Grouted Waste Leach Tests: Pursuit of Mechanisms and Data for Long-Term Performance Assessment
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GROUTED WASTE LEACH TESTS: PURSUIT OF MECHANISMS AND DATA FOR LONG-TERM PERFORMANCE ASSESSMENT R. Jeff Serne* *Pacific Northwest Laboratory, P. 0. Box 999 K6-81, Richland, WA 99352 ABSTRACT At Hanford, low-level liquid nuclear waste is being mixed with cementitious materials (grout) to form leach-resistant solid waste. Prior to grouting each liquid waste, an assessment must be performed to evaluate the long-term environmental impact. These predictions rely upon a diffusioncontrolled release model and short-term laboratory leach data on small grout samples. This paper describes size scale-up and inventory scale-up experiments that evaluate whether diffusion does in fact control the release of contaminants. The results of the volume scale-up test suggest that tests on grout cylinders between the sizes 3 cm dia. by 3 cm length and 30 cm dia. by 29 cm length yield comparable results. These data and other available literature suggest that extrapolation of leach results to large blocks of solidified waste is defensible. The inventory scale-up tests for I and selenium show a constant effective diffusivity regardless of the original inventory. This suggests that diffusion processes do in fact control the release of iodide and selenium from grout. The 'Tc effective diffusivities may not remain constant with changes in inventory. INTRODUCTION In the 1990s, disposal of the low-level radioactive portion of liquid wastes stored in double-shell tanks at the Hanford Site is scheduled to begin. The disposal begins by mixing liquid wastes with cementitious groutforming solids. The resulting slurry will be pumped into near-surface concrete vaults. Prior to grouting, an assessment is performed to evaluate the long-term environmental impact. These predictions rely upon a diffusioncontrolled release model and short-term laboratory leach data on small grout samples. This paper describes the results of size scale-up and inventory scale-up experiments that evaluate whether leaching changes with waste form size and whether diffusion does in fact control the release of contaminants. The diffusion model chosen to analyze leach data suggests that the cumulative fraction leached in any test is related to the ratio of the sample geometric surface area to geometric volume (S/V). Various sizes of grout samples are leached to test this theory. If diffusion theory accurately describes the leaching of contaminants from grout, then changing the initial inventory of a contaminant in the waste form should have no effect on the calculated effective diffusion coefficient. On the other hand, if a contaminant's leach properties are controlled by its solubility, then (if equilibrium is reached rapidly) the observed leachate concentration will reach a constant value each test period. If a fixed concentration is observed, then increasing the inventory would cause the calculated effective diffusion coefficient to drop. Thus, changing the inventory should be a sensitive way to identify whether diffusion or solubility controls release. METHODS AND MATERIALS In b
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