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Abstract

The disposal of radioactive waste is a topic with huge social importance. Understanding the complex geological, geophysical and geochemical conditions is indispensable for the long-term stability and the safe operation of subsurface nuclear waste repositories, regardless whether they are in salt rock, clay or granitoids. In this context, the subsurface temperature controls many natural processes and has mutual effects on the mineralogical and rheological conditions of the host rock and its neighboring rock mass. The characterization of the thermal field and of the governing heat-transfer processes are paramount for the layout of a multi-barrier repository and for its safe operation. Understanding the thermal field depends inter alia on the detailed knowledge of the surface heat flow, rock thermal properties (thermal conductivity, thermal diffusivity, heat capacity), and their variation in geological space. The latter requires a geological understanding of facies changes. Moreover, rock thermal properties need to be considered for effective in-situ pressure and temperature. In line of these requirements, research at GFZ, in the competence cluster ‘Earth Temperature Field’ is devoted to core-log integration techniques and to mapping and upscaling of data to different geological scale. In addition to indirect approaches to determine rock thermal properties from well-logging data, a particular focus is on laboratory measurements of thermal properties. Thus, a new laboratory vessel was recently tested that was designed and constructed by GFZ as part of the EU-funded ITHERLAB project. With this plethora of methods an overcome of the fundamental lack of data that still is present for all types of the targeted potential host rock types in Germany can be achieved. The data form the basis for the evaluation of both steady-state and transient heat transfer processes targeted on time-dependent solutions of the thermal field for any potential geological location. This is paramount for all scales, but of particular importance for the monitoring and controlling of the near-field temperature which is strongly affected by the technical storage construction and the heat-generating materials contained therein. For the waste-containment area surrounded and protected by clay as backfill material, our lab technology allows to thermally optimize the selected materials (e.g. bentonite mixtures) by studying their thermal behavior under simultaneously varying pressures, temperatures and fluid-saturating conditions