JPH0644069B2 - Method for forming oxide film on wetted metal material of nuclear power plant - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an oxide film on the surface of a water contact metal material such as a heater tube of a feed water heater used in a nuclear power plant such as a boiling water nuclear power plant. It relates to a method of forming.

[Background of the Invention]

Piping used in the primary cooling water system of a nuclear plant,
Pumps, valves, etc. are made of stainless steel, stellite, etc. When these metal materials are used for a long period of time, they are corroded and damaged, and some of them are eluted into the primary cooling water and brought into the reactor. Most of these eluted metals become oxides and adhere to the fuel rods, and upon irradiation with neutrons, they become radionuclides such as ⁶⁰ Co, ⁵⁸ Co, ⁵¹ Cr and ⁵⁴ Mn. The radionuclide attached to the fuel rod and activated is re-eluted in the primary cooling water and floats as an ion or an insoluble solid component (hereinafter referred to as a cladding). Some of the floating radionuclides are removed in the demineralizer, etc. of the reactor water purification system, but some of the radionuclides that have not been removed are contacted with water that mainly consists of stainless steel while partly circulating in the cooling water system. Adheres to the surface of metallic materials. For this reason, the dose rate on the surface of the structural material becomes high, which causes a problem of radiation exposure of the worker during maintenance and inspection.

Therefore, there is a method of suppressing the attachment of radionuclides by forming an oxide film on the surface of the water contact metal material.
-12390 publication. However, this method is extremely inefficient because it takes a long time to form an oxide film.

[Object of the Invention]

An object of the present invention is to provide a method for forming an oxide film having excellent corrosion resistance on a wetted metal material in a nuclear power plant at a relatively low temperature for a short time.

[Outline of Invention]

As a result of detailed investigation of the influence of anode (anode) and cathode (cathode) polarization on the corrosion behavior of stainless steel in high temperature water, the inventors have conventionally thought that it suppresses the formation of oxide film, Under cathodic polarization, which is applied to prevent the oxidation of metals, an oxide film is formed in high-temperature water at around 200 ° C for a relatively short time, and is naturally dipped, or is more corroded than the oxide film formed under anodic polarization. As a result, they found that the effect of suppressing

A feature of the present invention is that a water circulation path is formed in at least a portion of a wetted metal material of a nuclear power plant where an oxide film is to be formed, the portion is cathodic polarized, and a temperature of 150 ° C.
As described above, the water having a dissolved oxygen concentration of 10 ppb to 500 ppb is circulated in the circulation path to form an oxide film on the portion.

Example of Invention

 An embodiment of the present invention will be described below with reference to FIG.

Embodiment 1 FIG. 1 shows the basic structure and flow of a surface treatment apparatus to which the present invention is applied.

In FIG. 1, 1 is an autoclave, 2 is a cooler, and 3
Is a pressure maintaining valve, 4 is a dissolved oxygen concentration (DO) adjusting tank, 5 is a gas cylinder, 6 is a booster pump, 7 is a heater, 8 is a stainless steel plate, 9 is a perforated stainless steel plate, 10 is a galvanostat (constant current power supply). Equipment), 11 is a switch, and 12 is a corrosion monitor.

1 to 7 constitute a high temperature water circulation loop, and high temperature water having a constant temperature in which the dissolved oxygen concentration is adjusted is continuously supplied to the autoclave.

The two stainless steel plates 8 in the autoclave are subjected to surface treatment for corrosion control, and the procedure is as follows.

First, the switch 11 is closed, and a galvanostat (constant current electrolysis device) 10 is used to carry out constant current electrolysis so that the stainless steel plate 8 is cathode-polarized so as to have a lower potential than the intermediate perforated stainless steel plate 9. . The surface treatment of the stainless steel plate 8 is performed by continuing constant current electrolysis in high temperature pure water having a predetermined temperature and a dissolved oxygen concentration for a predetermined time. After the surface treatment was completed, the switch 11 was closed to stop the constant current electrolysis, and the corrosion rate of the surface-treated stainless steel plate 8 was measured by the corrosion monitor. The corrosion rate was measured online based on the frequency response of the AC impedance between the two stainless steel plates facing each other.

