Fe-saponite growth on low-carbon and stainless steel in hydrothermal-bentonite experiments

Hydrothermal experiments on engineered barrier system (EBS) materials were conducted to characterize high temperature interactions between bentonite and candidate waste container steels (304SS, 316SS, low-C steel) for deep geological disposition of nuclear spent fuel. Hydrothermal experiments were performed using Dickson reaction cells at temperatures and pressure of up to 300 °C and 15-16 MPa, respectively, for four to six weeks. Wyoming bentonite was saturated with a 1900 ppm K-Ca-Na-Cl solution in combination with stainless and low-C steel coupons. Authigenic Fe-saponite precipitated utilizing steel as a growth substrate with Fe being supplied by steel corrosion. Concurrent with Fe-saponite formation, sulfides precipitated from sulfide-bearing fluids, from pyrite dissolution, near the steel interface. Sulfide mineral formation is dependent on the steel substrate composition: stainless steel produced pentlandite ((Ni, Fe)9S8) and millerite (NiS), whereas low C steel generated pyrrhotite (Fe7S8). The presence of sulfides suggests highly reduced environments at the steel-clay barrier interface potentially influencing overall steel corrosion rates and (re)passivation mechanisms. Results of this research show that nuclear waste steel container material may act as a substrate for mineral growth in response to corrosion during hydrothermal interactions with bentonite barriers.