Bentonite evolution at elevated pressures and temperatures: An experimental study for generic nuclear repository designs

Abstract

Geologic disposal of spent nuclear fuel in high-capacity metal canisters may reduce the repository footprint, but it may yield high-thermal loads (up to 300 °C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally heated pressure vessel at 150-160 bars and temperatures up to 300 °C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Cl-bearing water (~1900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K + , and Ca 2+ concentrations decreased, whereas SiO 2(aq) , Na + , and SO 4 2-concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210 °C, generating H 2 S (aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixed-layer formation was significantly retarded in the closed system due to a limited K + supply along with high Na + and SiO 2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six weeks, 300 °C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al 3+ , thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~17% volume decrease (quartz formation) that translates into ~2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers' lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.

International Collaboration Activities in Different Geologic Disposal Environments

“…These experiments are important in evaluating the potential effects of secondary minerals in bentonite. Such effects have been identified in bentonite hydrothermal experiments conducted by UFD (Cheshire et al, 2014), where degradation of secondary phases could yields marked effects on the altered mineral assemblage and solution chemistry.…”

“…• The effect of soluble or unstable phases in the buffer/backfill clay matrix: Current UFDC experimental activities on clay interactions have revealed the effects of minor phases on the high temperature degradation of barrier clay material (Cheshire et al, 2014). Benchmark 2 has provided a data set on the effect of gypsum dissolution embedded with clay.…”

International Collaborations on Engineered Barrier Systems: Brief Overview of SKB-EBS Activities.

“…These experiments are important in evaluating the potential effects of secondary minerals in bentonite. Such effects have been identified in bentonite hydrothermal experiments (Cheshire et al, 2014) where degradation of secondary phases could yields marked effects on the altered mineral assemblage and solution chemistry. Figure 11.…”

Generic Argillite/Shale Disposal Reference Case

“…Reviews of the performance of bentonite backfill at temperatures exceeding 100 o C (e.g., Wersin et al, 2007;Pusch et al, 2010), modeling Zheng et al, 2014a), and experiment studies (e.g. Pusch et al, 2003;Caporuscio et al, 2012;Cheshire et al, 2013;Cheshire et al, 2014) of the mechanical and chemical changes at temperature up to 300 o C showed little or moderate deterioration of bentonite. While further analyses of the THMC alteration of EBS bentonite and argillite at high temperatures are warranted, these modeling (e.g.…”

“…Zheng et al, 2014a) and experimental (e.g. Cheshire et al, 2014) studies in the UFDC are suggestive that an argillite repository with EBS bentonite could sustain temperatures higher than a 100 o C. Thermal analyses with the Disposal System Evaluation Framework (DSEF) are needed to determine the drift and waste package spacing under higher thermal loads, e.g., 150 o C.…”