A method for removing plutonium and neptunium from uranium in a PUREX dilute process

Abstract

The present invention relates to a method for removing plutonium and neptunium from uranium in the PUREX thin process, which comprises the following steps: (1) Feed liquid pretreatment: Receiving the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, and obtaining the 2DF solution after sequential acid adjustment, heating, valence adjustment and acid adjustment treatments; (2) Extraction and washing: Using the detergent 2DS and the extractant 2DX to perform extraction and washing on the 2DF solution to obtain the organic phase 2DU' and the aqueous phase 2DW; (3) Reduction stripping: Using the reduction stripping agent 2DNS to perform reduction stripping on the organic phase 2DU' to obtain the organic phase 2DU and the aqueous phase 2DNW; (4) Stripping: Using the stripping agent 2EX to perform stripping on the organic phase 2DU to obtain the product liquid 2EU and the aqueous phase 2EW. Compared with the prior art, the present invention provides a technical route for uranium purification in a thin process, achieving the goal of removing plutonium and neptunium simultaneously in one uranium purification cycle and obtaining a qualified uranium product.

Description

Technical Field

[0001] The present invention relates to the technical field of spent fuel reprocessing, and particularly relates to a method for removing plutonium and neptunium from uranium in the PUREX thin process. Background Art

[0002] In the field of spent fuel reprocessing, the only process that has achieved large-scale commercial application at present is the PUREX (Plutonium Uranium Reduction Extraction) process, which mainly includes the uranium-plutonium co-decontamination cycle, the uranium purification cycle, and the plutonium purification cycle. For the uranium purification cycle, its task is to further purify the uranium-containing aqueous solution (1CU feed solution) from the co-decontamination separation cycle to reduce the radioactivity of the uranium product. Among the impurity elements of the uranium product, fission product elements with relatively high β and γ activities, such as ruthenium (Ru), can be well purified through the extraction and washing processes due to factors such as short half-life and small distribution coefficient in the extractant; while for elements with relatively high α activities, mainly neptunium (Np) and plutonium (Pu) elements, due to their numerous valence states and difficult control, it is very difficult to completely purify them. At the same time, due to the high specific α radioactivity of neptunium and plutonium elements, the requirements for the concentrations of neptunium and plutonium in the uranium product are very strict: the α activity concentration from plutonium in the uranium product must be lower than 166.7 Bq / gU (the converted mass concentration is about 1×10 -8 gPu / gU), and the α activity concentration from neptunium must be lower than 83.3 Bq / gU (the converted mass concentration is about 3×10 -6 gNp / gU).

[0003] For the uranium purification cycle process, there are two processes: the thick process and the thin process. The thick process is similar to the spent fuel reprocessing process of a production reactor. First, the uranium-containing aqueous solution from the co-decontamination separation cycle is evaporated and concentrated, and then subsequent extraction, washing, stripping, etc. are carried out; the thin process means that the uranium-containing aqueous solution from the co-decontamination separation cycle is not evaporated and concentrated, and only pretreatment and acid adjustment are carried out before extraction, washing, stripping, etc. Compared with the thick process, the thin process omits the intermediate evaporator, simplifies the operation mode, and avoids the risk of deposition of highly radioactive substances that may be generated during the evaporation and concentration of the uranium-containing aqueous phase in the evaporator. At present, the uranium purification technical routes adopted by the reprocessing facilities in China are all two-cycle processes, and it is very necessary to study the technology for simultaneously removing plutonium and neptunium in one uranium purification cycle. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for removing plutonium and neptunium from uranium in the PUREX thin process, so as to achieve the goal of simultaneously removing plutonium and neptunium in one uranium purification cycle and obtaining a qualified uranium product.

[0005] The object of the present invention can be achieved by the following technical solutions: A method for removing plutonium and neptunium from uranium in the PUREX thin process, comprising the following steps:

[0006] (1) Feed solution pretreatment: Receive the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, and obtain the 2DF solution after sequential acid adjustment, heating, valence adjustment, and acid adjustment treatments.

