Technical Report NTB 17-05
Chemistry of selected dose-relevant radionuclides
When looking at the dose rate calculations made as part of the provisional safety assessment in Stage 2 of the Sectoral Plan (Nagra 2014a), it is remarkable that only very few radionuclides contribute to the resulting dose rate. These radionuclides include C-14 (instant and congruent release), Cl-36, Se-79 and I-129. The reasons for this observation are the limited retardation of these radionuclides in clay- and cement-based environment and/or their unknown chemical speciation. This report gathers information regarding inventory, chemical speciation in the relevant waste sorts and transport processes in clay and/or cement (sorption and diffusion). This is done for each of the following radionuclides: Cl-36, Se-79, Ag-108m and I-129. As the research on C-14 speciation and retardation is ongoing, a separate report will deal with the contribution of C-14. Based on existing literature, the report investigates the origins of uncertainties, describes the complexity and nature of the waste in which the radionuclides are formed, evaluates the possible speciation for each element and makes suggestions of how the remaining uncertainties can be reduced in the future. The report comes to the conclusion that uncertainties in the inventory or in the speciation for Cl-36, Se-79 and Ag-108m could be further reduced by dedicated studies. For Cl-36, verifying the inventory of Cl-36 in spent fuel, cladding and stainless steel can decrease the dose contribution. In case of Se-79, thermodynamic and modelling exercises to understand the Se(cr) ⇌ Se(aq) system and the uptake of Se in sulphides can reduce the dose contribution. A similar study on the Ag(cr) ⇌ Ag(aq) system could also elucidate the speciation and solubility of Ag-108m. Finally, this report however estimates the chance of reducing the dose rate contribution of I-129 by doing further studies as very low, as the chemical speciation and the inventory are well-known and have been extensively characterised.