JPS6131999A - Volume-reducing solidifying treating process of radioactive waste liquor - Google Patents

Abstract

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

Description

[Detailed description of the invention]

purpose of invention

[Industrial Application Field 1] The present invention relates to an improvement in a process for volume reduction and solidification of radioactive waste liquid containing boron. BACKGROUND OF THE INVENTION Radioactive waste fluid discharged from nuclear power facilities, particularly PWR power plants, contains boron mainly in the form of boric acid, and it is appropriate to treat it by a cement solidification method. In recent years, it has become necessary to reduce the volume as much as possible, so it is necessary to evaporate and concentrate the waste liquid and mix as much solid content into the cement paste as possible. The present inventors have studied this problem and have already disclosed several inventions regarding its treatment method. Among them, Japanese Patent Application No. 57-120792 [Process for treatment of radioactive waste fluids] involves adding caustic soda to boron-containing waste fluids to adjust the H content, and then evaporating and concentrating the fluid to remove soluble calcium compounds. is added to precipitate insoluble calcium borate, and aged to reduce the I)H of the liquid to 12
At the same time, the precipitate particles are grown, solid-liquid separation is performed, and the precipitate is solidified as a concentrated solid slurry.
On the other hand, the separated liquid having a pH of 12 or higher is characterized in that it is recycled and reused for neutralizing the acidic waste liquid. Nuclear power plants often have equipment such as waste liquid evaporation concentration equipment, so it is desirable to use existing equipment without making any major modifications. The above process affects the operating conditions of this evaporation concentration step, and therefore, although it is effective as a treatment method for radioactive waste liquid containing boron, its adoption is limited.

