JPS62211597A - Method of decontaminating piping - 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 [Industrial Application Field] The present invention relates to a method for dissolving and removing metal oxides attached to the inner surface of plant piping, and in particular to a method for dissolving and removing metal oxides attached to the inner surface of atomic couplant piping. The present invention relates to a piping decontamination method that can highly efficiently dissolve and remove metal oxides containing metal oxides.

[Conventional technology]

In atomic couplers, oxides containing radioactive nuclides (Co, Mn, etc.) tend to adhere to secondary system piping as the operating time progresses, and the dose tends to increase. The radiation doses that workers receive during the operation and periodic inspection of nuclear couplants tend to increase, and decontamination technology that dissolves and removes metal oxides on the surface is required as one measure to reduce the radiation doses.

As a conventional method for removing metal oxides adhering to surfaces, there is a chemical decontamination method using chemical agents. Chemical decontamination methods, as typified by JP-A No. 53-731, use a solution containing an acid, a complexing agent, and a reducing agent as main components, and because the pH is relatively low, metal oxides on one surface are Although it is effective for dissolving, it is not preferable from the viewpoint of corrosion of constituent materials. Further, as a method for electrically dissolving and removing oxides attached to metal surfaces, an electrolytic polishing method is known, which is a method in which a counter electrode is provided to the object to be decontaminated and electrolysis is performed. The electrolytic polishing method uses a high current to dissolve not only the oxide but also the constituent materials, so it is not suitable for objects to be reused. In addition, this method focuses on valves, short pipes, etc. as objects to be decontaminated, and is not suitable for system decontamination of piping and the like. On the other hand, there is a method that uses a neutral solution to alleviate the corrosion of the constituent materials, and also electrolyzes the decontamination solution using a diaphragm electrolytic tank to regenerate the decontamination solution and dissolve the metal oxides attached to the surface. . (Unexamined Japanese Patent Publication No. 57-
(No. 85980) This method involves directly cathodically polarizing the object to be decontaminated, and supplying electrons to the object from the decontamination liquid side.

A method for dissolving surface oxides is listed.

The former method requires a DC power source for the device and also causes a large overvoltage when polarizing the cathode.

In addition, the latter method is not preferred in terms of energy efficiency, and requires consideration such as using an electrolytic cell to regenerate the reducing power of the decontamination solution and maintaining the electrolytic current at an appropriate setting value.

[Problem that the invention seeks to solve]

Chemical decontamination is suitable for system decontamination that involves decontaminating the entire reactor-subsystem piping. However, in conventional chemical decontamination, after forming a decontamination loop for the object to be decontaminated, decontamination is carried out by heating the decontamination solution to 100°C or higher and circulating the decontamination solution. The decontamination equipment itself will be a pressure vessel, and consideration will need to be given to permits and costs. Also,
When decontamination is carried out at a temperature of 100°C or lower, the decontamination speed is extremely reduced.

The purpose of the present invention is to achieve a high removal rate in systematic decontamination compared to conventional chemical decontamination even at a decontamination temperature of 100°C or lower, and to dissolve metal oxides on the surface without dissolving the base material. An object of the present invention is to provide a method for decontaminating piping that can be removed.

[Means for solving problems]

In order to achieve a higher removal rate than conventional chemical decontamination methods even at a decontamination temperature of 100°C or lower, the present invention is designed to provide a A flexible wire or plate-shaped electrode is inserted and brought into contact with the inner surface of the pipe to reduce and dissolve metal oxides. The wire or plate-shaped electrodes used in the present invention are made of carbon steel. The electrodes are inserted from the connection point when configuring the decontamination loop.

Also, it can be linear or plate-shaped! Instead of inserting a pole, iron particles or iron powder are used to form a decontamination loop, mixed with a decontamination liquid, and circulated around the piping to be decontaminated to lyse the particles and powder on the inner surface of the piping and dissolve metal oxides. The operation pattern for decontamination using the particulate electrode of the present invention is to circulate the decontamination solution containing the particulate electrode, then stop the circulation once, and perform an operation to deposit one particle on the inner surface of the pipe. Contact with metal oxides. Decontamination is performed by repeating this operation pattern. Decontamination using particulate electrodes can be carried out using existing drain ports and decontamination ports because they are mixed with decontamination liquid and circulated.

By the above means, even at a decontamination temperature of 100° C. or lower, a high removal rate can be easily achieved compared to conventional chemical decontamination methods.

[Effect]

The decontamination method of the present invention dissolves and removes metal oxides attached to the inner surface of piping, and in particular removes radioactive nuclides ("Co", "M") attached to the inner surface of the primary system piping of the atomic couplant.
It is characterized by easily and efficiently dissolving metal oxides containing metals such as n, etc.).

The metal oxides adhering to the inner surface of the primary cooling water system piping of the atomic couplant are mainly composed of Fe3O4 and Fe208, and these oxides contain radionuclides that contribute to the dose rate. These metal oxides can be dissolved by reduction.

