2002:17 Review of Project SAFE: Comments on biosphere conceptual model description and risk assessment methodology

The Swedish Nuclear Fuel and Waste Management Company’s (SKB’s) most recent assessment of the safety of the Forsmark repository for low-level and intermediatelevel waste (Project SAFE) is currently undergoing review by the Swedish regulators. As part of its review, the Swedish Radiation Protection Institute (SSI) identified that two components of SAFE require more detailed review: (i) the conceptual model description of the biosphere system, and (ii) SKB’s risk assessment methodology.

We have reviewed the biosphere system interaction matrix and how this has been used in the identification, justification and description of biosphere models for radiological assessment purposes. The risk assessment methodology has been reviewed considering in particular issues associated with scenario selection, assessment timescale, and the probability and risk associated with the well scenario.

There is an extensive range of supporting information on which biosphere modelling in Project SAFE is based. However, the link between this material and the biosphere models themselves is not clearly set out. This leads to some contradictions and mismatches between description and implementation. One example concerns the representation of the geosphere-biosphere interface. The supporting description of lakes indicates that interaction between groundwaters entering the biosphere through lake bed sediments could lead to accumulations of radionuclides in sediments. These sediments may become agricultural areas at some time in the future. In the numerical modelling of the biosphere carried out in Project SAFE, the direct accumulation of contaminants in bed sediments is not represented. Application of a more rigorous procedure to ensure numerical models are fit for purpose is recommended, paying more attention to issues associated with the geosphere-biosphere interface. A more structured approach to risk assessment would be beneficial, with a better explanation of the difference between conditional and overall risk. More specifically, the risk assessment should take account of climate change as part of the base scenario. Assumptions regarding the number of persons exposed in the well scenario are not well justified, and the reasoning behind the limitation of the assessment to a period of 10 000 years should be made more robust.