A method for removing rare earth elements from spent fuel by chlorination

Abstract

The present application belongs to the field of chemical separation and spent fuel reprocessing technology, and discloses a method for removing rare earth elements from spent fuel by chlorination. The spent fuel is first pretreated to obtain a mixture of triuranium octaoxide and rare earth oxides, then mixed with uranium tetrachloride and briquetted, and subjected to chlorination reaction to convert the rare earth oxides in the spent fuel into rare earth chlorides. Subsequently, the rare earth chlorides are volatilized and separated by vacuum separation to obtain a mixture of triuranium octaoxide and uranium dioxide from which the rare earth elements have been removed, and the volatilized substances are condensed and collected to obtain rare earth chlorides. The present application has the advantages of high safety, simple process flow, efficient removal of rare earth elements from spent fuel, no introduction of impurities, no waste liquid or waste gas generation, and good industrial application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of chemical separation and spent fuel reprocessing, and relates to a method for removing rare earth elements in spent fuel by chlorination. BACKGROUND

[0002] Nuclear fission energy is a mature clean energy with the advantages of high efficiency, low carbon and large-scale utilization, and is one of the effective ways to solve the sustainable supply of future energy. However, the current uranium resources are scarce, the utilization rate of nuclear reactor uranium resources is low, and a large amount of high-level spent fuel is stored, which has become a bottleneck restricting the development of nuclear power. In order to improve the utilization rate of uranium resources and reduce the emission of high-level fission products and long-lived radionuclides, Chinese researchers have innovatively proposed a new concept and research program of "Accelerator Driven Advanced Nuclear Energy System (ADANES)". The ADANES system is an advanced nuclear fuel closed cycle technology, which integrates nuclear fuel breeding, long-lived nuclide transmutation and nuclear power generation, can improve the utilization rate of uranium resources to about 95%, reduce the discharge of long-lived nuclear waste by more than 95%, and is expected to make fission nuclear energy a safe, reliable and clean strategic energy.

[0003] Spent fuel reprocessing is an important step in the ADANES system, and the spent fuel reprocessing process includes spent fuel and cladding separation, volatile fissile element separation, and rare earth element separation, etc. Among them, the spent fuel and cladding separation and volatile product separation and other pretreatment technologies are relatively mature, the device is simple, and it is relatively easy to industrialize. However, the separation of rare earth elements in spent fuel is more difficult, which is a major problem that needs to be solved for safe and efficient utilization of spent fuel. Rare earth elements have a large neutron absorption cross section, which will seriously affect the breeding of nuclear fuel and the transmutation of nuclear waste. Chinese patent application CN1186564A discloses a method for removing rare earth elements from spent nuclear fuel. The invention first oxidizes the uranium dioxide in the spent nuclear fuel to triuranium octaoxide at 200-800℃, then heats it in air at 1000-1600℃, so that the rare earth elements enter the fluorite phase and are separated from the recrystallized triuranium octaoxide, and then the rare earth elements are removed by ultrasonic dispersion-ultrafine filter cloth filtration. In the invention, the triuranium octaoxide (-10μm) and rare earth oxides (-1μm) after phase separation are very fine, and the two are closely combined. The method of ultrasonic dispersion-ultrafine filter cloth filtration is only suitable for laboratory operation, and it is difficult to realize in actual large-scale application. Chinese patent CN105195328B discloses a method for removing rare earth elements from spent fuel using a double organic phase flotation separation system containing an ionic liquid. The invention uses P507 (2-ethylhexyl phosphonic acid-2-ethylhexyl monoester) as a rare earth separation extractant, an ionic liquid as a synergistic extractant, and oleic acid / kerosene as a diluent. The rare earth elements are removed by a multi-stage cascade differential flotation process. The invention has a low single-pass rare earth removal rate, and requires multiple cycles of separation, resulting in a complex operation process. In addition, the use of a large number of liquid solvents makes it difficult to scale up the application. Chinese patent CN108538417B discloses a method for removing rare earth elements from spent fuel using an ionic liquid. The invention uses a specific functional ionic liquid to selectively dissolve and remove rare earth elements based on the difference in solubility of uranium dioxide and rare earth oxides in ionic liquids. Although the invention can directly remove rare earth elements, the use of ionic liquids is large, and the dissolved radioactive rare earth elements are difficult to recover, which poses a risk of nuclear proliferation.

