JPH0758349B2 - Method and apparatus for solidifying trialkyl phosphate containing radioactive substance - Google Patents

Description

The present invention relates to a method for solidifying an organic solvent containing a radioactive substance (hereinafter, simply referred to as a solvent), and particularly to a solvent suitable for solidifying an alkyl phosphate solvent. Solidification treatment method and solidification treatment device.

[Conventional technology]

For example, tributyl phosphate (hereinafter referred to as TBP), which is a trialkyl phosphate-based extraction solvent used in the reprocessing process of spent nuclear fuel, Brown and plutonium, is diluted with dodecane and kerosene, but nitric acid, etc. Deteriorates due to the chemical action and radiolysis of and is treated as waste after several recycles. At this time, the diluent and TBP are separated, and the diluent is incinerated, but TBP is often stored as a liquid. However, storage as a liquid is not desirable from the perspective of potential leakage and chemical stability due to the relatively high radioactivity contained in the waste solvent. Various solidification methods have been attempted in order to improve these points. A method of mixing with a plastic such as polyvinyl chloride to solidify, for example, the method described in JP-A-50-15998 is simple, but the chemical form of TBP itself does not change, so there remains a problem in chemical stability, In addition, the amount of plastic added increases the volume compared to storing TBP as it is, causing a problem of storage space.

[Problems to be solved by the invention]

The above-mentioned prior art does not consider conversion of trialkyl phosphate to a chemically stable form, and has a problem in volume reduction ratio (volume reduction ratio).

An object of the present invention is to convert a trialkyl phosphate solvent into a chemically stable form to obtain a solidified product having a high volume reduction rate.

[Means for solving problems]

In the method for solidifying trialkyl phosphate containing a radioactive substance, the above-mentioned object is converted into dialkyl phosphate, monoalkyl phosphate or a mixture thereof by hydrolyzing trialkyl phosphate, and This is accomplished by heating to the glass forming temperature in the presence of the glass forming agent.

[Action]

Taking the case of TBP as an example, this is (CH ₃ CH ₂ CH ₂
It is an organic compound having a molecular formula of CH ₂ O) ₃ PO. In order to make this a stable solid, it is conceivable to remove C and H by decomposition to form a phosphate. However, heat treatment at high temperature is necessary to perform such treatment, but TBP evaporates when it reaches the boiling point or higher and decomposes in the form of gas to form fine particles of phosphorus oxide or phosphate, which makes the equipment visible. It causes clogging. Further, since phosphorus oxide has a strong corrosiveness with respect to metal materials and ceramic materials, material corrosion due to scattered phosphorus oxide is also a problem. In order to eliminate such difficulties, it is necessary to suppress the evaporation of TBP.

On the other hand, TBP is hydrolyzed in an aqueous solution containing an oxidizing agent such as nitric acid and hydrogen peroxide, and is sequentially dibutyl phosphate (hereinafter DBP).
It is changed from monobutyl phosphate (hereinafter referred to as MBP) to phosphoric acid. Fig. 2 shows the results in 3N-nitric acid solution.
The time change of hydrolysis at 90 ° C is shown. When TBP is almost completely decomposed, it is a mixture of DBP, MBP and phosphoric acid. Unlike TBP, DBP and MBP have unstable molecular structures,
Decomposes easily at high temperatures. Therefore, hydrolyze TBP to D
Evaporation of TBP can be significantly suppressed by treating it at high temperature after converting it to BP, MBP and phosphoric acid. Evaporation is 1/10 in DBP compared to TBP
It can be reduced to about 0, and MBP reduces it. Hydrolysis can be achieved by suspending TBP in an aqueous solution containing an oxidizing agent or by mixing TBP with water and an oxidizing agent and heating since TBP dissolves in water to only about 200 ppm. As the oxidizing agent, nitric acid, oxygen, ozone potassium permanganate, perchloric acid, hydrogen peroxide, manganese dioxide, cerium oxide or the like can be used. It is desirable to be as high as possible in order to increase the hydrolysis rate of TBP, but the upper limit is determined by the boiling point of the solution.

The hydrolyzed decomposition product is converted into a stable chemical form by heat treatment at high temperature. When nothing is added, phosphorus oxide is produced, but it is chemically extremely unstable, so it is necessary to make it another compound as the final chemical form.

