Long Term Diffusion (LTD) Aims

The broad objectives of the LTD project are to provide quantitative information on matrix diffusion in fractured rock under in situ conditions.

The specific aims of LTD are:

• Clearly defining the degree of realism of the matrix diffusion and flow-wetted surface parameters employed in current PA calculations
  
• Demonstration of in situ radionuclide retardation in the rock matrix, especially for the suite of weakly- and non-sorbing elements (so minimising the over-conservatism in current PA calculations)
  
• In situ verification of long-term and large-scale diffusion concepts for radionuclides transported from a water-conducting system into a rock matrix under well-defined boundary conditions
  
• Verification of the conceptualisation in existing transport codes (via predictive and post mortem testing) of the transport of radionuclides across the flow-wetted surface and subsequent diffusion in the rock matrix
  
• Comparison of field and laboratory data
  
• The full characterisation of spatial distribution of porosity in the matrix, and its link with the mineralogy, in order to better identify microstructure-derived residence time distributions in the matrix
  
• The identification of flow-wetted surfaces in order to better define the spatial distribution of mass transfer coefficients controlling transport from fracture to matrix