Method for recovering enriched boron in radioactive waste liquid of nuclear power plant

Abstract

The invention provides a method for recovering enriched boron in radioactive waste liquid of a nuclear power plant, and relates to the technical field of nuclear industry. The method for recovering the enriched boron in the radioactive waste liquid of the nuclear power plant comprises the following steps: treating the radioactive waste liquid containing the enriched boron through a membrane separation process to obtain permeate liquid; carrying out distillation treatment on the permeate to obtain a concentrated solution; cooling and crystallizing the concentrated solution to obtain enriched borate crystals; the enriched borate crystals are subjected to dehydration treatment, and enriched boron oxide is obtained; and reducing the enriched boron oxide to obtain the enriched boron simple substance. According to the recycling method, forbidden elements in a primary loop of a nuclear power plant are not introduced, the recycling method has the characteristics of high recycling efficiency, environment friendliness, easiness in industrial production and thorough recycling of reaction byproducts, the recycled amorphous enriched boron simple substance is small in particle size, narrow in particle size distribution and high in purity, and the activity of radionuclide is lower than the control release level specified by relevant national standards.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear industry, and in particular to a method for recovering boron enriched in radioactive waste liquid from a nuclear power plant. Background Art

[0002] The primary coolant system of a pressurized water reactor (PWR) nuclear power plant is responsible for heat transfer and reactor cooling. Boric acid is added to the primary coolant to chemically compensate for reactivity, leveraging boron's ability to absorb thermal neutrons. Natural boron consists of two stable isotopes, B-10 and B-11, with abundances of 19.8% and 80.2%, respectively. B-10 is a highly efficient thermal neutron absorber, with a thermal neutron absorption cross-section of 3837 bar, while B-11 has a cross-section of only 0.005 bar. Enriched boric acid refers to boric acid with a B-10 abundance greater than 19.8%. PWR NWR plants utilizing boric acid enrichment technology can reduce the total boron concentration from 2500 ppm to 500-600 ppm while maintaining the same B-10 concentration. This reduces the amount of boron-lithium coordinator dosage and mitigates the risk of corrosion of primary structural materials and core deposits. The lithium hydroxide reagent used for boron-lithium coordinator water chemistry control must have a Li-7 isotope abundance of at least 99.9%.

[0003] For boron-enriched wastewater that does not meet the requirements of the operating technical specifications, the boron recovery system of nuclear power plants uses filtration, ion exchange, and evaporation technologies for classified collection and treatment. After the effluent is tested and qualified, it is recycled or discharged in compliance with the standards. However, during the boron removal process of conventional anion exchange resins, the exchanged and adsorbed boron is easily replaced by strong-affinity anions such as sulfate, nitrate, and chloride ions. The evaporation and concentration of boron-containing wastewater and cement solidification can easily lead to blockage of heating cycle components and serious radioactive contamination of downstream pipelines. Because B-10 and Li-7 are both national strategic resources and are expensive, it is necessary to develop more efficient and economical boron-enriched recovery and reuse technologies for nuclear power plants with boron-enriched wastewater that does not meet the operating technical requirements, especially for inland nuclear power plants with stricter wastewater discharge management requirements. Summary of the Invention

[0004] In view of the above problems existing in the prior art, the present invention provides a method for recovering boron enriched in radioactive waste liquid from nuclear power plants, the recovery method comprising the following steps:

[0005] The radioactive waste liquid containing enriched boron is treated by a membrane separation process to obtain a permeate;

[0006] distilling the permeate to obtain a concentrated solution;

[0007] Cooling and crystallizing the concentrated solution to obtain enriched borate crystals;

[0008] Dehydrating the enriched borate crystals to obtain enriched boron oxides;

[0009] The enriched boron oxide is reduced to obtain enriched boron element.

[0010] In one embodiment of the present invention, before the boron-enriched radioactive waste liquid is treated by a membrane separation process, the recovery method further comprises adjusting the pH value of the radioactive waste liquid to 3.0-4.0.

