JPS62129799A - Method of decomposing and processing radioactive wastedorganic solvent - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention mainly relates to a method for decomposing radioactive waste organic solvents generated from spent nuclear fuel reprocessing facilities.

[Technical blueprint of the invention and its problems] In nuclear power generation, in order to effectively utilize fuel resources, spent nuclear fuel is reprocessed to remove useful components such as uranium and plu-1-nium through nuclear fission, which is an unnecessary component. They are separated from the products and reused as nuclear fuel. This reprocessing method involves dissolving spent nuclear fuel in nitric acid, extracting this nitric acid solution with an organic solvent, and separating and recovering uranium and platonium (so-called P
urex method) is the usual method.

In this method, the extraction solvent is 1-butyl phosphate (
Hereinafter referred to as TBP. ) and other hydrocarbons such as n-dodecane in a diluent of 3:7 (
A mixed solvent is generally used in which the uranium and plutonium are extracted with this mixed solvent and then back-extracted with dilute nitric acid water. The used mixed solvent is radioactively contaminated and a portion of it has deteriorated due to radiation damage, so it is disposed of as radioactive waste.

The following two methods are used to treat this used waste organic solvent.

(a) A method in which a used waste organic solvent is treated with concentrated phosphoric acid to separate it into an organic phosphate component and a diluent component, and then the former is thermally decomposed and the latter is reused or incinerated [ET
R-287 (1980)].

(A method in which both components are separated by the same method as in (a) and the separated phosphate ester component is directly solidified with asphalt or thermoplastic resin [KFK-2212 (1974)
] In the case of the thermal decomposition method described in (a) above, anhydrous phosphoric acid and hydrocarbons are generated by decomposition, but the former may corrode equipment materials, and the latter may cause condensation, oil/water separation, incineration, etc. This method has the disadvantage that post-processing operations are required, which complicates the process.

In addition, in the case of the solidification method described above, there is a limit to the amount of waste (i.e., waste solvent) that can be mixed into the assimilation agent, and it is impossible to increase this too much, so it is difficult to reduce the volume. There's a problem. Moreover, the properties of the assimilated product are not as good as those of the assimilated product of inorganic solid waste.

[Object of the invention] The present invention was made in response to the above-mentioned situation, and
Under mild conditions that are easy to carry out, a method for decomposing radioactive waste organic solvents is provided, which almost completely decomposes and reduces the volume of the radioactive waste organic solvent to a precipitate-free product, and then converts it into a stable dry powder. The purpose is to provide

[Summary of the Invention] That is, the present invention involves wet oxidative decomposition of a radioactive waste organic solvent by reacting it with an oxidizing agent in water, and converting the decomposed liquid into a water-insoluble phosphate. This is a method for decomposing radioactive waste organic solvents, which is characterized by neutralizing using an alkali containing metal species as a neutralizing agent, and then drying and powdering the resulting phosphate.

[Embodiments of the invention] The present invention will be described in more detail below based on Examples.

Radioactive waste organic solvents to be decomposed include organic phosphate esters, hydrocarbons, and mixed solvents thereof, but the most typical among these is the aforementioned TBP and its diluent. Filter with n-dodecane or a mixture thereof.

Examples of waste phosphoric acid esters other than TBP include dibutyl phosphate (DBP>) and monobutyl phosphate (MBP>), which are partial decomposition products of TBP.

Such a radioactive waste organic solvent is reacted with an oxidizing agent in water to undergo oxidative decomposition, and the oxidizing agent used is preferably hydrogen peroxide or oxygen gas.

When hydrogen peroxide is used, the hydrogen peroxide is decomposed by metallic copper to generate OH radicals, and the waste solvent is oxidatively decomposed by the action of these radicals.

The reaction conditions are preferably atmospheric pressure and a temperature of about 80 to 100'C.

Since the metal copper used as a catalyst is insoluble in water, it is used in a suspended state in water.

Metallic copper of any shape can be used, but
From the viewpoint of increasing the surface area and making the reaction system more uniform by stirring, it is most preferable to use a fine powder.

On the other hand, when oxygen gas is used as the oxidizing agent, it is appropriate to use a method in which oxygen or a mixed gas containing oxygen is blown in at a temperature of 200 to 300'C and a pressure of 20 to ioatm without a catalyst or by using copper sulfate as a catalyst.

Such an oxidative decomposition reaction ultimately produces carbon dioxide and water, and if the waste solvent contains phosphoric acid esters, phosphoric acid is also produced. Part or all of the generated phosphoric acid reacts with part or all of the metallic copper catalyst present in the system due to the action of hydrogen peroxide to form the corresponding copper salt (
Copper phosphate) is dissolved in the decomposition solution to form a homogeneous aqueous solution.

For example, in the case of oxidative decomposition of a hydrocarbon solvent such as n-dodecane, the decomposition products are only carbon dioxide and water, and the metallic copper used as a catalyst remains as a precipitate, but TBP
In the case of oxidative decomposition, part or all of the catalytic copper becomes copper phosphate and dissolves.

The decomposition solution produced by oxidative decomposition of a waste solvent containing a phosphoric acid ester such as TBP contains the above-mentioned phosphoric acid and is therefore acidic. This is neutralized using an alkali to precipitate a phosphate, and then this phosphate is dried and powdered.

NaOH is usually used to neutralize acidic waste liquid, but when used to neutralize phosphoric acid, Na2l-IP04
- 12H20 is produced, which has a water solubility of 17w% and therefore needs to be evaporated and concentrated.

