Radionuclide transport through fractured media is usually calculated assuming that water flows in most of the fractures. Several observations in the field and the laboratory show that flow is very unevenly distributed in fractured crystalline rock. These observations indicate that most of the water flow takes place in a limited number of channels. The channels are seldom wider than a few meters and are often much narrower. This means that the surface of the fracture in contact with the flowing water (wetted surface) is less than one might expect.

This low value of the wet surface of the fracture may considerably influence the transport of radionuclides through fractured media. If the channels do not intersect over a certain distance, then the channels may be modelled as a bundle of independent channels. Channels with a large flow and small sorption surface will carry the tracer rapidly and in large amounts.

Calculations are performed for cases where channeling is assumed to take place. The most important entities to assess are the water flow distribution in the different channels, the wetted surface of the channels, the diffusivity into the rock matrix, and the sorption coefficient in the matrix. Experimental data for the water flow distribution are used and the transport of nuclides is calculated for the different channels. From these values the concentration of the effluent is determined. The results show that the retardation for the nonsorbing nuclides is negligible. Retardation is only important for the nuclides which are strongly sorbed on the granitic rock. Calculations are also done assuming other channel frequencies and other overall water flowrates.