JPS5821239B2 - Method for cleaning passages through which reactor coolant flows - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cleaning a passage through which a fluid flows, and more particularly to a method for cleaning a passage through which a fluid flows, which is suitable for cleaning a coolant passage in a nuclear reactor.

Conventionally, methods for cleaning coolant channels include a mechanical cleaning method in which scale within the pipes is physically and mechanically scraped off, and a chemical cleaning method in which scale is eluted using chemicals.

Adopting a chemical cleaning method to remove radioactive scale deposited on the inner walls of reactor recirculation system pipes is important because chemicals used in the layer may remain on the inner surface of the pipes or leak into the cooling water of the reactor core. This is not desirable because it may have an adverse effect on the reactor core and other parts.

For this reason, conventionally, mechanical cleaning methods have mainly been attempted for cleaning the pipes of this system.

Figures 1 and 2 show an overview of the reactor recirculation system piping.

In order to supply cooling water to the reactor core 2 within the reactor vessel 2, a recirculation system piping 4 equipped with a recirculation pump 5 is provided.

Cooling water discharged from the recirculation system piping 4 is guided to the reactor core 2 via the jet pump 3.

In the recirculation system piping 4, radioactive scale 7 carried by the coolant fluid 8 is deposited on the inner surface of the pipe wall 6.

As a result, the radiation dose on the surface of the piping increases, so when the nuclear reactor is shut down, this system is flushed and cleaned in order to reduce the radiation dose on the surface of the piping 4.

In this case, the discharge amount of the recirculation pump 5 is increased to its maximum capacity, and as a result, the flow rate in the pipes of this system is increased to 120-150% of normal operation.

However, according to conventional experience, the method of removing the scale 7 carried by the fluid and deposited on the inner surface of the pipe 4 by increasing the velocity of the fluid has an extremely small removal effect on the scale 7.

Great effects cannot be expected unless the flow rate is dramatically increased by about 10 times by increasing the flow rate of the coolant.

However, the recirculation system piping 4 is huge, with an inner diameter of about 50 cm.

Generally, the discharge amount of the recirculation pump 5 is close to the limit during rated operation, and the flow rate within this piping 4 is 1% of the normal recirculation flow rate.
In order to flow 0 times the flow rate, 1 of the current recirculation pump
A very large pump with a displacement of 0 times that amount is required.

If a recirculation pump with a large discharge rate is used to remove scale 7 in the piping 4 during maintenance and inspection, only one-tenth of the flow rate will be discharged during normal operation, resulting in an enormous waste of equipment.

On the other hand, in thermal power plants and the like, in order to remove scale adhering to the heat transfer tubes of condensers, sponge balls having an inner diameter equal to the inner diameter of the heat transfer tubes are circulated within the heat transfer tubes to prevent scale adhesion.

If this is applied directly to the recirculation system piping of a nuclear reactor, the inner diameter of the recirculation system piping 4 is approximately 50 cIrL, so it would be difficult to circulate a sponge ball with a diameter of 50 (m) inside the pipe.

Although it is possible to reduce the diameter of the sponge balls, the smaller the diameter, the more sponge balls are required, making it difficult to completely remove the sponge balls from within the recirculation system piping 4.

If sponge balls remain in the recirculation system piping 4 even after cleaning the recirculation system piping 4, these sponge balls will flow into the reactor core when the reactor starts operating, and the coolant in the fuel assembly will be destroyed. There is a risk of clogging the flow path.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the above-mentioned techniques and to provide a cleaning method for removing scale adhering to reactor coolant passages without leaving any foreign matter in the passages.

The present invention is characterized in that solid particles that melt as the temperature of the coolant increases are mixed into the reactor coolant, a mixed fluid of the coolant and the solid particles is flowed into the passage through which the coolant flows, and the Bringing solid particles into contact with scale attached to the passageway to remove the scale, separating the coolant containing the scale from the solid particles, removing the scale from the coolant, and removing the scale from the coolant. The coolant and solid particles are returned to the passageway to form a closed loop.

Preferably, the solid particles have a hardness that is greater than the scale deposited within the passageway and less hard than the material of the passageway.

A preferred embodiment of the present invention will be described based on FIGS. 3, 4, and 5.

Components that are the same as those shown in FIG. 1 are designated by the same reference numerals.

Valves 10 and 11 are installed in the recirculation system piping 4 at the upstream and downstream sides of the recirculation pump 5 and close to the reactor vessel 2.
is installed.

A piping 13 is attached to the recirculation system piping 4 at a portion between the recirculation pump 5 and the valve 11 and close to the valve 11.

This piping 13 is connected to a separator 16 via a valve 21.
The mixed liquid artificial nozzle 43 (FIG. 4) is connected to the mixed liquid artificial nozzle 43 (FIG. 4).

