JP3143839B2 - Treatment of radioactive liquid waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating radioactive waste liquid for efficiently removing radioactive waste from radioactive waste liquid containing uranium and beta decay nuclide which is a daughter nuclide of uranium.

[0002]

2. Description of the Related Art A process effluent discharged from a uranium hexafluoride conversion process in a nuclear fuel conversion facility contains radioactive substances of beta decay nuclides such as uranium and thorium, which are daughter nuclides of uranium. A method of adding water glass to this process waste liquid to coagulate and precipitate these radioactive substances and removing it from the process waste liquid is disclosed in JP-B-48-38320.
No. 6,086,045. This method utilizes the fact that water glass (sodium silicate) forms a precipitate of amorphous silica, which is a polar adsorbent having a large surface area and activity, in an ammonia solution or an acidic solution. The radioactive substances such as uranium and beta decay nuclide which are daughter uranium of uranium are adsorbed and collected, and these radioactive substances are removed and recovered from the waste liquid. According to this method, 50-200p
It is possible to remove the radioactive material from the process effluent containing the radioactive material of about pm to reduce the uranium concentration to about 100 ppb.

[0003] On the other hand, when nuclear fuel made from uranium recovered by reprocessing spent nuclear fuel is converted in the future, highly radioactive uranium (uranium uranium) is contained in the process effluent discharged from the conversion step. Isotope), and in order to reduce the radioactivity concentration of these waste liquids to the emission standard value, the uranium concentration should be 10 to 20 ppb.
It is necessary to reduce to the extent. From this, it is predicted that the above conventional water glass treatment method cannot sufficiently cope with the problem.

[0004]

A method of adding water glass to a process effluent to coagulate and precipitate these radioactive substances and removing the radioactive substances from the process effluent is disclosed in Japanese Patent Publication No. Sho.
Various methods other than the method of 8-38320 have been proposed (Japanese Patent Publication No. 60-636, Japanese Patent Publication No. 61-26040, and Japanese Patent Publication No. 33117). However, considering that the uranium radioactivity concentration in the waste liquid will increase in the future, Both methods still have room for improvement in uranium removal. On the other hand, when the process waste liquid is treated with water glass, the silica dissolves in the waste liquid, and as time passes, the silica becomes a precipitate and re-precipitates, which hinders the discharge of the treated waste liquid. There is also.

[0005] An object of the present invention is to provide a method for treating radioactive waste liquid which more efficiently removes radioactive waste from radioactive waste liquid containing uranium and β-decay nuclide which is a daughter nuclide of uranium.

[0006]

SUMMARY OF THE INVENTION The present invention is directed to an improved method for treating radioactive waste liquid by adding water glass to a radioactive waste liquid containing uranium and beta decay nuclide, which is a daughter nuclide of uranium, to remove uranium and beta decay nuclide. It is. The characteristic place is
It is to add alumina sol in addition to water glass.

Hereinafter, the present invention will be described in detail. The radioactive effluent treated in the present invention is a process effluent mainly discharged from the conversion step of uranium hexafluoride, and is a daughter nuclide of uranium such as uranium and thorium and protactinium.
Including decay nuclides. That is, this radioactive waste liquid is a filtrate (a normal conversion step waste liquid) obtained by filtering a slurry of ammonium biuranate (ADU) generated by adding aqueous ammonia to an aqueous uranyl fluoride solution. As another radioactive waste liquid, ammonium biuranate (AD) produced by adding aqueous ammonia to an aqueous uranyl nitrate solution
There is a filtrate (nitric acid waste liquid) obtained by filtering the slurry of U).

