Method for solidifying radioactive waste

Abstract

The invention provides a method for solidifying radioactive nuclear waste, and the method comprises the following steps: S1, uniformly mixing radioactive waste and dihydric phosphate to obtain a solid mixture, and then adding water into the solid mixture to obtain a raw material mixture, the radioactive waste comprising strontium chloride; s2, the raw material mixture is subjected to a supercritical hydrothermal reaction, a ceramic solidified body is obtained, and the supercritical hydrothermal reaction conditions include that the reaction temperature ranges from 390 DEG C to 400 DEG C, the pressure in a kettle ranges from 25 MPa to 27 MPa, the heat preservation time ranges from 9 min to 11 min, and the stirring rotating speed ranges from 400 rpm to 600 rpm. The method is easy to operate, good in curing effect and low in radionuclide leaching rate.

Description

Technical Field

[0001] This application relates to the field of radioactive waste treatment technology, specifically to a method for ceramic solidification of radioactive strontium waste using an apatite system. Background Technology

[0002] Molten salt electrolysis, a dry reprocessing method for spent fuel, uses eutectic salts such as LiCl–KCl or NaCl–KCl as electrolytes to directly electrochemically refine metallic fuels at approximately 500°C. This process is compact, has strong radiation tolerance, and can be directly coupled with fast reactor metallic fuels, making it a strategic alternative for closed fuel cycles. However, this process inevitably generates high-level radioactive molten salt waste rich in fission products, actinides, and alkali metal chlorides. Without solidification, long-term geological disposal safety cannot be guaranteed.

[0003] Therefore, a stable method for solidifying high-electroactive molten salt waste is needed. Summary of the Invention

[0004] The purpose of this application is to provide a method for solidifying radioactive waste, which can stably solidify radioactive waste and make it suitable for long-term disposal.

[0005] This application provides a method for solidifying radioactive waste, the method comprising the following steps: S1, Radioactive waste and dihydrogen phosphate are mixed evenly to obtain a solid mixture, and then water is added to the solid mixture to obtain a raw material mixture, wherein the radioactive waste includes strontium chloride; S2, the raw material mixture is subjected to a supercritical hydrothermal reaction to obtain a ceramic solidified body. The conditions for the supercritical hydrothermal reaction include: a reaction temperature of 390℃-400℃, an internal pressure of 25-27MPa, a holding time of 9-11min, and a stirring speed of 400-600rpm.

[0006] In some embodiments, step 1 further includes adding an alkaline compound to the raw material mixture to adjust the pH of the raw material mixture to 9-11.

[0007] In some embodiments, the dihydrogen phosphate is sodium dihydrogen phosphate.

[0008] In some embodiments, the molar ratio of Sr to P in the solid mixture is in the range of 1.65 to 1.69.

[0009] In some embodiments, the content of strontium chloride in the solid mixture is 65 wt.% to 70 wt.%.

[0010] In some embodiments, the solid-liquid ratio of the solid mixture and water in the raw material mixture is 1:4 to 1:6.

[0011] In some embodiments, step S2 further includes heating from room temperature to 390°C-400°C at a heating rate of 4-6°C / min.

[0012] In some embodiments, the ceramic solidified body comprises apatite.

[0013] The embodiments of this application have the following beneficial effects: This application provides a method for solidifying radioactive waste. The method involves mixing radioactive waste containing strontium chloride and dihydrogen phosphate, adding water, adjusting the pH, mixing thoroughly, and then subjecting the mixture to a supercritical hydrothermal reaction followed by drying to obtain a solidified apatite ceramic body containing radioactive strontium waste. This method has a simple process, produces a stable apatite ceramic body containing radioactive waste, and is suitable for long-term disposal of radioactive molten salt waste. Attached Figure Description