[0007] (2) Extraction and washing: Use the detergent 2DS and the extractant 2DX to extract and wash the 2DF solution to obtain the organic phase 2DU' and the aqueous phase 2DW.

[0008] (3) Reduction stripping: Use the reduction stripping agent 2DNS to perform reduction stripping on the organic phase 2DU' to obtain the organic phase 2DU and the aqueous phase 2DNW.

[0009] (4) Stripping: Use the stripping agent 2EX to strip the organic phase 2DU to obtain the product solution 2EU and the aqueous phase 2EW.

[0010] Preferably, the feed solution pretreatment process in step (1) is specifically: Receive the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, adjust the solution acidity to 0.5 - 1 mol / L with a 65% nitric acid solution, heat the solution temperature to 90 - 95 °C, keep it warm for 2 - 4 h, after the solution cools to 25 - 30 °C, add a hydroxylamine solution until the concentration is 0.1 - 0.15 mol / L, and finally add a 65% nitric acid solution until the solution acidity is 2 mol / L.

[0011] Preferably, the detergent 2DS in step (2) is a hydroxylamine acid-containing solution, and the extractant 2DX is TBP / OK.

[0012] More preferably, the detergent 2DS in step (2) uses 0.2 - 0.3 mol / L hydroxylamine and 0.3 - 0.5 mol / L HNO3, and the extractant uses 30% TBP / OK. 30% TBP / OK means the volume fraction of TBP is 30%, and the rest is kerosene.

[0013] Preferably, in the extraction and washing process of step (2), the flow ratio range is controlled at 2DF:2DX:2DS = 1:0.5 - 0.6:0.085 - 0.1, the extraction stage number is 8 - 10 stages, and the washing stage number is 6 - 8 stages.

[0014] Preferably, the reduction stripping agent 2DNS in step (3) is 0.3 - 0.5 mol / L hydroxylamine and 0.3 mol / L nitric acid.

[0015] Preferably, in the reduction stripping agent process of step (3), the temperature is 45 - 50 °C.

[0016] Preferably, in the reduction stripping agent process of step (3), the flow ratio range is controlled at 2DU':2DNS = 1:0.3 - 0.35, and the number of reduction stripping stages is 4 - 6 stages.

[0017] Preferably, the stripping agent 2EX in step (4) is nitric acid with a concentration of 0.01 - 0.07 mol / L.

[0018] Preferably, in the stripping process of step (4), the stripping temperature is 55 - 60 °C, the flow ratio range is controlled at 2DU:2EX = 1:1.1 - 1.2, and the number of stripping stages is 8 - 10 stages.

[0019] In the present invention, experiments were conducted on the process conditions of the feed liquid pretreatment, reduction stripping, etc. The effects of different process conditions such as hydroxylamine concentration, TBP concentration, price adjustment and acid adjustment sequence, acidity before extraction, price adjustment temperature and duration on the removal of plutonium and neptunium from uranium were explored respectively. Through the analysis and summary of the experimental results, the following optimal process technical route was obtained: mainly including processes such as feed liquid pretreatment, extraction washing, reduction stripping, and stripping. In the feed liquid pretreatment stage, the adjustment of neptunium valence state was achieved by controlling the feed liquid temperature and acidity: most of it was Np(V), and a small part was Np(VI). After the solution was cooled to room temperature, hydroxylamine solution was added to adjust Np(VI) to Np(V) and Pu(IV) to Pu(III), and then the acidity was adjusted to meet the requirements of subsequent extraction. In the reduction stripping stage, the effect of plutonium removal was enhanced by adding hydroxylamine solution and heating.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The present invention provides a process technical route for uranium purification with a short process flow, achieving the goal of removing plutonium and neptunium simultaneously in one uranium purification cycle and obtaining qualified uranium products;