FIG. 2 compares the corrosion rate after constant-current electrolysis for 24 hours at a water temperature of 260 ° C. and DO of 200 ppb using the cathodic polarization current value as a parameter.

As is clear from this figure, when compared with the case of naturally dipping in high temperature water without cathodic polarization, dipping in high temperature water under cathodic polarization increases the corrosion inhibition effect of the formed film, and the cathode polarization current density is 10 μA. It was found that the corrosion inhibiting effect was large when the amount was less than 1 cm ² / cm ² , and the inhibiting effect was decreased when the current was increased above 10 μA / cm ² .

FIG. 3 shows DO200 in water temperature 260 ° C. in order to investigate the remarkable corrosion inhibition effect under such cathodic polarization.
24 hours at ppb, cathode polarization current density of 10 μA / cm ²
r Secondary ionic current value (vertical axis) generated when the stainless steel surface is sputtered with Ar ions as analyzed by the ion microanalyzer (IMA) in the depth direction elemental denominator on the surface of the stainless steel after surface treatment Is a function of the number of spatterings (horizontal axis).

Generally, when a metal is converted into an oxide and ionized, the secondary ion generation efficiency is increased. Therefore, the region where the total secondary ion current in the surface layer is high corresponds to the oxide film layer formation region. Corresponding to this, the secondary ion current due to oxygen atoms also increases in the oxide film layer portion.

Further, it was found that the change of the total secondary ion current continuously decreased toward the inside of the base material, and the oxide film grew to a deep portion of the base material.

Further, looking at the change in Cr concentration in the oxide layer, there is a region where the Cr concentration (content) is higher than the concentration in the base metal from the center of the film to the base metal side, and Cr improves the corrosion resistance of the oxide film. Therefore, it is considered that such an increase in Cr concentration is also a cause of a large corrosion inhibiting effect.

FIG. 4 shows the effects of water temperature and immersion time under cathodic polarization on the corrosion inhibition effect. The corrosion inhibition effect was defined as the rate of reduction from the corrosion rate during natural immersion.

As a result, when the immersion time is 200 hours, the water amount is 200 ° C.
It has been found that an oxide film having a sufficient corrosion inhibiting effect is formed even in the low temperature region before and after.

FIG. 5 shows the relationship between the corrosion inhibiting effect at a water temperature of 200 ° C. and a dipping time of 200 hours under cathodic polarization and the dissolved oxygen concentration.

The corrosion inhibition effect increases with dissolved oxygen concentration, but it is 1pp
On the contrary, under a high dissolved oxygen concentration of m or more, the corrosion inhibiting effect is lowered.

FIG. 6 shows the effect of applying the surface oxidation treatment according to the present invention to a stainless steel heater tube used in a feed water heater of a BWR plant, and compares and evaluates the effect with the conventional method based on the effect of basic experiments.

In the absence of pre-oxidation treatment, stainless steel corrosion products (Fe
The concentration of oxides, Ni, Cr, Co ions, etc. increases at the beginning of plant operation, but the concentration gradually decreases as the oxide film grows. The formed film becomes thick and the film growth becomes constant after about 4000 hr when the film growth becomes sufficiently small. On the other hand, when an oxide film is formed under anodic polarization, the corrosion inhibition effect is exhibited when the anode current density is 0.1 μA / cm ² or more, but the corrosion product concentration reduction rate is 20%.
Small and small. Further, since the required anode current density is large, it is not practical to apply it to a feed water heater having a large surface area (about 100 m ² / column). On the other hand, when an oxide film is formed in high temperature water containing sufficient dissolved oxygen, an oxide film having the same corrosion inhibition effect as the present invention is formed at a water temperature of 285 ° C., which is the BWR reactor water temperature condition. However, since the design temperature of the feed water heater is 230 ° C., the oxidation treatment at such a high temperature cannot be applied.

