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ONLINE ISSN: 1881-9664
PRINT ISSN: 0917-0480

Zairyo-to-Kankyo Vol. 70 (2021), No. 12

  • Activity in an University and the JSCE under the COVID-19 Pandemic

    pp. 373-374

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    DOI:10.3323/jcorr.70.373

  • A Remark of “the Special Issue for the 10th Anniversary of Fukushima Dai-ich Accident.”

    pp. 375-376

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    DOI:10.3323/jcorr.70.375

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    1. Investigations of Corrosion Inhibitors for Past 70 Years Zairyo-to-Kankyo Vol.70(2021), No.11
  • Role of Corrosion Science and Engineering in Decommissioning Process of Fukushima Daiichi Nuclear Power Plant

    pp. 377-380

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    DOI:10.3323/jcorr.70.377

  • Corrosion Issues and Mitigation Activities After the Fukushima Daiichi Nuclear Accident

    pp. 381-391

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    DOI:10.3323/jcorr.70.381

    A catastrophic earthquake of magnitude 9.0 occurred off the Pacific coast of Tohoku in Japan on March 11, 2011. The subsequent tsunami lost the cooling function of nuclear fuel at the Fukushima Daiichi Nuclear Power Station, resulting in a core meltdown. This is the outline of the Fukushima Daiichi accident. As an emergency action for fuel cooling, seawater was injected into the reactors and the spent fuel pools, resulting in various corrosion issues. This article first presents corrosion issues and mitigation activities immediately after the accident. Various corrosion mitigation methods that were examined for application to the reactors and the spent fuel pools, their selection process, the timing of application, and the result of application are described in detail. After the accident was resolved, the decommissioning work of the Fukushima Daiichi Nuclear Power Station is proceeding based on four technical issues: fuel removal from the spent fuel pools, fuel debris retrieval, contaminated water management, and waste management. This article also outlines the corrosion issues that have become apparent in solving these technical issues and the potential corrosion issues that may emerge in the future.
  • Mechanism of Deoxidization Effect by Hydrazine under Gamma-ray Irradiated Condition

    pp. 392-395

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    DOI:10.3323/jcorr.70.392

    One of the corrosion mitigation methods, hydrazine (N2H4) addition to spent nuclear fuel pool (SFP) cooling water, was adopted in Fukushima Dai-ich nuclear power station (1F). It is well-known that deoxygenation of high temperature water by N2H4 is effective but the effect is smaller at the room temperature. However, gamma-ray irradiation would accelerate the deoxygenation reaction of N2H4. To clarify the acceleration effect, gamma-ray irradiation tests of N2H4 containing water were conducted in various conditions. Dissolved oxygen (DO) concentration rapidly decreased by gamma-ray irradiation, in both cases N2H4 added pure water and artificial sea water. The process was confirmed by the gamma-ray radiolysis model. These data pointed out a scientific meaning to the corrosion mitigation methods of 1F site.
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  • Study of Potential Corrosion Issues Concerning Spent fuel Pools and Its Countermeasures in 2011

    pp. 396-401

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    DOI:10.3323/jcorr.70.396

    A large amount of seawater and freshwater were injected into the nuclear reactors and spent fuel pools as emergency measures in response to the Fukushima Daiichi Nuclear Power Station accident that occurred on March 11, 2011. In response to this situation, the “Fukushima Daiichi Nuclear Power Station Corrosion Countermeasures Study Group” was established with members from academia, plant manufacturers, water treatment industry, and research institutes. The committee examined the countermeasures for potential corrosion problems in spent fuel pools, where the situation was known somewhat clearly. In the committee, major components and structures directly related to the safety function of the fuel pool were selected, and corrosion modes to be considered, possibility of occurrence, and countermeasures were discussed for each component. This paper describes the concept of the study and the outline of the results.
  • Study on Galvanic Corrosion of Aluminum Lack in Spent Fuel Pool

    pp. 402-408

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    DOI:10.3323/jcorr.70.402

    Seawater injection to the spent fuel pool has raised concerns over corrosion of fuel racks supporting fuel assemblies and pool liner serving as a boundary. Electrical conduction between the aluminum alloy racks and the stainless steel liner may cause galvanic corrosion of racks, at the same time, may inhibit corrosion of the liner with such the macro-cell effects. Boundary element analysis was performed to study such the macro-cell behaviors. With the highest conductivity and microbiological effects considered, a maximum corrosion increase of several mm/y was predicted for the ruck. However, lower conductivity significantly reduces galvanic corrosion. Thus, it has been verified quantitatively that decreased conductivity through water purification and microbiological control are effective in maintaining the soundness of the racks.
  • Structural Integrity Evaluation of RPV and PCV for Corrosion Loss

    pp. 409-415

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    DOI:10.3323/jcorr.70.409

