Over the last decades, passive safety systems have been gathering a noticeable international interest playing an essential role in tackling the safety-related design goals of advanced Nuclear Power Plants (NPPs). A crucial challenge, still to be deal with, is the quantification of uncertainty in passive safety systems reliability assessments. In particular, uncertainty may be present in some input parameters of the Thermal-Hydraulic (T-H) models used to predict the passive system behavior, which is typically quantified by “expert judgment”. The purpose of the present thesis is to develop an advanced Inverse Uncertainty Quantification (IUQ) framework to support expert judgment in the assessment of the (epistemic) uncertainty affecting T-H model input parameters.