[0014] Figure 1 These are the XRD patterns of the apatite solidified bodies of Example 1 and Comparative Examples 1-5 of this application; Figure 2 The element leaching rate curve of the apatite solidified body of Example 1 of this application is shown. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. The described embodiments should not be regarded as limitations on this application. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0016] Currently, the solidification methods for molten salt waste can be broadly divided into vitrification, glassization, and other methods. Ceramic curing and ceramic hardening. Borosilicate glass processes are mature and allow for continuous casting, but Cl... - Ions are easily volatilized at 800℃–1100℃ and separate from the glass network conditioner; phosphate or thiophosphate glasses can improve halogen retention, but often due to high Li content in the molten salt... + / K + The content leads to a sharp increase in viscosity and phase precipitation in glass. ceramics (such as sodalite) Glass composites improve durability by embedding halogen-containing framework crystals within a glass phase, but require hot pressing at 900°C–1000°C or cold pressing. Radiation sintering is energy-intensive, produces high residual stress, and requires demanding equipment. Ceramic curing, on the other hand, utilizes the mineral structure itself to dissolve long-lived nuclides and halogens, and is considered a method for treating high-Cl content ceramics. - Molten salt is the most promising solution.

[0017] In view of this, this application provides a method for solidifying radioactive waste, the method comprising the following steps: S1, Radioactive waste and dihydrogen phosphate are mixed evenly to obtain a solid mixture, and then water is added to the solid mixture to obtain a raw material mixture, wherein the radioactive waste includes strontium chloride; S2, the raw material mixture is subjected to a supercritical hydrothermal reaction to obtain a ceramic solidified body. The conditions for the supercritical hydrothermal reaction include: a reaction temperature of 390℃-400℃, an internal pressure of 25-27MPa, a holding time of 9-11min, and a stirring speed of 400-600rpm.

[0018] The method of this application involves mixing radioactive waste containing strontium chloride with dihydrogen phosphate, and then carrying out a solidification reaction in a supercritical hydrothermal reaction. The principle of the solidification reaction is as follows: 5Sr 2+ + 3H2PO4 - + Cl - + 6OH - = Sr5(PO4)3Cl↓+ H2O.

[0019] In this application, "supercritical hydrothermal reaction" refers to a hydrothermal reaction in which water exists in a supercritical state, i.e., a fluid that simultaneously possesses the characteristics of both liquid and gaseous states. Through the aforementioned solidification reaction, strontium chloride and dihydrogen phosphate in radioactive waste react to form a stable apatite ceramic solidified body, whose crystal structure has a natural advantage for ion "entry-lock-difficult migration". A typical apatite structure can be described as A5(BO4)3X (commonly A=Ca / Sr, B=P, X=F / Cl / OH, etc.): its framework consists of high-bond-energy [PO4] bonds. 3- The tetrahedrons and metal cation sites form a stable three-dimensional framework. The strong PO bonds and high structural rigidity mean that dissolution / hydrolysis in aqueous environments requires disruption of the stable phosphate framework, resulting in slow overall dissolution kinetics and high thermodynamic stability. Simultaneously, apatite contains two independent cation sites and anion channel sites (X-sites) along the c-axis: most radionuclides (such as Sr...) 2+ Rare earth / actinide ions, etc., can enter the M site through isovalent / isovalent substitution and achieve charge compensation through vacancies or coupled substitution, thus becoming part of the crystal lattice composition; halogen anions (Cl... - / I -(etc.) can occupy the X-site channel. Since nuclides are embedded in the crystal lattice rather than being adsorbed on the surface or existing as easily soluble salts, their diffusion and migration are constrained by the crystal lattice, resulting in a low effective diffusion coefficient. The leaching process is mainly controlled by slow dissolution and ion exchange on the solid surface, so the leaching rate is usually low. In addition, the apatite structure has a certain defect-accommodating capacity (it can absorb radiation damage through point defects, vacancies, and local distortions, making it less prone to catastrophic structural collapse), which further supports its long-term structural stability and long-term storage performance, which is beneficial for the long-term storage of radioactive waste after solidification.

