2014:54 Technical Note, Further Modelling Comparison of Simple Reference Biosphere Models with the LDF Modelling Approach – Main Review Phase

This Technical Note describes further use of simple reference biosphere models for SSM’s review of the Landscape Dose Factor (LDF) approach adopted by SKB in the SR‑Site safety assessment for the proposed final disposal of spent nuclear fuel at the Forsmark site.  Simple biosphere models for the Forsmark site under temperate climate conditions were previously developed for SSM’s review and are described in Walke (2014), along with comparisons against SKB’s LDF modelling approach.  The current study had two objectives.

  • Firstly, to extend SSM’s simple models of the Forsmark site to include the capability to represent warm and periglacial climate states.
  • Secondly, to further explore differences between the biosphere dose factors calculated with SSM’s simple models and those calculated using the LDF approach used by SKB in its SR-Site assessment.

The representation of the warmer systems in the simple biosphere models draws on the SR-Site descriptions of a warm climate at Forsmark to justify increased runoff, increased irrigation and increased occupancies.  Dose factors calculated for warm climate conditions are shown to be consistently higher than those for present-day climate conditions, principally due to increased irrigation and occupancies.

The SR-Site assessment includes a ‘global warming’ case.  Descriptive reports on the terrestrial, limnic and marine biosphere at Forsmark describe warmer conditions with increased runoff and present parameter values for increased productivity.  However, these are not taken into account in the dose assessment modelling, which instead simply represents a ‘global warming’ case as one where the interglacial conditions last longer than in the reference glacial cycle, without changing the parameterisation of the system.  As might be expected, LDFs for the ‘global warming’ case only differ for those long-lived radionuclides that have not reached equilibrium by the end of the interglacial conditions in the reference glacial cycle.