JP4528916B2 - Method for decontamination and treatment of spent alumina - Google Patents

Description

  The present invention can effectively decontaminate spent alumina contaminated with radioactive material generated from the nuclear fuel cycle, thereby facilitating the treatment and disposal of the spent alumina, and the final high-level radioactive waste. The present invention relates to a method for decontamination and treatment of used alumina, which can reduce the amount of generation.

  In the nuclear fuel cycle, alumina is used as a fluidized medium in a fluidized bed furnace used in the fluorination step of the uranium conversion process, and further in each exhaust gas treatment step such as a uranium conversion process, uranium enrichment process, reconversion process, and fluorine production process. Widely used as a fluorine adsorbent. This alumina is partly changed to fluoride with the operation of each process, and does not satisfy the specifications due to breakthrough or deterioration, and becomes radioactive waste. In the present specification, alumina that is used in the nuclear fuel cycle and does not satisfy specifications due to breakthrough or deterioration, is contaminated with radioactive substances, and fluoride is also present is referred to as “used alumina”.

At present, these used aluminas are only stored and managed until the radioactivity decays, and decontamination, treatment, and disposal methods have not been sufficiently studied. It is expected to become a big problem from the viewpoint of economic efficiency. As a general decontamination processing technique for radioactive waste, a wet method and a dry method are conceivable. However, when used alumina is processed by the wet method, further radioactive liquid waste is generated. In addition, it has been found that it is difficult to electrochemically separate uranium and the like from aluminum when the molten salt electrolysis dyeing method is used as a dry method (Non-patent Document 1).
Against this background, as a part of reducing the amount of radioactive waste generated, development of decontamination and treatment technology for spent alumina is desired.

Proceedings of the 2005 Atomic Energy Society of Japan Annual Meeting “J42: Study on Reprocessing Technology Using Fluoride Volatilization Method (4)”, published on August 12, 2005.

  Therefore, the present invention is used as a fluidized medium in a fluidized bed furnace in a fluorination process of a nuclear fuel cycle or a fluorine adsorbent in an exhaust gas treatment process, and is used by decontamination of spent alumina contaminated with radioactive substances. It was made for the purpose of providing a new decontamination and treatment method for used alumina that makes it easy to treat and dispose of alumina and reduce the amount of final high-level radioactive waste generated. It is.

  The present inventors pay attention to the physical properties of aluminum chloride that volatilizes at a low temperature compared with other substances among aluminum compounds, and chlorinate spent alumina to convert it to chloride, thereby lowering the boiling point of aluminum chloride. The present invention has been completed by finding that aluminum chloride can be selectively volatilized and aluminum chloride can be effectively separated and recovered from other chlorides having a high boiling point.

That is, the present invention converts spent oxides into chlorides by contacting spent alumina contaminated with radioactive material generated from the nuclear fuel cycle with chlorine gas at a predetermined reaction temperature, and converts aluminum chloride from the obtained chlorides. A spent alumina decontamination and treatment method comprising: a selective chloride conversion step, and an aluminum chloride recovery step in which the vaporized aluminum chloride is condensed by a cold trap and collected.

  In the present invention, the chloride conversion step comprises converting the fluoride coexisting with the used alumina into an oxide by contacting the used alumina contaminated with the radioactive material generated from the nuclear fuel cycle with water vapor at a predetermined temperature. It can also be provided as a pre-process of the conversion step, and the resulting oxide can be sent to the chloride conversion step.

  Furthermore, in the present invention, after the cold trap in which aluminum chloride is condensed and collected in the aluminum chloride recovery step is heated to a predetermined temperature to vaporize the aluminum chloride again, the aluminum chloride is converted into aluminum oxide by countercurrent contact with water vapor. An aluminum chloride reconversion step for conversion may be provided as a subsequent step of the aluminum chloride recovery step.

ADVANTAGE OF THE INVENTION According to this invention, the used alumina contaminated with the radioactive substance can be effectively converted into a chloride. Of the chlorides produced by this chlorination, the boiling point of aluminum chloride is significantly lower than the boiling point of chlorides of various radioactive materials coexisting with the used alumina, so aluminum chloride with a low boiling point is selected at a given temperature. By volatilizing automatically, only aluminum chloride can be effectively separated and recovered from other chlorides having a high boiling point, and decontamination can be performed effectively.
Furthermore, by reconverting the recovered aluminum chloride into a stable oxide, the subsequent treatment and disposal can be facilitated, and the final amount of high-level radioactive waste generated can be reduced.

