With PASTELS, European reference thermohydraulic codes will first be tested and validated by separate and combined effect test that include the interaction of a limited number of elementary phenomena. Thus, the performance of these codes will also be measured on the results of the integral test facilities (PASI and PKL). Coupling methodologies between system and CFD codes will be developed to improve the prediction of passive systems behaviour. Through the improved predictive numerical capabilities, it will be possible for European actors to propose optimised high-performance SACO and CWC for the future nuclear powerplants markets as the exploration and qualification of reliable solutions will be possible.