Solubility and Leaching of Radionuclides in Site Decommissioning Management Plan (SDMP) Slags (NUREG/CR-6632, PNNL-12205)

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Publication Information

Manuscript Completed: September 2001
Date Published: February 2002

Prepared by:
Andrew R. Felmy, Dhanpat Rai, Stacey A. Hartley, Virginia L. LeGore

Pacific Northwest National Laboratory
P.O. Box 999
Richland, Washington 99352

P.R. Reed, NRC Project Manager

Prepared for:
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

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Abstract

Samples of disposed wastes at three U.S. Nuclear Regulatory Commission (NRC) Site Decommissioning Management Plan (SDMP) sites 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 which may or may not be solubility controlled, and 5) the potential for radiocolloid formation. In order to achieve these goals, batch studies of ground or fractured samples 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. Three slag containing sites were selected for study. The results for slags at all three sites show that the major radionuclides present in the wastes were Th and U with their associated daughter products. Th daughters were in secular equilibrium with the parent Th-232 in all samples. U-238 daughters were in secular equilibrium in certain highly solidified non-porous samples, but escape of Rn-222 had occurred from more porous samples perturbing the U-238 secular equilibrium. Analysis of solution phase concentrations and solid phase composition indicated that aqueous Th concentrations are solubility controlled, most likely by thoranite, ThO₂(c), which sets an upper limit on the dissolved Th concentrations. Uranium also appears to be solubility controlled in certain waste samples (thoriated slags) high in pH and alkaline earth cations (Ca, Sr, Ba), upper limits on the observed solubilities apparently being set by the secondary formation of alkaline earth uranates. Comparisons of filtered and unfiltered analyses of samples from flow-through-columns did not show any evidence for the presence of radiocolloids. Maximum dissolved concentrations (solubility limits) and radionuclide leaching rates have been calculated for three sites for use in performance assessment calculations. Observed Th and U solubilities were quite low (maximum Th solubilities of 3.2x10^-8 M, 4x10^-8 M, and 3.2x10^-9 M; maximum U solubilities were 2.5x10^-8 M, 6.3x10^-8 M, and 8.9x10^-9 M all for sites A, B, and C, respectively). Observed leaching rates for the three sites were 100, 1, and 0.2 pCi/yr for Th and 260, 8, and 2 pCi/yr for U again for sites A, B, and C respectively. Chemical modeling of well water compositions at site C indicates that the low concentrations of soluble uranium are primarily present as anionic uranium carbonate complexes. Statistical analysis of the batch, column, and solubility data showed that the uncertainties in the dissolved Th and U concentrations can be described using a lognormal distribution with a meanlog of -17.7 and a sdlog of 0.57 for U and a meanlog of -19.5 and an sdlog of 1.7 for Th. These parameters are valid for all three sites.