METHOD FOR THE REMOVAL OF RADIONUCLIDES FROM AQUEOUS RADIOACTIVE WASTE

Abstract

The present invention discloses a method for the separation of radionuclides from an aqueous radioactive waste solution, the method comprising: receiving of an aqueous radioactive waste solution, adding at least one zirconium salt to the aqueous radioactive waste solution, changing the pH of the radioactive waste solution to obtain a precipitate P, and separating the precipitate P from the radioactive waste solution. The present invention also discloses the use of zirconium salts, preferably zirconium oxychloride, zirconium nitrate or a zirconium oxynitrate or any mixture thereof, for the treatment of aqueous radioactive waste solution, preferably acidic or alkaline intermediate or low level radioactive waste solution, preferably an acidic intermediate and/or low level radioactive waste solution.

Claims

1. A method for the separation of radionuclides from an aqueous radioactive waste solution, the method comprising: receiving of an aqueous radioactive waste solution, adding at least one zirconium salt to the aqueous radioactive waste solution, changing the pH of the radioactive waste solution to obtain a precipitate P, and separating the precipitate P from the radioactive waste solution.

2. The method according to claim 1, wherein the change in pH of the radioactive waste solution to obtain the precipitate P is done prior to adding at least one zirconium salt to the received aqueous radioactive waste solution.

3. The method according to claim 1, wherein the zirconium salt added to the aqueous radioactive waste solution is selected from zirconium salts with zirconium ions having an oxidation level of +4, preferably from the group comprising zirconium oxychloride, zirconium nitrate or a zirconium oxynitrate or any mixture thereof.

4. The method according to claim 1, wherein the at least one zirconium salt is added in its solid form and/or as an aqueous solution comprising the at least one zirconium salt.

5. The method according to claim 1, wherein the zirconium concentration of the solution obtained after adding at least one zirconium salt to the aqueous radioactive waste solution is in the range of 0.5 to 2.5 mg/l zirconium, preferably 1.0 to 2.25 mg/l zirconium, most preferably 1.5 to 2.0 mg/l.

6. The method according to claim 1, wherein the pH of the of the radioactive waste solution is changed to a pH range of 6 to 8, preferably to a pH of 6.5 to 7.5.

7. The method according to claim 1, wherein the aqueous radioactive waste solution is an acidic or alkaline intermediate or low level radioactive waste solution, preferably an acidic intermediate and/or low level radioactive waste solution.

8. The method according to claim 7, wherein the pH of the of the radioactive waste solution is changed by adding one of the additives selected from the group consisting of: at least one base to the received acidic aqueous low or intermediate level radioactive waste solution; and at least one acid to the received alkaline low or intermediate level radioactive waste solution.

9. The method according to claim 8, wherein the at least one base is a water soluble salt and/or an aqueous solution of a water soluble salt, wherein the water soluble salt is preferably an alkali metal hydroxide, more preferably sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, wherein sodium hydroxide is preferred and the at least one acid is hydrochloric acid (HCl), nitric acid (HNO.sub.3) or sulfuric acid (H.sub.2SO.sub.4), wherein 1 M nitric acid is preferred.

10. The method according to claim 1, wherein the radionuclides comprise ruthenium, preferably ruthenium with a mass number of 106, and antimony, preferably with a mass number of 125.

11. The method according to claim 1, wherein the precipitate P comprises or consists of ruthenium and/or antimony, preferably and wherein ruthenium and/or antimony are precipitated simultaneously.

12. The method according to claim 1, wherein the temperature during the process, during receiving of an aqueous radioactive waste solution, adding at least one zirconium salt to the aqueous radioactive waste solution, changing the pH of the radioactive waste solution to obtain a precipitate P, and separating the precipitate P from the radioactive waste solution, or during receiving of an aqueous radioactive waste solution, adding at least one zirconium salt to the aqueous radioactive waste solution, changing the pH of the radioactive waste solution to obtain a precipitate P, flocculating the precipitate P, after the changing of the pH, and separating the precipitate P from the radioactive waste solution, or during receiving of an aqueous radioactive waste solution, adding at least one zirconium salt to the aqueous radioactive waste solution, changing the pH of the radioactive waste solution to obtain a precipitate P, flocculating the precipitate P, after the changing of the pH, separating the precipitate P from the radioactive waste solution, and processing the solution obtained after separating the precipitate P to remove cesium and strontium prior to discharge, is kept at room temperature, preferably between 20 to 40° C.

