A process for the extraction of technetium from nuclear fuel reprocessing waste solutions by solvent extraction

Abstract

This invention relates to a method for extracting technetium from nuclear fuel reprocessing waste using an extraction method, comprising the following steps: preparing a technetium-zirconium co-extraction feed; co-extracting technetium and zirconium using a TBP-kerosene organic solution as the extractant; reducing and back-extracting the technetium in the TBP organic phase co-extraction solution using a reducing stripping agent; and back-extracting the zirconium in the remaining TBP organic phase using a complexing stripping agent. This method improves upon existing techniques for extracting technetium from nuclear fuel reprocessing waste using extraction methods, which generate a significant amount of organic phase radioactive waste other than tributyl phosphate and require an additional solvent treatment system. It avoids introducing additional solvents and metal ion groups into the reprocessing system and is better compatible with the main process flow and facilities of reprocessing plants using the PUREX process, thus making the technetium extraction process easier to implement in reprocessing plants. The method provided by this invention prevents technetium from entering vitrification and geological disposal, thus preventing long-term hazards to the biosphere.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel reprocessing technology, specifically relating to a method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method. Background Technology

[0002] Technetium-99 is a long-lived fission product with a relatively high yield in nuclear reactors, with a fission yield of approximately 6% and a half-life of approximately 2.11 × 10⁻⁶. 5 Technetium-99, during the vitrification of high-level radioactive waste in the PUREX reprocessing process, readily forms co-volatiles with the gamma-ray radionuclide Cs-137 in the main process, posing a highly adverse impact on process operation. In geological disposal, Technetium-99 is prone to migration, posing a potential long-term radiation hazard to the biosphere. Therefore, in spent fuel reprocessing, it is best to extract Technetium from radioactive waste to prevent it from entering vitrification and geological disposal processes.

[0003] The following are some reported or patented technologies for extracting technetium from post-processing techniques or waste liquids:

[0004] (1) The UREX+ process proposed by the United States and the PUREX process for separating technetium proposed by Liu Fang et al. These methods use hydrophilic acetohydroxy acid (AHA) as a key reagent and make significant changes to the main process flow framework of post-processing. They are not suitable for traditional PUREX process post-processing plants; the technetium products obtained have poor decontamination factors for fragmented elements and uranium and plutonium.

[0005] (2) Adsorption and ion exchange methods. Various ion exchange resins and COF / MOF functional materials have been developed domestically and internationally for technetium extraction. The post-treatment wastewater contains high concentrations of technetium, has a complex composition, and contains organic reagent degradation products, which can easily affect the lifespan of the ion exchanger. Therefore, adsorption is not suitable for extracting large amounts of technetium. Adsorption is more suitable for capturing low concentrations of technetium in discharged wastewater. However, the ion exchanger will generate a large amount of radioactive solid waste after discharge, which is detrimental to minimizing radioactive waste.

[0006] (3) Extraction is carried out using extractants such as trioctylamine, N235, Aliquat336, pyridine and its derivatives, butanone, NTAamide, and MIDOA. The extraction method requires extraction, washing, and back-extraction processes, which can achieve high technetium recovery rates and purification factors. However, this type of method generates a significant amount of organic phase radioactive waste liquid other than tributyl phosphate, requiring an additional solvent treatment system, which is very disadvantageous for reprocessing plants.

[0007] (4) Precipitation method. Large cation reagents such as tetraphenylpyridine acetate or tetrafluoroborate can form precipitates with pertechnetate anions, exhibiting strong selectivity. However, the precipitation method has many operational disadvantages in highly radioactive operations and introduces large molecular organic compounds such as tetraphenylpyridine into the waste liquid.

[0008] Therefore, it is evident that the existing technical solutions described above all present some insurmountable difficulties in terms of practicality for reprocessing plants using the PUREX process. Thus, there is an urgent need for a feasible technology for technetium extraction to better match the main process flow and facilities of reprocessing plants employing the PUREX process. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a method for extracting technetium from nuclear fuel reprocessing wastewater using an extraction method. This method employs tributyl phosphate (TBP) as the extractant to extract technetium-99 from wastewater discharged from the PUREX process. This avoids introducing solvents other than TBP into the technetium extraction process and does not significantly increase the types of additional organic waste. It can be better matched with the main process flow and facilities of reprocessing plants using the PUREX process.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is: a method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method, the method comprising the following steps:

[0011] S1. Preparation of technetium-zirconium co-extraction feed: Evaporate and concentrate the technetium source waste liquid, and add a certain amount of zirconium nitrate to prepare technetium-zirconium co-extraction feed;

[0012] S2. Co-extraction of technetium and zirconium: Technetium and zirconium in the technetium-zirconium co-extraction feed are extracted with an extractant, and the co-extraction solution is washed with a detergent to obtain a TBP organic phase co-extraction solution;

[0013] The extractant is a TBP-kerosene organic solution;

[0014] S3, Technetium back-extraction: Technetium in the TBP organic phase co-extraction solution is back-extracted using a reducing back-extraction agent to obtain the back-extracted technetium product;

[0015] S4. Back-extracting zirconium from the remaining TBP organic phase: Back-extracting zirconium from the remaining TBP organic phase using a complexing back-extracting agent.

