JPS6244699A - Decontaminating method using bivalent chromium ion reducing regenerating liquid - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is a method for removing a radioactive oxide film formed on the inside of piping, equipment, fuel assemblies, etc. that come in contact with the primary cooling water of a nuclear power plant. Regarding dyeing methods.

[Background of the invention]

Pipes and equipment that come into contact with the primary cooling water of nuclear power plants;
A radioactive oxide film is formed on the inside of fuel assemblies, etc.
Since this is a cause of increasing the surface attenuation rate of plants, it is necessary to remove these oxide films, that is, decontaminate them, from the standpoint of reducing exposure during periodic inspections. This removal usually requires that only the oxide film formed on the surface of the totally encapsulated material be selectively dissolved and released from f+, while the base material, that is, the metal material itself, can be used without being melted. .

These decontamination methods can be broadly divided into chemical methods.

Physical methods including mechanical methods, electrochemical methods, etc. can be considered. These methods have advantages and disadvantages. Chemical decontamination methods take into account the characteristics of the oxide film]
This method uses a decontamination agent that is a blend of selected acids, reducing agents, complexing agents, and inhibitors. Although this method is superior in terms of the dissolution rate of the iron oxide film, there remains the risk of dissolving the metal base material as well and the risk of corrosion due to residual liquid. Physical decontamination methods include brushing using friction with a brush, spraying high-pressure water, and mechanically peeling off the film by applying vibrations to the solution or metal base material using ultrasonic waves. The decontamination rate varies greatly depending on the shape of the object, and it may not be applicable if the object is a thin pipe or has a complicated shape. The electrochemical decontamination methods are represented by an anodic polarization method in which a direct current is applied to the metal base material 1c to make itself an anode, and a cathodic polarization method in which the metal base material 1c is used as an anode. The former corresponds to an electrolytic polishing method that dissolves the base material and the oxide film, and the latter corresponds to the reduction dissolution method that reduces and dissolves only the oxide film. These methods are ideal in principle, but in practice, there are instability causes such as ineffectiveness unless an appropriate electrolyte and four solution conditions are selected for the composition and shape of the target object. is left behind.

As described above, the various conventional decontamination methods described above have many problems in terms of stability and reliability when applied to radioactive equipment and the like.

The present invention is classified as a chemical decontamination method among the various methods described above. The closest known example can be found in technical literature. Attachment (Corrosion Prevention Technology, 32724 (1983)
The LOMI method (LOWQxidation 5ta) shown in
te Metal Ion), oxides are dissolved using V'', Cr'', and Fe2+ as reducing metal ions. Specifically, these metal ions are not used alone, but as a stable complex with a complexing agent.
dy/pe""/iTA.

■2"/An organic acid that is used as picolinic acid and has a potential close to the redox direct position of these complexes.

For example, formic acid is selected for V29, and oxalic acid is selected for Cr2+, so this is different from the present invention.

The difference is in the type of complexing agent and organic acid that coexist with Cr 2+ ions.In the decontamination process, the LOMI method uses the decontamination solution in the toilet, whereas the present invention uses the dissolution in the decontamination solution. The point is that a recirculation system is used, which includes a step of removing components and a step of decontamination and regeneration through electrolytic reduction. Furthermore, in the present invention, conditions such as the concentration, pH, and dissolved oxygen of the decontamination solution are set.

[Purpose of the invention]

The purpose of the present invention is to avoid melting the metal base material.

The object of the present invention is to provide a decontamination method using a divalent chromium ion reduction regenerating solution that can selectively, uniformly, and efficiently dissolve iron oxide films and deposits formed on complicated piping, equipment, etc.

[Summary of the invention]

The present invention is applicable to objects such as piping and equipment that dissolve and separate deposits mainly composed of iron oxide films.

Cr! By circulating an EDTA decontamination solution containing + ions, the deposits are reduced and dissolved. During the decontamination process
c” In order to maintain the ion concentration at a predetermined value, electrolytic reduction is carried out in an electrolytic bath equipped with limited electrodes, and dissolved metal ions dissolved in the decontamination solution are removed by electrodeposition by constant potential electrolysis operation. This decontamination method is characterized in that Cr ions are added to the decontamination solution by dissolving them by electrolysis using metal Cr as an anode.

