JP5713245B2 - Method for treating incinerated ash containing radioactive materials - Google Patents

Description

  The present invention relates to a method for treating incineration ash that reduces radioactive material in incineration ash containing radioactive material to a safe level in a living environment.

When contaminated by an accident such as a nuclear power plant, a large amount of pollutant is generated. If the degree of radioactive contamination is not high, a method of incinerating the pollutant is taken. Even in the 2011 Fukushima nuclear accident, the amount of pollutants was judged not to be so high and was incinerated.
However, the radioactive material in the incinerated ash is concentrated to a radioactivity level of about 50,000 Bq / kg, for example. Incineration ash is usually exposed to temperatures around 900 ° C. in order to suppress the generation of dioxins, and is a highly water-absorbing porous body having a fine mechanism. The radioactive material is considered to be trapped in the porous pores.
There are no reports that specifically show that radioactive materials have been extracted from incineration ash and reduced to a safe level in the living environment, and a large amount of such incineration ash has been left unattended. There's a problem.

  The present invention seeks to solve the above problems.

  The present invention contains incinerated ash containing a radioactive substance in a container, the temperature below the critical temperature of water, a water-soluble liquid or a mixture thereof existing in the container together with the incinerated ash in a sealed state, and saturated steam A step of heat-treating the incinerated ash in a pressure state equal to or higher than a pressure, a step of releasing the pressure, a step of separating the substance released from the container after the heat-treatment into a liquid component and a solid component, and coexisting during the heat treatment A method for treating incinerated ash containing a radioactive substance, comprising a step of adsorbing a radioactive substance with an adsorbent or with an adsorbent after heat treatment.

According to the present invention, the radioactive substance in the incinerated ash containing the radioactive substance can be removed or reduced by an industrial method, and the incinerated ash treated according to the present invention can be safely returned to the living environment. In particular, if the pressure is suddenly released after the heat treatment, the degree of decontamination can be further increased.
In the present invention, “radioactive cesium” or “cesium” includes radioactive cesium compounds.

[Incinerated ash containing an radioactive substance and an aqueous liquid are contained in a container, preferably water, a water-soluble liquid or a mixture thereof (hereinafter, “water, a water-soluble liquid or a mixture thereof” is referred to as “aqueous liquid”. Process to cover the incinerated ash]
In the present invention, the incinerated ash containing a radioactive substance and an aqueous liquid that is water, a water-soluble liquid, or a mixture thereof are first contained in a container, and preferably water-based to cover the incinerated ash before heat treatment. Soak in liquid. By doing in this way, an aqueous liquid can be made to infiltrate into the porous void | hole of the said incineration ash with the pressure which arises when the aqueous liquid is heated and vaporized under a closed system. On the other hand, when heating is performed under conditions where moisture does not exist in advance, such as steaming, even if steam for steaming is partially condensed, the amount of the steam is slight and enters the porous gaps of the incinerated ash. I can't. The effect of the penetration of the aqueous liquid will be described later. Here, “covering” is sufficient if the incinerated ash is covered with the aqueous liquid in a state of not higher than the critical temperature of the aqueous liquid and not lower than the saturated vapor pressure. Therefore, it is not necessary to immerse the incinerated ash with an aqueous liquid in advance before heating. For example, even if it does not cover preexisting moisture, moisture generated by the condensation of steam used for heating is added and heated. It may be sufficient to cover during processing. Further, the “covering degree” varies depending on the radioactivity of the radioactive material of the incineration ash, but in a state where 70% or more, more preferably 80% or more, more preferably 90% or more of the incineration ash is covered. In many cases, the penetration effect of the aqueous liquid can be expected. However, this range is also for the case where the container is fixed and varies depending on the conditions. For example, if the container rotates, the incineration ash is immersed in any time zone if the incineration ash has a small amount of aqueous liquid. The more the incineration ash is covered and the surface of the aqueous liquid is higher than the surface of the incineration ash, the more the amount of aqueous liquid increases, so the amount of radioactive material that dissolves in the aqueous liquid increases and the decontamination effect improves. Since more energy is required to raise the temperature, it is preferable that the optimum condition of “the extent to be covered” is determined by these balances. As one guideline, when the container is fixed, the volume of the incinerated ash is 1.5 to 5 times, more preferably 2 to 4 times. Here, water, methanol, ethanol, acetone or the like is suitably used as the aqueous liquid. An aqueous liquid containing a surfactant is also more preferably used because it easily penetrates into the fine porous structure of incinerated ash in a normal incinerator incinerated at around 900 ° C.

