JP2000028789A - Method for operating nuclear power generation plant of boiling water type - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to accelerate the higher effect than usual injection of hydrogen by injecting more excessive hydrogen than the quantity of hydrogen injected continuously ahead of the implementation of the continuous injection of hydrogen and controlling the property of an oxide film on the surface of a material. SOLUTION: Before hydrogen is continuously injected, a more excessive quantity of hydrogen injected continuously-for example, about twice of the most appropriate quantity in the hydrogen concentration on a reactor basis-is added for the time film-for approximately 20 hours, for instance-required to stabilize a certain value of the property of an oxide. As a result, the replacement of the oxide film with that of a reduced type leads to a condition where the film is a reduced type and the low concentration of oxidative species during the period when hydrogen is injected continuously afterward to secure the soundness of constituting members. As to the position where hydrogen is injected, it may be impregnated from a pipe of a feed water system downstream from a feed water heater 12 in a boiling water nuclear power plant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of operating a boiling water nuclear power plant (BWR power plant),
In particular, the present invention relates to an operation method for injecting hydrogen for ensuring the integrity of reactor structural materials.

[0002]

2. Description of the Related Art Hydrogen injection has been adopted as an effective means for ensuring the soundness of structural materials in a reactor pressure vessel of a BWR power plant, particularly in the lower part of the pressure vessel, and in particular, for the generation and suppression of stress corrosion cracking. . In hydrogen injection at the BWR power plant, hydrogen is injected from the feedwater heater outlet of the feedwater system,
Due to the direct reaction between the injected hydrogen and oxygen or hydrogen peroxide or the indirect reaction via OH radicals, etc., mainly in the downcomer section around the reactor core, oxygen or hydrogen peroxide This is to reduce the concentration. The injected hydrogen moves from the water layer to the steam layer at the reactor core, and the transferred hydrogen is exhausted from the turbine condenser by the steam extractor along with the main steam, and is converted to water by the oxyhydrogen recombiner. Will be returned.

[0003] Hydrogen injection has so far been employed in dozens of plants. FIG. 2 shows an example of the implementation. Hydrogen enters the reactor from the water supply system, mixes with the recirculated water,
We reach down kama part. Since the injection effect is determined by the hydrogen concentration in the downcomer section, the index of the injection amount is not the concentration in the feedwater but the concentration in the downcomer section, that is, the core-converted hydrogen concentration [H ₂ ] _eff defined by the following correlation: Is adopted.

Core equivalent hydrogen concentration [H ₂ ] _eff = [Hydrogen concentration in feed water] × [Feed water flow rate] / [Recirculation flow rate] As the core equivalent hydrogen concentration increases, the measured oxygen concentration in the reactor water decreases. Here, the concentration of oxygen is measured.However, in the furnace atmosphere, hydrogen peroxide has almost the same concentration as oxygen, and most of the hydrogen peroxide thermally decomposes before being measured through the sampling system. Instead of oxygen,
All of the measured concentrations are not oxygen in the furnace, but a mixture of oxygen and hydrogen peroxide. For this reason,
Hereinafter, the measured oxygen concentration is referred to as the effective oxygen concentration [O ₂ ] _eff
Is defined. That is, effective oxygen concentration [O ₂ ] _eff = [oxygen concentration] + (1/2) ×
[Hydrogen peroxide concentration] According to the findings so far, if the effective oxygen concentration can be reduced to 20 ppb or less, the occurrence and propagation of stress corrosion cracking of stainless steel will be suppressed. The first goal is to make the value equal to or less than this value (target value). On the other hand, one of the problems when hydrogen injection is adopted in a BWR power generation plant is the suppression of the dose rate of the turbine system. That is, a part of the radioactive nitrogen 16 generated as a result of the following water activation reaction in the core becomes a volatile ammonia form, and the rate of transfer to the turbine system accompanying the steam increases. The system dose rate increases.

¹⁶ O (n, p) ¹⁶ N [Reaction with High Energy Neutrons Above 10 MeV] FIG. 2 illustrates the turbine system dose rate as a result of hydrogen injection. An increase in the turbine system dose rate may cause an increase in the radiation dose received by the operating personnel, and a too large increase is not preferable.Therefore, the hydrogen injection amount is limited up to the allowable value of the turbine system dose rate. As a result, the optimum injection amount is determined as shown in FIG.

