The German reference concept for the final disposal of spent fuel elements comprises their emplacement in Pollux casks in backfilled drifts of a repository in salt rock. Since 3,5 years the Asse salt mine is the location of a large scale trial to demonstrate this storage technology and is intended in particular to allow assessment of long term thermomechanical behaviour of backfill and rock mass. As a result of heating to a maximum 200°C a considerable increase was measured in rock mass deformation and drift closure. Substantial thermally induced stresses result in the rock mass. The backfill pressure shows a steady increase, which is however far below the initial stress state in the host rock.