JPS63273097A - Removal of pollutant from system - Google Patents

Abstract

PURPOSE:To melt away an oxidation film without roughening the surface of a metal base material, by bringing an acid solution having metal compound highly oxidized and a fluoride dissolved into contact with a radioactive oxidation film adhering to the surface of the metal base material. CONSTITUTION:From a piping 1 to be treated with the internal surface thereof polluted by a radioactive substance, a product adhering to the internal surface thereof is removed by a pollution removing liquid 3 comprising an acid solution containing cerium trivalent ions and cerium quadrivalent ion. The pollutant removing liquid 3 herein generated shifts to an elution ion recovery tank 5 from a circulation return line 4. In the recovery tank 5, the pollutant removing liquid 3 is separated with a diaphragm 8 from an acid solution 7 containing no elution ion and a cathode 9 for recovery of elution metal ions is emmersed into the pollutant removing liquid 3 while an anode 10 for recovery of elution metal ions into the acid solution to apply a voltage. The pollutant removing liquid 3 is circulated between the recovery tank 5 and an electrolytic tank 6 and the pollutant removing liquid 3 is stored into the electrolytic tank 6. The anode 14 and the cathode 15 also applies a voltage to the electrolytic tank 6. A gas or the like generated in the recovery tank 5 and the electrolytic tank is treated with an upper exhaust gas line 19 or the like.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention is directed to the use of liquid primary coolant or solution generated when decommissioning nuclear power generation equipment or nuclear fuel cycle related facilities. This field relates to a system decontamination method that chemically dissolves and removes radioactive oxide films that have adhered to or accumulated on used parts such as metal piping, equipment, or structures.

(Prior Art) A radioactive oxide film is formed on the inner surface of piping, equipment, fuel fl assemblies, etc. used in nuclear power generation equipment or nuclear fuel cycle-related facilities, and this causes an increase in surface dose rate. From the standpoint of reducing radiation exposure during decommissioning and dismantling, it is necessary to remove these oxide films, or decontaminate them. This removal usually requires dissolving the oxide film formed on the surface of the material and the metal matrix, and dropping the attached or deposited corrosion products and fission products into a solution. Such decontamination methods can be broadly classified into chemical decontamination methods, physical methods including mechanical methods, and electrochemical methods. Among these methods, the chemical decontamination method uses a decontamination agent blended with an acid, a reducing agent, a complexing agent, and a non-inhibitor, taking into account the characteristics of the strong oxide film. For example, JP-A-61-1187
No. 00 discloses a method of removing iron oxide by chelating component metal ions in an oxide film using a chelating agent. In addition, ozone is used as an oxidizing agent to dissolve chromium oxide in the oxide film, and further,
A method is known in which an oxide film is dissolved and removed using an aqueous solution containing an acidic decontamination reagent containing an organic polycarboxylic acid as a component. (
For example, Japanese Patent Application Laid-Open No. 61-58800). These methods are excellent for dissolving radioactive oxide films.

(Problems to be Solved by the Invention) However, with the method described above, it was not possible to completely remove the oxide film and reduce the radioactivity level to below the unconstrained limit value. In addition, the decontamination agent has a low concentration and is difficult to regenerate, resulting in an increased amount of secondary waste.

The present invention was made in order to solve the above problems, and it dissolves and removes the radioactive oxide film and its metal base material without roughening the surface, and reduces the radioactivity level to below the unrestricted limit value. The objective is to provide a system decontamination method for decontamination.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above-mentioned problems, the system decontamination method according to the present invention is a system decontamination method according to the present invention, which decontaminates pipes, equipment, oval relic parts, etc. exposed to reactor coolant or solution. An acidic solution prepared by adding sodium fluoride to a decontamination solution containing oxidizing power, for example, cerium tetravalent ions in an acidic aqueous solution, is brought into contact with the radioactive oxide film attached or deposited on the metal base material. The oxide film is removed without roughening the surface of the metal base material to reduce the radioactivity to below the unrestricted limit value, and the metal ions in a low oxidation state and the metal ions in a high oxidation state due to the decontamination are removed. The method is characterized in that an anode and a cathode are immersed in a decontamination solution containing the decontamination liquid, and a current is passed between the re-polarized electrodes to oxidize metal ions in a low oxidation state at the anode and regenerate them into metal ions in a high oxidation state.

