[en] SKB's concept on deep geological disposal of spent nuclear fuel is based on a multi-barrier system for isolation of the spent fuel from the biosphere. The barriers are a low-solubility waste form, encapsulation of the fuel in a copper canister, a bentonite buffer surrounding the canister, and the host rock. In case of an initial canister damage, the retention capacity of the host rock for the short lived radionuclides such as Cs and Sr is important. For the Operating Phase of the Aespoe Hard Rock Laboratory the need for a better understanding of radionuclide transport and retention processes was recognized. This included enhancement of confidence in models to be used for quantifying transport of sorbing radionuclides in performance assessment. Further, to be able to show that pertinent transport data could be obtained from site characterization or field experiments and that laboratory results could be related to retention parameters obtained in situ. To resolve these issues SKB initiated the Tracer Retention Understanding Experiments (TRUE). (author)