When the injection amount is determined by the effective oxygen concentration in this way, the optimum value can be determined by the above-described process. However, as an index of the water quality environment of stress corrosion cracking of stainless steel, the corrosion potential becomes the mainstream. It is getting. FIG. 3 shows the evaluation factors of the corrosive environment. Taking the crack growth rate of stress corrosion cracking as an example, the main evaluation factors include corrosion potential, conductivity, pH, flow rate, temperature, and other organic impurity concentrations. Factors that determine the corrosion potential include oxygen, hydrogen peroxide concentration (typically, effective oxygen concentration), hydrogen concentration, and corrosive radical concentration. However, until now, there has been no decisive decision whether to use the effective oxygen concentration as the evaluation index or the corrosion potential as the evaluation index.

FIG. 4 shows the crack growth rate when the corrosion potential (ECP) is used as the evaluation index and when the effective oxygen concentration is used as the evaluation index. In all cases, there is a discrepancy in the concentration range, but the database supports the validity. FIG.
Also shows the corrosion potential as a result of hydrogen implantation, but the decrease in corrosion potential is slower than the effective oxygen concentration,
The optimum value of the hydrogen injection amount may differ depending on which evaluation index is used for the corrosive environment.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a BW
When hydrogen injection is continuously performed to ensure the soundness of the reactor structural material of the R power plant, the oxide film on the surface of the structural material is pre-conditioned in advance to provide an injection effect more than that of normal hydrogen injection. It is an object of the present invention to provide a method of operating a boiling water nuclear power plant capable of operating the nuclear power plant. Another object of the present invention is to provide a method for operating a boiling water nuclear power plant that optimizes a corrosive environment evaluation index of a structural material and can optimally control an appropriate amount of hydrogen injection. In other words, since the crack growth rate of stress corrosion cracking is the final target, the crack growth rate itself is used as a direct index to suppress the crack growth rate, rather than an indirect evaluation index such as corrosion potential or effective oxygen concentration. In addition, it is intended to optimize the hydrogen implantation amount or to promote the implantation effect.

[0009]

According to a first aspect of the present invention, there is provided a method for operating a boiling water nuclear power plant in which hydrogen is continuously injected. By injecting an excess of hydrogen from the amount of hydrogen to be continuously injected, the properties of the oxide film on the material surface are controlled.

A second aspect of the present invention relates to a method for operating a boiling water nuclear power plant in which hydrogen is continuously injected. By injecting hydrogen,
Control the properties of the oxide film on the material surface.

According to a third aspect of the present invention, there is provided a method for operating a boiling water nuclear power plant wherein hydrogen is continuously injected. By properly injecting, the properties of the oxide film on the material surface are repeatedly controlled.

In a fourth aspect based on any one of the first to third aspects, the control of the property of the oxide film is monitored by measuring a corrosion potential or measuring a crack growth rate.

Here, the definition of the corrosion potential is shown in FIG. The corrosion potential of stainless steel is defined as the potential at which the cathodic current associated with the reduction of oxygen and hydrogen peroxide on the metal surface and the anodic current associated with the oxidation reaction such as elution of metal ions naturally balance. It is known that the anodic current depends on the concentrations of oxygen and hydrogen peroxide on the water side, but the anodic current strongly depends on the surface properties of the metal. Oxygen and hydrogen peroxide have different reducing effects, and therefore have different degrees of influence on the corrosion potential.

Attempts to measure the corrosion potential in an oxygen atmosphere or a hydrogen peroxide atmosphere have been one of the major issues for corrosion researchers. At high temperatures, thermal decomposition of hydrogen peroxide, especially catalytic decomposition at the material surface, is large, making it difficult to create an atmosphere of hydrogen peroxide alone, and as a result, the quantitative evaluation of the effect of hydrogen peroxide on the corrosion potential is sufficient. I couldn't do it. The inventors have
Journal of Nuclear Science and Technology, Vol.
35, No. 4, pp. 301-308, April 1998.
The experimental method described in (1) was established in an atmosphere of hydrogen peroxide alone, and the corrosion potential and the crack growth rate in the atmosphere were measured using the hydrogen peroxide concentration as a parameter. As a result, the following new findings were obtained. The present invention has been made based on this finding.

