Radionuclide transport in fractured rock a new model: Application and discussion

Abstract

A model for predicting the concerntration of radionuclides transported through fractured rock was developed in several earlier papers as part of the CEC programme on management and storage of radioactive waste. In this report, we examine that theory in more depth and develop expressions for the system parameters in terms of the rock statistics. A review of related work on fracture statistics is given and its relevance to the present work highlighted. In particular, we note that the connectivity, i.e. the average number of intersections per fracture, plays a vital part in determining the ‘cross section’ and redistribution function at intersections. Expressions are developed for these cross sections in terms of fracture orientation distributions. For a single set of fractures, i.e. fractures of one particular type, we have obtained numerical solutions and have demonstrated how the concentration profile becomes more dispersive as the orientation distribution becomes more isotropic. The method shows considerable promise and has many advantages when compared with the classical advective-dispersive model. In particular, it enables microscopic rock parameters, e.g. fracture length and orientation, to be employed directly and avoids the effective continuum approximation common to many approximations. It also explains the scaledependent effect and can account in a consistent manner for dead-end pores and channelling.