JPH077097B2 - Method for decomposing radioactive waste organic solvent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention mainly relates to a method for decomposing radioactive waste organic solvent generated from a spent nuclear fuel reprocessing facility.

(Prior art) In nuclear power generation, in order to effectively use fuel resources, spent nuclear fuel is reprocessed to separate useful components such as uranium and plutonium from unnecessary fission products and reused as nuclear fuel. Is being done. The reprocessing method is usually a method in which spent nuclear fuel is dissolved in nitric acid, and this nitric acid solution is subjected to liquid-liquid extraction with an organic solvent to separate and recover uranium and plutonium (Purex method). In this method, a phosphoric acid ester such as tri-n-butyl phosphate (TBP) as an organic solvent for extraction and an aliphatic hydrocarbon such as n-dodecane which is a diluent thereof are mixed at a ratio of about 3: 7 (volume ratio). A mixed solvent mixed in is generally used. The uranium and plutonium extracted with this mixed solvent are then back-extracted with dilute nitric acid water, concentrated and processed.
On the other hand, the used waste solvent is radioactively contaminated and deteriorated due to radiation damage, and is treated as waste (waste solvent). Conventionally, the following two types of methods have been known as methods for treating the used waste organic solvent.

A method in which the used waste organic solvent is treated with concentrated phosphoric acid to separate it into an organic phosphate ester component and a diluted component, then the former is pyrolyzed and the latter is reused or incinerated. (ETR-2
87, (1980)), the two components are separated by the same method, and the separated phosphate ester component is directly solidified with asphalt or a thermoplastic resin. (KFK-2212, (1974)) However, in the case of the thermal decomposition method of the above two technologies, anhydrous phosphoric acid and hydrocarbons are generated by decomposition, but the former may corrode equipment materials. However, the latter has the problem that the post-processing operations such as condensation, separation of oil and water, and incineration are required, and the process becomes complicated.

Also in the case of the solidification method, the volume of waste (waste solvent) mixed with the solidifying agent cannot be increased so much that the volume reduction is low, and the property of the solidified product is that of the solidified product of the inorganic solid waste. There is a problem that it becomes considerably worse than in the case.

Under the following background, the present inventors
We have proposed a method for the wet oxidative decomposition treatment of waste organic phosphate ester solvents such as, and mixed organic solvents containing them (Japanese Patent Application No.
58-169205 and Japanese Patent Application No. 58-212592). These are aqueous solutions of copper (II) salt catalysts (copper sulfate or copper phosphate),
In this method, hydrogen peroxide is used as an oxidizing agent to decompose the waste solvent into inorganic substances such as water, carbon dioxide and phosphoric acid.

However, even with these wet oxidative decomposition methods, there is a problem that the decomposition liquid cannot be reused and becomes secondary waste in the method of using copper sulfate as a catalyst. Since copper acid is insoluble in water, it will be used as a dilute phosphoric acid solution, so this phosphoric acid acts as a stabilizer of hydrogen peroxide (decomposition inhibitor of hydrogen peroxide) and the peroxidation necessary for the reaction. There is a problem in that a very large amount of hydrogen water is required, resulting in high cost.

(Problems to be Solved by the Invention) A problem to be solved by the present invention is that in a method for decomposing a radioactive waste organic solvent by wet oxidative decomposition of a radioactive waste organic solvent using an oxidizing agent, a necessary amount of the oxidizing agent is required. And the amount of secondary waste generated.

[Structure of the Invention] (Means for Solving the Problems) The present invention solves the above problems by bringing a radioactive waste organic solvent into contact with an oxidizing agent in water in the presence of a copper oxide catalyst.

In the present invention, as the waste solvent to be subjected to oxidative decomposition,
The organic phosphate ester, the hydrocarbon, the mixed solvent of the both, and any other solvent mentioned above can be mentioned, but among them, the most representative of them is the waste after use in the reprocessing by the Purex method. It is a waste mixed solvent comprising waste TBP which is an extraction solvent and TBP and n-dodecane which is a diluent thereof.

As the oxidizer, hydrogen peroxide, oxygen gas, oxygen mixed gas, etc. can be used. In the case of hydrogen peroxide, it is preferable to carry out the reaction under atmospheric pressure at a reaction temperature of about 80 to 100 ° C. Temperature of 200 to 300 for oxygen gas or oxygen mixed gas
It is advisable to blow a high-temperature high-pressure gas at a temperature of 20 ° C. and a pressure of about 20 to 100 atm into water for the reaction.