[0022] 2. Compared with the uranium purification cycle process with a long process flow, the process of the present invention is simple to operate, eliminating the intermediate evaporator and its maintenance and operation costs, and avoiding the risks of organic phase degradation, increased interfacial dirt, and deposition of highly radioactive substances that may occur during the evaporation and concentration of 1CU feed liquid;

[0023] 3. Compared with the uranium purification process technical route with two cycles, the present invention realizes the purification of plutonium and neptunium simultaneously in one uranium purification cycle. Since one uranium purification cycle is reduced, the equipment costs, operation costs, and maintenance costs of the extraction equipment, feed liquid transfer equipment, instrument control equipment, etc. used in this cycle will all be eliminated, and the risk of temporary shutdown of the production line due to equipment failure is also reduced;

[0024] 4. Since one extraction and stripping cycle is reduced in the present invention, the volume of the raffinate is reduced by about 50%, greatly reducing the pressure on the intermediate-level radioactive waste evaporator. Brief Description of the Drawings

[0025] Figure 1 This is the process technical route diagram of the method of the present invention. Detailed Embodiments

[0026] The present invention will be described in detail below with reference to the drawings and specific embodiments. The following embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0027] Example 1

[0028] A method for removing plutonium and neptunium from uranium in the PUREX thin process. The process technical route is as Figure 1 shown, and the specific steps are as follows:

[0029] (1) Feed liquid pretreatment

[0030] Receive the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, pre-treat the 1CU feed liquid to obtain the 2DF solution, so that most of the Np in the 2DF solution exists in the Np(V) state, and most of the Pu exists in the Pu(III) state, and then enter the extraction process. The specific operations and process parameter controls of this process are as follows: adjust the solution acidity to 0.5 - 1 mol / L, heat the solution temperature to 90 - 95 °C, keep warm for 2 - 4 h. The solution vapor volatilized due to heating is condensed at the condensation reflux device above the seasoning device and refluxed to the seasoning tank. The purpose of this step is to adjust Np(IV) in the solution to mostly Np(V) and a small part of Np(VI) by controlling the acidity and temperature. After the solution is cooled to 25 - 30 °C, add hydroxylamine solution to a concentration of 0.1 - 0.15 mol / L. The purpose of this step is to reduce a small part of Np(VI) to Np(V) and reduce Pu(IV) to Pu(III). Finally, add 65% nitric acid solution to make the solution acidity 2 mol / L to ensure that U(VI) has a higher distribution coefficient in the subsequent extraction process.

[0031] (2) Extraction and washing

[0032] The 2DF solution is subjected to extraction washing to transfer uranium from the aqueous phase to the organic phase. During extraction, the saturation of uranium in the organic phase after extraction is controlled by regulating the solution acidity and the flow ratio, maintaining it at around 70%, which can ensure both the recovery rate of uranium and the purification effect of uranium on other nuclides. The detergent (2DS) uses a solution containing hydroxamic acid to reduce Np(VI) and Pu(IV) extracted into the organic phase back to the aqueous phase, enhancing the removal of plutonium and neptunium from uranium. The specific operations and process parameter controls of this process are as follows: The extractant is 30% TBP / OK, the detergent is 0.2 - 0.3 mol / L hydroxamic acid and 0.3 - 0.5 mol / L HNO3, the flow ratio range is controlled at 2DF:2DX:2DS = 1:0.5 - 0.6:0.085 - 0.1, the extraction stage number is 8 - 10 stages, and the washing stage number is 6 - 8 stages.

[0033] (3) Reductive stripping

[0034] In the extraction process, due to the low content of plutonium in the organic phase and the solution being at room temperature, the reductive stripping rate is low. Therefore, a reductive stripping process is added to strengthen the purification of plutonium in the organic phase 2DU’. The specific operations and process parameter controls of this process are as follows: The reductive stripping temperature is controlled at 45 - 50 °C, the reductive stripping agent 2DNS is 0.3 - 0.5 mol / L hydroxamic acid and 0.3 mol / L nitric acid, the flow ratio range is controlled at 2DU’:2DNS = 1:0.3 - 0.35, and the reductive stripping stage number is 4 - 6 stages.