[Process for heat treating incinerated ash at a temperature below the critical temperature of the aqueous liquid and at a pressure above the saturated vapor pressure]
Next, in a sealed state of the container, the incinerated ash is covered with the aqueous liquid and subjected to heat treatment at a temperature lower than the critical temperature of the aqueous liquid and at a pressure higher than the saturated vapor pressure. This condition is a kind of subcritical state. The subcritical state is a case where the aqueous liquid is water. Generally, the subcritical state is a high temperature of not less than the critical temperature of water and not more than the critical pressure (the critical temperature of water is 374 ° C. and the critical pressure is 22.1 MPa). Refers to the state of water vapor under pressure and the state of liquid water at medium temperature and medium pressure above the saturated water vapor pressure below the critical temperature of water. Perform in sub-critical state of liquid water. The ion product of water in such a subcritical state is very large compared to room temperature and atmospheric pressure. While the ion product at room temperature and atmospheric pressure is 10 ^-14 mol ² / kg ² , it is 10 ^-12 to 10 ^-11 mol ² / kg ^{2 in the} subcritical state, and ions at room temperature and atmospheric pressure. Since the product is 100 to 1000 times, the concentration of H ⁺ and OH ⁻ is about 3 to 30 times the value at room temperature, the hydrolysis power is very large, and the attack to the binding site where hydrolysis can occur is extremely large. Become. The same applies to water-soluble liquids other than water. In the case of a mixture of water and a water-soluble liquid, considering the hydrolysis power and component ratio of each component, it is carried out in a liquid state at a medium temperature and medium pressure below the critical temperature of any component and above the saturated vapor pressure. It seems that radioactive substances, especially cesium, are easily dissolved in aqueous liquids due to such strong hydrolytic power. When the temperature rises above 330 ° C, as the temperature rises, the ionic product of water decreases rapidly, so the hydrolytic power also declines sharply and the hydrolytic power disappears when it exceeds the critical point. It is good to do. Further, since the hydrolysis power is moderately lowered even at a temperature lower than 130 ° C., it is preferably 130 to 330 ° C., more preferably 180 to 300 ° C., still more preferably 230 to 280 ° C., and particularly preferably 240 to 240 ° C. Performed at 270 ° C. This reaction may be non-catalyzed, but is more effective when performed in the presence of a catalyst. An iron material such as iron powder is preferably used as the catalyst. Since the incineration ash is covered with an aqueous liquid and treated under the above conditions, the incineration ash is subjected to strong hydrolytic power, and radioactive cesium that is considered to exist in the porous of the incineration ash is strongly hydrolyzed. Receive power. As a result, it is considered that radioactive cesium is easily dissolved in an aqueous liquid. In contrast, in steaming, the aqueous liquid does not exist so much as to penetrate into the porous body, and the porous body is only in contact with the vapor. Therefore, radioactive cesium is dissolved in the aqueous liquid and is subject to hydrolysis. It is not preferable because it is not present or is scarce.

  As the pressure for pressurizing the incineration ash, the higher the pressure, the more easily the aqueous liquid penetrates into the pores of the incineration ash, which is desirably 3 atm (0.3 MPa) or more, preferably 5 atm (0.5 MPa) or more. Is 10 atm (1.0 MPa) or more. The heat treatment may take a method of heating the sealed space from the outside, or may be a method of adding a heating medium such as injecting steam into the sealed space. In the latter case, the amount of water is not enough with steam alone, so it is necessary to have some moisture in advance, and as described above, it is better to immerse in an aqueous liquid.

