JPH01320497A - Chemical decontamination method - Google Patents

Abstract

PURPOSE:To eliminate the danger of generation of stress corrosion cracking in the clevis part of a piping system even if the number of flushing times is decreased by heating the clevis part to >=150 deg.C and decomposing away the residual decontaminating agent. CONSTITUTION:For example, a heater 6 for joining the flanges of the piping is constituted by sandwiching a soft surface heating element 61 with heat resistant insulating sheets 62, is attached with leads and is mounted to the flange joint part so as to wind around said part nearly one round. For example, an aq. % oxalic acid soln. is filled and circulated in the piping in this constitution and thereafter, the heater 6 is energized in the state of removing said liquid to heat the joint part until the clevis part attains about 200 deg.C. The total org. carbon content in cleaning water after the clevis part is cooled and is cleaned with water after the heating. The danger of the stress corrosion cracking in the clevis part by the residual decontaminating agent of the piping system is eliminated in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a method of decontaminating a piping system that handles liquids, particularly liquids containing radioactive substances, using a chemical decontamination agent.

[Prior art] In facilities such as nuclear power plants, radioactive corrosion deposits are formed on piping systems through which liquids containing radioactive substances pass, such as secondary systems and waste liquid treatment equipment pipes, their flange joints, valves, pumps, etc. It is well known that decontamination using chemicals is an effective method for removing substances.

One of the decontamination methods is to use a chemical decontamination agent consisting mainly of an organic acid such as oxalic acid or citric acid with some surfactant added, and to apply the aqueous solution to the piping system. Some methods are carried out by circulating at 90°C.

By the way, there are "clevises" in piping systems where liquid may remain, such as flange joints, valves, and pumps.
There is always a part called.

(The edict of crevice is phlebas creva.
Like sse, it means "slit" and originally refers to a cut such as a flange joint, but it is widely used to cut together parts where liquid may remain. ) When designing a plant, it goes without saying that efforts are made to minimize the number of clevises, but it is still unavoidable that a considerable number of clevises will appear. For example, the safe end between the piping and nozzle when using a secondary jet pump, and the heat sleeve part provided in preparation for temperature changes are examples of characteristic recesses.

If the chemical decontamination agent remains in the recesses in this way, local corrosion may progress and cause undesirable phenomena such as stress corrosion cracking.

To eliminate such risks, after decontamination, the pipe system is usually flushed with pure water (this is called "flushing"). The effect of flushing is
Needless to say, it increases by repeating it many times,
That means more downtime for the plant for maintenance and higher costs.

[Problems to be Solved by the Invention] The purpose of the present invention is to improve the above-mentioned situation during chemical decontamination of piping systems, and to eliminate the risk of stress corrosion cracking in the clevis even if the number of flushing operations is reduced. The purpose of this invention is to provide a chemical decontamination method.

[Means for Solving the Problems] The chemical decontamination method of the present invention is a method for decontaminating a piping system that handles liquid using a solution of a chemical decontamination agent. It is characterized by heating to a temperature of 150°C or higher to decompose and remove residual decontamination agent.

This method is particularly useful when the piping system handles liquids containing radioactive materials.

The means for heating the clevis is optional, and the use of an electric heater will often be advantageous.

[Function] As mentioned above, the present inventors discovered that the chemical decontamination agent is composed mainly of an organic acid and contains some surfactant, and the latter is also an organic compound and is resistant to heat. Focusing on its weakness, we attempted to thermally decompose a 1% aqueous solution of a commercially available chemical decontamination agent.

As a result, thermal decomposition progresses as follows, and 0 chemical decontamination agent A
Heating to 150'C → C02 + H2O 0 Heating chemical decontamination agent B to 200'C → CO2 + 12Q + N20V Substantially completely decomposes chemical decontamination agents A and B, both of which have oxalic acid as their main component. It was confirmed that it can be removed.

The heating temperature and heating time to be applied will vary depending on the type of chemical decontamination agent used, the amount remaining in the crevice, etc., but if necessary, it is possible to find the appropriate conditions by conducting some preliminary experiments. can be determined.

[Example] In order to examine the degree of corrosion caused by chemical decontamination agents, 5US3
A crevice corrosion test piece made of 04 stainless steel and having a shape not shown in FIG. 1 was prepared.