[0011] In one embodiment of the present invention, the membrane separation process is any one of nanofiltration and reverse osmosis.

[0012] In one embodiment of the present invention, in the membrane separation process, the pressure difference across the separation membrane is 1.5-4.0 MPa.

[0013] In one embodiment of the present invention, the distillation process is a multi-effect distillation, the temperature of the multi-effect distillation is 60-70°C, and the temperature gradient of the multi-effect distillation is 1-4°C.

[0014] In one embodiment of the present invention, the enriched borate crystals are dehydrated to obtain enriched boron oxides, comprising the following steps: drying the enriched borate and placing it in an argon inert atmosphere, first heating it to 150°C and keeping it for 8 hours, then heating it to 350°C and keeping it for 24 hours to obtain enriched boron oxides.

[0015] In one embodiment of the present invention, reducing the enriched boron oxide to obtain enriched boron elemental substance comprises the following steps:

[0016] placing enriched boron oxide and lithium reducing agent in a reactor;

[0017] The reactor is replaced with inert gas and evacuated;

[0018] Heat the reactor to 130-200°C and keep it warm for 30-60 minutes.

[0019] In one embodiment of the present invention, the mass ratio of the enriched boron oxide to the lithium reducing agent is 1:(1.5-2.0).

[0020] In one embodiment of the present invention, after reducing the enriched boron oxide to obtain enriched boron elemental substance, the recovery method further comprises washing the enriched boron elemental substance with water.

[0021] In one embodiment of the present invention, the enriched boron element is washed with water and then dried at 105° C. for 60 minutes.

[0022] The present invention relates to a method for recovering enriched boron from radioactive waste liquid from nuclear power plants. A membrane separation process is employed to selectively remove high-valent radionuclides from the waste liquid. Distillation is then used to further remove volatile radionuclides. The concentrated liquid produced by the distillation process is cooled and crystallized to precipitate enriched borate crystals. The enriched borate crystals are then dehydrated and reduced to obtain amorphous enriched boron. This method, which avoids the introduction of prohibited elements into the primary circuit of nuclear power plants, features high recovery efficiency, is environmentally friendly, is easily commercializable, and allows for thorough recovery of reaction byproducts. The recovered amorphous enriched boron has a small particle size, a narrow particle size distribution, and high purity. The activity of the radionuclides is below the clearance level specified by relevant national standards. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 A flow chart of a method for recovering boron enriched in radioactive waste liquid from a nuclear power plant according to one embodiment of the present invention;

[0025] Figure 2 yes Figure 1 Step S5 in the illustrated embodiment is a flow chart in an exemplary embodiment. DETAILED DESCRIPTION

[0026] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0027] When numerical ranges are given in the examples, it should be understood that unless otherwise specified herein, both endpoints of each numerical range and any value between the two endpoints may be used. Unless otherwise defined, all technical and scientific terms used herein are consistent with the prior art as understood by those skilled in the art and the description of the present invention. Any prior art methods, devices, and materials similar or equivalent to those in the examples of the present invention may also be used to implement the present invention.

[0028] See also Figure 1 The present invention provides a method for recovering boron enriched in radioactive waste liquid from a nuclear power plant, the method comprising the following steps:

[0029] S1. treating the radioactive waste liquid containing enriched boron by a membrane separation process to obtain a permeate;

[0030] S2, distilling the permeate to obtain a concentrate;

[0031] S3, cooling and crystallizing the concentrated solution to obtain enriched borate crystals;

[0032] S4, dehydrating the enriched borate crystals to obtain enriched boron oxide;

[0033] S5. Reducing the enriched boron oxide to obtain enriched boron element.

