Reactive transport of Ra-226 in the sandy facies of Opalinus Clay

Abstract:

The low hydraulic conductivity and the large retention capacity for many cations qualify Opalinus clay (OPA) as barrier material for nuclear waste deposits. Thus, OPA is proposed as host rock formation for a geological disposal facility for nuclear waste in Switzerland and will potentially also be considered in the started site selection procedure for a high-level radioactive waste repository in Germany.

To license a final repository for radioactive waste, a profound understanding of how these barriers evolve with time is needed. Knowledge on reactive transport processes of Ra in clay rocks is so far scarce. Therefore, in the context of the deep geological disposal of nuclear waste, the main objective of this study is to predict the reactive transport of Ra in different OPA sandy facies samples with the utilization of iCP version 1.6 (interface COMSOL-PHREEQC).

The reactive transport model is implemented in 1D and uses the composition of OPA samples characterized with an amount of carbonates and total clay minerals ranging between 13 wt. % – 35 wt. % and 5 wt. % – 25 wt. %, respectively. The model uses reactive transport parameters determined experimentally via diffusion and sorption experiments (e.g. distribution ratio, effective diffusion coefficient and accessible porosity). The uptake behaviour of Ra is taken into account via cation exchange and surface complexation reactions. The formation of solid solutions of Ra- carbonates and -sulphates are considered to demonstrate their relevance in the model for the reactive transport of Ra, especially for systems as the OPA sandy facies. The results of the simulation are compared to experimental data such as the diffusion profile of Ra in OPA after 100 days of in-diffusion, within the same conditions of the reactive transport model (boundary conditions, chemical/physical composition of OPA).