When Al ₂ O ₃ or B ₂ O ₃ is added to phosphoric acid and heated and melted at a high temperature to be vitrified, a phosphate glass having excellent chemical durability is obtained. Therefore, when an aluminum compound or a boron compound is added to the hydrolysis product of the solvent, heated and melted at a high temperature and then cooled, a phosphate glass is produced. Since phosphate glass can incorporate various ions in its network structure, it can stably trap fission products and actinides in the waste solvent. It is also possible to mix it with other radioactive waste to form a phosphate glass vitrified body. Generally, since phosphorus is pentavalent, not all of the oxygen atoms in the phosphate glass can bond with two phosphorus atoms. That is, since there is a large amount of non-crosslinked oxygen, phosphate glass has an open structure rather than silicate glass, and chemical durability is poor. However, for phosphate glass
Addition of about 10 to 20% of Al ₂ O ₃ or B ₂ O ₃ results in very good durability. It is considered that this is because Al ³ + and B ³ + have the structure of ortho-aluminum phosphate (or boron) in the glass and are connected as shown in FIG. Therefore, the total valence of phosphorus atoms in PO ₄ tetrahedra and AlO ₄ (or B
O ₄ ), the deficiency of the aluminum atoms in the tetrahedron is offset by the formation of the tetrahedron of both species,
It is considered that non-bridging oxygen changes into bridging oxygen of both tetrahedra and strengthens the structure. For this reason, the leaching rate of aluminophosphate glass has a value as low as that of borosilicate glass used as a high-level waste solidified product, and is extremely excellent as the final form of waste. Moreover, borosilicate glass requires heating and melting at a temperature of 1000 ° C. or higher for vitrification, but with aluminophosphate glass
A temperature of about 800 ° C is sufficient. For this reason, the amount of waste food of the equipment material is small, and it is possible to heat and melt the glass in a metal container such as stainless steel to vitrify it, and seal the container by welding or the like to form a solid waste product.

In addition, since phosphorus itself in TBP is the main constituent of the solidified body, a very high volume reduction ratio can be realized. Compared to storing as TBP as it is, the volume of plastic is 3 times and that of calcium phosphate is 1 time, but the volume is 1/20 for aluminophosphate glass, so the storage space is very small. Phosphoric acid glass tends to devitrify when kept at a temperature of about 400 ° C, but since the radioactivity in the waste solvent is low, it does not generate heat that raises the temperature of the solidified body and is stable over the long term. Can realize a solidified body with little radioactivity leaching.

In addition to TBP, alkyl phosphoric acid such as di-2-ethylhexyl phosphoric acid is used as an extraction solvent in group separation and the like.
Since these solvents only have the butyl group of TBP replaced with other alkyl groups, a stable solidified product can be obtained by the same treatment.

〔Example〕

An example of the present invention will be described below. FIG. 1 is a schematic diagram of a solidification processing apparatus used in the following examples, and will be described below. The waste solvent 1 composed mainly of TBP, which is obtained by removing the diluent from the spent extraction solvent from the spent nuclear fuel reprocessing step, is stored in the waste solvent storage tank 2 and transferred to the hydrolysis tank 4 by the transfer pump 3. To be done. The hydrolysis tank 4 contains a 6N aqueous nitric acid solution. Further, the hydrolysis tank 4 is provided with a stirrer 5 and a heater 6, and while maintaining the aqueous solution in the mixing portion 7 at 90 ° C., hydrolyzing while suspending TBP in the aqueous solution. TBP is decomposed into DBP first. 6
Since TBP has a lower specific gravity and DBP has a higher specific gravity than the normal nitric acid aqueous solution, undecomposed TBP floats in the separation section 8 and DBP sediments to form a layer 9 of decomposition products. MBP and phosphoric acid have higher solubility in water than DBP. Therefore, the temperature of the decomposition product layer 9 is controlled to be 50 ° C. or lower to prevent further decomposition. 90 ° C 6
The reaction rate constant for the hydrolysis of DBP to MBP in normal nitric acid is 1.88 × 10 ^-2 h ^{-1 at} 50 ° C, 2.96 × 10 ^-4 h ^-1 , which is close to two digits, and below 50 ° C it changes from DBP to MBP. Changes hardly occur. However, DBP easily forms a complex with MBP and phosphoric acid, and in the DBP of the decomposition product layer 9, MBP generated in the decomposition layer 7
It also contains phosphoric acid. Layer 9 of decomposition products is a valve
Open 10 and feed to the decomposition product reservoir 11. Aluminum nitrate 14 is fed from the glass forming agent supplier 13 to the decomposition product storage layer 11 and mixed by the mixer 15. The generated slurry 12 is passed through the pump 16 to the canister joint part.
It is delivered to a canister 19 which is connected via 17 and a packing 18. A pipe 20 to the off gas system is connected to the canister joint portion 17, and the gas generated through this pipe 20 is sent to the off gas system. The slurry 21 sent to the canister is heated to 800 ° C. in the heating furnace 22 and vitrified. The generated gases such as butanol, H ₂ O, NOx, CO, and CO ₂ are processed by off-gas system. After vitrification is complete, remove the canister 19 from the canister joint 17 and cap it.
Replace 23 and weld at weld 24. By the above operation, the solidified aluminophosphate glass 25 contained in the stainless canister 26 can be formed. According to this example, the hydrolysis product of TBP can be easily separated from TBP, and the waste solvent can be converted into a stable form as a solidified aluminophosphate glass as shown in the following formula. By such treatment, the volume can be reduced to 1/20 as compared with the case of storing TBP as it is.