[0034] In step S1, the membrane separation process uses either nanofiltration or reverse osmosis. The membrane separation process can remove large molecular radionuclides, such as Co-58, Co-60, Cr-51, Nb-95, and Ag-110m, from radioactive wastewater containing enriched boron. Small molecular boric acid molecules and Li-7 lithium ions can pass through the pores of the separation membrane. The separation membrane has a rejection rate of 0-10% for boric acid molecules and Li-7 lithium ions, and the recovery rate of boron in the radioactive wastewater is greater than 75%. During the membrane separation process, the pressure difference across the separation membrane is 1.5-4.0 MPa, for example, any value within the range of 1.5-4.0 MPa, such as 1.5 MPa, 3.0 MPa, or 4.0 MPa. Exemplarily, in the membrane separation process, the separation membrane uses a nanofiltration membrane such as Dow NF90, Dow BW30XFR, Hydra Energy ESNA1, or Hydra Energy SWC4, or a brackish water reverse osmosis membrane. For example, the boron concentration in the boron-enriched radioactive waste liquid is 0.25-0.70%, and the boron concentration in the permeate after the membrane separation process is 0.30-0.95%.

[0035] To improve the membrane's efficiency in purifying radionuclides from boron-enriched radioactive wastewater, a two-stage membrane separation process is employed. The concentrated effluent from the secondary separation process is returned to the primary separation inlet for further membrane separation. This increases boron recovery to 85%, purifying the majority of radionuclides in the secondary reverse osmosis effluent. The membrane separation process retains 95-100% of high-valent radionuclides (such as Co-58, Co-60, Cr-51, Nb-95, and Ag-110m), reducing the radiation protection requirements during the distillation of boron-enriched radioactive wastewater. The membrane separation process offers the advantages of high separation efficiency, no phase change, simplicity, and high efficiency.

[0036] In some embodiments, the recovery method further includes adjusting the pH value of the radioactive waste liquid to 3.0 to 4.0 before the boron-enriched radioactive waste liquid is treated by membrane separation. Generally, the coolant of a nuclear power plant is weakly alkaline. When the waste liquid is alkaline, the boron is enriched in the form of B(OH)4 - In the membrane separation process, borate ions are easily repelled by the negatively charged surface of the separation membrane. Acidic conditions can alter the membrane's surface charge and hydrophilicity, while simultaneously enriching boron in an uncharged, undissociated boric acid state. Boric acid molecules are smaller than the lower pore size of the separation membrane, making them easily permeable. Appropriately increasing the acidity of the wastewater can improve the separation efficiency of the membrane separation process. For example, when the pH of the wastewater is adjusted to 4.0, over 85% of the boric acid in the wastewater passes through the separation membrane along with the water, and over 95% of the radioactive nuclides are retained in the concentrate. When the pH is adjusted to 3.0-3.5, over 90% of the boric acid in the wastewater passes through the membrane along with the water, while nearly 98% of the radioactive nuclides are retained in the concentrate. Since the pore size of the reverse osmosis membrane is smaller than that of the nanofiltration membrane, this application can select nanofiltration or reverse osmosis according to actual needs. For example, when the radioactive nuclides are complex and the recovery requirements are strict, a reverse osmosis membrane with a small pore size is selected to improve the nuclide interception efficiency while moderately reducing the boron recovery rate; when the radioactivity level of the enriched boron waste liquid is low, a nanofiltration membrane can be selected to improve the enriched boron recovery efficiency.

[0037] In some embodiments, the recovery method further comprises filtering the waste liquid before adjusting the pH value of the waste liquid to remove particulate impurities such as suspended matter in the waste liquid.

