JPH0713674B2 - Method for separating useful nuclides in radioactive liquid waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention is a radioactive substance in which radioactive metal ions, iron ions, nickel ions, chromium ions and cerium tetravalent ions are dissolved. The present invention relates to a method for separating useful nuclides in a radioactive waste liquid for separating cerium tetravalent ions from the waste liquid.

(Prior Art) As a method of decontaminating a substance to be decontaminated that is contaminated with radioactive substances, an acidic aqueous solution containing trivalent cerium ions and tetravalent cerium ions is used, and cerium trivalent ions are converted into cerium trivalent ions by an electrolytic oxidation reaction. There is known a method for decontaminating a radioactive substance by generating tetravalent ions and utilizing the oxidizing power when the generated cerium trivalent ions are converted to remove the radioactive substances from the surface of the substance to be decontaminated.

In this decontamination method, radioactive metal ions gradually accumulate in an acidic aqueous solution together with non-radioactive metal ions.

Therefore, the acidic aqueous solution had to be discarded when the radioactivity concentration or the metal ion concentration reached a predetermined value.

(Problems to be solved by the invention) Conventionally, as a method of disposing of this waste liquid, when a nitric acid aqueous solution in which metal ions containing cerium tetravalent ions having oxidizing power are dissolved is put in a drum and stored, Since it corrodes, iron materials, alkali metals such as magnesium, calcium, zinc, etc., or sulfurous acid and sulfite salts were added as reducing agents, reduced, and stored in drums. However, there is a problem that the amount of waste increases because the reducing agent is added.

The present invention has been made to solve the above problems,
Reduces the amount of radioactive secondary waste generated from the disposal of radioactive waste liquid in which radioactive metal ions, iron ions, nickel ions, chromium ions, and cerium tetravalent ions having oxidizing power are dissolved. It is an object of the present invention to provide a method for separating useful nuclides from a radioactive liquid waste.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the method for separating useful nuclides in a radioactive liquid waste according to the present invention is a method for separating radioactive metal ions, iron ions, and nickel. Nitric acid is added to radioactive waste liquid in which metal ions containing chromium ions and cerium tetravalent ions having oxidizing power are dissolved to form cerium tetravalent ions as complex ions, and then to a strongly basic anion exchange resin. It is characterized in that cerium tetravalent ions which are useful nuclides are separated by adsorption.

(Function) Oxidizing activity of radioactive waste liquid in which radioactive metal ions, iron ions, nickel ions, chromium ions, and cerium tetravalent ions having oxidizing power are dissolved without increasing the amount of oxidizing waste generated. It can be eliminated to reduce the amount of waste.

That is, since the cerium tetravalent ion having an oxidizing power in the radioactive waste liquid is made into a complex ion to be adsorbed and separated by the strongly basic anion exchange resin, the ion having an oxidizing power disappears from the radioactive waste liquid, and the cerium tetravalent ion is removed. As it is removed from the ions, the amount of secondary waste generated is reduced.

(Embodiment) FIG. 1 is a system diagram showing an example of an arrangement of suitable devices for carrying out the method of the present invention.

The figure shows that an acidic aqueous solution containing cerium trivalent ions and cerium tetravalent ions is used to generate cerium tetravalent ions from the cerium trivalent ions by an electrolytic oxidation reaction, and the generated cerium tetravalent ions are converted into cerium trivalent ions. A system diagram of a state in which a reduction treatment device is installed in a decontamination device that dissolves the surface of a substance to be decontaminated that has been contaminated with radioactive substances and removes radioactive substances from the surface of the substance to be decontaminated by utilizing the oxidizing power of changes It shows with.

In the figure, reference numeral 1 is a decontamination tank, and a radioactive waste liquid 2 is stored in the decontamination tank 1, and a metallic decontamination object 3 is immersed in the radioactive waste liquid 2. Reference numeral 4 is an ion exchange tower, and the ion exchange tower 4 is filled with an ion exchange resin 5. The ion exchange resin 5 is immersed in the radioactive waste liquid 2 composed of an acidic aqueous solution containing cerium trivalent ions and cerium tetravalent ions.

