JP2015215186A - Method for removing radioactive cesium in contaminated soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently separating radioactive cesium from an object to be treated such as soil or sludge in which radioactive materials have been accumulated and concentrating the radioactive cesium.SOLUTION: A method for removing radioactive cesium comprises: an addition step of adding a metallic oxide, a chlorine-based oxidizer, and an alkali to an object to be treated; a step of melting the object to be treated to which chemicals have been added; and a step of dissolving the object to be treated in water and then concentrating the object to be treated by using an ion exchange resin.

Description

  The present invention relates to a method for separating / removing and concentrating radioactive cesium from a workpiece contaminated with radioactive cesium.

  In recent years, there has been a demand for a technique for separating and concentrating radioactive materials from soil contaminated with radioactive materials leaked from nuclear power plants and the like and sludge. For this reason, various measures have been studied, and iodine and cesium, which are the main radiation materials at the time of the accident, are attracting particular attention. Among these, iodine has a short half-life and is a gas body, so that diffusion and decay from the field are fast. However, cesium is regarded as a problem because it has a long half-life of 30 years. In addition, cesium has a strong adsorptivity with clay minerals, and once adsorbed in clay minerals, it is not easily released even by acid washing, ignition, etc., and it has been reported from various studies that it is a major problem in processing. Has been.

Cesium is an alkali metal, and although it is a highly reactive metal due to its ionization tendency and the like, it has a large ionic radius and is an alkali metal that is difficult to move ions. Moreover, since the large ionic radius matches the crystal interval of the clay mineral and silicate mineral, it is sandwiched in the crystal and further prevents liberation.
From these things, the technique which removes a cesium efficiently is calculated | required and many techniques seen in the patent document 1, the patent document 2, etc. are disclosed.

  However, in order to liberate radioactive cesium, these conventional techniques require a high temperature of 1350 ° C. in Patent Document 1, and even if possible at the laboratory level, when the practical level is reached, the temperature can be maintained in terms of equipment and running. A big problem occurs in cost.

  In Patent Document 2, in order to adapt to an actual large-scale treatment, such as adding a high-concentration strong acid to a treatment object and leaving it for 30 days, treatment of acid gas generated when a strong acid is added to the treatment object. Since a strong acid is used, there are problems such as corrosion of the container during the reaction.

The radioactive cesium adsorbed on the soil is difficult to move because of its large ionic radius, despite the highest reactivity among alkali metals. Also, when adsorbed on clay minerals, it is sandwiched between crystals due to the size of its ionic radius and cannot be easily released. This is especially true for mica minerals.

JP 2013-242194 A JP2014-32110A

  An object of the present invention is to provide a method for removing radioactive cesium from highly contaminated soil in addition to clearing the conventional drawbacks as described above and adding new ideas not found in other conventional techniques.

  In order to efficiently separate and concentrate radioactive cesium contained in the object to be treated, which is the above object, the present invention includes an addition step of adding a metal oxide, a chlorine-based oxidant, and an alkali to the object to be treated; This is a method for removing radioactive cesium comprising a melting step in which a chemical is added to and a step of concentrating with an ion exchange resin after dissolving in water. (Claim 1)

  In more detail, the process is described in the previous configuration. Manganese dioxide is used as the metal oxide in the addition process. Sodium chlorate, potassium chlorate, hypochlorous acid, calcium chlorate, chlorine gas is used as the chlorinated oxidant. Any one of these or a mixture thereof is melted by heating to 400 ° C to 800 ° C after mixing using any of sodium hydroxide, potassium hydroxide, calcium hydroxide or a mixture thereof as an admixture for the melting process. Then, the radioactive cesium is dissolved in water as a manganate, then moved to the aqueous phase, and the radioactive cesium is captured by a cation exchange resin. (Claim 2)

In the present invention, radioactive cesium that is difficult to release adsorbed on the untreated material is converted into water-soluble manganate and cesium chloride by alkali melting due to the loss of manganese dioxide and chlorinated oxidant. Since the changed cesium is ionized, it becomes efficient and easy to capture the ion exchange resin.
In the present invention, the additive excluding manganese dioxide is water-soluble, and part of the manganese dioxide changes to a water-soluble manganate. It is an advantage not to cause a large increase, and it is also an effect of the invention.

Separation of radiation cesium from the object to be treated in the present invention is not a physical separation but a chemical separation, so it can be concentrated by a single cesium product and changed to a form that can be captured by an ion exchange resin. By doing so, it became possible to reduce the volume of pollutants to the limit. This has made it possible to separate radioactive cesium from a fine-grained material that has been said to be difficult until now.
Furthermore, it is one of the effects of the invention that the chemicals used are less harmful and do not require management of the processed product after removal.

The flowchart which shows the basic process of this invention.

  Embodiments of the present invention will be described below. FIG. 1 is a flowchart showing the basic steps of the present invention, and judgment A (1) returns to mixing before the melt is not green, and proceeds to the next when the melt is green. Decision B (2) continues by exchanging the cation exchange resin when radiation is detected in the filtrate that has passed through the cation exchange resin.

