOLUTION COMPRISING FLUORIDE IONS AND RADIONUCLIDES Technical field

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

[0002] More specifically, it relates to a method 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.

[0003] The invention finds application in the nuclear industry, in particular for the treatment of fluoronitric attack solutions of fission precipitates as well as fluoronitric attack solutions of dissolution fines. STATE OF THE PRIOR ART

[0004] In the spent nuclear fuel treatment plant at La Hague (France), the laboratories responsible for characterizing the radioactive solids produced during this treatment or during rinsing or sanitation operations of the installations in operation or being dismantled, are required to put these solids into solution by subjecting them to attack by concentrated mixtures of hydrofluoric acid and nitric acid, the operating methods for which are described in standard NF M60-323.

[0005] The following are thus produced: - solutions resulting from the dissolution (otherwise known as attack) of fission product precipitates (FP) by a mixture of 0.25 N hydrofluoric acid and 14 N nitric acid, followed by dilution, in particular to 4 / 5 by 0.5 N nitric acid, and - solutions resulting from a dissolution (otherwise called attack) of dissolution fines by a mixture of 11 N hydrofluoric acid and 7 N nitric acid, also followed by a dilution, in particular to 4 / 5th by 0.5 N nitric acid.

[0006] These very high acidity solutions are rich in fluoride ions, which result from a dissociation of hydrofluoric acid in an aqueous medium, and in various radionuclides, essentially emitters [3 / y including caesium 137 and caesium 134, or emitters a including americium 241 and curium 244. In addition, they can include cations of the molybdenum, zirconium, palladium, iron, etc. type, depending on the initial solid dissolved (precipitate or fines).

[0007] It turns out that only a small fraction of these solutions are used for the characteristics The waste and residues are likely to be transformed, without prior treatment, into solid waste by immobilization in a stable matrix.

[0008] With such immobilization, on the one hand, all the radiological activity of the residues would be found in the solid waste thus produced and, on the other hand, the volume of this solid waste would be relatively large.

[0009] With a view to optimising the management of nuclear waste, it would therefore be desirable to have a method which makes it possible to treat the residues of fluoronitric attack solutions of radioactive solids and, in general, any acidic aqueous solution which comprises fluoride ions together with radionuclides, so as to reduce as much as possible both the radiological activity and the volume of solid waste produced from this type of solution.

[0010] It would also be desirable for this method to be simple to implement and for its implementation not to require equipment other than that conventionally provided in the shielded chains and glove boxes of the laboratories of the nuclear fuel cycle installations. Statement of the invention

[0011] The invention aims precisely to propose a method for treating an acidic aqueous solution A, comprising hydrofluoric acid, fluoride ions resulting from a dissociation of the hydrofluoric acid and radionuclides, which method comprises at least the following steps: a) adjusting the pH of aqueous solution A to a value greater than or equal to 10; b) reacting 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 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 poor in fluoride ions is obtained, the radionuclides having been distributed between the precipitate and the aqueous phase according to their solubility in an aqueous medium at the pH of the mixture formed in step b); and wherein step b) is carried out simultaneously with step a) or after step a).

[0012] Thus, according to the invention, defluorination of 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 of the order of 5% to 10%, a reaction which is written: 2 F + |C.ai+ CaF2.

[0013] This reaction being carried out at a pH at least equal to 10, the radionuclides present in the aqueous solution A, which are insoluble or slightly soluble in an aqueous medium at the pH of the mixture formed in step b), are wholly or partly 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 the 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 be, for example, the case of caesium 137 and caesium 134 if, of course, these radionuclides are present in the aqueous solution A).

[0014] It follows that 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 would be presented by a waste obtained by immobilizing the whole 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 to a concentration unit (by evaporation), calcination and vitrification (i.e. immobilization of radionuclides in a vitreous matrix) or to an effluent treatment unit before discharge into the environment in compliance with administrative authorizations.

[0015] According to the invention, the aqueous solution A is preferably a solution which comprises hydrofluoric acid together with a mineral acid other than hydrofluoric acid, for example nitric acid, phosphoric acid or sulfuric acid, preference being given to nitric acid.

[0016] 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.