That is, FIG. 6 shows the results of measuring the corrosion potential of a stainless steel test piece whose surface was mechanically polished and the oxide film on the surface was removed in advance in an atmosphere of oxygen and hydrogen peroxide. From FIG. 6, it can be seen that the corrosion potential decreases due to the decrease in the concentration of the oxidizing species (oxygen or hydrogen peroxide), and the reduction of hydrogen peroxide is slower than that of oxygen. That is, even at a low concentration, the corrosion potential in the case of hydrogen peroxide is higher than in the case of oxygen.

However, the potential measurement was repeated in a hydrogen peroxide atmosphere, and as the oxide film formed of hydrogen peroxide was formed on the surface of the test piece, the corrosion potential showed hysteresis as shown in FIG. . That is, when there is a film formed of hydrogen peroxide, the anodic polarization characteristic shown in FIG. 5 shifts greatly to the right, and as a result, the corrosion potential maintains a high value independent of the concentration of hydrogen peroxide. .
This tendency is almost the same even when the atmosphere is changed to oxygen. Once a stable oxide film is formed with hydrogen peroxide, not only hydrogen peroxide but also oxygen exhibits a high corrosion potential as shown in FIG. Here, it should be noted that there is a difference in the oxide film formed between oxygen and hydrogen peroxide, and in the case of a hydrogen peroxide atmosphere,
This tendency is notable.

Next, FIGS. 8 and 9 show the results of measuring the crack growth rate in an atmosphere of oxygen and hydrogen peroxide. As shown in FIG. 8, the result measured in an oxygen atmosphere shows a monotonically decreasing correlation to the left, and the crack growth rate monotonically decreases as the oxygen concentration decreases. On the other hand, as shown in FIG. 9, in a hydrogen peroxide atmosphere, the crack growth rate does not depend much on the hydrogen peroxide concentration, and the crack growth rate becomes almost constant in a concentration range of slightly more than three digits. FIG. 10 shows this correlation using the corrosion potential as an index.
These are shown in an oxygen atmosphere and a hydrogen peroxide atmosphere, respectively. In an oxygen atmosphere, a downward-sloping correlation shown conventionally has been obtained, but in hydrogen peroxide, a substantially constant crack growth rate is shown regardless of the change in corrosion potential.
That is, it indicates that it is difficult to correctly evaluate the crack growth using the corrosion potential as an index.

As described above, it is very difficult to measure the respective concentrations of oxygen and hydrogen peroxide in a reactor pressure vessel because of thermal decomposition of hydrogen peroxide in a sampling pipe. Possible quantification methods rely on measuring the effective oxygen concentration and predicting the respective concentrations of oxygen and hydrogen peroxide based on a theoretical analysis of the radiolysis of water by a radiolysis model.

[0019] The inventors have been working on Nuclear Science and Engineer.
ing: 85, 339-349 (1983), FIG. 11 shows an overview of the radiolysis of water in the BWR and a theoretical model.
FIG. 12 shows an example of an analysis result by the present model. As a result of the hydrogen injection, the oxygen concentration drops sharply, but the hydrogen peroxide concentration exists for a relatively long time. However, even if the hydrogen peroxide concentration is sufficiently reduced, the corrosion potential does not readily decrease, and the amount of injected hydrogen tends to rapidly decrease beyond about 60 ppb in terms of core-converted hydrogen concentration. This suggests that the decrease in corrosion potential is due to an increase in hydrogen rather than a decrease in hydrogen peroxide.

That is, the hydrogen peroxide-dominated oxide film is reduced by hydrogen and changes to a new film property.
It is considered that a change occurs in the anodic polarization characteristics, and as a result, the corrosion potential escapes from the hydrogen peroxide dominant type. The same concept can be applied to the crack growth rate. In the case of an oxide film dominated in a hydrogen peroxide atmosphere, the crack growth rate is hardly dependent on the hydrogen peroxide concentration. , The crack growth rate depends on the concentration of the oxidizing species.

