The aim is to refine or develop scientific software, which can be used to calculate migration of organic species and radionuclides complexed by organic species. The software is based on fundamental thermodynamic principles and conceptual mechanisms, which are established during the project. The software can then be used to model, interpret and simulate the migration behaviour of radionuclides (RN) through reducing organic rich sediment at scales relevant for Performance Assessment.
The kinetics of radionuclide complexation to, and destabilisation from, organic matter, are potentially significant factors in radionuclides migration. The model, POPCORN, which evaluates the effects of POPulations of COlloids on RadioNuclide transport, has been developed to study these kinetic effects.
Status: in progress - the POPCORN model has been developed and tested, and initial applications have been made, in the TRANCOM II project.
Results:
POPCORN has been used to evaluate organic matter (OM) diffusion experiments and 241Am-14C-OM migration experiments. Model application at the repository performance assessment level may also be possible. Key model features are: representation of dissolved radionuclides and radionuclides associated with mobile and immobile OM; linear kinetic representations of radionuclide adsorption (complexation) to, and desorption (destabilisation) from, mobile and immobile OM; and representation of rates of attachment of mobile OM to the rock matrix and rates of detachment of immobile OM from the rock matrix.
The model reproduces the observations from the diffusion and migration experiments. The diffusion experiments were simulated assuming diffusion and filtration of OM by attachment to the surface of the clay matrix. The instability of the 241Am-OM complex was successfully characterised by kinetic adsorption constants for mobile and immobile OM, with the interaction of OM with the clay rock providing filtering. A sub-population of the original 241Am-OM complex in the larger size range, and characterised by a low dispersion and a greater potential for filtering, appears to be the most stable.
Relationships between the inverse of the rate terms that describe OM attachment and adsorption kinetics and the estimated peak advection-driven OM breakthrough time have been identified. The relationships offer potential means of extrapolating parameter values for model application to repository-scale problems.