Next, the present invention will be comprehensively explained. First, Figure 1 shows an overview of the primary cooling water system of an atomic coupler. reactor 1
The steam generated is worked in a turbine, cooled in a condenser 3 and becomes condensate 4, and this condensate is returned to the reactor. The cause of the increase in radioactivity in the reactor is due to impurities such as iron oxides contained in the feed water 5 supplied to the reactor. Therefore, a demineralizer 6 etc. is installed in the condensate system to purify the water. There is. Further, impurities in the reactor water inside the nuclear reactor are purified by a desalination device 8 installed in the reactor water circulation system 7.

By installing such a purification system, radioactive substances can be removed. However, in many plants, it cannot be said that even these purifying devices can sufficiently remove the pollutants. Systems where radioactivity increases are mainly nuclear reactor systems, especially recirculation systems. This is Co, which is a radioactive component on the surface of the fuel rods in the reactor core.

Ni ions adhere together with the iron oxide and are converted into radioactive components by irradiation with neutrons. This component adheres to equipment such as recirculation system piping and pumps.

・) Therefore, decontamination of nuclear couplants will mainly involve the reactor water recirculation system and its surrounding systems. Next, the main components of these radioactive deposits are magnetite (Fe304),
Made of hematite (α-FezO3).

There are also other iron oxides such as Fed and Fe0OH. Some of these components include radioactive ions, such as Co-60°Mn-
It is a deposit that contains ions such as 54 and has radioactivity. To dissolve these components, the oxides are reduced to reduce the valence of the refined iron ions to make them easier to dissolve, and then the metal ions are captured by a complexing agent and taken out to the outside.

Here, the reducing dissolving power of iron oxide is the power of the reducing component in the solution to give electrons to the iron oxide.

As an index, reduction potential can be given.

In this case, the reduction potential of the Cr14- ion, which is the reducing agent, is 10.68 Vvg in the Cr''/Cr2+ system.
, it is located in SCE and exhibits a fairly strong return power. Also C
-0.6V also when r forms a complex with EDTA
Shows nearby values. Therefore, the reducing power of a solution containing Cr2 is very strong. On the other hand, in order to dissolve iron oxide using these reducing powers, it is possible to consider the natural potential that each of the metal material and the iron oxide have as a reference. Among metal materials, SuB material is at approximately -0.5V mag, scE, and iron oxide F'e304 is at -0.15V vs, S
CE, α-FezO3 is at −0, IVv+, SCE. Therefore, in order to dissolve iron oxide and prevent corrosion of metal materials, the potential should be on the negative side of the natural potential of iron oxide, and the aforementioned C, l * EDTA M, may be lower than that potential. There is.

Next, when iron oxide is dissolved, Cr2+ is oxidized to Cr3". Therefore, in order to continue the decontamination ability, it is necessary to reduce Cr" to C%+. This includes Cr
It must be reduced at a potential lower than that of the "/Cr" system. In this case, using a reducing agent may be a bad idea from the point of view of reducing power.

Electrolytic reduction has the potential to adjust the reducing power. In this operation, selection of electrode material becomes a problem.

Among various electrode materials, we found that Cr'' can be reduced to Cr 2+ in electrolytes with large H2 overvoltage, such as lead, zinc, and graphite.However, in electrolytic operation, appropriate selection of electrode materials,
Under the arrangement, the initial decontamination solution components can be regenerated.

On the other hand, as decontamination progresses, the dissolution of iron oxides in the decontamination solution
e ions increase. The Fe ions are Fe2+ ions and Fe3+ ions. These ions are gDTA
It reacts with Fe to form EDTA, but as the number of Pe ions increases, the ability to complex with EDTA weakens. In order to maintain the chelating power, it is necessary to remove Fe ions. At the same time, Cr 2 + or Cr" must not be removed. Therefore, it is necessary to selectively remove Fe ions from Cr ions.
The electrodeposition potential of r-gDTA can be used.

Although the potential at which each electrodeposites is not well understood,
Cr-EDTA is at a lower potential. In order to realize this potential difference, K can separate Fe-EDTA by applying an appropriate potential difference in an electrolytic cell filled with conductive particles. on the other hand.

Although the Cr t + -EDTA solution has excellent reducing power, it is easily oxidized and unstable. Therefore, in order to maintain the concentration of Cr"EDTA produced in the electrolytic bath, it is necessary to remove oxygen from the solution. To do this, a degassing tank is installed in the circulation system of the decontamination solution, and VcR.

It is necessary to reduce and remove oxygen in a decomposition tank.

In the degassing tank, it is necessary to reduce Do to below so ppb by bubbling an inert gas such as nitrogen (N) or argon (Ar), and further to reduce Do to below 10 ppb by electrolytic reduction.