FIG. 1 is a process chart showing an embodiment of the method of the present invention. Each process will be described below in order.
(1) Oxidation conversion process Spent alumina generated from the nuclear fuel cycle coexists with various fluorides and oxyfluorides. The oxidation conversion step is a step of converting these fluorides and oxyfluorides into oxides, and the used alumina is brought into contact with water vapor at a reaction temperature of 400 to 600 ° C. At this time, the following reaction occurs and the fluoride is converted into an oxide.
2AlF ₃ + 3H ₂ O → Al ₂ O ₃ + 6HF

  When most of the used alumina is fluoride, or when it is difficult to chlorinate to the inside due to the fluoride film on the surface, it is necessary to perform an oxidation conversion step, but fluoride that coexists with used alumina. If the amount is small, this step can be omitted. If this step is omitted, a small amount of fluoride coexisting with spent alumina coexists as fluoride without being chlorinated in the next chloride conversion step. As can be seen from Table 1 below, these fluorides all have a high boiling point, and the difference from the boiling point (194 ° C.) of aluminum chloride produced by chlorination of alumina in the chloride conversion step is extremely large. Aluminum chloride can be selectively volatilized and separation from these fluorides can be facilitated.

(2) Chloride conversion step In this step, the oxide of each element produced in the oxidation conversion step is chlorinated, and the oxide is brought into contact with chlorine (Cl ₂ ) gas at a reaction temperature of 200 to 800 ° C. In the case of using Cl ₂ gas, carbon (C) powder is previously added to the oxide in order to remove the generated oxygen. Chlorination can be performed using carbon tetrachloride (CCl ₄ ) gas instead of Cl ₂ gas, and in this case, it is not necessary to use C powder. In addition, any gas other than Cl ₂ gas or CCl ₄ gas can be used as long as it is a halide gas capable of chlorinating oxides.
In this step, the following reaction occurs, and the oxide is converted to chloride.
When using Cl ₂ :
_{Al 2 O 3 + 3Cl 2 +} 3 / 2C → 2AlCl 3 + 3/2 CO 2
When using CCl ₄ :
_{Al 2 O 3 + 3 / 2CCl} 4 → 2AlCl 3 + 3/2 CO 2

As shown in Table 1, the boiling point of the produced aluminum trichloride (AlCl ₃ ) is 194 ° C., which is significantly lower than the boiling points of chlorides of other elements. Therefore, at a temperature higher than the boiling point of AlCl _3, by performing sufficient time chlorination reaction in AlCl ₃ is volatilized, since only AlCl ₃ is volatilized to generated, be separated from other substances (non-volatile residue) Can do. Therefore, the reaction temperature in the chloride conversion step is set to be equal to or higher than the boiling point of AlCl ₃ , preferably 200 to 800 ° C.

The other chloride that volatilizes at 200 ° C. is only stannic chloride (SnCl ₄ : boiling point 114 ° C.) as shown in Table 1. Therefore, if it contains Sn in the spent alumina, SnCl ₄ also generates chloride conversion step, at a reaction temperature of 200 ° C. SnCl ₄ also become volatilized with AlCl _3.
Therefore, by setting the reaction temperature at the beginning of the reaction in the chloride conversion step to 140 to 150 ° C. and holding it for a predetermined time, the generated SnCl ₄ is first volatilized and removed, and then the reaction temperature is 200 to 800 ° C. at the end of the reaction. And hold for a predetermined time to volatilize AlCl ₃ . Thus, the temperature of the chloride conversion step is set in two stages, and SnCl ₄ having a boiling point lower than that of AlCl ₃ is volatilized first, whereby SnCl ₄ can be effectively separated and removed.
Even when uranium (U) is contained in the used alumina, if the chlorination conversion step is performed at 200 ° C., the uranium chloride produced (UCl ₄ : boiling point 791 ° C., UCl ₅ : boiling point 527 ° C.) does not volatilize, It is not mixed in the volatile AlCl ₃ and can be separated effectively.