In contrast to these conventional methods, the present invention forms an oxide film having a sufficient corrosion inhibiting effect at a water temperature of 200 ° C. lower than the design temperature of the feed water heater. Further, since the required cathode current density is as low as 1 μA / cm ² or less, in addition to cathode polarization by an external power source, cathode polarization using a sacrificial electrode, which will be described later, is possible, which is very practical.

Embodiment 2 FIG. 7 shows an embodiment in which a sacrificial electrode is used for cathodic polarization instead of cathodic polarization by an external power source (galvanostat, potentiostat).

Here, 14 is a sacrificial electrode.

That is, a metal piece having a greater ionization tendency than stainless steel is attached to the end of the stainless steel plate 8 as the sacrificial electrode 14. From the sacrificial electrode 14, as shown by the following equation, the metal is dissolved as an ion on the water side, and the electron is accumulated on the electrode side.

M → M ^{n +} + ne ⁻ M: Sacrificial electrode metal These extra electrons flow into the surface of the stainless steel plate 8 to realize the same cathode polarization state as when an external power source is used.

Therefore, the current flowing under such cathodic polarization becomes equal to the corrosion current of the sacrificial electrode.

As a metal with a greater ionization tendency than stainless steel,
Fe, Zn, Al, etc. are available. The corrosion rate of these metals in high temperature pure water is 20 to 100 mg / dm ² / day.
It is 10 to 100 times the value of stainless steel (1 to ² mg / dm ² / day).

For example, when the corrosion rate is 30 mg / dm ² / day and the metal ions are eluted as divalent ions, the corrosion current density at that time is about 10 μA / cm ² . Therefore, Fe having a surface area of about 1/10 of the total surface area of stainless steel,
By attaching Zn and Al metals as sacrificial electrodes, the surface of stainless steel can be cathodically polarized at a constant current density of ¹ μA / cm ² .

FIG. 8 shows that a sacrificial electrode piece of 1/10 of the total surface area of stainless steel is screwed to a stainless steel plate and the water temperature is 260 ° C.
The corrosion rate after immersion for 24 hours with DO of 200 ppb is compared for each electrode material.

With Fe, Al and Zn metals, almost the same corrosion inhibition effect as when electrolyzing with a constant current of cathode current density of 1 μA / cm ^{2 by} an external power source was obtained.

The decrease in the corrosion inhibition effect in the case of Zn metal is considered to be because the corrosion rate of Zn is about one digit higher than that of Fe and Al, so that the cathode current density becomes too high.

Embodiment 3 FIG. 9 shows an embodiment for a pipe-shaped metal. Here, 15 is a metal pipe to be surface-treated.

By forming a metal having a greater ionization tendency than the pipe metal as the sacrificial electrode 14 on the outer surface of the pipe and immersing it in high temperature water, an oxide film having a large corrosion inhibiting effect is formed not only on the outer surface of the pipe but also on the inner surface. Is possible.

Embodiment 4 FIG. 10 shows a concrete embodiment in which the present invention is applied to a feedwater heater of a nuclear reactor plant.

FIG. 10 shows the flow of the primary system of the BWR plant. 16 is a nuclear reactor, 17 is a high pressure turbine, 18 is a low pressure turbine, 19 is a condenser, 20 is a condensate pump, 21 is a demineralizer, 22 is a low-pressure feed water heater, 23 is a feed pump, 24
Is a high-pressure feed water heater, 25 is an additional bypass line unique to the present invention, 26 to 29 are valves, 30 to 31 are valves unique to the present invention, and 32 is a feed water recirculation line.

The feature of this embodiment is that a high-pressure feed water heater having a feed water temperature of 150 ° C. or higher and a main source of corrosion products from a stainless steel heater tube is subjected to surface oxidation treatment according to the present invention, and a plurality of feed water heaters are used. By considering the carbon steel pipe connecting the two as a sacrificial electrode, an oxide film is formed on the inner surface of the stainless steel heater tube in the high-pressure feed water heater under cathodic polarization.

