Corrosion-Enhancing and Corrosion-Reducing Accessories in Bentonite Surrounding Copper-Shielded Containers for Nuclear Waste

Abstract

The standard design for the underground disposal of high-level nuclear waste consists of a carbon steel container that hosts the waste shielded by a 5-cm-thick layer of corrosion-resistant native copper (Cu°), which is surrounded by a 35-cm-thick bentonite buffer. The accessory minerals in the buffer strongly influence the corrosion depth of the Cu° shield. The reactive-transport modeling shows that a buffer containing calcium sulfates (gypsum or anhydrite) has a Cu° corrosion depth that is three to eight times greater than that without calcium sulfates, depending on the groundwater chemistry. The presence or absence of iron oxides (hematite, goethite, or lepidocrocite) has a minor effect according to reactive-transport modeling. However, mineral equilibrium thermodynamics implies that the presence of iron oxides significantly reduces corrosion depth. Such a model is supported by the genesis of natural native copper deposits: the hydrothermal fluids interacted with iron oxide while depositing native copper.