This paper deals with the improvement and validation of numerical tools for the simulation of design basis accidents in nuclear power plants equipped with passive safety systems. Numerical models are implemented in the framework of the 1-D thermal–hydraulic system code ATHLET developed by GRS. Experimental reference data for the validation were obtained at the INKA test facility, a model of the KERENA reactor, reproducing the passive safety systems nearly at full scale. The validation effort focuses firstly on the accuracy of the models for the single passive components, and secondly on the ability of the numerical simulation to reproduce the interaction of all components of the KERENA design under realistic conditions as reproduced in the INKA test facility. Thermal-hydraulic models are presented and validated for two passive components of the KERENA reactor: the passive pressure pulse transmitter and the pressure-controlled flooding valve. Finally, the full model of the INKA facility, including these and other passive components, is discussed and numerical results for simulations reproducing three different design basis accidents are validated by comparison with the corresponding experimental data.