A method for separating uranium zirconium oxide

Abstract

The present invention discloses a method for separating uranium-zirconium oxides. The method adopts carbonization of uranium oxides, high-temperature vacuum sintering of excess carbon, and molten salt electrolysis for separation, thereby achieving separation of uranium-zirconium oxides and obtaining pure UO2 and ZrO2. The method solves the problems of dissolution of uranium-zirconium oxides in molten salt and the more negative reduction potential of uranium than that of zirconium. The method improves the atmosphere restriction of traditional molten salt electrolysis requiring an inert atmosphere. The method of the present invention is simple in process and low in cost, providing new ideas and new directions for the post-processing of MOX spent fuel.

Description

Technical Field

[0001] The invention belongs to the technical field of spent fuel post-processing, and in particular relates to a method for separating uranium-zirconium oxides. Background Art

[0002] Spent fuel reprocessing can be divided into dry and aqueous methods. Typical dry reprocessing processes include electrorefining (electrorefining) for metal fuel and oxide electrodeposition (electrodeposition) for oxide fuel. To adapt electrorefining technology to the MOX spent fuel currently used in fast reactors and light water reactors, the oxide spent fuel must first be converted to metal. Metallothermic reduction (MTR) and electrodeoxidation are two methods for this conversion. Metallothermic reduction replaces the metal (primarily uranium) in the oxide spent fuel with active metals such as Li and Ca. However, the resulting oxides (Li2O and CaO) coat the surface of the oxide spent fuel, preventing further replacement. Large amounts of molten salt are required to dissolve the oxide layer to ensure continued reaction, generating significant amounts of waste salt. Electrodeoxidation involves electrolytic reduction of the oxide spent fuel in high-temperature molten salt, using the oxide spent fuel as the cathode. This ultimately produces a crude metal element, which can then be electrorefined to obtain a relatively pure metal.

[0003] However, the electrolytic reduction process using spent oxide fuel as the cathode has fatal flaws such as complex integrated cathode preparation process, high cost and severe loss of platinum anode materials, unstable uranium reduction rate, low current efficiency, and the need to ensure an inert atmosphere. Summary of the Invention

[0004] In view of the above technical problems existing in the prior art, the object of the present invention is to provide a method for separating uranium-zirconium oxides with simple process, low cost and effective separation of metal oxides.

[0005] To achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows: A method for separating uranium zirconium oxides comprises the following steps:

[0006] (1) uniformly mixing uranium zirconium oxide and graphite in appropriate proportions and pressing the mixture into tablets;

[0007] (2) vacuum sintering to carbon-oxidize the uranium oxide;

[0008] (3) High temperature vacuum sintering to remove carbon;

[0009] (4) Cooling to room temperature under vacuum and taking out the sintered sample;

[0010] (5) The sintered sample is electrolyzed and separated by molten salt to obtain UO2 and ZrO2 respectively.

[0011] Furthermore, in the step (1), U3O8:C=1:0.12wt%; UO2:C=1:0.09wt%.

[0012] Furthermore, in the step (1), the tableting pressure is 28 to 226 MPa, and the holding time is 1 to 15 minutes.

[0013] Furthermore, in the step (2), the vacuum sintering temperature is 1500-1650° C., the vacuum degree is below 100 Pa, and the sintering time is 0-8 hours.

[0014] Furthermore, in the step (3), the temperature of the high-temperature vacuum sintering is greater than 1650° C., the vacuum degree is below 100 Pa, and the sintering time is 0 to 8 hours.

[0015] Furthermore, in the step (5), the molten salt electrolysis environment is a chloride molten salt environment under an air atmosphere, and the electrolysis temperature is 400-800°C; the electrode system is a three-electrode system or a two-electrode system, and the electrodes are the sintered sample anode, Mo cathode, and Ag / AgCl reference electrode; and the electrolysis mode is constant current or constant potential.