FIG. 13 schematically shows this correlation. That is, in normal water quality (NWC), there is a sufficient oxygen concentration, and the crack growth rate is controlled by the oxygen concentration. However, when the coating is changed to a hydrogen peroxide dominated type by hydrogen injection (HWC), the crack growth rate becomes a constant value. Stop. When the amount of hydrogen injection further increases, the coating turns into a reduced type, and the crack growth rate again decreases.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Based on the results of the above experiments and analysis, the present invention solves the problem that the oxide film becomes a hydrogen peroxide type in normal hydrogen implantation and it is difficult to sufficiently reduce the crack growth rate by hydrogen implantation. It is characterized in that the oxide film properties are positively converted by injecting excess hydrogen.

In one embodiment of the method of operating a boiling water nuclear power plant according to the present invention, prior to continuous hydrogen injection, an excess amount of hydrogen, such as an optimum core concentration-converted hydrogen concentration, over the amount of hydrogen continuously injected. About twice as much hydrogen is required to stabilize the oxide film properties to a certain value, for example, about 20 times.
Add over time. As a result, by replacing the oxide film with the reduced type, the condition that the film is reduced and the concentration of the oxidized species is low during the subsequent continuous hydrogen injection period is created, and the soundness of the structural material can be secured. As the hydrogen injection position, for example, the hydrogen may be injected from a water supply pipe downstream of the water heater 12 of the boiling water nuclear power plant shown in FIG.

It is preferable from the viewpoint of ensuring the soundness of the structural material to continuously increase the hydrogen concentration and reduce the crack growth rate during the hydrogen injection period, but as described above, it is necessary to increase the turbine dose rate. Derived problems, not preferred. On the other hand, the conventional optimal hydrogen injection amount is not sufficient in securing structural material integrity. For this reason, as shown in FIG. 14, at the initial stage of hydrogen injection, excess hydrogen than usual, for example, about twice as much as the optimum value in core equivalent hydrogen concentration, is required to stabilize the oxide film properties at the start of the plant. For an appropriate time, for example, about 20 hours, and the oxide film is changed to a reduced form. Thereafter, by returning the hydrogen injection amount to a predetermined value, it is possible to create a condition that the film is reduced and the concentration of oxidized species is low.

As a result of the reduction of the oxide film, the crack growth rate can be sufficiently reduced even with the same amount of hydrogen injection, and when the amount of hydrogen injection is large, the furnace output is small, the steam supply amount to the turbine is small, and the Since the amount of generated nitrogen-16 is limited during the start-up, the turbine dose rate does not increase even in the rated operation. In addition, since the circulation flow rate at the time of startup is sufficiently smaller than that at the time of rating, even when hydrogen is injected from the hydrogen injection device prepared for the time of rating, it is possible to achieve a sufficiently excessive core-converted hydrogen concentration.

In another embodiment of the present invention, the time for changing the oxide film to the reduction type by increasing the hydrogen injection amount is not limited to the start-up time, and the hydrogen injection amount is appropriately increased. Thus, it is effective to increase the frequency of converting the oxide film properties to the reduced type. In this case, the turbine dose rate naturally increases during the period of increasing the hydrogen injection rate.However, if measures such as patrol restrictions on the turbine system are taken in advance in operation, the turbine dose rate temporarily increases temporarily. Even so, it is possible to achieve the soundness of the structural material while suppressing the radiation dose received by the worker.

In another embodiment, changes in film properties are constantly monitored by a corrosion potential monitor installed in the bottom drain system, and the amount of excess hydrogen and the excess hydrogen supply period required before continuous hydrogen implantation are monitored by the monitor values of the corrosion potential monitor. Determine using. This makes it possible to more reliably control the properties of the oxide film with the optimum amount of hydrogen and the optimum time. In addition, by appropriately increasing the amount of hydrogen implantation using the monitored value of the corrosion potential monitor, the property of the oxide film on the material surface can always be kept in the reduced type.

In another embodiment, a change in film properties is constantly monitored by a crack growth rate monitor installed in a bottom drain system.
The amount of excess hydrogen and the period of supply of excess hydrogen necessary before the continuous hydrogen injection are determined using the monitoring values of the crack growth rate monitor. This makes it possible to more reliably control the properties of the oxide film with the optimum amount of hydrogen and the optimum time.
In addition, by appropriately increasing the hydrogen injection amount using the monitored value of the crack growth rate monitor, the crack growth rate can be kept low at all times.

