[en] The SUCO program is a three-step series of scaled model experiments investigating the optional sump cooling concept of the EPR. This concept is entirely based on passive safety features. This report presents the basic physical phenomena and scaling criteria of decay heat removal from a large coolant pool by single-phase and two-phase natural circulation flow. The physical significance of the dimensionless similarity groups derived is evaluated. The report gives first measurement results of the 1:20 linearly scaled plane two-dimensional SUCOS-2D test facility. The real height SUCOT test facility that is in its building up phase is presented. (orig.)