REPOSITORY CANISTER STEEL CORROSION – INTERFACE MINERALOGY. EFFECTSOF WALL ROCK CHEMISTRY

In accordance with the U.S. Used Fuel Disposition campaign to evaluate various generic geological repositories for the disposal of spent nuclear fuel, our current engineered barrier system (EBS) research focuses on the effects of increased temperatures and pressures. This experimental work analyzed and characterized the canister corrosion and steel interface mineralogy of bentonite-based EBS 304 stainless steel (SS), 316 SS and low-carbon steel coupons in brine at higher heat loads and pressures. Experiments contrast EBS with and without an argillite wall rock.

Unprocessed bentonite from Colony, Wyoming was used as the clay buffer material and Opalinus Clay simulated the wall rock. Redox conditions were buffered at the magnetite-iron oxygen fugacity univariant curve. A K-Na-Ca-Cl-based brine was chosen to replicate generic granitic groundwater compositions, while Opalinous Clay groundwater was used in the wall rock series of experiments. The experiments were run at ~150 bar and 300°C, for 4 to 6 weeks. The two major experimental mixtures were 1) brine-bentonite clay- steel, and 2) brine-bentonite clay-Opalinus Clay- steel. Both systems were equilibrated at a high liquid/clay ratio. Mineralogy and aqueous geochemistry of each experiment were evaluated to monitor the reactions that took place.

The more notable clay mineral reactions occurred at the steel surfaces. Authigenic chlorite and Fe-saponite grew with their basal planes near perpendicular to the steel plate, forming a 10 – 100 μm thick ‘corrosion’ layer. Partial dissolution of the steel plates was likely the iron source for chlorite/saponite formation with steel plates acting as a substrate for chlorite/saponite growth. XRD and microprobe analyses of the silicate mantling on the low-carbon steel indicates the phase is a Fe saponite with a composition of (Na_0.09,Ca_0.03) (Fe_2.20Mg_0.12Al_0.86) (Al_0.58 Si_3.42)O₁₀(OH)₂. Stainless steel (304) is mantled by a chlorite/Fe saponite mixture. This phyllosilicate mix is high in Fe (33.99 wt% FeO), Cr (1.35 wt % Cr₂O₃) and Ni (1.34 wt % NiO).

No significant Fe-rich clay mineral alteration was apparent away from the steel-smectite clay interface. Results of this research show that the waste container may act as a substrate for mineral growth in corresponds to corrosion.