(Function) According to the present invention, it adheres to metal base materials such as piping, equipment, or large structural parts exposed to reactor coolant or solution.
The deposited radioactive oxide film can be completely decontaminated without damaging the metal base material. In other words, since sodium fluoride is added to the decontamination solution containing tetravalent cerium ions with oxidizing power in an acidic aqueous solution, the dissolving power of the metal base material by the tetravalent cerium ions is not reduced, and the sodium fluoride The effect of uniformly dissolving the surface of the metal base material increases, and since the surface is not roughened, contaminants do not adhere again.

Furthermore, the regeneration efficiency of the reaction for regenerating trivalent cerium ions whose oxidizing power has decreased after decontamination into tetravalent cerium ions having oxidizing power can be increased.

(Example) The figure is a system diagram showing an example of the arrangement of equipment suitable for carrying out the system decontamination method according to the present invention.

The figure shows that cerium tetravalent ions are generated from cerium trivalent ions through an electrolytic oxidation reaction using an acidic aqueous solution containing cerium trivalent ions, cerium tetravalent ions, and sodium fluoride. This shows a system diagram of a decontamination device that removes radioactive substances from the surface of a decontaminated object by dissolving the surface of the object contaminated with radioactive substances by utilizing the oxidizing power of the object when it changes into valence ions.

In the figure, reference numeral 1 denotes a pipe to be treated made of a metal base material, and a decontamination liquid 3 is circulated within this pipe to be treated 1 by a circulation supply line 2. After decontamination, the decontamination liquid 3 is sent to the circulation return line 4.
The ions are circulated through the eluted ion recovery tank. The decontamination liquid 3 from which the eluted ions have been collected is transferred to the electrolytic cell 6, where the decontamination liquid 3 is regenerated and conveyed again through the circulation supply line 4.

A decontamination liquid 3 and an acidic solution 7 containing no eluted metal ions are stored in the eluted ion recovery tank 5, and a diaphragm 8 is provided between these liquids. A cathode 9 for recovering eluted metal ions is immersed in the decontamination liquid 3, and an anode 10 for recovering eluted metal ions is immersed in the acidic solution 7 that does not contain eluted metal ions. A voltage can be applied between the anode 10 for recovering eluted metal ions and the cathode 9 for eluted metal. The eluted ion recovery tank 5 is connected to the electrolytic tank 6 via a circulation line 11.

A circulation pump 12 is provided in the middle of this circulation line, and the decontamination liquid 3 is circulated between the eluted ion recovery tank 5 and the electrolytic tank 6.

A decontamination liquid 3 made of an acidic aqueous solution containing the same trivalent cerium ions and tetravalent cerium ions as that stored in the eluted ion collection W15 is stored in the electrolytic tank 6. This decontamination liquid 3 has an anode 14 to which a positive voltage is applied from a DC power supply 13 and a cathode 15 to which a negative voltage is applied.
and are immersed opposite each other. The electrolytic cell 6 and the pipe 1 to be treated are connected by a circulation supply line 2, and a supply pump 16 is provided in the middle of this circulation supply line 2, and the decontamination liquid 3 is supplied to the electrolytic cell 6 and the pipe 1 to be treated. It is configured to circulate between the pipe 1 and the pipe 1.

The inner surface of the pipe 1 to be treated is contaminated with radioactive substances, and the inner surface is dissolved with a decontamination liquid 3 made of an acidic aqueous solution containing trivalent cerium ions and tetravalent cerium ions, and the radioactive substances are removed from the pipe 1 to be treated. removed from the inner surface. The pipe 1 to be treated and the eluted ion recovery tank 5 are connected by a circulation return line 4. A circulation pump 17 and a filter 18 are provided in this Wi ring return line 4.
The decontamination liquid 3 is transferred from the pipe to be treated 1 to the eluted ion recovery tank 5.