Another embodiment of the present invention will be described with reference to FIG. To the aqueous solution in the autoclave 1, 1 vo
Add about 1%. Then, the autoclave 31 is closed, heated by the heater 32, and maintained at 200 ° C. At this time, the pressure inside the autoclave is maintained at about 30 atm. Oxygen gas is supplied from the oxygen cylinder 33 into the autoclave 31 through the bubbler 34. The TBP is decomposed by being agitated and mixed by the mixer 35 installed in the autoclave. The saturated solubility of DBP in water is about 0.08M, and MBP and phosphoric acid have large solubilities, so when TBP is completely decomposed, it becomes a complete aqueous solution. The pressure is controlled to around 30 atm by the pressure regulator 36, and when TBP disappears, the pressure regulator 3
Return to normal pressure by 6. When the pressure in the autoclave 31 reaches normal pressure, the valve 37 is opened and the aqueous solution in the autoclave 31 is transferred to the concentrator 39 using the pump 38. In the concentrator 9, the aqueous solution is boiled and concentrated by using the heater 40. The evaporated vapor is condensed in the condenser 42, and the gas is sent to the off-gas processing system through the pipe 43. After concentrating about 10 times in the concentrator 39, the valve 44 is opened to send the concentrated liquid inside to the ceramic melter 45. The ceramic melter 45 is heated to 850 ° C by the high temperature heater 46, and DBP, MBP
Is decomposed and water and butanol are evaporated. From the glass forming agent adding device 48, aluminum metal powder is supplied as a slurry to the ceramic melter, and the Al ₂ O ₃ content of the final solidified product is adjusted to about 10 wt%. Aluminum is
It melts at 850 ° C and reacts with molten phosphorus oxide to form an aluminophosphate glass melt, which accumulates at the bottom of the ceramic melter 45. The melt is supplied to the stainless steel canister 50 through the nozzle 49. The stainless steel canister is air-cooled and the melt solidifies as aluminophosphate glass 51 to form a solidified body. According to this embodiment, TBP
It is possible to safely convert radioactive elements such as ruthenium, which easily volatilize in an oxidizing atmosphere, into a reducing atmosphere due to the decomposition of organic matter, and to convert them into a stable aluminophosphate vitrified body without volatilizing.

〔The invention's effect〕

According to the present invention, a trialkyl phosphate solvent containing a radioactive substance can be converted into a solidified product having a chemically stable form and a high volume reduction property.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a solidification treatment apparatus used in one embodiment of the present invention, and FIG. 2 is a graph showing the hydrolysis of TBP in a 3N nitric acid solution at 90 ° C. with time as a parameter, FIG. FIG. 3 is a schematic view of a solidification processing apparatus used in another embodiment of the present invention. 1 ... Trialkyl phosphate solvent, 2 ... Storage tank for the solvent, 3 ... Pump, 4 ... Hydrolysis tank, 5 ... Stirrer,
6 ... Heater, 7 ... Mixing section, 8 ... Separation section, 9 ...
Decomposition product layer, 10 ... Valve, 11 ... Decomposition product storage tank, 12 ... Slurry, 13 ... Glass forming agent feeder, 14 ...
… Aluminum nitrate, 15 …… Mixer, 16 …… Pump, 17
…… Canister joint, 18 …… Packing, 19 …… Canister, 20 …… Piping, 21 …… Slurry, 22 …… Heating furnace, 23 …… Cap, 24 …… Welding, 25 …… Aluminophosphate glass solidification Body, 26 …… Canister, 31 …… Autoclave, 32 …… Heater, 33 …… Oxygen cylinder, 34 ……
Bubbler, 35 …… Mixer, 36 …… Pressure regulator, 37 ……
Valve, 38 ... Pump, 39 ... Concentrator, 40 ... Heater,
41 …… Piping, 42 …… Condenser, 43 …… Piping, 44 …… Valve, 45 …… Ceramic melter, 46 …… High temperature heater, 47
...... Piping, 48 …… Glass forming agent addition device, 49 …… Nozzle, 50 …… Stainless steel canister, 51 …… Aluminophosphate glass solidified body.

Claims (6)

[Claims] 1. A method for solidifying a trialkyl phosphate containing a radioactive substance, which is converted into a dialkyl phosphate, a monoalkyl phosphate or a mixture thereof by hydrolyzing the trialkyl phosphate, and Is heated to a glass forming temperature in the presence of a glass forming agent, a method for solidifying a trialkyl phosphate containing a radioactive substance. 2. The method for solidifying trialkyl phosphate containing a radioactive substance according to claim 1, wherein the organic solvent is tributyl phosphate. 3. A method for solidifying a trialkyl phosphate containing a radioactive substance according to claim 1 or 2, wherein the aqueous solution is hydrolyzed by adding an aqueous nitric acid solution. 4. The glass forming agent according to claim 1, wherein the glass forming agent is aluminum or a compound thereof.
Item 4. A method for solidifying a trialkyl phosphate containing the radioactive substance according to Item 3 or 3. 5. The phosphoric acid containing a radioactive substance according to claim 4, wherein the trialkyl phosphate is a waste solvent of the trialkyl phosphate used as an extraction solvent for the dissolved solution of spent nuclear fuel. Method for solidifying trialkyl. 6. (a) An outlet for taking out a hydrolysis product at the bottom,
A hydrolysis treatment tank equipped with a mixer and a heater on the upper part, (b) a mixer equipped with a glass forming agent feeder, and (c) a trialkyl phosphate solidification treatment apparatus containing a radioactive substance in sequence having a heating furnace.