Publications

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For the interim storage of used nuclear fuel, the storage casks/containers will be exposed to conditions under which considerable dust and/or atmospheric aerosols may be deposited on the surface. These dust layers may contain a sizeable portion of water soluble salts, particularly in marine environments where many interim storage systems are located. These soluble salts will deliquesce if sufficient moisture is present, resulting in the formation of potentially corrosive brine on the material surface. Experimental results have illustrated that some stainless steels, such as 304SS (a common material of construction for interim storage containers) can and will undergo localized corrosion in elevated temperature conditions where a chloride rich brine has formed on the surface. Results presented here illustrate that it is possible that stifling of localized attack will result when limited reactant is present, but additional analysis is necessary before a definite conclusion can be made.

Once sufficiently cool, spent nuclear fuel is stored in dry storage cask systems, most commonly consisting of welded stainless steel containers enclosed in ventilated concrete or steel overpacks. As the United States does not currently have a viable disposal pathway for SNF, these containers may be required to perform their waste isolation function for many decades beyond their original design criteria. Failure by stress corrosion cracking due to deliquescence of deposited salt aerosols is a major concern. Parameters controlling deliquescence include the temperature and RH at the waste package surface, and the composition of deposited salts. The timing and duration of deliquescence under in situ conditions is poorly defined, because of uncertainties in thermal history, the large variability in temperatures over the storage container surface, and uncertainties in the composition of deposited salts. Storage installations in near-marine environments are of greatest concern because of exposure to significant quantities of chloride-rich sea salt aerosols. Published stainless steel corrosion studies with sea salt and sea salt components suggest that conditions conducive to localized corrosion initiation and propagation may exist on the surface of SNF storage containers in such environments at some point in their extended service life, and furthermore, that stress corrosion cracking may occur over a broad range of potentially relevant conditions. However, the studies were carried out with heavy salt loads and limited gas flow, which may limit the beneficial effects of brine/atmosphere exchange (e.g., acid degassing, CO2 exchange, degassing and thermal decomposition of ammonium phases). Gas exchange with the atmosphere will modify brine pH and chloride content, and will modify the deliquescent salt assemblage through precipitation of Ca and Mg carbonates, potentially limiting brine volumes or resulting in dryout. Nitrate-rich inland salt aerosols are considered less corrosive, but may have higher levels of potentially reactive pollutants. Moreover, the compositions of inland salt deposits on hot storage containers may have greater uncertainty, as ammonium- and nitrate-rich salt assemblages are subject to thermal decomposition and potential reactions with organics. For both inland and near-marine sites, little information is available on the dust/salt deposition rates, or the quantity of salt present on existing storage container surfaces. A sampling program for in situ dust deposits on current storage containers will provide critical compositional data for new stress corrosion cracking studies, and will allow evaluation of the applicability of existing studies of stainless steel stress corrosion cracking under conditions of dust deliquescence.

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In the supercritical CO2-water-mineral systems relevant to subsurface CO2 sequestration, interfacial processes at the supercritical fluid-mineral interface will strongly affect core- and reservoir-scale hydrologic properties. Experimental and theoretical studies have shown that water films will form on mineral surfaces in supercritical CO2, but will be thinner than those that form in vadose zone environments at any given matric potential. The theoretical model presented here allows assessment of water saturation as a function of matric potential, a critical step for evaluating relative permeabilities the CO2 sequestration environment. The experimental water adsorption studies, using Quartz Crystal Microbalance and Fourier Transform Infrared Spectroscopy methods, confirm the major conclusions of the adsorption/condensation model. Additional data provided by the FTIR study is that CO2 intercalation into clays, if it occurs, does not involve carbonate or bicarbonate formation, or significant restriction of CO2 mobility. We have shown that the water film that forms in supercritical CO2 is reactive with common rock-forming minerals, including albite, orthoclase, labradorite, and muscovite. The experimental data indicate that reactivity is a function of water film thickness; at an activity of water of 0.9, the greatest extent of reaction in scCO2 occurred in areas (step edges, surface pits) where capillary condensation thickened the water films. This suggests that dissolution/precipitation reactions may occur preferentially in small pores and pore throats, where it may have a disproportionately large effect on rock hydrologic properties. Finally, a theoretical model is presented here that describes the formation and movement of CO2 ganglia in porous media, allowing assessment of the effect of pore size and structural heterogeneity on capillary trapping efficiency. The model results also suggest possible engineering approaches for optimizing trapping capacity and for monitoring ganglion formation in the subsurface.

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