[0038] In step S2, the permeate is subjected to a distillation process to further remove difficult-to-separate radionuclides in the permeate, such as H-3 and C-14, while also increasing the concentration of enriched boron. Exemplarily, the distillation process is a multi-effect distillation process, wherein the temperature of the multi-effect distillation is 60-70°C, for example, any value within the range of 60-70°C, such as 60°C, 65°C, or 70°C, and the temperature gradient of the multi-effect distillation is 1-4°C, for example, any value within the range of 1-4°C, such as 1°C, 3°C, or 4°C. For example, the distillation process is a 4-7 effect distillation process. Under vacuum conditions of 0.03-0.07 MPa, the spray system sprays the permeate onto the first-effect heat exchange tubes of the evaporator. The permeate is heated and vaporized, and the slightly concentrated permeate enters the second-effect heat exchange tubes. Due to the higher vacuum and lower evaporation temperature in the second effect, the permeate is further concentrated. The permeate is repeatedly evaporated and sprayed in each effect, continuously concentrating the permeate and fully releasing potential radioactive gases. The resulting concentrated liquid is finally discharged from the bottom of the terminal evaporator. In one embodiment, condensate from all effects is collected and reused in the nuclear power plant's production system. The boron concentration in the concentrate can reach 3.0-5.0%, and the H-3 activity concentration is controlled below 1000 Bq / L. Multi-stage distillation utilizes waste heat at lower temperatures, significantly reducing energy consumption and operating costs. Low-temperature evaporation also reduces equipment scaling and corrosion, extending its service life. In some embodiments, the distilled water produced by the distillation process is discharged to the power plant's wastewater system for further evaporation and cement solidification.

[0039] In step S3, at room temperature of 25°C, the solubility of boric acid in water is about 5.74%. Taking advantage of the low solubility of boric acid in water, borates are separated and precipitated from the concentrated solution. Specifically, the concentrated solution is cooled to room temperature, the solubility of borates in the concentrated solution is reduced and precipitated, and enriched borate crystals are obtained, which are then dried. The mother liquor of the concentrated solution from which borates are precipitated is returned to the upper distillation system for repeated distillation treatment to improve the recovery rate of the boron element. Exemplarily, the main component of the enriched borate crystals is boric acid, and also includes a small amount of lithium borate and lithium tetraborate, and the impurity content is low. In one embodiment, the mother liquor of the concentrated solution from which borates are precipitated is repeatedly distilled to improve the recovery rate of the boron element.

[0040] In step S4, the dehydration treatment of the enriched borate crystals includes the following steps: placing the dried enriched borate in an argon inert atmosphere, first maintaining it at 150°C for 8 hours, then cooling it to room temperature and grinding it, and finally maintaining it at 350°C for 24 hours to fully dehydrate the enriched borate to obtain enriched boron oxide.

[0041] In step S5, reducing the enriched boron oxide to obtain the enriched boron element comprises the following steps:

[0042] S51, placing the enriched boron oxide and the lithium reducing agent in a reactor;

[0043] S52, replacing the reactor with inert gas and evacuating the reactor;

[0044] S53. Heat the reactor to 130°C to 200°C and keep it warm for 30 to 60 minutes.

[0045] In step S51, the enriched boron oxide is first placed in a reactor. The reactor is then purged with high-purity argon for 5 minutes to remove air and moisture from the reactor, maintaining a dry, oxygen-free atmosphere. A lithium reducing agent is then added. The mass ratio of the enriched boron oxide to the lithium reducing agent is 1:(1.5-2.0). For example, the mass ratio of the enriched boron oxide to the lithium reducing agent can be any value within the range of 1:(1.5-2.0), such as 1:1.5, 1:1.8, or 1:2.0. Exemplarily, the lithium reducing agent is elemental lithium or lithium borohydride.

[0046] In step S52, the reactor is evacuated to a vacuum degree of <5 Pa, and high-purity argon gas is introduced into the reactor until the pressure inside the reactor reaches 0.5 MPa. The above vacuuming and high-purity argon gas introduction operations are repeated 2 to 3 times to completely remove the oxygen and moisture in the reactor, and finally the vacuum degree in the autoclave is maintained at <5 Pa.

[0047] In step S53, the temperature of the reactor is set to any value between 130°C and 200°C, for example, 130°C, 150°C, or 200°C, and the holding time can be any value between 30 minutes, 40 minutes, 50 minutes, or 60 minutes. After the reaction is completed, the gas in the reactor is released to maintain the reactor at normal pressure. The enriched boron element is removed from the reactor after the temperature of the reactor drops to room temperature.