Although copper oxide used as a catalyst is insoluble in water, it may be used by suspending it in water. The copper oxide can be used in any shape, but fine powder is most preferable from the viewpoint that the surface area is large and the reaction system can be effectively made uniform by stirring.

(Operation) In the present invention, when the radioactive waste organic solvent is brought into contact with the oxidizing agent in water in the presence of the copper oxide catalyst, the radioactive waste organic solvent is easily oxidatively decomposed.

For example, when hydrogen peroxide is used as an oxidizing agent, OH radicals are generated by the catalytic action of copper oxide in the reaction system, and the waste solvent is oxidatively decomposed by the OH radicals.

When the radioactive waste organic solvent is oxidatively decomposed, carbon dioxide and water are produced in the case of a hydrocarbon solvent, and phosphoric acid is produced in addition to this in the case of an organic phosphate ester such as a TBP solvent.

When the TBP solvent is oxidatively decomposed, part or all of the generated phosphoric acid reacts with part or all of the copper oxide catalyst present in the system by the action of the oxidizing agent to form copper phosphate, and phosphoric acid It is dissolved in a decomposition solution containing a to form a uniform aqueous solution. Therefore, the composition of the oxidative decomposition liquid of the organic solvent containing TBP is a dilute phosphoric acid aqueous solution of copper phosphate, which is used for the oxidative decomposition of TBP using the previously proposed dilute phosphoric acid aqueous solution of copper phosphate as a catalyst. It has the same composition as that of the decomposition liquid used in the method, and can be reused as the catalyst liquid in the method.

On the other hand, when a hydrocarbon solvent such as n-dodecane is oxidatively decomposed, the decomposition products are only carbon dioxide and water, so that the copper oxide used as a catalyst remains unreacted as a precipitate.

(Example) An example of the present invention will be described.

Example 1 In an oxidative decomposition tank equipped with an external heater and a stirrer, 320 ml of pure water, 18.5 g of TBP and 1.0 g of copper oxide powder (0.054 g / g TBP,
The amount of copper was 0.043 g / g TBP) and the mixture was heated to 100 ° C. while being sufficiently stirred and mixed so as to be uniform. The water evaporated at this time is made to flow back into the reaction solution by a water-cooled condenser. Subsequently, 330 ml of 35% hydrogen peroxide solution was continuously added to this mixed solution at a constant rate for 2 hours using a pump.

When the reaction solution was kept under the same condition for another hour after the addition of the hydrogen peroxide solution, a pale blue transparent uniform aqueous solution was obtained. When the total organic carbon content (TOC) in the decomposed liquid was measured, it was reduced to less than 1/10 of the total carbon content in the TBP initially used, that is, the decomposition rate using TOC as an index.
It was confirmed that 0% or more was obtained.

Comparative Example 1 320 ml of 25% phosphoric acid solution of 0.033 mol / copper phosphate (amount of metal copper ion in the liquid: 2.04 g, 0.127 g / g TBP) was used as a catalyst solution, and peroxidation as an oxidant was used. TBP 16g was oxidatively decomposed by using hydrogen water in an amount twice that in the case of Example 1. The final TOC decomposition rate of the decomposition solution in this case was 90%, which was almost the same as in Example 1.

Comparative Example 2 Using the same amount of copper phosphate solution catalyst as in Comparative Example 1,
The amount of hydrogen peroxide used was 330 ml, which was the same as in Example 1, and an attempt was made to oxidatively decompose 16 g of TBP. However, the organic phase remained in the liquid after completion of the reaction, and two phases were separated. Therefore, it was revealed that TBP was not decomposed under this reaction condition.

Example 2 Under the same conditions as in Example 1, oxidative decomposition was carried out using a mixed solvent of TBP / n-dodecane = 9/1 instead of TBP. The decomposed solution was a uniform transparent aqueous solution as in Example 2, and the TOC decomposition rate was 90% or more.

Example 3 The oxidative decomposition of a mixed solvent in the same ratio as in Example 2 was carried out using Example 2.
It was carried out under the same conditions with an amount scaled up to 70 times. The TOC decomposition rate of the final decomposition solution was 90%.

