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, E R,CREV of Ti-palladium (Pd) alloys was measured and compared with the free corrosion potential, E SP = +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/m 2 , 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.

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Metallic canisters placed in concrete casks and containing spent nuclear fuel elements, will be exposed to a moist oceanic atmosphere while the decay heat generated in the fuel elements cools for more than fifty years. Thus, the surface environment of the metallic canisters will be wet and covered with chloride compounds. The canisters may suffer atmospheric stress corrosion cracking. Therefore, corrosion tests for some potential alloys were conducted in an aqueous bittern solution, containing 22% enriched chloride compounds simulating the expected surface conditions of the canisters, to aid in selecting appropriate alloys based on corrosion resistance. The results suggest that the corrosion resistance of ordinary stainless steels, such as SUS304 and SUS316, is not high enough to avoid ASCC (atmospheric stress corrosion cracking) in the environment. Thus, a higher-grade stainless steel, namely, NSSC270 (20Cr-18Ni-6Mo-0.2N-Low C)or SUS836L (23.5Cr-25Ni-5.5Mo-0.2N-Low C, equivalent to AL-6XN) has been selected for this application.

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