Spatial distribution, chemical state, solubility of rhenium in a reducing cement waste form: Implications for predicting technetium mobility in saltstone

Reducing cementitious materials (RCMs) are presently being developed for the long-term, subsurface disposal of low-level radioactive waste within the United States Department of Energy complex. Slag is included in the formulation of the RCM to facilitate the reductive precipitation of redox-sensitive radionuclides, particularly 99Tc. Using Re(VII)O4− as a chemical surrogate for Tc(VII)O4−, depth sequence Re distribution, solubility, and chemical state were evaluated to provide a spatial measure of the impact of atmospheric O2 migration into field aged (<300 d) RCM monoliths. Total digestion analysis showed that total Re concentration increased with decreasing depth, indicating more Re was concentrated at the surface. A synchrotron based X-ray microprobe energy map also supports this finding. There was no clear depth dependent trend in Re solubility. Overall 30-60% of the total Re was soluble at the depth up to ∼3 cm. Partial reduction of Re(VII) to Re(IV) was also observed at these depths. These results suggest that long-term immobilization of Tc in RCM waste forms may be more complicated than previous thought and may require the consideration of non-uniform distribution of redox sensitive Tc in RCMs.