Date of Award

12-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Earth Science

Advisor

Powell, Brian A

Committee Member

Coates , John T

Committee Member

DeVol , Timothy A

Committee Member

Kaplan , Daniel I

Abstract

Plutonium, neptunium, technetium, and iodine present appreciable risks at nuclear waste disposal sites around the world due to their potential mobility. Sorption of each of these radionuclides is profoundly influenced by oxidation/ reduction reactions. Therefore, the mobility of each radionuclide may be greatly influenced by redox conditions of the natural or engineered system. The primary focus of this study was to determine distribution coefficients (Kd) for each radionuclide for engineered concrete and saltstone systems with varying amounts of reducing slag (a cement additive to create a reducing environment), and for iodide/iodate in natural sediments. Saltstones are a mixture of cement, high ionic strength liquid wastes, and reducing slag used to sequester radionuclides. The engineered solids examined in this work are 1) aged cement: an 50 year old aged concrete from which the aggregate has been removed 2) Vault 2 concrete: a concrete formulation containing 17% reducing slag to be used as during vault construction at the SRS, 3) TR547: a saltstone formulation containing 45% reducing slag and 4) TR545: a saltstone formulation containing 90% reducing slag.The natural sediments used are two end member sediments (subsurface clayey and subsurfaec sandy) as well as a wetland sediment from Four Mile Branch.
Sorption experiments were performed under oxidizing and reducing conditions using Np(V), Pu(IV), Tc(VII), I(VII) and I(I-). Neptunium and plutonium both exhibited strong affinity for concrete and saltstones (a cementitious low-level nuclear waste form) under both oxidizing and reducing conditions. Distribution coefficient (Kd) values of >10 5 (mL/g) were calculated for all Np and Pu systems under oxidizing and reduction conditions. Experimental conditions had a far greater effect on Tc sorption. Under oxidizing conditions, Tc showed similar affinity for concrete and saltstone despite the presence of reducing slag, and Kd values remained around 10 (mL/g). However, under reducing conditions, Tc sorption increased relative to the oxidizing conditions, and the Kd values increased with increasing reducing slag concentration in the solid with values ranging from approximately 200 to 15,000 (mL/g). This behavior is consistent with reduction of Tc(VII) to Tc(IV). Similar reduction of Tc(VII) in the presence of saltstone was observed by Lukens et al. (2005). The rate of Tc sorption, from which reduction was infered, increased with increasing reducing slag content. It was determined there is a second order dependence on slag concentration, and steady state is reached around 3 weeks. Under oxidizing conditions, the iodide exhibited similar behavior to Tc, and had Kd values significantly lower than Np and Pu. However, unlike the Tc systems, increased sorption was not observed under reducing conditions thus indicating that the majority of iodine remains as the initially amended iodide in both systems.
Unlike with the highly reducing engineered systems above, when found in natural environments, iodine can exist as either iodate or iodide. Although easily reducible, iodate is known to experience stronger sorption to sediments than the reduced form iodide. This was shown to be true for all three soils under oxidizing conditions with the most sorption to the wetland soil, followed by the clayey, and then the sandy. There was no noticeable change in the iodate clayey soil Kd values under either oxidizing or reducing conditions indicating it remained as iodate. However, under reducing conditions, the wetland soil apparently reduced the iodate to iodide by the 4th day, which resulted in an 8 fold decrease in sorption. The final iodate equilibrium Kd value under reducing conditions was equal to that of iodide suggesting complete reduction of the iodate.

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