Technical Report NTB 83-06

A method for Modelling the transport of nuclides in fissured rock with diffusion into the solid matrix

A technique is described, whereby diffusion of a chain of radionuclides in the microfissures or pores of the matrix material adjacent to waterbearing fractures may be calculated. The method enables the process to be calculated on all scales:

– in the individual blocks penetrated by a single fracture

– in an assembly of characteristic block sizes in a representative volume in a model on a coarser scale

The technique involves an analytical solution to the diffusion -decay ­sorption equation in a block of simplified geometry. This is embedded into a numerical (finite difference) computer code which also handles the other mechanisms relevant to the transport of radionuclides in the geosphere.

The resulting program is verified against purely analytical solutions for matrix diffusion coupled with convective/dispersive transport of a single nuclide by means of benchmark calculations. Finally, the effect of matrix diffusion within blocks of varying sizes is demonstrated.