The Grimsel (Switzerland) migration experiment: integrating field experiments, laboratory investigations and modelling

For several years tracer migration experiments are performed at Nagra's Grimsel Test Site in the Swiss Alps as a joint undertaking of Nagra, PNC and PSI. The aim is to develop methods for field experiments at possible sites for nuclear waste repositories and to test radionuclide transport models.A hydraulic dipole field is generated in a well-defined fracture zone in granite. The tracers used are non-sorbing (uranine, ³He, ⁴He, ⁸²Br⁻, ¹²³I⁻), mildly sorbing (²²Na⁺, ²⁴Na⁺), and more strongly sorbing (⁸⁵Sr²⁺, ⁸⁶Rb⁺, ¹³⁴Cs⁺, ¹³⁷Cs⁺). These experiments have been complemented by extensive laboratory investigations on petrography, on water-rock and nuclide-rock interaction as well as by migration experiments with bore cores.The main questions addressed are: What are the relevant geometric factors and mechanisms for transport, how well can breakthrough curves be extrapolated from one dipole arrangement to another, which parameters are scale dependent, is there a difference in sorption values between laboratory and field experiments or between static and dynamic experiments. Evaluating the experimental results for the non-sorbing uranine and the mildly sorbing tracers sorption, Strontium, we show that a consistent picture of tracer transport, and specifically of tracer sorption, is obtained when exploiting all available experimental information and using not too simplistic models.