JP2624898B2 - Decomposition method of radioactive waste organic solvent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

 [Object of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decomposing radioactive waste organic solvent generated from a facility for reprocessing spent nuclear fuel.

[0002]

2. Description of the Related Art Generally, spent nuclear fuel generated from a nuclear power plant contains useful substances such as uranium (U) or plutonium (Pu). In order to effectively use these useful substances as fuel resources, it is common practice to reprocess spent nuclear fuel in reprocessing facilities, separate useful elements such as uranium and plutonium from fission products, and recover them as nuclear fuel materials. Have been done.

As a method of this reprocessing, a method of dissolving spent nuclear fuel in nitric acid, liquid-liquid extraction of the nitric acid solution with an organic solvent to separate and recover uranium and plutonium (PUREX method) is generally adopted. ing.

In this method, a phosphate ester such as tributyl phosphate (TBP) and an aliphatic hydrocarbon such as n-dodecane as its diluent are used as an organic solvent for extraction in a ratio of 3: 7.
A mixed solvent mixed at a ratio of about (volume ratio) is generally used. The uranium and plutonium extracted with this mixed solvent are then back-extracted with dilute aqueous nitric acid, concentrated and processed. On the other hand, the used solvent (waste solvent) is contaminated with radioactivity and is deteriorated by radiation damage, and is treated as waste (waste solvent). As a method of treating the used organic solvent (waste organic solvent), conventionally, the following two methods are known.

(1) A method of treating a used waste organic solvent with concentrated phosphoric acid to separate it into an organic phosphate ester component, then thermally decomposing the former, and reusing or incinerating the latter (ET)
R-267, 1980).

(2) A method in which both components are separated by a method similar to the above (1), and the separated organic phosphate component is directly solidified with asphalt or a thermoplastic resin (KFK-2
212, 1974).

However, of the above two methods,
In the case of the thermal decomposition method (1), anhydrous phosphoric acid and hydrocarbons are generated by decomposition, but the former may corrode equipment materials, and the latter may be post-treatment such as condensation, oil-water separation, and incineration. There is a problem that the operation is required and the process becomes complicated. Also,
In the case of the solidification method (2), the amount of waste (waste organic solvent) mixed with the solidifying agent cannot be increased so much. There is a problem that it is considerably deteriorated as compared with the case of a solidified product.

For this reason, the present inventors have proposed that radioactive waste TB
A wet oxidative decomposition treatment method using a waste organic phosphate ester solvent such as P or a mixed organic solvent containing these has been proposed (Japanese Patent Application Nos. 58-169205 and 58-2).
No. 12592). In this method, hydrogen peroxide is used as an oxidizing agent in an aqueous solution of a copper (II) salt catalyst (copper sulfate or copper phosphate) or an aqueous solution in which copper oxide powder is dispersed, and water, carbon dioxide and It decomposes to inorganic substances such as phosphoric acid. In this method, the temperature inside the reactor is maintained at the boiling point of water as a diluent, so that the decomposition treatment can be performed under relatively low temperature conditions.

[0009] In the wet oxidative decomposition method, the pressure in the reactor is approximately atmospheric pressure, and the temperature of the reaction solution is about 100 ° C, which is the boiling point of water as a diluent. Here, the reason why the temperature does not necessarily reach exactly 100 ° C. is that when handling radioactive materials, it is usual to operate at least at atmospheric pressure or less, that is, at a slightly negative pressure in order to prevent the contents from leaking out of the reactor. This is because the temperature of the reaction solution is slightly lower than 100 ° C.

[0010]

The waste organic solvent to be decomposed is mainly composed of TBP and nBP.
-Dodecane. In particular, the flash point of n-dodecane is about 70 ° C., well below the operating temperature of 100 ° C. which has been performed so far. Considering safety, the composition of the gas phase inside the reactor under normal operating conditions is almost water vapor, and the concentration of n-dodecane vapor is far outside the flammable region. Absent. However, if the concentration of the waste organic solvent inside the reactor increases for some reason, the partial pressure of vapor of n-dodecane may increase, and the n-dodecane may enter the combustible region. Various measures have been taken to prevent such conditions from being met in an actual device, but at present, drastic measures have been desired.

In view of the above, it is an object of the present invention to ensure the safety inside the reactor essentially by setting the temperature of the reaction solution inside the reactor below the flash point of the waste organic solvent. I do. [Configuration of the Invention]

[0012]

In order to achieve the above object, a method for decomposing a radioactive waste organic solvent according to the present invention comprises:
In a method for decomposing a radioactive waste organic solvent, in which a radioactive waste organic solvent is introduced into a reactor and brought into contact with an oxidant and an additive in water to oxidize and decompose, the boiling point of water in the reactor is lower than the flash point of the waste organic solvent. Reducing the pressure in the reactor to a temperature,
This decomposition reaction is performed at a temperature lower than the flash point of the waste organic solvent.

