4 research outputs found
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Effects of heterogeneity on actinide diffusion rates in tuffaceous rock
The pore structure and mineralogy of Topopah Spring Tuff are heterogeneous on scales less than one cm. This heterogeneity creates spatial variation in transport rates for aqueous actinide species both on the scale of tenths of microns and the scale of mm. The volumetric distribution of fluid paths having very different tortuosity, and potentially differing surface mineralogy and sorptive properties, must be considered in order to provide realistic predictions of transport rates. In addition, size and speciation of actinides in solution must be characterized since coexisting species can diffuse at different rates through the porous material due to both filtration effects and differences in sorption onto exposed mineral surfaces. 11 refs., 8 figs., 2 tabs
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Actinide transport in Topopah Spring Tuff: Pore size, particle size, and diffusion
Diffusive transport rates for aqueous species in a porous medium are a function of sorption, molecular diffusion, and sample tortuosity. With heterogeneous natural samples, an understanding of the effect of multiple transport paths and sorption mechanisms is particularly important since a small amount of radioisotope traveling via a faster-than-anticipated transport path may invalidate the predictions of transport codes which assume average behavior. Static-diffusion experiments using aqueous {sup 238}U tracer in tuff indicated that U transport was faster in regions of greater porosity and that apparent diffusion coefficients depended on the scale (m or {mu}m) over which concentration gradients were measured in Topopah Spring Tuff. If a significant fraction of actinides in high-level waste are released to the environment in forms that do not sorb to the matrix, they may be similarly transported along fast paths in porous regions of the tuff. To test this, aqueous diffusion rates in tuff were measured for {sub 238}U and {sub 239}Pu leached from doped glass. Measured transport rates and patterns were consistent in both systems with a dual-porosity transported moeld. In addition, filtration or channelling of actinides associated with colloidal particles may significantly affect the radionuclide transport rate in Topopah Spring tuff. 9 refs., 7 figs
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Uranium transport in Topopah Spring tuff: An ion-microscope investigation
We investigated the effect of different methods of surface preparation on ion-microscope profiles of uranium concentration (added to the sample by diffusion from an aqueous solution) vs depth in a welded, devitrified, tuffaceous rock from Yucca Mountain. The concentration profiles were used to study transport of uranium in the tuff. Four wafers of rock were prepared from primary drill core material and finished by polishing with increasingly finer abrasive material. Final polishes were made with 400 grit SiC, 600 grit SiC, 0.3 um alumina, and 0.05 um alumina. The polished tuff wafers were exposed for eight hours to a solution of groundwater doped with 2 ppM 235-U. The wafers were then examined by SEM and the ion microscope was used to measure the lateral and depth distributions of 235-U and other isotopes in the wafer. No systematic correlation of the measured 235-U concentration- vs-depth profiles with the degree of surface finish was observed, indicating that the polishing does not affect the measurable transport of U in the tuff. A zone of enhanced 235-U concentration was observed in the upper few microns, which we attribute to sorption onto surfaces of exposed pores. Concentrations of 235-U were elevated above background to depths >15 um, indicating that rapid transport paths exist. When the uranium distribution near the surface of the wafer was modelled by an error function, an upper limit for a slower transport path was defined by an apparent diffusion coefficient of approximately 10{sup {minus}13} cm{sup 2}/s. 5 refs., 5 figs., 1 tab