In the vicinity of a high-level waste repository, corrosion of carbon steel
overpacks will create a reducing environment. Reducing conditions are
expected to retard the migration of redox-sensitive radionuclides such as
technetium.
The apparent diffusion coefficients of technetium were measured in compacted
bentonites (Kunigel VI® and Kunipia F®, JAPAN) in contact with carbon steel
and its corrosion products under reducing conditions or without carbon steel
under oxidizing conditions for comparison. The apparent diffusion
coefficients measured were 10-12 to 10-13
m2/s under oxidizing conditions and 10-12 to
10-13 m2/s under reducing conditions. There were
significant effects of redox condition, dry density (0.2 to 2.3 g /
cm3) and montmorillonite content (50% for Kunigel VI or 100%
for Kunipia F) on the apparent diffusion coefficients. Montmorillonite
density could be a good index to explain density dependence of the diffusion
coefficients under both reducing and oxidizing conditions.