JP2012242092A - Processing method of radioactive cesium containing contaminated water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To construct a system for cyclic use of high-concentration nitric acid as a desorbent and safety disposal of radioactive cesium to be separated in a method for efficiently removing cesium from a radioactive cesium containing contaminated water by repeatedly using an absorbent containing a copper salt based insoluble ferrocyanide as an active ingredient.SOLUTION: In a first step, contaminated water is brought into contact with an absorbent, and the absorbent which has captured cesium is carried into a centralized processing facility 1 where steps after a second step are installed, to perform cesium desorption processing and absorbent reproduction processing. A radioactive cesium accumulated high-level waste liquid discharged from the second step is condensed and denitrated to such a state as to be thrown into a glass or the like stable solid manufacturing step, and nitric acid is collected. By using a salt-free reductant, no vitrification inhibitor component is mixed into the condensed high-level waste liquid. In the centralized processing facility 1, steps after a high-level waste liquid condensing step are shared with steps after a high-level waste liquid condensing step in a nuclear reprocessing plant, thereby presenting effects of the present invention to a maximum.

Description

The present invention relates to a method for treating radioactive cesium-containing contaminated water from nuclear facilities such as nuclear power generation facilities and nuclear fuel reprocessing facilities, particularly high-level radioactive cesium-containing contaminated water discharged in large quantities when a serious accident occurs.

Cooling of nuclear fuel rods in the core and spent nuclear fuel rods in the storage pool as seen in a power loss accident at a nuclear power plant caused by the Great Tsunami following the Great East Japan Earthquake on March 11, 2011 The occurrence of malfunction of the water circulation system may cause damage to the fuel cladding due to decay heat. In order to prevent damage progression and fuel body melting / re-criticality, continuous water flow from outside the system and water spraying are required, but as a result, elution of fission products contained in the broken fuel rods causes extremely high A large amount of radioactive contamination water is generated. If it continues to accumulate in underground water storage tanks and external water storage facilities, and the water storage capacity reaches the limit, leakage or overflow is unavoidable, leading to serious environmental pollution. Therefore, removal of radioactive substances from such contaminated water is extremely important not only for preventing environmental pollution but also for ensuring a safe working environment inside and outside the building with the aim of restoring the cooling water circulation function.

In the case of a severe accident involving melting of the fuel body or damage to the cladding tube at a nuclear power plant, contamination of cooling water by a large amount of gamma-ray generating nuclides makes urgent work such as cooling water circulation function recovery extremely difficult, It becomes an obstacle in the subsequent radioactive containment work. Even if the nuclide has a large amount of fission, the amount of gas substances that are easily released into the atmosphere and those with short half-lives such as radioactive iodine are small in the contaminated water, or the radiation intensity is rapidly attenuated. Nonvolatile nuclides such as radioactive strontium have a large production amount and a long half-life, but according to source term analysis, most of them are said to remain in the debris of the molten fuel (see Non-Patent Document 1). Also, since radioactive strontium only generates beta rays, the risk to field workers is low compared to gamma-ray-producing nuclides, as long as ingestion is avoided. Therefore, the radionuclide that is likely to be a serious obstacle to cooling water circulation function recovery work and radioactive confinement work is radioactive cesium in terms of fission production, half-life, gamma ray intensity, etc., and its removal from contaminated water Is particularly important.

Moreover, even after the cooling water circulation function is recovered, it is necessary to continue water supply until the cold temperature is stopped. Meanwhile, elution of radioactive cesium from damaged fuel rods into cooling water continues, so not only leakage prevention to the surrounding environment, but also to keep the working environment healthy by reducing the gamma dose around the buildings and facilities as much as possible. It is necessary to remove radioactive cesium from the circulating water continuously.

On the other hand, with regard to the decontamination technology of cooling water contaminated with extremely high levels of radioactive cesium, zeolitic inorganic ion exchangers such as mordenite are cheaper than ion exchange resins, and cesium is often used when salinity is low. Adsorption is known, and in March 1979, a zeolite-based adsorbent was used in an accident at the Three Mile Island nuclear power plant in the United States, and it was reported that it had a certain effect.

However, zeolitic inorganic ion exchangers have the drawback that their adsorption capacity tends to decrease in the presence of relatively high concentrations of metal salts such as seawater. In contrast, insoluble ferrocyanide-based inorganic ion exchangers such as potassium potassium ferrocyanide, nickel potassium ferrocyanide, iron ferrocyanide, and potassium potassium ferrocyanide have extremely high selectivity to cesium, and a large amount of seawater, etc. It is known that cesium adsorption capacity hardly decreases even in the presence of an electrolyte.

