CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2010-206960 filed on Sep. 15, 2010, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a method for monitoring corrosion protection in a power plant.

BACKGROUND

Corrosion is aging degradation that a metal structural material of an apparatus dissolves as ions into an environment, and important element that decides the apparatus lifetime. When corrosion proceeds, unexpected damage might be caused by the breakage of the apparatus, and corrosion protection measures are required from the viewpoint of safety. Therefore, the corrosion protection measures are taken for a variety of apparatuses used in an outdoor environment and a high temperature environment, and particularly for power plants.

The surface of a metal structural material of an apparatus to be used under a state of high temperature is generally covered with iron oxide such as Fe₂O₃. It is known that Fe₂O₃ has low solubility in water and anticorrosive effect when produced densely.

However, since the Fe₂O₃ film dissolves to disappear or its crystal system changes depending on the environment it is used, it might not be able to exhibit its original anticorrosion property. In view of such a problem, there is disclosed a method of the protection of the dissolution of the Fe₂O₃ film by using at least one of morpholine, alkanolamine and aliphatic cyclic amine as a pH adjuster to adjust the pH of water to which the Fe₂O₃ film is contacted. However, when the pH adjuster is used, its control is difficult, and there was a problem that the monitor of the corrosion was difficult.

There is also disclosed a method of protecting the corrosion of a boiler plant by controlling an amount of oxygen dissolved in the boiler, but there is no disclosure about the monitor of the corrosion protection of the Fe₂O₃ film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of the pressurized water reactor according to a first embodiment.

FIG. 2 is a graph showing an operation temperature and an oxygen introduction amount that can stabilize an Fe₂O₃ film.

FIG. 3 is a graph showing an operation temperature and an oxygen introduction amount that can stabilize an FeTiO₃ film.

FIG. 4 is a system diagram of a boiling-water reactor according to a second embodiment.

DETAILED DESCRIPTION

According to one embodiment, there is provided a method for monitoring corrosion protection in a power plant, including: forming a film with a composition of M1FeO₃ (M1: trivalent or tetravalent metal) or M2Fe₂O₄ (M2: divalent metal) on an inner wall surface of each of apparatuses of the power plant; and maintaining the film stably by controlling an amount of introduction of oxygen into each of the apparatuses commensurate with a temperature in each of the apparatuses.

First Embodiment

FIG. 1 is a system diagram of a pressurized water reactor (hereinafter abbreviated as “PWR”) according to this embodiment. In a PWR 10 of the embodiment shown in FIG. 1, a high-pressure turbine 12, a moisture separator/reheater 13, a low-pressure turbine 14 and a condenser 15 are sequentially connected to a steam generator (boiler) 11. Moreover, a low pressure feedwater heater 16 and a high pressure feedwater heater 17 are connected to the condenser 15, and the high-pressure feedwater heater 17 is connected to the steam generator 11. Furthermore, a degasifier 18 is disposed between the low pressure feedwater heater 16 and the high pressure feedwater heater 17.

For example, the PWR 10 can be operated as follows. Namely, the high-pressure turbine 12 is driven by steam generated by the steam generator 11. Then, the steam having driven the high-pressure turbine 12 is cooled down and is partly liquefied into water. Therefore, the steam is reheated by the moisture separator/reheater 13, and the liquefied water is vaporized and introduced into the low-pressure turbine 14 to drive the turbine 14. The steam which has driven the low-pressure turbine 14 is cooled by the condenser 15 and thus converted into water. The water is then heated through the low pressure feedwater heater 16 and the high pressure feedwater heater 17, introduced into the steam generator 11 and heated to be converted into steam again. The generated steam is introduced again into the high-pressure turbine 12 and the low-pressure turbine 14 to drive the turbines 12 and 14.

