Validation of matrix diffusion modeling

Crystalline rock has been chosen as the host medium for repository of highly radioactive spent nuclear fuel in Finland. Radionuclide transport takes place along water-carrying fractures, and matrix diffusion has been indicated as an important retarding mechanism that affects the transport of mobile fission and activation products. The model introduced here for matrix diffusion contains a flow channel facing a porous matrix with stagnant water into which tracer molecules advected in the channel can diffuse. In addition, the possibility of a finite depth of the matrix and an initial tracer distribution (‘contamination’) in the matrix are included in the model.In order to validate the developed matrix diffusion model, a relatively simple measuring system was constructed. Matrix diffusion was illustrated by observing the migration of 0.1 ml KCl pulses in the water flowing through a channel facing a porous matrix made of synthetic fibre felt. Migration of K+ and Cl− ions was monitored by measuring the electrical conductivity of the solution. The experimental system allowed also measurements on the concentration profile inside the porous matrix, but the focus is here on the input and output (breakthrough) pulses. Measurements were performed for two different initial distributions of KCl tracer in the porous matrix. There was excellent agreement between modeling and experimental results with consistent values for the diffusion coefficient used as the fitting parameter.