A strontium carbonate-containing radioactive waste solidification body and a method for solidifying the same

Abstract

This invention belongs to the field of radioactive waste treatment technology, specifically relating to a strontium-containing carbonate radioactive waste solidification body and its solidification method. The solidification method includes the following steps: S1, mixing magnesium salt solution, calcium salt solution, and strontium salt solution to obtain a mixed solution, adding a carbonate solution to react and obtain a strontium-magnesium-calcium mixed carbonate precipitate; S2, sequentially subjecting the strontium-magnesium-calcium mixed carbonate precipitate obtained in S1 to solid-liquid separation, washing, drying, and pulverizing to obtain strontium-containing carbonate powder; S3, mixing the strontium-containing carbonate powder obtained in S2 with a transient liquid phase to obtain a mixture to be sintered; S4, placing the mixture to be sintered obtained in S3 in a mold, cold sintering and holding at a temperature to obtain a strontium-containing carbonate radioactive waste solidification body. This strontium-containing carbonate radioactive waste solidification body has high density and physical strength, as well as excellent chemical stability, meeting the safety requirements for long-term radioactive waste disposal.

Description

Technical Field

[0001] This invention belongs to the field of radioactive waste treatment technology, specifically relating to a solidified body of radioactive waste containing strontium carbonate and its solidification method. Background Technology

[0002] The widespread application of nuclear energy and nuclear technology, while bringing enormous social benefits, also continuously generates various types of radioactive waste. Its safe handling and long-term reliable disposal are fundamental guarantees for the sustainable development of the industry. During the operation of nuclear power plants, nuclear fuel reprocessing, and radioactive decontamination, large amounts of radioactive wastewater are generated. Strontium (Sr) is a typical fission product nuclide. Due to its long half-life, chemical properties similar to calcium, and tendency to accumulate in organisms, it poses a significant risk to the environment and health. Currently, strontium-containing wastewater is mostly treated using methods such as precipitation and adsorption, resulting in secondary radioactive solid waste containing strontium, primarily composed of carbonates. This type of waste typically exhibits characteristics such as concentrated radioactivity, fine particles, and unstable chemical forms. If effective solidification is not achieved, it is highly susceptible to leaching and diffusion of nuclides during subsequent storage, transportation, and final disposal.

[0003] Currently, the mainstream technologies for solidification of radioactive waste include cement solidification, glass solidification, ceramic solidification, polymer solidification, and artificial rock solidification. Cement solidification is a mature and low-cost process, but the resulting solidified body has high porosity, a loose structure, a high rate of radionuclide leaching, and insufficient long-term stability. Glass solidification and ceramic solidification can form solidified bodies with excellent stability, but the processing temperature usually needs to be above 1000℃. Under these high-temperature conditions, the carbonate components in carbonate-containing waste are prone to thermal decomposition and gas release, which not only affects the densification of the solidified body but may also cause secondary release of radionuclides. Although polymer solidification can be carried out at room temperature or low temperature, its radiation resistance is poor, and it is prone to aging and decomposition after long-term irradiation, which may produce hydrogen gas and pose safety hazards. Artificial rock solidification technology has specific requirements for waste composition, is complex in process, expensive, and has a limited scope of application. Summary of the Invention

[0004] The present invention aims to provide a strontium carbonate-containing radioactive waste solidified body and a solidification method thereof. The strontium carbonate-containing radioactive waste solidified body has high density and physical strength, as well as excellent chemical stability, meeting the safety requirements for long-term disposal of radioactive waste.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for solidifying radioactive waste containing strontium carbonate includes the following steps: S1. Mix magnesium salt solution, calcium salt solution and strontium salt solution to obtain a mixed solution, add carbonate solution to react and obtain a strontium-magnesium-calcium mixed carbonate precipitate; S2. The strontium-magnesium-calcium mixed carbonate precipitate obtained in S1 is subjected to solid-liquid separation, washing, drying and pulverizing in sequence to obtain strontium-containing carbonate powder. S3. The strontium-containing carbonate powder obtained in S2 is mixed with the transient liquid phase to obtain the mixture to be sintered; S4. Place the mixture to be sintered obtained in S3 into a mold, perform cold sintering and heat preservation to obtain a solidified radioactive waste containing strontium carbonate.

