Numerical Simulation of Atmospheric Carbonation of Concrete Components in a Deep Geological Radwaste Disposal Site during Operating Period

Simulations of atmospheric carbonation of concrete intermediate-low level waste cell components were conducted to evaluate potential chemical degradations affecting these components during the operating period of a radioactive waste repository in a deep Callovo-Oxfordian clay layer. Two-phase liquid water-air flow is combined with gas components diffusion processes, leading to a progressive drying of the concrete and an array of chemical reactions affecting the cement paste. The carbonation process depends strongly on the progression of the drying front inside the concrete, which in turn is sensitive to the initial water saturation and to nonlinear effects associated with permeability and tortuosity phenomenological laws. Results obtained with a modified version of Tough-React-EOS4 to represent realistic tortuosity evolution of materials with clogging and saturation are presented and commented upon. Strong porosity clogging of the carbonated concrete is not observed in the simulations; slight porosity opening is in general predicted, except for high initial liquid saturation of the concrete, in which case a moderate porosity reduction is found. Carbonation depths on the order of 0.6 to 1.1*10-3 m.yr-1 are predicted for cementitious components. However, these values are probably overestimations both in depth and intensity of carbonation. The model of cement drying needs some revision to correctly weight diffusion control in the discretized representation of the cement/air boundary. Also, the kinetic model of mineral reactivity needs improvements with respect to the influence of liquid saturation on reaction rates, which are actually strongly decreased in dry materials, and with respect to the protective effect of secondary carbonates. (authors)