The decontamination tank 1 is a tank for dissolving the surface of the substance to be decontaminated 3 contaminated with a radioactive substance to remove the radioactive substance from the surface of the substance to be decontaminated. The radioactive waste liquid 2 composed of an acidic aqueous solution containing the same cerium trivalent ions and cerium tetravalent ions stored in the exchange tower 4 is stored. A substance to be decontaminated 3 contaminated with a radioactive substance is immersed in the radioactive waste liquid 2. The decontamination object 3 is suspended by a cable 7 from a fixing mechanism 6 arranged above the decontamination tank 1. The decontamination tank 1 and the ion exchange tower 4 are connected by a circulation line 8, and a circulation pump 9 is provided in the middle of the circulation line 8 to transfer the radioactive waste liquid 2 from the decontamination tank 1 to the ion exchange tower 4. You can move. Further, the radioactive waste liquid 2 in the ion exchange tower 4 can be drained through a drain line 10.

Reference numeral 11 is a cerium regeneration tank, and this cerium regeneration tank 11
A radioactive waste liquid 2 composed of an acidic aqueous solution containing the same cerium trivalent ions and cerium tetravalent ions stored in the decontamination tank 1 is stored therein. This radioactive waste liquid 2
Is a cerium regeneration anode 12 to which a positive voltage is applied.
And the cerium reproducing cathode 13 to which a positive voltage is applied, are soaked face to face. The cerium regeneration tank 11 and the decontamination tank 1 are connected by circulation lines 14 and 15, and in the middle of the circulation line 15, a circulation pump 16 and a filter.
17 is provided so that the radioactive liquid waste 2 circulates between the decontamination tank 1 and the cerium regeneration tank 11.

On the other hand, decontamination tank 1, ion exchange tower 4 and cerium regeneration tank
An exhaust gas line 18 is connected to the upper side surface of 11 and hydrogen or oxygen, steam and mist generated from the acidic aqueous solution 2 are processed through a exhaust gas line 18 by a condenser 19, a demister 20 and an exhaust blower 21. The liquid condensed in the condenser 19 passes through the return pipe 22 and is collected in the cerium regeneration tank 11.

Examples of the method for separating useful nuclides in the radioactive liquid waste according to the present invention will be described below.

In the examples, nitric acid first which is a cerium trivalent (Ce ³⁺ ) compound
Cerium [Ce (NO ₃ ) ₃ ] is used. It is important that the compound used as the decontaminating agent is a compound that is easily dissolved in water or acid.

Nitric acid (HNO ₃ ) is used as an acidic solution in which Ce ³⁺ (NO ₃ ) ₃ is dissolved.
Use a solution. Ce (NO ₃ ) ₃ concentration 0.8mol / l, HNO ₃ concentration 2m
Cerium recycling tank 11 for radioactive waste liquid 2 from ol / l nitric acid aqueous solution
Ce ³⁺ is exchanged with Ce ⁴⁺ under the influence of the electrolytic redox reaction shown below.

Anode ^{Ce 3+ → Ce 4+ + e -} ......... (1) 2OH - → H 2 O + (1/2) O 2 (↑) + 2e - ......... (2) cathode H ^{+ +} e ^- → (1/2 ) 2H ₂ (↑) (3) The radioactive waste liquid 2 with high Ce ⁴⁺ concentration is transferred to the decontamination tank 1 through the circulation line 14. This decontamination tank 1 is a cerium recycling tank
At the same time that the cerium tetravalent ions generated in 11 are exchanged for cerium trivalent ions, the oxidizing power at that time removes radioactive contamination from the metal base material surface tank of the decontamination target 3. Cerium tetravalent ion (Ce ⁴⁺ ) is exchanged with cerium trivalent ion (Ce ³⁺ ) while the metal surface of the decontamination target 3 is dissolved. On the street.

M + Ce ⁴⁺ → M ⁺ + Ce ³⁺ (4) About 90% of the radioactivity transferred to the radioactive waste liquid 2 due to dissolution of the metal base material surface tank of the material to be decontaminated 3 is present in the insoluble material. Therefore, the radioactive liquid 2 can be easily separated with a filter or the like.
However, the remaining about 10% is present as a metal ion in a state of being dissolved in the radioactive waste liquid 2, so that it is accumulated in the radioactive waste liquid 2 together with the radioactive metal ion.

By using the method for separating useful nuclides in the radioactive liquid waste according to the present invention, the cerium tetravalent ion in the radioactive liquid 2 can be decomposed.

In the radioactive waste liquid 2, the radioactive metal ions are C
o ^2+, Mn ²⁺ or MnO ₄ ^- is dissolved in the state. Iron and nickel ions, chromium ions are Fe ³⁺ , Ni ²⁺ , Cr ³⁺
Alternatively, it is dissolved in the state of Cr ₂ O ₇ ²⁻ and also contains cerium trivalent ions (Ce ³⁺ ) and cerium (Ce ⁴⁺ ). Nitric acid is added to this radioactive waste liquid 2 to adjust the nitric acid concentration to 2.0 mol / l. At this nitric acid concentration, dissolved metal ions C
_{^{o 2+, Mn 2+, MnO 4}} -, Fe 3+, Ni 2+, Cr 3+, Cr 2 O 7, Ce 3+ does not form a complex ion.

Meanwhile, cerium tetravalent ions, complex ions [Ce (NO _{3) 6]} as follows to form ^2-.

Ce ⁴⁺ + 6NO ₃ ⁻ → [Ce (NO ₃ ) ₆ ] ² -... (5) This solution is circulated through the ion exchange column and passed through the strongly basic anion exchange resin 5.

Strongly basic anion exchange resins have the property of adsorbing only anions with a large ionic radius, so complex ions [Ce (N
O ₃ ) ₆ ] ²⁻ is adsorbed, but other ions are not adsorbed and pass through the ion exchange resin 5. Therefore, it is important to adjust the nitric acid concentration to a concentration at which cerium tetravalent ions easily form complex ions. It is necessary to use a strongly basic anion exchange resin because it forms complex ions and adsorbs anions as the ion exchange resin.

Table 1 shows the results of measuring the efficiency of Ce ⁴⁺ adsorption for the examples of the method for separating useful nuclides in the radioactive liquid waste according to the present invention.

Ce ⁴⁺ concentration 0.3mol / l, HNO ₃ concentration 2.0mol / as radioactive waste liquid
A solution having a Fe (NO ₃ ) ₃ concentration of 1.6 mol / l was used. As the ion exchange resin, a strong basic anion exchange resin, a weak basic anion exchange resin, a strong basic anion exchange resin and a weak basic anion exchange resin were used.

The amount of adsorption was determined by measuring the Ce ⁴⁺ concentration before and after passing through the ion exchange column. The adsorption efficiency x is as follows.

x = (ab) / aa: Ce ⁴⁺ concentration before passing through the ion exchange column (mol / l) b: Ce ⁴⁺ concentration after passing through the ion exchange column (mol / l) According to the examples of the present invention from Table 1, Ce ⁴⁺ can be adsorbed by the strongly basic anion exchange resin with an efficiency of 99.8%. The Ce ⁴⁺ concentration is 0.001 to 0.4 instead of 0.3 mol / l.
mol / l, HNO ₃ concentration is 1.0-3.0 mol / l instead of 2.0 mol / l,
The Fe (NO ₃ ) ₃ concentration may be 0.1 to 2.0 mol / l instead of 1.6 mol / l.

An example of a method for separating useful nuclides in a radioactive waste liquid according to the present invention and a case of discarding a conventional radioactive waste liquid will be described by comparing the amounts of secondary waste generated. Obtain the amount of secondary waste generated by the disposal of 1000 l of radioactive liquid waste. There is no secondary waste generated in the examples of the present invention except radioactive waste liquid. On the other hand, the amount of secondary waste generated when the radioactive waste liquid is discarded by adding the reducing agent of the conventional example is determined as the iron agent reducing agent.

The reduction reaction follows the following reaction formula.

Ce ⁴⁺ + e → Ce ³⁺ (1/3) Fe → (1/3) Fe ³⁺ + e As radioactive liquid waste, Ce (NO ₃ ) ₃ concentration 0.8mol / l, HNO ₃ concentration 2.
0 mol / l, Ce ⁴⁺ concentration of 0.3 mol / l aqueous solution, and radioactive waste liquid
The results of examination of the amount of secondary waste generated by radioactive waste liquid treatment when 1000 l is discharged are shown in comparison.

