JPH036477B2 - - Google Patents

Description

[Detailed description of the invention]

(Purpose and Background) The present invention processes an aqueous solution containing radioactive iodine, particularly a radioactive iodine-containing waste liquid generated from the reprocessing process of spent nuclear fuel, and converts most of the waste liquid into a permeated liquid with reduced radioactive iodine. , relates to a treatment method for separating a small amount of concentrated liquid containing iodine. At reprocessing facilities that recover uranium and platonium from spent fuel, the fuel bodies are cut and melted, and then a complex and lengthy chemical separation process is performed, which generates a large amount of process waste liquid. This waste liquid contains iodine, especially ^129I , which has a long half-life.
Conventionally, waste liquid with low radioactivity levels generated from reprocessing plants is evaporated and concentrated, and the evaporated and condensed treated liquid is released into the environment under certain conditions, while the concentrated liquid is solidified with asphalt. In addition to the radionuclides such as iodine, Ce, Ru, and Cs, the process waste liquid mentioned above contains
It also contains small amounts of U and Pu nuclides, which are fissile materials. Conventional evaporation concentration processing is generally carried out in a pH range of slightly acidic to neutral to ensure safety, such as the criticality problem of Pu nuclides. Iodine nuclides in water become ionic and non-volatile chemical forms in alkaline conditions, but tend to become volatile molecular iodine in weakly acidic to neutral conditions.
Therefore, in the evaporative concentration treatment of neutral to weakly acidic wastewater, iodine volatilizes together with water vapor and transfers to the condensate, making it impossible to effectively separate radioactive iodine from water. In order to compensate for these drawbacks, an adsorption treatment method using silver-adsorbing activated carbon has been introduced (Japanese Patent Application Laid-Open No. 1983-1999).
156898). This treatment method operates at room temperature and pressure, so it is more energy-saving than evaporation treatment. However, when the concentration of iodine is low, the amount of silver used is relatively large, and a large amount of used activated carbon is generated. There are some drawbacks, such as the difficulty of In recent years, reverse osmosis membrane treatment technology has been widely used for radioactive waste liquid treatment. The present inventors conducted research on the technology for the purpose of removing radioactive iodine. As a result, they found that the removal efficiency of radioactive iodine largely depends on the chemical form of iodine, and found that while ionic chemical forms can be efficiently removed, molecular iodine is difficult to remove. Therefore, in view of the above situation, the inventors of the present invention
As a result of intensive research into a treatment method that is energy-saving, room-temperature treatment, produces as little secondary waste as possible, and sufficiently reduces the amount of radioactive iodine in the emitted treatment solution, we have completed the present invention. (Structure of the Invention) That is, in the present invention, a reducing agent is added to an aqueous solution containing radioactive iodine, which is then treated with a reverse osmosis membrane device, and separated into a permeated liquid with a reduced amount of radioactive iodine and a concentrated liquid with an increased amount of radioactive iodine. It depends. Suitable reducing agents to be added to the aqueous solution containing radioactive iodine in the present invention include sulfurous acid, sulfites, and hydrazine (hydrates, sulfates). Further, the pH during the addition treatment is preferably in the weakly acidic to neutral range of 4 to 7. In principle, the amount of reducing agent added may be in excess of the chemical equivalent of iodine present in the aqueous solution;
Preferably, the amount is 2 to 10 times equivalent. As the reverse osmosis membrane device used in the present invention, a module composed of a commercially available reverse osmosis membrane can be used. Among these, preferred reverse osmosis membranes include cellulose acetate-based reverse osmosis membranes and PBIL reverse osmosis membranes. The structure of the module may be a tubular type, a spiral type, a hollow fiber type module, a ribbon type module, etc., and a hollow fiber type module having a large membrane area per module is particularly preferred. When implementing the treatment method of the present invention, it is more effective to separate iodine by assembling the reverse osmosis membrane device as a multi-stage system using two or more stages. As an example, a case of three stages will be explained with reference to the accompanying drawings. In Figure 1, symbols 1, 2, and 3 are the first