(3) Aluminum chloride recovery process The AlCl ₃ vaporized in the chloride conversion process is condensed, collected and recovered by sending it to a cold trap set at room temperature or below the room temperature. At this time, since Cl ₂ gas (boiling point: −34.04 ° C.) accompanying AlCl ₃ is not condensed, it is separated from AlCl ₃ by passing through a cold trap and recycled to the chlorine gas supply step.

(4) Aluminum chloride reconversion step After the AlCl ₃ condensed and collected by the cold trap in the aluminum chloride recovery step is volatilized again by heating the cold trap to the boiling point of AlCl ₃ or higher, preferably 200 to 300 ° C, By contacting with water vapor, it is converted into a stable oxide (Al ₂ O ₃ ) by the following reaction. By this re-volatilization operation, other chlorides accompanying AlCl ₃ remain in the cold trap, so that final separation of AlCl ₃ and other radioactive substances becomes possible.
2AlCl ₃ + 3H ₂ O → Al ₂ O ₃ + 6HCl
The hydrogen chloride (HCl) gas generated simultaneously in this reaction is recovered as hydrochloric acid, electrolyzed into chlorine gas, recycled to the chlorine gas supply step, and reused in the chloride conversion step. When the AlCl ₃ recovered in the aluminum chloride recovery process is stored as it is or used for another purpose as it is, it is not necessary to perform the aluminum chloride reconversion process.

  FIG. 2 shows an example of an apparatus for carrying out the method of the present invention. The main equipments are (1) an oxidation conversion device for performing an oxidation conversion step, (2) a chloride conversion device for performing a chloride conversion step, (3) an aluminum chloride recovery device for performing an aluminum chloride recovery step, and (4) an aluminum chloride reconversion. It consists of an aluminum chloride reconverter that performs the process.

  The used alumina to be decontaminated is first put into a used alumina supply hopper, and is introduced from this hopper into an oxidation conversion device. The oxidation conversion device is maintained at a reaction temperature of 400 to 600 ° C., and the water vapor supplied from the water vapor supply device and the used alumina are countercurrently contacted in the device, so that various fluorides coexisting with the used alumina, Converts acid fluoride to oxide.

The oxide produced | generated by the oxidation converter is once stored in an oxide supply hopper, and introduce | transduced into the lower part of a chloride converter from here. During this introduction, carbon powder supplied from a carbon supply hopper is added to the oxide. On the other hand, Cl ₂ gas supplied from the chlorine gas supply device is also introduced into the lower portion of the chloride conversion device. The chloride conversion apparatus is maintained at a reaction temperature of 200 to 800 ° C., and the oxide and Cl ₂ gas come into contact with each other in the apparatus, and the chlorination of the oxide proceeds. As a result, the generated AlCl ₃ volatilizes at the temperature in the apparatus and is led from the upper part of the apparatus to the aluminum chloride recovery apparatus. Other chlorides that do not volatilize at the temperature in the apparatus are discharged as a non-volatile residue from the bottom of the apparatus to the residue receiving tank.

The aluminum chloride recovery device is provided with a cold trap maintained at room temperature or a temperature below room temperature, and AlCl ₃ introduced into the device is cooled and condensed when passing through the cold trap. To be collected. The Cl ₂ gas introduced into the apparatus accompanying the AlCl ₃ passes through the cold trap without being condensed, is discharged outside the apparatus, and is recycled to the chlorine gas supply apparatus.

The AlCl ₃ condensate collected in the cold trap in the aluminum chloride recovery unit is volatilized again in the aluminum chloride recovery unit by heating the cold trap to 200 to 300 ° C., and the volatilized AlCl ₃ is reconverted to aluminum chloride. Install into the device. In the aluminum chloride re-conversion device, the water vapor supplied from the water vapor supply device and AlCl ₃ are countercurrently contacted to convert AlCl ₃ into stable Al ₂ O ₃ and stored in an aluminum oxide receiving tank. The HCl gas generated at this time is recycled to the chlorine gas supply device.

