2003:12 Analysis of Critical Issues in Biosphere Assessment Modelling and Site Investigation

The aim of this document is to present a critical review of issues concerned with the treatment of the biosphere and geosphere-biosphere interface in longterm performance assessment studies for nuclear waste disposal in Sweden. The review covers three main areas of investigation:

  • a review of SKB’s plans for undertaking site investigations at candidate locations for the development of a deep geological repository for spent fuel;
  • identification of critical uncertainties associated with SKB’s treatment of the geosphere-biosphere interface in recent performance assessments; and
  • a preliminary modelling investigation of the significance of features, events and processes in the near-surface environment in terms of their effect on the accumulation and redistribution of radionuclides at the geosphere-biosphere interface.

Overall, SKB’s proposals for site investigations are considered to be comprehensive and, if they can be carried out to the specification presented, will constitute a benchmark that other waste management organisations will have to work hard to emulate. The main concern is that expertise for undertaking the investigations and reporting the results could be stretched very thin. The authors have also identified weaknesses in the documentation concerning the collection of evidence for environmental change and on developing scenarios for future environmental change.

A fundamental assumption adopted in the renewed assessment of the SFR 1 repository, which is not discussed or justified in any of the documentation that has been reviewed, is that radionuclides enter the water column of the coastal and lake models directly, without passing first through the bed sediments. The modelling study reported herein suggests that SKB’s models are robust to range of alternative conceptual descriptions relating to the geosphere-biosphere interface. There are however situations, in which contaminated groundwater is released via sediment rather than directly to the water column, which may lead to significantly higher doses than indicated by the SKB models. It is recommended that alternative groundwater discharge and system evolution models should therefore be considered in future assessments. It is also recommended that care should be taken to ensure that releases from the geosphere to the biosphere are represented in a consistent manner, based on careful integration of processes at the interface.