[0048] The present application adopts a lithium thermal reduction method to reduce enriched boron oxide to enriched boron element, which can form a liquid phase at a lower temperature, enhance the mass transfer process, simplify the reaction conditions, and improve the yield and recovery rate. The recovery rate of the boron element is higher than 90%. In addition, the lithium reducing agent is an element originally present in the radioactive waste liquid. When reducing the enriched boron oxide, no prohibited elements in the primary circuit of the nuclear power plant are introduced, thereby ensuring the purity of the enriched boron.

[0049] In one embodiment of the present invention, the recovery method further comprises washing, filtering, and drying the enriched boron element to remove water-soluble impurities in the enriched boron element, thereby obtaining an amorphous enriched boron element powder. To improve the efficiency of impurity removal, the water washing temperature can be increased, for example, to 90°C. The waste liquid after water washing is a lithium hydroxide solution with a low concentration, and the water washing wastewater includes Li-7. The waste liquid generated by water washing can be reused in the nuclear power plant production system to achieve the reuse of Li-7 and reduce the pollution of wastewater to the environment. The activity of radioactive nuclides in the enriched boron element after water washing is lower than the clearance level specified by relevant national standards, and it is exempt from radiation protection supervision, thereby achieving the recycling of enriched boron resources. After water washing, the enriched boron element is dried. Exemplarily, it is dried at 105°C for 60 minutes.

[0050] The recovery method described in this application fully recycles B-10 and Li-7 from nuclear power plant wastewater, boasting high recovery efficiency, moderate operating temperatures, and minimal equipment requirements, promising broad industrial development prospects. The recovered amorphous enriched boron powder contains ≥98.5% boron, has radionuclide activity below the clearance level specified by relevant national standards, and exhibits high neutron absorption, making it an ideal raw material for nuclear reactor control, regulating rods, accident rods, safety rods, and shielding rods.

[0051] The technical solutions of the present invention are described in detail below through several specific examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0052] Example 1

[0053] This embodiment is applied to the working condition where silicon impurities exceed the standard in the spent fuel pool of a nuclear power plant. The radioactive waste liquid is simulated by using water with excessive silicon impurities in the spent fuel pool of the nuclear power plant. The concentration of boron enrichment in the simulated water is 2527 mg / L, the concentration of silicon dioxide is 2 mg / L, and the radionuclide Co 2+ The ion concentration is 3mg / L, and the total amount of simulated water is 5m 3 .

[0054] The recovery method of enriched boron in this embodiment comprises the following steps:

[0055] (1) Nitric acid is added to the radioactive waste liquid to adjust the pH of the waste liquid to 3.5, so that the enriched boron is in the state of uncharged and undissociated boric acid molecules. The waste liquid is subjected to membrane separation treatment using a two-stage reverse osmosis device to obtain a permeate. The separation membrane is Dow BW30XFR brackish water reverse osmosis membrane. The pressure difference on both sides of the reverse osmosis membrane is 1.7 MPa. The concentrated wastewater produced by the second-stage reverse osmosis is recovered to the first-stage reverse osmosis water inlet tank for further treatment to obtain a permeate. When the recovery rate of boron element after membrane separation process is 80%, the radioactive nuclide Co 2+ The removal rate of nitrite is 100%, the removal rate of silicon dioxide is 95%, and the concentration of boron enriched in the permeate is 3010 mg / L.

[0056] (2) The permeate is distilled and concentrated using a molecular distillation apparatus simulating low-temperature multi-effect distillation technology to obtain a concentrated solution. During the distillation process, the vacuum is maintained at 100 kPa and the evaporation temperature is 60°C, so that the enriched boron concentration is increased to about 50,000 mg / L.

[0057] (3) The concentrated solution is cooled to room temperature, and the solubility of boric acid in the concentrated solution is reduced and precipitated to obtain enriched borate crystals.