On the other hand, one exhaust gas line is connected to the upper part of the eluted ion recovery tank 5 and the electrolytic tank 6, and the hydrogen or oxygen, steam, and mist generated from the decontamination liquid 3 are passed through the exhaust gas line 19 to the capacitor 20, the demister 21, and Exhaust blower 2
Process in 2. The condensed liquid in the condenser 20 passes through the condensate return line 23 and is collected in the eluted ion recovery tank 5.

A first embodiment of the system decontamination method according to the present invention will be described below.

In the first example, cerium is selected as a metal having a multivalent oxidation state in an aqueous solution, and cerous nitrate Ce (a trivalent cerium compound) is selected as a metal compound with a low oxidation state.
NJ) 3 is used. In addition, the fluoride used to prevent the surface of the metal base material from becoming rough is sodium fluoride (NaF).
Use.

It is important that the cerium trivalent compound and fluoride are easily soluble in water or acid.

A nitric acid solution is used as the acidic solution in which cerous nitrate is dissolved. The concentration of cerous nitrate is 0.81o
The anode made of titanium coated with platinum and the cathode made of titanium coated with platinum are immersed in an acidic solution of l/J2 sodium fluoride 0.61101 #2. A voltage is applied between the anode and the cathode to flow a current with a current density of 0.2 A/d"I:' to generate a decontamination solution.

Next, the operation of the first embodiment will be explained.

In an acidic solution in which trivalent cerium salt is dissolved, trivalent cerium ions (Ce
3″) is converted to cerium tetravalent ion (Ce4+) and C
The state is a mixture of e'+ and Ce'10.

(@pole) Ce” −+ Ce” + e −−−−−−−−
--(1)2+120 -02 (↑) +411+
+28−−−・−・(2)(Cathode) H”+e−→ (1/2)02 (↑) −(3)
Ce4++e −−Ce” −・・−−
-<4) The cerium tetravalent ion (Ce4+) generated by the above formula (1) has extremely strong oxidizing power, and when it comes into contact with the object to be decontaminated, it is removed from the surface of the metal base material together with radioactive contaminants. The surface of the material is melted.

The reaction formula in this case is as shown below, assuming that M is the metal base material.

M + Ce"→HJ"+Ce" ・-
---・(5) However, sodium fluoride, which prevents the surface of the metal base material from becoming rough, forms a protective film on the surface of the metal base material to repair the melted area, and also reduces the DH of the melted area due to its buffering effect. This prevents the dissolution current of Fe from decreasing, and the oxygen in the atmosphere acts as a passivating agent.

C6J) and Ce'+ are dissolved in a mixed state in the used decontamination liquid from which radioactive substances and the surface of the metal base material have been dissolved. The anode and cathode used for the generation of Ce4+ are immersed in this decontamination solution, and a voltage is applied between the anode and cathode to flow a current at a current density of 0.2 A/d. , the electrolytic redox reactions of formulas (1) to (4) occur, and C6j) is regenerated to Ce'+ at the anode.

If the system decontamination method of the above embodiment is used, cerium tetravalent ions are efficiently generated and regenerated near the anode by an electrolytic oxidation reaction caused by a voltage applied between the anode and the cathode. By the way, at the anode, the reaction for producing tetravalent cerium ions and the reaction for producing oxygen compete with each other.

This oxygen generation reaction occurs when OH- adsorbed on the anode surface is applied with voltage according to equation (2).

On the other hand, the [-] of sodium fluoride added to prevent roughening of the surface of the metal base material is the gold Ii of the pipe to be treated! j Displaces 011- and adsorbs it not only to the surface of the base material but also to the surface of the anode. OH- adsorbed on the anode surface decreases, and the oxygen generation reaction decreases. Instead of 0, the generation/regeneration reaction of (:64+) (Equation (1)) becomes predominant, and the generation/regeneration of cerium tetravalent ions decreases. Improves current efficiency.

Therefore, it is important to add sodium fluoride.

