JPS636839B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cleaning equipment contaminated with alkali metals, particularly sodium used as a coolant in fast breeder reactors.

Sodium, which melts at about 98 degrees Celsius, melts at about 300 to 550 degrees Celsius.
It is used as a coolant in fast breeder reactors and flows through reactor vessels, pumps, heat exchangers, steam generators, etc.

Equipment such as the pumps and heat exchangers mentioned above are used for quite a long time, and during that time they may be inspected and repaired periodically or as necessary.
Preferably, the device is then reused.

Since the above-mentioned inspections and repairs are generally performed in the atmosphere, the liquid sodium is discharged beforehand, but sodium still adheres to the parts that were in contact with the sodium. If the internal structure of the above equipment (heat exchanger tube assembly, etc. in the case of a heat exchanger) is directly exposed to the atmosphere with sodium attached to its outer surface, it may react with moisture and generate hydrogen gas. , sodium hydroxide may remain on the surface, which is undesirable for inspection and repair.

For this reason, cleaning (removal of adhering sodium) is performed before exposing sodium-adhered internal structures to the atmosphere, but rough cleaning (primary cleaning) is performed using water vapor, humidified gas, alcohol, etc., and then secondary cleaning is performed. A conventional general cleaning process has been to immerse the structure in pure water and rinse it.

However, the internal structure has a complex structure, and it is normal for the surface to have a narrow entrance of about 200 μm and a blind space (hereinafter referred to as a clevis) that is slightly wider inside. When such a crevice exists, sodium remains deep inside the crevice even after the above-mentioned cleaning, and an aqueous solution of sodium hydroxide tends to remain at the inlet.

When inspecting or repairing internal structures in the atmosphere,
The sodium hydroxide aqueous solution at the clevis inlet reacts with carbon dioxide gas to become sodium carbonate, but the sodium carbonate acts as a seal and unreacted sodium hydroxide aqueous solution remains inside.

As mentioned above, if an aqueous sodium hydroxide solution remains in the crevices on the surface of the internal structure, a problem arises in that alkaline stress corrosion cracking occurs during preheating for reuse.

The present invention was made in order to prevent the above-mentioned alkali stress corrosion cracking during reuse, and as the final cleaning, the secondary cleaning is performed by physically eliminating boiling bubbles by reducing pressure and by reacting with carbon dioxide gas to eliminate harmlessness. The purpose of the present invention is to provide a cleaning method that can completely remove sodium hydroxide in the crevice by using chemical conversion to carbonate.

The method of the present invention will be explained below with reference to specific examples.

FIG. 1 is a system diagram of an apparatus implementing the method of the invention.

In FIG. 1, a porous support plate 11 on which the internal structure of the device, that is, the object to be cleaned 1 is placed, is installed at the bottom of a pot-shaped cleaning tank 10, and a heater 13 is attached to the outer periphery of the porous support plate 11.

After the object to be cleaned 1 is inserted and placed, the cleaning tank 1
The upper opening of 0 is closed with a lid 15, and an exhaust pipe 22 equipped with a valve 21 is connected to the lid 15. A drain pipe 25 with a valve 24 leads to the bottom of the cleaning tank 10, while an evacuation pipe 29 with a vacuum pump 27 and a valve 28 leads to its top.

In addition, supply pipes 31 and 33 for introducing cleaning agents such as steam and pure water are connected as shown in the figure, and valves 3 and 33 are connected to each other as shown in the figure.
5 and 36 are provided.

In the illustrated apparatus, the process of cleaning the object 1 to be cleaned will be explained. As shown in the figure, the object 1 to be cleaned is set and the lid 15 is attached.

As the first cleaning, first open the valve 28, start the vacuum pump 27 to exhaust the air inside the cleaning tank 10 through the vacuum exhaust pipe 29, then close the valve 28 and preheat the inside of the cleaning tank 10 with the heater 13. .
After that, the valves 35 and 36 are opened, and the supply pipes 31 and 3 are opened.
Inject water vapor etc. from step 3. Water vapor containing hydrogen gas is continuously exhausted from the exhaust pipe 22. After the operation is completed, the valve 24 is opened and the condensed water containing the removed sodium (compound) is discharged through the drain pipe 25 to a waste water tank (not shown).

Note that the primary cleaning may be performed using humidified gas, alcohol, or the like instead of water vapor.

Next, the secondary cleaning will be explained with reference to FIG. 2 showing the state inside the clevis of the object 1 to be cleaned.