[0058] (4) The solid material of the enriched borate crystals was first heated at 150°C for 8 hours to remove free moisture, cooled and crushed, and then placed in an inert atmosphere furnace for a second time, and then heated to 350°C and heated for 24 hours to obtain enriched boron oxide.

[0059] (5) The enriched boron oxide is placed in a reactor and purged with high-purity argon for 5 minutes. In a dry and oxygen-free environment, lithium borohydride reducing agent is added according to the mass ratio of enriched boron oxide to lithium borohydride of 1:2.0. High-purity argon is introduced into the reactor to increase the pressure of the reactor to 0.5 MPa. After maintaining the pressure for 10 minutes, the reactor is evacuated to a vacuum degree of <5 Pa. After the enriched boron oxide and lithium borohydride are evenly mixed, the temperature of the reactor is increased to 200°C and maintained for 30 minutes. The enriched boron oxide undergoes lithium thermal reduction reaction. After the reaction is completed, the gas in the reactor is released, the reactor is maintained at normal pressure and the reactor is cooled to room temperature to obtain a block of boron-enriched elemental substance.

[0060] (6) The boron-enriched lumps were crushed and washed with 90°C high-purity water to remove water-soluble impurities on the surface of the boron-enriched lumps. The lumps were then filtered and dried at 105°C for 60 min to obtain an amorphous boron-enriched powder. XRD phase analysis showed that the boron content of the amorphous boron-enriched powder was 98.9%. SEM observation showed that the boron-enriched powder was agglomerated particles with a uniform particle size of 0.75 to 0.88 μm.

[0061] Example 2

[0062] This embodiment is applied to a nuclear power plant boron and water supply system where a high-concentration boric acid solution is contaminated by impurities. The wastewater in this embodiment contains the following components: a radioactive nuclide tritium H-3 activity concentration of 10,000 Bq / L and a boron concentration of 7,000 mg / L. Because the wastewater in this embodiment does not contain large molecular radionuclides, the membrane separation process can be omitted. The boron enrichment recovery method in this embodiment includes the following steps:

[0063] (1) The pH value of the wastewater is adjusted to 4.0 and the radioactive waste liquid is subjected to low-temperature multi-effect distillation and concentration treatment. In this embodiment, the low-temperature multi-effect distillation consists of 6 evaporators connected in series, and the maximum evaporation temperature does not exceed 70°C. The wastewater is heated to 69°C in a preheater, and the evaporation temperature of each effect is lowered in turn by gradually increasing the vacuum degree of each effect, thereby achieving low-temperature distillation. The multi-effect distillation gradually concentrates the boron-containing water and fully releases H-3. Finally, the boron-enriched concentrate is discharged from the terminal evaporator. The concentration of boron enrichment in the concentrate is 4.0%.

[0064] (2) The concentrated solution is cooled to room temperature, and the solubility of boric acid in the concentrated solution decreases and precipitates, thereby obtaining enriched borate crystals. After the enriched borate crystals in the concentrated solution are collected, the activity concentration of H-3 in the crystallized salt is 1000 Bq / L. The mother liquor is returned to the evaporator for continued circulation, heating and evaporation.

[0065] (3) The solid material of the enriched borate crystals was first heated at 150°C for 8 hours to remove free moisture, cooled and crushed, and then placed in an inert atmosphere furnace for a second time, and then heated to 350°C and heated for 24 hours to obtain enriched boron oxide.

[0066] (4) The enriched boron oxide is placed in a reactor and purged with high-purity argon for 5 minutes. In a dry and oxygen-free environment, lithium borohydride reducing agent is added according to a mass ratio of enriched boron oxide to lithium borohydride of 1:1.8. The reactor is pressurized to 0.5 MPa with high-purity argon. After the pressure is maintained for 10 minutes, the reactor is evacuated to a vacuum degree of <5 Pa. After the enriched boron oxide and lithium borohydride are evenly mixed, the temperature of the reactor is raised to 180°C and maintained for 45 minutes. The enriched boron oxide undergoes a lithium thermal reduction reaction. After the reaction is completed, the gas in the reactor is evacuated and the reactor is cooled to room temperature under normal pressure to obtain a block of boron-enriched elemental substance.

