(Invited) Effect of Bentonite Dry Density on Corrosion of Embedded Copper
Authors: Tully Claire Samantha , Zagidulin Dmitrij , Binns Jeff , Noel James 
Published: December 2020

As per international consensus on the best practice for managing used nuclear fuel (UNF), the NWMO, plan to isolate and contain UNF within a multiple-barrier system, underground in a deep geological repository (DGR) [1]. In the proposed design, used fuel bundles will be sealed in copper-coated carbon steel used fuel containers (UFC), encased in blocks of highly compacted bentonite clay, and emplaced ~500 m below ground in the DGR. Any gaps between the rock walls and the bentonite blocks will then be filled with a bentonite gapfill material (GFM).

Due to its small pore size, high swelling pressures, and cation exchange properties [2], the bentonite will significantly limit the transport of active species to and from (in the case of a UFC failure) the UFCs. The high swelling pressure can suppress a potential UFC corrosion induced by sulphate reducing bacteria (SRB) by decreasing the clay pore space and lowering water activity [3]. Those properties of the bentonite clay are a function of several physical and chemical parameters. The goal of this work is to investigate the effect of bentontie dry density, presence of oxygen, and evolution of conditions on corrosion of copper in contact with bentonite, and on SRB viability, by varying water composition and GFM dry density.

In this work we are conducting a series of experiments in bentonite-filled modules exposed to ocean conditions (ocean modules, OM) and more controlled laboratory-based conditions (pressure vessels, PC). The OM are porous vessels containing copper coupons embedded in GFM that has been compacted to various dry densities. The modules are then placed in the Pacific Ocean at up to 2.6 km depth for months-to-years at a time where they are exposed to seawater with hydrostatic pressures representative of potential DGR pressures. PC experiments contain copper coupons embedded in GFM are pressurized to 100 bar with Type-1 water for varying durations up to 1 year. After exposure, copper coupons are analyzed by SEM, XPS, FIB, and AES. The change in topography is evaluated using CLSM, the water activity of the bentonite assessed, and 16s RNA analysis used to determine the presence of SRB.

Two OMs, both with cold-spray (Cucs) and wrought (Cuw) copper coupons, with GFM (dry density of 1.25 or 1.45 g/cm3) were exposed to seawater at 90 meters deep for 6 months. Analysis found the copper surface, post-experiment, to be non-uniformly corroded, possibly due to non-uniform wetting, and subsequently swelling, of the clay. The surface corrosion products consisted of Cu2O (~85 %at), Cu2S (~ 5 %at), CuCl (~3 %at), and CuO (~ 3 %at). Cuw had higher amounts of Cu2O and lower amounts of Cu2S at both GFM dry densities. Both the Cuw and Cucs exposed to the higher density GFM had lower amounts of Cu2S and higher amounts of Cu2O on the surface. This may be the result of non-uniform swelling, decresed mobility of the sulphide or a decrease in the number of metabolically active SRBs, or a combination of those factors [3]. Mass loss measurements, determined that Cucs had a higher corrosion rate than Cuw; however, Cuw corrosion rates were less affected by the GFM dry density variation. Overall, the increase in bentonite dry density resulted in a decrease in copper corrosion rates, with the Cucs corrosion rate being decreased more significantly than that of Cuw. Profilometry measurements of the surface, performed after removal of the corrosion products, showed a decrease in surface roughness on Cucs as the GFM dry density was increased, but the surface roughness of Cuw was not significantly affected.

The experiments performed in PCs for 1 month duration, found similar results. Cross-sections of the film formed at the copper surface revealed the presence of copper, oxygen, and sulphur, unevenly distributed across the copper surface, accounting for the non-uniform corrosion observed. The corrosion rates were significantly higher than those in the OM experiments, due to higher oxygen concentration. Profilometry measurements concluded the Cucs and Cuw surfaces had lower roughness than those in the OM experiments.

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