[Problem to be solved by the present invention 1] The present invention solves the above-mentioned problems by using the existing equipment of the nuclear power plant almost as is and adding the equipment necessary for the subsequent treatment process. , provides a treatment process that can be carried out while benefiting from the previous invention. Structure of the Invention (Means for Solving the Problems) The volume reduction and solidification treatment process for radioactive waste liquid containing boron of the present invention involves evaporating and concentrating the waste liquid, adding caustic soda and soluble calcium compounds to the concentrated liquid, and adding insoluble calcium compounds to the concentrated liquid. Calcium borate is precipitated, aged to grow precipitate particles, solid-liquid separated, the precipitate is solidified with cement as a concentrated solid slurry, and the separated liquid is evaporated and concentrated, then insoluble boric acid It is characterized in that it is returned to the calcium precipitation step for cyclical reprocessing. [Operation] By doing so, it is possible to eliminate the mutual connection between the evaporation concentration of the waste liquid and the steps after the precipitation of insoluble calcium compounds. The benefits of the previous invention as a method for treating waste liquid containing boric acid are maintained. (Example) As shown in FIG. Acidic radioactive waste liquid containing boric acid is evaporated and concentrated.The concentration of boric acid in this type of waste liquid is 2.100 in terms of boron.
Typically, the concentration is around 1111 m, and evaporation concentration is at least above the saturation concentration at 25°C, that is, the boron content is 9.
oooppm or more, preferably 21. This is done so that the amount is approximately OOOppm. Then, a caustic soda aqueous solution is added to the concentrate to adjust the pH to 7-8. This I)H adjustment is carried out to first react in an alkaline region, since the precipitation reaction of insoluble calcium in the next step is extremely slow in an acidic region, and is adjusted by adding caustic soda as necessary. However, since adding too much sodium has an adverse effect on the cement solidification process in the final step, it is desirable to limit the amount to the minimum amount necessary for the precipitation reaction of insoluble calcium borate. From this point of view, the appropriate pH of the liquid is 7 to 8.
Under conditions where the boron concentration of the concentrated solution is 21,000 pI), the Na/B (molar ratio, the same hereinafter) is 0.2 to 0.
．． 3, this pH value is obtained. Condensate obtained by evaporation concentration can be recycled and reused. Next, adjust the pH to t=? A soluble calcium compound is added to the lI condensate to precipitate insoluble calcium borate. As the calcium compound, calcium hydroxide or calcium oxide is preferably used from the viewpoint of avoiding accumulation of other ions in the system and increase of solid content due to evaporation and concentration. Calcium oxide may be added in the form of powder, slurry, or aqueous solution, but calcium oxide should be used with care to avoid rapid heat generation due to contact with water. Naturally, the precipitation reaction of the insoluble salt proceeds more quickly at higher temperatures, and a temperature of 40° C. or higher is practical. On the other hand, as a result of the reaction, a paste-like material is produced, and if the temperature is high, it becomes hard, which is disadvantageous in operation. The permissible limit with normal equipment is about 70°C, and the preferred temperature is 60°C.
It is as follows. Furthermore, this step must be carried out under stirring, and a particularly suitable stirring device is one that has at least two sets of stirring blades, one above the other, and which simultaneously rotates and moves up and down. At the beginning of the precipitation of the insoluble salt, the liquid is in the form of a slurry, but as the precipitation progresses and the amount increases, it becomes paste-like. When this change no longer progresses, the liquid is cooled to below the precipitation temperature and ripened. Continued stirring is preferred, but not essential. Due to aging, the precipitated calcium borate crystals grow into large plate-like or columnar shapes, and the liquid changes from a paste-like state to a slurry-like state again. Moreover, as the boron concentration in the liquid decreases, the sodium concentration recovers to the concentration at the beginning of the reaction. Accordingly, the pH of the liquid increases. The pH of the liquid should be as high as possible, that is, the N in the liquid should be
We would like to increase the concentration of a as much as possible and use the solution for pH adjustment in the calcium borate precipitation process. The Na9 degree in the solution after aging is closely related to the amount of calcium compound added in the calcium borate precipitation process, and as the value of Ca / B (molar ratio, the same applies hereinafter) increases, , increases as the amount of calcium compound added increases, and tends to be saturated when Ca/B reaches a certain value. NalA in this solution
From the viewpoint of stability, it is desirable that the amount of the calcilram compound added be equal to or greater than the value of Ca/B at the saturation point. The value of Ca/B at the saturation point differs slightly depending on the initial Na/B concentration of the reaction solution, but in the pH adjustment range of the normal concentrated solution mentioned above, Ca/B = 0.35 to 0.45F, and the value of Ca/B is 0.35 to 0.45F. The amount of calcium compound to be used should be at least Ca
/B=0.3 or more is desirable. In actual operation, we take into account the concentration of porium in the waste liquid concentrate, the amount of caustic soda added, and the actual production and aging of insoluble salts.
A calcium compound may be added after finding an appropriate Ca/B value. The matured liquid is sent to a solid-liquid separator and separated into a concentrated solid slurry and a separated liquid. Separation equipment is selected depending on the moisture content of the slurry, such as a conventional filter or a centrifugal separator. Although it is better for the water content to be low in order to increase the volume reduction ratio, the optimum value is set from the viewpoints of ease of transfer and handling to the next solidification step, physical properties of the solidified material, etc. Usually, a solid content temperature range of 30 to 80% by weight is appropriate. The concentrated solids are mixed with cement and water and undergo a cement solidification process. The combing method may be performed using a known technique. Portland cement is suitable as cement. A suitable amount of solids to be encapsulated in the cement assimilate is 30 to 70% (based on dry weight). On the other hand, the separated liquid is concentrated and returned to the insoluble calcium borate precipitation step for cyclic reprocessing. The separated liquid contains most of the caustic soda that was added to adjust the I)H of the waste liquid concentrate, so by circulating and reprocessing the separated liquid, the caustic soda added thereafter will be able to reduce the loss. It will be good enough to replenish. The condensate generated during concentration of the separated liquid is reused. This concentration can be carried out either by evaporation or by reverse osmosis. In this step, since the already concentrated separated liquid is simply concentrated, the scale of the apparatus can be small.

