Effect of heat on CO2 (g) generation from Boom Clay: Experimental and modeling assessments

The Belgian radioactive waste management program considers a geological disposal of vitrified high-level and spent fuel waste in clay formations as an option. The decay of the vitrified high-level and spent fuel radioactive waste will produce heat so the temperature of the clay as a geological barrier will increase. The Boom Clay, studied as one of the potential host rocks for geological disposal, contains relatively high amount of organic matter of low maturity. This organic matter will release CO2 (g) due to temperature increase, thus affecting the pore water composition and mineral equilibrium. This report presents experimental and geochemical modelling data in order to evaluate the effect of heat on the CO2 (g) generation from Boom Clay. The long-term batch experiments performed at T of 80C is of specific importance, because this temperature is considered as the maximum temperature limit reached at the interface between the concrete lining and clay due to presence of heat-emitting waste. Upon heating to 80°C, CO2 (g) yields reach a maximum of 40 mg/g TOC corresponding to pCO2 of 0.18 atm. The experimental data are in relatively good agreement with the mineral assemblage model, which predicts hundredfold increase of pCO2 (g) from 0.0024 atm under undisturbed conditions to 0.24 atm at 80C. The mass balance calculations suggest a partitioning of the Corg released by thermal decomposition of kerogen between the gas phase, the dissolved organic carbon in the liquid phase and inorganic carbon in the solid phase. As such the results of the mass balance calculations confirm the role of carbonates in the buffering of the thermally produced CO2 (g). Upon heating, the stable isotopes data (13C and 18O) further support the organic origin of the CO2 (g) and its ultimate transformation to inorganic carbon in the solid phase. A mineral assemblage model considering the coexistence of minerals calcite, dolomite, kaolinite, quartz, and chlorite explains well the experimentally measured pH and pCO2 (g). As combination of modelling and experimental results suggests that the CO2 (g) produced by thermal decomposition of the Boom Clay organic matter is buffered by the inorganic minerals present in Boom Clay, no net accumulation of CO2 (g) is expected.