Thermodynamic Framework for Analysis of Waste Containment Barrier Materials

Abstract

In order to analyze the fate and estimate the transport rates of contaminants through a barrier system, textural parameters such as the specific surface, density, permeability, diffusion coefficient, and flow path tortuosity are usually measured or estimated. The magnitudes of transport parameters of barrier systems are expected to change in response to physicochemical reactions and other environmental stresses, the intensities of which may grow or wane over time. In essence, when discrete catastrophic events (for example, earthquakes) are discounted, the flaws that develop are macroscopic manifestations of microlevel processes. Processes such as crystallization and precipitation add solid material to pore spaces in barriers and can improve barrier performance. Conversely, processes that cause changes in state from solid to liquid (for example, material dissolution) degrade barriers through the creation of larger flow channels. An appreciation of the thermodynamics of contaminant/barrier interactions under various environmental (temperature, pressure, and moisture) conditions is a prerequisite for establishing the bounds for textural changes and estimating contaminant release rates from containment systems. Then, process kinetics can be used to estimate the rate at which such texture-controlling processes may occur. The alternative approach is to conduct numerous “test-and-see” factorial experiments of limited utility, in which one parameter is changed at a time. The latter approach consumes resources excessively, relative to an approach that involves the use of thermodynamics to minimize the number of tests. In this paper, long-term deterioration mechanisms are analyzed, and a framework for their assessment within the context of barrier system performance modeling is presented.