As shown in FIG. 4, the separator 16 includes a body 41, a mesh 42, a mixed liquid inlet nozzle 43, a replenishment nozzle 44,
It consists of a particle outlet nozzle 45 and a fluid outlet nozzle 46.

Refill nozzle 44 is connected to ice maker 19 via piping 23 .

One end of the piping 24 is connected to the fluid outlet nozzle 46, and the other end of the piping 24 is connected to the pure water tank 17 via the filter 18.

A cooler 25 is inserted into the pure water tank 17.

The particle outlet nozzle 45 is connected to the pipe 26 and the valve 2
2 to the particle inlet nozzle 35 (FIG. 5) of the ejector 15.

The detailed structure of the ejector 15 is shown in FIG.

The ejector 15 includes a differential user 31, a throat 32, a suction chamber 33, an ejection nozzle 34, and a particle inlet nozzle 35, as shown in the figure.

The jet nozzle 34 is connected to the pure water tank 17 via a pipe 27.

The pump 14 is attached to the pipe 27.

A pipe 12 is attached to the differential user 31 of the ejector 15.

This piping 12 is connected via a valve 20 to a recirculation pump 5
It is attached to the recirculation system piping 4 between the valve 10 and the valve 10 and close to the valve 20.

During normal operation, the valve 1 installed in the recirculation system piping 4
0 and 11 are open, valves 20 and 21 are closed, and the cooling water discharged from the recirculation pump 5 is sent to the core section 1 through the jet pump 3.

When the reactor is shut down for periodic reactor inspections, the valves 10 and 11 are closed on the recirculation system piping 4 to be cleaned, and the recirculation system piping 4 is isolated from the reactor vessel 2 containing the reactor core 1. This prevents the influence from reaching the reactor core 1 and damaging the fuel and the like.

Next, valves 20 and 21 are opened, and water containing ice particles is sent from pipe 12 through valve 20 to recirculation system pipe 4.

As shown in FIG. 6, a mixture of water 8 and ice particles 9 flows in the pipe 4.

This mixed water is taken out from the recirculation system piping 4 to the piping 13 through the valve 21.

In this way, the scale 7 adhering to the inner wall 6 of the recirculation system piping 4 is scraped off by the ice particles 9, and the scraped scale is spilled out of the system together with the mixed water, thereby cleaning the recirculation system piping 4. Do the following.

For cleaning, approximately 10 tons of pure water constantly stored in the reactor condensate tank is introduced into the pure water tank 17.

Here, the pure water is cooled to about 5° C. by the cooler 23.

This water then passes through piping 27 and is sent to ejector 15 by pump 14.

The pure water from the pump 14 is ejected from the ejection nozzle 34 to the throat 32, where the ice particles 9 with a diameter of about 3 to 100 m are stored in the separator 16 and sucked through the particle inlet nozzle 35 and the suction chamber 33 by the ejector principle. This results in a mixed flow of ice particles 9 and pure water.

The mixed flow of ice particles and pure water exiting the ejector 15 is transferred to the pipe 12.
, and is fed into the recirculation system piping 4 through the valve 20, and as shown in FIG. 6, the scale 7 adhering to the wall 6 is scraped off by interaction with the particles 9.

Experiments have confirmed that it is possible to remove scale in about 20 seconds by mixing particles with a particle size of about 10107I with pure water at a mixing ratio of 100 particles/liter and flowing the mixture through a pipe at a flow rate of 4 m for 7 seconds. .

Therefore, in the embodiment, about 10 tons of pure water and about 500 Kp of ice are mixed in order to clean about 10 m of the recirculation system piping 4 having a diameter of about 50 crrL.

Example 1 of mixing ice particles and pure water in the ejector 15
In order to adjust to 5Ky/10 tons, the valve 22 is
Valve 2 is used to adjust the flow velocity in the pipe to 4 m/sec.
Use 0.

When mixed water flows through a pipe of approximately 10 m length at a flow rate of 4 m/sec, it takes 2.5 seconds for the mixed water to pass through the pipe section.

Since the recirculation system piping 4 is usually kept warm, there is about 5
Even if the mixed water is passed through at a temperature of 20°C, the temperature rise of the mixed water is negligible due to the temperature difference of about 15°C from the outside air when the room temperature is 20°C.

Moreover, the lifespan of ice with a diameter of 10 m in water at 5°C is approximately 30
Since it is a second, more than 90 percent of the ice particles mixed in the ejector 15 are washed in the recirculation system piping 4 and then returned to the separator 16.

The mixed water of ice particles and pure water after cleaning the recirculation system piping 4 flows in from the mixed liquid nozzle 43 and is separated into pure water and ice particles by the mesh 42.