  If the size of the container is large, the internal temperature tends to be non-uniform, so it is desirable to make the size of the container small or to stir. In the former case, a small one having a capacity of about 30 to 200 L, preferably about 30 to 100 L may be used, and the processing time varies depending on the temperature in the subcritical state, but if it is a preferable temperature, a few seconds is sufficient. However, considering the case where the desired temperature is not reached depending on the apparatus, a few seconds to 60 minutes, and in many cases 2 to 30 minutes are sufficient. When a large amount of processing is necessary, a plurality of such small containers are prepared. By interlocking a plurality of such small containers, it is possible to reach a predetermined temperature in a short time compared to the temperature raising time until the large container is brought to a predetermined temperature, and the temperature distribution in the container can be made uniform. Combined, it is possible to perform a large amount of processing more than processing using a large container. For example, when the light sensor detects that the incineration ash placed on the conveyor has been weighed, a predetermined amount of incineration ash is fed into the container. The incineration ash and steam inlets and outlets of the mutual containers are opened and closed when a predetermined condition is satisfied by a limit switch, and are sequentially heat-treated.

  When the temperature is below the critical temperature of the aqueous liquid and above the saturated vapor pressure, there is no redox power like supercritical water. And oxygen, which has a factor to accelerate corrosion. However, it is possible to prevent the corrosion itself by causing oxygen to not be included in the heat treatment. In addition, air (oxygen, nitrogen, etc.) in the container does not become liquid unlike water vapor when the pressure is suddenly released. Is desirable. As such means, it is preferable to use pure water, whether it is water used for steam or pre-existing water, which is heated to about 80 ° C. and even if oxygen enters temporarily. It is also a preferable means that the air adsorbed on the incinerated ash is expelled from the blow tank or outside the system with, for example, about 0.5 to 0.8 MPa of water vapor so that the gas in the system is only water vapor. Moreover, it is also a preferable means not to contain inorganic phosphorus. With such considerations, if the pressure release after the heat treatment is performed with a rapid pressure drop as will be described later, since everything inside the container is blown away, the inside of the container is cleaned, so the relative Can be used over a long period of time, and is preferable from the viewpoint of durability of the container for long-term use. As the material of the container used in the present invention, stainless steel such as austenite, martensite, and double-layer alloy, high alloy steel, and the like are preferably used, but iron or the like can also be used. However, since cesium hydroxide is produced in the course of this reaction, caution is required for its strong alkalinity when the cesium concentration is high.

[Process of releasing pressure]
After the heat treatment, the pressure is released. The pressure may be released suddenly or gradually. When the pressure is gradually released without releasing the pressure suddenly, the ions of the radioactive substance such as cesium activated under the heating condition are dissociated, but it is gradually cooled under high pressure. Therefore, since the possibility of recombination with the partner before dissociation increases, a method in which an adsorbent that adsorbs a radioactive substance during heat treatment is used in order to prevent recombination as much as possible. Examples of the adsorbent that can withstand a high temperature below the critical temperature include inorganic adsorbents such as clay and zeolite. The adsorbent will be described later.

  When the above-described substance that adsorbs the radioactive substance is not allowed to coexist during the heat treatment, the pressure is rapidly released in the step of releasing the pressure. If the pressure is suddenly released after the heat treatment, the aqueous liquid impregnated into the porous pores of the incinerated ash, which is thought to have adsorbed radioactive materials at that time, will not only thermally expand, but will also vaporize. A sudden volume expansion occurs, and the hole is further expanded by receiving a sudden pressure. As a result, the radioactive substance can jump out of the incineration ash together with the aqueous liquid. Compared to the case where there is only a little liquid due to condensation, such as steaming, more liquid is impregnated in the porous pores. Volume expansion occurs.

After the heat treatment, if the pressure is released rapidly, the condition is changed from the condition that is easily hydrolyzed to the condition that is difficult to hydrolyze, so the possibility that the radioactive substance is recombined is reduced, and it is released with the vaporization of the aqueous liquid or the aqueous liquid. The possibility increases. In this sense, it is preferable to release the pressure rapidly. Here, “abruptly releasing the pressure” means that the ratio of the area (cm ² ) of the opening for releasing the pressure at a time to the volume (cm ³ ) of the sealed space subjected to the heat treatment is 0.0002 / cm. That's all. The higher this ratio is, the better, preferably 0.005 / cm or more, more preferably 0.001 / cm or more, still more preferably 0.005 / cm or more, and particularly preferably 0.00. 01 / cm or more. This is defined when the pressure is released at atmospheric pressure and the opening is opened at once, but under other conditions, the pressure difference and the moving speed are converted as appropriate. To do. In this sense, it is preferable to make the incinerated ash fine powder before the heat treatment.

