Articles techniques et présentations

The disposal of highly radioactive waste in geological environment is not the possibility studies anymore as the countries such as Finland, Sweden, France with more advanced radwaste disposal programs are actually implementing it nowadays. In such disposal system, passive safety does not rely on the geological environment only, but on engineered barriers also. Concrete is among the materials often considered for engineered barriers. In this study the following hydro-mechanical-chemical (HMC) processes in concrete were considered: advective water flow including relative permeability’s and capillary pressure depending upon liquid saturation; advective/dispersive and diffusive transport of dissolved CO2 and in gaseous phase (respectively); aqueous complexation, advective/dispersive transport of dissolved species (solutes); dissolution/precipitation of minerals; poroelasticity and evolution of material stiffness due to chemical damage. Numerical HMC model of concrete was implemented in COMSOL Multiphysics and coupled to PHREEQC via iCP interface developed by Amphos21 (Spain) (Nardi et al, 2014). For modelling of concrete drying-wetting and mass transport under repository conditions (excavation phase, ventilation phase, post closure phase), COMSOL Multiphysics flexibility to solve predefined and user defined partial differential equations was effectively employed. Developed 2D numerical model considered the disposal tunnel of 9.7 m diameter at the depth of 520 m, the 0.5 thick concrete liner, surrounding excavation disturbed zone and part of clayey host rock. ½ of the liner, EDZ and the host rock was modelled due to symmetry. Modelling such a complex system is high computational resources demanding task; the simulations were done up to 10 000 years. Modelling results showed that during ventilation phase the concrete and surrounding EDZ will become unsaturated. When the EDZ was re-saturated again (after ventilation is finished), the pH value at the concrete-EDZ interface did not return to the initial pH value of COx porewater (~7). Within certain distance from the liner-EDZ interface the higher pH values were observed in the EDZ. Within simulation time (10 000 years) no significant pH decrease in the concrete (to 11 and lower) was observed. Thus, it should not impose the chemical degradation of rebars in the concrete and the mechanical strength of the structure. Relating the mechanical properties of concrete (Young modulus) to calcium concentration in solid skeleton, the changes of elasticity modulus would be expected within limited extent (<20 cm) from the liner external boundary. Concrete material weakening in terms of decreasing Young modulus was observed within 5 cm from liner-EDZ interface by the end of simulation (10 000 yr). Transport of solutes and potential leaching of calcium from the concrete could be affected not only by porosity change induced by mineral precipitation/dissolution. Impact of mechanical strains on diffusivity and permeability upscaled from smaller scale to safety structure scale are expected to enhance the current model formulation. The developed HMC model could help to predict the design lifetime of concrete structures and transport of radionuclides, considering their sorption and speciation depending on prevailing chemical conditions (pH, redox, etc.). Analysis of these processes was a part of LEI activities in EC programme EURAD Work package MAGIC (Grant Number 847593).