[0016] The beneficial effects brought about by the technical solution of the present invention are as follows: a method for separating uranium-zirconium oxides, which adopts carbon oxidation of uranium oxides, high-temperature vacuum sintering of excess carbon, and molten salt electrolysis to separate uranium-zirconium oxides, realizes the separation of uranium-zirconium oxides and obtains pure UO2 and ZrO2; solves the problems of dissolution of uranium-zirconium oxides in molten salt and the reduction potential of uranium being more negative than the reduction potential of zirconium; improves the atmosphere limitation of traditional molten salt electrolysis requiring an inert atmosphere; and the method of the present invention is simple in process and low in cost, providing new ideas and new directions for the post-processing of MOX spent fuel. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of a material column before separation of uranium-zirconium oxides according to Example 1 of the present invention;

[0018] Figure 2 Schematic diagram of a material column after sintering of uranium-zirconium oxides by the method of Example 1 of the present invention;

[0019] Figure 3 is the XRD diffraction spectrum of the product after sintering zirconium oxide by the method of Example 2 of the present invention;

[0020] Figure 4 This is the XRD diffraction spectrum of the product after sintering uranium oxide using the method of Example 3 of the present invention;

[0021] Figure 5 This is the XRD diffraction spectrum of the anode product after separating uranium zirconium oxide using the method of Example 4 of the present invention;

[0022] Figure 6 This is the XRD diffraction spectrum of the cathode product after separating uranium zirconium oxides using the method of Example 4 of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0024] Example 1

[0025] An embodiment of the present invention provides a method for separating uranium zirconium oxides, comprising the following steps:

[0026] (1) uniformly mixing uranium zirconium oxide and graphite in appropriate proportions and pressing the mixture into tablets;

[0027] (2) vacuum sintering to carbon-oxidize the uranium oxide; wherein the vacuum sintering temperature is 1500-1650°C, the vacuum degree is below 100 Pa, and the sintering time is 0-8 hours; under these temperature conditions, the complete carbon oxidation of the uranium oxide is ensured, while the carbon oxidation of the zirconium oxide is prevented, which would prematurely consume the graphite therein; the vacuum conditions can satisfy the carbon oxidation of the uranium oxide while avoiding the re-oxidation of the uranium carbon oxide; the change in the pressure in the high-temperature vacuum furnace can determine whether the uranium oxide is completely carbon-oxidized; when the pressure in the furnace is reduced and maintained within the initial pressure range, the uranium oxide is almost completely carbon-oxidized;

[0028] (3) High-temperature vacuum sintering for decarbonization; the temperature of high-temperature vacuum sintering is greater than 1650°C, the vacuum degree is below 100 Pa, and the sintering time is 0 to 8 hours; the embodiment of the present invention removes excess graphite by carbon oxidation of zirconium oxide, while preventing the re-oxidation of uranium carbon oxide;

[0029] (4) Cooling to room temperature under vacuum and taking out the sintered sample;

[0030] (5) Separating the sintered sample by molten salt electrolysis to obtain UO2 and ZrO2, respectively. The molten salt electrolysis environment is a chloride molten salt environment under an air atmosphere, and the electrolysis temperature is 400-800°C; the electrode system is a three-electrode system or a two-electrode system, and the electrodes are the sintered sample anode, Mo cathode, and Ag / AgCl reference electrode; the electrolysis method is constant current or constant potential.

[0031] Preferably, in step (1), U3O8:C=0.12-0.192:1; UO2:C=0.09-0.192:1, and the content ratio of graphite is slightly greater than that of uranium zirconium oxide to ensure complete carbon oxidation of uranium oxide;

[0032] Preferably, in step (1), the tableting pressure is 28-226 MPa and the holding time is 1-15 min. Under these conditions, the tableting of the uranium zirconium oxide powder is ensured to have a certain mechanical strength while preventing damage to the mold due to overpressure.

[0033] Preferably, in step (2), the vacuum sintering temperature is 1550° C., the vacuum degree is 0.1 Pa, and the sintering time is 2 h.

[0034] Preferably, in step (3), the temperature of the high-temperature vacuum sintering is above 1750° C., the vacuum degree is 0.1 Pa, and the sintering time is 4 hours.

[0035] Preferably, in step (5), the chloride molten salt is LiCl-KCl molten salt, and the electrolysis temperature is 500° C.; oxides are added to the molten salt before electrolysis; and after electrolysis, ZrO 2 is collected at the anode and UO 2 is collected at the cathode.

[0036] Preferably, in step (5), Li2O is added to the molten salt before electrolysis.

[0037] Refer to the attached Figure 1 、 2 The appearance of the uranium zirconium oxide column remains basically unchanged before and after sintering. After weighing, the reduction weight loss rate is 26.91%. Compared with the theoretical weight loss rate of 28.58%, the reduction rate reaches 94.16%.

[0038] Example 2

[0039] The method of Example 1 of the present invention is used to separate uranium zirconium oxides, comprising the following steps:

[0040] (1) Weigh 1 g of ZrO2 and 0.19 g of graphite, mix thoroughly, and press into a 15 mm mold at 113 MPa for 3 min.