Therefore, in the case of the present invention, an alkali containing a metal species which produces a water-insoluble phosphate upon neutralization is used as a neutralizing agent. As a result, the produced phosphate can be separated from the neutralized solution by sedimentation, centrifugation, etc., and evaporation and concentration operations are not necessary. Furthermore, when this neutralized phosphate is precipitated, it can be expected that coprecipitation of the activation dissolved in the decomposition solution will occur.

Metal species that form water-insoluble phosphates include Ca,
(3a, etc.) Therefore, as the alkali used for neutralizing the decomposition liquid, hydroxides or carbonates of each of the above-mentioned metals, specifically Ca(OH1)2, CaCO3
, Ba (OH) 2 , and BaCOx can be cited as representative examples, but of course the present invention is not limited to these four types. The phosphates produced when using the above neutralizing agent are respectively Ca 6 (PO4)In
(OH) 2 , 13a HPO4, both of which are almost insoluble in water, have a specific gravity greater than 2.5, and have the common properties of being thermally stable, making them easy to separate and subsequently dry and powder. can be implemented.

The dry powdered phosphate can be stored as it is, or if necessary, it can be treated by adding a solidifying agent to solidify and stabilize it.

On the other hand, the supernatant liquid after separating the precipitate is treated in a normal low-level waste liquid treatment process.

Next, a specific example of the method for decomposing radioactive waste organic solvent of the present invention will be explained. The figure is a flow diagram illustrating the operation of this example.

In addition, the 15Ii and waste organic solvent to be treated was a mixed solvent of TBP and rhododecane in a volume ratio of 3:7,
The volume ratio of TBP and n-dodecane is not limited to the above value.

In an oxidative decomposition tank equipped with an external heater and stirrer, 10λ of pure water, 31.8a of metallic copper powder, and 154 of TBP
．． A mixed solvent of 5 ml and 360.5 mβ of n-dodecane was sufficiently stirred and mixed to be uniform, and then heated to 100°C. The water that evaporates is refluxed into the reaction solution in a water-cooled condenser.

Subsequently, 30λ of 35% hydrogen peroxide solution was continuously added to this mixed solution at a constant rate over a period of 2 hours.

After the addition of the hydrogen peroxide solution was completed, the reaction solution was kept under the same conditions for an additional hour, and the decomposition solution of 33.9.2, which was a pale blue, transparent, homogeneous aqueous solution, was raised. When the total organic carbon content (hereinafter referred to as TOC) in the decomposition solution was measured, it was found to be 1/100 of the total carbon content in the TBP originally used.
It was confirmed that the decomposition rate using TOC as an index was approximately 100%.

Next, add this decomposition solution to 1w% calcium hydroxide suspension solution 3.
．． 4f! When completely neutralized by adding , a pale blue precipitate of copper hydroxide and hydroxyhexanoic acid + calcium (Ca
Neutralized solution 37 in which a white precipitate of l[l (PO4) 6 (Ot-1) 2 ) is formed and the supernatant liquid is colorless and transparent.
．． 3λ was obtained.

33.6m 12. The supernatant liquid was discarded, and the remaining precipitate at 3.7°C was heat-dried at 100°C for 7Ill, and 78.9C1 of dry powder with no tide VR properties was obtained in 2 to 3 hours. This dry powder has an M ratio of 115 or less and an apparent volume ratio of 17'.
It was confirmed that the number had decreased to 3 or less.

[Effect of the invention 1 As described above, according to the present invention, radioactive waste organic solvent, for which no effective treatment method has been established, can be oxidized and decomposed easily with a simple device and at a high decomposition rate. Can be made into a stable inorganic dry powder. Since the oxidative decomposition conditions of the present invention are relatively mild, there is little burden on equipment and equipment side slopes, and there is almost no transfer of radioactivity into the generated exhaust gas. Further, since the decomposition reaction proceeds until the waste solvent is finally decomposed into carbon dioxide, water (steam), and inorganic acid, only water-soluble inorganic compounds remain in the treated decomposition liquid. Furthermore, the phosphate produced by the neutralization of this aliquot is insoluble in water and has a sufficiently large structure, so that it becomes a precipitate and can be easily separated. Since the separated phosphate is thermally stable, it can be easily handled by drying it into a powder.
The volume is reduced to less than /3. The dry powdered phosphate can be stored as it is, or if necessary, it can be solidified and stabilized by adding a solidifying agent, allowing for flexible processing.

[Brief explanation of drawings]

The drawing is a flow diagram illustrating a specific example of the present invention.

Claims (5)

[Claims] (1) Radioactive waste organic solvent is oxidatively decomposed by reacting it with an oxidizing agent in water, and then the decomposed liquid after this oxidative decomposition reaction is neutralized with a neutralizing agent containing a metal species that forms a phosphate that is insoluble in water. 1. A method for decomposing a radioactive waste organic solvent, which comprises performing a neutralization treatment, separating a precipitate consisting of a phosphate produced by the neutralization treatment, and then drying and powdering the separated produced phosphate. (2) The method for decomposing a radioactive waste organic solvent according to claim 1, wherein the radioactive waste organic solvent has an organic phosphate ester as a main component. (3) The oxidative decomposition reaction is carried out at a temperature of 80 to 100° C. and under atmospheric pressure using hydrogen peroxide as an oxidizing agent and metallic copper as a catalyst. Processing method. (4) The radioactive waste organic solvent according to claim 1, wherein the oxidative decomposition reaction 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 oxidizing agent. Decomposition processing method. (5) Neutralizing agents include Ca(OH)_2, CaCO_3, B
1 selected from a(OH)_2 and BaCO_3
The method for decomposing radioactive waste organic solvents according to claim 1, wherein the method is one or more kinds of radioactive waste organic solvents.