JP2013178218A - Purification method of radioactive contaminant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of inexpensively and efficiently separating and removing a large quantity of radioactive contaminant generated in large quantities due to an accident in a nuclear power plant and capable of concentrating and collecting radioactive substances to be a cause of contamination.SOLUTION: In a radioactive contaminant purification method, radioactive contaminant such as soil containing radioactive elements such as radioactive cesium is treated under conditions of 600 to 1450°C and partial water vapor pressure of 0.1 atm or less and vaporized radioactive elements are separated and removed from the radioactive contaminant.

Description

  The present invention mainly removes radioactive elements from radioactive contaminants such as soil, incineration ash, sludge and other radioactive contaminants by evaporating and separating the radioactive elements in the radioactive contaminants. Further, the present invention relates to a method for purifying radioactive contaminants in which radioactive elements separated by evaporation are concentrated and recovered.

  Nuclear utilization technology that has greatly advanced in the 20th century is an extremely important technology for the use of energy such as nuclear power generation, and does not require fossil fuel, so it does not involve the generation of carbon dioxide, a global warming gas. As is well known, it is widely used as a low-cost and effective means. Regarding the use of nuclear energy, it is essential to isolate radioactive materials from the external environment so that harmful radioactive materials generated by nuclear use do not leak to the external environment, and various safety measures have been taken. .

  However, when using nuclear power generation, it is difficult to completely prevent leakage of radioactive materials to the external environment, such as a large amount of radioactive materials leaked to the outside from a power plant damaged by the 2011 tsunami. There is also a possibility. And when such a situation occurs, radioactive materials are scattered over a wide area, causing problems such as soil contamination in a very large area, setting restrictions on use as agricultural land and setting semi-permanent restricted areas. It will cause various problems such as having to. In particular, in addition to being said to have a half-life of about 30 years, radioactive cesium usually has a large amount of soil contaminated at high concentrations. Therefore, for example, even if the contaminated soil up to the surface of 50 mm can be removed, the storage amount is assumed to reach at least tens of millions of tons (t) level.

  By the way, as a management technology for substances contaminated by radioactivity, for example, a first container made of activated carbonaceous material and containing waste (radiocontaminant) and a zeolite made of zeolite surround the first container. Strict management facilities are required, such as an underground waste storage facility (see Patent Document 1) composed of a second container that performs and the soil that covers the second container. Management of contaminated soil has extremely difficult problems in terms of land and management facilities. Moreover, these radioactive pollutants are not only soil, but also incinerated ash, sludge, etc., and it is an extremely difficult problem to purify such radioactive pollutants in large quantities and economically.

  On the other hand, methods for purifying radioactive contaminants contaminated by radioactivity include, for example, a method in which high temperature steam is blown into the soil through a pipe (see Patent Document 2), a method using microorganisms (see Patent Document 3), and the like. Proposed. However, in the method described in Patent Document 2, since the high-temperature steam blown in is immediately cooled in the soil, it is removed in a very limited region and is not practical. Moreover, since the method described in Patent Document 3 uses microorganisms, its processing speed is slow, and it is considered difficult to apply to the processing of a large amount of radioactive contaminants.

  Recently, means for removing contaminated soil by steaming at high temperature (see Non-Patent Document 1) has been proposed, but the effect and the like are not disclosed.

JP 62-269098 A JP 2004-243195 A JP-A-4-204295

Asahi Shimbun No. 45096, page 6, October 27, 2011

  Therefore, the present invention has been found by conducting a detailed investigation and study on the chemical existence form of the radioactive element which is the cause of contamination and the chemical reaction based on it, and utilizes the decomposition reaction of the radioactive element in the radioactive pollutant. Thus, an object of the present invention is to provide a technique for separating and removing radioactive elements from industrially large amounts of radioactive contaminants and purifying them.

  The present inventors have conducted a detailed investigation and research on a method for purifying radioactive contaminants such as soil, incinerated ash, sludge, etc. containing radioactive elements leaked and scattered from nuclear power plants, thereby efficiently purifying radioactive elements. The technology has been developed. The gist of the present invention is as follows.