    In order to evaluate structural integrity of RPV and PCV after severe accident in Fukushima Daiichi nuclear power station, immersion corrosion tests were performed using diluted artificial sea water simulating PCV stagnant water. Based on the result of parabolic law approximation for corrosion test data and others, corrosion loss of RPV and PCV was predicted in 40 years after the severe accident considering the time of fuel debris removal. Evaluation of structural integrity of RPV and PCV main equipment was performed using such as the predicted corrosion loss and load condition based on earthquake response analysis results. It was confirmed that generated stresses for RPV and PCV main equipment of unit 1, 2 and 3 satisfied with allowable values in 40 years after the severe accident.
  • Long Term Integrity of Fuel Assemblies Removed from Spent Fuel Pool

    pp. 416-420

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    DOI:10.3323/jcorr.70.416

    The fuel assembly stored in the spent fuel pool (SFP) of Fukushima Daiichi Nuclear Power Station Units 1 to 4 at the time of the Great East Japan Earthquake is in an unusual environment due to the contamination of rubble generated by seawater injection and damage to the building. It has a history. Therefore, in order to store spent fuel extracted from SFP for a long period of time in the service pool, it is essential to evaluate the effect of their environmental history on the health of fuel assemblies. It was evaluated that the soundness of the fuel assembly was maintained from the viewpoint of structural integrity and cladding tube tightness.
  • Corrosion Suppression Technology for the Primary Containment Vessel

    pp. 421-426

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    DOI:10.3323/jcorr.70.421

    The addition of corrosion inhibitor was considered as one of corrosion countermeasures for the primary containment vessels of Fukushima Daiichi nuclear power station. For various candidate corrosion inhibitors, immersion corrosion test, inhibitor interim addition test, and evaluation of localized corrosion resistance by electrochemical measurements were carried out. Based on the results of these tests, sodium pentaborate, sodium tungstate and two types of phosphate-based corrosion inhibitors were selected. Furthermore, the management procedure was established for the selected inhibitors.
  • Corrosion Behavior of Carbon Steel in Seawater and Freshwater Environments

    pp. 427-430

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    DOI:10.3323/jcorr.70.427

    In order to obtain knowledge of the corrosion behavior of carbon steel in the operating environment of TEPCO’s Fukushima Daiichi Nuclear Power Plant (1F), water quality change behavior was analyzed using water quality data published by TEPCO. Based on the results of the analysis, a full-scale corrosion test of carbon steel pipes was conducted in an environment simulating that of 1F’s spent fuel pool cooling system. Based on the test results and previous studies, a uniform corrosion growth prediction model was proposed applicable to static and flowing water conditions in steel pipes from seawater to freshwater (including diluted seawater).This paper presents an overview of the research carried out on the evaluation of the corrosion behavior of carbon steel in seawater and freshwater environments.
  • Corrosion Behavior of Carbon Steel in Flowing Water Contaminated by Seawater

    pp. 431-435

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    DOI:10.3323/jcorr.70.431

    At the spent fuel pool of the Fukushima Daiichi Nuclear Power Plant, it was necessary to reduce the possibility of corrosion caused by salt contamination. For the carbon steel piping used in the alternative system for cooling the spent fuel pool, careful consideration must be given to corrosion caused by the flow of diluted seawater. In this paper, studies on the corrosion behavior of carbon steel in flowing water containing seawater are reviewed. In fluid-diluted artificial seawater, the corrosion rate of carbon steel decreases significantly with the test time, and the difference in corrosion behavior depending on the flow rate is relatively small. The addition of inorganic corrosion inhibitors such as sodium nitrite and sodium pentaborate is effective in suppressing the corrosion of carbon steel. The effect of flow rate on the critical concentration of the inorganic corrosion inhibitor required for corrosion control depends on the inhibitor. The critical concentration of nitrite ion is affected by the flow rate, whereas that of pentaborate ion is not. Purification of water is an effective in controlling carbon steel corrosion under various flow conditions.
  • Corrosion Behavior of Carbon Steel at Water Line under Gamma Ray Irradiation

    pp. 436-440

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    DOI:10.3323/jcorr.70.436

