JPS5835270B2 - Rust prevention method for liquid metal equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for rust-proofing liquid metal equipment, and more particularly to a method for rust-proofing such equipment during manufacture and assembly.

The main liquid metal equipment is the steam generator of the liquid metal cooled fast breeder reactor, which aims to generate high-temperature, high-pressure steam through heat exchange between high-temperature liquid metal and water. Of these, the steam generator heat transfer tube is the part where high temperature liquid metal and water directly exchange heat.

The steam generator has a structure as schematically shown in FIG. 1, and includes an upper body 1, a lower body 2, an upper head plate 3, and a lower head plate 4.
, a heat shielding structure network 6, an internal shroud 7, and heat exchanger tubes 8 and 9 are the main components.

In such a steam generator, the heat generated in the reactor vessel is transferred once to non-radioactive liquid sl-I in an intermediate heat exchanger.
After being heat exchanged with sodium (generally liquid metal), it flows into the steam generator through the liquid sodium inlet pipe 11.

This hot liquid sodium passes through the sodium inlet pipe 11, outside the internal shroud 7 and inside the heat shielding structure network 6, travels downward along the rising heat exchanger tubes 9, and exits the steam generator through the sodium outlet nozzle 14. and returned to the intermediate heat exchanger.

On the other hand, the water in the steam generator is first supplied to the water supply ring header 1.
2 into the steam generator body, and the upper and lower body 1
, 2 and the outside of the heat shield structure network 6.
The steam passes through the spiral rising heat exchanger tube 9 located inside the inner shroud 7 and outside the inner shroud 7, and is sent out as steam from the steam outlet flange 10.

This steam is directed to a turbine to rotate a generator.

Note that heat exchange between liquid sodium and water is performed in the descending heat exchanger tube 8 and the ascending heat exchanger tube 9, but in the case of the evaporator of a steam generator, the ascending heat exchanger tube has a much larger heat transfer area. Water mainly turns into steam within the rising heat exchanger tubes 9.

Normally, in a fast breeder reactor steam generator, the sodium inlet temperature is about 500°C, the sodium outlet temperature is about 320°C, the feed water temperature is about 300°C, and the steam outlet temperature is about 450°C.
It is.

In this way, the steam generator is a device in which high-temperature liquid I-IJ coexists with high-temperature, high-pressure water or steam, so it is technically the most technically difficult and important of the fast breeder reactor plant components. Moreover, heat exchanger tubes are particularly important parts of steam generators because water flows on the inside of the tubes, liquid sodium flows on the outside of the tubes, and heat exchange takes place through the tube walls.

Because the heat transfer tubes of a steam generator are such an important part, the superheater, reheater, etc. are made of stainless steel, while the evaporator is made of chromium-molybdenum steel (2
, 25wt% chromium-1wt% molybdenum steel), and it is becoming a worldwide trend to use chromium-molybdenum steel not only for evaporators but also for superheaters and reheaters.

Now, if we trace the manufacturing and assembly process of the heat exchanger tube part of the fast breeder reactor steam generator, it will be as shown in Figure 2.

That is, as shown in Figure a, first, a long heat exchanger tube 9 is obtained by welding straight tubes.

At this time, the welded portion of the heat exchanger tube is subjected to post-heat treatment, and its soundness is checked by X-ray inspection.

Next, the straight heat exchanger tube 9 is processed into a coil shape as shown in the figure, placed in the inner shroud 7 as shown in the figure C, and the heat exchanger tube 9 is fixed with the heat exchanger tube support 16.

Next, as shown in Figure d, both ends of the heat transfer tube 9 are welded to add an upper end tube 9a and a lower end tube 9b of the riser tube.

At this time, the welded portion is subjected to post-heat treatment and X-ray inspection.

Next, as shown in Figure e, the heat shielding structure side 6 is inserted into the outside of the heat exchanger tube group, the lower end tube 9b of the riser tube and the descender heat exchanger tube 8 are welded, and the upper end tube 9a of the riser tube and the steam communication pipe 17 are welded. do.

Figure 1 shows a nearly completed steam generator, but welding of the descending heat exchanger tube 8 and the water supply ring header tube 12a, and welding of the steam communication tube 17 and the steam tube sheet tube 18 was performed, and the welded parts at this time were Both are subjected to post-heat treatment and X-ray inspection.

Finally, the upper fuselage 1 and lower fuselage 2 are matched to complete the construction.

