Synchrotron-Based Micro-CT Investigation of Oxic Corrosion of Copper-Coated Carbon Steel for Potential Use in a Deep Geological Repository for Used Nuclear Fuel

Thalia Standish,David Shoesmith,Dmitrij Zagidulin,James Noël,Sridhar Ramamurthy,Peter Keech

doi:10.3390/geosciences8100360

Thalia Standish, David Shoesmith + Show 4 more

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https://doi.org/10.3390/geosciences8100360

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Abstract

Within the multi-barrier system proposed for the permanent disposal of used nuclear fuel, the primary engineered barrier is the sealed metallic container. The present Canadian container design utilizes a carbon steel vessel coated with Cu for corrosion protection. In the event of a defect in the Cu coating that exposes the steel substrate, galvanically accelerated corrosion of steel is, in principle, possible. In this work, the progression of corrosion at a simulated through-coating defect in 3.0 mol/L NaCl solution containing dissolved O2 was monitored using electrochemical measurements and imaged non-destructively using synchrotron X-ray micro computed tomography (micro-CT). The damage volume at the base of the simulated defect was measured from the 3D micro-CT data and used to calculate the amount of O2 used to drive steel corrosion. The results demonstrate that the availability of O2 determines the rate and overall extent of corrosion, while the coatings produced using different deposition and treatment methods (cold spray deposition, heat-treated cold spray deposition, electrodeposition) lead to different corrosion propagation geometries, with the distribution of damage depending on the quality of the Cu/steel interface.

Highlights

The proposed long-term management method for used nuclear fuel in Canada is disposal in a deep geological repository (DGR), which has multiple barriers in place to ensure safe isolation and containment
These processes, which have been described in more detail elsewhere [1], include corrosion by residual groundwater oxidants, radiolytic corrosion, corrosion by remotely produced sulphide, and galvanic corrosion of carbon steel coupled to Cu
In the presence of O2, steady-state galvanic corrosion conditions are eventually reached after transitioning through the formation and subsequent corrosion conditions are eventually reached after transitioning through the formation and breakdown of partially protective oxide films or corrosion product deposits

Summary

Introduction

The proposed long-term management method for used nuclear fuel in Canada is disposal in a deep geological repository (DGR), which has multiple barriers in place to ensure safe isolation and containment Within this proposed multi-barrier system, the key engineered barrier is the robust used fuel container, which consists of a carbon steel inner vessel coated with ~3 mm of Cu, applied via cold spray deposition and electrodeposition. The conditions in a DGR will evolve over time, beginning as warm, dry and oxic, and evolving to cool, wet and anoxic The transitions among these conditions could take anywhere from a few months to many hundreds or thousands of years, and during these phases there are a number of corrosion processes that could occur.

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