    In the primary containment vessel (PCV) of Fukushima Daiichi Nuclear Power Station (1F), corrosion data for understanding and predicting the corrosion behavior at the water line under the gamma-ray irradiation environment, which could be a potential problem, were obtained by the corrosion test. The results of corrosion tests clearly showed that the corrosion was accelerated by the superposition of water film effect and irradiation effect. From the viewpoint of the corrosion acceleration mechanism, it was considered that the oxygen supply from the atmosphere was more pronounced near the water line and the concentration of hydrogen peroxide produced by the radiolysis of water was higher, both of which contributed to the corrosion acceleration.
  • Effect of Gamma-ray Irradiation on Corrosion of Stainless Steel Contacted with Zeolite Particle

    pp. 441-447

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    DOI:10.3323/jcorr.70.441

    Zeolite particles have been used in a Cs adsorption vessel for purification of contaminated water in Fukushima Dai-ich nuclear power station (1F). The used Cs adsorption vessels were kept in storage space on the 1F site. The risk of localized corrosion of stainless steel used in the vessel was worrying. To evaluate the risk of localized corrosion, using specially designed electrochemical testing apparatus was used under gamma-ray irradiation test. And, real size mock-up test was conducted. The results showed that the presence of zeolite particles decomposed hydrogen peroxide generated by radiolysis and consequently suppressed the ennobled corrosion potential of stainless steel by γ-ray irradiation. Moreover, the concentration of chloride ions in the remaining water was not increased by the decay heat of radioactive Cs adsorbed on the zeolite. These data indicate the minimizing risk of localized corrosion of the Cs adsorption vessel.
  • Corrosion Monitoring of Carbon Steels under Humid Air Environment with Ozone Simulating Irradiation

    pp. 448-456

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    DOI:10.3323/jcorr.70.448

    The effects of gamma-ray irradiation on the corrosion of carbon steels in humid gas-phase environments investigated by introducing ozone as a simulated oxidant produced by radiolysis under gamma-ray irradiation and by monitoring corrosion using corrosion sensors are introduced. The corrosion rate was increased with both relative humidity and ozone concentration. This is because the reduction reaction and the dissolution reaction of ozone in water were easier than those of oxygen, which resulted in the acceleration of the cathodic reaction. The introduction of ozone is considered to be effective as a corrosion acceleration test in a non-irradiated environment simulating gamma-ray irradiation.
  • Effect of Oxidants on Corrosion of Carbon Steel under Irradiation Conditions

    pp. 457-461

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    DOI:10.3323/jcorr.70.457

    The Primary Containment Vessels (PCV) in Fukushima Daiichi Nuclear Power Plants (1F) have been exposed to a corrosive environment containing seawater components under irradiation condition. In such conditions, hydrogen peroxide (H2O2) is generated by water radiolysis as an oxidant in addition to dissolved oxygen. The H2O2 concentration is able to be estimated by radiolysis calculations.The corrosion tests under gamma-ray irradiation were performed. It was confirmed that the N2 gas purging performed in the PCV is effective in inhibiting the corrosion of carbon steel under irradiation conditions.The weight losses of carbon steel specimens obtained in the corrosion tests under gamma-ray irradiations, are evaluated using O2 and H2O2 concentrations estimated by radiolysis calculations. As a result, it is confirmed that the corrosion rate of carbon steel under irradiation is determined by the sum of diffusion limiting currents of O2 and H2O2 as well as that of carbon steel in the O2 and H2O2 coexistent conditions under non-irradiation.
  • Effects of Environmental Factors to the General Corrosion of Steel Under Irradiation

    pp. 462-467

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    DOI:10.3323/jcorr.70.462

    The effects of various environmental factors on the general corrosion of steel in an aqueous chloride solution under gamma-rays irradiation are reviewed, focusing on the experimental results obtained by the author’s group. Under irradiation, the corrosion of steel is accelerated by oxygen and hydrogen peroxide generated by the radiolysis of water. The rate of corrosion under irradiation increase with the concentration of chloride. The corrosion rate under an air atmosphere is higher than that in an Ar-gas atmosphere. The corrosion rate in a N2-gas atmosphere is at an intermediate level between the air atmosphere and the Ar-gas atmosphere. In the Ar-gas atmosphere, the corrosion rate is minimal at around the pH of 8 and maximal at around that of 10. Under irradiation, Br ions greatly accelerate the corrosion even existing in trace amounts in the solution. The corrosion rates of pure iron, SQV2A low alloy steel and SGV480 carbon steel under irradiation are roughly equal.
  • Radiolysis Effects, Which should be Taken into Account for Safe Decommissioning of Fukushima-Daiichi Nuclear Power Station

    pp. 468-473

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    DOI:10.3323/jcorr.70.468

