Corrosion studies on selected packaging materials for disposal of heat-generating radioactive wastes in rock-salt formations. Final report 1991-1994

In previous corrosion studies, carbon steels and the alloy Ti 99.8-Pd were identified as promising materials for heat-generating nuclear waste containers that could act as a barrier for immobilization of radionuclides in a rock-salt repository. For this reason, these materials are subject to more detailed investigations. In the present study, the long-term corrosion behaviour of three preselected carbon steels has been investigated in the liquid and vapor phase of disposal relevant brines at 150 C-170 C without radiation and in the presence of a gamma radiation field. Stress corrosion cracking studies (SCC) were also performed on the steels in an MgCl2-rich brine at 25 C-170 C and slow strain rates of 10-4-10-7 s-1. In addition to these laboratory-scale experiments, long-term in-situ experiments on Fe-base alloys, Ti 99.8-Pd and Hastelloy C4 were performed in the Asse salt mine. Both metal sheets and tubes of these materials with selected container manufacturing characteristics (e.g. sealing technique, corrosion protection of steel with either Ti 99.8-Pd or Hastelloy C4) were examined in rock salt and rock salt plus brine at 32 C-200 C. Both in the liquid and in the vapor phase of the brines the steels investigated (unalloyed TStE 355 steel, low-alloyed TStE 460 and 15MnNi 6.3 steels) are resistant to pitting corrosion. The liquid-phase corrosion rates (36-71 μm/a in NaCl-rich brine, 65-203 μm/a in MgCl2-rich brines) are significantly higher than the vapor-phase corrosion rate (10 μm/a), but they imply acceptable corrosion allowances for thick-walled containers. The gamma dose rate of 10 Gy/h and the submerged arc welding (SAW) do not accelerate the corrosion rates of the steels in NaCl-rich brine. In MgCl2-rich brines, the corrosion rates in the irradiated environment are a factor of about 1.5 higher than in the unirradiated system. (orig./MM)