  (1) A radioactive material characterized by subjecting a radioactive contaminant containing a radioactive element to a steam treatment in an atmosphere having a temperature range of 600 to 1450 ° C. and a partial pressure of water vapor of 0.1 atm or more to vaporize and separate the radioactive element. Pollution purification method.

  (2) A method for purifying radioactive contaminants in (1), wherein the radioactive element vaporized and separated is radioactive cesium.

  (3) In (1) or (2), the radioactive cesium contained in the radioactive contaminant is a radioactive oxide characterized in that 30% by mass or more thereof exists as a composite oxide or composite hydroxide containing silicon. Pollution purification method.

  (4) In any one of (1) to (3), the radioactive pollutant purification method is characterized in that the steam treatment of the radioactive contaminant is performed for an integrated time of 10 minutes or more.

(5) In (3) or (4), the radioactive pollutant purification method characterized by performing the steam treatment of the radioactive pollutant under the condition of the formula (1).
M _x / M _H2O <0.03 (1) Formula [M _X : mass of radioactive element to be treated (g), M _H2O : total mass of water vapor supplied to treatment (g)]

  (6) The method for purifying radioactive contaminants according to any one of (1) to (5), wherein the vaporized and separated radioactive elements are concentrated and recovered.

  According to the present invention, it is possible to purify a large amount of radioactive contaminants generated by scattering of radioactive substances and the like and containing radioactive elements in a large amount quickly and economically. In addition, the removed radioactive materials can be concentrated and recovered at a high concentration, which can be reduced in volume and stored, thus greatly reducing the load on the site and facilities for storage, and for a long period of time. Monitoring and management for long storage can also be facilitated.

FIG. 1 is an explanatory diagram for explaining a process of purifying contaminated soil using a rotary kiln in the method for purifying radioactive contaminants of the present invention.

  An embodiment of the present invention will be described with reference to FIG. The present invention is not limited to the rotary kiln system as shown in FIG. 1, and the radioactive pollutant is heated using electricity, or is treated by burner heating using a hydrogen-containing gas, etc. It can be implemented by an appropriate method.

  In FIG. 1, a contaminated soil 4 containing radioactive elements is inserted into a rotary kiln 1 through a hopper 2 from the vicinity of an outlet hood at the upper part thereof. The furnace gas (atmosphere gas) 6 is heated to a predetermined temperature by the burner 3, and the inserted contaminated soil 4 reacts with water vapor in the atmosphere gas 6 while moving in the kiln 1 to perform steam treatment. To be purified. The purified purified soil 5 moves from the lower part of the kiln 1 into the soil container 7, and after the predetermined amount is accumulated in the soil container 7, it is carried out of the system.

  At this time, it was experimentally found that the atmospheric gas 6 needs to have a water vapor partial pressure of 0.1 atm or higher, preferably 0.5 atm or higher. As a method for achieving a water vapor partial pressure of 0.1 atm or higher, the burner 3 is made into a type using a hydrogen-containing gas such as hydrogen or propane gas, or an external heating method using an electric heater or the like is used. Examples of a simple and effective method include a method of appropriately supplying the inside of the kiln to ensure the condition of the water vapor partial pressure of 0.1 atm or more.

  The radioactive material in the contaminated soil 4 exists in the form of an oxide or hydroxide, but is separated as a gas phase such as CsOH in the high-temperature steam atmosphere by the steam treatment of the present invention. By cooling in the apparatus 8, the pollutant can be reduced to a concentrated state and recovered. Contaminants concentrated in the agglomeration device can be collected as dust, etc., and can be collected in the state of cesium hydroxide aqueous solution, etc. along with the agglomeration of water vapor. In addition to further volume reduction by evaporative separation, a method such as precipitation concentration separation as a slightly water-soluble salt such as cesium perchlorate by a neutralization reaction can be employed.