As shown in each of the above examples, in the case of decomposition of TBP, when copper phosphate was used as a catalyst, the catalyst amount was required to be at least 0.127 g per 1.0 g of TBP in terms of copper ions. On the other hand, when the copper oxide according to the present invention was used as a catalyst, even if the amount of copper was 0.043 g per 1.0 g of TBP sample, it could be sufficiently decomposed.

In this case, if the amount of catalytic copper is 0.064 g / g TBP (0.05 mol /) or less, all of the catalytic copper oxide used becomes copper phosphate and dissolves in the liquid, but if it is used more than that, excess The copper will remain unreacted as a precipitate as in the case of decomposing hydrocarbons such as n-dodecane.

Regarding the amount of hydrogen peroxide required for decomposition, when the target waste solvent is TBP only, the method using a copper phosphate catalyst as a catalyst requires 120 ml or more of a 30% aqueous solution of hydrogen peroxide for 1.0 g of the sample. However, this can be reduced to 60 ml in the method using copper oxide as a catalyst according to the present invention.

As described above, the decomposition treatment method according to the present invention can reduce the amount of chemicals used as compared with the conventional method using copper phosphate as a catalyst, which can reduce the cost and the target waste solvent. Excellent in both aspect ratio.

Further, as described above, when TBP is oxidatively decomposed in the present invention, the decomposition liquid is a dilute phosphoric acid solution of copper phosphate, and therefore, as it is or after concentration, a new waste organic solvent is oxidized. It can be reused as a catalyst solution for decomposition.

Furthermore, if the copper ions dissolved therein are separated and recovered as metallic copper from the oxidative decomposition liquid of the present invention by any method, the recovered metallic copper contains almost no radioactivity, so It is also possible to reuse as a catalyst in a new decomposition reaction. Although any method can be applied as a method for separating and recovering copper ions, the electrolysis method is the most suitable in that the amount of secondary waste is small, the operation is simple, and the cost is low. For example, using platinum for the anode and copper, graphite, etc. for the cathode,
It can be implemented by applying a DC voltage of about 3 V between both electrodes. As a result, the copper ions dissolved in the decomposition solution are easily deposited as metallic copper on the surface of the cathode plate and recovered. Since the steps of separating, recovering and reusing copper can be repeated any number of times, a large amount of waste solvent can be decomposed with a small amount of copper used for one reaction.

The oxidative decomposition liquid after the copper is recovered can be reused as a separating agent for the waste organic solvent described in the section of the prior art. If the radioactivity level is high and it is difficult to reuse, adjust the pH as necessary, evaporate the water content, and add a fixing agent to the obtained solid residue to solidify and stabilize it. It is also possible to do. In this case, since copper is recovered from the decomposition liquid and then solidified, it is expected that the volume reduction property is further increased as compared with the method of solidifying the decomposition liquid as it is.

On the other hand, as described above, in the case of oxidative decomposition of a hydrocarbon solvent or the like, the copper oxide used as a catalyst remains unchanged as a precipitate, so this is filtered and recovered, and after washing, a new waste solvent is used. It can be reused as a catalyst in the oxidative decomposition reaction of.

[Effects of the Invention] As described above, according to the present invention, a radioactive waste organic solvent whose effective treatment method has not been established conventionally can be easily oxidatively decomposed to an inorganic substance under mild conditions with a simple device. can do. Further, the present invention has an advantage that both the amount of the copper catalyst and the amount of the oxidizing agent can be smaller than those of the wet oxidative decomposition methods proposed so far.

Further, the decomposition liquid generated by the decomposition treatment of the present invention can be reused as it is or after being subjected to a concentration treatment and reused as a separating agent or a catalyst for the radioactive waste organic solvent. The amount generated can be greatly reduced.

Claims (4)

[Claims] 1. A method for decomposing a radioactive waste organic solvent, which comprises contacting the radioactive waste organic solvent with an oxidizing agent in water in the presence of a copper oxide catalyst. 2. The radioactive waste organic solvent is an organic phosphate ester,
The method of decomposing a radioactive waste organic solvent according to claim 1, which is a hydrocarbon or a mixed solvent of both of them. 3. A temperature of 80 to 80 using hydrogen peroxide as an oxidant.
The method for decomposing a radioactive waste organic solvent according to claim 1, which is carried out at 100 ° C. under atmospheric pressure. 4. The method of decomposing a radioactive waste organic solvent according to claim 1, which is carried out at a temperature of 200 to 300 ° C. and a pressure of 20 to 100 atm using oxygen or a mixed gas containing oxygen as an oxidant.