A method for monitoring corrosion protection in the PWR 10 shown in FIG. 1 is described below. FIG. 2 is a graph showing an oxygen concentration range in which an Fe₂O₃ film can be maintained stably within a range of 25° C. to 225° C., which is a typical operation temperature of the PWR 10. Moreover, FIG. 3 is a graph showing an oxygen concentration range in which an FeTiO₃ film can be maintained stably within a range of 25° C. to 225° C., which is a typical operation temperature of the PWR 10. Namely, as shown in FIG. 2 and FIG. 3, the Fe₂O₃ film and the FeTiO₃ film depend on the operation temperature and the amount of introduction of oxygen and can be maintained stably without being dissolved and corroded when the films are in the range surrounded by the curves and straight lines.

The Fe₂O₃ film and the FeTiO₃ film are conventionally known that their solubility in water is low and known as films which provide anticorrosive effect when formed densely.

The graphs shown in FIG. 2 and FIG. 3, that is, regions which depend on the operation temperature and the amount of introduction of oxygen under which the Fe₂O₃ film and the FeTiO₃ film can be maintained stably, are obtained by extensive research and devoted study performed by the present inventors.

Thus, it is seen that the Fe₂O₃ film and the FeTiO₃ film to be formed on the apparatuses of the PWR 10 shown in FIG. 1, that is, the power plant, can be maintained stably by controlling the amount of introduction of oxygen within the film stable region as shown in FIGS. 2 and 3 commensurate with the operation temperature. In other words, it is seen that the Fe₂O₃ film and the FeTiO₃ film used for the power plant can be maintained stably by a quite simple and easy method that controls the amount of introduction of oxygen within the film stable region commensurate with the operation temperature without using a chemical agent such as a pH adjuster which adversely affects the apparatuses. Therefore, it becomes possible to maintain the anticorrosion property of the PWR 10 under high reliability.

In this embodiment, in the application of corrosion protection to the PWR 10, the Fe₂O₃ film or the FeTiO₃ film is formed as the anticorrosive film on the inner wall surface of each of the apparatuses composing the PWR 10, that is, the inner wall surfaces of the steam generator 11 through the high pressure feedwater heater 17 and the degasifier 18.

Since each of the apparatuses contains iron, the Fe₂O₃ film can be formed by, for example, immersing each of the apparatuses in water that is held at pH 9.3 or more under atmosphere at room temperature, or by introducing oxygen into each of the apparatuses. Moreover, the Fe₂O₃ film can be formed naturally by arranging each of the apparatuses in such a condition as the Fe₂O₃ film can be maintained stably, the condition being set by adjusting the operation temperature as shown in FIG. 2, that is, the temperature of water or steam and adjusting the amount of introduction of oxygen.

The FeTiO₃ film is formed by directly coating the FeTiO₃ film or by coating the TiO₂ film or the like because each of the apparatuses composing the PWR 10 contains iron (Fe). In the latter case, iron ion or iron oxide contained in the structural material making each of the apparatuses is reacted with the TiO₂ film or the like and converted into the FeTiO₃ film.

Referring to FIG. 2 and FIG. 3, the operation temperature of the PWR 10, that is, the operation temperature of each of the apparatuses and the amount of introduction of oxygen in each of the apparatuses, are controlled so that the Fe₂O₃ film or the FeTiO₃ film can be maintained stably. In the PWR 10 shown in FIG. 1, the condenser 15 has the lowest operation temperature within a range of 25° C. to 50° C., and the steam generator 11 and the high pressure feedwater heater 17 have the highest operation temperature within a range of 200° C. to 225° C.

Therefore, when the Fe₂O₃ film is formed on the inner wall surface of the condenser 15, the amount of introduction of oxygen in the condenser 15 is controlled within a range of about 1.0×10⁻⁵ ppm to 1.0×10² ppm with reference to FIG. 2. Meanwhile, when the Fe₂O₃ film is formed on the inner wall surface of the steam generator 11, the amount of introduction of oxygen into the steam generator 11 is controlled within a range of about 1.0×10⁻¹ ppm to 1.0×10⁵ ppm.

Meanwhile, when the FeTiO₃ film is formed on the inner wall surface of the condenser 15, the amount of introduction of oxygen in the condenser 15 is controlled within a range of about 1.0×10⁻⁸ ppm to 0.5×10⁻⁴ ppm with reference to FIG. 3. Meanwhile, when the FeTiO₃ film is formed on the inner wall surface of the steam generator 11, the amount of introduction of oxygen into the steam generator 11 is controlled within a range of about 1.0×10⁻⁶ ppm to 1.0 ppm.