【Example】

Hou II! (H3BO3) was dissolved in water to prepare a simulated concentrated waste liquid containing boron. Add casei soda to this to adjust the pH, 874 degrees 21.0 OOpl] m, N
a /B (-tel/ratio) = 0.2617) Attach the waste liquid 11 3 . Calcium hydroxide powder is added to this adjusted waste liquid, Ca / B
= 0.5 (molar ratio), the liquid was kept at 60°C while stirring uniformly, and after the liquid turned into a paste, it was cooled to below 40°C and aged to obtain a concentration of 13. .6
% calcium borate slurry was obtained. This was subjected to suction filtration to separate solid and liquid, and the separated liquid was concentrated approximately 10 times using a forced circulation type evaporative concentrator. Concentrated separated liquid is 8111 degrees 20
, 500ppm SNa/B (molar ratio), =26.
It is 2. This was added to the simulated concentrated waste solution 11II3 containing 21.000 ppm boron. The same amount of calcium hydroxide powder as above was added to this mixed solution, precipitated in the same manner, and aged to obtain a calcium borate slurry. The concentration of the slurry was 137%, and the properties were the same as those obtained directly from the simulated concentrated waste liquid described above. Thereafter, it was possible to repeat the reprocessing in which the separated liquid was similarly concentrated and circulated to the insoluble calcium borate precipitation step. On the other hand, 33 parts by weight of Pol 1 to Land Cement and 11 parts by weight of water were added to 100 parts by weight of the concentrated solid content obtained by the above solid-liquid separation and kneaded, and the mixture was filled into a container and hardened to obtain a solidified product. . The density of the solidified material after curing for 28 days at room temperature is
It was 1.8 Kg/cm3. Separately, a test piece was prepared according to the method specified in JIS, and the compressive strength was measured after curing for 3 months and found to be 213 Kg/cm2. Effects of the Invention According to the treatment process of the present invention, the separated liquid generated after the waste liquid evaporation concentration process is recycled and reprocessed in one system up to the assimilation process, so that the evaporation concentration process upstream from this is not affected. I won't give it. Therefore, in order to carry out the treatment process of the present invention in a nuclear power plant that has an existing evaporation concentration process, it is sufficient to construct equipment for the process after the process of precipitating the insoluble calcium compound. This is not only efficient in terms of equipment costs, but also means that both can be operated independently, and furthermore, the neutralization operation is omitted, which simplifies the operation. Furthermore, in the treatment process of the present invention,
Only a small amount of caustic soda is added, which not only reduces the amount of chemicals used, but also reduces the amount of waste to be disposed of. Furthermore, since almost no sodium ions are transferred to the cement solidification process, the amount of sodium ions encapsulated in the cement solidified body is extremely reduced, and the physical properties of the solidified body are improved.

[Brief explanation of the drawing]

The drawing is a block diagram showing the volume reduction and solidification treatment process of radioactive waste liquid according to the present invention. Patent applicant Kyushu Electric Power Co., Ltd. JGC Corporation

Claims (4)

[Claims] (1) In the volume reduction and solidification process of radioactive waste liquid containing boron, the waste liquid is evaporated and concentrated, caustic soda and soluble calcium compounds are added to the concentrated liquid to precipitate insoluble calcium borate, and the precipitate is aged. The particles are grown, solid-liquid separated, the precipitate is treated as a concentrated solid slurry for cement solidification, and the separated liquid is evaporated and concentrated, then returned to the insoluble calcium borate precipitation process for circulation and reprocessing. Featured processing process. (2) The amount of caustic soda and calcium compounds added to boric acid in the waste liquid concentrate is such that Na/B (molar ratio of sodium to boron in the waste liquid concentrate) is at least 0.2, and Ca/B (the molar ratio of sodium to boron in the waste liquid concentrate) is at least 0.2. A process according to claim 1, wherein the molar ratio of calcium to boron in the concentrate is selected to be at least 0.3. (3) Precipitation of insoluble calcium borate at a temperature of 40 to 7
The treatment process according to claim 1, wherein the treatment process is carried out at 0° C. with stirring, and the liquid is matured at a temperature below the precipitation temperature. (4) The treatment process according to claim 1, wherein calcium hydroxide or calcium oxide is used as the soluble calcium compound.