[0016] Furthermore, in step S1, the waste liquid from the technetium extraction source is evaporated and concentrated more than 20 times;

[0017] The nitric acid concentration in the prepared technetium-zirconium co-extraction feed is 4–5 mol / L.

[0018] Furthermore, the extractant contains 15-50% TBP by mass.

[0019] Furthermore, the detergent is nitric acid with a molar concentration of 4–5 mol / L.

[0020] Furthermore, in step S2, the co-extraction of technetium-zirconium adopts a series connection of 6-10 extraction stages and 1-8 washing stages.

[0021] Furthermore, in step S3, the reducing stripping agent is hydrazine or methylhydrazine.

[0022] Furthermore, in step S3, after reducing the technetium in the TBP organic phase co-extraction solution using a reducing stripping agent, the following steps are also included:

[0023] Zirconium in the back-extracted technetium product is extracted using a supplementary extractant to obtain the technetium product.

[0024] Furthermore, the extraction agent is a TBP-kerosene organic solution, wherein the mass percentage of TBP is 15-50%.

[0025] Furthermore, in step S3, a 4-10 stage back-extraction technetium section is connected in series with a 4-8 stage supplementary extraction zirconium section.

[0026] Furthermore, the complexing and back-extracting agent is ethylhydroxamic acid or a hydrophilic polyamide amine reagent.

[0027] The beneficial effects of this invention are as follows: The method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method provided by this invention includes the following steps: preparing technetium-zirconium co-extraction feed, co-extracting technetium-zirconium using a TBP-kerosene organic solution as an extractant, reducing and back-extracting the technetium in the TBP organic phase co-extraction solution using a reducing back-extracting agent, and back-extracting the zirconium in the remaining TBP organic phase using a complexing back-extracting agent; this method improves upon the existing technology where the extraction of technetium from nuclear fuel reprocessing waste liquid using an extraction method generates a large amount of organic phase radioactive waste liquid other than tributyl phosphate and requires an additional solvent treatment system. It avoids introducing additional solvents and metal ion groups into the reprocessing system and can be better matched with the main process flow and facilities of reprocessing plants using the PUREX process, thereby making the technetium extraction process easier to implement in reprocessing plants. The method provided by this invention can extract technetium from nuclear fuel reprocessing waste liquid, reduce the α and γ activities in technetium products, and enable technetium to be stored safely and conveniently for a long time; it can avoid technetium from entering vitrification and geological disposal, and prevent it from causing long-term harm to the biosphere. Attached Figure Description

[0028] Figure 1This is a schematic flowchart of a method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method, as described in an embodiment of the present invention.

[0029] Figure 2 This is a schematic diagram of the PUREX post-processing process described in an embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the acidity curve of the distribution ratio in the technetium-zirconium co-extraction feed system described in the embodiments of the present invention;

[0031] Figure 4 The reduction of TcO4 with methylhydrazine (MMH) as described in the embodiments of the present invention. － The reaction kinetics curve;

[0032] Figure 5 This is a flow chart of the technetium extraction process using TBP-kerosene organic solution as the extractant, as described in an embodiment of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be further described clearly and completely below with reference to the accompanying drawings and examples. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0034] Considering that existing technical solutions all face insurmountable difficulties when applied to reprocessing plants using the PUREX process, the inventors believe that the optimal solution is to propose a technetium-99 extraction process based on tributyl phosphate (TBP) as the extractant. This process extracts technetium-99 from the wastewater discharged from the PUREX process, avoiding the introduction of solvents other than TBP into the extraction process and preventing a significant increase in additional organic waste types. This approach is better suited to the main process flow and facilities of reprocessing plants using the PUREX process.

[0035] like Figure 1 As shown in the figure, an embodiment of the present invention provides a method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method, the method comprising the following steps:

[0036] S1. Preparation of technetium-zirconium co-extraction feed AF: The waste liquid from the technetium source is evaporated and concentrated more than 20 times, and a certain amount of zirconium nitrate is added to assist the extraction of technetium, thus preparing technetium-zirconium co-extraction feed AF.

