The engineering barriers suggested to be used for the disposal of low and intermediate level wastes in Finland are concrete and crushed rock or mixtures of crushed rock and bentonite. In the repository the barriers are saturated by groundwater and radionuclides may be released by diffusion through the barries. For safety analysis, the mechanisms by which the nuclides migrate and corresponding parameters should be known. In this study diffusion measurements on different types of concrete and mixtures of sodium bentonite and crushed rock were carried out. Radioactive isotopes of cesium, strontium, carbon and nickel were used as tracers. The apparent diffusivities (Dsub(a)) were evaluated on the basis of the measurements. The apparent diffusivity of cesium in concretes was 10-14...10-15m2/s. Strontium was mainly sorbed on cement where it diffuses slowly. Part of strontium propably penetrates in the rock ballast by diffusion. The diffusivities of carbon and nickel in the concrete was low. The upper limit was evaluated to be Dsub(a) < 10-14m2/s. The diffusivity of cesium in the mixtures of crushed rock and bentonite varies between 0.5x10-12 and 7x10-12m2/s. Cesium was mainly sorbed on the rock. The diffusivity of strontium was 2x10-11...2x10-12m2/s. Strontium was mainly sorbed on bentonite. The diffusion of the sorbed ions (surface diffusion) seems to be a additional migration mechanism in the case of cesium and strontium in the mixture of bentonite and crushed rock. The diffusivity of carbon in the mixtures of crushed rock and bentonite was 6x10-11...4x10-12m2/s. No sorption was found in the case of carbon. The measured Dsub(a) of nickel in the mixtures of crushed rock and bentonite was 4x10-14...2x10-15m2/s. The experimental arrangement was not, however, in the stationary state and the more correct values would propably be 10-13...10-14m2/s. No surface diffusion was found in the case of nickel.
|Place of Publication||Helsinki|
|Number of pages||70|
|Publication status||Published - 1986|
|MoE publication type||D4 Published development or research report or study|
|Series||Voimayhtiöiden ydinjätetoimikunta YJT|