Radionuclide migration was studied in a natural fissure in a granite core. The fissure was oriented parallel to the axis in a cylindrical core 30 cm long and 20 cm in diameter. The traced solution was injected at one end of the core and collected at the other. Breakthrough curves were obtained for the nonsorbing tracers, tritiated water, and a large-molecular-weight lignosulphonate molecule and for the sorbing tracers, cesium and strontium. From the breakthrough curves for the nonsorbing tracers it could be concluded that channeling occurs in the single fissure. A 'dispersion' model based on channeling is presented. The results from the sorbing tracers indicate that there is substantial diffusion into and sorption in the rock matrix. Sorption on the surface of the fissure also accounts for a part of the retardation effect of the sorbing species. A model which includes the mechanisms of channeling, surface sorption, matrix diffusion, and matrix sorption is presented. The experimental breakthrough curves can be fitted fairly well by this model by use of independently obtained data on diffusivities and matrix sorption. BACKGROUND The migration of radionuclides in various kinds of rocks has become an area of large interest in the last decade because of various national and international efforts in studying the final disposal of radioactive wastes from nuclear power plants. In the Swedish studies [KBS Nuclear Fuel Safety Project, 1977; 1978], crystalline rock has been selected as the most suitable bedrock in which to build a repository. In crystalline rock the water moves in fissures which may be fairly far apart at larger depths. The radionuclides, carried by the water, will interact in various ways with the rock. They may be strongly retarded by sorption on the surface of the fissures and, given time, may also penetrate the intercrystalline microfissures of the matrix of the rock. The present study aims at obtaining experimental results from radionuclide migration in a single fissure under well defined conditions. Such results should be useful in understanding and possibly predicting radionuclide migration in fissured crystalline rock. THE EXPERIMENT Flow SystemThe rock used in this study was a 30-cm-long granitic drill core (20-cm diameter) taken from the Stripa mine at a depth of 360 m below ground level. The core has a natural fissure which runs parallel to the axis. The cylindrical surface of the drill core was sealed with a coat of urethane lacquer to prevent any water leaving the rock except through the outlet end of the fissure. The granite cylinder was thereafter mounted between two plexiglas end plates containing inlet and outlet channels slightly wider than the fissure (Figure 1).Artificial groundwater with a tracer was fed to the upper channel by means of a four-channel peristaltic pump (Istma-