Solidification method for radio-active waste

Abstract

To provide a solidification method for a radio-active waste that can improve the fluidity of a mixture containing a radio-active waste and cement to achieve a high filling level.SOLUTION: A solidification method for a radio-active waste comprising dry powder containing sodium sulfate or borate, has: a process of preparing a solidifying materia composition by contacting cement and a polycarboxylic acid dispersant without contacting the radio-active waste; and a process of solidifying the radio-active waste by using the solidifying materia composition.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for solidifying radioactive waste. [Background technology]

[0002] Known methods for treating radioactive waste include solidifying and storing radioactive waste consisting of concentrated radioactive liquid waste or dried radioactive powder (see, for example, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-70599 [Patent Document 2] US Patent Application Publication No. 2016 / 0276049 [Patent Document 3] Japanese Patent Publication No. 63-243798 [Patent Document 4] Japanese Patent Application Publication No. 8-179095 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to increase the amount of radioactive waste stored in storage facilities, it is necessary to increase the loading capacity of the radioactive waste stored in the storage containers. To achieve this, it is necessary to obtain a dry powder of the radioactive waste to reduce the moisture content, and after mixing with cement as a solidification agent, to improve the fluidity of the mixture containing the radioactive waste and cement.

[0005] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for solidifying and treating radioactive waste that can improve the fluidity of a mixture containing radioactive waste and cement, thereby enabling high loading of radioactive waste. [Means for solving the problem]

[0006] A first aspect of the present invention is a method for solidifying radioactive waste consisting of a dry powder containing sodium sulfate or borate, comprising the steps of: preparing a solidification material composition by contacting cement and a polycarboxylic acid-based dispersant without contacting the radioactive waste; and solidifying the radioactive waste using the solidification material composition.

[0007] A second aspect of the present invention is the method for solidifying radioactive waste according to the first aspect, characterized in that the dry powder is obtained by concentrating and drying radioactive liquid waste. Effect of the Invention

[0008] According to the present invention, the fluidity of a mixture containing radioactive waste and cement can be improved, thereby enabling high loading of radioactive waste. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the present invention will be described based on preferred embodiments.

[0010] The solidification method of the present embodiment is a method for solidifying radioactive waste consisting of dry powder containing sodium sulfate or borate.

[0011] Sodium sulfate may be anhydrous, represented by Na2SO4, or hydrated. Radioactive waste consisting of dry powder containing sodium sulfate includes, for example, radioactive waste obtained by treating radioactive liquid waste discharged from a boiling water reactor (BWR).

[0012] The borate may be a salt such as a sodium salt, for example, sodium orthoborate, sodium metaborate, etc. The borates may be a mixture of two or more kinds, and may be anhydrous or hydrated. Radioactive waste consisting of a dry powder containing a borate may be, for example, radioactive waste obtained by treating radioactive liquid waste discharged from a pressurized water reactor (PWR).

[0013] The dry powder of radioactive waste can be obtained by concentrating and drying radioactive liquid waste. Examples of radioactive liquid waste include aqueous solutions such as cleaning liquid, decomposition liquid, and condensate. The radioactive waste may contain inorganic substances such as inorganic salts, and organic substances such as resins. Additives such as neutralizing agents, pH adjusters, and antifoaming agents may be added to the radioactive liquid waste or radioactive waste.

[0014] The dry powder may be a mixture of two or more kinds of dry powders. For example, the dry powder may include powders obtained by filtering or pulverizing various solids. More specifically, examples of the dry powder include powders such as sludge, incineration ash, and pellets, and particles such as granules.

[0015] The solidification material composition used for solidifying radioactive waste contains cement as a solidification material. The ratio of cement in the solidification material composition is not particularly limited, but it is preferable that the components other than radioactive waste and water are mainly composed of cement.

[0016] As the cement, a material known as a solidifying agent can be used. Specific examples include Portland cement, blast furnace cement, slag cement, alumina cement, etc. These cements have hydraulic properties and can be solidified by mixing with water and curing.

