Abstract: The molten pool stratification morphology within lower head under various operating conditions is crucial for optimizing the IVR (in-vessel retention) strategy and enhancing safety measures during severe accidents. In recent decades, many prototype tests are conducted to investigate the influence of chemical reactions and composition of corium mixture on molten pool stratification, such as RASPLAV, MASCA and CESEF. In addition to tests, various thermodynamic models based on different theories are also employed to predict the behavior of corium. The prediction of the molten pool stratification requires density data of core materials at high temperatures while existing models and databases often rely on a weighted average of the single material densities. NUCLEA15 is a nuclear material database developed by IRSN, when combined with thermodynamic calculation software Thermo-Calc, it can be used for thermodynamic calculation of any equilibrium state in severe accidents. But volume data is not contained in this database either, making it impossible to analyze the molten pool stratification. This work presented a novel thermodynamic analysis method by integrating a dedicated volumetric database to NUCLEA15 to calculate densities of stratified liquid phases at high temperatures. The CALPHAD (CALcalculation of PHAse Diagram) method was the fundamental principle of establishing a thermodynamic database for multi-component system. A volume database could be integrated into the NULCEA15 using the method, which initiated by establishing reasonable volume models for both pure substances and solution phases. This study compiled test data for the required pure substances to fit a volume model. Subsequently, the pure component model was extended to the solution phase model. Various stratification patterns under changing conditions of U/Zr atomic ratios, Zr oxidation degree, and steel content were investigated. To validate this method and the database, the calculation results were compared with the data from MASCA and CESEF tests. The results show that the calculated results about the molten pool stratification morphology, representing the relative positioning of the metal layer and the oxide layer, consistent with all test observations. The calculated element distribution results in each layer are in agreement with the MASCA test. The element distribution in the metal layer of CESEF test has a deviation, which may because the element distribution results in MASCA are axial average values, while CESEF test provides local measurement values. The element migration process and density changes of metal and oxide layers are consistent with the test results. It is concluded that the thermodynamic calculation method proposed and the volume database built in this paper are applicable for analyzing the molten pool stratification morphology following the reaction between corium and steel. It can be used to determine the equilibrium molten pool layering, illustrate the distribution of molten pool elements, and calculate the density of the molten pool.