The mesh 42 consists of a perforated plate through which ice having a diameter of 3 m or less passes through, so that the ice having a diameter of 3 m or less flows out of the fluid outlet nozzle 46 along with pure water.

Since the life of ice having a diameter of 371a in 5° C. water is as short as approximately 0.3 seconds, it completely disappears within the approximately 10 m pipe between the fluid outlet nozzle 46 and the filter 18.

If necessary, a heater with a small calorific value may be attached to the piping 24 on the upstream side of the filter 18.

In this case, the cooling capacity of the cooler 25 increases.

Only the water passes through the filter 18, where the radioactive scale carried with the removed tag mixed water from the pipe walls is removed.

On the other hand, ice particles with a diameter of 3w/1 or more left inside the mesh 42 of the separator 16 are produced by the ice maker 19, and are sucked by the ejector 15 together with the ice particles with a diameter of 10101L replenished from the replenishment nozzle 44 and sent to the particle outlet nozzle. 45 and is again subjected to cleaning.

After pipe cleaning, pipe 1 that was entangled with recirculation system pipe 4
Valves 20 and 21 of 2 and 13 are closed, valves 10 and 11 are opened, and the recirculation system piping 4 is connected to the reactor core 1 as before.

In this case, a small amount of ice particles used for cleaning may remain in the recirculation system piping 4, but it is possible that they will completely melt before reactor operation and be brought into the reactor core as ice. There is no gender.

In addition, after cleaning, the ice remaining on the separator is left alone and easily melts and returns to water, so no radioactive solid waste is generated, and the entire cleaning device is not contaminated by radiation at the exit of the separator 16. Excluding the influence of scale that may adhere, it is minute and the possibility of exposure during handling is extremely small.

Note that the same effect as described above can be obtained by using low melting point paraffin instead of ice as the particles used for cleaning.

In other words, the cleaning effect is the same as that of ice, plastic particles, etc., and it completely melts as the temperature of the active water increases, and the melted material is composed of carbon, hydrogen, and oxygen, which are irradiated in a nuclear reactor. It also has no toxic effect.

As usable low melting point paraffins, solids insoluble in water having a melting point of about 60 to 140°C are preferred, such as acenaphthylene (C1°H18) and acenaphthene (Cl2H1O).

In the embodiment shown in FIG. 3, the valve 11, valve 12, and piping 13 are omitted, and the mixed water of ice particles and pure water injected through the valve 21 is passed through the recirculation system piping 4.
After being used for cleaning, it is also possible to use the fuel assembly for cleaning by injecting it into the reactor core 1 from the jet pump 3.

In this case, ice injected into the reactor core is not recovered and melts in the reactor water.

According to this example, it is possible to remove scale deposits in coolant channels such as recirculation system piping by installing simple equipment such as a pump, ejector, and separator without damaging the core of the reactor due to cleaning. As a result, the surface dose of the piping can be significantly reduced, and the amount of radiation exposure for employees during periodic reactor inspections can be reduced to less than half of the current level.

In addition, by regularly performing main cleaning, it is possible to suppress the growth of radioactive scale deposited on the pipes, and it is possible to prevent the surface dose of the recirculation system pipes from increasing over time.

According to the present invention, the mixed solid particles are melted in the passage through which the reactor coolant flows, so no solid particles remain in the coolant, and it is possible to form a closed loop, which simplifies the cleaning work. This has the effect of preventing the removed scale from returning to the reactor.

[Brief explanation of the drawing]

Figure 1 is a schematic system diagram of conventional reactor recirculation system piping, Figure 2
The figures are a sectional view of the recirculation system piping shown in Fig. 1, Fig. 3 is a schematic system diagram of the recirculation system piping to which a preferred embodiment of the present invention is applied, and Fig. 5 is a schematic diagram of the ejector. 4 is a detailed sectional view of the separator shown in FIG. 3, FIG. 5 is a detailed sectional view of the ejector shown in FIG. 3, and FIG. 6 is a sectional view of the inside of the recirculation system piping shown in FIG. 3. Explanation of symbols, 4... Recirculation system piping, 12, 13
...Piping, 24,26...Piping, 27
...Piping, 15...Ejector, 16...
...Separator, 17...Pure water, 18...
... Filter, 19 ... Ice maker.

Claims (1)

[Claims] 1. Mix solid particles that melt when the temperature of the coolant increases in the reactor coolant of a nuclear power plant, and flow a mixed fluid of the coolant and the solid particles into the passage through which the coolant flows, and the solid particles removing the scale by bringing particles into contact with the scale adhering to the passage; separating the coolant containing the scale from the solid particles; removing the scale from the coolant; A method for cleaning a reactor coolant flow passage, characterized in that the coolant and solid particles are returned to the passage to form a closed loop.