Experimental and Modeling Indications for Self-Sealing of a Cementitious Low- and Intermediate-Level Waste Repository by Calcite Precipitation

Abstract

In the vicinity of a cementitious nuclear waste repository, mineral reactions will change the hydraulic conditions and the parameters describing radionuclide transport with time during the cement degradation phase. Porosity changes due to mineral and cement reactions will influence permeability and diffusivity. Formation water rich in CO2 will lead to calcite precipitation in the water-conducting zones surrounding the cementitious waste repository. This will have an impact on the radionuclide release from the cementitious repository into the host rock environment. The sequentially coupled flow, transport, and chemical reaction code MCOTAC is used to include such processes in the modeling. A porosity-permeability relation and a porosity-diffusivity relation are used for describing cement degradation and related secondary mineral precipitation and their coupling to reactive transport modeling. Two-dimensional model calculations are used to predict the temporal evolution of transport parameters for radionuclides within a “small-scale” near field of a cementitious waste repository. Reduced solute transport is calculated in the repository near field due to porosity and permeability changes at the rock-repository interface. Within the small-scale porous medium approach, coupling of chemical reactions and hydrodynamic parameters indicates a self-sealing barrier at the host rock–repository interface for several scenarios. This barrier might persist for very long times and effectively contain radionuclides within the engineered repository system. Taking into account flow path and barrier-specific heterogeneity will be a further step to improve the understanding of coupled processes in the vicinity of a real cementitious near field.