CAREM is an Argentine project to achieve the development, design and construction of an innovative, simple and small Nuclear Power Plant (NPP). This NPP has an indirect cycle reactor with some distinctive and characteristic features (e.g. integrated primary cooling system by natural circulation, self-pressurized primary system and passive safety systems, etc.). In the frame of an IAEA forwarded HUMAN RESOURCES DEVELOPMENT - "Advice on CAREM passive emergency condenser" at the Centro Atómico in San Carlos de Bariloche (Argentina) German experimental as well analytical investigation of passive safety systems - mainly performed at the research centers Jülich and Rossendorf were presented. During this visit it was decided that FZR should determine the emergency condenser capacity by using the ATHLET code with and without the condensation module extension KONWAR (condensation inside horizontal tube). These results should be compared and assessed with existing RELAP calculations. The ATHLET and ATHLET/KONWAR calculations of the emergency condenser were used for minimizing the user influence performed with exactly the same nodalization scheme and the same initial and boundary conditions than the RELAP calculations. The comparison of all three computational results shows that the required condenser capacities were achieved. The magnitudes were in order of up to 12%. An additionally performed evaluation of local flow parameters show large deviations especially for the heat transfer coefficients at the inner and outer tube wall. Due to the good validation progress of KONWAR by an extensive module validation against NOKO and HORUS experiments, the focus of the further investigation were concentrated on the boiling heat transfer. A comparison of the ATHLET boiling model (according to Forster and Zuber in combination with the equation of Thom for the determination of temperature difference between wall and saturated fluid) with other models from literature (Borishansky, Fritz, Gormenflo, Stefan and Abdelsalam) shows a strong underestimation of heat transfer coefficients in low pressure conditions. Therefore the model of Forster and Zuber is replaced against the model of Stefan and Abdelsalam. The calculations with ATHELT/KONWAR were repeated and show now a better agreement (deviations less equal than 3 %) with the RELAP results.