A Channel-Network-Model for Radionuclide Transport in Fractured Rock-Testing Against Field Data
- PDF / 492,733 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 27 Downloads / 189 Views
A CHANNEL-NETWORK-MODEL FOR RADIONUCLIDE TRANSPORT IN FRACTURED ROCK- TESTING AGAINST FIELD DATA. Bjom Gylling, Luis Moreno and Ivars Neretnieks Department of Chemical Engineering and Technology Royal Institute of Technology S-100 44 Stockholm, Sweden.
ABSTRACT A new model concept for describing flow and transport in fractured rock has been tested on some field tracer experiments. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. For times of interest for deep geological repositories for nuclear waste it has been found that the volume of the channels, i.e. the flow porosity of the rock, has no practical influence on the Residence Time Distribution of the nuclides. It is totally dominated by the matrix diffusion effects. The specific Flow Wetted Surface, FWS, of the channels therefore needs to be known. A method to evaluate the FWS from hydraulic measurements in boreholes is presented. Tracer tests at the Swedish ASPO rock laboratory have been predicted using this model and data obtained in the way outlined in the paper. The main tracer test was predicted reasonably well without using any adjustable parameters. Some other tracers also used were not detected in the tests as they were expected to be. We discuss some of the possible reasons for this in relation to geological observations made in the expected pathways for these tracers. The sensitivity of results to uncertainties in parameter values is also discussed.
SCOPE Our aim is to devise a model where hydraulic data and conducting fracture frequencies are essentially the only information needed to predict the migration of species that interact with the matrix of the rock. This would obviate the need to use tracer tests to determine flow porosities and hydrodynamic dispersion properties. The basic idea is that for interacting species, it is the flow wetted surface, FWS, that determines the residence time in a pathway. The variability of conductivities of different pathways in the channel network in the rock will determine the Residence Time Distribution, RTD, over distances that include many different paths. INTRODUCTION AND BACKGROUND A new type of model concept for describing flow and transport in fractured rock has been devised and tested on some field tracer experiments. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. The channels are characterized by lengths, widths, apertures and transmissivities. The rock matrix is assumed to be porous and the nuclides can diffuse into the matrix. For the times of interest for deep geological repositories for nuclear waste it has been shown that the residence time of the nuclides is determined by the diffusion into the matrix porosity. This applies for non-sorbing as well as
Mat. Res. Soc. Symp. Proc. Vol. 353 C 1995 Materials Research Society
396
sorbing nuclides. The sorbing nuclides are, in addition, strongly retarded by the sorption on the pore surfaces in th
Data Loading...
