Application of a simple model for assessment of underground radionuclide migration. Part II. Assessment in rock-fracture systems

Abstract

The performance of a geological layer as a natural barrier to retard migration of radionuclides is assessed using simple mathematical models. A sensitivity analysis is presented on barrier performance when radionuclides discharged from the repository are spread in a certain area of the hydrologically static rock-fracture system. Transport of the nuclides to the biosphere is expected to be governed by diffusion to and/or pumping through other layers rather than by groundwater convection within the layer. Radionuclide concentration in the fracture may be predicted by simple first-order model equations with diffusion-equivalent rate parameters, the solution of which is expressed in a simple exponential form. The time for radionuclide concentration in the fracture to attenuate below a safety standard, which can be obtained from the solution quite easily is used as a criterion of barrier performance. Geological layer characteristics which make the criterion smaller are considered to be more favorable in decreasing the probability of high public exposure caused by the polluted groundwater. The sensitivity analysis showed that the criterion is smaller for larger matrix diffusion coefficients, but not always smaller for higher sorption capacities of the geological medium. For comparison, a sensitivity analysis is presented on barrier performance when the nuclide is transported to the biosphere by groundwater convection as well as diffusion-sorption to rock matrices in a rock fracture system. The solution for a single fracture-rock system was obtained and the distance for the radionuclide plume to attenuate below a certain level was calculated as a criterion of barrier performance. Geological layer characteristics which make the distance shorter were considered to be more favorable in decreasing the probability of high public exposure. Results of the calculation showed that the criterion became smaller for larger matrix diffusion coefficients, and higher sorption capacities of the geological medium.