[0041] (2) Sintering at 1750°C for 4 h under vacuum pressure of 0.1 Pa;

[0042] (3) Cool down to room temperature under vacuum and take out the sintered sample.

[0043] The rest is the same as in Example 1.

[0044] Refer to the attached Figure 3 The sintered sample was subjected to XRD testing, and the results showed that zirconium oxide was converted into zirconium carbon oxide after high-temperature vacuum sintering.

[0045] Example 3

[0046] The method of Example 1 of the present invention is used to separate uranium zirconium oxides, comprising the following steps:

[0047] (1) Weigh 1 g of UO2 and 0.09 g of graphite, mix thoroughly, and press into a 15 mm mold at 113 MPa for 3 min.

[0048] (2) Sintering at 1550°C for 2 h under vacuum pressure of 0.1 Pa;

[0049] (3) Cool down to room temperature under vacuum and take out the sintered sample.

[0050] The rest is the same as in Example 1.

[0051] The samples before and after sintering were weighed, and the weight loss was 0.200 g. Compared with the theoretical weight loss of 0.207 g, the reduction rate was 96.6%.

[0052] Refer to the attached Figure 4 The sintered samples were subjected to XRD testing, and the results showed that uranium oxide was converted into uranium carbon oxide after high-temperature vacuum sintering.

[0053] Example 4

[0054] The method of Example 1 of the present invention is used to separate uranium zirconium oxides, comprising the following steps:

[0055] (1) Weigh 1 g ZrO2, 1 g UO2, and 0.09 g graphite, mix thoroughly, and press into a 15 mm mold at 113 MPa for 3 min.

[0056] (2) Sintering at 1550℃ for 2h under vacuum pressure of 0.1Pa to completely carbonize UO2;

[0057] (3) Sintering at 1750°C and 0.1 Pa pressure for 4 h;

[0058] (4) Cooling to room temperature under vacuum and taking out the sintered sample;

[0059] (5) In an air environment at 500°C, the sintered sample was used as the anode, the Mo sheet as the cathode, and the Ag / AgCl as the reference electrode. Electrolysis was performed at a constant potential of 0.5 V. During the electrolysis process, a total of about 0.25 g of Li2O was added in batches.

[0060] After the separation, ZrO2 was collected at the anode and its vicinity, and UO2 was collected at the cathode and its vicinity. After washing, drying and weighing, a total of 0.95g of ZrO2 and 0.98g of UO2 were collected.

[0061] Refer to the attached Figure 5 、 6 , indicating that the products separated by the method of the embodiment of the present invention are ZrO2 and UO2.

[0062] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for separating uranium zirconium oxides, characterized in that: The following steps are involved: (1) uniformly mixing a mixture of uranium oxide and zirconium oxide with graphite and pressing the mixture into tablets, wherein the mixture of uranium oxide and zirconium oxide and graphite are mixed in an appropriate ratio; (2) vacuum sintering to carbon-oxidize the uranium oxide; ensuring complete carbon oxidation of the uranium oxide and preventing carbon oxidation of the zirconium oxide; (3) High-temperature vacuum sintering for decarbonization; removing excess graphite by carbon oxidation of zirconium oxide while preventing re-oxidation of uranium carbon oxide; (4) Cool down to room temperature under vacuum and take out the sintered sample; (5) The sample is subjected to molten salt electrolysis to separate UO2 and ZrO2, respectively.

2. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (1), the tableting pressure is 28-226 MPa, and the holding time is 1-15 min.

3. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (2), the vacuum sintering temperature is 1500-1650° C., the vacuum degree is below 100 Pa, and the sintering time is below 8 hours.

4. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (3), the temperature of the high-temperature vacuum sintering is greater than 1650° C., the vacuum degree is less than 100 Pa, and the sintering time is less than 8 hours.

5. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (5), the molten salt electrolysis environment is a chloride molten salt in an air atmosphere, and the electrolysis temperature is 400-800°C; the electrode system is a three-electrode system or a two-electrode system, and the electrodes are the sintered sample anode, Mo cathode, and Ag / AgCl reference electrode; and the electrolysis mode is constant current or constant potential.

6. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (5), the chloride molten salt is LiCl~KCl molten salt, and the electrolysis temperature is 500°C; oxides are added to the molten salt before electrolysis; and after electrolysis, ZrO2 is collected at the anode and UO2 is collected at the cathode.

7. The method for separating uranium zirconium oxides according to claim 1, wherein: In the step (5), Li2O is added to the molten salt before electrolysis.

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