A physically based approach to modelling radionuclide transport in the biosphere

Abstract

Calculations of radiological risk are required to assess the safety of any potential future UK deep underground repository for intermediate-level and certain low-level solid radioactive wastes. In support of such calculations, contaminant movement and dilution in the terrestrial biosphere is investigated using the physically based modelling system SHETRAN. Two case studies are presented involving modelling of contaminants representing long-lived poorly sorbed radionuclides in the near-surface aquifers and surface waters of hypothetical catchments. The contaminants arise from diffuse sources at the base of the modelled aquifers. The catchments are characterised in terms of detailed spatial data for topography, the river network, soils and vegetation. Simulations are run for temperate and boreal climates representing possible future conditions at a repository site. Results are presented in terms of the concentration of contaminants in the aquifer, in soils and in surface waters; these are used to support the simpler models used in risk calculations.

A mathematical model for the behaviour of Se-79 in soils and plants that takes account of seasonal variations in soil hydrology

“…This is the approach adopted in various catchmentscale models developed for water resource analyses and contaminant transport studies (see, e.g. Parkin et al 1999, Hughes and Liu 2008, Towler et al 2011 and in the detailed onedimensional (1D) models developed for interpretation of experimental lysimeter and column studies (Wheater et al 2007). However, for modelling on very long timescales a simpler approach is appropriate, as the seasonal hydrological cycle can only be represented in very general terms (BIOCLIM 2004).…”

Modelling radionuclide transport in the environment and calculating radiation doses

“…However, in practice, the interest may be in radionuclide distribution and transport at the scale of a surface-water catchment. Over the last 15 years, the SHETRAN model has been extensively developed to investigate this issue (Abbot et al, 1986;Ewen et al, 2002;Parkin et al, 1999;Birkinshaw et al, 2005), as has the MIKE-SHE model (Vikström and Gustafsson, 2006). When such complex, physically based models are used, issues arise concerning their verification and validation.…”

A model for evaluating radiological impacts on organisms other than man for use in post-closure assessments of geological repositories for radioactive wastes

“…The water table is assumed to be well defined and may lie within the substrate or surface soils. For more detailed studies, it may be more appropriate to replace this simple onedimensional compartmental model by a physically based, three-dimensional surface-water catchment model such as SHETRAN (Parkin et al 1999).…”