In addition to the heat transfer tube assembly process, the steam generator assembly process includes the assembly process of the Nato IJ Umlinda header in the upper section, the downcomer pipe installation process in the upper fuselage, and the lower fuselage assembly process.

In addition, there are about 1000 welded parts per unit, and all welded parts are subjected to post-heat treatment and X-ray inspection, so it takes about 2.5 years to manufacture the steam generator.

As described above, since it takes a long time to manufacture a steam generator, by the time the steam generator is completed using chrome-molbran steel, there is significant red rust on the area surrounding the road. This is the biggest problem in the manufacturing process.

In particular, large structures such as steam generators are often manufactured in factories near the coast for transportation purposes, and in such cases, the development of red rust that progresses during the approximately 2 to 5 years of manufacturing is difficult. It is very difficult to wear, and furthermore, it is not easy to remove the rust from the heat exchanger tubes housed inside the steam generator after the steam generator is completed.

When steam generator heat exchanger tubes generate significant rust, the wall thickness of the tube becomes thinner, and in some cases, the rust progresses deeply locally, making the surface of the heat exchanger tubes extremely uneven, which reduces the strength of the heat exchanger tubes. There is a risk of a large fire due to a sodium-water reaction.

Additionally, when liquid sodium is poured into a steam generator that has rusted, corrosion products in the liquid sodium, especially iron, increase.
In addition, the increased oxygen concentration in the liquid sodium may accelerate corrosion of the entire secondary system equipment materials.
The increase in corrosion products in liquid sodium may reduce the lifespan of cold trap equipment for purifying liquid sodium.

The present invention aims to provide a rust prevention method for liquid metal equipment that solves these problems, and which does not cause any trouble during operation of these equipment. At least one step in the process of processing and assembling equipment from component materials for liquid metal equipment made of steel, a process of coating the surface of chromium-molybdenum steel with a low melting point metal made of at least one type of metal, and this equipment. After the assembly is completed, the deposited low melting point metal is removed by heating at a temperature above its melting point, and the low melting point metal remaining in this step is eluted into the liquid metal. be.

The present invention was obtained as a result of studies by the inventors on rust prevention methods, and the results will be explained below.

First, as a rust prevention method, there is a method using non-metallic coating. For example, black rust (Fe 304 ) is applied to the surface of the heat exchanger tube by
There is a so-called chemical skin covering method that covers the surface of the product, but red rust (Fe203) occurs in the presence of moist air for a long period of 2.5 years, and this spreads over a wide area, so it is not a good idea. No results are obtained.

Further, methods using nonmetallic coating include paints using pigments such as metal powder, red lead, and basic lead chromate, and organic paints using polymeric materials such as rubber, plastic, and vinyl chloride.

However, paints using pigments and anticorrosive agents using organic paints are not suitable for steam generating pipes.

That is, these rust inhibitors must be removed before immersing the steam generator heat transfer tubes in liquid I-IJ, and for this purpose organic solvents such as acetone and alcohol are used. Organic solvents are flammable substances and require large amounts of water, making them extremely dangerous, and
It is difficult to completely remove these rust preventives from the surface of the heat exchanger tubes, and if these paints remain, there is a risk that they will react with liquid sodium during operation of the steam generator and cause an explosion.

Therefore, a non-metallic coating method is not suitable as a rust prevention treatment method for steam generator heat exchanger tubes, and therefore a metal coating method in which the heat exchanger tubes are coated with a metal or alloy having better corrosion resistance than the chromium-molybdenum steel is suitable.

Metal coating methods include electroplating, chemical plating, hot-dip plating, metal spraying, diffusion infiltration, and alloying methods.Among these, electroplating uses an electrolytic solution, and chemical plating uses a residual reducing agent. , which becomes a problem in liquid sodium, and the diffusion osmosis method requires high-temperature, long-time treatment, which further increases the number of days required to manufacture the steam generator and increases costs.

Furthermore, the alloy method has problems with the heat transfer characteristics of the heat transfer tube and the welding method.

From these points of view, hot-dip plating or metal spraying is an easy method for applying rust prevention to fast breeder reactor steam generator heat exchanger tubes in terms of the number of process days and processing operations.

The present invention was obtained as a result of such consideration, and low-melting point metals such as tin, bismuth, and lead are used as metal materials for hot-dip plating or metal spraying, but these only have low melting points. It is better that the viscosity is lower in the molten state.