[0017] The solidification material composition used in the embodiment includes a polycarboxylic acid dispersant. The ratio of the polycarboxylic acid dispersant in the solidification material composition is not particularly limited, but the polycarboxylic acid dispersant may be added at an appropriate ratio according to a conventional method. In a preferred solidification treatment method, as described in detail below, the dispersant and cement are added to the mixing water, and after mixing, the radioactive waste is added and mixed.

[0018] The polycarboxylic acid dispersant is not particularly limited, but may be a compound having two or more carboxylic acid groups in one molecule. The compound may be a long-chain compound or a polymer compound. The carboxylic acid group may be a free carboxylic acid group (-COOH) or a neutralized carboxylic acid group (-COOM, where M is a cation such as a metal).

[0019] The cation of the neutralized carboxylic acid group is the sodium ion (Na + ), potassium ion (K + ) and the like. The cation M may be one type or two or more types. The carboxylic acid groups contained in the compound may be all or partly neutralized. The polycarboxylic acid dispersant compound may become alkaline by neutralizing the carboxylic acid groups.

[0020] The solidification material composition used in the embodiment is prepared by contacting at least cement and a polycarboxylic acid-based dispersant without contacting radioactive waste. Other additives can be added to the solidification material composition as necessary. The additive added to the cement may be only the polycarboxylic acid-based dispersant.

[0021] In preparing the solidifying material composition, the order of contacting the mixing water, cement, polycarboxylic acid dispersant, optional additives, etc. is not particularly limited and can be set appropriately. A method of dissolving or dispersing the polycarboxylic acid dispersant in the mixing water, then adding the cement and kneading is preferred. An aqueous solution of the polycarboxylic acid dispersant may be added to the mixing water.

[0022] Dissolving or dispersing the polycarboxylic acid dispersant in water before adding the cement makes it easier to dissolve or disperse the polycarboxylic acid dispersant, but this does not exclude adding at least a portion of the cement to the mixing water before adding at least a portion of the polycarboxylic acid dispersant to the mixing water.

[0023] In the solidification material composition used in the embodiment, at least the cement and the polycarboxylic acid dispersant are contacted without contacting with radioactive waste. This can suppress the decrease in fluidity even if the cement contains calcium ions. When the solidification material composition comes into contact with sulfates, the sulfates may be adsorbed to the cement before kneading and affect the fluidity. In the present invention, the state of not contacting with radioactive waste does not exclude a state in which a deposit containing radioactive waste remains on the kneading device, nor does it exclude a state in which a small amount of radioactive waste comes into contact with the cement due to the deposit. Here, the small amount means an amount in which the effect of the deposit can be ignored with respect to the total amount of the solidification material composition obtained by kneading, for example, 3 mass % or less, 1 mass % or less, or 0.5 mass % or less of the total amount.

[0024] The solidifying material composition before contacting with the radioactive waste is preferably homogenized by kneading. The kneading method and kneading device are not particularly limited, and can be appropriately selected from known methods and devices. The solidifying material composition may be a cement composition mainly composed of kneading water and cement.

[0025] In the solidification step of solidifying the radioactive waste, the radioactive waste is brought into contact with the solidification material composition. The method of bringing the radioactive waste into contact with the solidification material composition is not particularly limited and can be appropriately set.

[0026] When the radioactive waste is brought into contact with the solidifying composition, it is preferable to mix the radioactive waste with the solidifying composition to obtain a mixture containing the radioactive waste and cement. The mixture is preferably a homogeneously mixed mixture. The kneading method and kneading device are not particularly limited, and can be appropriately selected from known methods and devices.

[0027] The mixture containing radioactive waste and cement may be a mixture of radioactive waste and a solidification material composition. If necessary, other additives can be added after contacting the radioactive waste with the solidification material composition.

[0028] By kneading the mixture containing the radioactive waste and cement, a slurry or paste-like mixture is obtained. By curing the mixture, the radioactive waste can be solidified. The curing conditions are not particularly limited, but room temperature or higher is preferable.

