Nuclear Waste Repositories in Crystalline Rock- an Overview of Flow and Nuclide Transport Mechanisms
- PDF / 1,132,548 Bytes
- 14 Pages / 414.72 x 648 pts Page_size
- 27 Downloads / 170 Views
AN
IVARS NERETNIEKS Royal Institute of Technology, Department of Chemical Engineering and Technology, S-100 44 Stockholm, SWEDEN ABSTRACT Nuclides eventually escaping from nuclear waste repositories in crystalline rock will move with the seeping water in the fracture network. Most important nuclides interact physically or chemically with the rock and are expected to be considerably retarded allowing them to decay to insignificant concentrations. Velocity variations may allow some portions of the nuclides to move faster. Matrix diffusion and sorption on the surfaces of the rock are by far the most powerful retardation mechanisms and depend, in addition to the sorption and diffusion properties, directly on the magnitude of the "flow wetted surface" which is the contact surface between the mobile water carrying the nuclides and the fracture surfaces over which the nuclides diffuse into the matrix. A number of field experiments have been performed over the last 15 years to help validate the concepts and models and to obtain data. A number of such experiments are described and discussed in relation to the above issues. INTRODUCTION AND BACKGROUND Many countries are seriously considering siting final repositories for nuclear waste in water saturated crystalline rocks, at depths in excess of 500 m. The large mass of rock between the repository and the biosphere will ensure that disturbances at the surface and erosion will not influence the integrity of the repository. At large depths the flow rate of water is low because the hydraulic gradient and the hydraulic conductivity of the rock are low. This ensures that the transport capacity of the water passing the repository is very small. Low solubility of the waste matrix and of many of the individual nuclides further decreases the amount of nuclides that can escape. Many important nuclides will sorb or react by ion exchange and may be considerably retarded in relation to the water velocity. They may then have sufficient time to decay to insignificant concentrations before reaching the biosphere. Fast pathways due to channeling or strong dispersion can partly counteract these effects. Fracture zones in the "good rock" may form paths with higher flow rates. The rock minerals act as chemical buffers neutralising chemical changes in pH and Eh due to intrusion of acid and oxidising waters and from chemical disturbances caused by the repository materials, disturbances that might change the solubilities and sorptive properties of the nuclides. The matrix of the rock is porous, giving the nuclides access to the micropore surfaces in the interior of the rock for sorption and ion exchange, provided the nuclides can diffuse into the matrix from the mobile water in the channels in the fractures of the rock. The "flow wetted surface" between the flowing water in the channels and the stagnant water in the rock matrix will strongly influence the uptake. Matrix diffusion and sorption have been found to be the most important retardation mechanism. High flow rates extending over long distan
Data Loading...
