Impact of rock heterogeneity in matrix diffusion process : a single fracture case at the Äspö granitic site (Sweden)
Abstract
Within the framework of nuclear spent fuel storage, special concern is put on experimentation and modelling work to improve the modelling capacities for the transfers of radionuclides within natural fractured media. Several aspects make it a challenging task, among which the heterogeneity of the system, the scarcity of the available information, the strong contrasts in the parameter values between mobile and immobile zones. One of the key retention process is related with diffusion in immobile zones within the fractured medium (fractions of the plume are temporally retained from the main flow paths by diffusion processes into these zones). The importance of this mechanism increases with the contact time of the plume with these zones. This means that for a post closure situation matrix diffusion plays the larger role. We provide here with results obtained within the SKB Task Force (Task6) in relation with the single fracture tracer experiments (TRUE1 Project - feature A at Äspö site in Sweden). The purpose of this task, involving several other modelling teams, is to provide a bridge between detailed SC (Site Characterization) models operating at experimental time scale and more simple PA (Performance Assessment) models operating at large time scales. A good level of realism is obtained in the definition of the task by relying on the very extensive and valuable Äspö site data base. We conducted the study with different representations of the system (involving deterministic and stochastic approaches). Our simulation code is Cast3M with a mixed and hybrid finite element scheme. We show here our main conclusions concerning the role played by different matrix zones (fracture coating, fracture infilling, altered and non altered rock ), the ability to identify the heterogeneous matrix zone properties from tracer tests or independent measurements, the type of zones accessed by the plume as a function of the time scale of the test, the homogenisation of transport processes leading to simplified representation of the matrix zones.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....6922G