This is a module for a stage, second stage, and third stage reverse osmosis membrane device. The radioactive iodine-containing aqueous solution to which a reducing agent has been added is pressurized and supplied to the first stage module 1 from the raw material supply line 4, the permeate with reduced iodine is sent from the line 11, and the concentrated liquid with increased iodine is sent to the line 1. It is taken out from 12. The first-stage permeate taken out from the line 11 is supplied to the second-stage module 2, and the permeate containing even less iodine than the first-stage permeate is taken out from the line 21. On the other hand, the second-stage concentrate taken out from the line 22 is circulated to the raw material supply line 4 and processed again by the first-stage module. In addition, the first stage concentrate taken out from line 12 is supplied to the third stage module 3, and the concentrate containing even more iodine than the first stage concentrate is taken out from line 32 and sent to another processing system. , for example, to an asphalt solidification treatment system. line 31
The permeate from the third stage taken out is circulated to the raw material supply line 4 and processed again by the first stage module. When performing such three-stage treatment, it is possible to use approximately 90% of the supplied wastewater as a treatment liquid, and to reduce the iodine concentration to 1/50 to 1/100 of that of the supplied wastewater. The above is an explanation of the basic principle of the system; if the concentration of salts in the concentrated liquid in the first stage is high enough, asphalt treatment may be performed as is, and the permeated liquid in the second stage may also contain radioactive iodine. If the concentration is still high, further treatment may be performed with another module. The present invention overcomes the drawbacks of conventional evaporative concentration treatment methods.
It provides a treatment method that solves all problems such as the inability to sufficiently concentrate radioactive iodine due to restrictions on waste liquid PH due to Pu criticality limitations, etc. It can be treated at room temperature in a weakly acidic to neutral range, This method realizes a treatment method that reduces the amount of radioactive iodine to an extent that allows release into the environment. Furthermore, since the energy required for the present invention is only that required for pressurizing the aqueous solution, it also has advantages such as significant energy savings compared to evaporative concentration treatment. Furthermore, unlike adsorption treatment, the present invention does not limit the amount of iodine treated to a certain limit depending on the capacity of the adsorbent, and does not generate a large amount of secondary waste. The effects of the present invention will be described below based on Examples. Example 1 Sodium sulfite was added to a simulated reprocessing low-level waste liquid containing 5 ppm of stable isotope iodine (I ₂ ) corresponding to a concentration of 10 ^-3 μCi/Ml ¹²⁹ I and 500 ppm of NaNO ₃
It was added to a concentration of 10 ppm. This amount added corresponds to about 5 times the equivalent of I ₂ . This pretreated simulated liquid was pressurized to 50 kg/cm ² and subjected to reverse osmosis treatment using a hollow fiber type module of a cellulose acetate semipermeable membrane (Toyobo HR-5255). The treatment was continued until the amount of liquid that permeated through the semipermeable membrane became 80% of the amount of the supplied liquid, and the iodine concentrations of the permeated liquid and concentrated liquid at that time were measured. Comparative Example 1 All experiments were carried out under the same conditions as in Example 1 except that sodium sulfite was not added. Table 1 shows the iodine concentration and decontamination coefficient values of the permeate under different pH conditions for Example 1 and Comparative Example 1. Example 2 Hydrazine instead of 10 ppm sodium sulfite
The same test as in Example 1 was conducted except that 2 ppm was added. The results are shown in Table 2.

[Table] Concentration in supply liquid Decontamination factor (DF) =

Claims (1)

[Claims] 1. A reducing agent is added to an aqueous solution containing radioactive iodine, which is then treated with a reverse osmosis membrane device, and separated into a permeated liquid with a reduced amount of radioactive iodine and a concentrated liquid with an increased amount of radioactive iodine. A method for treating aqueous solutions containing radioactive iodine. 2. The treatment method according to claim 1, which comprises using sodium sulfite or hydrazine as a reducing agent. 3. The treatment method according to claim 1 or 2, which comprises using two or more stages of reverse osmosis membrane devices.