[0020] In this application, the conditions for the supercritical hydrothermal reaction include: a reaction temperature of 390℃-400℃, an internal pressure of 25-27MPa, a holding time of 9-11 min, and a stirring speed of 400-600 rpm. By setting the hydrothermal reaction temperature to 390℃-400℃ and the internal pressure to 25-27MPa, the water in the reaction system is in a supercritical state, exhibiting characteristics of both liquid and gaseous states. This effectively addresses the common problems of slow reaction and dependence on strong alkali in conventional hydrothermal processes for apatite solidification. Furthermore, holding the reaction at the specified temperature for 9-11 min ensures sufficient reaction of the substances in the system to form a solidified apatite body, promotes the growth of apatite crystal structures, improves the structural stability of the solidified body, and reduces the leaching rate of radioactive elements. Setting the stirring speed to 400-600 rpm ensures more uniform heating of the reactants, prevents sedimentation, and ensures a more complete reaction.

[0021] In some embodiments, the dihydrogen phosphate includes dihydrogen phosphates of alkali metals or alkaline earth metals. For example, as an example, the dihydrogen phosphate includes one or more of calcium dihydrogen phosphate and sodium dihydrogen phosphate. In some embodiments, the dihydrogen phosphate is sodium dihydrogen phosphate. Using sodium dihydrogen phosphate as a reactant is advantageous for providing a phosphorus source for the reaction, and the raw material is widely available, low in cost, and has good safety and operability.

[0022] In some embodiments, the molar ratio of strontium to phosphorus in the solid mixture is in the range of 1:1.65 to 1:1.69. By maintaining this molar ratio within this range, it indicates an excess of phosphorus in the reaction system, which facilitates the complete consumption of strontium through the aforementioned solidification reaction. This, in turn, helps to fully solidify the radioactive elements in the radioactive waste and reduces the leaching amount of radioactive elements. For example, the molar ratio of strontium to phosphorus in the solid mixture can be 1:1.65, 1:1.66, 1:1.67, 1:1.68, 1:1.69, or a value within a range of any two of these values.

[0023] In some embodiments, the content of strontium chloride in the solid mixture is 65 wt.% to 70 wt.%. Maintaining the strontium chloride content in the solid mixture within this range helps ensure that the strontium-to-phosphorus molar ratio in the raw material is approximately the stoichiometric ratio of 1.67. Exemplarily, the strontium chloride content in the solid mixture can be 65 wt%, 66 wt%, 67 wt%, 68 wt%, 69 wt%, 70 wt%, or a value within a range of any two of these values.

[0024] In some embodiments, the solid-liquid ratio of the solid mixture to water in the raw material mixture is 1:4 to 1:6. Maintaining the solid-liquid ratio of the solid mixture to water within this range helps to ensure sufficient fluidity and mass transfer efficiency of the system while avoiding insufficient product per synthesis due to excessive solution, thus balancing the controllability and yield of the reaction process. For example, the solid-liquid ratio of the solid mixture to water in the raw material mixture can be 1:4, 1:4.5, 1:5, 1:5.5, 1:6, or a value within a range of any two of these values.

[0025] In some embodiments, step 1 further includes adding an alkaline compound to the raw material mixture to adjust the pH of the raw material mixture to 9-11. Adjusting the pH of the raw material mixture within the above range facilitates the solidification reaction of strontium chloride and dihydrogen phosphate and avoids excessive alkali metal contamination.

[0026] In some embodiments, step S2 further includes heating from room temperature to 390℃-400℃ at a heating rate of 4-6℃ / min. In the supercritical hydrothermal reaction process, heating the reaction system to the target temperature at the aforementioned heating rate is beneficial to the safety of the reaction, as well as to the stability of temperature and pressure evolution, a more uniform temperature field, and to improving the formation stability and phase purity of the target apatite phase.

