Geochemical Transport Modeling of Tc and Re Leaching from Cementitiuos Waste Forms.

Cementitious waste forms are often used to stabilize radioactive waste resulting from plutonium (Pu) production by the U.S. Department of Energy. Technetium is an abundant fission product of particular concern because of its long half-life, and the high solubility and mobility of Tc(VII), pertechnetate (TcO₄^-), the stable form of Tc in aerobic environments. Saltstone is a cementitious waste form developed to immobilize contaminants from low-level liquid waste, and is being used at the Department of Energy’s Savannah River Site (SRS). The waste form is produced by mixing liquid waste with cement, fly ash, and blast furnace slag.  The blast furnace slag is used in the formulations to create a reducing environment to immobilize redox sensitive elements, such as Tc. During the service life stage, the release of substances due to contact with the water is the main potential hazard source, as a consequence of the leaching phenomenon. I order to assess this release leaching experiments for the Rhenium (Re, used as an stable analogue for Tc) and ⁹⁹Tc spiked Saltstone materials were conducted using EPA Method 1315, Mass Transfer Rates of Constituents in Monolithic or Compacted Granular Materials Using a Semi-Dynamic Tank Leaching Procedure under aerobic and anoxic conditions. A leaching model was developed using Geochemist Work Bench (GWB), coupled with Thermoddem V1.10 Code version 1.07_2.06 thermochemical Database from the BRGM institute (French Geological Survey) to simulate the leaching of Tc and Re from the monoliths. The development of the model is based on the iterative process of comparing the experimental results with the simulated results up to an acceptable fit. The model utilizes both kinetic terms and equilibrium thermodynamics of the relevant  compounds and provides information on leachate and precipitate speciation. Our model indicated the high pH (≈12.4) is mainly controlled by the dissolution of C-S-H gel (Ca_0.8SiO_2.8:1.54H₂O). Model results suggest that Tc levels in the leachate were governed by the solubility of TcO₂:2H₂O, while Re appears to leach at a faster rate possibly due to limited reduction or rapid oxidation during testing. This suggests that Re is not a reliable chemical analog for evaluating Tc immobilization.