By performing the above operations, iron oxides can be effectively dissolved and decontaminated. To recover the decontamination solution after decontamination, use an anion exchange resin to collect Cr-EDTAFe-EDTA.
is removed by ion exchange, and radioactive substances are also removed.

[Implementation sequence of the invention]

The implementation IHJ of the present invention will be described below.

FIG. 2 shows a schematic diagram of the decontamination flow when the method of the present invention is carried out to remove iron oxide deposits adhering to the inner surface of a pipe.

In this example, iron oxides are efficiently and selectively decontaminated by circulating a Cr"EDTA decontamination solution around the object to be decontaminated.
This is what we do. As shown in FIG. 1, a decontamination solution is applied to a decontamination object 9 of a plant from which iron oxides are to be removed.

It is adjoined by a system 10 that recirculates. Recirculation system 10
Raw water F111.0 with a heating source and a deaerator between both ends.
Li'e iono recovery vessel 12. A diaphragm electrolytic cell 13 and a circulation pump 14 are provided. As an accessory device to these devices, there is a Cr2° generation electrolytic tank 15 that is supplied to the raw water tank. There is an ion exchange resin tank 16 for recovering decontamination solution after decontamination. In these devices, the diaphragm electrolyzer 4'u13 is partitioned by an ion exchange membrane.

A V is formed to allow decontamination solution to flow into the cathode chamber. The anode is any insoluble electrode such as a platinum plate, whereas the IS
At the pole, the H2 overvoltage of graphite, lead, zinc, etc. is large, and cr”
An electrode suitable for reducing Cr to Cr is provided. on the other hand,
The C, 2-producer electrolytic cell 15 supplied to the raw water tank 11 has the same configuration as the diaphragm electrolytic cell, and is degassed with an inert gas to maintain the VC cathode chamber in a reducing atmosphere. There is. Note that the Fe ion recovery device 12 is constituted by an electrolytic cell filled with granular activated carbon.

In such a system and device, when decontaminating the object 9 to be decontaminated, first the Cr of the decontamination solution is removed from the Cr2 "producing electrolytic tank 15".
The EDTA solution is electrolytically reduced to produce Cr"EDTA and supplied to the raw water tank 114C. This decontamination solution is circulated to the object to be decontaminated 9 by the pump 14. Decontamination progresses.

The Fe ions generated at the same time become FeEDTA combined with E DrA, which is removed by desorption and adsorption in the li"e ion collector.Furthermore, the Cr"EDTA consumed and oxidized during decontamination is converted into Cr"EDTA. However, this is reduced in the diaphragm electrolytic cell 13 to produce Cr2"EDT.
Return to A. By processing the decontamination liquid in such a system, iron oxides in the object to be decontaminated can be dissolved without weakening the decontamination power. When these decontamination operations are completed, the decontamination solution contains C.
r"ED'l'A and some Fe"EDTA remain, but these are negative io7 (CCrEDTA]-,
Since it is CFeEDTA)-) and has radioactivity, the entire amount is recovered using an anion exchange resin 16. After recovery, the resin will be stored separately.

Next, an experimental example showing the effectiveness of the method of the present invention will be explained. .

First, the method for preparing the Cr''EDTA liquid will be described.

The reagent containing Cr is chromium sulfate (Cr 2 (804) 3
).

Chromium chloride (CrCl3), chromium acetate ((CH3)
COO]3Cr), chromic acid (Cr03), etc.
These are components that contain anions or are difficult to dissolve in the solution. Therefore, in order to contain as few inorganic coexisting ions as possible and considering the treatment of the shallow distillate after dissolution, metallic Cr powder was added to oxalic acid and dissolved at high temperature. Dissolution conditions are 0.15M/l oxalic acid 0.61JC
It was prepared by adding 0.05M of Cr powder, and was sufficiently deoxidized with Ar gas in advance, then placed in an autoclave, heated to 150C, and held for 1 hour.

The liquid taken out after cooling completely dissolves and does not decompose the phosphoric acid. By processing in this way, a solution with any desired chromium oxalate concentration can be produced.

In the following experiments, we will use this method to obtain arbitrary ED'T'All
.

A liquid prepared by mixing with was used. .

Next Electrolytic reduction experiment of K Cr ” ” E DTA W C
r”!i:1) The method for creating a TA will be described.