[0006] Preferably, in S1, the magnesium salt is one or both of magnesium chloride and magnesium nitrate, and the calcium salt is one or both of calcium chloride and calcium nitrate.

[0007] Preferably, in S1, the molar ratio of strontium ions, magnesium ions and calcium ions in the mixed solution is (5:3:1)-(2:1:0.4).

[0008] Preferably, in S1, the specific conditions for the reaction of adding carbonate solution are: pH 7.5-8.5 and temperature 25-30℃.

[0009] Preferably, in step S2, the drying temperature is 80-90°C and the drying time is 12-24 hours.

[0010] Preferably, in step S3, the transient liquid phase is an aqueous sodium carbonate solution, the amount of the transient liquid phase added is 5-15% of the mass of the strontium carbonate powder, and the concentration of the aqueous sodium carbonate solution is 0.5-2.0 mol / L.

[0011] Preferably, in S4, the uniaxial pressure of the cold sintering treatment is 100-500 MPa, the temperature of the cold sintering treatment is 100-200℃, and the holding time is 10-60 min.

[0012] The present invention also provides a solidified body of radioactive waste containing strontium carbonate obtained by the solidification method.

[0013] Compared with the prior art, the present invention has the following advantages and technical effects: This invention discloses a solidified body of strontium-containing carbonate radioactive waste and its solidification method. This solidification method achieves highly dense solidification of strontium-containing carbonate waste at temperatures not exceeding 200°C, avoiding carbonate decomposition and volatilization. Furthermore, through the synergistic effect of low temperature, high pressure, and a transient liquid phase, the dissolution-reprecipitation and grain rearrangement processes of the precursor powder are promoted, resulting in an extremely dense solidified body. Examples show that its relative density can reach over 95%, its maximum compressive strength exceeds 700 MPa, and its Vickers microhardness exceeds 2.0 GPa.

[0014] Furthermore, the leaching index of strontium in this strontium-containing carbonate radioactive waste solidification can reach over 9, and the leaching rate can be as low as 1×10⁻⁶. -5 g·m -2 ·day -1 This meets the safety requirements for the long-term disposal of radioactive waste.

[0015] This solidification method has a simple process flow, low energy consumption, and does not require the introduction of additional secondary waste, such as inorganic binders, fluxes, or glass phase precursors. It is suitable for the large-scale treatment of secondary radioactive waste containing carbonates.

[0016] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0017] Figure 1 The images show the scanning electron microscope (SEM) morphology and energy-dispersive X-ray spectroscopy (EDS) elemental distribution of the strontium-containing carbonate powder in Example 1 of this invention. Figure 1 Image (a) is a SEM morphology image with a scale bar of 30 μm. Figure 1 (b) in the diagram is the distribution map of Ca element. Figure 1 (c) in the diagram is the distribution map of Mg element. Figure 1 (d) in the diagram represents the distribution of element O. Figure 1 (e) in the diagram represents the distribution of element C. Figure 1 (f) in the figure is the distribution map of Sr elements; Figure 2 Statistical analysis of the grain size of strontium carbonate powder; Figure 3 EDS elemental analysis of strontium carbonate powder; Figure 4 The effects of sintering time, sintering temperature, sodium carbonate solution content, and applied pressure on the bulk density of the solidified body were investigated. Figure 5 The effect of applied pressure on the compressive strength and micro Vickers hardness of the cured body; Figure 6 SEM images of the cured bodies under different applied pressures and temperatures are shown. Figure 6 Image (a) shows the SEM morphology of the solidified body under an applied pressure of 100 MPa and a sintering temperature of 25 °C. The scale bar is 2 μm. Figure 6 Image (b) shows the SEM morphology of the solidified body under an applied pressure of 300 MPa and a sintering temperature of 100 °C. The scale bar is 2 μm. Figure 6 (c) in the figure is a SEM image of the solidified body under an applied pressure of 500 MPa and a sintering temperature of 200 °C, with a scale bar of 2 μm; Figure 7The image shows the leaching results of CS-Sr-MCS, where... Figure 7 In the figure (a), the normalized leaching rate of strontium after 42 days of CS-Sr-MCS leaching is... Figure 7 (b) shows the leaching percentage of strontium after 42 days of CS-Sr-MCS leaching. Figure 7 (c) shows the surface SEM image of CS-Sr-MCS after immersion in deionized water at 25°C for 42 days, with a scale bar of 1 μm. Figure 7 Image (d) shows a surface SEM image of CS-Sr-MCS after leaching at 0.1 mol / L NaCl and 25 °C for 42 days, with a scale bar of 1 μm. Figure 7 Image (e) shows a surface SEM image of CS-Sr-MCS after leaching with 0.1 mol / L NaOH at 25°C for 42 days, with a scale bar of 1 μm. Figure 7 (f) in the figure is a surface SEM image of CS-Sr-MCS after leaching at 0.1 mol / L HNO3 and 25 °C for 42 days, with a scale bar of 1 μm. Detailed Implementation

