GEOCHEMICAL ANALOGUES TO TRANSPORT FEATURES AND PROCESSES OF A MINED GEOLOGIC REPOSITORY

Geochemical natural analogues provide one of the multiple lines of evidence intended to increase confidence in the safe geologic disposal of high-level radioactive waste. Analogues provide an understanding of processes at temporal and spatial dimensions that cannot be tested by laboratory or field-scale experiments. For example, analogues add valuable insight to understanding long-term waste-form degradation processes through the record left behind in secondary minerals and groundwater chemistry. In addition, measurement of the concentration of fission products as tracers in rock and groundwater surrounding uraninite provides a satistfactory approach to estimating natural dissolution rates, as was tested at a number of sites.

In the event of waste mobilization and migration away from emplacement drifts, the rate of radionuclide transport through the unsaturated zone is determined by the percolation flux and by the hydrologic and sorption properties of the rock units. Fractures act both as transport pathways and as places of retardation at a number of unsaturated analogue sites in North America and Europe. In the saturated zone, advective transport along fractures has been identified as a more significant transport mechanism than matrix diffusion in all the analogues studied. Matrix diffusion in crystalline rock is generally limited to only a small volume of rock close to fractures, but even a small volume can make a significant difference in radionuclide retardation. In most studies of natural systems, a proportion of the total metal concentration in groundwater was associated with colloids. Colloid transport appears to be an important factor for migration of thorium in one open unsaturated system, but not in others where filtration of colloids is effective.