On the other hand, a drawback common to ion exchangers having high selectivity for such specific ions is that desorption, which is a reverse ion exchange reaction, is generally difficult due to high selective adsorption. Therefore, these cesium separation materials can be used only once, and their utility is only to transfer and concentrate the radioactive cesium of the target nuclide from the contaminated water to the solid inorganic ion exchanger. In other words, new high-level radioactive waste that requires safe isolation from the environment for a long time as a one-time waste adsorbent is generated at the contaminated water purification site. When the amount of contaminated water is small and the concentration of radioactive cesium is low, it is clear that it is not preferable as a method for treating an extremely large amount of contaminated water containing a relatively high level of radioactive cesium, even if effective in achieving the purpose. Such a defect is also applicable to the radioactive cesium coagulation precipitation method. Despite the urgent need to develop efficient treatments for large volumes of contaminated water containing fairly high levels of radioactive cesium, techniques to overcome these deficiencies are not known.

The present inventor has already limited the copper salt-based insoluble ferrocyanide among the insoluble ferrocyanide-based inorganic ion exchangers, and its crystal structure and redox function are suitable for oxidative desorption of adsorbed cesium by nitric acid and regeneration by a reducing agent. By using a 1-7 M nitric acid solution for desorption and a dilute nitric acid solution of hydrazine for reduction, an adsorption / desorption / regeneration cycle can be constructed. We have invented a cesium separation method that can be used repeatedly more than once (see Patent Documents 1 to 6 and Non-Patent Document 4).

Japanese Patent No. 1946448 Japanese Patent No.1991489 Japanese Patent No. 2021973 Japanese Patent No. 2560253 Japanese Patent No. 2810981 Japanese Patent No.3749941 Japanese Patent No.3733417

"Research report on maintenance of containment function in the latter half of FY2007" (National Nuclear Safety Infrastructure Agency) July 2008, pages 2.2.2-7 Ryohei Kiyose, “Chemical Engineering of Fuel Reprocessing and Radioactive Waste Management” Nikkan Kogyo Shimbun, December 1983 The Chemical Society of Japan “Radioactive Substances” Maruzen 1951 Journal of Radioanalytical and Nuclear Chemistry Vol.185 No.1 p.57 (1994)

The known method using the adsorption / desorption / regeneration function of an adsorbent containing copper salt-based insoluble ferrocyanide as an active ingredient for removing radioactive cesium from contaminated water is extremely dangerous with little use. There has been a difficulty in collecting radioactive cesium by being concentrated in a high-concentration nitric acid solution through a desorption treatment step. In order for the method of removing radioactive cesium from contaminated water using the adsorbent to be practical, a safe disposal method for recovered radioactive cesium and recovery and reuse of nitric acid from the desorption treatment solution are indispensable. The present invention is intended to solve such problems.

The present inventor has a concentration of nitric acid in the high-level radioactive liquid waste discharged from the Purex co-decontamination process of the nuclear fuel reprocessing plant of 2.5 to 3M, and the concentration of radioactive cesium from the desorption process of the present invention It was noted that the concentration of nitric acid was almost the same as that of nitric acid solution. Conventionally, group separation has attracted attention from the standpoint of developing an ideal treatment method for high-level waste liquid, and the separation and recovery of radioactive cesium has been studied, but in the present invention, the results of reviewing various processes from the opposite viewpoint In the concentrated radioactive cesium concentrated nitric acid solution discharged from the desorption process of the present invention, platinum group metals such as palladium and molybdenum which cause deterioration of the quality of the vitrified body during the production of the vitrified body Therefore, it is predicted that there will be no problem even if it is mixed with the high-level waste liquid discharged from the reprocessing step, and the present invention has been achieved.

That is, the present invention relates to a copper salt-based insoluble ferrocyanide or an adsorbent containing the same as an active ingredient (hereinafter abbreviated as “adsorbent” including use as an adsorbent alone) and radioactive cesium-containing contaminated water. A first step of contacting and adsorbing cesium; a second step of desorbing cesium by bringing the concentrated cesium adsorbent into contact with a 1-7M aqueous nitric acid solution; the desorption that has been oxidized to ferricyanide and has lost its adsorption capacity The cycle up to the third step of regenerating ferrocyanide by bringing the treated product into contact with a dilute nitric acid solution containing a reducing agent is defined as one cycle, and the adsorbent regenerated in the third step is reused to perform the cycle of adsorption / desorption / regeneration. Repeatedly removes radioactive cesium from the contaminated water, denitrifies the concentrated radioactive cesium waste liquid discharged from the second step, collects nitric acid in the concentration step, glass, etc. Characterized by concentrated high level liquid waste as a raw material for producing a constant solidified, a method for treating a radioactive cesium-containing contaminated water (see Figure 1).

