Technical Report NTB 17-11
Estimates of the influence of radionuclide solubility limits and sorption competition on the sorption values in the SDBs of MX-80 bentonite and Opalinus Clay
In a geologic repository for radioactive waste a wide variety of metals from various sources, e.g. from backfill materials, from canister corrosion and from the dissolution of spent fuel and vitrified waste will be in solution in the porewaters. These stable and radioactive isotopes are simultaneously present and are an integral part of a realistic repository system.
Two effects need to be distinguished. The first is that even though individual radionuclides may be present at trace levels it is the total concentration (radioactive + stable isotopes) which determines the sorption. The second effect is that dissolved metals can compete with one another for the available sorption sites on the backfill material and the host rock. Both effects, higher total concentrations and sorption competition, lead to reduced sorption values.
It is clearly an inherently important issue to quantify their influence on the transport of released radionuclides through the multi-barrier system in a deep repository. The only practical means of assessing the influence of competitive sorption in MX-80 bentonite and Opalinus Clay, and bound its effects, is via modelling. This has been addressed in this report based on experimental results and modelling with the 2 site protolysis non-electrostatic surface complexation cation exchange (2SPNE SC/CE) sorption model. Generally, competitive sorption arises between metals which are chemically similar (valence state, hydrolysis behaviour) and which fit on the same Linear Free Energy Relationship (LFER). Three main groups of dissolved metals were considered: divalent transition metals, trivalent actinides / lanthanides and tetravalent actinides. Competitive sorption occurs between the metals within each group but not with metals from different groups.
The main conclusions drawn were:
- For divalent transition metals the main competitive metal ion is ferrous iron, arising from the saturation of siderite, and leads to a reduction in sorption of approximately two orders of magnitude compared with trace concentrations in MX-80 bentonite and Opalinus Clay.
- Competitive sorption is also taking place between trivalent actinides and lanthanides. The effect of the competition i.e. sorption reduction, depends on the sum of their local concentrations. In a worst case, calculated reductions of a factor of 10 to 100 may be anticipated in both MX-80 bentonite and Opalinus Clay compared with the values at trace concentration.
- For tetravalent actinides sorption competition is taking place, and sorption reductions of factor ~ 10 and ~ 3 may be anticipated in MX-80 bentonite and Opalinus Clay, respectively, compared with the values at trace concentration.