The water glass of the present invention includes alkali silicates such as sodium silicate and potassium silicate. The alumina sol added to the radioactive waste liquid in addition to the water glass is a colloid liquid of alumina hydrate using water as a dispersion medium. The amount of each of the water glass and the alumina sol varies depending on the total amount of the radioactive liquid waste and the content of uranium and β-decay nuclide which is a daughter nuclide of uranium. When the addition amount of water glass is 1, the addition amount of alumina sol is desirably in the range of 0.1 to 1 in volume ratio. If it is less than 0.1, the effect of adding the alumina sol is poor, and if it exceeds 1, only a radioactive substance decontamination coefficient equivalent to 1 can be obtained. In order to increase this decontamination coefficient, it is preferable to add alumina sol after adding water glass, and it is more preferable to add water glass after adding alumina sol twice.

[0009]

When water glass and alumina sol are added to radioactive waste liquid, silica particles and alumina particles are dispersed as colloid particles in the waste liquid. Positively charged alumina particles adsorb and neutralize negatively charged silica particles. As a result, in the waste liquid, the silica particles and the alumina particles aggregate and precipitate due to the electric suction force. At this time, the coagulation effect of silica is further improved due to the action of alumina as compared with the case of water glass alone, and the trapping effect of uranium and β-decayed nuclides increases accordingly. Therefore, a higher decontamination effect can be obtained as compared with the conventional case of using water glass alone, and uranium and β
The concentration of the decay nuclide can be further reduced. This can also be confirmed from the fact that the concentration of silica remaining (dissolved) in the waste liquid decreases when alumina sol is added. This also has the effect of reducing the concentration of silica in the waste liquid. The uranium collected by precipitation is later eluted with nitric acid and easily recovered.

[0010]

EXAMPLES Next, examples of the present invention will be described in order to show specific embodiments of the present invention. The embodiments described below do not limit the technical scope of the present invention. <Example 1> A process waste liquid (a waste liquid from a normal conversion step) having a uranium concentration of 20 ppm discharged from a conversion step of uranium hexafluoride was prepared as a radioactive waste liquid containing uranium and β-decay nuclide which is a daughter nuclide of uranium. . This waste liquid 100
mL was placed in a beaker, 0.94 mL of water glass containing 200 g / L of sodium silicate was added to the waste liquid, and the mixture was stirred for 5 minutes using a magnetic stirrer. Next, 0.94 mL of the same water glass was added, followed by stirring for 5 minutes.

Next, alumina sol (trade name:
"Alumina sol-100", manufactured by Nissan Chemical Co., Ltd.)
88 mL was added and stirred for 10 minutes. The silica particles negatively charged and the alumina particles positively charged by the addition of the water glass and the alumina sol aggregated and precipitated by an electric attraction force. This aggregated precipitate was suction-filtered using a Buchner funnel. Among the uranium and β-decayed nuclides contained in the obtained filtrate, β-decayed nuclides are present in only a very small amount in the waste liquid, so the uranium concentration was measured using a fluorometer on behalf of uranium, and the measured values were measured. The decontamination factor was calculated based on The silica concentration of the filtrate was measured using an inductively coupled plasma emission spectrometer. Table 1 shows the results. “Alumina sol-100” is a colloidal solution of boehmite-based alumina hydrate using water as a dispersion medium, having an alumina content of 10 to 11% by weight, a pH of 2.5 to 4.5, and a specific gravity (20 ° C.). 1.09-1.14, average particle size 100-1
0000 nm, viscosity (25 ° C.) 100 to 10,000 CP
And the charge of the alumina particles is positive.

Example 2 The procedure of Example 1 was repeated except that 0.24 mL of the same alumina sol was added to the same mixture as in Example 1. Table 1 shows the results.

Comparative Example 1 Example 1 was repeated except that no alumina sol was added to the same mixture as in Example 1.
Was repeated. Table 1 shows the results.

[0014]

[Table 1]

As is clear from Table 1, in Comparative Example 1, 2
Although the uranium concentration of 0 ppm could only be reduced to about 50 ppb (decontamination coefficient: about 420), the uranium concentration was reduced to about 15 pb in Examples 1 and 2.
pb (decontamination coefficient: about 1300) was able to be reduced. This was about a little over 1/3 that of Comparative Example 1. Further, while the silica concentration of Comparative Example 1 was 373 ppm, the silica concentration of Examples 1 and 2 was 13
2 ppm and 199 ppm, about 1/3 of Comparative Example 1.
It shows that the value was significantly reduced.
Further, in Comparative Example 1, silica precipitate was precipitated several minutes after the addition of water glass, whereas in Examples 1 and 2, no silica precipitate was deposited even when left for one week.

