Using a generalised Power Law for simulating the feedback effect of dissolution/precipitation on diffusive transfer in ToughReact

André Burnol (1) , Francis Claret (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

In the case of variable porosity calculations (due to precipitation/dissolution in a calculation coupled with chemistry), the permeability and diffusion coefficients in different porous media vary with porosity. In the case of nuclear waste storage, solute transport is supposed to be diffusion-controlled because of the extremely low permeability of surrounding materials, like concrete or clay. Various experiments and also different reactive transport modeling show a possible clogging due to the alkaline perturbation at the concrete/bentonite or concrete/clay interfaces (Gaucher et al., 2004; Burnol et al. 2006; Trotignon et al., 2007; De Windt et al., 2008). The decrease of porosity will therefore impact the extent of diffusion of all chemical elements, including the radionuclides, through the engineered and geological barriers. Taking into account the feedback effect of porosity change due to the chemical reactivity is therefore an important issue to simulate the migration of radionuclides out of the disposal drift. The chemical conditions found in deep nuclear waste storage rise some concern about the migration of radionuclides at the interface between the engineered and geological barriers where the alkaline perturbation could cause a "clogging" and therefore could impact the diffusion-controlled process itself.In this poster, a new general "Power Law" which connects the effective diffusion coefficient and the porosity evolution is described and tested. The conclusion is that if the version V1.0/V1.2 is used to study the diffusion of radionuclides in the surrounding zone of a waste nuclear disposal, the effective diffusion could be overestimated even with the Millington Law. A modified version of TouhgReact with diffusion harmonic weighting gave the same results as HYTEC code, both in the case of weak feedback(power=1/3) or strong feedback (power=2) in the new "Power Law". It should be noticed that the Power Law described in this poster is only verified for the aqueous phase but the same kind of law was developed for the gaseous phase in a two-phase system.

Mots clés: reactive transport; diffusion; harmonic mean; archie law; TOUGHREACT

Domaines: Chimie-Physique [physics.chem-ph]; Géochimie; Milieux et Changements globaux

André Burnol, Francis Claret. Using a generalised Power Law for simulating the feedback effect of dissolution/precipitation on diffusive transfer in ToughReact. TOUGH Symposium 2012, Sep 2012, Berkeley, United States. pp.Board 44. ⟨hal-00722057v2⟩