[0027] In some embodiments, the ceramic cured body comprises apatite. Apatite is a type of phosphate mineral with the general chemical formula A5(PO4)3X, where A is a divalent cation and X is one of F, Cl, or OH. In this application, the ceramic cured body comprises Sr5(PO4)3Cl, which is a hexagonal crystal system, and its main structure is composed of Sr 2+ Cations and PO4 3- The phosphate ion-based framework, PO4³ - The tetrahedron is a rigid element, obtained through Sr². + Ion bonding allows radioactive Sr elements to be stably fixed within the crystal structure of apatite, thereby stabilizing the radioactive elements and reducing their leaching rate.

[0028] The radioactive waste solidification method of this application involves mixing radioactive waste containing strontium chloride with dihydrogen phosphate, and then subjecting the mixture to a supercritical hydrothermal reaction to obtain apatite ceramic solidified body containing radioactive strontium waste. This method has a simple process, and the resulting apatite ceramic solidified body containing radioactive waste has a stable structure, making it suitable for long-term disposal of radioactive molten salt waste.

[0029] The implementation methods of this application are further described in detail below with reference to specific embodiments. It is understood that the purpose of providing the following embodiments is to better understand the technical solutions of this application, and not to constitute a limitation on this application.

[0030] Example Example 1 S1. Strontium chloride containing radioactive strontium and sodium dihydrogen phosphate are mixed to obtain a solid mixture. Deionized water is added to the solid mixture, and an appropriate amount of 1 mol / L NaOH standard solution is added to adjust the pH of the reaction system to 10. The raw material mixture is stirred evenly. The composition is 68.8 wt.% strontium chloride and 31.2 wt.% sodium dihydrogen phosphate, and the ratio of solid mixture to deionized water is 1:5.

[0031] S2. The uniformly mixed raw material mixture was poured into a supercritical reactor for a supercritical hydrothermal reaction. The heating rate was 5℃ / min, the reaction temperature was 390℃, the holding time was 10min, and the stirring rate was 500rpm. After the reaction, the mixture was allowed to cool naturally to room temperature, and the product was washed with deionized water, centrifuged, and dried to obtain a radioactive strontium-containing apatite ceramic solidified body.

[0032] Comparative Example 1 Strontium chloride containing radioactive strontium was cured using a method similar to that in Example 1, except that the supercritical reaction time was 5 minutes.

[0033] Comparative Example 2 Strontium chloride containing radioactive strontium was cured using a method similar to that in Example 1, except that the supercritical reaction time was 30 min.

[0034] Comparative Example 3 Strontium chloride containing radioactive strontium was cured using a method similar to that in Example 1, except that the supercritical reaction time was 60 min.

[0035] Comparative Example 4 Strontium chloride containing radioactive strontium was cured using a method similar to that in Example 1, except that the supercritical reaction time was 180 min.

[0036] Comparative Example 5 Strontium chloride containing radioactive strontium was cured using a method similar to that in Example 1, except that the supercritical reaction time was 360 min.

[0037] Table 1

[0038] XRD testing of apatite ceramic cured body The prepared radioactive strontium-containing apatite ceramic solidified body was subjected to XRD testing. The testing methods included: The prepared solidified body was fixed in a fixture and placed on a test stage. X-ray diffractometer (Rigaku SmartLab SE, Japan) was used for testing. The test parameters were set as follows: scanning range 5°–80°, scanning rate 5° / min, target material Cu target, voltage 40 kV, and current 40 mA. After setting the parameters, the scanning program was started to acquire XRD pattern data. The acquired data was processed and analyzed to obtain the XRD pattern of the apatite ceramic solidified body.

[0039] The XRD patterns obtained from the radioactive strontium-containing apatite ceramic solidified bodies obtained in Example 1 and Comparative Examples 1-5 are as follows: Figure 1 As shown in the figure, the main diffraction peaks of the solidified body are those of chlorapatite (Sr5(PO4)3Cl). The chlorapatite peaks obtained in Example 1 have the best shape.

