In case of a temporal undefined failure of the active cooling system of a spent fuel pool (SFP), the water inside the pool heats up depending on the decay heat of the stored spent fuel. In the worst case, this leads to a boil-off scenario with a water level decrease and, after appropriate time, to a progressive uncovery of the fuel assemblies (FA). The cooling of the uncovered structures is limited by the steam production in the lower still wetted part of the FA, radiative heat transfer, radial heat conduction and convection inside the FA and interaction with ambient air depending on the storage conditions. As a result, the removal of decay heat can be endangered and, by reaching certain cladding temperatures, the integrity of the cladding tubes can be compromised. The detailed heat and mass transfer mechanisms of the boil-off scenario are not yet sufficiently investigated – especially experimentally. Therefore, reliable conclusions about the development of the cladding temperatures cannot be made. Against this background, the SINABEL project was launched in 2013 at TU Dresden for an improved understanding of the heat transport processes inside a SFP under accident conditions. Studies are performed both experimentally on the test facility ALADIN and theoretically by using computational fluid dynamics (CFD) calculations and integral codes for severe accidents. The test facility simulates a generic boiling water reactor (BWR) FA on almost original scale with regard to the boundary and ambient conditions within a SFP. Through an appropriate unique net of instrumentation, the distribution and development of radial and axial cladding temperatures and the development of the water level can be investigated in detail. Experiments were carried out for various decay heats respectively rod powers and changing storage distances. The thermal hydraulics of the heat-up and the boil-off phase with the occurrence of subcooled boiling in the beginning followed by geysering-like instabilities are described in the present paper. The higher the heat load, the faster the heat-up and, as boiling temperature is reached, the lowering of the water level. The maximum limit of the water level and the influence by the radial heat conduction, convection and radiative heat transfer is shown and explained. The experimental data and thermohydraulic findings provide the basis for the phenomenological understanding of this accident scenario and are used for the validation and improvement of numerical simulations.