  As an apparatus for carrying out the present invention, a workpiece and a feeder for quantitatively delivering a drug, a rotary kiln that melts them while mixing them, a water tank that melts the melted workpiece, dissolved cesium and a workpiece And a filter press for separating the cesium and an exchange resin tower for adsorbing cesium in the filtrate.

  In the present invention, the particle diameter of the object to be treated is not particularly limited, and it is sufficient that the particle diameter is such that the added drug can be mixed. However, since the chemical to be added is an oxidizing agent, it is preferable to remove the combustible material by preliminary firing. Manganese dioxide to be added is 2% to 5% by mass of the object to be processed. The chlorine-based oxidizing agent added at the same time is 10% to 20% by mass. The melting temperature varies depending on the alkali used, but is 400 ° C to 800 ° C. The amount of alkali added varies depending on the particle size of the workpiece, but is preferably 20% to 50% by mass of the workpiece.

  In the present invention, the amount of the agent added to release radioactive cesium from the object to be processed varies somewhat depending on the particle size of the object to be processed. Manganese dioxide, which is an additive, may not be able to come into contact with an object to be processed unless it is added in an amount of about 5% when the object has a large particle size (5 mm or more on a sieve). The amount of chlorinated oxidant may be large, but if there are many flammables, the loss will increase. Remelting of is desirable. Since the amount of alkali for melting is absorbed as the particle size of the object to be processed decreases, the amount of alkali added increases as the particle diameter of the object to be processed decreases.

  Among the additive agents used in the present invention, the chlorine-based oxidizing agent and alkali are most preferable to add sodium chlorate and sodium hydroxide having a low melting temperature because the melting temperature is as low as about 400 ° C. As other chlorinated oxidants, any of perchlorates, chlorates, chlorine gas, and the like can be used, but there are safety problems such as an increase in melting temperature and explosion risk. Similarly, any other alkali, potassium hydroxide, calcium hydroxide, calcium carbonate or the like can be melted, but the melting temperature tends to increase.

  Conditions for insufficient release of radioactive cesium from the object to be treated include the case where the added chlorine-based oxidant is consumed by the combustible material of the object to be treated and the alkali is absorbed by the object to be treated. It is considered that the contact between the chlorine-based oxidizing agent and the alkali in the presence of manganese dioxide in the object to be treated is insufficient. In these cases, the melt becomes black or brown, and becomes green when it is sufficiently melted.

  When the release of radioactive cesium from the object to be processed is insufficient, the recovery rate is increased by reheating by adding a chlorine-based oxidant and an alkali again. In this case, manganese dioxide is present in the object to be treated and does not need to be added again.

  The melted treated product is dissolved in water and then filtered. The filtration residue becomes a treated product from which radioactive cesium has been removed, and the filtrate is passed through a cation exchange resin to adsorb the radioactive cesium. Although a large amount of alkali is present in this filtrate, since the cesium salt has a higher ionization tendency than other metals, it is preferentially captured by the ion exchange resin.

  The example which implemented this invention is demonstrated below. The soil in Fukushima after the Great East Japan Earthquake was sampled with a particle size of 5 mm or less and 2 mm or more under the sieve, and 5 mass% manganese dioxide, 10 mass% sodium chlorate, and 30 mass% sodium hydroxide were mixed. And melted at 400 ° C. for 15 minutes. After cooling, the radiation concentration in the soil where the salt was separated by dissolving in water decreased by 75%.

  In addition, sludge collected in the side groove of Fukushima after the accident was sampled with a particle size of 2 mm or less under the sieve, and 5% by mass of manganese dioxide, 10% by mass of sodium chlorate, and 40% by mass of sodium hydroxide. Mix and melt at 400 ° C. for 15 minutes. After cooling, the radiation concentration in the soil dissolved in water and separated from the salt decreased by 86%.

  Radiation (registered trademark) PA-1000, a scintillation counter manufactured by HORIBA, Ltd. was used as the radiation dose measuring instrument in both Examples 1 and 2 above.

The present invention is widely used in decontamination plants as a trump card for removing cesium in contaminated soil, and hopes that recovery from the Great East Japan Earthquake will be realized at an early stage by spreading the efficient method.

Claims (2)

  A method for efficiently separating and concentrating radioactive cesium contained in an object to be processed, an addition step of adding a metal oxide, a chlorine-based oxidant and an alkali to the object to be processed, and an agent added to the object to be processed The radioactive cesium removal method which consists of a melt process of this and the process of concentrating with ion exchange resin after melt | dissolving in water. Manganese dioxide as the metal oxide in the addition step, sodium chlorate, potassium chlorate, hypochlorous acid, calcium chlorate, chlorine gas, or a mixture thereof as the chlorinated oxidant, or a mixture thereof, is mixed in the melting process. After mixing with sodium hydroxide, potassium hydroxide, calcium hydroxide or a mixture of these as a chemical, heating and melting to 400 ° C to 800 ° C and dissolving radioactive cesium in water as manganate The method for removing radioactive cesium according to claim 1, wherein the radioactive cesium is moved to an aqueous phase and captured by a cation exchange resin.

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