Biosphere Modelling in Used Fuel Repository Safety Assessments: Comparing Swedish and Canadian Perspectives

Abstract

In repository safety assessments, biosphere models are the last in a modelling chain that simulates hypothetical radionuclide release, transport to the surface, and estimated exposure to future residents. The complexity of ecosystems and the long assessment timescales require modelling simplifications and assumptions. In this paper, the Swedish (SKB) and Canadian (NWMO) biosphere models are compared and the differences are discussed. While the models are fundamentally similar, differences exist with respect to ecosystem evolution and the approach to predict a bounding dose rate from the modelled geosphere discharges. Both models are dynamic compartment models that predict concentrations throughout environmental media. However, the SKB model includes ecosystem evolution to calculate a time-independent factor which, when applied to a geosphere release flux, represents the overall bounding dose rate over time. In contrast, the NWMO model assumes a steady-state ecosystem, and, using the modelled dynamic flux from the repository, calculates the bounding dose rate as a function of time in the compartment model.