[0037] Specifically, in such Figure 2In the PUREX post-processing flow shown, technetium mainly enters the tailings of technetium washing and uranium-plutonium supplementary extraction (1AXXW) and plutonium wire tailings (2AW). The main components of these two technetium extraction source waste liquids (1AXXW and 2AW) are shown in Table 1, both containing relatively high nitric acid concentrations (>3.5 mol / L). During the technetium extraction process, the main targets for decontamination are trace amounts of uranium-plutonium, as well as strontium, cesium, ruthenium, and other components.

[0038] Table 1. Main components of waste liquid from two extraction sources.

[0039]

[0040] In the technetium source waste liquid (1AXXW, 2AW), the concentration of technetium is relatively low, which is not conducive to the extraction process of technetium-zirconium co-extraction. In this embodiment, the technetium needs to be concentrated by about 20 times and the nitric acid concentration is controlled at 4-5 mol / L to obtain the concentrated technetium source waste liquid, which is used to prepare the technetium-zirconium co-extraction feed AF.

[0041] Technetium (TcO4) was extracted directly from nitric acid medium using TBP. － The distribution ratio of ) is very low (less than 0.6), but the inventors discovered that there are high-valence metal ions (such as Zr) 4+ Pu 4+ In the presence of ), technetium can form co-extraction with high-valence metal ions, resulting in a high partition ratio, up to nearly 2, such as... Figure 3 The acidity curves show the distribution ratios in the technetium-zirconium co-extraction feed system. Therefore, adding a certain amount of stable zirconium isotopes to the concentrated technetium source waste liquid can enhance technetium extraction. Zirconium can also be extracted under high acidity conditions, with only trace amounts entering the tail liquid; that is, the concentrated technetium source waste liquid and the added zirconium are combined to form the technetium-zirconium co-extraction feed AF.

[0042] S2, Co-extraction of Technetium and Zirconium (A process section): Technetium and Zirconium in the feed AF are extracted using extractant AX, and the co-extract is washed with detergent AS to obtain TBP organic phase co-extract BF;

[0043] Specifically, in step S2, the extractant AX is a TBP-kerosene organic solution; the detergent AS is nitric acid.

[0044] Optionally, the extractant AX contains 15-50% TBP by mass.

[0045] Optionally, the molar concentration of the nitric acid is 4 to 5 mol / L.

[0046] Specifically, the co-extraction of technetium and zirconium (process section A) adopts a series connection of 6-10 extraction stages and 1-8 washing stages; in the final TBP organic phase co-extraction solution BF, the recovery rate of technetium reaches over 98% and the recovery rate of zirconium reaches over 98%.

[0047] S3, Technetium Back-extraction (B Process Section): Technetium in the organic phase co-extraction liquid BF of TBP is reduced and back-extracted using the reducing back-extraction agent BS to obtain the back-extracted technetium product;

[0048] Specifically, in step S3, the reducing stripping agent BS is hydrazine or methylhydrazine.

[0049] The organic phase co-extract (BF) of TBP contains technetium and zirconium, requiring selective back-extraction of technetium. Technetium in its +7 valence state can be extracted, while lower valence states (+6, +5, +4) are not extracted by TBP. Therefore, technetium can be reduced and back-extracted using organic reducing agents such as hydrazine or methylhydrazine under acidity conditions with a hydrogen ion concentration of 2–3 mol / L, while zirconium and trace amounts of uranium and plutonium remain in the TBP organic phase, achieving separation of technetium from these impurities. TcO4 is reduced using methylhydrazine (MMH). － The reaction kinetic curves are as follows Figure 4 As shown.

[0050] Optionally, in order to prevent zirconium from entering the back-extracted technetium product stream, after using the reducing back-extracting agent BS to reduce the technetium in the organic phase co-extractant BF of TBP in step S3, the following step is also included: using the supplementary extraction agent BX to supplement the zirconium in the back-extracted technetium product to obtain the technetium product BP.

[0051] Specifically, the extractant BX is a TBP-kerosene organic solution, wherein the mass percentage of TBP is 15-50%.

[0052] Specifically, step S3 employs a 4-10 stage technetium back-extraction section connected in series with a 4-8 stage zirconium supplementation section, achieving a technetium recovery rate of over 99.9% for the technetium product BP. After the technetium back-extraction and zirconium supplementation processes in step S3, most of the technetium in the TBP organic phase co-extraction solution BF has been extracted, but the remaining zirconium in the TBP organic phase CF and the TBP-kerosene organic solution can still be recycled.

