Corrosion Mitigation Activities Performed After the Fukushima Daiichi Accident

Abstract

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.

Ten years after the NPP accident at Fukushima : review on fuel debris behavior in contact with water

“…After decontaminating the water by the cesium adsorption device installed in the CRw/B, and further removing salts by reverse osmosis the obtained freshwater is recycled by injection into the PCVs as cooling water. In order to prevent corrosion of metallic materials in the PCV, nitrogen gas is blown into the freshwater and hydrazine is injected to avoid an oxidizing atmosphere [55]. On the other hand, waste water enriched with salts and radionuclides, which is generating in the reverse osmosis treatment is temporarily stored and then decontaminated except for 3 H by the installed Multiple Radionuclides Removal System (MRRS, or often called as Advanced Liquid Processing System; ALPS).…”

“…Already a year later, the quality of the water collected from the PCVs is similar to purified water, as shown in Table 5, though 137 Cs was detected, which was dissolved into the injected water [55]. And, the concentrations of major elements in seawater such as Na, Mg, and Ca were below the detection limits in the accumulated water in PCVs [57].…”