  An apparatus for performing a series of heat treatment and a process of rapidly releasing pressure in a closed system includes a container for heat treatment, a blow tank connected to the container through a valve, and a silencer as necessary. In the case where the amount of processing is large and a plurality of containers are prepared, the subsequent step of rapidly releasing the pressure may be a common blow tank, and the heat-treated incineration ash is fed there. The volume of the mixture of incinerated ash and liquid in the container is 10 to 90% by volume, preferably 15 to 85% by volume, more preferably 20 to 80% by volume. Open the valve, send the mixture of incinerated ash and liquid into the blow tank at normal pressure, and release the pressure by a sudden pressure change. The radioactive material considered to be confined in the porous pores of the incineration ash is scattered with the aqueous liquid due to the sudden release of pressure, but stays in the blow tank and does not diffuse to the outside. In this process, some radioactive materials dissolve in the aqueous liquid, but some radioactive materials are crushed by suddenly releasing the pressure and some are attached to the wall of the blow tank. The latter radioactive material can be dissolved in water by occasionally washing the inside of the blow tank and sent to the next processing step. In addition, if the amount of incinerated ash in the container is less than the above preferred range, the treatment efficiency is only bad, but conversely if it exceeds the above range, the effect of releasing the pressure suddenly falls, so when only expecting the thermal effect It is not necessary to stick to the above range. The time before and after the treatment can be shortened by interlocking with a device that releases the pressure rapidly.

[Removal process of radioactive material in gas, if necessary]
In the case of the heat-treated material, for example, if it is naturally cooled after heat treatment in an airtight container, most radioactive substances are dissolved in the aqueous liquid and a little in the gas. It is not necessary to remove the radioactive material inside. However, when the pressure is suddenly released, the radioactive substance is released together with the vaporized water gas, and the radioactive gas is dissolved in the aqueous liquid in the liquid state or is accompanied by the water in the aqueous liquid. Divided into those that disperse or settle. As the degree of rapid release of the pressure is significant, or as the pressure is rapidly released and then rapidly cooled, the ratio of the liquid state becomes higher than that in the gas state. Even in such a case, if the incineration ash before the treatment according to the present invention has a high radiation intensity, the radiation intensity in the gas cannot be ignored and may not be released into the living environment as it is. Come. In such a case, the radioactive material must be recovered by carrying out the process of the present invention in a closed system so that the radioactive material present together with the vaporized water gas is not released to the outside. As the method, a method of cooling a gas using a known heat exchange means such as a flash condenser to form an aqueous liquid, or a method of adsorbing a radioactive substance by passing the gas through an adsorption means such as an adsorbent column. Alternatively, a known method such as a method of dissolving a radioactive substance in an aqueous liquid by passing a gas through the aqueous liquid is employed. Thereby, the gas after heat treatment can be safely released into the atmosphere.

[Step of separating liquid and solid]
Next, the heat-released material released from the pressure is separated into a liquid content and a solid content. The solid-liquid separation means for the solid content and the liquid content is not particularly limited, and for example, known separation means such as squeeze, screw press, filter press, centrifugal separation or filtration are used. Here, “solid content” specifically refers to incinerated ash and adsorbents when adsorbents are used in combination with heat treatment, and those that have been crushed by the above-mentioned process or remain solid without being crushed Since it is a residual material and has a fine fracture, it is often difficult to recognize it as a solid at first glance. Therefore, the “solid content” as used in the present invention is often referred to as a drunk liquid rather than a solid content, and the “solid content” as used in the present invention includes such a state. Further, depending on the solid-liquid separation means, there are naturally those that move to the liquid component while being solid because they are fine particles. Although it is preferable to move to such a solid content, it is not necessary to do so. However, when adsorbent is mixed at the time of heat treatment, it must be easy to separate the adsorbent that has adsorbed the radioactive material and the decontaminated incineration ash. One method is to use the difference in specific gravity. Depending on the type of incineration ash, there are those with a specific gravity exceeding 1 and those with a specific gravity smaller than 1. When the specific gravity is larger than 1, the adsorbent having a specific gravity smaller than 1 is selected and the specific gravity of the incineration ash is 1 In the following cases, separation is possible by selecting an adsorbent having a specific gravity greater than 1. However, the present invention is not limited to this. When the specific gravity of the incineration ash is less than 1, the adsorbent is selected to have a specific gravity smaller than 1, and only the adsorbent that has adsorbed the radioactive substance is precipitated with the flocculant in the later step. The incinerated ash decontaminated in step 1 may be separated from the adsorbent that has adsorbed the radioactive material.