[0053] S4. Back-extraction of zirconium in the remaining TBP organic phase CF (C process section): The zirconium in the remaining TBP organic phase CF is back-extracted using the complexing back-extraction agent CX.

[0054] The zirconium-containing aqueous phase is concentrated after the complexing agent is destroyed and returned to step S1 to prepare the technetium-zirconium co-extraction feed AF; the remaining TBP-kerosene organic solution in the TBP organic phase CF is sent to the solvent treatment unit of the post-processing plant for regeneration.

[0055] Specifically, the remaining TBP organic phase CF, including the TBP-kerosene organic solution and zirconium, needs to be recycled. The recycling method is as follows: First, zirconium in the remaining TBP organic phase CF is back-extracted using a complexing back-extractant CX. The zirconium back-extracted into the aqueous phase is concentrated and then used to prepare the technetium-zirconium co-extraction feed AF. Then, the remaining TBP organic phase is sent to the solvent treatment unit of the post-processing plant, where it is regenerated through washing, distillation, and other processes, and then used to prepare the extractant AX in step S2 and the supplementary extractant BX in step S3.

[0056] Optionally, to avoid zirconium hydrolysis and polymerization, when back-extracting the remaining TBP organic phase CF in step S4, zirconium back-extraction is carried out under high acidity conditions via complexation. Specifically, acetohydroxamic acid (AHA) or hydrophilic polyamide amine reagents (NTAamide (C2HE), MIDEA, etc.) are used as complexing back-extraction agents (CX) to selectively complex and back-extract tetravalent elements, thereby achieving zirconium recovery.

[0057] A complete flow chart of technetium extraction process using TBP-kerosene organic solution as extractant is shown below. Figure 5 As shown.

[0058] Example 1

[0059] In this embodiment 1, a method for extracting technetium from nuclear fuel reprocessing waste liquid using an extraction method is provided. The tailings 1AXXW and 2AW of the technetium washing and uranium-plutonium supplementary extraction from the PUREX reprocessing process are used as the technetium source waste liquid. The method includes the following steps:

[0060] S1. Preparation of technetium-zirconium co-extraction feed AF: Evaporate and concentrate the waste liquid from the technetium extraction source by more than 20 times, and add a certain amount of zirconium nitrate to assist in the extraction of technetium;

[0061] The concentrated waste liquid from the technetium source was mixed with added zirconium nitrate to prepare technetium-zirconium co-extraction feed AF. The main components of the prepared technetium-zirconium co-extraction feed AF include: zirconium with a mass concentration of 1.8 g / L, technetium with a mass concentration of 0.4 g / L, and nitric acid with a molar concentration of 4.5 mol / L.

[0062] S2, Co-extraction of Technetium and Zirconium (Process Section A): TBP-kerosene organic solution is used as extractant AX to extract technetium and zirconium from the co-extraction feed AF; then the co-extraction solution is washed with nitric acid as detergent AS to obtain TBP organic phase co-extraction solution BF;

[0063] The process employs an 8-stage extraction stage and a 4-stage washing stage connected in series. The feed flow ratio of technetium-zirconium co-extraction feed AF, extractant AX, and detergent AS is 1:1:0.25. In the final TBP organic phase co-extraction solution BF, the recovery rate of technetium reaches over 98%, and the recovery rate of zirconium reaches over 98%.

[0064] Specifically, the TBP-kerosene organic solution contains 30% TBP by mass.

[0065] Specifically, the molar concentration of the nitric acid is 4–5 mol / L.

[0066] S3, Technetium back-extraction (B process section): First, hydrazine or methylhydrazine is used as the reducing back-extraction agent BS to reduce and back-extract technetium in the TBP organic phase co-extraction solution BF; then, in order to prevent zirconium from entering the technetium product liquid stream, a supplementary extractant BX is used to supplement the extraction of zirconium, wherein the supplementary extractant BX is a TBP-kerosene organic solution.

[0067] Specifically, under acidity conditions with a hydrogen ion concentration of 2–3 mol / L, 0.5 mol / L methylhydrazine is used as the reducing stripping agent BS, and the mass percentage of TBP in the stripping agent BX is 30%.

[0068] The process employs a 10-stage back-extraction technetium section and a 6-stage supplementary extraction zirconium section connected in series. The feed flow ratio of TBP organic phase co-extractant BF: back-extractant BS: supplementary extractant BX is 1:0.25:0.25, and the technetium recovery rate of technetium product BP is over 99.9%.

[0069] S4. Back-extraction of zirconium from the remaining TBP organic phase (C process section): Back-extract zirconium from the remaining TBP organic phase CF using the complexing back-extraction agent CX.

