This paper presents an experimental study of surface wettability on a stainless plate in a high-temperature, high-pressure environment. Using a pressure vessel, we measured contact angles of water droplets at temperatures from 20 to 300°C and a constant pressure of 15 MPa, as an indicator of macroscopic surface wettability. Measured contact angles decreased with temperature below 250°C, clustering around a straight line at temperatures below 120°C and around another line in the range from 120 to 250°C. At temperatures above 250°C, on the other hand, the contact angles remained constant, independent of temperature, and contrary to the existing theoretical model, no highly hydrophilic condition or null contact angle condition was achieved.
This synopsis describes corrosion issues and mitigation activities shortly after the Fukushima Daiichi Nuclear Power Station accident. An earthquake of magnitude 9.0 occurred on March 11, 2011; the subsequent tsunami removed the cooling capacity of fuels in both the reactors and spent fuel pools (SFPs). Seawater was temporarily used for emergency fuel cooling, which induced various corrosion issues. Just after the accident, the temperatures within the reactors of Units 1 to 3 increased to several hundred degrees Celsius and the water quality of the cooling water seems to have become similar to that of concentrated seawater. To stabilize the fuel cooling, corrosion mitigation actions were required for mainly carbon steel components. The following corrosion mitigation measures were applied to the reactors: (a) temperature decreases, (b) dissolved oxygen removal from feedwater via deaeration, (c) dissolved oxygen removal from cooling water in the reactors via nitrogen gas injection, (d) salt removal from cooling water, and (e) sterilization of feedwater by hydrazine addition. The temperatures of SFP water in Units 1 to 4 were between 47°C and 93°C just after the accident. The maximum chloride ion concentration was approximately 2,000 ppm and the pH was in the range from 7.5 to 11.2. The mitigation of localized corrosion of the stainless steel pool liners and alkaline corrosion of the aluminum fuel racks was the top priority. In addition to (a), (b), and (d) listed above, (f) dissolved oxygen removal and sterilization by hydrazine addition and (g) pH control were applied to the SFPs. In the six years since the accident, no major corrosion problems have yet arisen. However, continued efforts to increase plant stability are underway for the longterm goal of decommissioning.
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