0.25 of a commercially available chemical decontamination agent whose main component is oxalic acid.
% aqueous solution was prepared and heated to 90° C., the above test piece was immersed in the solution and held while circulating the solution.

When I took out the test piece after 3000 hours and examined it, I found that
Some corrosion was observed in the gap.

Next, in order to investigate the behavior of the chemical decontamination agent remaining in the crevice of the piping system, the liquid in the tank 1 was circulated by a pump 4 through a pipe with a flange connection 2 and a valve 3 as shown in Fig. 2. We assembled an experimental device configured to do this.

After flowing a 1% oxalic acid aqueous solution for 50 hours,
Handled liquids from the piping system. When the remaining liquid in the clevis was collected and the oxalic acid concentration was measured, it was found to be approximately twice as high as the circulating liquid. The chemical decontamination agent was concentrated in the crevice, and combined with the results of the preliminary experiment described above, it became clear that it was necessary to remove the chemical decontamination agent from the crevice.

Therefore, as a means to heat the clevis, electric heaters for flange joints, valves, and pumps were manufactured. The heater 6 for the flange joint shown in FIG. 3 has a structure in which a flexible planar heating element 61 is sandwiched between heat-resistant insulating sheets 62 and a lead is attached so as to wrap around the flange joint approximately once. The valve heater 7 shown in Fig. 4 is made of a metal plate and has a heater acid 73 in the shape of a hollow body with a slight thickness cut into pieces that follows the outer shape of the valve, and the shell 73 is filled with magnesia powder. Heating wire 7 with material 72
It has a structure in which 1 is embedded.

Although not shown, an electric heater with a structure similar to that for the valve was also fabricated for the pump (volute type).

Each part of the experimental apparatus described above was thoroughly cleaned and reassembled. Each heater was attached, and the above 1% aqueous oxalic acid solution was circulated again for 50 hours.

While the liquid was being soaked, electricity was applied to the heater to heat each crevice until it reached approximately 200'C, and this was maintained for approximately 30 minutes.

After cooling, the device was disassembled and the clevis was washed with a small amount of water. TOC (free carbon) in the wash water was measured and found to be below the detection limit. This indicates that the chemical decontamination agent remaining in the Talebis area was completely decomposed and removed.

[Effect of the Invention 1] According to the chemical decontamination method of the present invention, after chemical decontamination of a piping system is performed using a commonly used chemical decontamination agent, the remaining decontamination agent can be substantially completely decomposed and removed.

This effect can be achieved by reducing the number of times the piping system is flushed with pure water, especially when a high degree of removal of residual decontamination agents is required. Achieved with fewer flushes. In this way, it can be said that the risk of stress corrosion cracking in the clevis due to residual decontamination agent in the piping system has virtually been eliminated, reducing equipment maintenance time, and
Benefits include reduced costs and improved safety.

Therefore, the present invention is particularly useful when applied to plants that handle liquids containing radioactive substances, and can also be applied to chemical plants in general.

[Brief explanation of the drawing]

FIG. 1 shows the shape of a test piece used in a preliminary experiment in an example of the present invention, in which A is a plan view and B is a side view. Fig. 2 is a conceptual flowchart showing the configuration of the experimental apparatus used in the embodiment of the present invention. Fig. 3 shows the electric heater for the flange joint used in the embodiment of the present invention. A is a vertical cross-sectional view, B is a vertical cross-sectional view, and
is a cross-sectional view. FIG. 4 shows an electric heater for a valve used in an embodiment of the present invention, where Δ is a longitudinal cross-sectional view and B is a cross-sectional view. 1...Tank 2...Flange joint 3
...Valve 4...Pump 6...Flange joint heater 61...Sheet heating element 62...Heat-resistant insulation sheet 1~
7...Valve heater 71...Heating wire 72...Filling material 73...
heater acid

Claims (2)

[Claims] (1) In a method of decontaminating a piping system that handles liquid using a chemical decontamination agent, the clevis of the piping system after decontamination is heated to a temperature of 150°C or higher to decompose and remove residual decontamination agent. A chemical decontamination method characterized by: (2) The chemical decontamination method according to claim 1, wherein the liquid contains a radioactive substance.