[0017] 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 possibly being in hydrated form or not.

[0018] Among these, 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 Calcium is highly recommended.

[0019] According to the invention, the source of calcium ions may be in the form of an aqueous suspension, which will be the case in particular if the source of calcium ions is calcium hydroxide. Indeed, it is preferred to use calcium hydroxide in the form of a milk of lime previously obtained by supersaturating water with slaked lime (for example, at a rate 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.

[0020] In this respect, it is recalled that slaked lime is the product of the reaction of quicklime with water, quicklime itself being the product of a thermal decomposition of limestone (for example, by calcination) and being, as such, essentially composed of calcium oxide, CaO. By reaction with water, the calcium oxide of quicklime is transformed into calcium hydroxide, Ca(OH)2, which is therefore the main constituent of slaked lime.

[0021] Alternatively, the source of calcium ions may also be in a particulate form, typically a powder, which will be the case in particular if the source of calcium ions is calcium nitrate, calcium carbonate, calcium phosphate or calcium sulfate.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] In which case, step a) comprises adjusting 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.

[0026] In accordance with the invention, the method may 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 fluorine seeds formed will increase until grains are obtained which can be more easily separated by solid-liquid separation during step c) while depleting the solution in fluoride ions.

[0027] Step c) can be carried out by any technique allowing a precipitate to be separated from the aqueous medium in which it is immersed, for example by filtration, in particular under vacuum or pressure, by decantation, by centrifugation or even by decantation followed by centrifugation, the latter technique being preferably used in shielded chains or glove boxes in laboratories of nuclear fuel cycle installations.

[0028] Furthermore, step c) may be followed by one or more washes of the precipitate with water or with a weakly concentrated sodium hydroxide solution, for example 0.2 N, each wash itself being completed by a solid-liquid separation.

[0029] Preferably, the aqueous solution A is: - a solution resulting, directly or after possible dilution, from an attack by hydrofluoric acid, alone or in a mixture with another mineral acid such as nitric acid, of one or more precipitates produced during the treatment of spent nuclear fuel and, in particular, a solution resulting, directly or after possible dilution, from an attack of one or more PF precipitates by a mixture of hydrofluoric acid, for example 0.25 N, and nitric acid, for example 14 N, or - a solution resulting, directly or after possible dilution, from an attack of dissolution fines by a mixture of hydrofluoric acid, for example 11 N, and nitric acid, for example 7 N, or - a mixture of these.

[0030] By PF precipitates is meant residues based on zirconium molybdate and / or cesium phosphomolybdate, which are obtained by concentration, typically by evaporation, of aqueous solutions resulting from the operations of extraction, separation and purification of uranium and plutonium typically implemented in the treatment of spent nuclear fuel.

[0031] By dissolution fines is meant 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 using pendulum centrifugal decanters.

[0032] Other characteristics and advantages of the invention will emerge from the additional description which follows.

[0033] It goes without saying, however, that this additional description is given only as an illustration of the subject of the invention and must in no case be interpreted as a limitation of this subject.

[0034] DETAILED DESCRIPTION OF PARTICULAR METHODS OF IMPLEMENTATION

[0035] Example I: Treatment with calcium nitrate of a mixture of solutions for attacking a PF precipitate with HE and HNO3:

[0036] The following example relates to a test carried out with 100 mL of a solution, hereinafter referred to as “solution SI”, previously obtained by combining four remainders of 25 mL each, of solutions resulting all four from the attack of a precipitate of fission products by 40 mL of a mixture of 0.25 N hydrofluoric acid and 14 N nitric acid, and the volume of which was adjusted to 50 mL by the addition of 0.5 N nitric acid.

[0037] 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.

[0038] The activity of each of these radionuclides in SI solution, as measured by γ spectrometry, is shown in Table 1.