After the first cleaning, the supply pipe 31 is connected to a carbon dioxide gas supply source by being switched by a valve (not shown).

After that, (a) the valves 21, 24, 35, and 36 are closed, the valve 28 is opened, and the vacuum pump 27 is started to evacuate the inside of the cleaning tank 10 to reduce the pressure to the target value.

FIG. 2A schematically shows an enlarged view of the inside of the clevis 3 of the object to be cleaned 1 when the pressure is reduced in step (A). That is, due to the reduced pressure, so-called reduced pressure boiling occurs, and water vapor bubbles D are created in the sodium hydroxide aqueous solution A.
occurs, and this leaves the clevis 3. At the same time, part of the aqueous solution A is also pushed out.

(b) After the pressure is reduced, the valve 28 is closed, the valve 36 is opened, pure water is injected into the cleaning tank 10 from the supply pipe 33, and the cleaning object 1 is kept immersed therein for an appropriate period of time.

When water is being filled under reduced pressure, residual sodium B also reacts with water due to the supply of water from the outside of the clevis 3 and changes into hydrogen gas and sodium hydroxide.

Hydrogen gas escapes to the outside in the same way as water vapor bubbles D.

In this way, at the end of step (b), the second
As shown in the figure, only a considerably diluted low concentration sodium hydroxide aqueous solution A remains in the clevis 3. However, since the concentration diffusion to the outside of the clevis 3 is performed only at a low speed, the sodium hydroxide concentration is hardly reduced even if it takes a long time.

(c) Next, open the valve 24 to discharge the pure water from the cleaning tank 10, and at the same time open the valve 35 to supply carbon dioxide (a mixture of carbon dioxide and inert gas) from the supply pipe 31 into the cleaning tank 10. and keep it filled with carbon dioxide gas.

After carrying out the operation in step (c), the situation is generally as shown in FIG.

If you repeat step (a) again, the second
As water vapor bubbles D are generated and separated as shown in Figure D, most of the sodium hydroxide aqueous solution A and sodium carbonate C are also pushed out.
In the state of step (B), a small amount of sodium carbonate C and a more dilute aqueous sodium hydroxide solution A remain as shown in FIG. 2E. If step (c) is further performed here, as shown in FIG. 2, sodium carbonate C is produced coarsely at the inlet, and even at the back, it is produced quite close to the bottom. This is because the concentration of aqueous solution A is already dilute in Figure 2 (e), so the evaporation of water into the carbon dioxide atmosphere precedes the reaction between sodium hydroxide and carbon dioxide gas in step (c). It's for a reason. For this reason, the sodium hydroxide aqueous solution A remaining deep inside
becomes negligible. By repeating steps (a) to (c) in this way, a small amount of sodium carbonate (C) will eventually be found in the clevis 3, as shown in Figure 2 (g).
Sodium hydroxide aqueous solution A remains.
This means that no alkaline stress corrosion cracking occurs even if preheating is performed during reuse.

According to the inventor's experiments, if steps (a), (b), and (c) are repeated at least three times on an actual machine, the second
It was confirmed that the situation as shown in the figure was reached.

Note that in step (c) of the above specific example, carbon dioxide gas was introduced and held as it was, but after completion of drainage, the vacuum pump 27 sucked and discharged carbon dioxide gas to once reduce the pressure inside the cleaning tank 10, and then the carbon dioxide gas was introduced again. When gas is introduced through the supply pipe 31 and maintained, boiling under reduced pressure occurs in the state shown in FIG.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of an apparatus for carrying out the method of the present invention, and FIG. 2 is a diagram explaining the state at each step of the method of the present invention. 1...Object to be cleaned, 10...Cleaning tank, 15...
Lid, 21, 24, 28, 35, 36...Valve, 22
... Exhaust pipe, 25 ... Drain pipe, 27 ... Vacuum pump, 29 ... Vacuum exhaust pipe, 31, 33 ... Supply pipe, A ... Sodium hydroxide aqueous solution, B ... Residual sodium, C ... Carbonic acid Sodium, D... water vapor bubbles.

Claims (1)

[Claims] 1 After the object to be cleaned stored in the cleaning tank is primarily washed, the cleaning waste liquid is discharged from the tank, and then the pressure inside the tank is reduced, pure water is injected into the tank, and the object to be cleaned is washed with the purified water. 1. A method for cleaning an alkali metal-adhered structure, which comprises repeating a predetermined number of secondary cleaning steps in which the structure is immersed in water and held, and the pure water in the tank is replaced with carbon dioxide gas and held.