Modelling simultaneous transport of natural and anthropogenic radionuclides in fractured media – Diffusion into the heterogeneous layered rock matrix for an arbitrary length decay chain.
Our former colleague Batoul Mahmoudzadeh (Ramboll) and current colleague James Crawford (Kemakta Konsult AB) have published an article in Journal of Contaminant Hydrology addressing a notable gap in the literature for modelling of radionuclide transport. With the financial support of Svensk Kärnbränslehantering AB (SKB), the authors have developed a comprehensive analytical model that enables simulation of both natural and human made radionuclides in fractured bedrock.
Typical models used in safety assessment of radioactive waste repositories focus solely on anthropogenic sources of radionuclides, neglecting naturally occurring ones found in the rock and fracture minerals. Predictions made by such models are difficult to corroborate, however, since they concern hypothetical releases many thousands or tens of thousand of years into the future after repository closure. Including natural radionuclides in these models can offer valuable insights by providing an internal standard to compare against present day measured natural groundwater levels, helping to interpret the natural analogue data, and deriving key transport parameters for comparison with lab estimates.
The new model addresses this gap by simultaneously accounting for the transport of radionuclides from both sources. It considers flow and diffusive-sorptive interactions with the rock matrix incorporating decay chains of arbitrary length. Heterogeneity in the rock can be included in the form of different rock types distributed along a transport path as well as layered alteration rims adjacent to the fracture surface, each with individualised material property descriptions and concentrations of natural radionuclides.
Aside from its applications in safety assessment, the model has potential applications in other areas such as geological storage of carbon dioxide or geothermal heat extraction from hot rock where studying disturbances in the distribution of natural radionuclides in the groundwater systems and timescales associated with return to steady-state conditions can lead to important insights.
The authors’ work is an important step forward in the field of radioactive waste management. By taking both natural and anthropogenic radionuclides into account, their model offers a more complete picture of transport of solutes in fractured rock. This, in turn, can help improve confidence in models used in the safety and performance assessment of deep geological repositories for radioactive wastes, ultimately protecting both the environment and human populations.