[The process of washing radioactive substances adhering to the surface of incinerated ash with water if necessary]
If the radiation intensity in the solid content is already at a safe level at the solid-liquid separation stage, the solid content is returned to the living space. It can be said that the radioactive substance taken in the incineration ash was released to the outside by the above-described treatment.

  On the other hand, when the radiation intensity of the solid component exceeds the safe level, it is a case where the radioactive substance adheres to the solid component. Therefore, the solid component is washed with water to remove the radioactive material adhering to the surface with water. It can be brought to a safe level by dissolving in. The water washing is preferably stirring washing. It is also effective to vibrate the solid content during washing with water. If the safety level is still exceeded, it is possible to reach a safer level by repeating washing with water, or by repeating heat treatment and pressure release treatment on the solid content before or after washing. . The water after washing is combined with the above-mentioned liquid and subjected to the treatment described later.

[Step of adsorbing radioactive material with adsorbent]
Since the radioactive substance such as cesium is dissolved in the liquid, the radioactive substance is aggregated by adsorbing the radioactive substance with an adsorbent. The adsorption of the radioactive substance may be chemical adsorption or physical adsorption. Examples of the adsorbent include ferrocyanide, zeolite, activated carbon, silica gel, activated alumina, and ss made of clay mineral. Some of these adsorbents float in the liquid and others precipitate due to their apparent specific gravity, and are appropriately used as described later. The adsorbent may be brought into contact with the column by allowing it to pass through the liquid component, or the adsorbent in a necessary amount may be charged into the liquid component and stirred. If it is stirring, it can perform also as the above-mentioned washing | cleaning process. In addition, when filling the former column and allowing a liquid part to pass through, the adsorbent must have a particle size that can be filled in the column and does not flow out. In addition, as described above, the adsorbent is not limited to being used after being separated into a liquid component and a solid component, but is heated in a state below the critical temperature of the aqueous liquid and above the saturated vapor pressure before separating the liquid component and the solid component. You may use during the process to process. Due to the difference in specific gravity between the incinerated ash from which the radioactive material has been removed by heat treatment and the adsorbent, it is possible to separate them as described above.

  The adsorbent adsorbed with the radioactive material and the liquid component may be transported to the final disposal site as they are to evaporate the water, but the liquid radioactive material is almost negligible. Even if the adsorbent and liquid are separated and only the adsorbent is transported to the final disposal site, the liquid can be excreted or reused without affecting the environment. The separation of the adsorbent and the liquid component is performed by a method such as centrifugation or filtration in consideration of the physical properties of the adsorbent.

The adsorbent adsorbing the radioactive substance is stored and stored in a container that can suppress the radiation intensity. The material of the container may be any material that can significantly suppress the degree of radiation emission to the outside, such as concrete and lead, and concrete is preferably used. Here, the container becomes a storage container at the final disposal site when the radiation dose of the adsorbent adsorbing radiation is large, but if the radiation dose of the adsorbent is small, the container may be in the form of a container until the adsorbent that adsorbs the radioactive substance is accommodated. What is necessary is just to seal a mouth with sealing materials, such as concrete, after accommodation, and if it is another use, for example, if it is concrete, it will be used for the use in which concrete is originally used, for example, a civil engineering building material. In order to store the adsorbent adsorbing the radioactive substance in the container, a method of mixing the adsorbent together with the raw material water of the raw kon is suitably used.
Alternatively, the adsorbent adsorbing the radioactive substance can be further solidified with a resin, and it can be used as a sealed radiation source for effective use of radiation, for example, for sterilization or as a γ knife.

