JPH01138497A - Method for decontaminating radiation metal waste - Google Patents

Abstract

PURPOSE:To prevent decontamination performance from deteriorating by reducing permanganate ions into a deposit of manganese dioxide and separating and removing the manganese dioxide before used liquid for chemical decontamination is neutralized. CONSTITUTION:Radioactive metal wastes in a complicate shape such as a valve are dipped in a 1st chemical decontaminating tank 1 which contains an acid solution, etc., containing potassium permanganate and dissolved by oxidation, and the wastes are decontaminated in a 2nd chemical decontaminating tank 2 by using a solution of mineral acid such as sulfuric acid as decontaminating liquid. Further, radioactive metal wastes in a simple shape such as a straight pipe are decontaminated in an electrolytic polishing decotaminating tank 6 which contains an electrolyte consisting of a solution of neutral salt such as a sodium sulfate solution. Permanganate ions which does not contribute to the reaction in the decontaminating tank 1, on the other hand, are reduced into the deposit of MnO2 by feeding the decontaminating liquid from the decontaminating tank 1 into the permanganate ion reducing tank 3 and reducing the ions into MnO2. Then this liquid is fed into a neutralizing tank 4, mixed with the used liquid in the decontaminating tank 2, and fed into a solid-liquid separating tank 5, so that a separated supernatant is sent to the decontaminating tank 6 for reused.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for decontaminating radioactive metal waste whose surface is contaminated and which is generated from facilities handling radioactive materials such as nuclear power plants. This article relates to a decontamination method for reducing the volume of radioactive waste by decontaminating metal waste to a level of radioactivity comparable to that of general waste.

(Prior Art) Conventionally, this type of radioactive metal waste has been cut into pieces, canned in drums, and stored in a radioactive waste storage. However, the number of drums in storage is increasing year by year, and when nuclear power plants are decommissioned and dismantled in the future, it is expected that tens of thousands of drums will be generated from radioactive metal waste alone.

For this reason, it is desired to reduce the volume of the radioactive metal waste, and methods for decontaminating this radioactive waste in the same manner as general waste are being studied.

Two methods are used: an electrolytic decontamination method that electrochemically dissolves the surface of the object to remove radioactive materials, and a chemical decontamination method that chemically dissolves the surface of the object and removes the radioactive material. It has been known.

Although the electrolytic polishing decontamination method has a fast decontamination speed, it has the disadvantage that it is difficult to apply to objects with complex shapes because it requires a counter electrode to be placed opposite the object to be decontaminated. On the other hand, the chemical decontamination method is a method that dissolves the metal surface through the chemical action of a decontamination agent.
Although the decontamination speed is slow, it has the advantage of being applicable to objects with complex shapes.

Therefore, by taking advantage of the advantages of both the electrolytic polishing decontamination method and the chemical decontamination method, it is possible to deal with objects of any shape and produce less secondary waste. Decontaminated by electrolytic polishing decontamination method using a neutral salt solution,
A method has been proposed in which complex-shaped objects are decontaminated using chemical decontamination methods, the used solution from chemical decontamination is neutralized, radioactive substances are removed by a fraction, and the solution is reused as an electrolyte for electrolytic polishing decontamination. ing.

(Problem to be solved by the invention) However, since the electrolytic polishing decontamination method is a method of electrochemically dissolving the surface, most of the materials generated from nuclear power plants, such as carbon steel and stainless steel, However, since chemical decontamination methods use chemical action, it is necessary to select a decontamination solution depending on the material of the object to be decontaminated. For example, in the case of carbon steel, the oxide film that contains most of the radioactive substances is said to be made of magnetite (Fc, On) and hematite (Fewo2), which is made of sulfuric acid (HfSO4). Chemical decontamination using sulfuric acid can be applied because the base material of carbon steel itself is also easily dissolved. However, in the case of stainless steel, the oxide film that contains most of the radioactive substances is magnetite (FeJ4), hematite (FezO+
), it is said to be made of chromium ferrite (FeCr 204), and chromium ferrite (FeCrzOt), which is a component of this oxide film, is made of sulfuric acid (llzs
Since chromium does not dissolve in mineral acids, a method of oxidizing and dissolving chromium from trivalent to hexavalent before immersing it in these mineral acids to change the film to a form that is easily soluble in mineral acids is being considered, and this is the preferred method. As a possible method, immersion in an acidic or alkaline solution containing potassium permanganate has been considered.