[0067] (5) The boron-enriched lumps were crushed, washed with high-purity water, filtered, and dried at 105° C. for 60 min to obtain an amorphous boron-enriched powder. The purity of the amorphous boron-enriched powder was 99.1%, and the particle size was 0.70 to 0.85 μm.

[0068] Example 3

[0069] This embodiment is applied to a working condition where an enriched boric acid solution in a nuclear power plant reactor pool is contaminated by impurities. The nuclear power plant uses enriched boric acid with a B-10 enrichment of 40% to regulate water quality. Foreign matter has been introduced into the reactor pool due to maintenance, resulting in localized black turbidity in the water. The contaminants are suspected to be metal oxides and dust. Water samples were collected from the contaminated area, and the total boron concentration in the water sample was 960 mg / L, the enriched boron concentration was 384 mg / L, the Li-7 concentration was 3.4 mg / L, the suspended solids concentration was 2 mg / L, the silica concentration was 2 mg / L, and the total radioactive activity concentration was 8.1 MBq / L.

[0070] The recovery method of enriched boron in this embodiment comprises the following steps:

[0071] (1) Nitric acid was added to the water sample to adjust its pH to 3.0, so that the enriched boron was in the form of uncharged, undissociated boric acid. The water sample was first processed through a 0.45 μm security filter and then entered a nanofiltration membrane purification device. The Hyde Energy ESNA1 nanofiltration membrane was selected, and the pressure difference across the nanofiltration membrane was 3 MPa. The secondary nanofiltration concentrated wastewater was recovered to the primary nanofiltration inlet tank for further treatment to obtain the permeate. After nanofiltration treatment, the boron recovery rate of the wastewater was 75%, the radioactivity in the permeate was reduced to 0.17 MBq / L, the silica concentration was reduced to 0.2 mg / L, the silicon removal rate was 90%, and the enriched boron concentration in the permeate was 488 mg / L and the Li-7 concentration was 3.1 mg / L. The concentrated wastewater produced by the nanofiltration was concentrated with radionuclides and collected in the power plant wastewater treatment system for further evaporation and cement solidification.

[0072] (2) The permeate is concentrated using a low-temperature multi-effect distillation technique to further remove the radioactive nuclide H-3 and increase the boron concentration in the permeate to obtain a concentrated solution. In this embodiment, the low-temperature multi-effect distillation consists of five evaporators connected in series, with the maximum evaporation temperature not exceeding 70°C. The secondary steam generated by the previous effect serves as the heating source for the next effect. During the multi-effect distillation process, the vacuum degree of each effect is gradually increased, so that the evaporation temperature of each effect is successively reduced, thereby achieving low-temperature distillation and gradually increasing the boron concentration of the permeate. In this embodiment, the boron concentration in the concentrated solution is 4.0%, the Li-7 concentration is 0.2%, and the radioactivity is 40,000 Bq / L. The distilled water generated by each evaporator flows through each effect in the direction of decreasing pressure and temperature until it is extracted as fresh water in the final condenser and collected in the nuclear power plant waste liquid storage system.

[0073] (3) The concentrated solution is cooled to room temperature, and the solubility of boric acid in the concentrated solution decreases and precipitates, thereby obtaining enriched borate crystals.

[0074] (4) The enriched borate crystals were first heated in an inert atmosphere furnace at 150°C for 8 h, cooled and crushed, and then placed in an inert atmosphere furnace for a second time and heated at 350°C for 24 h to obtain enriched boron oxide.

