Experimental and Theoretical Analyses of Small-Scale Radionuclide Migration Field Experiments
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EXPERIMENTAL AND THEORETICAL ANALYSES OF SMALL-SCALE RADIONUCLIDE MIGRATION FIELD EXPERIMENTS
K. L. ERICKSON AND D. R. FORTNEY Sandia National Laboratories, Albuquerque,
New Mexico,
87185,
U.S.A.
ABSTRACT Analyses have been completed which provide guidance for conducting radionuclide migration field experiments. Characterization of nonwelded tuffs and laboratory experiments defining dominant chemical phenomena were used to develop a model for describing migration in fractured porous rock. Criteria for obtaining optimum experimental conditions were developed in terms of the key variables dominating migration in a given rock type, namely the fracture aperture, distribution coefficient, and average fluid velocity. For simple dissolved species, which are reversibly sorbed, variations in fracture aperture and fluid velocity affect experiment results much more than variations in distribution coefficient. Therefore, the experiment should be designed to optimize hydrogeologic conditions rather than sorption properties. INTRODUCTION Los Alamos, Sandia, and Argonne National Laboratories will cooperatively conduct small-scale, radionuclide migration field experiments in G-tunnel at the Nevada Test Site as part of the Nevada Nuclear Waste Storage Investigations project. This paper discusses results from preliminary laboratory experiments and radionuclide transport analyses, conducted to provide guidance for obtaining optimum operating conditions, selecting sites, and designing equipment for such field experiments. PRELIMINARY EXPERIMENTAL RESULTS The nonwelded tuff in G-tunnel is a high porosity (0.2-0.4), high surface 2 area (35-40 m /gm) rock having a complex pore-size distribution. Sorption equilibria for various radionuclides have been investigated using samples of crushed tuff in batch equilibration experiments. Sorption rates also have been investigated using batch experiments in which rock tablets were contacted with well-mixed solutions. For nuclides which usually exist as simple ions in solution, such as 22Na, 137Cs, and 85Sr, available data indicate that sorption rates from well-mixed solutions are limited by molecular diffusion through the pore water in the rock and simultaneous sorption by the solid phases. It also appears that the pore water and solid phases can be treated as a quasi-homogeneous medium for which sorption equilibrium between solution and solid phases in the bulk rock and at the interface between rock and external solution is a reasonable approximation. Initial experiments indicate similar phenomena dominate sorption from solutions moving slowly through narrow joints. Finally, the porosity of the tuff appears sufficiently connected that the rock bounding a joint can be considered semi-infinite in extent in small-scale experiments [1].
372 EXPERIMENT MODEL Based on the above results, the following idealized joint and rock mass, (1) an aqueous solution in steady-state, laminar shown in Fig. 1 is defined: flow in the z-direction is contained by a joint of constant rectangular cross se
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