[0004] Therefore, through process and technological innovation, safe and efficient removal of rare earth elements from spent fuel is the key to realizing the recycling of spent fuel in China. SUMMARY

[0005] In view of the problems existing in the prior art, the purpose of the present application is to provide a method for removing rare earth elements from spent fuel, which is safe and reliable, green and environmentally friendly, and has a simple process. The method can efficiently remove rare earth elements from spent fuel, has high safety, does not introduce impurities, does not produce waste liquid and waste gas, has a simple process, is easy to operate, and is suitable for large-scale treatment.

[0006] To achieve the above object, the present application adopts the following technical solutions:

[0007] A method for removing rare earth elements from spent fuel by chlorination, comprising the following steps of pretreatment, chlorination, vacuum separation and condensation:

[0008] (1) The spent fuel is pretreated by shearing, oxidation and high temperature to obtain a pretreated product;

[0009] (2) The pretreated product obtained in step (1) is mixed with uranium tetrachloride to obtain a briquetted mixture;

[0010] (3) The briquetted mixture obtained in step (2) is subjected to chlorination to obtain a chlorination product;

[0011] (4) The chlorination product obtained in step (3) is subjected to vacuum separation to obtain a mixture of triuranium octaoxide and uranium dioxide from which the rare earth elements are removed, and volatile substances are collected by condensation to obtain a rare earth chloride-containing product.

[0012] In step (1), the content of uranium dioxide in the spent fuel is greater than 94%, the content of rare earth elements is 0.1-5%, and the content of volatile fissile elements is 0.1-1%. The rare earth elements refer to one or more of La, Pr, Nd, Sm, Yb and Gd. The volatile fissile elements refer to one or more of H, I, Xe, Kr, C, Cs, Te and Mo.

[0013] In step (1), the pretreated product is a mixture of triuranium octaoxide and rare earth oxides.

[0014] In step (2), the ratio of the added molar amount of uranium tetrachloride to the molar amount of rare earth elements in the spent fuel is (0.75-0.825):1.

[0015] In step (3), the chlorination temperature is 400-900℃, the chlorination time is 0.5-5h, and the system pressure is 80-150kPa.

[0016] In step (4), the vacuum separation temperature is 600-1600℃, the vacuum separation time is 1-10h, and the vacuum degree is 0.00001-9000Pa.

[0017] Compared with the prior art, the present application has the following outstanding advantages:

[0018] (1) The present application uses uranium tetrachloride as a chlorination agent, and can efficiently remove rare earth elements from spent fuel by chlorination-vacuum separation. The process flow and equipment structure are simple, the operation is simple, and the present application is easy to scale up and has good industrial application prospect.

[0019] (2) The present application does not introduce impurities, does not add solvents or produce waste liquid and waste gas, and the separated rare earth elements and uranium oxides are in solid phase, which is convenient for recycling, avoids the risk of nuclear proliferation, and is high in safety. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A flow chart of a method for removing rare earth elements from spent fuel by a chlorination method. DETAILED DESCRIPTION

[0021] Any feature disclosed in this specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Each feature disclosed in this specification is one example only of a generic series of equivalent or similar features. Numerous specific details are disclosed herein in order to provide a thorough understanding. However, in the interest of not obscuring the application, the application is not limited to the specific details disclosed.

[0022] The application will be further described with reference to the drawings and specific examples.

[0023] It should be particularly noted that, since the spent fuel is highly radioactive, the example uses a non-radioactive or extremely low radioactive simulation material with the same composition to replace the spent fuel.

[0024] Example 1

[0025] A method for removing rare earth elements from spent fuel by a chlorination method, the method specifically comprising the following steps: (1) after the spent fuel is sheared, oxidized and pretreated at high temperature, a pretreated product is obtained; (2) the pretreated product obtained in step (1) is mixed and briquetted with uranium tetrachloride to obtain a briquetted mixture; (3) the briquetted mixture obtained in step (2) is subjected to a chlorination reaction to obtain a chlorination product; (4) the chlorination product obtained in step (3) is subjected to vacuum separation to obtain a mixture of triuranium octaoxide and uranium dioxide from which the rare earth elements are removed, and volatile substances are collected by condensation to obtain a rare earth chloride-containing product.

