2014:35 Technical Note, Modelling comparison of alternative biosphere models with LDF models and evaluation of selected parameter values used in the biosphere dose assessment - Main review phase

This report has been prepared as part of the SSM’s Main Review Phase of SKB’s SR-Site performance assessment of the long-term safety of the KBS-3 geological disposal facility (GDF) proposed for construction at Forsmark. The review addresses the methodology employed in the dose assessment calculations of SR-Site; specifically issues of transport, accumulation and transfers of radionuclides in the near surface environment and the way in which doses to future human and non-human populations can arise.

The issues addressed here are: representation of hydrology within the SR-Site assessment; comparison of SKB’s dose assessment modelling with an independent alternative biosphere modelling approach; and an evaluation of key parameters used in the SKB biosphere model, including whether transfer rates and selected Kd values and concentration ratios are appropriate.

Water flows in the near surface environment are the main drivers of contaminant transport. The review of the surface hydrological modelling in the SKB dose assessment model finds that there is a lack of justification of the hydrology assumed for basins in the future Forsmark landscape, and that the information derived from a detailed hydrological model of the future landscape is not used to best advantage in deriving the hydrological underpinning of the dose assessment model, with potentially significant parameters and processes discarded. The interpretation of the surface hydrology used in the dose assessment modelling is thus suitable only for a certain class of landscape object as a snapshot of conditions at a particular time. Consequently, it is hard to state with confidence that the hydrological representations in the dose assessment model are fit for purpose.