Steel Structure Corrosion and Corrosion Prevention
Major steel structures under study
(Source: IRID published material, 2013)
In proceeding with the work for decommissioning of the Fukushima No. 1 nuclear power plant steadily and safely, it is necessary to ensure its structural strength, keep cooling the fuel and maintain the confinement of radioactive materials for the required period in a reliable manner. In addition, it is also necessary to prevent the loss of these functions in the event of a large-scale aftershock by being aware of this likelihood. Important to achieving this is the systematic maintenance of the reactor containers and the coolant piping systems. While use of water is essential to keeping the fuel cooled and maintaining the radiation shield, it also means that the metal equipment and structures have to remain in an aqueous environment for a long time. Under normal conditions, highly-controlled ultra-pure water is used in nuclear power plants. However, the environment of the Fukushima No. 1 nuclear power plant after the accident deviates from that assumed in design. In such a situation, the biggest factor in age deterioration of structural materials is considered to be corrosion.
Our studies in the Steel Structure Corrosion and Corrosion Prevention Task Group (TG(1)-1) focus mainly on prediction and evaluation methods of corrosive deterioration of the steel structures such as reactor containers and coolant piping systems, as well as on the development of deterioration control technologies. Teachers and graduate students from Tohoku University (Quantum Science and Energy Engineering, Material Engineering) and teachers from Fukushima University and National Institute of Technology, Fukushima College participate in TG(1)-1.
In TG(1)-1, we study "the technologies addressing corrosion issues according to rules and principles" and through this try to develop "human resources capable of appropriately breaking down the on-site issues and making research topics of them, and feeding back the outcomes to the site."
One of the features of our study is the unique and diversified environmental conditions. There is a wide range of major influencing factors such as temperature, Cl concentration, pH, oxidant concentration (potential), ion species and concentration, flow rate, radiation, and the effects of deteriorated coating films. The most important thing for predicting and evaluating corrosion is determining the corrosion mode, that is, determining whether occurrence of uniform corrosion or local corrosion should be considered, as the basic concept behind each mode is different. The basic concept for uniform corrosion is the control and inhibition of the corrosion propagation rate, and, for local corrosion, the clarification and maintenance of conditions that will stop corrosion. In the Fukushima No. 1 nuclear power plant, there is the possibility of injecting a boric-acid solution or sodium pentaborate solution into the reactor to preventing recriticality, which can lead to a change in the corrosion mode (local/uniform).
In both corrosion modes, the greatest possible reduction of the amount of oxidants in the environment (cathode reaction) should be effective. Currently, in the plant, efforts are being made to inhibit the cathode reaction by reducing the dissolved oxygen concentration through constant deaeration by nitrogen gas. The effective cathode reaction rate (corrosion rate) is considered to increase under a radioactive and flowing environment and, therefore, evaluation of the reaction rate is important. In addition, when opening the hatches of the reactor containers to remove debris, there is the possibility of the dissolved oxygen concentration (corrosion rate) temporarily increasing. It is necessary to evaluate the effect of this in advance.
Application of corrosion inhibitors can be considered as one of the powerful options for long-term corrosion protection technologies in the future. It is important for such inhibitors to be effective under complex and diverse environmental conditions including radiation and not to have secondary adverse effects. A broad range of ideas and precise evaluation are necessary.
In TG(1)-1, studies for precise prediction of corrosion and development of mitigation technologies are performed from these perspectives. Participating students have gained an understanding of basic theories such as electrochemistry and materials science, and are engaged in the study of various subjects from experimental or computational science approaches.