In the present invention, the copper salt-based insoluble ferrocyanide or an adsorbent containing the same is used as an active ingredient, and the process is repeated by a known method from the first step to the third step. The installation location after the second step does not need to be in the vicinity of the site where the contaminated water is generated, and is not intended for the recovery and use of cesium, so it is installed in a nuclear fuel reprocessing plant (see FIG. 2). The effect is maximized. Therefore, the adsorbent adsorbed radioactive cesium in the first step is from the adsorption treatment site to the nuclear fuel reprocessing plant, and the recycled adsorbent is from the nuclear fuel reprocessing plant to the contaminated water generation site in the first step, respectively. It is packed and transported. In that case, an adsorbent using a spherical resin or silica gel as a carrier is preferable because it does not require time and effort for refilling and transporting compared to a simple adsorbent exhibiting powder or mud.

As the destination of the concentrated radioactive cesium-containing nitric acid solution generated from the second step, a high-concentration nitric acid-containing high-level waste solution generated from the nuclear fuel reprocessing step, preferably a co-decontamination step in the Purex process reprocessing A concentration process of high-level waste liquid having a nitric acid concentration of 2.5 to 3 M discharged from the factory is selected. Nitric acid can be recovered from the concentration step and the condensate fractionation step associated therewith. The concentrated high-level waste liquid is provided as a raw material for producing a glass solid or ceramic solid by a known method. The recycling wastewater discharged from the third step is a low-level radioactive wastewater that is close to zero. By using salt-free hydrazine as a reducing agent, it is concentrated together with the washing water in each step and added to the high-level wastewater. be able to. By thoroughly washing with water after desorption treatment of radioactive cesium in the second step, the used waste adsorbent generated from the second step of the final cycle can be treated as a medium level radioactive waste. In particular, in the case of a waste adsorbent using silica gel as a carrier, the cyanide can be separated into nitric acid acidic solution of metal salt and silica gel by thermal decomposition at around 420 ° C and then extraction with nitric acid. After the concentration step, it becomes a raw material for vitrified glass together with the latter as concentrated high-level waste liquid (see Patent Document 7).

According to the present invention, a copper salt-based insoluble ferrocyanide capable of adsorption / desorption / regeneration cycles or an adsorbent containing the same as an active ingredient is repeatedly used and a high-level radioactive liquid waste treatment process generated from a nuclear fuel reprocessing plant Can effectively remove radioactive cesium from a large amount of contaminated water, consume very little adsorbent per unit of contaminated water, and easily collect a large amount of nitric acid to be used as a desorbent. -It is possible to contain radioactive cesium in a stable solidified material such as glass without generating new high-level radioactive solid waste that is difficult to dispose of.

FIG. 1 is a flow sheet of the method of the present invention. FIG. 2 is a flow sheet of the method of the present invention when a part of the nuclear fuel reprocessing process is utilized.

A supplementary explanation will be given for the method of carrying out the present invention. In the present invention, as an adsorbent containing the copper salt-based insoluble ferrocyanide in the first step as an active ingredient, a porous anion exchange resin, a slightly polar adsorbent resin, or silica gel rather than a copper salt-based insoluble ferrocyanide alone Those in which a copper salt-based insoluble ferrocyanide is supported in each of these pores are excellent in solid-liquid separation and are preferably used (see Patent Documents 4 to 6). These adsorbents have a small decrease in adsorption performance due to regeneration, and can be used repeatedly at least 10 times or more. In particular, the adsorbent carrying a copper salt-based insoluble ferrocyanide in each pore of a slightly polar adsorbent resin or silica gel uses an aqueous solution of potassium ferrocyanide or sodium ferrocyanide and an alcohol solution of cupric salt. When prepared by this method, a product uniformly supported in the pores is obtained and used preferably (see Patent Document 6).

Either the batch method or the column method can be adopted as the adsorption method in the first step. However, in the case of using only a copper salt-based insoluble ferrocyanide, it is a batch method, each of porous resin or silica gel. An adsorbent carrying a copper salt-based insoluble ferrocyanide in the pores is excellent in solid-liquid separation, and is preferably used in either the column method or the batch method. In addition, the elution amount of iron and copper from the copper salt-based insoluble ferrocyanide into the treated water in the first step can be made to be below the water quality standard (0.3 ppm and 1 ppm, respectively) of the water supply.

