Research and Development on Geological Disposal of High-Level Radioactive Waste

Estimation of Long-Term Metallic Corrosion Using Archaeological Sample–Application of Analogize Phenomena in Nature on GeologicalIsolation Research–

Iron platecross section

moundCorrosion depth (mm)

2-3

adzes
Laboratory
experiment

Predicted
carbon steel
corrosion after
1000years(31.8mm)
JNC(2000)
Iron plate

Extrapolated line based on
experimental data

Time (year)

Fig.2-5 Condition of iron plate buried for 1500 years andits excavation site

Fig.2-6 Conservatism of prediction of corrosion after
1000 years based on data of excavated artifacts

Carbon steel material being considered for use as anengineered barrier system (overpack) is expected to keep its
strength for more than 1000 years. We estimated maximum
corrosion depth of the overpack as 31.8 mm for 1000 years in
the case of vertical setting in rock. For the purpose to obtainsome data as second evidence for the long stability estimation,
we investigated corrosion phenomena for iron material ofarchaeological sample which buried for more 1000 years andtheir corroded environmental condition.
Two adzes buried in clay for 750 years were dug out fromthe Izumo-Oyashiro-keidai ruins (Taisha-machi, Shimane).
One of candidate metal material of the overpack is carbon steel
which will be surrounded by clay in a proposed concept ofJapanese high-level waste disposal. The environment of these
excavated samples is similar to the proposed Japanese disposalenvironment.
Yamato 6th tumulus is famous as a ruin from which 872
iron plate artifacts were excavated. We examined the corrosion
depth of these samples using X-ray CT. Although they wereburied for 1500 years, they had metallic brightness in theircross sections (Fig.2-5).

Corrosion data for these artifacts such as thickness anddensity of the rust were analyzed non-destructively using ahigh-power X-ray computer tomography system, becauseof their historical importance. The density distribution wasobtained from the X-ray CT image, and thickness of the rust
was determined by the density difference between the base ironmaterial (ρ=8.0) and their rust (magnetite:ρ=4.0). In Fig.2-6,
the corrosion data of the artifacts is shown together with datafrom laboratory experiments with corrosion periods of severalyears. The red square symbol ■ is carbon steel corrosion
depth, 31.8 mm, after 1000 years predicted by extrapolation of
the laboratory data (red line). Two green lines extrapolations
based on experimental corrosion data under slightly oxidizingor reducing conditions. The upper line corresponds to a 0.001
mm/y corrosion rate. It is clear from these results that thepredicted corrosion data is conservative.
We will continue to obtain corrosion data of iron artifacts
as secondary evidence for the long term stability estimation ofcarbon steel and work to develop a high reliability estimation
method.

ReferenceYoshikawa, H. et al., Application of Archaeological Analogues for a Repository Safety Case : Arguments Supporting the Waste Container Lifetime, Safety
Cases for Deep Geological Diposal of Radioactive Waset : Where Do We Stand ? , Symposium Proceedings, Paris, France, 2007, OECD-NEA, 2008,
p.365-371.

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JAEA R&D Review 2008