THE CORROSION OF MORE THAN 176 MILLION YEAR OLD NATIVE COPPER PLATES FROM A DEPOSIT IN MUDSTONE IN SOUTH DEVON; UNITED KINGDOM

摘要

Copper is the selected material for the corrosion barrier in the canister for encapsulation of spent nuclear fuel from the Swedish power reactors. These canisters will be buried in a deep geological repository in granitic rock at a depth of 400 to 700 m. At these depths, the groundwater will be reducing and copper will be immune to corrosion in the absence of dissolved sulphides. Under these conditions the spent fuel canisters are expected to meet with a very high margin SKB's design goal of 100 000 years. Extrapolations of short-term experimental data to such long times will always have some degree of uncertainty. Therefore, natural analogues are sometimes used to bridge the gap between laboratory time scales and the very long times for which a repository is designed. In the present paper, we present an analysis of the corrosion of native copper plates that have survived in a water-saturated clay environment for more than 176 million years. Although the native copper is affected by corrosion, the study shows that a significant proportion (30-80 percent of the original thickness) of the copper sheets is preserved in the saturated compacted clay environment of the Littleham Mudstone. Apart from the recent weathering effects due to exposure at outcrop, petrographical studies demonstrate that most of the observed corrosion and alteration of the native copper is geologically old (i.e. predating the main sediment compaction) and also occurred before the end of the Lower Jurassic. This demonstrates that the native copper can remain stable in a saturated and compacted clay environment for geological timescales well in excess of the timescales considered for performance assessment of a deep geologic repository for spent nuclear fuel.