Regarding the first embodiment and the conventional example, Table 1 shows Ce'+
Table 2 shows the results of measuring the current efficiency regarding the generation of Ce.
The results of measuring the current efficiency regarding e-regeneration are shown for comparison.In the case of the conventional example, sodium fluoride was not added. The current efficiency η regarding the production of cerium tetravalent ions is as follows.

77: <96485xCe' + production amount (1101/
, 12)×electrolyte amount (bu))/(electrolysis time (SeC)
×Current (A)) (Hereinafter in the margin) Tables 1 and 2 show that the current efficiency for regenerating cerium tetravalent ions in the example of the present invention is approximately 1.3 times higher than that in the conventional example. It is recognized that this can be obtained.

This is because F- is adsorbed on the anode surface by the addition of sodium fluoride, and F- is not oxidized by the application of voltage, so the adsorption sites for 0 - are relatively reduced.
This is because the proportion of current used for oxygen generation becomes smaller as ``'' decreases.

As explained above, in this example, by adding sodium fluoride, the amount of oxygen generated is reduced, and cerium tetravalent ions can be efficiently generated and regenerated.

Note that cerous sulfate [Ce(SO+ > 31,
Cerous ammonium nitrate (Ce(NO3) 3
・2(NH4NO3)], cerium carbonate [(C(32(
CO3) 3, ceric oxalate [Ce 2 (
C204) It is also possible to use 31. The concentration of the cerium trivalent compound is 0.8 mo1#! Instead, it can be used at 0.001 to 2.0 IIQI/J2.

As the fluoride, it is also possible to use potassium fluoride, F, or lithium fluoride LiF instead of sodium fluoride, NaF. The concentration of the fluoride is 0.611o
l/,! 0.01~1.0IO1# instead of 2! Available in

As the acidic solution in which the metal compound in a low oxidation state is dissolved, a sulfuric acid solution, a carbonic acid solution, or an oxalic acid solution can be used instead of a nitric acid solution.

Further, the concentration of the acidic solution is o, oos to 10. It can be used within the range of On+ol/i. The current density of the voltage applied between the anode and cathode is 0.01 to 2.0 instead of 0.2^/d.
It can also be used with A/d.

Next, a second embodiment of the present invention will be described.

In the second example, chromium is selected as the metal having a multivalent oxidation state in an aqueous solution, and the metal compound with a low oxidation state is chromium sulfate Cr2 (
SO4) 3 is used. In addition, sodium fluoride (NaF
) is used. It is important that the trivalent chromium compound and fluoride be compounds that are easily soluble in water or acid.

An acidic solution is used as the acidic solution in which the WJL chromium acid is dissolved. Said W, chromate concentration 0.11 Iol
#2 Sodium fluoride, 6nol/A, R acid concentration 1.
0nol#! in the electrolyte, with lead dioxide as the anode,
Titanium coated with platinum is immersed as a cathode. Said qlIf! A voltage is applied between the electrode and the cathode to flow a current at a current density of 0 and 1 A/c7.

Next, the operation of the second embodiment of the system decontamination method will be explained.

In an acidic solution in which trivalent chromium salt is dissolved, trivalent chromium ions (crJ+) are generated by the electrolytic oxidation reaction shown below.
is converted to dichromate ion (Cr20?2-).

(Anode) 2Cr" + 71+20 → cr20 y 2- +
1411” +6e−・・・・・・・・・(6) 2H20−021+48++28− −−−−
・−(7)(Cathode) H”+e−→ (1/2)02 (↑) ・・
...(8) Cr207'-+14H÷+6e-→2
Cr" +7820... (9) Dichromate ion (Cr
2Q72-) has an extremely strong oxidizing power, and when brought into contact with an object to be decontaminated, the surface of the metal base material is dissolved together with radioactive contaminants from the surface layer of the metal base material.

The reaction is as shown below, with the chemical symbol of the metal being M.

M + (1/6) Cr20 y 2-+ (7/3)H
"-M" + (1/3) Cr" + (7/6) H20
......(1G) However, sodium fluoride, which prevents the surface of the metal base material from becoming rough, forms a protective film on the surface of the metal base material to repair the melted area, and also has a buffering effect that prevents the melted area from becoming rough. This prevents the pH from decreasing, so the Fe dissolution current becomes small, and a trace amount of oxygen acts as a passivating agent.