That is, the lower the viscosity (viscosity coefficient), the smoother the removal can be performed when the low melting point metal plated on the heat transfer tube is heated to a temperature equal to or higher than its melting point.

In addition, the solubility in liquid sodium is good, which means that even after the low melting point metal plated on the surface of the heat transfer tube is removed at a temperature above its melting point, there will be a slight thickness on the surface of the heat transfer tube. This is because a low melting point metal remains, and if this residual low melting point metal has a high solubility in liquid sodium, it is easily eluted into liquid sodium.

For example, tin is one of the low melting point metals, but its melting point is 2.
The temperature is 31.9° C., and tin has excellent rust prevention in the atmosphere, and molten tin easily wets chromium-molybdenum steel and has good solderability.

The alloy layer formed in the hot-dip tin coating is mainly composed of FeSn2, but this alloy layer has a high hydrogen overvoltage and therefore does not accelerate the corrosion rate of the chromium-molybdenum steel.

Figure 3 shows the temperature dependence of tin dissolved in liquid sodium, with temperature on the horizontal axis and solubility on the vertical axis.

As is clear from the figure, for example, at 240°C, about 2% of tin dissolves in liquid sodium, and at 300°C, about 7 to 8% of tin dissolves.

Therefore, it is a metal that is extremely easily soluble in liquid sodium.

In this way, when tin is used, the steam generator heat exchanger tubes can be easily hot-dipped, and the tin plated on the heat exchanger tubes can be easily removed after the steam generator is assembled. , the tin remaining in the heat transfer tubes can be easily eluted into liquid sodium during the steam generator test run.

Therefore, the integrity of the heat exchanger tubes can be guaranteed without rusting during the 2.5 years of manufacturing the steam generator.

EMBODIMENT OF THE INVENTION Hereinafter, as an example of the present invention, a rust prevention method for heat exchanger tubes of a fast breeder reactor steam generator will be described.

Figure 2 is an assembly process diagram that mainly consists of heat exchanger tubes until the completion of a purified air generator, as usual. When the heat exchanger tubes 9 are received, or before they are welded and joined to lengthen them, hot-dip tin plating is applied, or hot-dip plating is applied in the coiled state shown in the figure.

When performing hot-dip plating, the outer surface of the heat transfer tube 9 is only plated by blindly plugging both ends of the tube in advance to prevent the plating from entering the inside of the tube.

There is no limit to the thickness of the hot-dip plating layer at this time, and it may be any thickness that can prevent the heat exchanger tubes from rusting during the manufacturing period of the steam generator.

Thus, the hot-dip tin-plated ascending heat exchanger tube 9 was fixed as shown in Figure C, and then as shown in Figures d and e,
Welding of the heat exchanger tube 9 and the upper end tube of the riser tube 9a, welding of the heat exchanger tube 9 and the lower end tube of the riser tube 9b, and the upper end tube of the riser tube 9a and the steam communication tube 17
After the welding and welding of the descending heat exchanger tube 8 and the rising tube lower end tube 9b, molten tin is sprayed onto the surface thereof.

There is no particular limit to the thickness of the tin when thermally spraying the welded portion, as long as rust does not occur during the manufacturing period of the steam generator.

In addition, the welding of the steam communication pipe 17 and the steam tube sheet pipe 18 and the welding joint of the descending heat exchanger pipe 8 and the water supply ring header pipe 12a shown in FIG.

In this way, during the assembly of the steam generator, the heat exchanger tube group is protected from the atmosphere by the tin plating and does not rust.

The entire steam generator is completed and the main body is transported to the site for installation, but before that, the tin plating on the heat exchanger tubes needs to be removed.

In this operation, saturated steam heated to 240 DEG C. or higher is flowed inside the steam generator heat exchanger tube, thereby melting the tin deposited on the outside and discharging it from the Na1-IJ tin nozzle 14.

Saturated vapor pressure is 40.5 kg/i at 250℃, 310℃
However, since the fast reactor steam generator heat tube can sufficiently withstand this level of internal pressure, there is no problem in terms of strength.

Moreover, the advantage of using saturated steam is that there is little temperature fluctuation and the heating temperature can be easily controlled.

In addition, in the case of a steam generator, tin can be easily removed since the steam generator originally has a structure in which liquid sodium circulates.

After the tin plating is removed in this way, tin with a thickness of about several microns remains in the heat exchanger tube.

The amount of remaining tin is approximately 10 kg per steam generator.

The steam generator, which has a layer of tin several microns thick on the surface of its heat transfer tubes, is transported to the site and installed at the plant.

