Long term stability of iron for more than 1500 years indicated by archaeological samples from the Yamato 6th tumulus

One of the candidate materials for overpack in the Japanese engineered barrier system for high-level radioactive waste (HLW) is iron and therefore its long-term stability for at least 1000 years is very important for safety analysis of the repository system. Therefore, several of the iron artifacts excavated from the Yamato 6th tumulus (ancient tomb) in Nara prefecture were analyzed using X-ray computed tomography (CT) to determine corrosion depth. The samples analyzed, both of two large and 11 smaller iron artifacts are called ‘Tetsutei’. The thickness of each rust layer was measured from a cross-section image of the sample and the difference in material density between rust and iron was shown by the image density by the X-ray CT. In the case of pitting corrosion in the sample, the depth of the pits was measured directly and estimated as total corrosion depth with general corrosion layer. The corrosion depths are 0.5–2.1 mm. These data indicate conservative predictions for the extrapolations based on experimental studies. Such corrosion data from archaeological samples are useful in analogue studies of high-level radioactive waste disposal as evidence of long-term stability of a waste container.