Figure 3 shows the electrolytic cell used in the electrolytic reduction experiment. The electrolytic cell is divided into one chamber 18 and a writing chamber 1 separated by an ion exchange membrane 9.
9! /l: Separated. In the 6-electron 11fK, an aeration pipe 20 for injecting Ar gas to remove dissolved oxygen in the liquid is installed at the bottom. Furthermore, the heating of the electrolyte VCd l
A monitor 21 is provided. The anode 14 in the anode chamber is made of platinum plate, and the cathode 23 in the cathode chamber is made to be exchangeable with various electric charges. For electrolysis, an external DC current [
A voltage is applied from 15.

Electrolytic reduction experiments were conducted using the electrolytic cell described above.

The electrolyte in the cathode chamber contains 0.006M/l Cr oxalate and 0
．． 008M/1gDT human/2NH4 liquid, 6 L
,' The pH was adjusted to 5,560C by sufficient degassing. A lead plate and a graphite plate were used as electrodes. Electrolysis conditions include constant current electrolysis wt flow 0.7 A (ti density 0.
025A/ffl). The cr" ion was analyzed by redox titration with Fe" ion. result 1
Shown in Figure 4. From the same figure, both the lead electrode and graphite tti are Cr2
+ ions are generated with electrolysis time. The reason why Cr i + ions are generated at these electrodes is considered to be because of the good adsorption of Cr 3 + ions and because the reduction reaction progresses by suppressing the decomposition of water. Therefore, in these '1' equations, depending on factors such as current density setting, WpHa degree, and dissolved gas deaeration level, Cr! + can be generated.

Next, the characteristics of actually dissolving iron oxide using the Cr1-containing electrolyte described above will be shown. The iron oxide used was Fen 04
This is a sintered body (20φ plate). The cathode chamber of the electrolytic cell shown in FIG. 2 was used for dissolution. That is, a Fe5Oi plate was inserted into a Cr2+ ion liquid produced by electrolysis. Fe
The solubility characteristics of 3O4 are shown in FIG. In the same figure, Crl +
Indicates the initial concentration of ions. From this, it can be seen that in an electrolytic solution that does not contain Cr2+ ions, there is almost no dissolution, but as the tc'' ion concentration increases, the amount of pe dissolved increases.

However, in all Cr1 ion-containing electrolytes, the increase in dissolved amount tends to decrease with time. This is because C,2' ions are consumed in the reduction and dissolution of Fe304.

Next, the decontamination characteristics of radioactive iron oxides adhering to the surface of the cooling water pipes of the atomic couplant were determined through experiments. The radioactive test material had a deposit mainly composed of Fe5O4 with a thickness of 20 μm on 5U8304 steel.
The surface of the InX1crn square with deposits was left and the rest was covered with an insulating material.

The surface radiation dose has approximately 0.8 m R/h. The noodle electrolytic cell shown in Figure 2 above was used in the experiment. The decontamination characteristics were determined by preparing a solution containing Cr l * in the FI decomposition tank in advance, immersing the sample material in it, and measuring the radiation dose of the sample material at each time.
It was measured. An example of the test results is shown in Figure 6. Test items are 0.003M/l Cr citric acid, 0.005M/l
LEDTA・2NHa liquid, 0.7t, pH5.5,
The temperature was 80C, a lead plate (2'8i) was used as the cathode material, and a current of 1.5A was applied. C in the electrolyte before immersing the test material.
rp ions are 10 ppm. C, l after experiment
4-ion is 13 ppm. Figure 5 shows the radioactivity removal characteristics. The removal rate increases rapidly due to electrolysis1
After 20 min, the characteristic exceeds 90 dIJ. By the way, in the same figure, the removal rate is 10 for an electrolytic solution containing Cr 3+ ions with the same composition but not containing Cr 2° ions.
It is around 1. Therefore, it can be seen that the Cr1 ion-containing electrolytic solution greatly contributes to the dissolution of the radioactive iron oxide.

Next, the characteristics of selective adsorption and removal of Cr ions and pe+ ions will be described. The trial outfit is shown in Figure 7. The device is made of transparent acrylic plate and has a stirring section at the bottom of the electrostatic folding tank 25.
Granular activated carbon 28 is filled between the positive IM 26 and the cathode 27. The electrolyte is 0.008M/L EDTl't.
2NHs solution KF'63° (each ion of 2r3 + is o
, no2M/ was used. The experimental method was to place the electrolytic solution in a tank and apply voltage from both electrodes to electrolyze it. The amount of decrease in Fe and C'' in the electrolyte was determined in 200ml of electrolytic solution at a voltage of 8V and a current of 0.4AK.The measurement results are shown in Figure 48.

