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Published online by Cambridge University Press: 16 January 2017
At the Savannah River Site (SRS) low activity salt solution is stabilized via encapsulation within a grout termed saltstone. Saltstone is emplaced into large (multi-million gallon) concrete storage facilities referred to as Saltstone Disposal Units (SDUs). Technetium-99 (99Tc) is a long-lived radionuclide contained in the low activity salt waste and subsequently incorporated into the grout waste form: it is considered a significant contributor to risk with respect to the long-term radiation exposure of the environment surrounding the SDUs. In the reducing, high pH environment within the grout, 99Tc is expected to be relatively immobile since it exists in a reduced Tc(IV) oxidation state in the form of sparingly soluble sulfides (TcSx) or hydrated oxides (TcO2.xH2O). However, in the presence of O2 (associated with the future infiltration of air or oxygenated ground waters into the saltstone monolith) it is possible for redox-sensitive Tc(IV) to transition into highly soluble (and mobile) Tc(VII) species, pertechnetate (TcO4-), which is more readily transported to the surrounding environment. Traditional approaches to quantifying the leaching behavior of 99Tc from cementitious matrices have involved partitioning experiments using size-reduced (crushed/ground) saltstone samples, and determination of the 99Tc fraction immobilized by the cementitious solids. Such experiments create artificially high solid-solution contact areas that likely result in higher 99Tc leachate concentrations than would be expected for intact, monolithic samples. In the current study a new technique, termed the Dynamic Leaching Method (DLM), is being used to investigate the 99Tc leaching behavior of monolithic saltstone samples. The data derived using this technique is intended to inform the SRS Saltstone Disposal Facility (SDF) Performance Assessment (PA) which models the long-term transport of radionuclides from the SDUs to the environment. The DLM utilizes a flexible-wall permeameter to achieve saturated leaching under an elevated hydraulic gradient in an effort to simulate the transport of groundwater through saltstone. Initial findings indicate that the 99Tc concentrations in the leachate are on the order of 1E-08 mol/L which suggests that the saltstone leaching behavior is controlled by the solubility of TcO2.xH2O compounds.