[0026] Example 2

[0027] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method described in Embodiment 1. The content of UO2 in the simulation material is 98.88%, the content of Nd2O3 is 0.12%, the content of I is 1%, the mass fraction of Nd element is 0.1%, the amount of UCl4 added accounts for 0.2% of the mass of the simulation material, and the molar ratio of UCl4 to Nd element is 0.75:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture, and then the briquetted mixture is chlorinated in argon at 400℃ for 5h, the system pressure is 80kPa, and then the chlorination product is heated at 600℃ and a vacuum degree of 0.00001Pa for 10h to obtain U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a rare earth chloride-containing condensate.

[0028] Embodiment 3

[0029] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method described in Embodiment 1. The content of UO2 in the simulation material is 94.07%, the content of Nd2O3 is 5.83%, the content of I is 0.1%, the mass fraction of Nd element is 5%, the amount of UCl4 added accounts for 10.86% of the mass of the simulation material, and the molar ratio of UCl4 to Nd element is 0.825:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture, and then the briquetted mixture is chlorinated in argon at 900℃ for 0.5h, the system pressure is 150kPa, and then the chlorination product is heated at 1600℃ and a vacuum degree of 9000Pa for 1h to obtain U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a rare earth chloride-containing condensate.

[0030] Embodiment 4

[0031] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method described in Embodiment 1. The content of UO2 in the simulation material is 96%, the content of Nd2O3 is 3.5%, the content of I is 0.5%, the mass fraction of Nd element is 3%, the amount of UCl4 added accounts for 6.32% of the mass of the simulation material, and the molar ratio of UCl4 to Nd element is 0.8:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture, and then the briquetted mixture is chlorinated in argon at 500℃ for 4h, the system pressure is 90kPa, and then the chlorination product is calcined at 1200℃ and a vacuum degree of 200Pa for 8h to obtain U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a rare earth chloride-containing condensate.

[0032] Example 5

[0033] This example uses the method of removing rare earth elements from spent fuel by chlorination described in Example 1. The simulated material used has a UO2 content of 98.13%, a La2O3 content of 1.17%, and a Mo content of 0.7%, with the mass fraction of La being 1%. The amount of UCl4 added is 2.11% of the mass of the simulated material, and the molar ratio of UCl4 to La is 0.77:1. First, the simulated material is subjected to shearing, oxidation, and high-temperature pretreatment to obtain a mixture of triuranium octaoxide and rare earth oxides, which is then mixed with uranium tetrachloride and pressed into a briquette to obtain a briquetted mixture. The briquetted mixture is then chlorinated in argon at 750°C for 2.5 h, with the system pressure being 100 kPa. The chlorination product is then calcined at 1000°C and a vacuum of 9 Pa for 6 h to obtain a U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1 ‰, and a condensed and collected rare earth chloride.

[0034] Example 6

[0035] This example uses the method of removing rare earth elements from spent fuel by chlorination described in Example 1. The simulated material used has a UO2 content of 95.06%, a Sm2O3 content of 4.64%, and a Cs content of 0.3%, with the mass fraction of Sm being 4%. The amount of UCl4 added is 7.57% of the mass of the simulated material, and the molar ratio of UCl4 to Sm is 0.75:1. First, the simulated material is subjected to shearing, oxidation, and high-temperature pretreatment to obtain a mixture of triuranium octaoxide and rare earth oxides, which is then mixed with uranium tetrachloride and pressed into a briquette to obtain a briquetted mixture. The briquetted mixture is then chlorinated in argon at 650°C for 4 h, with the system pressure being 85 kPa. The chlorination product is then calcined at 1400°C and a vacuum of 1000 Pa for 1.5 h to obtain a U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1 ‰, and a condensed and collected rare earth chloride.

