title: Post-closure biosphere assessment modelling: comparison of complex and more stylised approaches.
abstract: Geological disposal facilities are the preferred option for high-level radioactive waste, due to their potential to provide isolation from the surface environment (biosphere) on very long timescales. Assessments need to strike a balance between stylised models and more complex approaches that draw more extensively on site-specific information. This paper explores the relative merits of complex versus more stylised biosphere models in the context of a site-specific assessment. The more complex biosphere modelling approach was developed by the Swedish Nuclear Fuel and Waste Management Co (SKB) for the Formark candidate site for a spent nuclear fuel repository in Sweden. SKB's approach is built on a landscape development model, whereby radionuclide releases to distinct hydrological basins/sub-catchments (termed 'objects') are represented as they evolve through land rise and climate change. Each of seventeen of these objects is represented with more than 80 site specific parameters, with about 22 that are time-dependent and result in over 5000 input values per object. The more stylised biosphere models developed for this study represent releases to individual ecosystems without environmental change and include the most plausible transport processes. In the context of regulatory review of the landscape modelling approach adopted in the SR-Site assessment in Sweden, the more stylised representation has helped to build understanding in the more complex modelling approaches by providing bounding results, checking the reasonableness of the more complex modelling, highlighting uncertainties introduced through conceptual assumptions and helping to quantify the conservatisms involved. The more stylised biosphere models are also shown capable of reproducing the results of more complex approaches. A major recommendation is that biosphere assessments need to justify the degree of complexity in modelling approaches as well as simplifying and conservative assumptions. In light of the uncertainties concerning the biosphere on very long timescales, stylised biosphere models are shown to provide a useful point of reference in themselves and remain a valuable tool for nuclear waste disposal licencing procedures.
authors.affiliation: Quintessa Limited, The Hub, 14 Station Road, Henley-on-Thames, United Kingdom. Electronic address: russellwalke@quintessa.org.
authors.forename: Russell C
authors.initials: RC
authors.lastname: Walke
authors.affiliation: University of Hamburg/ZNF, Beim Schlump 83, 20144, Hamburg, Germany. Electronic address: gerald.kirchner@uni-hamburg.de.
authors.forename: Gerald
authors.initials: G
authors.lastname: Kirchner
authors.affiliation: Swedish Radiation Safety Authority, SE-171 16, Stockholm, Sweden. Electronic address: shulan.xu@ssm.se.
authors.forename: Shulan
authors.initials: S
authors.lastname: Xu
authors.affiliation: Swedish Radiation Safety Authority, SE-171 16, Stockholm, Sweden. Electronic address: bjorn.dverstorp@ssm.se.
authors.forename: Björn
authors.initials: B
authors.lastname: Dverstorp
mesh_terms: D008962:Models, Theoretical; D011834:Radiation Monitoring / Q000379:methods*; D011850:Radioactive Waste / Q000032:analysis*; D012037:Refuse Disposal; D013548:Sweden; D018505:Waste Management
chemical_list: D011850:Radioactive Waste
keywords: Biosphere; Dose assessment; Environmental change; Radioactive waste; Radionuclides