An Updated Sorption Database for Use in Swiss Biosphere Modelling

pdf NAB 23-14 An Updated Sorption Database for Use in Swiss Biosphere Modelling(9.86 MB)

At the time of writing, Nagra is in Stage 3 (SGT E3) of the Swiss process to select a site or sites for deep geological disposal of HLW and ILW/LLW. At the commencement of the project reported here, three siting regions were being considered with potential for hosting a deep geological repository (or repositories). However, in September 2022, near the end of the project, Nördlich Lägern in Northern Switzerland was recommended for repository siting. In 2024, Nagra anticipates submitting documents justifying site selection and for the general licence application (RBG) for a proposed repository site. Safety assessments for potential repositories in the three siting regions will form part of the site selection process.

Post-closure safety assessment includes modelling the behaviour of radionuclides potentially released from deep geological disposal to the biosphere. Nagra’s biosphere modelling is reflected in the SwiBAC code (Walke & Keesman 2013). It is intended that biosphere modelling for the RBG application will divide the biosphere into compartments, representing:

• top soil
• deep soil
• an unsaturated zone
• a local shallow aquifer
• river bed sediments; and
• surface water (including suspended sediment)

For the RBG application, SwiBAC will be used to assess a reference biosphere based on an agricultural area within a Northern Swiss valley, with a river, underlying Quaternary aquifer, and a present-day climate. In addition to the reference biosphere, based on a river valley, the same compartment structure can be adapted to represent alternative biosphere systems, including, for example:

• the same system, but with a warmer, drier climate, and
• a wetland area drained for agriculture

The aim of this report is to provide a sorption database for use in SwiBAC in SGT E3. The database updates Nagra’s biosphere sorption database documented in (Tits & van Dorp 1999). Like this earlier database, the updated database provides sorption coefficients (“Kd” values) describing linear sorption isotherms for radioelements that may be important in the waste inventory. However, whereas the previous database focussed on sites being considered in the late 1990’s, in central and Northern Switzerland, the updated database is targeted at the conditions (compositions of sediments, soils, shallow groundwaters, and surface waters) found in the three siting regions being considered in SGT E 3. The revised database also incorporates newinformation that has become available since (Tits & van Dorp 1999) was produced.