In another embodiment, as shown in FIG. 15, a hydrogen injection amount is temporarily increased by using an accumulator 15 for hydrogen injection. In this case, a part of the hydrogen from the hydrogen generator 14 is stored in the accumulator 15 prior to the injection, and the stored hydrogen is used to cope with a short-term increase in the injection amount.

[0030]

According to the present invention, it is possible to reduce the oxide film and reduce the crack growth rate without any significant impact on the turbine dose rate by improving the operation method using ordinary hydrogen injection equipment. Become.

[Brief description of the drawings]

FIG. 1 is a plant configuration diagram for implementing one embodiment of a method of operating a BWR power plant according to the present invention.

FIG. 2 is a diagram showing an example of a hydrogen injection effect.

FIG. 3 is a diagram showing evaluation factors for a corrosive environment.

FIG. 4 is a correlation diagram between a crack growth rate, a corrosion potential, and an effective oxygen concentration.

FIG. 5 is a diagram showing a definition of a corrosion potential.

FIG. 6 is a correlation diagram between effective oxygen concentration and corrosion potential.

FIG. 7 is a correlation diagram between effective oxygen concentration and corrosion potential.

FIG. 8 is a correlation diagram between an oxygen concentration and a crack growth rate.

FIG. 9 is a correlation diagram between a hydrogen peroxide concentration and a crack growth rate.

FIG. 10 is a correlation diagram between a corrosion potential and a crack growth rate.

FIG. 11 is a schematic diagram of radiolysis of water in a BWR and a theoretical model.

FIG. 12 is a diagram showing an example of analysis of oxygen, hydrogen peroxide, and corrosion potential in a nuclear reactor during hydrogen injection.

FIG. 13 is a correlation diagram between the effective oxygen concentration and the crack growth rate.

FIG. 14 is a diagram showing a control operation of an oxide film.

FIG. 15 is a plant configuration diagram for implementing another embodiment of the method of operating a BWR power plant according to the present invention.

[Explanation of symbols]

14 ... hydrogen generator, 15 ... accumulator, 16 ...
Hydrogen injection device.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahiko Tachibana 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Power & Electricity Development Division, Hitachi, Ltd. (72) Inventor Atsushi Watanabe Omika-cho, Hitachi City, Ibaraki Prefecture 7-2-1, Hitachi, Ltd. Power and Electricity Development Division

Claims (8)

[Claims] 1. A method for operating a boiling water nuclear power plant in which hydrogen is continuously injected, wherein an excess amount of hydrogen is injected before the continuous hydrogen injection is performed. A method for operating a boiling water nuclear power plant, wherein the property of an oxide film on a material surface is controlled by the method. 2. A method for operating a boiling water nuclear power plant in which hydrogen is continuously injected, wherein an excess amount of hydrogen is injected before the output of the plant start-up operation reaches a rated value. A method for operating a boiling water nuclear power plant, wherein the properties of the oxide film on the surface of the material are controlled. 3. A method for operating a boiling water nuclear power plant in which hydrogen is continuously injected, wherein during the continuous hydrogen injection, an excess amount of hydrogen is injected as needed in excess of the amount of hydrogen continuously injected. A method for operating a boiling water nuclear power plant, wherein the properties of an oxide film on a material surface are repeatedly controlled by the method. 4. The method for operating a boiling water nuclear power plant according to claim 1, wherein the control of the property of the oxide film is monitored by measuring a corrosion potential. 5. The method for operating a boiling water nuclear power plant according to claim 1, wherein the control of the property of the oxide film is monitored by measuring a crack growth rate. 6. A method for operating a boiling water nuclear power plant having a hydrogen injection facility, wherein an accumulator is provided at an outlet side of the hydrogen injection facility, and excess hydrogen stored in the accumulator is used. 5. A method for operating a boiling water nuclear power plant, wherein the operation according to any one of 5 is performed. 7. A method for operating a boiling water nuclear power plant which continuously performs hydrogen injection, wherein the amount of hydrogen injected is controlled by measuring a crack growth rate. how to drive. 8. A boiling water nuclear power plant, wherein said accumulator is provided at an outlet side of a hydrogen injection facility for carrying out the operation of claim 6.