The IAEA facilitated the co-operation on research and development, including the development and validation of computer codes for design and safety analysis of NPPs, to bring together the experts from the Member States with water cooled reactor technologies experienced in developing and using the severe accident codes to further advance the state-of-knowledge on uncertainty propagation in severe accident simulation and modelling analyses, as well as increase the sophistication and competency of the practitioners in this field. This publication is one out of five in the TECDOC Series addressing specific reactor lines and it provides descriptions by contributing institutions of their utilized uncertainty and sensitivity methods for severe accidents analysis in large Pressurized Water Reactors (PWRs) and in Integral PWR-type Small Modular Reactors. This publication is intended for nuclear engineers, vendors, researchers, code developers, university professors and university students, regulatory experts, and nuclear power plant designers and planners.