Solubility and Leaching of Radionuclides in Site Decommissioning Management Plan (SDMP) Soil and Ponded Wastes (NUREG/CR-6821)
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Manuscript Completed: May 2003
Date Published: June 2003
A.R. Felmy, V.L. LeGore, and S.A. Hartley
Pacific Northwest National Laboratory
P.O. Box 999
Richland, Washington 99352
P.R. Reed, NRC Project Manager
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code W6409
Samples of disposed wastes from one former and two current U.S. Nuclear Regulatory Commission Site Decommissioning Management Plan Sites, labeled Sites D, E, and F, were studied to determine 1) the key radionuclides and their concentrations present in the waste, 2) the solubility limits and solubility limiting phases for these radionuclides, 3) the rate of attainment of solubility equilibrium, 4) the observed leaching rate for radionuclides that may or may not be solubility controlled, and 5) identify, if possible, the potential for radiocolloid formation. To achieve these goals, batch studies were conducted over ranges of solution pH values (2–12), solid to solution ratio, and particle size. In addition, flow-through column studies were conducted of selected samples to help verify the mechanisms and predictive relations identified in the batch experiments. The results show that the major radionuclides present in the wastes varied widely between the three sites.
At all three sites, comparisons of filtered and unfiltered analyses of samples from either flow-through columns or well waters did not show any evidence of radiocolloids. At Site F, certain well samples contained very high concentrations of iron oxide particles. However, these particles did not show any clear evidence of high radionuclide content.
Maximum dissolved concentrations (solubility limits) and radionuclide-leaching rates have been calculated for use in performance assessment calculations at all three sites. Observed Th concentrations at Site D were quite low, the maximum Th solubility was 3.2 x 10-8 M. U solubilities at Site D, 1.6 x 10-5 M, were the highest observed at any of the sites. The leaching rates at Site D for Th and U were calculated to be 3.1 x 10-9 g U/hr and 1.4 x 10-6 g U/hr, respectively. At Site E, the Th solubilities and leach rates were also quite low, 3.16 x 10-9 M, and 1.5 x 10-10 g Th/hr, respectively. The solubilities and leach rates for U at Site E were also quite low, 3.16 x 10-9 M and 1.6 x 10-10 g U/hr, respectively. At Site F, the principal radionuclides retained in the soil were Cs-137 and Ra-226. The maximum observed solubilities for these radionuclides were 1.8 x 10-14 M and 8.5 x 10-12 M, respectively. Maximum leach rates were 1.5 x 10-17 g/hr and 2.2 x 10-13 g/hr for Cs-137 and Ra-226 at Site F.
Statistical analysis of the observed solubilities was hampered by the generally very low solubilities (at or below the analytical detection limit) found for the radionuclides. Only the following means and standard deviations, assuming a normal distribution, were calculated:
- Site D — U (8.9 x 10-6 M ± 6.9 x 10-6)
- Site F — Cs-137 (1.6 x 10-14 M ± 2.4 x 10-15), Ra-226 (3.0 x 10-12 M ± 3.9 x 10-12)