[0075] (5) The enriched boron oxide is placed in a reactor and purged with high-purity argon for 5 minutes. In a dry and oxygen-free environment, lithium borohydride reducing agent is added according to a mass ratio of enriched boron oxide to lithium borohydride of 1:1.5. High-purity argon is introduced into the reactor to increase the pressure of the reactor to 0.5 MPa. After maintaining the pressure for 10 minutes, the reactor is evacuated to a vacuum degree of <5 Pa. After the enriched boron oxide and lithium borohydride are evenly mixed, the temperature of the reactor is increased to 130°C and maintained for 60 minutes. The enriched boron oxide undergoes lithium thermal reduction reaction. After the reaction is completed, the gas in the reactor is evacuated, and the reactor is cooled to room temperature under normal pressure to obtain a block of boron-enriched elemental substance.

[0076] (6) The boron-enriched lumps were crushed, washed with 90°C high-purity water, filtered, and dried at 105°C for 60 minutes to obtain an amorphous boron-enriched powder. XRD phase analysis results showed that the boron content was 98.8% with a B-10 abundance of 40%.

[0077] The present invention relates to a method for recovering boron enriched in radioactive waste liquid from nuclear power plants. The method employs a membrane separation process to selectively remove high-valent radionuclides from the waste liquid, further removes volatile radionuclides through distillation, cools the concentrated solution produced by the distillation process, and crystallizes to precipitate enriched borate crystals. The enriched borate crystals are then dehydrated and reduced to obtain an amorphous boron-enriched elemental powder. The recovery method of the present invention is characterized by high recovery efficiency, high production efficiency, environmental friendliness, ease of industrial production, and thorough recovery of reaction byproducts. The recovered amorphous boron-enriched elemental powder has a small particle size, a narrow particle size distribution, and high purity, and the activity of the radionuclides is below the clearance level specified by relevant national standards. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and practical significance.

[0078] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A method for recovering boron enriched in radioactive waste liquid from a nuclear power plant, characterized in that: The steps include: The radioactive waste liquid containing enriched boron is treated by a membrane separation process to obtain a permeate; distilling the permeate to obtain a concentrated solution; Cooling and crystallizing the concentrated solution to obtain enriched borate crystals; Dehydrating the enriched borate crystals to obtain enriched boron oxides; The enriched boron oxide is reduced to obtain enriched boron element.

2. The recycling method according to claim 1, wherein: Before the boron-enriched radioactive waste liquid is treated by the membrane separation process, the recovery method further comprises adjusting the pH value of the radioactive waste liquid to 3.0-4.

0.

3. The recycling method according to claim 1, characterized in that The membrane separation process is selected from either nanofiltration or reverse osmosis.

4. The recycling method according to claim 1, characterized in that In the membrane separation process, the pressure difference between the two sides of the separation membrane is 1.5 to 4.0 MPa.

5. The recycling method according to claim 1, characterized in that: The distillation process is a multi-effect distillation, the temperature of the multi-effect distillation is 60-70°C, and the temperature gradient of the multi-effect distillation is 1-4°C.

6. The recycling method according to claim 1, characterized in that The enriched borate is crystallized and dehydrated to obtain enriched boron oxide, comprising the following steps: drying the enriched borate and placing it in an argon inert atmosphere, first heating it to 150° C. and keeping it for 8 hours, then heating it to 350° C. and keeping it for 24 hours to obtain enriched boron oxide.

7. The recycling method according to claim 1, characterized in that: Reducing the enriched boron oxide to obtain enriched boron elemental substance comprises the following steps: placing enriched boron oxide and lithium reducing agent in a reactor; The reactor is replaced with inert gas and evacuated; Heat the reactor to 130-200°C and keep it warm for 30-60 minutes.

8. The recycling method according to claim 7, characterized in that: The mass ratio of the enriched boron oxide to the lithium reducing agent is 1:(1.5-2.0).

9. The recycling method according to claim 1, characterized in that: After reducing the enriched boron oxide to obtain enriched boron elemental substance, the recovery method further comprises washing the enriched boron elemental substance with water.

10. The recycling method according to claim 9, characterized in that: The enriched boron element was washed with water and then dried at 105° C. for 60 minutes.

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