    In order to estimate corrosive environment in the contaminated water at Fukushima Daiichi Nuclear Power Station, effects of oxidants, such as H2O2, which were generated from water radiolysis, should be taken into account due to the irradiation field in the reactor building. The process of water radiolysis and the amounts of these oxidants can change depending on the conditions of water and types of radiation. After the accident, a variety of factors, which can affect water radiolysis, such as seawater constituents, surface of oxides, and α-radionuclides, had been discussed. In this paper, these effects on radiolysis are reviewed for the better understanding of the corrosive environment in the contaminated water.
  • Electrochemical Study of the Effect of Adding Molybdate and Nitrate on the Corrosion of Carbon Steel in Diluted Artificial Seawater

    pp. 474-479

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    DOI:10.3323/jcorr.70.474

    The effect of adding Na2MoO4 and NaNO3 was investigated in diluted artificial seawater as countermeasures for suppressing corrosion of internal structures after the accident at the Fukushima Daiichi Nuclear Power Station. Addition of Na2MoO4 suppressed the anodic reaction, suggesting that the addition decreased the corrosion current density for carbon steel. Addition of NaNO3 suppressed the anodic reaction while accelerated the cathodic reaction, suggesting that the addition might accelerated the corrosion. Mixed addition suppressed the anodic and cathodic reaction, suggesting that the mixed addition decreased the corrosion current density.
  • Corrosion of Carbon Steel in the Simulated Air/Solution Interface Environment

    pp. 480-486

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    DOI:10.3323/jcorr.70.480

    This is a comprehensive paper of the corrosion of carbon steel in air/solution alternating condition.From cross-sectional observation and analysis of the iron rust layer formed on the surface of carbon steel in the alternating condition, it was found that a multilayered iron rust layer composed of red rust layer (γ-FeOOH), rust crust layer (Fe3O4), inner crystal (Fe3O4), and inner rust layer was formed on carbon steel. The multi-layered iron rust layer would accelerate the cathodic oxygen reduction reaction, and the reason why the corrosion rate of the carbon steel in the alternating condition was accelerated.The effect of artificial seawater (ASW) composition on the corrosion rate of carbon steel in air/solution alternating condition was investigated. It was found that the corrosion rate increased with increasing concentration of ASW from pure water to 200 times diluted ASW, and decreased with increasing concentration from 20 times diluted ASW to no diluted ASW. The Mg and Ca ions in ASW precipitated on the reaction interface and formed a metal cation layer, which inhibited the oxygen reduction reaction, and thus the corrosion of carbon steel was inhibited in the highly concentrated ASW.
  • Corrosion Issues from a Bird's-eye View of Fukushima Daiichi NPP Decommissioning

    pp. 487-490

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    DOI:10.3323/jcorr.70.487

    In order to complete the decommissioning of the Fukushima Daiichi Nuclear Power Plants, it is essential to maintain the structure for a long period of 30 years or more, and in particular, it is important to evaluate the deterioration due to corrosion and take countermeasures.In this paper, after taking a bird’s-eye view of the whole decommissioning process, the issues of corrosion events in reactor pressure vessels, primary containment vessels, storage vessels of the secondary wastes of contaminated water and fuel debris, and vessels used during the treatment/disposal of radioactive wastes will be explained. In addition, the corresponding concepts of how to deal with these events, will be discussed, taking into account the opinions of various experts.
  • A Task of Microbiologically Influenced Corrosion in Fukushima Daiichi Decommissioning

    pp. 491-496

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    DOI:10.3323/jcorr.70.491

    After Fukushima Daiichi Nuclear Power Station accident, various corrosion mitigating activities have been treated, and severe corrosion incident have never taken placed. On the other hand, the facilities were exposed sea water, and some of them have continuously exposed to ground water. The exposure of metal materials to environmental water has a risk of microbiologically influenced corrosion (MIC). In this paper, we summarize the latest knowledge of MIC and the task of MIC in the decommissioning of Fukushima Daiichi Nuclear Power Station.

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