[0035] (4) Stripping

[0036] The uranium in the organic phase is stripped into the aqueous phase by controlling the acidity of the stripping agent for subsequent evaporation concentration and denitration. The specific operations and process parameter controls of this process are as follows: The stripping temperature is controlled at 55 - 60 °C, the stripping agent 2EX is 0.01 - 0.07 mol / L nitric acid, the flow ratio range is controlled at 2DU / 2EX = 1:1.1 - 1.2, and the stripping stage number is 8 - 10 stages.

[0037] Example 2

[0038] Under the optimal process conditions, a single-stage test of the complete uranium purification process is carried out. The specific process condition parameters are shown in Table 1. According to the test results, the α activity per gram of uranium in the final product liquid 2EU is 23.5 Bq / gU, of which 14 Bq / gU is contributed by neptunium and 9.5 Bq / gU is contributed by plutonium, meeting the index requirement of α activity concentration ≤ 250 Bq / gU in uranium products.

[0039] Table 1 Test condition parameters

[0040]

[0041] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for removing plutonium and neptunium from uranium in the PUREX dilute process, characterized in that, It includes the following steps: (1) Feed liquid pretreatment: Receive the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, and obtain the 2DF solution after acid adjustment, heating, valence adjustment, and acid adjustment in sequence; (2) Extraction and washing: Use the detergent 2DS and the extractant 2DX to perform extraction and washing on the 2DF solution to obtain the organic phase 2DU’ and the aqueous phase 2DW; (3) Reduction stripping: Use the reduction stripping agent 2DNS to perform reduction stripping on the organic phase 2DU’ to obtain the organic phase 2DU and the aqueous phase 2DNW; (4) Stripping: Use the stripping agent 2EX to perform stripping on the organic phase 2DU to obtain the product liquid 2EU and the aqueous phase 2EW; The specific process of the feed liquid pretreatment described in step (1) is: Receive the uranium-containing aqueous solution 1CU from the co-decontamination separation cycle, adjust the acidity of the solution to 0.5 - 1 mol / L with a 65% nitric acid solution, heat the solution temperature to 90 - 95 °C, keep warm for 2 - 4 h, after the solution cools to 25 - 30 °C, add a hydroxylamine solution until the concentration is 0.1 - 0.15 mol / L, and finally add a 65% nitric acid solution until the acidity of the solution is 2 mol / L; In the process of the reduction stripping agent described in step (3), the flow ratio range is controlled at 2DU’:2DNS = 1:0.3 - 0.35, and the reduction stripping stage number is 4 - 6; In the stripping process described in step (4), the stripping temperature is 55 - 60 °C, the flow ratio range is controlled at 2DU:2EX = 1:1.1 - 1.2, and the stripping stage number is 8 - 10; 2. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 1, wherein The detergent 2DS described in step (2) is a hydroxylamine acid-containing solution, and the extractant 2DX is TBP / OK.

3. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 2, characterized in that, The detergent 2DS described in step (2) uses 0.2 - 0.3 mol / L hydroxylamine and 0.3 - 0.5 mol / L HNO3, and the extractant uses 30% TBP / OK.

4. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 1, characterized in that, In the extraction and washing process described in step (2), the flow ratio range is controlled at 2DF:2DX:2DS = 1:0.5 - 0.6:0.085 - 0.1, the extraction stage number is 8 - 10, and the washing stage number is 6 - 8; 5. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 1, characterized in that, The reduction stripping agent 2DNS described in step (3) is 0.3 - 0.5 mol / L hydroxylamine and 0.3 mol / L nitric acid.

6. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 1, characterized in that, In the process of the reduction stripping agent described in step (3), the temperature is 45 - 50 °C.

7. The method for removing plutonium and neptunium from uranium in the PUREX dilute process according to claim 1, characterized in that, The stripping agent 2EX described in step (4) is 0.01 - 0.07 mol / L nitric acid.

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