Post-Closure Biosphere Assessment
Walke, R., Newson, R., Bruffell, L., Bykov, V. & Hunkeler, P. (2024): Post-Closure Biosphere Assessment. Nagra Arbeitsbericht NAB 24-06.
pdf NAB 24-06 Post-Closure Biosphere Assessment(3.73 MB)
This work takes place in the context of the quantitative estimation of the radionuclide release from the system of engineered and geological barriers of the deep geological repository and the calculation of the resulting radiological consequences.
ENSI-G03 protection criteria refer to dose rates to average individual within the population group most affected by the potential impacts of the repository. To address these requirements, Nagra calculates dose rates (to this population group) resulting from radionuclide release from the geosphere.
This work contributes to these dose rate calculations by i) creating a model of the biosphere on the surface, including self-sufficient use of local resources by people, and ii) defining the biosphere dose conversion factors (BDCFs) to convert radionuclide fluxes (in Bq/a) from transport calculations into individual dose rate (mSv/a). These BDCFs are then used by the consequence analysis report (NAB 24-18) to calculate the dose rates for various scenarios.
The created biosphere model starts with the radionuclides released from the groundwater to a local shallow aquifer, and models all of the major exposure pathways. This model (i) simplifies the behavior of the released radionuclides in the human habitat and (ii) determines the potential radiation exposure for a representative member of the most affected population based on the major exposure pathways.
In this work, the biosphere model from SGT Stage 2 was updated to better reflect the current understanding of climate, geomorphology, and local and regional hydrogeology, also with regard to the geological long-term development. Specifically, the model newly includes additional compartments for the unsaturated zone above the aquifer and below the deep soil and the new model of plant interception of irrigation water. The transport of C-14 has also been refined, with changes relating to transport through plant canopy, degassing from surface-water bodies and spray irrigation water. (This work will be documented in NAB 23-25.) The underlying sorption database has also been updated (and will be documented in NAB 23-14), as were the element data (such as soil to crop transfer factors) and the dose coefficients for ingestion, inhalation, and external irradiation.
Compared to the SGT Stage 2 biosphere model, the new BDCFs for the majority of radionuclides are within a factor of 0.5–2, with the greatest changes driven by the updated biosphere sorption coefficients. The comparison can be seen in the Fig. 1 below. Preliminary dose rate calculations, shown in Fig. 2 below, show that the new BDCFs lead to a large decrease of the C-14 peak, but an increase of the I-129 peak (albeit not significant, given how far the value is from the 0.1 mSv/a limit).