[0018] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0020] Source of experimental materials: In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0021] Strontium-containing secondary radioactive waste generated during nuclear wastewater treatment was used as the experimental material. During the experiment, for operational safety reasons, stable strontium isotopes were used as chemical simulants of radioactive strontium for verification.

[0022] Example 1 A method for solidifying radioactive waste containing strontium carbonate includes the following steps: S1. Weigh 4.16 g of anhydrous calcium chloride and 7.06 g of anhydrous magnesium chloride, and dissolve them separately in 100 mL of deionized water to obtain a 0.74 mol / L magnesium chloride solution and a 0.37 mol / L calcium chloride solution. Mix them thoroughly, and add 1 mL of a 1 mol / L strontium chloride solution to obtain a mixed solution. The strontium chloride concentration in the mixed solution is... 2+ :Mg 2+ :Ca 2+The molar ratio of sodium carbonate was 5:3:1. Under continuous stirring, a 1 mol / L sodium carbonate solution was slowly added dropwise to the above mixed solution. The pH of the system was adjusted to 8.0. The mixture was stirred at 25°C for 12 hours. The precipitate of strontium magnesium-calcium mixed carbonate was obtained by ultrasonic treatment (ultrasonic power of 60W). S2. The strontium-magnesium-calcium mixed carbonate precipitate obtained in S1 is subjected to solid-liquid separation. The precipitate is repeatedly washed with deionized water until the filtrate is neutral. The washed precipitate is dried at 90°C for 24 hours, ground, and passed through a 200-mesh sieve to obtain strontium-containing carbonate powder. Grain analysis of the strontium-containing carbonate powder yielded the following results: Figure 1-3 As shown.

[0023] Depend on Figure 1 It can be seen that CaCO3 spherulite crystals tend to aggregate into clusters, while Sr... 2+ It has been evenly distributed within the carbonate powder.

[0024] Depend on Figure 2 It can be seen that the grain statistical analysis shows that most of the strontium-containing magnesium-calcium carbonate powders have a nanostructure with an average size of about 0.75 μm and regular particle morphology.

[0025] Depend on Figure 3 It can be seen that the atomic ratio of Mg, Ca and Sr in the obtained powder is about 2:1:0.36, which is consistent with the expectation and provides a good raw material basis for subsequent cold sintering densification.

[0026] S3. Mix 0.5g of the strontium carbonate powder obtained in S2 with 0.075mL of a 1mol / L sodium carbonate aqueous solution to obtain the mixture to be sintered; S4. The mixture to be sintered obtained in S3 is loaded into a cylindrical stainless steel mold with an inner diameter of 12.7 mm. It is compacted and shaped for 10 min at room temperature under a single-axis press at 200 MPa. After compaction, it is heated from room temperature to 200℃ at a pressure of 500 MPa at a heating rate of 20℃ / min. It is then sintered at this temperature for 60 min to obtain a solidified body of radioactive waste containing strontium carbonate.

[0027] Example 2 The curing method is the same as in Example 1, except that in S3, 0.5g of the strontium carbonate powder obtained in S2 is mixed with 0.025mL of a 1mol / L sodium carbonate aqueous solution to obtain the mixture to be sintered.

[0028] Example 3 The curing method is the same as in Example 1, except that in S3, 0.5g of the strontium carbonate powder obtained in S2 is mixed with 0.05mL of a sodium carbonate aqueous solution with a concentration of 1mol / L to obtain the mixture to be sintered.

[0029] Example 4 The solidification method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 200°C at a pressure of 100 MPa and a heating rate of 20°C / min, and sintered at this temperature for 60 min to obtain a solidified body of radioactive waste containing strontium carbonate.