Experimental example

Using a small rotary kiln (reaction part inner diameter 100 mm, length 300 mm), an oxidation conversion test and a chloride conversion test were conducted.
[Oxidation conversion test]
Fluoride (AlF ₃ ) coexisting with spent alumina was converted to an oxide under the following test conditions. The reaction product was analyzed by X-ray diffraction. The test results are shown in Table 2. From Table 2, it is understood that the reaction temperature for the oxidative conversion is preferably 400 to 600 ° C.
Reaction temperature: 200-800 ° C
Reactant supply conditions: Solid (AlF ₃ ) supplied in batch mode
The gas (water vapor) is continuously ventilated using a nitrogen carrier gas.
Rotary kiln speed: 1rpm
Rotary kiln operation time: 1 hour

[Chlorination conversion test]
Alumina (Al ₂ O ₃ ) was converted to an oxide under the following test conditions. The recovery rate of the reaction product AlCl ₃ was determined by recovering volatile substances and measuring the weight. The test results are shown in Table 3. Table 3 shows that an AlCl ₃ recovery rate of 90% or more can be obtained at any reaction temperature of 200 to 800 ° C. During the test, the test was performed without ensuring that the rotary kiln was rotated for safety, while ensuring the airtightness of each part.
Reaction temperature: 200-800 ° C
Reactant supply conditions: Solid (Al ₂ O ₃ , carbon) supplied in batch
Gas (chlorine) is continuously ventilated using nitrogen carrier gas.
Rotary kiln speed: 0rpm
Rotary kiln operation time: 2 hours

  As can be seen from the above description, according to the present invention, it is possible to effectively achieve decontamination, treatment and disposal of used alumina contaminated with radioactive substances. As a result, the amount of used alumina as high-level radioactive waste that becomes a problem in the fluorination treatment in the uranium conversion process using a fluidized bed furnace can be greatly suppressed. In domestic uranium conversion operation, it is assumed that a further amount of used alumina will be discharged from the reprocessing process, considering the fact that 30 to 40 kg of used alumina is generated per ton of uranium oxide. The present invention greatly contributes to reducing the amount of high-level radioactive waste generated.

It is process drawing which shows one Example of this invention method. It is explanatory drawing which shows an example of the apparatus structure for implementing this invention method.

Claims (8)

By contacting spent alumina contaminated with radioactive material generated from the nuclear fuel cycle with chlorine gas at a predetermined reaction temperature, the oxide is converted to chloride, and aluminum chloride is selectively vaporized from the resulting chloride. A chloride conversion step ;
An aluminum chloride recovery step of condensing and collecting the vaporized aluminum chloride by a cold trap ;
A method for decontaminating and treating spent alumina, comprising:   The oxidation conversion step in which spent alumina contaminated with radioactive material generated from the nuclear fuel cycle is brought into contact with water vapor at a predetermined temperature to convert fluoride coexisting with the used alumina into oxides is a step preceding the chloride conversion step. 2. The method for decontaminating and treating spent alumina according to claim 1, wherein the oxide obtained is sent to the chloride conversion step.   Reconversion of aluminum chloride that heats the cold trap that condenses and collects aluminum chloride in the aluminum chloride recovery step to a predetermined temperature to re-evaporate the aluminum chloride, and then makes countercurrent contact with water vapor to convert the aluminum chloride to aluminum oxide. 3. A method for decontaminating and treating spent alumina according to claim 1 or 2, wherein a step is provided as a step after the aluminum chloride recovery step.   The method for decontaminating and treating spent alumina according to claim 2, wherein the predetermined temperature in the oxidation conversion step is 400 to 600 ° C.   The predetermined reaction temperature of the said chloride conversion process shall be 200-800 degreeC, The decontamination and processing method of the used alumina of any one of Claims 1-3 characterized by the above-mentioned.   The used alumina according to any one of claims 1 to 3, wherein the predetermined reaction temperature in the chloride conversion step is 140 to 150 ° C at the beginning of the reaction and 200 to 800 ° C at the end of the reaction. Decontamination and processing method.   The method for decontaminating and treating spent alumina according to any one of claims 1 to 3, wherein the temperature of the cold trap in the aluminum chloride recovery step is set at room temperature or below room temperature.   The method for decontaminating and treating spent alumina according to claim 3, wherein a predetermined temperature of the cold trap in the aluminum chloride reconversion step is 200 to 300 ° C.

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