  The treatment temperature during the steam treatment is set to 600 ° C. or higher and 1450 ° C. or lower, preferably 700 ° C. or higher and 1100 ° C. or lower. The lower limit of the temperature is defined as 600 ° C. When the radioactive cesium, which is the most problematic of the radioactive materials, is separated as cesium hydroxide vapor, a temperature higher than the temperature at which the generation of the vapor pressure is promoted is secured. Because it is necessary to do. On the other hand, the upper limit of 1450 ° C. is defined as the result of the melting point measurement experiment conducted by the present inventors on ordinary field and rural soil, and the obtained melting point is 1450 ° C., which exceeds this melting point. When heated, the radioactive element compound in a precipitated state as a highly concentrated compound quickly diffuses and melts in the surrounding material in a molten state, the activity decreases significantly, and the progress of the separation reaction by high-temperature steam is greatly hindered It is to be done. The reason why the upper limit of the preferable treatment temperature is defined as 1100 ° C. is that the melting point of the composite oxide of cesium and silica is about 1100 ° C. When this temperature is exceeded, the composite oxide melts in the soil. This is because it diffuses to form a solid solution, causing a decrease in cesium activity and a decrease in reaction.

  Here, the processing temperature means a gas atmosphere temperature in contact with the radioactive contaminant to be processed, and can be measured by a thermocouple or the like.

  In addition, the upper limit of the partial pressure of water vapor of the atmospheric gas in contact with radioactive pollutants is not specified, but if it exceeds 10 atm, it is disadvantageous in terms of equipment because a special high pressure resistant device is required, and a completely sealed structure is required. Since maintaining it is not easy in terms of equipment, the water vapor partial pressure is preferably 0.1 atm to atmospheric pressure. In addition, the gas components in the furnace are not specified, but in order to maintain a stable reaction, it is desirable that the main component is an inert gas such as nitrogen or carbon dioxide, and in order to suppress adverse effects such as reoxidation. It is desirable to suppress the oxygen partial pressure in the furnace to 0.03 atm or less.

Incidentally, the method of the present invention can be applied to pollutants containing various radioactive elements as purification targets, and examples of radioactive elements include radioactive strontium, radioactive iodine, etc. in addition to radioactive cesium. but in particular, radioactive cesium half-life is difficult, especially separated off in long-term ^{[Cs134 (134 Cs), Cs137 (} 137 Cs) and the like] can be suitably applied to purifying. Examples of radioactive contaminants include soil containing the above-mentioned radioactive elements, incinerated ash, sludge, dust, dust, and the like.

In addition, when the substance to be separated and removed is radioactive cesium, cesium mixed in soil, incineration ash, sludge, etc. easily reacts with a large amount of silica in the surrounding area to form stable complex oxides and complex hydroxides. Forming. Therefore, for example, even when radioactive cesium is present as Cs ₂ O · 4SiO ₂ which is extremely thermodynamically stable, when the steam treatment is performed according to the purification method of the present invention, the following (2) According to the formula, Cs ₂ O · 4SiO ₂ is decomposed, and the generated radioactive cesium is separated from soil components as metal vapor and removed. Therefore, such a reduction reaction process using a relatively high temperature is particularly effective for a substance that is chemically and stably bonded to silica.
Cs ₂ O · 4SiO ₂ + H ₂ O → 2CsOH ↑ + 4SiO ₂ (2)

That is, according to the experimental findings of the present inventors, when 30% by mass or more of radioactive cesium is contained in a radioactive contaminant as a composite oxide or composite hydroxide containing silicon, CsOH or CsCO It has been found that it is difficult to separate and remove by a simple method such as removal by water washing, which is applied when ₃ or the like is present alone. However, according to the present invention, even when 30 mass% or more of radioactive cesium is contained as a composite oxide or composite hydroxide containing silicon, radioactive cesium can be vaporized and separated and removed. It is particularly effective to perform the process utilizing the high temperature steam reaction according to the present invention.

  In order to advantageously proceed with the removal of radioactive substances by the above means (for example, the formula (2)), it is preferable to secure the treatment time of the steam treatment as 10 minutes or more as continuous or intermittent integration time. It is possible to obtain a better removal rate. There is no upper limit for the integrated processing time at this time, but even if processing exceeding 5 hours is performed, the processing efficiency per unit time decreases and the efficiency of removal rate also decreases. There should be.