Example 3 A slurry of ammonium biuranate (ADU) produced by adding aqueous ammonia to an aqueous uranyl nitrate solution as a radioactive waste liquid containing uranium and β-decay nuclide which is a daughter nuclide of uranium is obtained by filtration. A filtrate (nitrate waste liquid) having a uranium concentration of 20 ppm was prepared. 100 mL of the waste liquid was placed in a beaker, and water glass containing 200 g / L of sodium silicate was added to the waste liquid at a rate of 0.94 m.
L was added, and the mixture was stirred for 5 minutes using a magnetic stirrer. Next, 0.94 mL of the same alumina sol as in Example 1 was added to the mixture, and the mixture was stirred for 10 minutes. By the addition of water glass and alumina sol, a coagulated precipitate was formed in the same manner as in Example 1. This aggregated precipitate was subjected to suction filtration in the same manner as in Example 1. Among the uranium and β-decay nuclides contained in the obtained filtrate, the uranium concentration of uranium was measured in the same manner as in Example 1, and the decontamination coefficient was calculated based on the measured values. Further, the silica concentration of the filtrate was measured in the same manner as in Example 1. Table 2 shows the results.

Comparative Example 2 An agglomerated precipitate was formed in the same manner as in Example 3 except that no alumina sol was added. Uranium concentration, decontamination coefficient, and silica concentration were determined in the same manner as in Example 3. Table 2 shows the results.

[0018]

[Table 2]

As apparent from Table 2, in Comparative Example 2, 2
Although the uranium concentration of 0 ppm could be reduced only to about 401 ppb (decontamination coefficient: about 130), in Example 3, the uranium concentration was reduced to about 55 ppb (decontamination coefficient: about 912). Was completed. Further, while the silica concentration of Comparative Example 2 was 360 ppm, the silica concentration of Example 3 was 150 ppm, which was about 2/5 of that of Comparative Example 2, indicating that the silica concentration was significantly reduced. .
Further, in Comparative Example 2, silica precipitate was precipitated several minutes after the addition of water glass, whereas in Example 3, no silica precipitate was deposited even after being left for one week.

[0020]

As described above, according to the present invention, by adding alumina sol to radioactive waste liquid containing uranium and β-decay nuclide which is a daughter nuclide of uranium in addition to water glass, water glass alone is sufficient. Uranium and β-decayed nuclides that could not be completely captured can be captured more efficiently. Furthermore, since the silica particles and the alumina particles aggregate and precipitate in the waste liquid due to the electric attraction force, the silica concentration can be further reduced as compared with the prior art, and the reprecipitation of the silica precipitate can be prevented. As a result, there is no problem in discharging the treated waste liquid.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Kenji Nishimura, Nakamachi, Naka-machi, Naka-gun, Ibaraki Pref. 142199 (JP, A) JP-A-53-2319 (JP, A) JP-A-56-109825 (JP, A) JP-A-57-172298 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) G21F 9/10 G21F 9/06

Claims (4)

(57) [Claims] 1. A method for treating a radioactive waste liquid in which water glass is added to a radioactive waste liquid containing uranium and β-decay nuclide which is a daughter nuclide of uranium to remove uranium and β-decay nuclide, wherein alumina sol is added to the water glass A method for treating a radioactive waste liquid, comprising adding. 2. The method according to claim 1, wherein the alumina sol is added after the water glass is added once or twice. 3. The method for treating a radioactive liquid waste according to claim 1, wherein the β-decay nuclide includes thorium or protactinium. 4. The method for treating a radioactive waste liquid according to claim 1, wherein the volume ratio of the added amount of water glass to the added amount of alumina sol is 1: 0.1 to 1.