In the present invention, a linear or plate-shaped electrode made of carbon steel having a base potential is brought into electrical contact with the surface metal oxide on the inner surface of the pipe to be decontaminated, thereby creating a gap between the electrode and the object to be decontaminated. It forms a battery and dissolves and removes metal oxides.

This method is more effective than conventional chemical decontamination even at low decontamination temperatures. These reactions proceed as follows.

Dissolves metal oxides.

Fs→F e"++ 2 e--(1) F e aoa
+8 H÷+2 e--+3 F e”++4 Hz
O... (2) Also, by utilizing this effect, metal oxides on the surface can be dissolved and removed by mixing particulate electrodes with the decontamination liquid instead of linear electrodes and circulating it. I can do it. This method uses a cyclically granular electrode. As the granular electrode, iron particles, iron powder, and particles coated with iron are used.

When the above electrode is brought into contact with the surface of the metal oxide under voltage, the reaction (1) occurs, and the oxide progresses as shown in (2). The electrodes are installed in a linear manner.

The plate-shaped electrode is inserted from the decontamination device connection point when constructing the decontamination loop for the piping to be decontaminated. Since flexible electrodes are used, they can be inserted and installed if the piping is long.

Furthermore, since the particulate electrode is mixed with the decontamination liquid and injected into the pipe, there is no need to install it in the pipe in advance. In this decontamination operation, the decontamination solution is heated to a predetermined temperature and sufficiently degassed, and then particles or powder are mixed into the decontamination solution and circulated and injected into the relevant piping, and the circulation pump is stopped during the circulation. The particles or powder are deposited on the inner surface of the pipe to make electrical contact. The operation button repeats this cycle → stop → circulation to dissolve oxides on the metal surface.

The decontamination solution consists of a solution whose main component is a complexing agent.
Use solution No. 7. In addition, during application, the decontamination solution should be sufficiently deaerated and the decontamination temperature should be kept below 100°C.

〔Example〕

The present invention will be described below with reference to Examples.

First, FIGS. 1 and 2 show decontamination flows when the present invention is applied to an actual plant.

Figure 1 shows that a flexible linear or plate-shaped electrode 2 is inserted into the piping of the object to be decontaminated 1, and the electrode 2 is placed on the inner surface of the piping.
This shows the decontamination flow when metal oxides are dissolved and removed by bringing them into contact with each other. The metal oxide of the object to be decontaminated 1 is reduced and dissolved as the electrode 2 is dissolved. The decontamination liquid 3 is adjusted in a decontamination liquid tank 4, heated to a predetermined temperature (below 100°C) by a heater 5, and inert gas (Nz
yAr gas) 6 is injected. In the decontamination flow, decontamination liquid 3
A circulation pump 7 is provided for injecting and circulating the water into the object 1 to be decontaminated. In addition, the peeled cladding component is removed by filter 8.
will be removed. After chemical decontamination, the system and object 1
Finally, the washing water is passed through a hot bed type ion exchange resin column 9 to be purified and then drained to a drain system. Furthermore, the decontamination tank 4 is equipped with a drain port 10 and a vent hole 11. After decontamination is completed, electrode 2 is removed.

In FIG. 2, a granular or powdered electrode 12 is mixed with a decontamination liquid 3 and is injected and circulated into an object 1 to be decontaminated, thereby reducing and dissolving metal oxides on the surface. In this decontamination operation, the circulation pump 7 is stopped during the circulation, and a single particle of the electrode 12 is deposited on the inner surface of the object, thereby making close electrical contact with the electrode and promoting dissolution of the metal oxide. In such removal using a granular or powdered electrode, an operation pattern of circulation→stop→circulation is repeated. The temperature of the decontamination liquid 3 is raised in a decontamination tank 4 by a heater 5 as in FIG. 1, and Ar or Nz gas 6 is injected to remove dissolved oxygen. However, the granular or powdered electrode 12 is heated to a predetermined temperature (below 100° C.), then put into a tank and stirred by a stirrer 13. Thereafter, it is injected into the object 1 using the circulation pump 7. After the decontamination is completed, remaining particles and powder are removed through a filter 8.

The electrodes used for the above decontamination are flexible carbon steel wires and plates, or bundles of these. Also 1
For particulate electrodes, recyclable iron particles and iron powder or iron coated particles are used. When connecting the decontamination equipment to the piping to be decontaminated, the wire or plate-shaped electrodes are inserted into the piping from the connection point in advance. In the case of granular electrodes, there is no need to install them in advance, and they can be mixed with the decontamination solution and injected and circulated.

Next, examples of the present invention and their results will be described.