[0030] Example 5 The curing method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 200°C at a pressure of 300 MPa and a heating rate of 20°C / min, and sintered at this temperature for 60 min to obtain a solidified body of radioactive waste containing strontium carbonate.

[0031] Example 6 The solidification method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 100°C at a pressure of 500 MPa at a heating rate of 20°C / min, and sintered at this temperature for 60 min to obtain a solidified body of radioactive waste containing strontium carbonate.

[0032] Example 7 The curing method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 150°C at a pressure of 500 MPa at a heating rate of 20°C / min, and sintered at this temperature for 60 min to obtain a solidified radioactive waste containing strontium carbonate.

[0033] Example 8 The solidification method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 200°C at a pressure of 500 MPa and a heating rate of 20°C / min, and sintered at this temperature for 10 min to obtain a solidified radioactive waste containing strontium carbonate.

[0034] Example 9 The curing method is the same as in Example 1, except that in S4, after compaction, the material is heated from room temperature to 200°C at a pressure of 500 MPa and a heating rate of 20°C / min, and sintered at this temperature for 30 min to obtain a solidified body of radioactive waste containing strontium carbonate.

[0035] The effectiveness of the strontium carbonate-containing radioactive waste solidification body provided in the above embodiments was verified.

[0036] The physical properties of strontium carbonate-containing radioactive waste solidification were determined using the following experimental procedure: The bulk density of the sample was determined using the Archimedes method in deionized water at T=25℃. The dry mass of the sample was obtained after drying at 90℃ for 6 hours. The suspended mass of the sample is obtained by immersing it in the medium. Sample volume density according to calculate( The density of the medium is 1 g / cm³. 3 ), the result is as follows Figures 4-6 .

[0037] Depend on Figure 4 It can be seen that with the increase of various factor values, the bulk density and relative density of the carbonate solidified body increase significantly, but the growth trends of each factor are different. The sintering time, i.e., the holding time, has the least impact on the density of the solidified body. With the increase of the amount of transient liquid phase added, the density of the solidified body increases significantly, but when the amount added exceeds a certain range, the increase in density tends to level off. With the increase of cold sintering temperature and pressure, the compressive strength of the solidified body gradually increases, indicating that the synergistic effect of pressure and temperature during cold sintering is beneficial to the dissolution-reprecipitation and grain rearrangement between powder particles, thereby improving the mechanical properties of the solidified body.

[0038] Depend on Figure 5 It can be seen that as the applied pressure increases from 100 MPa to 500 MPa, the relative density of the solidified body increases from 1.6 g / cm³. 3 Increased to 2.64 g / cm³ 3 The compressive strength and micro Vickers hardness are also dramatically improved. The compressive strength of the cured body reaches 719 MPa and the micro Vickers hardness reaches 2.69 Gpa after sintering at 500 MPa and 200℃ for 1 h. The compressive strength far exceeds that of other materials, and the relative density of 96% is also significantly better than that of existing carbonate-based cured bodies.

[0039] Depend on Figure 6 It can be seen that the initial pressure is applied to make the particles come into close contact; then the pressure is increased and the temperature is raised, the small particles gradually dissolve in the transient liquid phase, and the dissolved substances are deposited on the surface of the large particles; in the later stage, the grains continue to grow, and the adjacent grains gradually merge through grain boundary migration, eliminating grain boundary pores and forming a continuous large grain structure.

[0040] Leaching performance tests (MCC-1) were performed on the strontium carbonate-containing cold-sintered solidified body provided in Example 1: (1) Referring to the MCC-1 standard, four parallel samples of the cold-sintered solidified body (shaped approximately equal to a standard cylinder with a mass of approximately 0.5g, a diameter of 12.7mm, and a height of approximately 1.8mm) were respectively immersed in 35mL of ultrapure water, 0.1M NaCl, 0.1M NaOH, and 0.1M HNO3 (the solvent volume was 10 times the sample surface area).

[0041] (2) Use a thin thread made of polyvinyl fluoride to suspend the sample in the leaching solution, seal it in a PTFE container, and place it in an electric heating drying oven at 25°C and 90°C.