A closed loop formed by installing a bypass line 25 connecting the upstream side of the water supply pump 23 and the downstream side of the valve 28 in the water supply recirculation line 32 in order to obtain high temperature water of around 200 ° C. necessary for forming an oxide film Heat to the required temperature by the heat generated from the water supply pump.

Carbon steel is used as the material of the bypass line 25, and is used as a sacrificial electrode like the carbon steel pipe of the water supply system. In this case, the total surface area of the carbon steel pipe is about 500 m ² . On the other hand, the total inner surface area of the heater tube of the high pressure feed water heater is about 10,000 m ² . The temperature of the water supply is 150 ℃ ~ 200
The corrosion rate of carbon steel is 30 to 40 mg / dm ² / day when the temperature and DO are as low as 20 ppb. Therefore, the cathode current density on the surface of the heater tube is about 0.5μ.
It becomes A / cm ² , and an oxide film can be formed to the same extent as when cathodic polarized at a current density of 1 μA / cm ² .

Example 5 FIG. 11 shows a specific example in which a primary system carbon steel pipe was not used as a sacrificial electrode and a sacrificial electrode was attached to the barrel side of a feed water heater. 3 in FIG.
3 is a feed water heater, 34 is a heater tube, 35 is an end plate, 36 is a tube plate, and 14 is a sacrificial electrode. Since the total surface area of the heater tube of the feed water heater is approximately 2000 m ² / tower, for example, the width 1 made of carbon steel wire with a diameter of 1 mmφ
By attaching a mesh filter with a thickness of 0 cm and a thickness of 3 cm to the upper tube plate of the water heater (diameter of about 2.2 m) and the lower tube plate of 3 pieces, the surface area of the sacrificial electrode becomes about 200 m ² , and the inner surface of the heater tube is 1 μA / A cathode current can be continuously supplied at a current density of cm ² . Further, the sacrificial electrode of this example can be used together as an auxiliary sacrificial electrode for compensating for the small surface area of the carbon steel pipe used as the sacrificial electrode in Example 4.

Sixth Embodiment Hereinafter, an embodiment at the manufacturing stage of the present invention will be described with reference to FIG. The system configuration of the treatment device includes a boiler 37 that produces high-temperature water, a circulation pump 38 that circulates the high-temperature water, and a purification device 39 that guides and purifies a part of the circulation water. The operation method is as shown in FIG.
One or more are installed and connected to the circulation line of treated water. Then, the treated water is filled from the treated water tank 40 into the treated water circulation system and the heat exchanger 41 to be treated through the purifying device 39.

When the filling of the treated water is completed, the boiler 37 is used to heat and set the target temperature.

Further, DO during the processing period is adjusted by injecting each gas from the nitrogen gas tank 42 and the oxygen gas tank 43. High DO (concentration in injected water 8ppm) to low D
Degassing to O (concentration in treated water, 200 ppb or less) is performed by evaporating the treated water through the degassing line 44 and enclosing nitrogen gas. On the other hand, DO is increased by injecting oxygen gas. The water quality during the treatment period is monitored by the sampling lines 45 and 46. Purification of the treated water is carried out by passing the treated water to 60 ° C. or lower in the cooler 47 and then passing the purifying device 39 filled with the organic ion exchange resin.

After the oxidation treatment is completed, a recovered water line 48 and a recovered water pump 49 branching from the return line of the treated water,
It collects in the treated water tank 40 via the purifier cooler 47.

Oxidation treatment at the manufacturing stage using the present invention can be realized by the treatment method described above.