(Reducing agent amount) = (Separation amount of Fe) x 1/3 x (0.3mol / l) x
(1000l) = 56 × 0.3 × 1000/3 = 5.6kg Fe ³⁺ dissolved in radioactive liquid must be neutralized and discarded. Neutralization follows the formula:

^{Fe 3+ + 3OH - → Fe (} OH) 3 and concentrated radioactive waste liquid after neutralization Fe (OH) ₃ becomes Fe ₂ O ₃ powder having water of crystallization. The amount of Fe ₂ O ₃ powder generated, ignoring the water of crystallization, is as follows.

(Amount of generated Fe ₂ O ₃ powder) = (5.6 kg) × (molecular weight of Fe ₂ O ₃ ) / 2 / (molecular weight of Fe) = 5.6 × 160/2/56 = 8.0 kg The results of examining the amount of secondary waste generated in the waste liquid treatment in the case of discarding the radioactive waste liquid of the first embodiment and the conventional example are shown in comparison.

Table 2 shows that, when 1000 l of radioactive waste liquid is treated as waste liquid, the amount of secondary waste is 0 kg in the case of the embodiment of the present invention and 8 kg when the reducing agent is added to the radioactive waste liquid of the conventional example and discarded. There is.

As described above, according to the method for separating useful nuclides in the radioactive liquid waste according to the present invention, since the reducing agent is not added to the radioactive liquid waste, the secondary radioactive substance is compared with the case where the reducing agent is added and discarded. It is clear that less waste will be generated.

[Advantages of the Invention] (1) Add a reducing agent to a radioactive waste liquid in which metal ions containing radioactive metal ions, iron ions, nickel ions, chromium ions, and cerium tetravalent ions having oxidizing power are dissolved. Therefore, the amount of secondary waste generated is smaller than that in the case of reduction.

(2) Efficient separation of cerium tetravalent ions in radioactive waste liquid in which radioactive metal ions, iron ions, nickel ions, chromium ions, and metal ions containing oxidizing cerium tetravalent ions are dissolved. it can.

(3) The [Ce (NO ₃ ) ₆ ] ²⁻ adsorbed on the strongly basic anion exchange resin and separated can be dissolved and reused, so that cerium, which is a useful nuclide, can be effectively recovered.

By the method for separating useful nuclides in the radioactive waste liquid according to the present invention, the radioactive waste liquid loses its oxidizing property, so that it can be dried in a dryer and solidified by a plastic solidification method, a cement solidification method, an asphalt solidification method, or the like. You can

[Brief description of drawings]

FIG. 1 is a system diagram showing an embodiment of a method for separating useful nuclides in a radioactive liquid waste according to the present invention. 1 ... Decontamination tank 2 ... Radioactive waste liquid 3 ... Decontamination object 4 ... Ion exchange tower 5 ... Ion exchange resin 6 ... Fixing mechanism 7 ... Cable 8 ... Circulation line 9 ... Circulation pump 10 ... … Waste liquid line 11 …… Cerium regeneration tank 12 …… Cerium regeneration anode 13 …… Cerium regeneration cathode 14 …… Circulation line 15 …… Circulation line 16 …… Circulation pump 17 …… Filter 18 …… Exhaust gas line 19 …… Condenser 20 …… Demister 21 …… Exhaust blower 22 …… Return pipe

Claims (1)

[Claims] 1. An acidic aqueous solution containing cerium trivalent ions and cerium tetravalent ions is used to produce cerium trivalent ions into cerium tetravalent ions by an electrolytic oxidation reaction, and the oxidizing power of the produced cerium tetravalent ions is increased. In the method for separating useful nuclides in radioactive waste liquid by removing radioactive substances from the surface of the substance to be decontaminated, radioactive metal ions, iron ions, nickel ions, chromium ions, and cerium trivalent ions generated after decontamination are used. Nitric acid is added to radioactive waste liquid in which cerium tetravalent ions are dissolved to form cerium tetravalent ions as complex ions, and then adsorbed on a strongly basic anion exchange resin to produce cerium tetravalent ions, which are useful nuclides. A method for separating useful nuclides in a radioactive liquid waste characterized by separating.