Appearance of water hammer in thermal-hydraulic systems was widely studied employing different state of-the-art thermalhydraulic codes. Before carrying out the water hammer analysis, it is very important to match the model and to perform the analysis of its sensitivity. The paper presents an analysis of the water hammer experimental test performed at the Fraunhofer UMSICHT Institute using the RELAP5/Mod3.3 thermal hydraulic code. The model sensitivity study was performed by using the Fourier

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... litude sensitivity test (FAST) method. The FAST method aims to determine the most important input parameters that are major contributors to the model output uncertainty. Such information can be used further for a more detailed system study and development of improvements or preventive actions.