Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Corrosion or damage prevention
Reexamination Certificate
2001-04-23
2003-08-12
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Corrosion or damage prevention
C376S306000
Reexamination Certificate
active
06606368
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of operating a nuclear power plant and the nuclear power plant and a method of controlling water chemistry of the nuclear power plant in which occurrence of stress corrosion cracking in metallic components in contacting with reactor cooling water of the nuclear power plant is suppressed.
2. Prior Art
Presently, hydrogen injection to reactor water is widely applied to a boiling water reactor as one of measures for preventing occurrence of intergranular stress corrosion cracking (hereinafter, referred to as IGSCC) in a metallic component material of a nuclear reactor component (hereinafter, appropriately referred to as nuclear reactor component material) in contact with the reactor water such as a reactor pressure vessel, a nuclear reactor internal components or piping. There is an electrochemical corrosion potential (hereinafter, referred to as ECP) of a metallic component material as a potential of IGSCC, and it is said that the potential of IGSCC increases as the ECP becomes higher when the ECP exceeds a critical value of ECP (in a case of stainless steels used in the most reactor components, the critical value of ECP is about −230 mV vs.SHE). The hydrogen injection has a function to decease the ECP of metallic component materials.
As a technology of effectively lowering the ECP with injecting a small amount of hydrogen, noble metal injection is disclosed in Japanese Patent Application Laid Open No.7-198893, Japanese Patent Publication No.2818943. In this technology, a solution containing a noble metal typical of platinum, rhodium, palladium is injected into reactor water, and hydrogen is also injected into the reactor water. The injected noble metal is deposited in surfaces of the nuclear reactor components such as the reactor pressure vessel and the reactor internal components, and recombination of oxygen and hydrogen is accelerated by catalysis of the noble metal to form water molecules. As the result, an amount of oxygen is decreased, and the ECP of the nuclear reactor internal components can be decreased lower than the critical value of ECP with a small amount of injected hydrogen.
On the other hand, as technologies of lowering the electrochemical corrosion potential of the nuclear reactor component material without injecting hydrogen, non-noble metal injection is disclosed in Japanese Patent Application Laid-Open No.7-311295, Japanese Patent Application Laid-Open No.8-43587, Japanese Patent Application Laid-Open No.10-197684 and Published Japanese Translation of PCT International Publication for Patent Application No.9-502533. The technologies decrease the ECP of nuclear reactor component materials without injecting hydrogen by doping non-noble metallic spicies typical of zirconium into oxide films on surfaces of the nuclear reactor component materials.
Further, Japanese Patent Application Laid-Open No.8-226994 discloses both of technologies that the electrochemical corrosion potential of the nuclear reactor component materials is decreased with a small of injecting hydrogen by injecting a noble metal and hydrogen into reactor water of a nuclear reactor, and that the electrochemical corrosion potential of the nuclear reactor component materials is decreased by injecting a non-noble metal and hydrogen into reactor water of a nuclear reactor. It is said that the latter technology is applicable even to the case without hydrogen injection.
Presently, hydrogen injection to reactor water is widely applied to a boiling water reactor as one of measures for preventing occurrence of IGSCC. However, a large amount of hydrogen is necessary to be injected in order to decrease the ECP down to a value lower than the critical value of ECP. Increase in the amount of hydrogen injection causes increase in an amount of radioactive nitrogen exhausted to the main steam system, which increases the radiation dose rate of the main steam system.
According to the technologies injecting the solution containing noble metal disclosed in Japanese Patent Application Laid Open No.7-198893, Japanese Patent Application Laid-Open No.8-226994 and so on, the ECP of the nuclear reactor component material can be decreased to a value below the critical value of ECP with a smaller amount of hydrogen injection compared to the case without the noble metal injection. However, in the noble metal injection, it is necessary to perform control in taking an effect on corrosion of the cladding tubes when the injected noble metal is attached onto the surfaces of fuel cladding tubes. In addition to this, there is a problem in that the operating cost is increased due to use of the noble metal.
In the technologies injecting the non-noble metal disclosed in Japanese Patent Application Laid Open No.7-311295, Japanese Patent Application Laid-Open No.8-226994 and so on, zirconium, hafnium, tantalum, niobium, yttrium and so on are shown as the non-noble metals, and acetylacetonato zirconium, zirconium nitrate, zirconyl nitrate are shown as the chemical compounds containing zirconium. By injecting these chemical compounds into reactor water, the ECP can be decreased without hydrogen injection. However, the patents do not disclose any sufficient knowledge on decrease of the ECP, and it cannot said, as far as the contents of Detailed Description of the Invention, that the ECP of the treated material is decreased down to the critical value of ECP capable of suppressing IGSCC. Further, there is a possibility that increase in the conductivity of the cooling water of nuclear reactor due to nitrate group at injection treatment may increase the burden of water chemistry control of the plant.
Further, according to a test conducted by the inventors of the present invention, it is found that a phenomenon completely opposite to decrease of the ECP when zirconium hydroxide is used. That is, when zirconium hydroxide is injected into reactor water of the nuclear reactor without hydrogen injection, the ECP is not decreased but inversely increased, which is different from the phenomenon described in the above-mentioned patents.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of operating a nuclear power plant, the nuclear power plant and a method of controlling water chemistry of the nuclear power plant in which in the nuclear power plant, by injecting an amount of hydrogen small enough not to increase a radiation dose rate of the main steam system, ECP of metallic component materials composing a nuclear reactor can be decreased to suppress the potential of occurrence of IGSCC, and the control can be easily performed, and the operating cost can be suppressed to increase.
(1) In order to attain the above object, in the present invention, a method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein an electrochemical corrosion potential of the metallic component material is decreased by injecting zirconium hydroxide and hydrogen into the reactor cooling water.
As described above, according to the result of the test conducted by the inventors of the present invention, it is found that on the contrary, the ECP is increased when only zirconium hydroxide is injected into reactor cooling water (hereinafter, referred to as reactor water). However, according to the result of a further test conducted by the inventors of the present invention, it is found that when both of zirconium hydroxide and hydrogen are injected into reactor water, the ECP is substantially decreased compared to in the case of injecting only hydrogen.
Furthermore, as a result of studying these test results, it is estimated that the decrease in the ECP by injecting zirconium hydroxide is different in principle and mechanism from those in the prior art, that is, the decrease in the ECP in the case of injecting only hydrogen or the noble metal and hydrogen into the reactor water, or the decrease in the ECP in the
Asakura Yamato
Hayashi Kouji
Hosokawa Hideyuki
Nakamura Masato
Sakai Masanori
Carone Michael J.
Keith Jack
Mattingly Stanger & Malur, P.C.
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