As is apparent from the aforementioned description, it is required that the amount of introduction of oxygen is set larger as the operation temperature is set higher as in the steam generator 11 and the like. Therefore, when the amount of oxygen in the system of the PWR 10 is small, oxygen is supplied in order to set the amount of introduction of oxygen within the aforementioned range, e.g., at the position indicated by an arrow 19a in the drawing.

On the other hand, the amount of introduction of oxygen in the condenser 15 and the like must be decreased as the operation temperature is set lower therein. Therefore, if a large amount of oxygen is contained in the system of the PWR 10, for example, a reducing agent is supplied at the location indicated by an arrow 19b in the drawing to reduce the amount of oxygen contained in the system, thereby setting the amount of introduction of oxygen in the above-described range.

As is apparent from FIG. 2 and FIG. 3, the margin of the Fe₂O₃ film relating to the amount of introduction of oxygen is set larger than the margin of the FeTiO₃ film relating to the amount of introduction of oxygen. For example, therefore, when the Fe₂O₃ film is formed for all of the apparatuses of the PWR 10 under the condition that the amount of introduction of oxygen into the PWR 10 is 1.0 ppm, the Fe₂O₃ film is maintained stably at the operation temperature of each of the apparatuses. Therefore, the anticorrosion property of the PWR 10 can be held under high reliability.

For example, when the amount of introduction of oxygen is set to 5×10⁻³ ppm for the FeTiO₃ film, the FeTiO₃ film can be maintained stably in a relatively large operation temperature range of 100° C. to 150° C. Therefore, if there are a plurality of apparatuses having respective operation temperatures within a range of 100° C. to 150° C., the FeTiO₃ film can be maintained stably by setting the amount of introduction of oxygen to 5×10⁻³ ppm for the apparatuses.

In the PWR 10 shown in FIG. 1, the high-pressure turbine 12, the moisture separator/reheater 13, and the low pressure feedwater heater 16 are operated in the above temperature range, that is, in the temperature range of 100° C. to 150° C.

It is not required that in the PWR 10 the Fe₂O₃ film or the FeTiO₃ film is formed on the inner wall surfaces of all of the apparatuses uniformly, but the Fe₂O₃ film may be used for some of the apparatuses and the FeTiO₃ film for the other apparatus.

In this embodiment, although the Fe₂O₃ film or the FeTiO₃ film was used as a film to be formed on the inner wall surfaces of the apparatuses of the PWR 10, the present embodiment is not limited to the above description and can be also applied to a film with a composition of a general formula M1FeO₃ (M1: trivalent or tetravalent metal) or M2Fe₂O₄ (M2: divalent metal). As the metal M1 in this case, Y and La can be exemplified as the trivalent metal, and Ti, Zr and Hf can be exemplified as the tetravalent metal. As the metal M2, Ni, Co, Mn and the like can be exemplified.

Second Embodiment

FIG. 4 is a system diagram of a boiling-water reactor (hereinafter abbreviated as “BWR”) according to this embodiment. In a BWR 20 of this embodiment shown in FIG. 4, a high-pressure turbine 22, a moisture separator/reheater 23, a low-pressure turbine 24 and a condenser 25 are sequentially connected to a nuclear reactor 21. Moreover, a low pressure feedwater heater 26 and a high pressure feedwater heater 27 are connected to the condenser 25, and the high pressure feedwater heater 27 is connected to the nuclear reactor 21. The nuclear reactor 21 is provided with a reactor water cleanup system 28.

