Sorption of Tc(IV) to Cementitious Materials Associated with a Geological Disposal Facility for Radioactive Waste - 11129

Technetium-99 is one of the most important isotopes likely to be disposed of in the proposed UK Geological Disposal Facility (GDF) for higher-activity radioactive wastes. This is due to its long half-life, high fission yield, and its ability to migrate through the geosphere when in its oxidised, pertechnetate form. However, much of the technetium in the GDF is likely to be in the lower oxidation state of Tc(IV) due to the low Eh in the near field caused by the corrosion of the stainless steel canisters. Therefore, an important aspect of the behaviour of technetium in the nearand far-fields of a GDF is its sorption to cementitious materials in this reduced oxidation state. Batch sorption experiments have been performed on technetium in the presence of Ordinary Portland Cement (OPC), Nirex Reference Vault Backfill (NRVB) and Plutonium Contaminated Material Grout (PCMG), all of which may be used in a deep geological disposal facility in the UK. Tc(IV) solutions were used at trace concentrations from 10 to 10 mol dm to avoid the possibility of precipitation as TcO2. After equilibration, the technetium concentration in the supernatant solutions was measured using gamma counting. In these studies, the values for the partition coefficient (Rd) were found to range from 70 to 5500 cm g. It was noticeable that Rd was heavily dependent on pH in all cases. Surface complexation modelling of the data generated has been performed. In general, using bidentate surface complexation constants in the hyperalkaline region, and a combination of monoand bidentate ones below ca. pH 11.5 give best agreement between models and data. These data will inform the performance assessment for the behaviour of technetium in the near-field of the UK’s planned higher-activity wastes GDF.