JPH04364500A - Removing method of radioactive iodic ion - Google Patents

Abstract

PURPOSE:To remove iodic ions in radioactive waste by making them into the form of a stable compound. CONSTITUTION:Radioactive iodic ions contained in an NaI solution and a bismuth oxide (alpha-Bi2O3) are made to react directly with each other and thereby the radioactive iodic ions are immobilized as alpha-Bi5O7I and removed. It is preferable that the NaI contains the radioactive iodic ions of high concentration of 5X10<-2>mol.dm<-3> or above. By adjusting the ratio of the gram-molecular weight of the bismuth oxide to the gram-ion number of the radioactive iodic ions to be in a range from 4:1 to 1:2, a single phase of alpha-Bi5O7I is obtained irrespective of the concentration of the iodic ions. Accordingly, the iodic ions of 99.8% in the NaI solution can be removed as a stable solid.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing radioactive iodine ions, and more particularly, to a method for removing iodine ions from radioactive waste liquid produced in nuclear power plants.

0002

[Prior Art and Problems to be Solved by the Invention] In nuclear power plants, radioactive iodine gas produced by nuclear fission in the reactor is used during fuel inspection and replacement, and even when fuel is handled in an accident or in the reactor. There is a risk of sudden release due to a runaway accident, and continuous release during operation of a fuel reprocessing plant. In response to these various situations, methods for treating radioactive iodine gas, such as cleaning treatment methods, physical/chemical treatment methods using solid adsorbent filling, and treatment methods using ion exchange agents, are being considered.

However, in the cleaning treatment method using a liquid adsorbent, it is stored as a liquid for a long period of time (half-life is 17
1,000,000 years), there are many problems in terms of quantity and safety. In addition, iodine captured by the physical and chemical treatment method filled with solid adsorbents is constantly exposed to the possibility of exchange with other gases, and also has the disadvantage that it easily releases adsorbed substances when the temperature rises. be. Furthermore, in the treatment method using an ion exchange agent, the heat resistance stability of the exchange resin is maintained up to about 100° C., and sufficient performance is not exhibited at higher temperatures. Furthermore, since it is itself flammable, it poses a safety management problem.

On the other hand, from another perspective, attempts have been made to react iodine ions in a solution with other elements to extract and preserve them as stable compounds. This method is
Low concentration iodine ion solution made by dissolving NaI in water (
α-Bi2O3 in <5×10-2 mol・dm-3)
This is a method in which a mixture of . The purpose of mixing α-Bi5O7I into the raw materials is to reduce the time required for crystal nucleation and to speed up the reaction rate. (Note that bismuth oxide includes α-phase, β-phase, γ-phase
There are polymorphs of phase and δ-phase, where the α phase is a low-temperature stable phase, the δ phase is a high-temperature stable phase, and the others are metastable phases. Bi5O7I also has α and β polymorphisms. Hereinafter, when "Bi2O3" and "Bi5O7I" are described, unless otherwise specified, both represent α-form compounds)

However, in this method, the iodine ion concentration in the NaI solution is reduced to 5×10-2 mol·dm-3.
If the concentration is higher than that, compounds other than Bi5O7I will be generated, so the iodine ion concentration must be kept below that level. Therefore, there is a drawback that a large amount of solution must be processed in order to process a large amount of iodine ions. In addition, due to the low concentration, if anions such as HCO3-, SO4-2, and Cl- coexist in the solution, the growth of Bi5O7I will be hindered, and furthermore, the nucleation of Bi5O7I will take a long time. This requires an extra step of adding Bi5O7I to the raw materials in advance to carry out the reaction. Furthermore, in this method, even after repeated use, the Bi used as a raw material
At least 30% of the 2O3 remains unreacted, and it is impossible to separate and reuse it, resulting in large losses of raw materials and an increased amount of solids to be stored.

The object of the present invention is to eliminate the drawbacks of the above-mentioned conventional methods and to provide a method by which radioactive iodine ions can be easily removed and stored as a stable solid with high efficiency.

[0007]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventor has conducted intensive research on a method for extracting radioactive iodine ions contained in NaI solution as a stable solid, and has hereby devised the present invention. It is completed.

That is, the present invention combines radioactive iodine ions and bismuth oxide (α-Bi2O) contained in a NaI solution.
3) by directly reacting with α-B to convert radioactive iodine ions into
The gist of this invention is a method for removing radioactive iodine ions, which is characterized by immobilizing and removing i5O7I.

The present invention will be explained in more detail below.

[Effect]

Compounds of bismuth, oxygen and iodine include α-Bi5O7I, β-Bi5O7I, Bi7O9I.
3. Five types of compounds have been reported: Bi4O5I2 and BiOI. Each of these has a different crystal structure and is solid at room temperature. Among these compounds, α-Bi5O7I is the most stable to water and heat, for example in non-acidic solutions (2
The concentration of iodine ions released by decomposition at 5℃) is approximately 10
It is about -17 mol·dm-, which is an extremely small amount. Furthermore, it is stable up to about 550° C. when heated in air, and the stability is incomparably higher than that of other methods.

In the present invention, the stable α
-Bi5O7I, which is stored separately from the solution.