For example, the BWR 20 can be operated as follows. Namely, the high-pressure turbine 22 is driven by steam generated by the nuclear reactor 21. Then, the steam having driven the high-pressure turbine 22 is cooled down and is partly liquefied into water, so that the steam is reheated by the moisture separator/reheater 23, and the liquefied water is vaporized and introduced into the low-pressure turbine 24 to drive the turbine 24. The steam which has driven the low-pressure turbine 24 is cooled by the condenser 25 to be converted into water. The water is then heated through the low pressure feedwater heater 26 and the high pressure feedwater heater 27 and introduced into the nuclear reactor 21 to be converted into steam again. The generated steam is introduced again into the high-pressure turbine 22 and the low-pressure turbine 24 to drive the turbines 22 and 24.

A method for monitoring corrosion protection in the BWR 20 shown in FIG. 4 is described below, but is basically same as that of the PWR 10 shown in FIG. 1 except that the apparatuses are somewhat different from the ones in the PWR 10.

Namely, a typical operation temperature of the BWR 20 shown in FIG. 4 can be also set to a temperature range of 25° C. to 225° C. substantially same as that of the PWR 10. Therefore, a range of oxygen concentration in which the Fe₂O₃ film can be maintained stably at every operation temperature can be recognized by referring to FIG. 2, and a range of oxygen concentration in which the FeTiO₃ film can be maintained stably at every operation temperature can be recognized by referring to FIG. 3.

Depending on whether the film formed on the inner wall surface of each of the apparatuses of the BWR 20 shown in FIG. 4 is the Fe₂O₃ film or the FeTiO₃ film, therefore, the amount of introduction of oxygen in which the Fe₂O₃ film and the FeTiO₃ film are maintained stably at the operation temperature of each of the apparatuses is identified by referring to FIG. 2 and FIG. 3, and the Fe₂O₃ film and the FeTiO₃ film formed on the inner wall surface of each of the apparatuses can be maintained stably by setting the amount of introduction of oxygen to the identified amount of oxygen.

In other words, it is seen that the Fe₂O₃ film and the FeTiO₃ film to be used for the power plant can be maintained stably by an extremely simple method that controls the operation temperature and the amount of introduction of oxygen without using a chemical agent such as a pH adjuster which is difficult in control. As a result, the anticorrosion property of the BWR 20 can be held under high reliability.

For example, the condenser 25 has the lowest operation temperature within a range of 25° C. to 50° C., and the nuclear reactor 21 and the high pressure feedwater heater 27 have the highest operation temperature within a range of 200° C. to 225° C.

Therefore, when the Fe₂O₃ film is formed on the inner wall surface of the condenser 25, the amount of introduction of oxygen is set within a range of about 1.0×10⁻⁵ ppm to 1.0×10² ppm with reference to FIG. 2. Meanwhile, when the Fe₂O₃ film is formed on the inner wall surface of the nuclear reactor 21, the amount of introduction of oxygen in the nuclear reactor 21 is set within a range of about 1.0×10⁻¹ ppm to 1.0×10⁵ ppm.

When the FeTiO₃ film is formed on the inner wall surface of the condenser 25, the amount of introduction of oxygen in the condenser 25 is set within a range of about 1.0×10⁻⁸ ppm to 0.5×10⁻⁴ ppm with reference to FIG. 3. On the other hand, when the FeTiO₃ film is formed on the inner wall surface of the nuclear reactor 21, the amount of introduction of oxygen in the nuclear reactor 21 is set within a range of about 1.0×10⁻⁶ ppm to 1.0 ppm.

It is required that the amount of introduction of oxygen is set larger as the operation temperature is set higher as in the nuclear reactor 21 or the like. Therefore, when the amount of oxygen contained in the system of the BWR 20 is small, oxygen is supplied, e.g., at the position indicated by an arrow 29a in the drawing in order to set the amount of introduction of oxygen in the above range.

Meanwhile, it is required that the amount of introduction of oxygen is set smaller as the operation temperature is lower as in the condenser 25 and the like. Therefore, if a large amount of oxygen is contained in the system of the BWR 20, for example, a reducing agent is supplied at the location indicated by an arrow 29b in the drawing to reduce the amount of oxygen contained in the system, thereby setting the amount of introduction of oxygen in the above-described range.

It is to be understood that other characteristics and advantages are similar to those of the PWR 10 in the first embodiment related to FIG. 1, and their descriptions will be omitted.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.