[0070] Specifically, to avoid zirconium hydrolysis and polymerization, the zirconium back-extraction of the remaining TBP organic phase CF in step S4 is carried out under high acidity conditions, using 0.2 mol / L acetohydroxamic acid (AHA) as the complexing back-extraction agent CX. The feed flow ratio of the remaining TBP organic phase CF to the complexing back-extraction agent CX is 1:0.5, and the zirconium recovery rate is over 99.9%.

[0071] The method for extracting technetium from nuclear fuel reprocessing waste liquid provided in this embodiment can extract technetium from nuclear fuel reprocessing waste liquid, reduce the α and γ activities in the technetium product, and enable safe and convenient long-term storage of technetium. It can avoid technetium entering vitrification and geological disposal, preventing long-term hazards to the biosphere. This embodiment changes the current situation in the prior art where the extraction method for extracting technetium from nuclear fuel reprocessing waste liquid generates a large amount of organic phase radioactive waste liquid other than tributyl phosphate and requires an additional solvent treatment system. It can avoid introducing additional solvents and metal ion groups into the reprocessing system, and can be better matched with the main process flow and facilities of reprocessing plants using the PUREX process, making the technetium extraction process easier to implement in reprocessing plants.

[0072] The methods described in this invention are not limited to the specific embodiments described above. The embodiments are merely illustrative examples of this invention, and this invention can also be implemented in other specific ways or forms without departing from the spirit or essential characteristics of this invention. Therefore, the described embodiments should be considered illustrative rather than limiting in any respect. The scope of this invention should be defined by the appended claims, and any variations equivalent to the intent and scope of the claims should also be included within the scope of this invention.

Claims

1. A method for extracting technetium from a nuclear fuel reprocessing waste solution by an extraction process, characterized in that, The method comprises the following steps: S1, preparing a co-extraction feed of technetium and zirconium: evaporating and concentrating a technetium-extraction source waste liquid, and adding a certain amount of zirconium nitrate to prepare a co-extraction feed of technetium and zirconium; In step S1, the technetium-extraction source waste liquid is evaporated and concentrated by more than 20 times; The concentration of nitric acid in the prepared co-extraction feed of technetium and zirconium is 4-5 mol / L; S2, co-extracting technetium and zirconium: extracting technetium and zirconium in the co-extraction feed of technetium and zirconium by using an extractant, and washing the co-extraction liquid by using a washing agent to obtain a TBP organic phase co-extraction liquid; The extractant is a TBP-kerosene organic solution; S3, stripping technetium: reducing and stripping technetium in the TBP organic phase co-extraction liquid by using a reducing stripping agent to obtain a stripped technetium product; In step S3, the reducing stripping agent is hydrazine or methylhydrazine; under the acidity condition of a hydrogen ion concentration of 2-3 mol / L, the reducing stripping agent is used to reduce and strip technetium, while zirconium and a small amount of uranium and plutonium are still retained in the TBP organic phase; S4, stripping zirconium in the remaining TBP organic phase: stripping zirconium in the remaining TBP organic phase by using a complexing stripping agent.

2. The method for extracting technetium from nuclear fuel reprocessing waste liquid by extraction method according to claim 1, characterized in that, In the extractant, the mass percentage content of TBP is 15-50%.

3. A process for the extraction of technetium from nuclear fuel reprocessing waste solutions by solvent extraction as claimed in claim 1 wherein, The washing agent is nitric acid, and the molar concentration thereof is 4-5 mol / L.

4. The method of claim 1, wherein the method is characterized by: In step S2, the co-extraction process of technetium and zirconium adopts a series connection mode of 6-10 stages of extraction sections and 1-8 stages of washing sections.

5. The method as claimed in claim 1, wherein the method for extracting technetium from the nuclear fuel reprocessing waste liquid by using the extraction method is characterized by, In step S3, after the reducing stripping agent is used to reduce technetium in the stripped TBP organic phase co-extraction liquid, the following step is further included: A make-up extraction agent is used to make up zirconium in the stripped technetium product to obtain a technetium product.

6. The method of claim 5, wherein the method is characterized by, The make-up extraction agent is a TBP-kerosene organic solution, and the mass percentage content of TBP is 15-50%.

7. A process for the extraction of technetium from nuclear fuel reprocessing waste solutions by solvent extraction as claimed in claim 5 wherein, In step S3, 4-10 stages of technetium stripping sections and 4-8 stages of zirconium make-up sections are connected in series.

8. A process for the extraction of technetium from nuclear fuel reprocessing waste solutions by solvent extraction as claimed in any one of claims 1 to 7, wherein: The complexing stripping agent is ethylhydroxamic acid or a hydrophilic multi-amido amine reagent.

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