Cr6 and Cr2 are present in the used decontamination solution in which the radioactive materials and the surface layer of the metal base material are dissolved. 2- is dissolved in a mixed state. The anode and cathode used to generate Cr20,2- are immersed in this decontamination solution, and a voltage is applied between the anode and cathode to flow a current at a current density of 0.1^/d. At the anode and cathode, the electrolytic redox reactions of formulas (6) to (9) occur, and at the anode, Cr'+ changes to Cr20
7-4: Regeneration Sarel.

If the system decontamination method of the second embodiment is used, dichromate ions are efficiently generated and regenerated in the vicinity of the anode by an electrolytic oxidation reaction caused by a voltage applied between the anode and the cathode. By the way, at the anode, the dichromate ion production reaction and the oxygen production reaction compete.

This oxygen generating reaction is adsorbed on the qJ pole surface.

011- occurs according to equation (7) by applying a voltage.

On the other hand, the F- of sodium fluoride added to prevent roughening of the surface of the metal base material is adsorbed not only on the surface of the metal base material of the pipe to be treated, but also on the anode surface, displacing 0■-. do. OH- adsorbed on the PJ1m surface decreases, and the oxygen generation reaction decreases. Instead, the production/regeneration reaction of Cr2072- (Equation (6)) becomes dominant, and the current efficiency of production/regeneration of dichromate ions becomes good. Therefore, it is important to add sodium fluoride.

Regarding the second embodiment of the present invention and the conventional example, Table 3 shows C
Table 4 shows the results of measuring the current efficiency regarding the generation of r2072- and the results of measuring the current efficiency regarding the regeneration of Cr2072. The current efficiency η regarding acid ion generation/regeneration is as follows.

77 = (3x96485x Cr20? 2-Generation/Regeneration JL (+101/J2) x Electrolyte volume 1)/(Electrolysis time (sec) x Current (A)) (Left below margin) Table 3 Table 4 Table 3 It can be seen from the Table and Table 4 that the current efficiency for dichromate ion production and regeneration is approximately 1.4 times higher in the example of the present invention than in the conventional example.

This is because the addition of sodium fluoride causes Bi to be adsorbed on the anode surface, and Bi is not oxidized by the application of voltage, so the adsorption sites for OH'''' are relatively reduced.
This is because - decreases, so the proportion of current used for oxygen generation becomes smaller.

As explained above, in the examples of the present invention, the amount of oxygen generated is reduced by adding sodium fluoride, and dichromate ions can be efficiently generated and regenerated.

Note that it is also possible to use chromium sulfate (Cr(Nlh)!) and chromium hydroxide (Cr(Oth)) instead of chromium sulfate (Cr2(SO4)3) used as the trivalent chromium compound in the second example. The concentration of the trivalent chromium compound can be 0.001 to 1.0 nol/g instead of 0.111 ol#!.A sulfuric acid solution is used as the acidic solution in which the metal compound in a low oxidation state is dissolved. Instead, it is also possible to use a nitric acid solution or a hydrochloric acid solution.Also, the concentration of the acidic solution is 1,
Olo l / 0.005-10 instead of J2.
It can be used within the range of 0nol/J2.

It is also possible to use lead instead of lead dioxide as the anode.

As the fluoride, potassium fluoride, F, or lithium fluoride can be used instead of sodium fluoride, NaF. The concentration of the fluoride is 0.61101/J
0.01 to 1.0nol/J2 can be used instead of 2.

As the acidic solution in which the metal compound in a low oxidation state is dissolved, a sulfuric acid solution, a carbonic acid solution, or an oxalic acid solution can be used instead of a nitric acid solution.

Moreover, the concentration of the acidic solution can be used in the range of 0.005 to 10.0 mol/J instead of 2.0101/J. The current density of the voltage applied between the anode and cathode is 2. 0.01 to 2.0 instead of OA/-/
It can also be used with ^/cI.

Next, a third embodiment of the present invention will be described.