Therefore, according to the present invention, objects with simple shapes are decontaminated by the electrolytic polishing decontamination method using neutral salts, objects with complex shapes are decontaminated by the chemical decontamination method, and the used liquid in chemical decontamination is removed. In a decontamination method that separates and removes radioactive substances after neutralization and reuses them as an electrolyte for electropolishing decontamination, sulfuric acid (IhSOt) and bisulfuric acid are used to chemically decontaminate complex-shaped objects made of stainless steel. Before an object to be decontaminated is immersed in a mineral acid such as soda (NaHSO3,), it is necessary to immerse it in an acidic or alkaline solution containing potassium permanganate.

However, potassium permanganate is generally added in excess in order to provide sufficient reactivity, and unreacted permanganate ions (MnO6) remain in the solution after the reaction. Therefore, when the used solution is neutralized and used as an electrolyte for electropolishing decontamination, the permanganate ions (
There was a problem that MnO4) inhibited the electrolytic dedying performance.

An object of the present invention is to eliminate the drawbacks of the prior art and to neutralize a chemical decontamination solution using an acidic solution containing potassium permanganate or an alkaline solution and a mineral acid, and to use it as an electrolytic solution for electrolytic polishing decontamination. In a method for decontaminating radioactive metal waste to be reused, the objective is to prevent deterioration in decontamination performance during electrolytic polishing decontamination.

(Means for Solving the Problems) The present invention shows that the decrease in efficiency in electropolishing decontamination is caused by the use of spent neutralization of chemical decontamination using acidic or alkaline solutions containing potassium permanganate and mineral acids. It was confirmed through experiments that this was due to permanganate ions (MnO4) contained in the electrolytic polishing decontamination electrolyte solution, and after removing this permanganate ion (MnO4-), chemical This system is designed to reuse the used decontamination solution as an electrolytic solution for decontamination.

That is, in the decontamination method for radioactive metal waste according to the present invention, objects with simple shapes are decontaminated by an electrolytic polishing decontamination method using a neutral salt solution, and objects with complex shapes are decontaminated with an acidic solution containing potassium permanganate. Alternatively, chemical decontamination is performed using an alkaline solution and a mineral acid solution, and the spent chemical decontamination solution is neutralized.
In a radioactive metal waste decontamination method in which most radioactive materials are separated and removed as precipitates and the supernatant liquid is reused as an electrolyte for electropolishing decontamination, before neutralizing the spent chemical decontamination liquid. permanganate ion (MnO4-) is reduced to manganese dioxide (MnO□) and precipitated, and the manganese dioxide (MnO□) is removed in the same process as the radioactive substance separation process.
It is characterized by being separated and then reused as an electrolyte for electrolytic polishing decontamination.

Next, the present invention will be specifically explained based on the drawings. 1st
The figure is a system diagram showing one embodiment of the decontamination device according to the present invention. This device consists of a first chemical decontamination tank 1, a second chemical decontamination tank 2, a permanganate ion reduction tank 3, a neutralization tank 4, a solid-liquid separation tank 5, and an electropolishing decontamination tank 6.

In the apparatus shown in Figure 1, radioactive metal waste with complex shapes such as valves and pumps is immersed in the first chemical decontamination tank containing an acidic or alkaline solution containing potassium permanganate. After oxidizing and dissolving chromium as hexavalent ions from the oxide film containing chromium, the chromium is immersed in the second chemical decontamination tank 2 using a solution of mineral acids such as sulfuric acid, sodium bisulfate, and nitric acid as the decontamination liquid. Radioactive metal waste with simple shapes such as straight pipes and tees is decontaminated in an electrolytic polishing decontamination tank 6 using a neutral salt solution such as sodium sulfate solution or sodium nitrate solution as the electrolyte. be done.

The concentrations of alkaline and acidic solutions containing potassium permanganate, mineral acids used as chemical decontamination solutions, and the concentrations of sodium sulfate or sodium nitrate used as electrolytes in electrolytic polishing decontamination solutions are determined by these decontamination methods. It may be at a concentration commonly used for.

On the other hand, since potassium permanganate is used as an oxidizing agent in the first chemical decontamination tank 1, permanganate ions that do not contribute to the reaction are dissolved. If this permanganate ion mixes into the electrolytic solution for electrolytic polishing decontamination, the efficiency of electrolytic polishing decontamination will decrease, so this permanganate ion is reduced and precipitated as manganese dioxide (MnO The decontamination liquid in the first chemical decontamination tank 1 is introduced into the permanganate ion reduction tank 3 in order to separate it from the permanganate ion reduction tank 3. At this time, if the decontamination liquid in the first chemical decontamination tank 1 is an alkaline solution such as a sodium hydroxide solution or a sodium nitrate solution,
Oxalic acid ((COOH) t) may be added as is as a reducing agent, but if it is an acidic solution such as sulfuric acid or nitric acid, after making it a strong alkaline solution with sodium hydroxide,
Add oxalic acid ((Cool)!). Through this operation, permanganate ions are reduced to manganese dioxide (MnO□) and precipitated by the following reaction.