[0013]

According to the present invention constructed as described above, the temperature of the reaction solution substantially coincides with the boiling point of water, which is the main component. By keeping the temperature below the flash point, the operating temperature can be lower than the flash point of the waste organic solvent, thereby essentially ensuring the safety inside the reactor.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of a decomposition processing apparatus for explaining an embodiment of a decomposition processing method according to the present invention.
In FIG. 1, a double reactor 1 includes a stirrer 2, a jacket 3, a temperature indicator 4, a pressure indicator 5, and a liquid level indicator 6.
The waste organic solvent tank 7, the dilution water tank 8 containing the additive, and the hydrogen peroxide water tank 9 are connected to the reactor 1 via respective supply pipes 10, 11 and 12. Supply pipes 10 and 12 connected to the waste organic solvent tank 7 and the hydrogen peroxide water tank 9 respectively have a pressure regulating valve 1.
3 and 14 are provided to suppress fluctuations in the flow rate due to a pressure difference between the inside and the outside of the reactor 1. Further, on the downstream side of the reactor 1, a condenser 15 and a condensed water extracting pump 16 are provided, and a condensed water receiving tank 17 is provided. Next, an operation of decomposing a waste organic solvent using the above-described apparatus will be described.

First, the inside of the reactor 1 is set to a predetermined pressure, that is, a pressure at which the boiling point of water in the reactor 1 becomes lower than the flash point of the waste organic solvent to be decomposed, for example, 186.
At a pressure of 7 mmHg (the boiling point of water at this pressure is 65 ° C.), a predetermined amount of a diluent (water) containing an additive (copper oxide: CuO) and a waste organic solvent are fed into the reactor 1 through the supply pipes 10 and 11. Then, the stirring is started by the stirrer 2 and the heating is also started. Then, while confirming that the temperature of the reaction solution reached the boiling point of water and became substantially constant (65 ° C.) while stirring the reaction solution with the stirrer 2, the waste organic solvent (normal reprocessing) was placed in the reactor 1. Composition of waste organic solvent used in the facility, TBP 30%, n-dodecane 70%: n
Dodecane has a flash point of about 70 ° C.) and an oxidizing agent (aqueous hydrogen peroxide: concentration 35%) is supplied through the respective supply pipes 10 and 12 to start oxidative decomposition of the waste organic solvent.

The gas generated by the above-described oxidative decomposition operation leaves the reactor 1 and reaches the condenser 15, where water vapor in the gas is condensed. Then, the condensed water generated here is discharged to the condensed water receiving tank 17 by the condensed water extraction pump 16, and the condensed water extraction pump 16
The condensed water is discharged while being controlled so that the liquid level inside the reactor 1 is always constant, in conjunction with the liquid level gauge 6 provided in the reactor 1. The pressure conditions up to the condensed water receiving tank 17 are the same as those of the reactor 1, that is, for example, 186.7 mmHg. When the supply of the predetermined amount of the waste organic solvent is completed and the supply of the predetermined amount of the oxidizing agent is completed, the decomposition processing of the waste organic solvent is completed.

As a result of the decomposition treatment of the waste organic solvent as described above, it was confirmed that a decomposition rate of 95% or more, which is almost the same as in the case where the pressure inside the reactor 1 was at atmospheric pressure, could be obtained. Have been.

[0019]

Since the present invention has the above configuration,
Decompose the radiation waste organic solvent generated from the reprocessing facility by oxidative decomposition using hydrogen peroxide water under operating temperature conditions below the flash point of n-dodecane, which poses a safety problem. Thus, safety in the reactor can be essentially ensured.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a decomposition processing apparatus for explaining a decomposition processing method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reactor 4 Temperature indicator 5 Pressure indicator 6 Liquid level indicator 7 Waste organic solvent tank 8 Dilution water tank containing additives 9 Hydrogen peroxide water tank 15 Condenser 16 Condensate extraction pump

Claims (1)

(57) [Claims] 1. A method for decomposing a radioactive waste organic solvent, which comprises introducing a radioactive waste organic solvent into a reactor and bringing it into contact with an oxidizing agent and an additive in water to oxidatively decompose the waste organic solvent. A process for decomposing a radioactive waste organic solvent, wherein the pressure in the reactor is reduced so as to be a temperature not higher than the flash point of the waste organic solvent, and the decomposition reaction is carried out at a temperature not higher than the flash point of the waste organic solvent.