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

Se-79 is a long-lived radionuclide of potential radiological significance in relation to the deep geological disposal of solid radioactive wastes. In the context of release to the terrestrial environment, its main radiological impact is delivered through food chain pathways. Therefore, its accumulation in soils and uptake by plants is an important consideration in post-closure safety assessment studies. However, representation of its behaviour in the soil-plant system requires consideration of the multiple valence states that it can exhibit under different redox conditions and its susceptibility to volatilisation. A simple model is described that includes seasonal variations in soil hydrology and their effects on the mobility and root uptake of Se-79. Illustrative calculations are undertaken with the model, to demonstrate its capabilities for interpreting experimental data on the behaviour of Se-79 in soils and plants, and for making projections on the long-term behaviour of Se-79 transported to soils.