Aspects of Modeling of the Degradation of Engineered Safety Barriers Based on Portland Cement in Radionuclide Migration Problems

Abstract

Materials based on Portland cement and concrete are widely used for radioactive waste solidification and construction of engineered safety barriers at radioactive waste repositories. As compared to other candidate matrices, cement also exhibits the potential for chemical immobilization by reacting with many radionuclides and binding them. The water permeation through the bulk of concrete leads to its degradation, to a change in the composition of its pore water, and to transformations of solid mineral phases constituting the concrete. The long-term degradation of concrete can be estimated using geochemical modeling. A procedure for forecasting the degradation of materials based on Portland cement concrete is described. The model takes into account the stages of cement hydration and chemical degradation. The geochemical changes in the course of concrete degradation were calculated using numerical modeling with the PhreeqC code. Schemes of the stepwise concrete degradation are considered for estimating the radionuclide transport. Relevant radionuclide distribution coefficients are applied to each stage. The duration of concrete degradation stages largely depends on the filtering water composition.