Civil Engineering ETDs

Publication Date

7-1-2016

Abstract

Granular salt is likely to be used as backfill material and a seal system component within geologic salt formations serving as a repository for long-term isolation of nuclear waste. Pressure from closure of the surrounding salt formation will promote consolidation of granular salt, eventually resulting in properties comparable to native salt. Understanding the consolidation processes dependence on stress state, moisture availability, and temperature is important for demonstrating sealing functions and long-term repository performance. This study includes the characterization of laboratory-consolidated salt by means of microstructural observations, measurement of physical properties related to the pore structure, and quantification of pore sizes areas under differing conditions. Samples for this study were obtained from mine-run granular salt from the Waste Isolation Pilot Plant (WIPP) and Avery Island which were consolidated hydrostatically with varying conditions of stress up to 38 MPa, temperatures up to 250C, and moisture additions of 1%. Porosities achieved from consolidation ranged between 0.01 and 0.22. Microstructural observations using optical and scanning electron (SEM) microscopes were made to provide direct insight into deformation mechanisms during consolidation. Porosity, specific surface area, permeability, and tortuosity factor were quantified through multiple techniques including point counting, petrographic image analysis (PIA), porosimetry, and steady-state gas permeametry. Pore area distributions categorized into micropores (<1000 m2) and macropores (>1000 m2) were developed from Back-Scattered Electrons (BSE) SEM images analyzed in Fiji. Overall, the addition of moisture produces a higher degree of cohesion among grains, lower permeabilities and porosities as well as higher specific surface areas and lower macropore frequency at higher temperatures. A higher stress was also seen to lower porosity, increase specific surface area, and lower the frequency of micropores. Higher temperature samples experienced low porosities, more grain boundary cohesion, and, in WIPP samples, a higher frequency of macropores in the range from 1000 to 2500 m2. From microstructural observations, samples with 1% added moisture or those which were unvented during consolidation demonstrated clear pressure solution processes with tightly cohered grain boundaries and areas of occluded fluid pore spaces. Samples consolidated without additional moisture exhibited mainly cataclastic and plastic deformation. Recrystallization was also observed in samples consolidated at temperatures of 90C with added moisture and 250C. Porosities obtained from methods that measured both total and connected porosity were similar, suggesting a connected pore network within samples. From image analysis, a general trend of increase in specific surface area with a decrease in porosity was observed. Permeability values decreased with decreasing porosity and are comparable to permeability-porosity relationships for rock salt published by others. The tortuosity factor was calculated from the Carman-Kozeny model, which incorporates permeability, porosity, and specific surface area, and generally increased with decreasing porosity. Pore area analysis reveals porosities consisting predominately of macropores and minor changes in pore area frequencies with respect to consolidation conditions. It is well known that stress, temperature, and moisture affect the behavior of salt consolidation, but complete studies on deformation mechanisms and the evolving pore structure over a large range of conditions is not abundant. Information provided here enhances the current understanding of granular salt consolidation by offering direct insight into micro-mechanic processes and transformation of pore structure components.

Keywords

Granular Salt Consolidation, Nuclear Waste Repository Research, Microscopic Observations of Deformation Mechanisms, Pore Structure Analysis

Sponsors

Sandia National Laboratories, Department of Energy- Nuclear Energy University Programs

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Advisor

Stormont, John

First Committee Member (Chair)

Hansen, Frank

Second Committee Member

Thomson, Bruce

Third Committee Member

Bauer, Stephen

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