In the third example, vanadium is selected as the metal having a multivalent oxidation state in an aqueous solution, and a tetravalent vanadium compound wL vanadyl vo
Use so. In addition, sodium fluoride (NaF) is used as the fluoride to prevent roughening of the surface of the metal base material. It is important that the vanadium tetravalent compound and fluoride are compounds that are easily soluble in water or acid. A sulfuric acid solution is used as the acidic solution in which vanadyl acid is dissolved. Vanadyl sulfate concentration 0.2 mol/J2, sodium fluoride 0.6 n+ol/JQ, sulfuric acid concentration 4.011
An anode made of titanium coated with platinum and a cathode made of titanium coated with platinum are immersed in an acidic solution of No. 01/J2. A voltage is applied between the anode and the cathode to flow a current at a current density of 0.3^/d to generate a decontamination solution.

Next, the operation of the third embodiment will be explained.

In an acidic solution in which a tetravalent vanadium salt is dissolved, vanadyl and ions (VO'
) is converted to pervanadium ion (VO2'').

(Anode) VO2 + 112 0 -VO2” +28”
+e −−−−−−−(11)2t120 =
021+4H"+2e-・-------・-<12) (1
2) (Yin'!fl) H"1e" → (1/2)02 (t)
・・・・・・(13) VO2+ +28” 10e
” →V(F + H20・−−・・−(
14) The pervanadyl ions (in VO2) generated by the above formula (11) have extremely strong resistance to deterioration, and when they come into contact with objects to be decontaminated, they dissolve the metal surface together with radioactive contaminants from the surface layer of the metal base material. The reaction formula is as shown below, assuming that the chemical symbol of the metal is M.

H+VO2+ +211+ -+ H+ VO2 total +t
120 ... (15) However, sodium fluoride, which prevents the surface of the metal base material from becoming rough, forms a protective film on the metal surface to repair the melted area, and also prevents the pH of the melted area from dropping due to its buffering effect. The dissolution current of Fe becomes small, and a trace amount of oxygen comes to work as a passivating agent.

The radioactive contaminants and the metal base material surface are dissolved in the decontamination solution in a mixed state with the VO meter and VO2.

The anode and cathode used in the first example are immersed in this decontamination solution. A voltage is applied between the anode and the cathode to flow a current at a current density of 0.3 A/d. At the @ electrode and the cathode, the electrolytic redox reactions of equations (11) to (14) occur, and at the anode, %1Qtt and Played during VO2.

If the decontamination solution regeneration method according to this embodiment is used, pervanadyl ions can be efficiently regenerated in the vicinity of the anode through an electrolytic oxidation reaction caused by a voltage applied between the anode and the cathode.

If the system decontamination method of the above embodiment is used, pervanadyl ions are efficiently generated and regenerated near the anode through an electrolytic oxidation reaction caused by a voltage applied between the anode and the cathode. By the way, at the anode, the pervanadyl ion production reaction and the #M element production reaction compete.

This oxygen generation reaction is caused by the 011-
occurs according to equation (12) by applying voltage.

On the other hand, F''' of sodium fluoride added to prevent the surface of the metal base material from becoming rough displaces OH'' and adsorbs it not only on the surface of the metal base material of the pipe to be treated but also on the anode surface.I1% 1. OH- adsorbed on the surface decreases, and the oxygen generation reaction decreases. Instead, the generation/regeneration reaction in VO2 (
Equation (11)) becomes dominant, resulting in the production of pervanadyl ions and
It is important to add sodium fluoride to improve the current efficiency of regeneration.

Table 5 shows v0 regarding the third embodiment of the present invention and the conventional example.
The results of measuring the current efficiency regarding 2+ generation are shown in Table 6.
In the case of the conventional example shown in Figure 2, comparing the results of measuring the current efficiency regarding O juice regeneration, sodium fluoride was not added.

The current efficiency η regarding generation and reproduction is as follows.

77= (96485XVO2+generation/regeneration amount (nol
#! ) x electrolyte volume (,12)/(electrolysis time (sec
)×Current (A) (Hereafter, blank space) Table 5 Table 6 As is clear from Tables 5 and 6, the example of the present invention has about 1.5 times more pervanadyl than the conventional example. It can be seen that current efficiency regarding ion generation and regeneration can be obtained.