[0039] 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 container; 2. a volume of 100 mL of the SI solution is gradually introduced with stirring into the container 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, is gradually added to the SI solution at the adjusted pH; 4. the mixture obtained in point 2 is left to settle for at least 48 hours to obtain, by maturing, a precipitate of sufficient size for the solid / liquid separation of the next step; 5. the mixture obtained after ripening is transferred several times into two centrifuge tubes, each transfer being followed by centrifugation (3,200 or 3,600 rpm) and then by removal of the supernatant; 6. the precipitate obtained in point 4, separated by centrifugation in the two tubes in point 5, is subjected to two successive washes, each wash being carried out by introducing 10 mL of desiliconized water into each centrifuge tube, subjecting each tube to centrifugation (3,200 or 3,600 rpm) and removing the supernatant.

[0040] 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 solution S1 after treatment.

[0041] A first sample of this solution is subjected to a dosage of fluoride ions by ion chromatography while a second sample is subjected to an analysis by y spectrometry to measure the activity of the different radionuclides it contains.

[0042] 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.

[0043] As for the activity of the radionuclides, it is reported in Table 1 below.

[0044] [Tables 1] Radionuclides Activity before treatment of SI solution (Bq per 100 ml) Activity after treatment of SI solution 1Bq per 180 ml*} “-?Cs 5;44,1Ü- 5,41,10® 4,92.103 4,92.108 Vi kD. c 2.1G7 9,62.107 -e ■- .s 1S6Ru 4,81.107 Cq CO O '<4 3S&Rh 4,81.107 8,88.10' 4,44.107 < 2,03.10' 4,44,107 < 2,03.101, * Volume of supernatants obtained at points b and 6 combined

[0045] 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.

[0046] Knowing that the activity of cesium 137 and 134 alone represented approximately 93% of the activity (3 / y of the SI solution before treatment, the majority of the activity (3 / y of the Si solution is therefore found in this solution after treatment.

[0047] 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 partially or almost totally been immobilized in the precipitate.

[0048] 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 shielded 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:

[0050] The following example relates to a test carried out with 80 mL of a solution, hereinafter referred to as “solution S2”, corresponding to the mixture of different solution residues resulting from the attack of dissolution fines by a mixture of hydrofluoric acid 11 N and 7 N nitric acid.

[0051] Solution S2 comprises 170 g / L of fluoride ions and has an activity [3 / y of 1.0 .1010 Bq / L, mainly related to ruthenium 106 (45%), the other radionuclides being rhodium 106, antimony 125 and caesium 137.

[0052] For the treatment of this solution S2 with lime milk, the procedure is as follows: 1. prepare 275 mL of lime milk with an initial pH of 13 in a jug; 2. a volume of 80 mL of solution S2 is introduced into the jug 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, an adjustment of the pH to 13 by adding 20 M sodium hydroxide is carried out; 3. the mixture is left to settle for 24 hours, resulting in a suspended precipitate and a little clear supernatant liquid; 4. the pH of the clear supernatant liquid is checked to be greater than 11 and, if not, this pH is readjusted to 13 by adding sodium hydroxide and then this liquid is subjected to centrifugation (2200 rpm) 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 the collection of the centrifugation supernatants (30 mL and 74 mL respectively for washes 1 and 2).

[0053] 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 being solution S2 after treatment.

[0054] A first sample of this solution is subjected to a dosage of fluoride ions by ion chromatography while a second sample is subjected to an analysis by y spectrometry to measure the activity of the different radionuclides it contains.

[0055] The dosage of fluoride ions shows that their concentration in solution S2 after treatment is 10.6 mg / L, i.e. 99.99% of fluoride ions immobilized in the precipitates.

[0056] As for the activity of the radionuclides, it is reported in Table 2 below.

[0057] [Tables2] Radionuclides Activity before treatment of solution 52 (Bq per 80 ml) Activity after treatment of solution- S2 (Bq per 151 ml*) 3.64.10* 5.95.105 ÎG6Rh 3.64.10* 5.95.105 3.37.107 6.40.104 2.82.107 8.31.105 2.46.10* 6.31.105 3.90.105 8.11.102 Volume of supernatants obtained at points 4 and 5 combined

[0058] Example III: Treatment by different sources of calcium ions of a mixture of HF and HNO3 attack solutions:

[0059] 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 referred to as “solution S3”, corresponding to a mixture between: - various solution residues (40 mL) resulting from attacks on precipitates or other solid samples from different locations in the La Hague spent nuclear fuel treatment plant by mixtures of 0.25 N hydrofluoric acid and 14 N nitric acid, the volumes of which were adjusted to 50 mL by the addition of 0.5 N nitric acid; and - various solution residues (40 mL) resulting from attacks on precipitates or other solid samples from different locations in the La Hague spent nuclear fuel treatment plant by mixtures of 11 N hydrofluoric acid and 7 N nitric acid, the volumes of which were also adjusted to 50 mL by the addition of 0.5 N nitric acid.

