[en] The TRUE Block Scale project was performed at the Aespoe Hard Rock laboratory as an international partnership funded by ANDRA, ENRESA, JNC, Nirex, Posiva and SKB. The project, initiated mid 1996, was divided in a series of defined stages; Scoping Stage, Preliminary Characterisation Stage, Detailed Characterisation Stage, Tracer Test Stage and the Evaluation and Reporting Stage. The specific objectives were to: 1) increase understanding of tracer transport in a fracture network and improve predictive capabilities, 2) assess the importance of tracer retention mechanisms (diffusion and sorption) in a fracture network, and 3) assess the link between flow and transport data as a means for predicting transport phenomena. Characterisation in included drilling, core logging, borehole imaging, borehole radar, 3D seismic surveys, hydraulic tests (flow logging, single hole tests, cross-hole interference tests), tracer dilution tests, hydrogeochemical analyses of groundwater samples and various types of mineralogical, geochemical and petrophysical measurements on drill core samples. Drilling and characterisation of each new borehole was followed by analysis and decision with regards to need and geometry of a subsequent borehole. The main set of tools for determining the conductive geometry and the hydro-structural model was a combination of borehole television (BIPS), high resolution flow logging and pressure responses from drilling and cross-hole interference tests. The constructed hydro-structural model was made up of a set of deterministic sub-vertical structures mainly oriented northwest. Hydraulic features not part of the deterministic set were included in a stochastic background fracture population. Material properties and boundary conditions were also assigned to the developed model. Characteristics and properties measured in the laboratory were integrated in generalised microstructural models. Hypotheses formulated in relation to defined basic questions were addressed in the in situ radioactive sorbing tracer tests and in the subsequent evaluation using numerical models. The in situ tracer test programme was crowned by four injections of cocktails of radioactive sorbing tracers in three different source-sink pairs over distances ranging between 15 and 100 m, as integrated along the deterministic structures of the hydro-structural model, defining flow paths of variable complexity. Numerical modelling using a variety of concepts/codes constituted an important and integrated component of the project. A major accomplishment in this context was the development of a common conceptual basis for transport and retention. The fractured crystalline rock volume was here conceptualised as a dual porosity medium (mobile-immobile). Model predictions of the sorbing tracer tests were followed by evaluation modelling where the various modelling results were used for elevating understanding of block scale transport and retention and relative role of processes. Diffusion to the immobile pore space, sorption in the immobile pore space and surface sorption on the fracture surfaces along the transport paths were interpreted as the main retention processes in the prediction and evaluation models applied. This interpretation was supported both by the characteristics of in situ breakthrough curves and modelling, where in the latter case the measured residence time distributions were reproduced more accurately with diffusional mass transfer invoked. Geological information from the site also provides support for the assumption of multiple immobile zones along the investigated flow paths