Assessment of Crevice Corrosion and Hydrogen Induced Stress Corrosion Cracks of Ti-Pd Alloys for HLW Overpack In Deep Un
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Assessment of Crevice Corrosion and Hydrogen Induced Stress Corrosion Cracks of Ti-Pd Alloys for HLW Overpack In Deep Underground Water Environments Guen NAKAYAMA, Koichi MURAKAMI, and Masatsune AKASHI Research Laboratory, Ishikawajima-Harima Heavy Industries, Co., Ltd. ABSTRACT Crevice corrosion and hydrogen-induced stress corrosion cracking tendencies of titanium (Ti) alloys in deep underground water environments are investigated in the present paper. The crevice corrosion repassavation potential, ER,CREV of Ti-palladium (Pd) alloys was measured and compared with the free corrosion potential, ESP = +0.32 V vs. SHE. The minimum Pd content of the alloys for suppressing crevice corrosion in the envisaged most corrosive high-level radioactive waste (HLW) disposal environment, corresponding to the seawater originated underground water of 0.6 mol/L [NaCl] at 100 °C, is 0.05 %, Ti-Gr.17 alloy. The Pd content of 0.01 % is found to be sufficient for realizing satisfactory crevice corrosion resistance in transuranium (TRU) disposal environment of 0.6 mol/L [NaCl] at 80 °C. The hydrogen-induced stress corrosion cracking tendencies of the Ti alloys caused by the formation and cracking of hydride layers on alloy surfaces were assessed by galvanostatic and constant-load tests conducted in completely reducing environments. Test results indicate that Ti-Pd alloys are immune to stress-corrosion cracking in HLW underground disposal condition for 1000 years. Total electrical charge density in TRU waste overpacks for 60,000 years, estimated from analyses of thermal history in waste emplacement drifts, was found to be 7.5 MC/m2, suggesting the hydride layers formed were 30 µm thick and the crack depth was 15 µm on Ti-Gr.17. Thus, the depth of the stress-corrosion cracks is too small to be taken into engineering consideration. The above results suggest that Ti-Pd alloys with Pd content over 0.05 % for HLW and 0.01 % for TRU waste can be utilized as corrosion-resistant layers of waste-disposal containers of HLW and TRU waste. INTRODUCTION HLW is to be disposed of in underground emplacement drifts of 1000 meters deep after being encapsulated in metallic containers, or overpacks, with surrounding bentonite layers as buffer materials. The steady state deep underground water environment is essentially reducing, although the environment becomes acidic when oxygen is introduced into the environment through a drift shaft during charging of the overpacks to underground emplacement drifts. The origin of the underground water can be either the seawater or rainwater. The disposal overpacks are so placed in the underground drifts as not to be heated mainly by nuclear decay heating above 100 °C for the bentonite layers to be active at the drifts. The temperature is estimated to be reduced to 45 to 55 ℃ at the later stage of the emplacement because the decay heating is gradually diminishing. The emplacement drift floor for TRU waste is designed to be of concrete, and thus, the maximum temperature is restricted to be 80 °C. Furthermore, the
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