[0060] Solution S3 comprises 18.7 g / L of fluoride ions. It has an activity [3 / y of 2,085.107 Bq / L, mainly linked to caesium 137 (98%) - the other emitting radionuclides [3 / y detected by y spectrometry being europium 154 and cobalt 60 - as well as an activity a of 1.27.107 Bq / L linked to americium 241.

[0061] 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).

[0062] 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.

[0063] For the treatment with lime milk, the procedure is as described in Example II above, except that there is also no washing operation.

[0064] The characteristics of these tests are summarized in Table 3 below.

[0065] [Tables3] Test 1 - lime milk Test 2 - CafNChh Test 3 - CaCOj; Volume of solution S3 [Mi 100 100 120 Volume of milk of lime (ml] 2W — Volume of NaOH 20 N (ml) ___ 100 100 pH (verified on pH paper) 13 13 13 Mass of Ca provided by Cs ( IWsh or CaCOs (si 17 g 32.8 g + 6.5 g in the supernatant Maturing time > 24 h (several days) o 24 h (several days) Final concentration measured eh F' (mg / L| 30.6 41.7 4.n

[0066] Regarding the activity of the radionuclides, it is reported in Table 4 below.

[0067] [Tables4] Radionuclides Activity before treatment of solution S3 (Bq per 100 ml) Activity after treatment of solution S3 Test 1 - Milk of lime (Bq per 151 ml] Test 2 CalNOi); (Bq per 93 ml) Test 3 - CaCOj (Bq per 30 ml) ^Cs 2 04.10î: 7.36.105 / / Al / 3.87.1 / 1S4Eu 2.51.104 not detected not detected not detected sCo 161. W4 not detected nc-n detected not detected

[0068] Americium 241, europium 154 and cobalt 60 were immobilized in the precipitated since they are not detected in the S3 solution after treatment, regardless of the source of calcium ions used.

Claims

Claims

1. A method of treating an acidic aqueous solution A, comprising hydrofluoric acid, fluoride ions resulting from a dissociation of the hydrofluoric acid and radionuclides, which method 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 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 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 poor in fluoride ions is obtained, the radionuclides having been distributed between the precipitate and the aqueous phase according to 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. A method according to claim 1, wherein the aqueous solution A is a solution which comprises hydrofluoric acid and a mineral acid other than hydrofluoric acid, preferably nitric acid, phosphoric acid or sulfuric acid and, more preferably, nitric acid.

3. A method according to claim 1 or claim 2, wherein 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, more preferably, between 13 and 14.

4. A method according to any one of claims 1 to 3, wherein the source of calcium ions is an inorganic calcium salt, preferably calcium hydroxide, calcium nitrate, calcium carbonate, calcium sulfate or calcium phosphate, the salt possibly being in hydrated form.

5. A method according to claim 4, wherein the source of calcium ions is calcium hydroxide which is used in the form of a milk of lime previously obtained by supersaturating water with slaked lime and to which aqueous solution A is added.

6. A method according to claim 4 or claim 5, wherein the source of calcium ions is a milk of lime and step b) is carried out simultaneously with step a).

7. A method according to claim 4, wherein the source of calcium ions is calcium nitrate, calcium carbonate, calcium sulfate or calcium phosphate, the pH of 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. A method according to any one of claims 1 to 7, which further comprises, between steps b) and c), a step of allowing 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 in mixture with another mineral acid such as nitric acid, of one or more precipitates produced during the treatment of spent nuclear fuel.

10. 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 of one or more precipitates of fission products by a mixture of hydrofluoric acid and nitric acid, a solution resulting, directly or after possible dilution, from an attack of dissolution fines by a mixture of hydrofluoric acid and nitric acid, or a mixture thereof.