[0040] Leaching rate test of radioactive elements The testing method includes the following steps: According to the Product Conformity Test (PCT-B standard), the leaching characteristics of simulated nuclear waste element Sr were studied using a static leaching test (ASTM Committee, 2002). Samples with a particle size of 75-150 μm were immersed in deionized water (40 mL), and then the reactor containing the samples was placed in an environment of 90 °C. The leaching results of Sr were calculated using formula (1):

[0041] in NR i This represents the normalized leaching rate of Sr, in g·m³. - ²·d - ¹. Sr concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7800, USA). c i ). f i The mass fraction of Sr (wt.%). Specific surface area of ​​the powder ( SA m 2 ) and the volume of leachate ( V m 3 The ratio is approximately 2000 m -1 .t The leaching experiment time was set to 1, 3, 5, 7, 14, 21 and 28 days (d).

[0042] The radioactive strontium-containing apatite ceramic solidified body obtained in Example 1 was subjected to radioactive element leaching rate testing using the above method. The 28-day leaching rate curve of the radioactive strontium-containing apatite ceramic solidified body of Example 1, tested according to the above method, is shown below. Figure 2 .from Figure 2 It can be seen that the radioactive element leaching rate of the apatite solidified body prepared in Example 1 was less than 1.8 × 10⁻⁶ after 28 days. -6 g·m -2 ·d -1 .

[0043] The radioactive element leaching rate of the radioactive strontium-containing apatite ceramic solidified body obtained in Example 1 is shown in Table 2 below.

[0044] Table 2

[0045] The results above show that the leaching rate of radioactive elements gradually decreases and stabilizes with increasing leaching time. The apatite solidified body obtained in Example 1 exhibits good anti-leaching effect. The XRD test results show that the apatite solidified bodies obtained in Comparative Examples 1-5 have poor synthesis effects. Compared to Example 1, the apatite solidified bodies obtained in Comparative Examples 1-5 have poor crystallinity and do not achieve the purpose of radionuclide solidification, nor can they be used as solidified bodies. Therefore, leaching rate testing is unnecessary.

[0046] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, and improvements made within the spirit and scope of this application are included within the scope of protection of this application.

Claims

1. A method for solidifying radioactive waste, characterized in that, The method includes: S1, Radioactive waste and dihydrogen phosphate are mixed evenly to obtain a solid mixture, and then water is added to the solid mixture to obtain a raw material mixture, wherein the radioactive waste includes strontium chloride; S2, the raw material mixture is subjected to a supercritical hydrothermal reaction to obtain a ceramic solidified body. The conditions for the supercritical hydrothermal reaction include: a reaction temperature of 390℃-400℃, an internal pressure of 25-27MPa, a holding time of 9-11min, and a stirring speed of 400-600rpm.

2. The method according to claim 1, characterized in that, Step 1 further includes adding an alkaline compound to the raw material mixture to adjust the pH of the raw material mixture to 9-11.

3. The method according to claim 1 or 2, characterized in that, The dihydrogen phosphate salt is sodium dihydrogen phosphate.

4. The method according to claim 1 or 2, characterized in that, In the solid mixture, the molar ratio of Sr to P is in the range of 1.65 to 1.

69.

5. The method according to claim 1 or 2, characterized in that, The content of strontium chloride in the solid mixture is 65 wt.% to 70 wt.%.

6. The method according to claim 1 or 2, characterized in that, In the raw material mixture, the solid-liquid ratio of the solid mixture and water is 1:4 to 1:

6.

7. The method according to claim 1 or 2, characterized in that, Step S2 further includes heating from room temperature to 390℃-400℃ at a heating rate of 4-6℃ / min.

8. The method according to claim 1 or 2, characterized in that, The ceramic solidified body includes apatite.

Images