The decontamination test device in which the effectiveness of the present invention was investigated is shown in FIG. 3. The device shown in FIG. This is a study of the dyeing effect. The test equipment is roughly divided into potentiometer 16 and reaction tank 17.
．． It is composed of a heating heater 8. The decontamination liquid 19 was heated to 90° C. and degassed by injecting Ar gas through the injection pipe 2o. A saturated material reference electrode 21 is installed to measure the potential of the constituent materials and the test piece, and a potentiometer 16 is installed.
The potential was measured using In this test, the decontamination solution used was a solution whose main components were a complexing agent and an organic acid. The amount of dissolved metal oxide was quantified using an atomic absorption spectrophotometer. FIG. 4 shows a summary of test results using a simulated test piece (FeaOa) from the viewpoint of potential dependence. In FIG. 4, the vertical axis shows the dissolved amount of Fe+sO+ (g/%), and the horizontal axis shows the potential (Vvs, S CE ). As shown in the figure, the amount of FegOa dissolved shows potential dependence, and as the potential becomes more base than the natural potential, the amount of FegOa dissolved tends to increase. Under these conditions, the potential of the test piece FeaO4 is around -0.1 V at the saturated material electrode. On the other hand, carbon steel, which is an electrode, has a potential of around -0.7V, and by contacting or connecting it to Faao, which is the object to be decontaminated, it can be shifted to the potential in the dissolution range of FaagO. . As a result, better decontamination effects can be obtained even at low temperatures (below 100° C.) compared to ordinary chemical decontamination. Similar results can be obtained when this electrode is made into particles or powder.

In addition, in order to further demonstrate the effectiveness of the present invention, a comparison was made with a conventional chemical decontamination method under the same conditions using a piping test piece removed from an actual atomic couplant.

The test conditions and equipment were the same as in the experiment using the mock test piece. The decontamination effect was measured using a Ge (Li) semiconductor detector, and the removal rate (%) of the radionuclide BOCO, which has a large contribution to the dose
) was evaluated. The results are shown in Figure 5. Figure 5 shows the case in which carbon steel electrodes are in contact with each other, and Figure 5 shows the case in which carbon steel electrodes are in contact with each other. Figure 5 shows conventional chemical decontamination in Figure 5. Under low temperature conditions (below 100°C), the Co removal rate is slow and the decontamination efficiency is low.In contrast, by adding an operation in which carbon steel electrodes are brought into contact with conventional chemical decontamination, A high 6° Go removal rate can be obtained.

The present invention is for dissolving and removing metal oxides FeaOa (magnetite) and α-Fezes (hematite) formed under oxidizing atmosphere conditions, or spinel-type oxides containing Ni, and for removing the primary system of atomic couplants. It can be widely applied not only to the decontamination of piping systems, but also to the decontamination of piping systems in thermal power plants and industrial plants.

〔Effect of the invention〕

According to the present invention, metal oxides attached to the inner surface of the pipe can be easily and efficiently dissolved and removed.

In particular, in the primary system piping of an atomic couplant, metal oxides containing radionuclides can be efficiently dissolved and removed, reducing the radiation dose of the piping and suppressing corrosion of the base material during decontamination. I can do it. Also, decontamination temperature 1
It is possible to obtain a higher removal rate than conventional chemical decontamination methods even at temperatures below 100°C, and the decontamination equipment itself decontaminates at temperatures below 100°C, making it suitable from the standpoint of licensing and economic efficiency.

Therefore, during the disassembly and inspection of the atomic coupler and periodic inspection work of the atomic coupler, the radiation dose received by the workers can be reduced, and the reliability of the work and the reliability of the plant can be improved.

[Brief explanation of drawings]

FIGS. 1 and 2 are system diagrams when this device according to an embodiment of the present invention is applied to an actual plant, FIG. 3 is a schematic diagram of a test device for demonstrating the effectiveness of the present invention, and FIG. The figure shows potential and F
FIG. 5 is a diagram showing the relationship between the amount of dissolved eaOa, and FIG. 5 is a diagram showing the decontamination effects of the decontamination method of the present invention and the conventional chemical decontamination method. DESCRIPTION OF SYMBOLS 1... Decontamination target object, 2... Linear electrode, 4... Decontamination liquid tank, 5... Heater, 7... Circulation pump, 8...
...Filter #1 Figure/-*fn*a z-ti image; 7-single pole 3-f
8 to f-so (4--1 To Hitoshi So 5 tan 7 Jin-''!#~
Ofutsu” Kanshato Rz answer 4m σ-1,0-a, g -6,6-a, 4-a,
Z 09ll (Vvs3CE)

Claims (1)

[Claims] 1. A chemical decontamination method that consists of configuring a decontamination loop for the piping system to be chemically decontaminated to dissolve and remove metal oxides attached to the internal surfaces of piping and equipment in a piping system consisting of piping and equipment. A method for decontaminating piping, which comprises forming an electric circuit between the piping to be decontaminated and a linear or plate-shaped electrode installed in the piping.