Then, during a test run of the steam generator, tin remaining on the surface of the heat transfer tube is eluted into liquid sodium.

In addition, to remove tin from the heat transfer tubes in the cover gas layer of the steam generator, the liquid sodium is temporarily raised above the normal liquid level and eluted.

By dissolving tin into the liquid sodium, the liquid sodium contains impurity tin.

However, the amount of sodium in the secondary cooling system used for one steam generator is approximately 300 tons.

Therefore, if 10 kg of tin is dissolved in 300 tons of sodium, the concentration of tin in liquid sodium will be approximately 3Qpp.
It is about m.

In general, the impurity control value for sodium in the secondary cooling system is, for example, 3Qpp for nonmetals such as oxygen, carbon, and chlorine.
m or less.

On the other hand, metal impurities in liquid sodium do not have as bad an effect on secondary cooling system equipment materials as non-metals.

Therefore, even if about 30p11111 of tin is dissolved in the liquid sl-IJ, it does not cause any problems in terms of heat transfer characteristics and material strength.

In the above embodiments, tin is used as the low melting point metal, but other low melting point metals or low melting point alloys can be used. It can also be applied when chromium-molybdenum steel is used in containers, superheaters, etc.

The inside of the steam generator heat transfer tube can be prevented from forming rust like the outside surface of the tube by putting a blind stopper in the middle of the manufacturing process, and during operation of the steam generator, high-temperature, high-pressure water is used instead of liquid sodium. Or because water vapor flows,
Some rust can be washed away by steam or the like, so there is no need to hot-dip the inside of the heat transfer tube to prevent rust.

In this way, the steam generator heat exchanger tube manufactured according to this example can prevent the occurrence of cauldron during the long manufacturing period of 2.5 years, so there is no need for the difficult work of removing rust before assembly. There is no reduction in the strength of the heat exchanger tube due to this generation, and furthermore, tin used for rust prevention can be easily deposited and removed, and tin remaining on the surface does not have an adverse effect on the liquid sodium.

Furthermore, the amount of iron corrosion generated in liquid sodium can be reduced, and the oxygen concentration in liquid sodium is reduced, which has the effect of suppressing corrosion of the entire secondary system material.As a result, the cold trap, which is a liquid sodium purifier, Longer life is also possible.

As described above, the rust prevention method for liquid metal equipment of the present invention enables rust prevention during the assembly and production of these equipment over a long period of time, and also enables a rust prevention method that does not cause any trouble during operation of the equipment. As a result, this type of device has excellent effects in terms of manufacturing and soundness, and is an industrial advantage.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view showing the schematic structure of the fast breeder reactor steam generator, Figure 2 is a process diagram showing the assembly status of the steam generator heat exchanger tubes, and Figure 3 is the temperature dependence of tin dissolved in sodium. FIG. Explanation of symbols, 6...Heat shielding structure body, 7...
...Inner shroud, 8...Downward heat transfer tube, 9
... Rising heat transfer tube, 10 ... Steam outlet flange, 11 ... Sodium inlet pipe, 12 ...
...Water supply ring header 13...Discharge system nozzle, 17...Steam communication pipe, 18...
Steam tube sheet pipes.

Claims (1)

[Scope of Claims] 1. In at least one step of processing and assembling liquid metal equipment from component materials for liquid metal equipment made of chromium-molybdenum steel, at least one metal element is added to the surface of the chromium-molybdenum steel. a step of depositing a low melting point metal, a step of heating and removing the low melting point metal deposited in the step after completing the assembly of the device at a temperature equal to or higher than the melting point of the metal, and a step of removing the low melting point metal remaining in the step. A method for removing rust from equipment for liquid metals, comprising the step of eluting the metal into the liquid metal. 2. The method for rustproofing liquid metal equipment according to claim 1, wherein the step of depositing the low melting point metal is carried out by hot-dip plating and thermal spraying of the low melting point metal. 3. The liquid metal according to claim 1 or 2, wherein the step of removing the low melting point metal is carried out with saturated steam having a melting point higher than the melting point of the low melting point metal from inside the equipment to which the low melting point metal is attached. Rust prevention method for equipment. 4. The rust prevention method for liquid metal equipment according to claim 1, 2, or 3, wherein the low melting point metal is a metal with a low viscosity coefficient and high solubility in the liquid metal. 5. The rust-proofing method for liquid metal equipment according to claim 4, wherein the metal is tin.