JP4665103B2 - Vitrification method of radioactive liquid waste - Google Patents

Description

  The present invention relates to a method for converting a radioactive liquid waste into a physically and chemically stable vitrified body, and more specifically, by using an alkoxide as a main glass raw material, a homogeneous glass at a temperature below the softening point. The present invention relates to a method capable of producing a solidified body.

  Normally, radioactive liquid waste generated from nuclear facilities such as reprocessing plants is concentrated, then supplied to the glass melting furnace together with the glass raw material, melted at a high temperature of 1000 ° C or higher, and then physically and chemically stable glass. It is processed into a solidified body. The obtained vitrified body is stored in a radioactive waste storage facility. Such techniques are well known in the art. Various glass melting furnaces have been developed depending on the heating method, furnace material, type, and the like. As a typical example, there is a direct energization method in which a main electrode is installed so as to face the side wall of the furnace and melted by directly energizing glass. About the vitrification process of the radioactive waste liquid by the glass melting furnace of a direct electricity system, patent document 1 etc. have description, for example. However, since molten glass is very corrosive, the melting furnace needs to be designed and maintained in response to erosion, which increases the cost.

By the way, about 98% of the radioactivity of the high-level radioactive liquid waste is occupied by cesium, and since the boiling point of cesium is about 700 ° C., the conventional glass melt solidification processing method requires volatilization measures such as cesium recovery. It has become. For this reason, a facility for recovering radioactive elements such as cesium has to be attached, and in that respect, there is a drawback that the cost required for the facility and maintenance increases.
JP 2007-24816 A

  The problem to be solved by the present invention is to reduce the erosion of equipment materials and volatilization of radioactive elements by devising vitrification at a low temperature below the softening point of glass during vitrification treatment of radioactive liquid waste. And to be able to significantly reduce maintenance costs.

  In the present invention, a water-soluble raw material for obtaining glass properties necessary as a vitrified body and a solvent for promoting hydrolysis of an alkoxide are added to a radioactive liquid waste, and a lipophilic alkoxide raw material is further added. Mixing, proceeding with hydrolysis and polymerization reaction to gel the sol, drying the resulting wet gel, and sintering the dried gel at a temperature below the glass softening point by heat treatment It is the vitrification method of the radioactive waste liquid characterized.

Here, when drying the wet gel, a stress is applied between the wet portion and a dry portion of the gel, than which may cause cracking, drying by grinding a moist gel. After calcining Drying gel, and compression molded into a predetermined shape, by sintering, Ru obtain a dense vitrified large.

  In the present invention, for example, nitrate is preferably used as a water-soluble raw material, alcohol is used as a solvent for promoting hydrolysis of an alkoxide, and trimethyl borate and ethyl silicate are preferably used as a lipophilic alkoxide raw material.

  The vitrification method for radioactive liquid waste according to the present invention can produce a vitrified body at a temperature below the softening point (about 600 ° C. for borosilicate glass) by mainly using alkoxide as a glass raw material. It becomes. Thereby, it is not necessary to handle molten glass having high erodibility, erosion of equipment materials is suppressed, and maintenance costs can be reduced. The high-level radioactive liquid waste contains a large amount of cesium, but in the present invention, it can be handled at less than the boiling point of cesium throughout the entire process, so that the recovery equipment can be simplified.

  The present invention mainly includes composition adjustment with a water-soluble raw material, addition of a solvent for promoting alkoxide hydrolysis, composition adjustment with a lipophilic raw material, formation of a sol forming a gel skeleton, gelation, drying, and heat treatment. It consists of a process. The radioactive waste liquid to be treated in the present invention may be an aqueous solution containing volatile waste, a sol, a suspension, or the like.

  FIG. 1 is a process diagram showing an example of a vitrification method according to the present invention. This is an example of a bulk drying method. A water-soluble raw material for obtaining the necessary glass physical properties (transition temperature, melting point, strength, etc.) is added to the radioactive liquid waste 10. For example, metal nitrate is used as the water-soluble raw material. Further, the lipophilic alkoxide raw material to be added next is mixed with a water-soluble waste liquid, and a solvent for promoting hydrolysis is added. An alcohol such as ethanol or methanol is used as this solvent. Thereafter, in order to obtain the necessary glass properties, the lipophilic alkoxide raw material such as ethyl silicate and methyl silicate is added and mixed as in the case of the water-soluble raw material.

  Then, it is heated to an appropriate temperature and subjected to hydrolysis / polymerization reaction by adding ultrasonic waves, etc. to gel the sol. Next, the obtained wet gel is dried. At that time, stress is applied between the dry part and the wet part in the gel, which may cause cracks. Therefore, as a simple method for preventing cracks, the amount of gel to be processed at one time is limited and gradually dried by heating. As other crack preventing means, a technique such as supercritical drying or freeze drying may be used. Finally, the dried gel is heat-treated at a temperature equal to or lower than the glass softening point, and crystal water and solvent in the gel are decomposed, volatilized, and sintered to obtain a dense glass solidified body 12.

  In this vitrification method, the metal nitrate is particularly preferable as the water-soluble raw material because the metal nitrate is easy to dissolve in the radioactive liquid waste and is denitrated by heating, which is advantageous in obtaining a homogeneous vitrified body. For example, glass containing alumina has a property that is difficult to crystallize. If aluminum nitrate is added to the radioactive liquid waste, it is denitrated by a subsequent heat treatment, and a vitrified body containing alumina uniformly can be synthesized. In addition to aluminum nitrate, addition of calcium nitrate (homogeneously present as calcium oxide in the vitrified body) produces an alkali metal elution preventing effect. The metal nitrate may be a hydrate.

