Radionuclide Transport in Fractured Media

by M. M. R. Williams, Electrowatt Engineering Services, (UK) Ltd, Horsham, United Kingdom,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1993


Until recently, the classical advective-dispersive transport equation was considered to be an adequate model for describing the motion of a solute (e.g. radionuclides) in porous and fractured media. In this model, the dispersion coefficient is either obtained from microscopic model of the porous medium or by carefully controlled experiments. As a result of such experiments, a large body of data has been accumulated on the dispersivity. Detailed examination of these data has resulted in a curious phenomenon being discovered; namely, that the longitudinal dispersion length is 'scale-dependent'. That is to say the value deduced depends on the 'size' of the experiment, i.e. on the distance over which measurements are made. Several interesting attempts have been made to develop theories which explain this phenomenon, all based on treating the velocity of the water in the porous medium as a spatially random variable, but retaining the advective-dispersive balance equation. In this work we present an entirely new approach to the problem of solute transport in fractured media based upon an analogy with neutron transport. The new method has several advantages over the previous theories and these will be explained below. Results from the new theory are in agreement with experimental trends and do not require any further adjustment to explain the scale-dependent effect.

Subject Headings: Cracking | Radioactive materials | Porous media | Rock mechanics | Advection | Equations of motion | Distance measurement | Water treatment

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