〔The invention's effect〕

According to the present invention, an oxide film having a high corrosion inhibiting effect, which was conventionally formed in high temperature water of 200 ° C. for about 2000 hours, can be accelerated in about 200 hours at the same temperature. As a result, the generation of corrosion products in the water supply system at the beginning of nuclear plant operation is suppressed, and the pipe surface dose rate in the plant due to the activation of corrosion products brought into the reactor is further reduced. It

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a basic device configuration for carrying out the present invention. FIG. 2 is a curve showing the correlation between the corrosion rate and the cathode polarization current density. FIG. 3 is a curve showing the formed film thickness and the element distribution in the film. FIG. 4 is a curve showing the relationship between the corrosion rate reduction rate and the water temperature, using the immersion time as a parameter. Fifth
The figure is a curve showing the relationship between the corrosion rate reduction rate and the dissolved oxygen concentration.
FIG. 6 is a curve showing that the treatment method of the present invention is the best in terms of treatment conditions and effects by comparing the temporal changes in the concentration of SUS corrosion products in the feed water for each pretreatment method.
FIG. 7 is a diagram showing a modified example of the present invention. FIG. 8 is a diagram quantitatively showing the effect of implementing the modified example of the present invention. FIG. 9 is a diagram showing another modification of the present invention. FIGS. 10 to 12 are views showing a concrete apparatus configuration when the present invention is applied to a nuclear power plant. FIG. 10 is a carbon steel pipe, and FIG. 11 is a carbon steel filter. FIG. 12 shows an apparatus configuration in the case where the pre-oxidation treatment is performed during the operation of the nuclear reactor, and FIG. 1 ... autoclave, 2 ... cooler, 3 ... pressure-holding valve, 4 ... D
O adjustment tank, 5 ... gas cylinder, 6 ... booster pump, 7 ...
Heater, 8 ... Stainless steel plate, 9 ... Perforated stainless steel plate, 10 ... Galvanostat, 11 ... Switch, 12 ...
Corrosion monitor, 13 ... Sacrificial electrode piece, 14 ... Sacrificial electrode, 1
5 ... metal pipe, 16 ... reactor, 17 ... high pressure turbine,
18 ... Low-pressure turbine, 19 ... Condenser, 20 ... Condensate pump, 21 ... Demineralizer, 22 ... Low-pressure water heater, 23 ... Water pump, 24 ... High-pressure water heater, 25 ... Additional bypass line, 26-29 ... valve, 30-31 ... valve, 32 ... water supply recirculation line, 33 ... water supply heater, 34 ... heater tube, 35 ... end plate, 36 ... tube plate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masayoshi Kondo 1168 Moriyama-cho, Hitachi, Hitachi, Ibaraki Prefecture Energy Research Laboratory, Hiritsu Manufacturing Co., Ltd. (72) Inventor Motohiro Aizawa 3-2-1, Saiwai-cho, Hitachi, Hitachi Inside Engineering Co., Ltd.

Claims (5)

[Claims] 1. A water circulation path is formed in at least a portion where an oxide film is to be formed in a wetted metal material of a nuclear power plant,
The part is subjected to cathodic polarization, the temperature is 150 ° C. or higher, the dissolved oxygen concentration is 10 ppb to 500 p
A method for forming an oxide film on a wetted metal material of a nuclear power plant, wherein an oxide film is formed on the portion by circulating water of pb through the circulation path. 2. Oxidation of the water contact metal material of the nuclear power plant to the water contact metal material according to claim 1, wherein the water contact metal material is a stainless steel heater tube for heating the primary water supply of the nuclear power plant. Method of forming a film. 3. The method of forming an oxide film on a wetted metal material of a nuclear power plant according to claim 1 or 2, wherein the cathodic polarization is performed in water having a pH of 7 or less. 4. The cathode polarization is performed by using a metal having a higher ionization tendency in water at 150 ° C. or higher than stainless steel as a sacrificial electrode and electrically connecting the sacrificial metal material to the sacrificial metal material. The method for forming an oxide film on a wetted metal material of a nuclear power plant according to claim 1 or 2. 5. A carbon steel bypass line is provided from the downstream side to the upstream side of a high-pressure feed water heater of a nuclear power plant to form a closed loop including the bypass line and the feed water heater, and the temperature is 150 ° C. or higher and the dissolved oxygen is dissolved. Concentration 10ppb-500p
Pb water is circulated in the closed loop, the bypass line is used as a sacrificial electrode, and the heater tube of the feed water heater is cathodically polarized to form an oxide film. Method of forming oxide film of.