In the desorption treatment in the second step, the copper salt insoluble ferrocyanide is oxidized to copper ferricyanide with 1 to 7 M nitric acid, preferably 3 to 5 M nitric acid, and cesium ions such as radioactive cesium are desorbed. When the concentration of nitric acid is 4M or less, cesium ions are likely to remain slightly in the vacancies of the copper ferricyanide crystal, and sufficient washing with water is necessary. The oxidation reaction requires the coexistence of a small amount of nitrous acid, and a nitric acid aqueous solution containing about 0.001 M of nitrous acid is preferably used for stable treatment (see Non-Patent Document 4). The high concentration nitric acid solution discharged from the second step and desorbed and concentrated with radioactive cesium is concentrated in the high-level waste solution with a nitric acid concentration of 2.5-3M discharged from the co-decontamination step of the nuclear fuel reprocessing plant. By introducing it into the process, the treatment according to the present invention can be carried out efficiently, but the treatment system for radioactive cesium-containing contaminated water according to the present invention can be operated without any trouble even if the co-decontamination process is suspended in the reprocessing process. is there.

In the regeneration process of the third step, copper ferricyanide is reduced to a copper salt-based insoluble ferrocyanide with a dilute nitric acid solution containing an equivalent amount or more of hydrazinium ions as a reducing agent and regenerated as an adsorbent. The nitric acid concentration may be such that hydrazinium ions are sufficiently stable. In addition, since nitrogen gas is generated with the reduction reaction by hydrazinium ions, it is not preferable to employ the column method in the third step.

To perform safe and efficient adsorption treatment operation in the first step where contaminated water is generated and transporting the radioactive cesium concentrated adsorbent to the nuclear fuel reprocessing plant in the second step, an adsorbent-filled cartridge Adoption of a flow-through type using a formula column is preferred. In that case, the adsorbable amount of radioactive cesium on the adsorbent is limited to an amount capable of maintaining the temperature rise due to decay heat below the decomposition start temperature of the adsorbent. The adsorbable amount can be determined by adjusting the amount of the copper salt-based insoluble ferrocyanide supported or by the cartridge structure. When the amount of radioactive cesium adsorbed is large, it is possible to prevent deterioration of the adsorbent due to decay heat if the column body is transferred to a water-sealed container and transported from the first step to the second step. The adsorbent regenerated in the third step is refilled in the cartridge and returned to the adsorption processing site in a ready-to-use state.

By installing a cartridge filled with an adsorbent at a site where radioactive cesium-contaminated water may be generated, such as a nuclear power plant, it is possible to quickly respond to a contaminated water spill accident. In addition, there is a possibility that it can be used to purify trace amounts of radioactive cesium-contaminated water and raw milk by deploying them at water purification plants and milk collection sites. Possibility of realizing a system that can intensively bring the adsorbed cartridges used in a distributed manner to a high-level radioactive liquid waste treatment facility such as a nuclear fuel reprocessing facility, desorb cesium, regenerate the adsorbent, and return the refilled cartridge. There is enough. In that case, the cartridge has a structure that shields gamma rays, and a gamma ray measuring instrument is attached, so that the amount of radioactive cesium adsorbed can be managed according to the application.

1 Flow sheet in the central processing facility where the processes after the second process are installed 2 Flow sheet within the nuclear fuel reprocessing plant where the processes after the second process are installed

Claims (2)

Adsorb cesium by contacting copper salt-based insoluble ferrocyanide or adsorbent containing it as an active ingredient (hereinafter abbreviated as “adsorbent” including the use of adsorbent alone) and contaminated water containing radioactive cesium. A first step, a second step of desorbing cesium by bringing the concentrated cesium adsorbent into contact with a 1 to 7M aqueous nitric acid solution, and a deoxidation treatment product that has been oxidized by ferricyanide and has lost its adsorption ability Radioactive cesium by repeating the cycle of adsorption / desorption / regeneration by reusing the adsorbent regenerated in the third step, with one cycle from the third step to regenerate ferrocyanide by contacting with dilute nitric acid solution. Is removed from the contaminated water, the concentrated radioactive cesium waste liquid discharged from the second step is denitrated in the concentration step to recover nitric acid, and the raw material for the production of stable solidified materials such as glass Characterized by concentrated high level liquid waste to be, method for treating a radioactive cesium-containing contaminated water The high-level waste liquid discharged from the second step is a concentration step of the high-level waste liquid discharged from the co-decontamination step of the nuclear fuel reprocessing by the Purex method. Method for treating contaminated water containing radioactive cesium

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