Modeling Solute Transport in Fractured Rocks-Role of Heterogeneity, Stagnant Water Zone and Decay Chain

Sammanfattning: A model is developed to describe solute transport and retention in fractured rocks. It accounts for the fact that solutes not only can diffuse directly from the flowing channel into the adjacent rock matrix composed of different geological layers but can also at first diffuse into the stagnant water zone occupied in part of the fracture and then from there into the rock matrix adjacent to it. Moreover, the effect of radioactive decay-chain has also been studied in the presence of matrix comprising different geological layers. In spite of the complexities of the system, the analytical solution obtained for the Laplace-transformed concentration at the outlet of the flowing channel can conveniently be transformed back to the time domainby use of e.g. De Hoog algorithm. This allows one to readily include it into a fracture network modelorachannelnetwork model to predictnuclide transport through channels in heterogeneous fracturedmedia consisting of an arbitrary number of rock units withpiecewise constant properties. Simulations made in this study indicate that, in addition to the intact wall rock adjacent to the flowing channel, the stagnant water zone and the rock matrix adjacent to it may also lead to a considerable retardation of solute in cases with a narrow channel. The results further suggest that it is necessary to account for decay-chain and also rock matrix comprising at least two different geological layers in safety and performance assessment of the repositories for spent nuclear fuel. The altered zone may cause a great decrease of the nuclide concentration at the outlet of the flowing channel. The radionuclide decay, when accounted for, will drastically decrease the concentration of nuclides, while neglecting radioactive ingrowth would underestimate the concentration of daughter nuclides.