Elucidation of corrosion mechanism under radiation
or air/solution environments
Interviewee
Dr Kyohei Otani
Research purpose and contents
The previous assumption of corrosion of metals in nuclear reactors was based on pure water (light water) under normal operating conditions of the nuclear power station. Therefore, there was almost no data from which the impact and risk for sea water (salinity) could be estimated in an unanticipated environment created by the accident at the Fukushima Daiichi Nuclear Power Station (hereafter FDNPS). It is considered that the risks of corrosion deterioration for the equipment and structural materials in the FDNPS will be increased as time passes and the debris removal work progresses. The purpose of this study is to accumulate wide range data while simulating corrosion mechanisms under the environments of FDNPS because of the situation inside the reactor and building of FDNPS have not been clarified completely.
Corrosion tests under various environments which simulated FDNPS special
environments (*1)
Analysis of corrosion acceleration mechanisms under various environments
In order to collect and organize corrosion data under special environments of FDNPS, it was measured the corrosion rate at the γ-irradiation and non-irradiation under five conditions: immersion, semi-immersion, air phase after immersion, air phase, and air phase with falling droplets. The results clarified that the corrosion rate accelerated under the semi-immersion condition and the air phase with falling droplets condition. In addition, it was found that the corrosion rate in the semi-immersion condition and the falling droplets condition during γ-irradiation was especially accelerated because a thin water film was formed on the steel surface in the conditions.(*1) It has been reported that the corrosion rates of steels under a thin water film or the wet/dry cyclic condition were faster than that in aqueous solutions. (*2,3)
Analysis of corrosion acceleration mechanisms in air/solution alternating condition
The internal investigation of the Fukushima Daiichi Nuclear Power Station (FDNPS) Unit 1-3 reactor confirmed that the inner wall of the reactor is currently exposed to the air/solution alternating environment (the environment means that metals exposed in air and solution alternately) near the air/solution interface in the reactors.(*4,5) However, corrosion research on the air/solution interface that simulates the FDNPS environment has not been carried out, therefore, corrosion tests of steels under the air/solution alternating environments were carried out to estimate the corrosion rate of steels near the air solution interface in the reactors. The corrosion tests were carried out in the diluted artificial seawater which simulated the solution in the reactor for a month by a rotating corrosion test equipment which can control solution temperature and oxygen concentration. For comparison, the tests were performed in the air/solution alternating condition and the immersion condition. The tests clarified that the corrosion rate in the alternating condition were more than three times faster than in the immersion condition. (*6) It was also clarified that the structure of the iron rust layer which has not been reported previously.
Rotation corrosion test equipment
Specimen fixing jig in immersion condition
Schematic diagram of air/solution alternating environment
Materials and environments for corrosion resistance
With the progress in decommissioning of FDNPS, it is considered that the risks of corrosion deterioration in the FDNPS will be increased as time passes and the debris removal work progresses. In order to minimize the corrosion risks, it is necessary to take appropriate measures according to the situation such as reducing oxygen, maintaining lower salt concentrations, maintaining lower electroconductivity, and the injection of appropriate corrosion inhibitors. Furthermore, it is important to establish standards that can be used for selecting materials take into account the installing facilities and equipments in FDNPS environments. The facilities and equipments should be maintenance-free without assuming replacement, and it is necessary to select materials with higher corrosion resistance. We would like to establish standards for the selection of materials in accordance with the difference of dose rates and solution quality. The corrosion progresses slowly with time, hence, the corrosion often be found only after a hole is opened beyond a certain threshold and huge troubles occurs. By determining the standards for the threshold, it is possible to predict corrosion deterioration of equipments and structural materials and to proceed appropriate maintenance premeditatedly.
Related information
| Researcher | Kyohei Otani（researchmap） |
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| References | *1…T. Sato, FRC-Corrosion(2019) *2…A. Nishikata, T. Takahashi, Hou Bao-Rong and T. Tsuru, Zairyo-to-Kankyo., 43, p.188(1994) *3…M. Yamamoto, H. Katayama and T. Kodama,J. Jpan Inst. Met. Mater., 65, pp.465-469(2001) *4…Y. Fukaya, T. Hirasaki, K. Kumagai, T. Tatuoka, K. Takamori, and S. Suzuki, Corrosion, 74, pp.577-587(2018) *5…Tokyo Electric Power Company Holdings, http://www.tepco.co.jp/en/index-e.html [Accessed January 30, 2019] *6…K. Otani, T. Tsukada and F. Ueno, Zairyo-to-Kankyo, 68, pp.205-211 (2019) |
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