13. The method according to claim 1, wherein the separation of the precipitate P from the radioactive waste solution comprises the filtration or centrifugation of the solution obtained after changing the pH of the radioactive waste solution to obtain a precipitate P or flocculating the precipitate P, after the changing of the pH.

14. The method according to claim 1, wherein activity in Becquerel % milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the radioactive waste solution is at least 20 times, preferably 100, more preferably at least 300 times, even more preferably 500 times lower than the activity of the received aqueous radioactive waste solution, and wherein the activity in Becquerel/milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the acidic radioactive waste solution is at least 100 times, preferably 300, more preferably at least 500 times lower than the activity of the received aqueous radioactive waste solution, and wherein the activity in Becquerel/milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the alkaline radioactive waste solution is at least 20 times, preferably 30 times lower than the activity of the received aqueous radioactive waste solution.

15. The method according to claim 1, wherein activity in Becquerel/milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the radioactive waste solution is at least 20 times, preferably 100, more preferably at least 300 times, even more preferably 500 times lower than the activity of the received aqueous radioactive waste solution, or wherein the activity in Becquerel/milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the acidic radioactive waste solution is at least 100 times, preferably 300, more preferably at least 500 times lower than the activity of the received aqueous radioactive waste solution, or wherein the activity in Becquerel/milliliter caused by ruthenium 106 and antimony 125 in the supernatant obtained after separating the precipitate P from the alkaline radioactive waste solution is at least 20 times, preferably 30 times lower than the activity of the received aqueous radioactive waste solution.

16. A method using zirconium salts, preferably zirconium oxychloride, zirconium nitrate or a zirconium oxynitrate or any mixture thereof, for the treatment of aqueous radioactive waste solution, preferably acidic or alkaline intermediate or low level radioactive waste solution, preferably an acidic intermediate and/or low level radioactive waste solution.

Description

[0032] Further features and advantages of the invention will be apparent from the following description, in which examples of embodiments of the invention are explained by means of schematic drawings, without thereby limiting the invention.

[0033] FIG. 1 Flow chart of the method according to an embodiment of the present invention.

[0034] FIG. 2 Flow chart of a preferred embodiment of the present invention for the simultaneous removal of ruthenium and antimony from acidic intermediate level radioactive waste.

DETAILED DESCRIPTION

[0035] According to embodiments of the invention as claimed, provided is a method for the separation of radionuclides from an aqueous radioactive waste solution, the method comprising: a) receiving of an aqueous radioactive waste solution, b) adding at least one zirconium salt to the aqueous radioactive waste solution, c) changing the pH of the radioactive waste solution to obtain a precipitate P, and d) separating the precipitate P from the radioactive waste solution.

[0036] FIG. 1 shows a flow chart of the method according an embodiment of the present invention. In this flow chart four consecutive steps are shown. In a first step a), the aqueous radioactive waste solution is received. In a second step b), the at least one zirconium salt is added to the aqueous radioactive waste solution. In a third step the pH of the radioactive waste solution is changed to obtain a precipitate P. In the last fourth step d), the precipitate P is separated from the radioactive waste solution. It can also be possible to change the pH of the radioactive waste solution prior to adding the at least one zirconium salt, meaning that step c) is conducted before step b).

[0037] The operations of the method can be especially effective in an in-situ process for the separation of the radionuclides ruthenium 106 and antimony 125 from acidic intermediate level radioactive waste and low level radioactive waste. Ruthenium 106 and antimony 125 constitute a significant hazard in intermediate level radioactive waste and low level radioactive waste management, as they are radiotoxic and cytotoxic.

[0038] Since the aqueous radioactive waste contains significant fractions of corrosion products, a pH change results in the precipitation of various iron hydroxides and iron oxyhydroxides, which can adsorb radionuclides, in particular ones like ruthenium and antimony. The inventors have recognized that the use of zirconium salts increases the efficiency of the adsorption process.