2 Mid, +4! hO+ 3 C2O4,”
−→2 Mn0z + 80H−+ 6 coz Next,
This liquid is introduced into the neutralization tank 4 and mixed with the used liquid in the second chemical decontamination tank 2. Here, since the second chemical decontamination tank 2 uses mineral acids such as sulfuric acid, sodium bisulfate, and nitric acid as decontamination liquid, the used liquid from the second chemical decontamination tank 2 contains radioactive materials. Metals are dissolved as ions, but since the spent liquid in the first chemical decontamination tank introduced from the permanganate ion reduction tank 3 is alkaline, radioactive metals become metal hydroxides in the neutralization tank 3. , is precipitated.

However, in this operation, if the balance between acid concentration and alkali concentration is biased to either side, acid or alkali is added to adjust pl+ to neutrality.

At this time, the same mineral acid contained in the second chemical decontamination tank 2 is used as the acid. The neutralized decontamination liquid thus obtained is introduced into the solid-liquid separation tank 5 in the same manner as the used liquid in the electropolishing decontamination tank 6. The supernatant liquid separated in the solid-liquid separation tank 5 is
It is transferred to the electrolytic polishing decontamination tank 6, where it is circulated and reused as an electrolyte.

Further, the concentrated sludge is introduced into a solidification treatment step 7 and treated.

(Example) Next, the present invention will be described in detail based on Examples, but the present invention is not limited thereto.

Example 1 Pump contaminated with radioactive materials (Material: 5IJS304
) and straight pipes (material: 5S41) were decontaminated by the method of the present invention shown in Fig. 1 and the conventional method without a permanganate ion reduction tank shown in Fig. 2, followed by electropolishing decontamination. The decontamination performance of straight pipes was compared.

The decontamination conditions for each process are as follows.

First chemical decontamination conditions: Decontamination liquid NaOH3.6 mol/lKMnO4
1 mole/1 Temperature: 80°C Time: 2 hours Conditions for second chemical decontamination: Decontamination liquid H2SO, 1.8 mole/2 temperature
80℃ Time: 1 hour Permanganate ion reduction tank Oxalic acid addition amount: 1.5 mol/1 Electrolytic polishing decontamination conditions Counter electrode Carbon steel electrolyte Na25Ot O, 9 mol/1 Current density: 0.3 A/cnl Alternating conditions
The radioactive surface density of the cathode pump for 60 seconds and the anode pump for 60 seconds is
After decontamination, the detection limit (IXIO-'μCi/ctA
) or less.

Figure 3 shows the results of electrical decontamination of straight pipes.

In conventional methods, the radioactive areal density is below the detection limit (lXIO-
In contrast, in the method of the present invention, the detection limit was reached in about 30 minutes.

In this example, an alkaline solution was used as the decontamination solution in the first chemical decontamination tank. However, if an acidic solution is used, oxalic acid is added after making it alkaline in the permanganate ion reduction tank. The same effect can be obtained by doing so.

(Effects of the Invention) According to the present invention, after neutralizing a chemical decontamination solution using an acidic solution containing potassium permanganate or an alkaline solution and a mineral acid, it can be reused as an electrolyte for electropolishing decontamination. ,
There is no deterioration in decontamination performance, and radioactive metal waste can be efficiently decontaminated to the same level of radioactivity as general waste.

[Brief explanation of the drawing]

Figure 1 is a system diagram of an apparatus for carrying out the method of the present invention, Figure 2 is a system diagram showing a conventional method, and Figure 3 is a diagram showing the relationship between electrolysis time and radioactive surface density when straight pipes are electrolytically destained. It is a graph diagram. Explanation of symbols 1...First chemical decontamination tank, 2...Second chemical decontamination tank, 3
... Permanganate ion reduction tank, 4... Neutralization tank, 5
...Solid-liquid separation tank, 6...Electrolytic polishing decontamination tank

Claims (1)

[Claims] (1) In order to decontaminate radioactive metal waste of various shapes generated from facilities handling radioactive materials, simple shaped objects are decontaminated by electrolytic polishing decontamination method using a neutral salt solution, and complicated shaped objects are decontaminated. , chemical decontamination using an acidic or alkaline solution containing potassium permanganate and a mineral acid solution;
After neutralizing this spent chemical decontamination solution, radioactive substances are separated and removed, and the supernatant liquid is reused as an electrolyte solution for electrolytic polishing decontamination. Permanganate ions (Mn
O_4^-) is reduced to manganese dioxide (MnO_2) and precipitated, and after the manganese dioxide (MnO_2) is separated and removed in the same process as the radioactive substance separation process, it is reused as an electrolyte for electropolishing decontamination. A method for decontaminating radioactive metal waste characterized by: