JPS62214399A - Method of processing radioactive waste organic 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 [Object of the Invention] (Industrial Application Field) The present invention mainly relates to a method for treating radioactive waste organic solvent generated from a spent nuclear fuel reprocessing facility.

(Prior art) In nuclear power generation, in order to effectively utilize fuel resources, spent nuclear fuel is reprocessed to separate useful components such as uranium and plutonium from fission products, which are unnecessary components.
Efforts are being made to reuse these 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 plutonium (so-called Pu
rex method) is the usual method.

In this method, an organic phosphoric acid ester such as tributyl phosphate (hereinafter referred to as TBP) and a hydrocarbon such as n-dodecane as a diluent are mixed in a 3:7 (extraction) container.
A mixed solvent is generally used in which the uranium and plutonium are mixed at a ratio of about 100% by volume, and after the uranium and plutonium are extracted with this mixed solvent, they are 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 the used waste organic solvent is treated with concentrated phosphoric acid to separate it into a phosphoric acid nisple and a diluent component, the former is then thermally decomposed, and the latter is reused or discarded [ETR
-287 (1980.

(b) A method in which both components are separated using a method similar to (a), and the separated phosphate ester component is directly solidified with asphalt or thermoplastic resin [KFK-2212 (1974)]
] There is.

(Problems to be Solved by the Invention) Among these, the thermal decomposition method (a) generates anhydrous phosphoric acid and hydrocarbons due to decomposition, or the former may corrode the equipment materials. The latter has the disadvantage that it requires post-processing operations such as condensation, oil/water separation, and incineration, making the process complicated.

Furthermore, in the case of the solidification method described above, there is a limit to the amount of waste (i.e. waste dissolved There is a problem. Moreover, the properties of the assimilate are also less reliable than those of the assimilate of inorganic solid waste.

The present invention has been made to address the above-mentioned situation, and provides a highly reliable method for treating radioactive waste organic solvents.

[Structure of the Invention] (Means for Solving the Problems) That is, the present invention involves reacting a radioactive waste organic solvent with an oxidizing agent to oxidatively decompose the organic solvent in a wet manner, and then adding the decomposed liquid to water. Neutralize the inorganic phosphate using an alkali containing a metal species that forms an insoluble inorganic phosphate as a neutralizing agent, convert the resulting inorganic phosphate into a dry powder, granulate this dry powder into pellets, and then granulate the powder. This is a method for processing radioactive waste organic solvent, which is characterized by filling a storage container with body particles.

In the present invention, the radioactive waste organic solvent to be treated includes organic phosphate esters, hydrocarbons, and mixed solvents thereof, but the most typical among these is the above-mentioned TBP and its dilution. agent, n-dodecane, or a mixture thereof. - Waste phosphoric acid esters other than rBP include dibutyl phosphate (DBP>), which is a partial decomposition product of TBP, and monobutyl phosphate (MB
P) etc.

Such a radioactive waste organic solvent is reacted with an oxidizing agent to undergo oxidative decomposition, and the oxidizing agent used is preferably hydrogen peroxide or oxygen gas. When a hydrogen peroxide solution is used, the hydrogen peroxide solution is decomposed by the metal steel to generate OH radicals, and the waste solvent is oxidized and decomposed by the action of these radicals. The reaction conditions are atmospheric pressure, approximately 80 to 100
A temperature of °C is preferred.

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

Any shape can be used as metallic copper, but
In view of the size of the surface area and the purpose of making the reaction system more uniform by shading, it is most preferable to use a fine powder.

On the other hand, when using oxygen gas as an oxidizing agent, oxygen or a mixed gas containing oxygen is used without a catalyst or with copper sulfate as a catalyst at a temperature of 200-300°C and a pressure of 20-1100°C.
The appropriate method is to blow in with at.

Such an oxidative decomposition reaction ultimately produces carbon dioxide and water, and also produces phosphoric acid if the waste solvent contains a phosphate ester neck. Some or all of the generated phosphoric acid forms a copper salt (copper phosphate) that reacts with some or all of the metallic copper catalyst present in the system by the action of hydrogen peroxide, and is dissolved in the decomposition solution. to a homogeneous aqueous solution as a solvent.

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 after oxidative decomposition of waste solvents containing phosphoric acid esters such as TBP contains the above-mentioned phosphoric acid, so it is acidic (raw). This is neutralized using an alkali. The phosphate is then precipitated as a phosphate, which is then dried and powdered.

NaOH is usually used to neutralize acidic waste liquid, but when used to neutralize phosphoric acid, Na2HP O4.1
2H20 is produced, which needs to be evaporated to give a water solubility of 17 w%.

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 precipitation, centrifugation, etc., and evaporation and concentration operations are not necessary.

Furthermore, when the neutralized phosphate is precipitated, coprecipitation of the radioactivity dissolved in the decomposition solution can be expected to occur.

Metal species that form water-insoluble phosphates include Ca,
[3a, etc.] Therefore, as the alkali used for neutralizing the decomposed liquid, hydroxides or carbonates of each of the above-mentioned metal species, specifically Ca(OH)2, CaCO3, etc.
, Ba (OH) 2 , and BaCO 3 are representative examples, but of course the present invention is not limited to these four types.

The phosphates produced when the above neutralizing agent is used are Ca1n (PO4) 6 (01-1>2,
BaHPO4, all of which have the common properties of being almost insoluble in water, having a specific gravity greater than 2.5, and being thermally stable, allowing for easy separation and subsequent drying and powdering. .

A binder is added to the dry powdered phosphate and the mixture is stirred and granulated into pellets, and the resulting powder particles are filled into a storage container. The pellets filled in the storage container can be stored as is, or they can be filled with thermoplastic resin, asphalt,
It can also be solidified by adding an assimilating agent such as glass.

The binder used here is a thermoplastic resin such as chlorinated polyethylene, and the amount of the binder added to the dry powder is about 5 to 20%.

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

(Function) As described above, according to the present invention, it is possible to oxidize and decompose radioactive waste organic solvent, which has not been produced in the past, with a simple device and with a high decomposition rate. It can be easily stored and stored by converting it into a stable inorganic dry powder and filling it into a storage container in the form of rough pellet-like powder particles.The oxidative decomposition conditions of the present invention are relatively mild. , less burden on equipment and equipment materials,
Furthermore, there is almost no transfer of radioactivity into the generated waste gas.

Moreover, 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. In general, the phosphate produced by neutralizing this decomposition solution is insoluble in water and has a sufficiently large specific gravity, so that it becomes a precipitate and can be easily separated. Since the separated phosphate is thermally stable, it can be dried and powdered, and a binder can be added to form stable pellet-like powder particles and filled into storage containers, which will help future radioactive waste disposal trends. Can be easily stored until clear. Also,
If necessary, a solidifying agent can be added to solidify and stabilize the material, allowing for flexible processing.

(Example) Next, a specific example of the method for treating radioactive waste organic solvent of the present invention will be described. FIG. 1 is a flow diagram illustrating the process of this example.

The radioactive waste organic solvent used for the treatment was a mixed solvent of TBP and n-dodecane with a volume ratio of 3, but
-The volume ratio of dodecane is not limited to the above value.

In an oxidative decomposition tank equipped with an external heater and stirrer, 10 g of pure water, 31.8 g (1 g) of metallic copper powder, and 154 TBP
．． A mixed solvent of 5 mλ and n-dodecane 360.5 mβ was heated at 100°C while stirring the mixture to make it homogeneous.
heated to. This evaporated water is refluxed into the reaction solution through a water-cooled condenser. Next, add 17.5J2.60% hydrogen peroxide solution to this mixture. was added continuously at a constant rate over 2 hours. After the addition of the hydrogen peroxide solution was completed, the reaction solution was kept under the same conditions for another 1 hour, and the result was 21.4. A decomposition solution of e was obtained as a pale blue, transparent, homogeneous aqueous solution. The amount of total organic carbon contained in the decomposed liquid (hereinafter referred to as TOC).

), it was found that the amount of carbon contained in TBP had decreased to less than 1/100 of the total carbon content in the initially used TBP.
It was confirmed that the decomposition rate using C as an index was almost 100%.

Next, this decomposed solution was mixed into a 1W% calcium hydroxide suspension 3.
．． When completely neutralized by adding 4°C, a pale blue precipitate of copper hydroxide and calcium hydroxyhexanoate (Ca+o
A white precipitate of (PO4) s (OH) 2 ) was formed, and a neutralized solution 24.8β of which the supernatant was clear and colorless was obtained.

The supernatant liquids of 2i, i, and c were discarded, and the remaining precipitates of 3 and 7β were dried by heating at about 100''C, and 78.9 g of dry powder with no deliquescent property was obtained in 2 to 3 hours.

Furthermore, when 9.9 g of chlorinated polyethylene was added to this dry powder of 78.9 (J) and granulated in a stirring granulator, powder particles in the shape of pellets of 88.8 (L) were obtained. .

It was confirmed that the powder particles had a weight ratio of 174 or less.

[Effects of the Invention] As described above, according to the present invention, radioactive waste organic solvent, for which no effective treatment method has been established, can be treated using a simple device and under mild conditions that are easy to implement. The radioactive waste organic solvent is almost completely decomposed and reduced in volume into inorganic substances, then turned into a stable dry powder, granulated into pellets, and the granulated powder particles are filled into a storage container. We were able to provide a method for treating radioactive waste organic solvents.

[Brief explanation of drawings]

FIG. 1 is a flow diagram showing a specific example of the present invention. Applicant: Toshiba Corporation Applicant: Japan Atomic Energy Corporation Representative Patent Attorney Sasa Suyama - Waste organic/solvent Figure 1

Claims (5)

[Claims] (1) Radioactive waste organic solvent containing organic phosphate esters and hydrocarbons generated in the process of regenerating spent fuel is oxidized and decomposed using an oxidizing agent, and the resulting decomposition liquid is used to generate phosphates that are insoluble in water. The resulting precipitate containing phosphate as the main component is separated, then dried and pulverized, and the resulting powder containing phosphate as the main component is mixed with a binder. 1. A method for treating radioactive waste organic solvent, the method comprising: adding the organic solvent to the organic solvent, stirring and granulating the solvent, and filling it into a storage container. (2) 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. Method. (3) The oxidative decomposition reaction is carried out using oxygen or a mixed gas containing oxygen as an oxidizing agent at a temperature of 200 to 300°C and a pressure of 20 to 100 atm. How to treat organic solvents. (4) Neutralizing agents include Ca(OH)_2, CaCO_3, B
1 selected from a(OH)_2 and BaCO_3
The method for treating radioactive waste organic solvents according to any one of claims 1 to 3, wherein the method is one or more kinds of radioactive waste organic solvents. (5) The method for treating radioactive waste organic solvent according to any one of claims 1 to 4, wherein the binder is a thermoplastic resin.