From the same figure, both Fe and Cr decrease with '/C'tFL cape time, but the decrease in Fe(7) is large.
A)-, (Cr-EDTA:]-,
i! ! Electrical attractive force acts on the bipolarized activated carbon inside the decomposition tank, causing it to be absorbed. The absorption rate at this time is Fe'EDTA
The ionic diameter, charge amount, dissociation state of each Cr-EDTA ion, and the rQ generated on the surface of activated carbon particles.
, 4th place, and other factors. The experiment here was conducted for 1 minute.

Although the removal rate etc. differ in the flow treatment, Cr ions can be removed preferentially over Fe ions from a complexing agent solution containing both pe and Cr ions.

Note that a strongly acidic cation exchange resin can also be used for selective adsorption and removal of l'e ions. In this case (1;”e-
ED'I']- with the acidic group on the surface of the ion exchange group
It cuts off ions and removes them by adsorption. (Cr
-EDTA)- is immediately oxidized to Cr even if separated with acid.
It becomes an anion such as O3'- and is not adsorbed by the cation tree. Therefore, the separability of Fe ions and Cr ions is better than that of the previous electrolytic method.

However, in the ion exchange resin method, as the Fe ions are removed, acidic groups are released and the pH decreases, so it is necessary to neutralize with an alkali.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to selectively, uniformly and efficiently remove only the iron oxide on the surface of the base material without dissolving it, and moreover, it is possible to remove iron oxide on the surface of the base material selectively, uniformly and efficiently. It can be applied to objects to be decontaminated such as piping equipment, etc.
There is no need to worry about corrosion due to residual liquid, and the electrolytic conditions and operations are simple.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an atomic couplant, Fig. 2 is a schematic diagram showing an embodiment of the present invention for removing iron oxide from a decontaminated body, and Figs. 3 and 7 are diagrams showing the method of the present invention. Figures 4, 5, 6 and 8 showing the experimental apparatus are the experimental results necessary to demonstrate the present invention, and Figure 4 shows the cathode electrode material used to reduce Cr ions. Figure 5 shows the amount of Cr2 ions produced by electrolytic reduction.
A diagram showing the dissolution characteristics of Fe5Oi using a °ion-containing liquid. Figure 6 is a diagram showing the decontamination characteristics of a radioactive component adhering material using a solution containing C, l * ions generated by electrolytic reduction, and Figure 8 is a diagram showing the preferential removal of Fe ions from a solution containing Fe and Cr. FIG. 11... Raw water tank, 12... Fe' ion recovery device,
13...Diaphragm electrolytic cell, 14...Circulation bonder, 15.
...Cr l *Production electrolytic cell, 16...Ion exchange resin tank.

Claims (1)

[Claims] 1. In a method for decontaminating iron oxides that adhere to and accumulate on the surface of primary cooling water piping of a nuclear power plant and equipment in contact with it, divalent chromium ions (Cr^2^+) are used. The containing complexing agent solution is brought into contact with the object to be decontaminated to reduce and dissolve only iron oxides, and the decontamination solution after dissolution is regenerated using ion exchange resin and a diaphragm electrolytic cell to maintain a constant concentration of divalent chromium ions. 1. A decontamination method using a divalent chromium ion reduction regenerating solution, characterized in that the decontamination solution is kept in a vacuum and circulated in a cycle. 2. In the invention set forth in claim 1, the divalent chromium ion is produced and regenerated using a diaphragm electrolytic cell equipped with a cathode made of graphite, lead, etc. Decontamination method using ion reduction regeneration solution. 3. In the invention described in claim 1 or 2, the iron ions dissolved during the decontamination are removed by selective adsorption of iron ions with a strongly acidic cationic resin or by constant potential electrolysis. A decontamination method using a divalent chromium ion reduction regenerating solution, characterized in that the decontamination method is carried out by electrodeposition recovery of chromium ions. 4. The invention according to claim 2 or 3, characterized in that the trivalent chromium ion-containing liquid is produced by a high-temperature dissolution method using an organic acid solution of chromium metal powder. A decontamination method using a divalent chromium ion reduction regeneration solution. 5. In the invention according to any one of claims 1 to 4, the decontamination liquid contains a complexing agent such as EDTA and divalent chromium ions, and the concentration of the complexing agent is such that the concentration of the complexing agent is divalent chromium. A decontamination method using a divalent chromium ion reduction regenerating solution, characterized in that the molar ratio to the ion concentration is 1 or more and the pH is in the neutral range of 5 to 8.