  By the way, as a glass raw material, for example, boric acid has an effect of lowering the softening point. However, even if boric acid is added to the radioactive liquid waste, it is not completely dissolved, and the vitrified body becomes inhomogeneous. Therefore, in the present invention, an alkoxide such as trimethyl borate is used as a boron raw material instead of water-soluble boric acid. If an alkoxide is added, it can mix well with the alcohol already added, and a homogeneous glass solid can be obtained. In particular, a compound in which two or more alkoxides are coordinated to a central element such as trimethyl borate and ethyl silicate is mixed at the end because it undergoes polymerization and gelation upon addition.

  Note that if the alkoxide raw material is a highly reactive substance such as trimethylborate, alcohol is generated from the alkoxide during the reaction, so it is not always necessary to add an alcohol solvent, but usually it promotes hydrolysis. It is preferable to add ethanol or the like. In the subsequent drying process, stress is applied to the dry and wet parts of the gel, which may cause cracking, but by replacing part of the alcohol with dimethylformamide, it is possible to prevent cracking during drying, It is also possible to suppress phase separation of the glass during the heat treatment.

  FIG. 2 is a process diagram showing another example of the vitrification method according to the present invention. This is an example of a pulverization drying method. As in the above example, a water-soluble raw material and a solvent (alcohol) are added to the radioactive waste liquid 10. Furthermore, a lipophilic alkoxide raw material is added and mixed. The hydrolysis / polymerization reaction proceeds to gel the sol. In this example, the obtained wet gel is pulverized and dried. Finally, the dried gel is heat treated at a temperature below the glass softening point. First, by calcination, the crystal water and solvent in the gel are decomposed and volatilized, the calcined powder is compression-molded into a predetermined shape, and the compression-molded body is sintered to sinter the large dense glass solidified body. Set to 12.

  As described above, in the drying process, stress is applied to the dry portion and the wet portion, which may cause cracks. However, in this example, since the wet gel is pulverized, it is not necessary to take measures against cracks, the amount of gel to be dried at a time can be increased, and the drying time can be shortened. However, since the dried gel is a powder, a new compression molding step is added to produce a glass solid.

  The present invention can be applied to a wide range of fields such as vitrification treatment of high-level radioactive liquid waste and low-level radioactive liquid waste.

  By the bulk drying method shown in FIG. 1, 28000 MWD / t burned fuel in a pressurized water reactor was cooled for 0.5 years and then reprocessed, and a simulated radioactive waste liquid cooled for 5 years was vitrified.

First, in order to obtain necessary glass characteristics (transition temperature, melting point, strength, etc.) in 8.18 ml of the waste liquid, a nitrate having the following composition was added as a water-soluble raw material.
・ LiNO ₃ ... 0.69g
・ Ca (NO ₃ ) ₂ / 4H ₂ O 0.63 g
・ Zn (NO ₃ ) ₂ .6H ₂ O 0.55 g
・ Al (NO ₃ ) ₃ · 9H ₂ O ... 1.84 g
In addition, all addition amount is the raw material weight per 5 g of glass solidification bodies.

  Next, ethanol was added as a solvent for promoting hydrolysis. Further, 8.09 g of ethyl silicate and 2.51 g of trimethyl borate, which are lipophilic alkoxide raw materials that cause polymerization, were added.

  The mixture thus obtained was stirred for 1 hour in a 35 ° C. ultrasonic cleaner. The hydrolysis and polymerization reaction proceeded to gel the sol. Thereafter, the mixture was allowed to stand at room temperature for 2 days, and the gelation was sufficiently advanced. And it was made to dry in a 100 degreeC bathtub, and the dried gel was obtained. The dried gel was once heated at 300 ° C. for 3 hours, further heated to 600 ° C. and heated for 1 hour to sinter, thereby obtaining 5.00 g of a glass solidified body.

The glass-solidified material produced as described above is pulverized, measured for specific surface area, immersed in a 500 ml hot water bath at 95 ° C. for 1 day, and measured for elution weight, so that the elution rate per unit area can be determined by melting existing glass. Compared with solidified body. As a result, the dissolution rate of the vitrified produced by the present invention is 0.263 g / m ² day, confirmed to have a dissolution rate 0.232 g / m ² day performance equivalent existing glass melting solidified It was done.

Process drawing which shows an example of the glass melting processing method which concerns on this invention. Process drawing which shows the other example of the glass melting processing method which concerns on this invention.

Explanation of symbols

10 Radioactive waste liquid 12 Glass solidified body

Claims (2)

In the radioactive liquid waste, a water-soluble raw material for obtaining glass properties necessary as a glass solidified body and a solvent for accelerating hydrolysis of the alkoxide are added, and a lipophilic alkoxide raw material is further added and mixed. The decomposition / polymerization reaction is advanced to gel the sol, and the resulting wet gel is pulverized and dried, and the dried gel is calcined to decompose and volatilize the crystal water and solvent in the gel, and then calcined powder A method for vitrifying radioactive waste liquid, comprising compacting a body and sintering the compacted body at a temperature below the glass softening point to form a dense vitrified body. The nitrate as a water-soluble material, an alcohol as a solvent for promoting the hydrolysis of the alkoxide, vitrification method for treating a radioactive liquid waste according to claim 1 Symbol placement using trimethyl borate and ethyl silicate as an alkoxide material lipophilic .

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