Further, in order to further promote the removal of radioactive substances (eg, the formula (2)), it is more preferable to increase the amount of water vapor with respect to the mass of the target radioactive element, and the present inventors have studied. As a result, it was experimentally found that the treatment under the conditions defined by the following formula (1) is particularly desirable.
M _X / M _H2O <0.03 (1) Formula [M _X : mass of radioactive element to be treated (g), M _H2O : total mass of water vapor (g) supplied to treatment]

  In the method of the present invention, the radioactive element vaporized and separated from the radioactive contaminants is preferably concentrated and recovered, and is concentrated and recovered by a dust collection method such as dry or wet as appropriate. . Here, examples of the dry dust collection method include a method in which a radioactive element separated by vaporization is adsorbed on a zeolite and recovered. On the other hand, as a wet dust collection method, the radioactive element separated by vaporization is submerged in water. A method of collecting and collecting dust by blowing into the water and then collecting the water by evaporating it can be exemplified. Further, in the steam treatment of the present invention, the radioactive substance is vaporized and separated as a hydroxide in the high-temperature steam. In addition to dust collection as described above, the radioactive substance in the steam is directly converted into an aqueous hydroxide solution by gas cooling. Furthermore, it is possible to further increase the volume reduction efficiency by drying the aqueous hydroxide solution or recovering the hydroxide as a salt. In any method, since the radioactive substance can be concentrated and recovered, the volume of the radioactive element-containing material to be processed can be reduced.

  Next, the present invention will be further described with reference to examples. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is an example of this one condition. It is not limited to. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[Examples 1-7 and Comparative Examples 1-4]
In order to confirm the effects of the present invention, the following experiments (Examples 1 to 7 and Comparative Examples 1 to 4) were performed using a 3 kg scale test rotary kiln as a purifier and the total pressure at atmospheric pressure.

  The kiln was heated using pure oxygen + hydrogen burner and electric heating, the temperature measurement during the experiment was performed using a thermocouple, and the gas atmosphere in the kiln was collected by suction gas sampling. The amount of water vapor generated by the combustion of the hydrogen burner at this time was set to 161 g / min because the complete combustion conditions were the hydrogen flow rate 200 NL / min + the oxygen flow rate 100 NL / min. This is 200 NL per minute (calculated as 17.9 g when calculated with a gas volume of 22.4 L at a molecular weight of 2, 0 ° C. and 12.4 at 1 atm), and 100 NL per minute (calculated similarly at a molecular weight of 32, 142.9 g. ).

Prior to this experiment, water vapor caused by evaporation of a small amount of water contained in advance in an experimental sample to be described later, or water vapor in leak gas inevitably mixed in the apparatus should be less than 0.5 g / min. Has been confirmed in a preliminary experiment. In addition, since no intentional water vapor mixing is performed, the water vapor in the atmosphere can be regarded as water vapor generated by burner combustion in the entire amount (2H ₂ + O ₂ → 2H ₂ O ↑). Therefore, the experiment was conducted on the assumption that the flow rate of water vapor passed through the apparatus was 161 g / min.

  Moreover, the change of the water vapor partial pressure in Examples 1-4 and Comparative Examples 1-3 is changing nitrogen gas flow volume so that it may become a partial pressure of each level with respect to the water vapor amount generated by the burner combustion. The water vapor partial pressure is changed. At this time, about the influence of the temperature fall by nitrogen gas mixing, etc., it controlled to the temperature of each level by changing the amount of electric heating. Furthermore, the water vapor partial pressure in the atmospheric gas is confirmed by sampling the gas (nitrogen + water vapor) into a container during the experiment, cooling the collected gas to room temperature, and condensing it as liquid water at room temperature. And by measuring the residual gas volume. As a result, it was confirmed that each level had a desired water vapor partial pressure.

In addition, the contaminated soil contains silica in a proportion of 90% by mass or more, and in other soils containing magnesia, alumina, moisture, etc., 2 kg of the calcined cesium compound Cs by mixing with a reagent instead of radioactive cesium ₂ O.4SiO ₂ (Cesium carbonate and silica were mixed at a molar ratio of 1: 4 and fired under conditions of 800 ° C. × 60 minutes in an Ar atmosphere. When identified by powder X-ray, Cs ₂ The simulated contaminated soil prepared by adding and mixing 20 g of O · 4SiO _{2 (} approximately 100% by mass) (therefore, the mass of Cs in the simulated contaminated soil is 10.2 g).