[0029] The mixture containing the radioactive waste and the cement may be cured and solidified in a predetermined storage container. The step of preparing the solidifying material composition or the step of kneading the mixture may be carried out in the storage container or in a container different from the storage container.

[0030] A solidified body is obtained by solidifying a mixture containing radioactive waste and cement. The loading rate of the radioactive waste in the obtained solidified body is not particularly limited, but it is possible to achieve a high loading rate of, for example, 30 wt% to 50 wt%.

[0031] According to this embodiment, the mixture containing radioactive waste and cement contains a polycarboxylic acid-based dispersant, and the cement and the polycarboxylic acid-based dispersant are brought into contact with each other without being brought into contact with the radioactive waste, so that a highly fluid mixture can be obtained.

[0032] According to the present embodiment, since the mixture containing radioactive waste and cement has high fluidity, it is easy to mix more uniformly and a solidified body having sufficient strength can be stably obtained. In addition, it is possible to suppress voids in the mixture and the solidified body, and it is also possible to improve the handling properties of the mixture, such as mixing and filling.

[0033] Sources of radioactive waste include, but are not limited to, nuclear power plants, nuclear fuel reprocessing facilities, nuclear research facilities, and various facilities that use radioisotopes.

[0034] The present invention has been described above based on a preferred embodiment, but the present invention is not limited to the above-mentioned embodiment, and various modifications are possible without departing from the gist of the present invention. EXAMPLES

[0035] The present invention will now be described in detail with reference to examples.

[0036] Example 1 A mixture containing waste and cement was prepared by a method (represented as "Method A" in the table) in which a dispersant and cement were added to mixing water, mixed, and then the waste was added and mixed.

[0037] The waste material used was dry powder of sodium sulfate (referred to as "Na sulfate" in the table). As the dispersant, a polycarboxylic acid-based dispersant (product name "SN Dispersant 5034", manufactured by San Nopco Limited, indicated as "SN5034" in the table) was used.

[0038] The ratio (wt%) of waste and dispersant in the mix indicates the percentage of waste in the mixture containing waste and cement. The water-cement ratio (W / C) in the mix indicates the weight ratio of mixing water to cement.

[0039] To evaluate the physical properties (fluidity) of the kneaded product, the flow value (mm) of the kneaded product was measured according to JIS R 5201 (physical testing method for cement). The flow value of the kneaded product was measured within 15 minutes after kneading.

[0040] The results of Example 1 are shown in Table 1.

[0041] [Table 1]

[0042] Comparative Example 1 A mixture containing waste and cement was prepared by a method (represented as "Method B" in the table) in which a dispersant and waste were added to mixing water, mixed, and then cement was added and mixed.

[0043] In Comparative Example 1, the same waste material and dispersant as those in Example 1 were used. The composition and physical properties of Comparative Example 1 were measured in the same manner as in Example 1.

[0044] The results of Comparative Example 1 are shown in Table 2.

[0045] [Table 2]

[0046] In Example 1 in which Method A was used, the fluidity of the kneaded product was improved compared to Comparative Example 1 in which Method B was used, even though the amount of the dispersant added was relatively small.

[0047] Comparative Example 2 A mixture containing waste and cement was prepared in the same manner as in Example 1, except that a sulfonic acid-based dispersant (product name "Mighty (registered trademark) 150", manufactured by Kao Corporation, indicated as "M150" in the table) was used. Furthermore, the flow value (mm) of the mixture was measured by the method used in Example 1. The flow value of the mixture was also measured within 15 minutes after mixing.

[0048] The results of Comparative Example 2 are shown in Table 3.

[0049] [Table 3]

[0050] In Example 1, in which a polycarboxylic acid-based dispersant was used, the fluidity of the kneaded product was remarkably improved depending on the amount of dispersant added, compared to Comparative Example 2, in which a sulfonic acid-based dispersant was used. When the same compositions as those in Comparative Examples 2-3 and 2-4 were kneaded by Method B, the flow values ​​were 207 mm and 211 mm, respectively.