The above reaction proceeds according to the following formula. 5α-Bi2O3+2I-+H2O→2α-B
i5O7I+2OH-... (1)

0013
] As a NaI solution, high concentration (≧5×10-2 mol
- It is preferable to use a NaI solution containing iodine ions of dm-3). If the concentration is high, nucleation can be easily performed, so 100% of the raw material bismuth oxide is reacted, and unreacted bismuth oxide is not generated, and there is no need to add Bi5O7I as in the past. , and the influence of interfering ions present in trace amounts can also be ignored.

[0014] Even if the concentration of the NaI solution is high, the product is α-Bi5O7 when performing this chemical reaction.
In order to prevent only I from forming or coexisting with other compounds, the ratio of the number of gram molecules of bismuth oxide involved in the reaction to the number of gram ions of iodine ions should be adjusted from 4:1 to 4:1. It is best to adjust the ratio between 1:2. Even if the proportion of iodine is higher than this, it is also at least α-Bi5O
Compounds other than 7I begin to grow.

Next, examples of the present invention will be shown.

[0016]

[Example 1] Approximately 233 mg of bismuth oxide (5 x 10-4
gram molecule) and 1000 μl of a 0.2 mol/dm-3 sodium iodide solution (2
The reaction was carried out at 0°C. No stirring was performed. After a certain period of time, the solid was separated from the solution, dried, and identified by powder X-ray diffraction. This is the solid (Bi2O3) powder X before the reaction.
The progress of the reaction was examined by comparing it with the line diffraction pattern. When the reaction is completed according to chemical formula (1),
The remaining amount of Bi2O3 should become zero and the X-ray diffraction pattern should disappear. An example is shown in FIG.

As is clear from the results shown in Table 1, the reaction rate varies somewhat depending on the temperature; at 50°C, the reaction is completed within 48 hours, but at 25°C, it seems to take longer. In either case, the amount of Bi2O3 remaining after a predetermined reaction time is zero or very small.

[0017]

[Example 2] Approximately 233 mg of bismuth oxide (5 x 10-4
gram molecule) and 250 μl of 0.8 mol/dm-3 sodium iodide solution (2 x 10-4 gram ion)
The reaction was carried out at 0°C and the progress was examined. The experimental procedure was the same as in Example 1.

As is clear from the results shown in Table 2, the reaction was completed within 24 hours.

[0018]

Example 3 Approximately 233 mg of bismuth oxide and 1000 μl of sodium iodide solutions of various concentrations were reacted at 50° C. for 96 hours. The result

[Table 3] Shown below.

[0019]

Example 4 Approximately 233 mg of bismuth oxide and 250 μl of sodium iodide solutions of various concentrations were reacted at 50° C. for 48 hours. The result

[Table 4] Shown below.

As is clear from the results of Examples 1 and 2,
Changing the volume ratio of the raw material solid to the reaction solution also changes the reaction rate. It can be seen that as the volume of the solution increases, the reaction rate slows down. Furthermore, as is clear from the results of Examples 3 and 4, the conditions necessary to generate only Bi5O7I are B
It can be seen that it is preferable to maintain the ratio of the number of gram molecules of i2O3 to the number of gram ions of I- within a certain range.

[0021]

[Example 5] The reaction rate of iodine ions was investigated. Approximately 255 mg of bismuth oxide (5.5 x 10-4 gram molecules) and 0
．． 2 mol/dm-3 sodium iodide solution 1000
After reacting .mu.l (2.times.10.sup.-4 gram ions) at 50.degree. C. for 72 hours, the concentration of iodine ions remaining in the solution was analyzed. As a result, the remaining iodine ions were about 0.2% of the solution before reaction, and 99.8% of the iodine ions were converted into solid.

[0022]

[Effects of the Invention] As detailed above, according to the present invention,
Since the radioactive iodine ions contained in the NaI solution react with bismuth oxide and are converted into α-Bi5O7I, which is taken out and stored, stability against water, heat, various gases, etc. is significantly improved. Furthermore, since this compound is a solid crystal, it is not bulky and is easy to store and handle.

Furthermore, when the iodine ion concentration in the NaI solution is low, the volume of the liquid can be reduced to 1/100 or less by concentrating it, and the reaction vessel is extremely compact compared to the conventional technology. It only needs to be small, and the labor involved can be greatly reduced. In addition, since nucleation occurs easily when reacted in a highly concentrated solution, there is no need for the conventional step of adding Bi5O7I to the raw material bismuth oxide before the reaction, and there is no need for the effect of interfering ions present in trace amounts. Can be ignored. Furthermore, under high concentrations, the raw material bismuth oxide
Since the reaction can be carried out at 0%, unreacted bismuth oxide is not generated.

[Brief explanation of the drawing]

[Figure 1] Raw material (Bi2O3) and product (α-Bi5O
7I only), where A is the case of the raw material (Bi2O3) and B is the case of the product.

Claims (3)

[Claims] 1. Radioactive iodine ion, characterized in that the radioactive iodine ion contained in the NaI solution and bismuth oxide (α-Bi2O3) are directly reacted to immobilize and remove the radioactive iodine ion as α-Bi5O7I. How to remove. [Claim 2] The NaI solution is 5 x 10-2 mol・d
The method according to claim 1, which contains radioactive iodine ions at a high concentration of m-3 or more. [Claim 3] The ratio of the number of gram molecules of bismuth oxide to the number of gram ions of radioactive iodine ions is 4:1 to 1:
3. The method according to claim 1 or 2, wherein the method is adjusted between 2 and 3.