The diffusion of nonsorbing species in different rock materials and fissure coating materials has been studied on a laboratory scale. The nonsorbing species were iodide, Uranine, and Cr-EDTA. The results show that the effective diffusivity of iodide in rock materials with fissure coating material is of the same magnitude as or higher than the effective diffusivity of iodide in rock materials without fissure coating material. The results also show that the variations in the rock material are too large to give one value of the diffusivity in a rock material from a certain area. The estimated effective diffusivity of iodide in rock materials without fissure coating material was found to be in the range 1 x 10 -•'• m2/s to 70 x 10 -•'• m:•/s.

A significant rztardation of radionuclides transported by flowing water from an underground repository can be expected if the nuclides are able to diffuse into the water filled micropores in the rock. This diffusion into the pores will also increase the surface available to interactions between the nuclides in the groundwater and the rock material, such as sorption. To calculate the retardation, it is necessary to know the sorption properties and the diffusivities in the rock matrix for the radionuclides. Diffusion experiments with cesium and strontium in biotite gneiss samples have been performed. Both the transport of strontium and cesium through rock samples and the concentration profiles of cesium and strontium inside rock samples have been determined. The result shows that diffusion of cesium and strontium occurs in the rock material. A diffusion model has been used to evaluate the diffusivity. Both pore diffusion and surface diffusion had to be included in the model to give good agreement with the experimental data. If surface diffusion is not included in the model, the effective pore diffusivity that gives the best fit to the experimental data is found to be higher than expected from earlier measurements of iodide diffusion in the same type of rock material. This indicates that the diffusion of cesium and strontium (sorbing components) in rock material is caused by both pore diffusion and iurface diffusion acting in parallel. 10-TM to 10-•m•/s.The diffusivity of sorbing species in micropores in rock materials has also been studied previously. Skagius et al. [1982] studied the diffusion of cesium and strontium in crushed granite. The effective diffusivity of both cesium and strontium was found to be of the order of 10 -•'-to 10-•Xm2/s. Effective diffusivities of the same order of magnitude were found when studying the diffusion of cesium and strontium in granite plates [Skagius and Neretnieks, !982].['1986] studied the diffusion of cesium and strontium in sandstone and found the effective diffusivities to be of the order of 10-• x to 10-'øm2/s. ; Bradbury and Stephen, 1986] the diffusivity of iodide, cesium, and strontium has been determined in the same type of rock material. The bulk phase diffusivity of cesium and strontium in water is approximately the same as that of iodide. The measured effective diffusivity of strontium and cesium in the rock materials was, however, found to be about 10 times higher than the effective diffusivity of iodide in the same type of rock materials. This could be due to a diffusignal transport of the adsorbed component at the pore surfaces, surface diffusion, acting in parallel with the pore diffusion. This paper describes two types of diffusion experiments with cesium and strontium in a biotite gneiss. The results are compared with iodide diffusion data from experiments with the same type of biotite gneiss [Skagius and Neretnieks, 1986]. EXPERIMENTAL In some of the above-mentioned investigations ['Skagius and

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The handling and safe disposal of spent fuel from nuclear power plants has been an issue since the 1950s when the first suggested method, geological storage in salt formations, was proposed in the US. Since then a number of methods have been developed for different types of bedrocks and waste types. One common need applicable to all these methods is to describe features and processes essential in repository design and demonstrations of longterm safety. So far, most methods have not described, nor emphasized, the importance of site-specific understanding of key parameters related to a specific repository design. Furthermore, the need of interdisciplinary research and the benefits gained when handling the site as a unified connected and mutually interrelated system (from bedrock to surface) have not been fully discussed. During a 30-year period, research has been performed in Sweden to demonstrate feasibility and long-term safety of underground geological disposal of spent nuclear fuel. In this paper, the overall strategy and discipline-specific modelling methods used in the site description of a final repository in Sweden are described, as exemplified by the Forsmark site. The resulting site description covers understanding of the historical evolution of the site, site data describing the current situation as well as spatially variable models needed to design the repository and evaluate long-term safety after closure. Finally, lessons learnt from this work are summarized, which are important when employing this method in the future.

If radionuclides are released from an underground repository and enter the groundwater, they would be transported with the moving groundwater in fissures in the rock. Besides fissures the rock also contains micropores filled with stagnant groundwater. If the nuclides are able to diffuse into these micropores, a significant retardation of the nuclides can be expected. At repository depths, the rock is exposed to rather high stresses caused by the large overburden of rock. When drill cores are taken up from the gound, this overburden no longer exists. As a result of this, there might be an increase in the porosity of the rock samples. The effective diffusivity measured in rock samples under atmospheric pressure in the laboratory would then be higher than the effective diffusivity in the rock "in situ." To simulate the stress that may exist in the bedrock at large depths, diffusion experiments with iodide and electrical resistivity measurements in rock materials under mechanical stress have been performed. It was found that the diffusivity in rock samples at 300-350 bars stress was reduced to 20-70% of the value in the samples under atmospheric pressure. This reduction of the diffusivity in stressed rock is probably of minor importance because of the rather large variation in diffusivity between samples. /

Laboratory experiments to determine the sorption and the rate of diffusion of cesium and strontium in pieces of granite have been performed. The effective diffusivity, Dp ep was found to be I-2, 10-12 m2/2 for both cesium and strontium.

The Swedish Nuclear Power Inspectorate is developing a new methodology for the construction of scenarios for radiological consequence analysis as part of its SITE-94 performance assessment project. SITE-94 involves the incorporation of site specific data from the Äspö site into a performance assessment (PA) of a hypothetical high-level waste repository. This paper describes a systems analysis approach that has been developed based on the concept of organising all the events and processes which need to be taken account of in PA into a ‘process system’ and a much smaller residual group which are used to generate scenarios. The methodology used for developing scenarios, producing calculation cases and addressing the various types of uncertainty involved in PA consequence analysis is described.