  Since the same result can be obtained with a radioactive element, which is an isotope chemical reaction, in this experiment, a pseudo test using a reagent cesium was performed without using radioactive cesium for safety. Incidentally, in the experiment using radioactive elements, the concentration measurement may be evaluated by the amount of generation of gamma rays or the like.

In each of Examples 1 to 4 and Comparative Examples 1 to 3, 2 kg of the simulated contaminated soil was charged into a kiln and subjected to steam treatment under the conditions shown in Table 1, and the cesium concentration in the soil after this steam treatment Was determined by chemical analysis and the removal rate was calculated.
The results are shown in Table 1.

  Examples 1 and 2 of the present invention were processed at 800 ° C. for 20 minutes with water vapor partial pressures of 1.0 atm and 0.2 atm, respectively, and a high removal rate of 80% or more was obtained. . Further, in Example 3 of the present invention, the treatment was performed at 800 ° C. for 60 minutes at a water vapor partial pressure of 1.0 atm, but the removal rate was further improved by extending the treatment time as compared with Example 1. . In Example 4, the treatment time was further extended to 300 minutes (6 hours) and treated at 630 ° C., but the effect of improving the removal rate was relatively small compared to Example 3. In Examples 5, 6, and 7, the experimental temperature was 1000 ° C. or higher. In Example 5 (1050 ° C.), a particularly good removal rate (93.3%) was obtained. In Examples 6 and 7, which are high-temperature conditions as described above, there was an adverse effect of a decrease in activity due to partial soil diffusion due to the melting of the cesium compound, and the removal rate tended to decrease with increasing temperature, but the target removal A 70% rate has been achieved.

  On the other hand, in Comparative Example 1, the processing temperature is 550 ° C., but the removal rate is only low because the processing temperature is low. In Comparative Example 2, the decomposition reaction is insufficient due to the water vapor partial pressure being too low, and only a low removal rate is obtained. Moreover, in the high temperature conditions of Comparative Examples 3 and 4, the soil is in a molten state, and the cesium compound seems to have uniformly melted, so the removal rate is significantly reduced. In these Comparative Examples 1 to 4, the removal rate is less than 50% in any case.

  In addition, water vapor and vaporized and separated cesium hydroxide were recovered as an aqueous cesium hydroxide solution by a cooling device installed on the exit side of the kiln during the experiment. It is obtained from the cesium concentration in the recovered aqueous solution obtained from the cesium concentration in the aqueous solution measured by chemical analysis and the amount of the aqueous solution, and from the analytical value of the cesium concentration in the dust remaining in the kiln and the dust amount. The amount of residual cesium was determined, and the recovery efficiency of cesium in this experiment was evaluated from the amount of cesium in these recovered aqueous solutions and the amount of residual cesium. As a result, it was confirmed that cesium vaporized and separated from the simulated contaminated soil can be efficiently collected in the aggregating apparatus without being discharged out of the system.

1: Rotary kiln 2: Hopper 3: Burner 4: Contaminated soil 5: Purified soil 6: Atmospheric gas 7: Soil container 8: Aggregation device

Claims (6)

  A radioactive pollutant containing a radioactive element is treated with water vapor in an atmosphere having a temperature range of 600 to 1450 ° C. and a partial pressure of water vapor of 0.1 atm or more, and the radioactive element is vaporized and separated. Purification method.   The method for purifying radioactive contaminants according to claim 1, wherein the radioactive element to be vaporized and separated is radioactive cesium.   The method for purifying radioactive contaminants according to claim 2, wherein 30% by mass or more of radioactive cesium contained in the radioactive contaminant is present as a composite oxide or composite hydroxide containing silicon.   The method for purifying a radioactive substance according to any one of claims 1 to 3, wherein the steam treatment of the radioactive pollutant is performed for 10 minutes or more in an integrated time. The method for purifying a radioactive substance according to claim 3 or 4, wherein the steam treatment of the radioactive contaminant is performed under the condition of the formula (1).
M _X / M _H2O <0.03 (1) Formula [M _X : mass of radioactive element to be treated (g), M _H2O : total mass of water vapor (g) supplied to treatment]   The method for purifying radioactive contaminants according to any one of claims 1 to 5, wherein the radioactive elements separated by vaporization are concentrated and recovered.

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