[0042] (3) At fixed times (1, 3, 7, 14, 21, 28, 35 and 42 days), 5 mL of the leachate was taken out, and the concentration of strontium in the leachate was determined by inductively coupled plasma optical emission spectrometry, and the normalized leaching rate of strontium was calculated. and leaching percentage The formula is as follows: ; ; In the formula, : The mass (g) of element i in the leachate; : The mass fraction of element i in the sample; Geometric surface area of ​​the sample (m²) 2 ); Time (d); Concentration of Sr in the leachate (mg / L); : Mass fraction of Sr in sintered products (wt%); : Volume of leachate (L); : Weight of sintered product (g).

[0043] Depend on Figure 7 The leaching rates of Sr at 25℃ and 90℃ are: 0.1M HNO3 > 0.1M NaCl > H2O > 0.1M NaOH. The leaching percentages of Sr in deionized water, 0.1M NaCl, 0.1M NaOH, and 0.1M HNO3 are 0.15%, 4.58%, 0.015%, and 47% at 25℃, respectively; and 0.98%, 6.75%, 0.045%, and 92% at 90℃, respectively.

[0044] Sr has low solubility in deionized water (neutral pH), but water, as a polar solvent, can still slowly leach Sr through diffusion. 2+ The surface of the solidified body has some cracks, but they are not very obvious. In a 0.1M NaCl solution, the Na in NaCl... + With Sr in the cured body 2+ Competitive adsorption occurs, releasing Sr through ion exchange. 2+ The surface of the solidified body is mottled, and the ions on the outermost surface tend to be replaced and leave at any time. Therefore, Sr 2+ The leaching rate is higher than that of deionized water. In a 0.1M NaOH solution (i.e., high pH), the leaching rate of Sr is higher. 2+ With OH - The reaction produces a sparingly soluble Sr(OH)₂ precipitate, significantly reducing the Sr concentration in the solution. 2+At this concentration, the surface of the solidified body remains almost unchanged, relatively smooth and without cracks. In a 0.1M HNO3 solution (low pH), HNO3 reacts with carbonates, leading to Sr... 2+ The large release of acid and the appearance of numerous pores and cracks on the surface of the solidified body indicate that the solidified body has been subjected to severe acid corrosion.

[0045] The results show that the normalized leaching rate of strontium in the strontium-containing carbonate solidified body prepared by cold sintering is low during the leaching process and tends to be stable over time, indicating that strontium is effectively fixed inside the solidified body and the solidified body has good long-term chemical stability.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for solidifying a radioactive waste containing strontium carbonate, characterized in that, Includes the following steps: S1. Mix magnesium salt solution, calcium salt solution and strontium salt solution to obtain a mixed solution, add carbonate solution to react and obtain a strontium-magnesium-calcium mixed carbonate precipitate; S2. The strontium-magnesium-calcium mixed carbonate precipitate obtained in S1 is subjected to solid-liquid separation, washing, drying and pulverizing in sequence to obtain strontium-containing carbonate powder. S3. The strontium-containing carbonate powder obtained in S2 is mixed with the transient liquid phase to obtain the mixture to be sintered; S4. Place the mixture to be sintered obtained in S3 into a mold, perform cold sintering and heat preservation to obtain a solidified radioactive waste containing strontium carbonate.

2. The curing method according to claim 1, characterized in that, In S1, the magnesium salt is one or both of magnesium chloride and magnesium nitrate, and the calcium salt is one or both of calcium chloride and calcium nitrate.

3. The curing method according to claim 1, characterized in that, In S1, the molar ratio of strontium ions, magnesium ions and calcium ions in the mixed solution is (5:3:1)-(2:1:0.4).

4. The curing method according to claim 1, characterized in that, In S1, the specific conditions for the reaction of adding carbonate solution are: pH 7.5-8.5 and temperature 25-30℃.

5. The curing method according to claim 1, characterized in that, In S2, the drying temperature is 80-90℃ and the drying time is 12-24h.

6. The curing method according to claim 1, characterized in that, In S3, the transient liquid phase is an aqueous sodium carbonate solution, the amount of the transient liquid phase added is 5-15% of the mass of the strontium carbonate powder, and the concentration of the aqueous sodium carbonate solution is 0.5-2.0 mol / L.

7. The curing method according to claim 1, characterized in that, In S4, the uniaxial pressure of the cold sintering treatment is 100-500MPa, the temperature of the cold sintering treatment is 100-200℃, and the holding time is 10-60min.

8. The solidified radioactive waste containing strontium carbonate obtained by the solidification method according to any one of claims 1-7.

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