This is because F- is adsorbed on the anode surface by the addition of sodium fluoride, and F- is not oxidized by the application of voltage, so the number of adsorption sites for OH- is relatively reduced.
This is because - decreases, so the proportion of current used for oxygen generation becomes smaller.

As explained above, in the examples of the present invention, the amount of oxygen generated is reduced by adding sodium fluoride, and pervanadyl ions can be efficiently generated and regenerated.

In the third example, vanadyl sulfate (VOSO4> was used as the vanadium tetravalent compound, but vanadium oxide (VO2>, vanadyl oxalate (VOC20
4) can also be used. The concentration of the vanadium tetravalent compound is 0.00 instead of 0.21101/J2
It can be used within the range of 1 to 1.01101/J2.

As the fluoride, it is also possible to use potassium fluoride, KF, or lithium fluoride LiF instead of sodium fluoride NaF. The concentration of the fluoride is 0.6 mo
It can be used at 0.01 to 1.01101/ffl instead of 1/bu.

For decontamination in which the metal compound in a low oxidation state is dissolved, it is also possible to use a nitric acid solution or an oxalic acid solution instead of a sulfuric acid solution. The concentration of the decontamination solution is 4.
0.005 to 10.0 mol instead of On+ol/bu
#! It can be used within the range of The current density of the voltage applied between the anode and cathode is 0.3A/ci instead of 0.
．． 01 to 2.0^/d can also be used.

[Effects of invention According to the present invention, there are the following effects.

(1) Since the radioactive oxide film and its metal base material are dissolved using a decontamination solution containing dissolved fluoride, the metal base material can be dissolved without roughening its surface.

(2) Compared to conventional systematic decontamination methods, since the radioactive oxide film and its metal base material are dissolved using a solution containing metal ions in a highly oxidized state, the ability to remove the oxide film is strong; The contamination level can be brought below the unconstrained limit.

(3) Since the decontamination solution in which the fluoride is dissolved is used, the proportion of electric current used for the reaction of converting metal ions in a low oxidation state to metal ions in a high oxidation state increases. In other words, the current efficiency regarding the oxidation reaction that converts metal ions in a low oxidation state to metal ions in a high oxidation state is improved.

[Brief explanation of the drawing]

The figure is a system diagram showing an example of an apparatus for explaining a method for generating and regenerating a decontamination liquid in a system decontamination method according to the present invention. 1...... Elute ion collection tank 2...
... Decontamination liquid 7 ...... Filter 8 ...... Piping to be treated 9 ...... Electrolytic tank

Claims (4)

[Claims] (1) An acidic solution in which a highly oxidized metal compound and fluoride are dissolved is brought into contact with a radioactive oxide film attached to a metal base material such as piping, equipment, or structural parts, and the highly oxidized metal ion is After decontaminating the radioactive oxide film and the surface of its metal base material by dissolving the radioactive oxide film and the surface of its metal base material by the oxidizing power when it changes to metal ions in a low oxidation state, and reducing the radioactivity level to below the unrestricted limit value, An anode and a cathode are immersed in a decontamination solution containing metal ions in a low oxidation state and metal ions in a high oxidation state due to the decontamination, and a current is passed between the two electrodes to remove the metal in a low oxidation state at the anode. A systematic decontamination method characterized by oxidizing ions and regenerating them into highly oxidized metal ions. (2) The metal compound in a low oxidation state dissolves in water or acid and becomes a metal ion in a high oxidation state Ce^4
The system decontamination method according to claim 1, characterized in that it is either ^+, Cr_2O_7_2_-, or VO_2^+. (3) The metal ion in the low oxidation state is Ce^ in the solution.
3^+, Cr^3^+, or VO^2^+, the systematic decontamination method according to claim 1. (4) The system decontamination method according to claim 1, wherein the acidic solution is a solution containing any one of nitric acid, sulfuric acid, hydrochloric acid, carbonic acid, and oxalic acid.