[0039] The flow chart shown in FIG. 2 illustrates a preferred embodiment of the present invention for the simultaneous removal of ruthenium and antimony from acidic intermediate level radioactive waste (ILW), comprising the steps: [0040] a) Receiving 500 ml acidic aqueous intermediate level radioactive waste in a container, [0041] b) Adding 1.0 mg/l of a water soluble zirconium salt solution, [0042] c) Adding 25 ml of 10 M sodium hydroxide solution to the solution to gradually increase the pH of the solution to a range of 6 to 8, to obtain a precipitate and ruthenium 106 and antimony 125 lean aqueous intermediate level radioactive waste. [0043] d) The precipitates bearing ruthenium 106 and antimony 125 species are separated from the ruthenium 106 and antimony 125 lean aqueous intermediate level radioactive waste by centrifugation. [0044] c) Ruthenium 106 and Antimony 125 lean aqueous intermediate level radioactive waste can be taken for further processing such as cesium and strontium removal prior to discharge and the precipitate P can be immobilized, for example by incorporation into a solid cement block for long time storage.

[0045] Due to this process a ruthenium 106 and antimony 125 bearing phase of much smaller volume than the original provided acidic aqueous intermediate level radioactive waste can be stored for further processing. The decontamination factor (DF) for Ruthenium 106 and Antimony 125 significantly exceeds 300, so that the intermediate level radioactive waste can be discharged after cesium and strontium removal.

[0046] Further, exemplary embodiments of the process according to the invention are described below.

TABLE-US-00001 TABLE 1 A detailed waste composition which is treated with the method according to the invention. Typical characteristics of acidic raffinate post PUREX, actinide and fission product removal HNO3 molarity (M) 3.7 Gross αradiation (Bq/ml) 21 Gross Beta radiation 4.4 × 10.sup.5 (Bq/ml) .sup.137Cs (Bq/ml) 4.1 × 10.sup.3 .sup.90Sr & .sup.90Y (Bq/ml) <4.1 × 10.sup.3 .sup.134Cs (Bq/ml) <4.1 × 10.sup.3 .sup.125Sb (Bq/ml) 2.2 × 10.sup.5 − 3,0 × 10.sup.5 .sup.106Ru (Bq/ml) 7.4 × 10.sup.4 − 1,1 × 10.sup.5 PUREX means that plutonium-uranium recovery by extraction was already aqueous radioactive waste, as well as actinide and fission product removal.

[0047] The test method for determination of the decontamination factor (DF) is specified below:

[00001]$\mathrm{Decontamination}\mathrm{factor}\left(\mathrm{DF}\right)=\frac{\mathrm{Actívity}\mathrm{of}\mathrm{the}\mathrm{received}\mathrm{radíoactive}\mathrm{waste}\mathrm{solution}}{\begin{array}{c}\mathrm{Activity}\mathrm{of}\mathrm{the}\mathrm{radioactive}\mathrm{waste}\mathrm{solution}\mathrm{after}\\ \mathrm{seperation}\mathrm{of}\mathrm{the}\mathrm{precipitate}P\end{array}}$

[0048] The detection was performed by gamma spectroscopy using a calibrated multi-channel analyzer (MCA) to detect the 428 keV gamma emission of antimony 125 and 511 keV gamma emission of ruthenium 106. Typically, 0.5 ml of the solution to be analyzed were taken in a gamma vial and placed in the analyzer for measurement. The higher activity wastes were measured for around 10 minutes, while the low activity solutions, e.g. the solution after precipitation and separation of the ruthenium and antimony species, required hour of measurement to accumulate enough data. Each measurement was repeated at least three times to ensure accuracy. The limit of detection with the applied gamma spectroscopy is around 19 Bq/ml.

[0049] Example 1: Effect of pH on ruthenium 106 and antimony 125 removal by in-situ precipitation of zirconium (IV) hydroxide (Zr(OH).sub.4). The first batch of studies were carried out on a 50 ml scale using cesium lean intermediate level radioactive waste to ascertain the effect of pH on ruthenium and antimony removal. In all experiments shown in Table 1, a constant zirconium dosing of 1.0 mg/l zirconyl dichloride (ZrOCl.sub.2) was maintained. Starting pH was varied between 4 to 12. Ruthenium 106 activity in the feed was 9.6×10.sup.4 Bq/ml, while antimony 125 activity was 2.3×10.sup.5 Bq/ml. Table 2 summarizes the effect of pH on the decontamination factor.

TABLE-US-00002 TABLE 2 Effect of the pH on the Ruthenium 106 and Antimony 125 decontamination factor. Decontamination factor of Supernatant activity of pH antimony 125 ruthenium 106 4 415 342 6 1000 below limit of detection 8 810 1368 10 270 162 12 220 13

[0050] Table 2 shows that the Decontamination Factor (DF) obtained are highest for both antimony and ruthenium between pH 6 to 8, indicating that the process works most optimally at circumneutral pH.

[0051] Example 2: Effect of the zirconium concentration on ruthenium and antimony removal by in-situ precipitation of zirconium hydroxide (Zr(OH).sub.4). In these experiments, the concentration of zirconium in the solution obtained using zirconyl dichloride (ZrOCl.sub.2) was varied at a constant pH of 6 of the solution and the effect on ruthenium and antimony removal was observed. Ruthenium 106 activity in the feed was 9.6 Bq/ml, while antimony 125 activity was 2.3×10.sup.5 Bq/ml. The results are collected in Table 3.

TABLE-US-00003 TABLE 3 Effect of the zirconium concentration on the ruthenium 106 and antimony 125 decontamination factor according to example 2. Solution activity of Solution activity of ruthenium Zirconium antimony 125 in Bq/ml 106 in Bq/ml concentration after separation of the after separation of the in mg/l precipitate precipitate 0.5 281 89 1.0 229 below limit of detection 1.5 93 below limit of detection 2.0 56 below limit of detection 2.5 56 below limit of detection

[0052] It is shown that zirconium concentrations of 2.0 mg/l or greater allows most efficient removal of ruthenium 106 and antimony 125.

[0053] Example 3: Effect of zirconium source on ruthenium 106 and antimony 125 removal by in-situ precipitation of zirconium (IV) hydroxide (Zr(OH).sub.4). The effect of various zirconium sources to achieve a zirconium concentration of 1.0 mg/l in the acidic intermediate level radioactive waste at pH 6 was investigated. Ruthenium 106 activity in the feed was 2.1 Bq/ml, while antimony 125 activity was 9.5 Bq/ml. The results of these studies are collected in Table 4.

TABLE-US-00004 TABLE 4 Effect of zirconium source on ruthenium 106 and antimony 125 removal according to example 3. Solution activity Decon- Solution activity Decon- of antimony 125 tamination of ruthenium 106 tamination in Bq/ml after factor in Bq/ml after factor separation of the antimony separation of the ruthenium precipitate 125 precipitate 106 Zr(NO.sub.3).sub.4 52 4000  52 1828 ZrO(NO.sub.3).sub.2 78 2666 178  533 ZrOCl.sub.2 41 5090  63 1505

[0054] The results shown in Table 4 demonstrate high removal efficiency for all three tested zirconium salts.

[0055] Example 4: Decontamination factors achieved for intermediate alkaline radioactive waste with a pH of 12. In this experiment, the concentration of zirconium in the solution obtained using zirconyl dichloride (ZrOCl.sub.2) was adjusted to 1.0 mg/l, followed by the addition of 1 M nitric acid to reduce the pH of the radioactive waste solution to pH 6 to 7. Ruthenium 106 activity in the feed was 9.6 Bq/ml, while antimony 125 activity was 2.5×10.sup.5 Bq/ml. The results are collected in Table 5.

TABLE-US-00005 TABLE 5 Effect on the activity of intermediate alkaline radioactive waste after ruthenium 106 and antimony 125 removal according to example 4. Decon- Decon- Solution activity of tamination Solution activity of tamination antimony 125 in Bq/ml factor ruthenium 106 in Bq/ml factor after separation of the antimony after separation of the ruthenium precipitate 125 precipitate 106 303 83 311 31

[0056] In the dependent claims, in the description and in the figures, preferred aspects, embodiments and examples have been described for the invention, which invention being defined by the appended independent claims. In case these dependent claims, aspects, embodiments and examples comprise features which are not recited in the appended independent claims, these features are optional features that are not essential, but may be beneficial, for the invention. One or more of these optional features may be combined with each other and with any one of the appended claims.