[0036] Example 7

[0037] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method in Example 1. The content of UO2 in the simulation material is 94.11%, the content of Yb2O3 is 5.69%, and the content of Te is 0.2%. The mass fraction of Yb element is 5%, and the amount of UCl4 added accounts for 8.78% of the mass ratio of the simulation material. The molar ratio of UCl4 to Yb element is 0.8:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, and then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture. Then, the briquetted mixture is placed in argon at 680°C for chlorination for 3.5h, and the system pressure is 120kPa. Then, the chlorination product is calcined at 900°C and a vacuum degree of 0.1Pa for 2.5h to obtain U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a condensed and collected rare earth chloride.

[0038] Example 8

[0039] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method in Example 1. The content of UO2 in the simulation material is 94.11%, the content of Yb2O3 is 5.69%, and the content of Te is 0.2%. The mass fraction of Yb element is 5%, and the amount of UCl4 added accounts for 8.78% of the mass ratio of the simulation material. The molar ratio of UCl4 to Yb element is 0.8:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, and then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture. Then, the briquetted mixture is placed in argon at 680°C for chlorination for 3.5h, and the system pressure is 120kPa. Then, the chlorination product is calcined at 900°C and a vacuum degree of 0.1Pa for 2.5h to obtain U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a condensed and collected rare earth chloride.

[0040] Example 9

[0041] The embodiment adopts the method for removing rare earth elements in spent fuel by chlorination method in embodiment 1. The simulation material used contains 97.59% UO2, 2.31% Gd2O3 and 0.1% C, wherein the mass fraction of Gd element is 2%, the amount of UCl4 added accounts for 3.96% of the mass of the simulation material, and the molar ratio of UCl4 to Gd element is 0.82:1. First, the simulation material is sheared, oxidized and pretreated at high temperature to obtain a mixture of triuranium octaoxide and rare earth oxides, then the mixture is fully mixed with uranium tetrachloride and briquetted to obtain a briquetted mixture, and then the briquetted mixture is chlorinated in argon at 750°C for 3h, and the system pressure is 130kPa, and then the chlorination product is calcined at 700°C under a vacuum of 0.0001Pa for 1.5h to obtain a U3O8 and UO2 mixed powder with a rare earth element content of less than 0.1‰ and a rare earth chloride-containing condensate collected.

[0042] The upper and lower limits of the process parameters (such as temperature, time, etc.) of the present application and the interval values can all achieve the method, and therefore the embodiments are not listed one by one.

[0043] The contents not described in detail in the present application can all adopt the conventional technical knowledge in the field.

[0044] Finally, it should be explained that the above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application is described in detail with reference to the embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the spirit and scope of the technical solutions of the present application, and all of them should be covered in the scope of the claims of the present application.

Claims

1. A method for removing rare earth elements from spent fuel by chlorination, the method comprising the following steps: 1) subjecting the spent fuel to shearing, oxidation and high-temperature pretreatment to obtain a pretreated product; 2) mixing the pretreated product obtained in step 1) with uranium tetrachloride and pressing into a briquette to obtain a briquetted mixture; 3) subjecting the briquetted mixture obtained in step 2) to a chlorination reaction to obtain a chlorination product; 4) subjecting the chlorination product obtained in step 3) to vacuum separation to obtain a mixture of triuranium octaoxide and uranium dioxide from which the rare earth elements have been removed, and simultaneously condensing and collecting the volatiles to obtain a rare earth chloride-containing product.

2. The method of claim 1, wherein, In step 1), the spent fuel contains more than 94% of uranium dioxide, 0.1-5% of rare earth elements and 0.1-1% of volatile fissile elements; the rare earth elements refer to one or more of La, Pr, Nd, Sm, Yb and Gd; and the volatile fissile elements refer to one or more of H, I, Xe, Kr, C, Cs, Te and Mo.

3. The method of claim 1 or 2, wherein, In step 1), the pretreated product is a mixture of triuranium octaoxide and rare earth oxides.

4. The method of claim 1 or 2, wherein, In step 2), the ratio of the added molar amount of uranium tetrachloride to the molar amount of rare earth elements in the spent fuel is (0.75-0.825):

1.

5. The method of claim 1 or 2, wherein, In step 3), the chlorination temperature is 400-900℃, the chlorination time is 0.5-5h, and the system pressure is 80-150kPa.

6. The method of claim 1 or 2, wherein, In step 4), the vacuum separation temperature is 600-1600℃, the vacuum separation time is 1-10h, and the vacuum degree is 0.00001-9000Pa.

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