[0051] (Example 3 and Comparative Example 3) A mixture containing waste and cement was prepared in the same manner as in Example 1-3 or Comparative Example 1-2, except that the ratio of waste was changed to 40 wt% or 50 wt%. Furthermore, the flow value (mm) of the mixture was measured by the method used in Example 1. The flow value of the mixture was also measured within 15 minutes after mixing.

[0052] The resulting mixture was cured and solidified, and the compressive strength (MPa) of the resulting solidified product was measured at 7 days of age according to JIS R 5201 (physical testing method for cement). Note that MPa is expressed in N / mm 2 is equal to.

[0053] The results of Example 3 and Comparative Example 3 are shown in Table 4. Table 4 also shows the results of Examples 1-3 and Comparative Examples 1-2 described above.

[0054] [Table 4]

[0055] Even if the ratio of waste materials (filling rate) is increased, it was confirmed that the fluidity and compressive strength tend to be higher when using Method A than when using Method B. In addition, the results of kneading using Method B in the same manner as Comparative Examples 1-2, 3-1, and 3-2, except that no dispersant was added, are as follows. Comparative Example 3-3: Waste material 30wt%, flow value 175mm, compressive strength 33.1MPa Comparative Example 3-4: Waste material 40wt%, flow value 209mm, compressive strength 27.2MPa Comparative Example 3-5: Waste 50wt%, flow value 206mm, compressive strength 26.0MPa

[0056] (Example 4 and Comparative Example 4) In order to compare the solidified bodies obtained by Method A and Method B, a kneaded material containing waste and cement was prepared in the same manner as in Example 1 or Comparative Example 1, except that the cement ratio was changed so that the flow value of the kneaded material was about 300 mm when the amount of waste packed in a 200 L drum was about 100 kg and the kneading volume was 180 L. Specifically, in Example 4, the ratios per drum were 81.9 kg of water, 1.6 kg of dispersant, and 202.7 kg of cement, and the density of the kneaded material was 2.163 g / cm. 3 In Comparative Example 4, the ratio of water to cement was 124.4 kg and the density of the mixture was 1.659 g / cm 3Furthermore, the flow value (mm) of the kneaded product was measured by the method used in Example 1, and the compressive strength (MPa) of the solidified body was measured by the method used in Example 3. Furthermore, the flow value of the kneaded product was measured within 15 minutes after kneading.

[0057] The results of Example 4 and Comparative Example 4 are shown in Table 5.

[0058] [Table 5]

[0059] In Example 4 in which Method A was used, the amount of cement added could be increased to adjust fluidity, and the compressive strength increased, compared to Comparative Example 4 in which Method B was used.

[0060] (Example 5 and Comparative Example 5) A mixture containing waste and cement was prepared in the same manner as in Example 1 and Comparative Examples 1 and 2, except that the waste was sodium metaborate dihydrate (represented as "NaBO2" in the table). Furthermore, the flow value (mm) of the mixture was measured by the method used in Example 1. The flow value of the mixture was also measured within 15 minutes after mixing.

[0061] The results of Example 5 and Comparative Example 5 are shown in Table 6.

[0062] [Table 6]

[0063] In Example 5, in which Method A and a polycarboxylic acid-based dispersant were used, the fluidity of the kneaded product was remarkably improved compared to Comparative Example 5-3 in which Method B was used, even though the waste material was a borate. In Comparative Examples 5-4 and 5-5, in which a sulfonic acid-based dispersant was used, no significant difference in fluidity was observed due to the difference in the kneading method.

Claims

1. A method for solidifying radioactive waste consisting of dry powder containing sodium sulfate or borate, comprising the steps of: a step of contacting cement and a polycarboxylic acid-based dispersant without contacting the radioactive waste to prepare a solidification material composition; a solidification step of solidifying the radioactive waste using the solidification material composition; 1. A method for solidifying radioactive waste, comprising:

2. 2. The method for solidifying radioactive waste according to claim 1, wherein the dry powder is obtained by concentrating and drying radioactive liquid waste.

Images