Example 1
Incinerated ash 30.6 g (radiation intensity 45300 Bq / kg) and hot water 160.4 g having a radioactivity of 1385 Bq were placed in a kettle of a Yoshimura cereal expander (produced by Pon Confectionery Co., Ltd.). The kettle has a circular shape with an opening of about 5 cm in diameter and a capacity of 600 cm ³ inside the kettle. The kettle is closed with a lid and heated rapidly with propane gas from the outside, and the kettle surface temperature is about 240 ° C. The pressure was set to 2 MPa, and this state was maintained for 5 minutes. The surface temperature of the kettle is obtained by measuring the temperature around the kettle with a non-contact thermometer. After this, tap the pawl of the stopper that keeps the hook body and the lid of the hook in close contact with a wooden mallet, and open the steel box with an opening toward the expander and the recovery polystyrene container as an interior. The pressure was released. The ratio of the opening area of about 50 cm ^{2 to} the heat treatment space volume of 600 cm ³ of the expander kettle is about 0.08 / cm. The polystyrene container has a thickness of 5 mm, a size of a rectangular parallelepiped having a size of 55 cm × 55 cm × depth of 120 cm, and the steel box has a thickness of 1.2 mm, and a size of a rectangular parallelepiped having a size of 60 cm × 60 cm × 120 cm. The treated incinerated ash containing water adhering to the wall surface of the box was collected by wiping with Kimwipe (registered trademark) to obtain 31.9 g. The radiation intensity was 8390 Bq / kg. In addition, there was no residue in the kettle, and most of the charged hot water was diffused as steam. In this embodiment, the diffused vapor is not cooled and liquefied and collected, but it goes without saying that radioactive cesium dissolved in water can be adsorbed by the adsorbent. The recovered 31.9 g was dispersed in hot water 301.2 g, stirred, and solid-liquid separated using a filter paper (No. 1). The filtrate was 251.2 g, had a radioactivity of 181 Bq, and the radiation intensity was 720 Bq / kg. Radiocesium dissolved in the filtrate was recovered by adsorption with an adsorbent. On the other hand, although the filtration residue (solid content) contained water, it was 72 g, the radioactivity was 97 Bq, and the radiation intensity was 1340 Bq / kg. The initial radiation intensity of the incinerated ash, 45300 Bq / kg, decreased to 1340 Bq / kg, and about 97% of the radioactivity contained in the incinerated ash could be removed.

(Example 2)
After the same heat treatment as in Example 1, it was naturally cooled overnight and returned to room temperature and normal pressure. When filtration and washing were performed in the same manner as in Example 1, the radiation intensity of the filtration residue (solid content) was 2984 Bq / kg, and about 93% of the radioactivity contained in the incinerated ash could be removed.

(Comparative Example 1)
This shows how far the radioactivity can be reduced only by washing with water without performing the heat treatment as in Example 1. Disperse 30.6 g of the same incineration ash as in Example 1 in 74.9 g of hot water, stir, filter using No. 1 filter paper, add 16.5 g of hot water to the residue, stir in the same manner, Filtered. The residue has a radioactivity of 325 Bq, 46.0 g containing water, and its radiation intensity is 7065 Bq / kg. Therefore, only about 85% of the radioactivity of the incinerated ash was removed. The incinerated ash after washing with water was further washed with water in the same manner, but it was also found that the radiation intensity did not decrease below that.

  The present invention can be used to bring incineration ash contaminated with radioactive materials to a safe level.

Claims (2)

The ash containing the radioactive material into the container to be connected to a blow tank to yield volume via a valve, water present together with ash in the container while sealing the container, criticality of the water-soluble liquid or a mixture thereof The step of heat-treating the incinerated ash at a temperature lower than the temperature and a pressure state higher than the saturated vapor pressure, the area of the opening (cm ² ) for releasing the pressure at once with respect to the volume (cm ³ ) of the container By opening the valve at a ratio of 0.01 / cm or more, the pressure is released at a time by sending incineration ash and water, water-soluble liquid, or a mixture thereof into the blow tank, which is lower in pressure than in the container. A step of separating the substance released from the container after the heat treatment into a liquid content and a solid content, and an adsorbent that coexists during the heat treatment or by an adsorbent after the heat treatment. A method for treating incinerated ash containing a radioactive substance, comprising the step of: The method for treating incinerated ash containing a radioactive substance according to claim 1, wherein water, a water-soluble liquid, or a mixture thereof in the heat treatment step is soaked as to cover the incinerated ash.