129I is one of the more hazardous nuclides occurring in radioactive waste. In the form of I-, its most likely speciation, it is poorly sorbed on most geologic media. Several workers have suggested the use of silver to precipitate I- as the insoluble AgI, in a cemented waste form, or as a "getter". The efficacy of this procedure is examined by experiment, in conjunction with thermodynamic predictions. The addition of AgNO3 to Portland cement leads to coprecipitation with C-S-H, with low Ag solubilities (∼ 10 μmg/L); 2-;3 orders of magnitude lower than predicted (from Ag2O). AgI is stable in these matrices, with low aqueous I concentrations (<2 mg/L). In 85% BFS-15% OPC pastes, AgI is unstable due to redox and complexation reactions, with much I- passing into solution; concentrations up to 900 mg/L were observed. It is shown that repository conditions, on closure, are also likely to induce solubilisation of I- from AgI. It is concluded that the use of Ag is unlikely to significantly improve the immobilisation properties of the near field for radioiodine.
- radioactive waste management