Corrosion Rates of Zircaloy-4 by Hydrogen Measurement under High pH, Low Oxygen and Low Temperature Conditions
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Corrosion Rates of Zircaloy-4 by Hydrogen Measurement under High pH, Low Oxygen and Low Temperature Conditions Tomofumi SAKURAGI, Hideaki MIYAKAWA, Tsutomu NISHIMURA1 and Tsuyoshi TATEISHI2 Repository Engineering and EBS Technology Research Project, Radioactive Waste Management Funding and Research Center, 1-15-7 Tsukishima, Chuo City, Tokyo 104-0052, Japan 1 Kobe Steel, Ltd., 4-7-2 Iwaya-Nakamachi, Nada-ku, Kobe 657-0845, Japan 2 Kobelco Research Institute, Inc., 1-5-5 Takatsukadai, Nishi-ku, Kobe 657-2271, Japan ABSTRACT Corrosion tests of Zircaloy-4 were performed in a dilute NaOH solution (pH =12.5) at 303 K for 90 days using the gas flow system (oxygen; < 1 ppb) and a batch method (oxygen; < 0.1 ppm). The corrosion rate was determined by measuring gaseous hydrogen and the hydrogen absorbed into Zircaloy-4 assuming the following reaction: (2x+1)Zr + 2H2O o ZrO2 + 2xZrH2 +2 (1-x)H2n where x represents the Zircaloy-4 hydrogen absorption ratio. The initial hydrogen content in the Zircaloy-4 specimen was controlled to be below 10 ppm. The corrosion rate decreased with time (90-day values: 2.46u10-3 and 2.37u10-3 Pm/y for the gas flow method and 6.72u10-2 Pm/y for the batch test). The Zircaloy-4 hydrogen absorption ratio during corrosion was over 90%. The large amount of hydrogen absorbed in Zircaloy-4 will play an important role in the long-term safety for the disposal of irradiated Zircaloy materials. INTRODUCTION Corrosion behavior is a key issue in assessing the disposal performance assessment of activated wastes such as spent fuel assembles, i.e. hulls and endpieces, because corrosion is expected to be the source of radionuclides (e.g. C-14) leaching from such wastes. The second progress report on research and development for TRU waste disposal in Japan [1] (TRU-2 report), projected that the hulls and endpieces will be disposed in a deep underground repository that is expected to be anticorrosive for metals due to the low-oxygen, high-alkaline (influence of cement materials) and the low temperature. Although numerous studies were performed at high temperatures and extrapolating their data to the corrosion rate in a low temperature was reported [2], the sufficiency of the extrapolation to the corrosion behavior in the disposal environment has not yet been investigated. Several studies have covered attempts to obtain the corrosion rate of metals in repository environments [3 - 6]. Hansson [3] attempted electrochemical acquisition of the Zircaloy corrosion rate in an alkaline and anoxic environment, and estimated the corrosion rate to be less than 3u10-2 Pm/y, based on the detection limit of the corrosion current. Franker and Harris [4] also studied Zircaloy corrosion electrochemically, in temperature conditions between 295 K and 368 K. From the cyclic polarization curve, they concluded that the corrosion rate is low or negligible due to passivity. Kreis [5] pointed out that electrochemical study is not well suited and alternatively measured the amount of hydrogen gas that evolved from corrosion of iron wires und
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