Hydrogen, a more sustainable source of energy, is a potential substitute for hydrocarbon fuel for power generation. The Very High Temperature Reactor (VHTR) concept can produce hydrogen with high efficiency and in large quantities. In order to acquire licensing eligibility for VHTRs, analyses of VHTRs under operation conditions and accident scenarios need to be performed. In this book, MELCOR, a severe accident code, was used to analyze one of the VHTR designs - a prismatic core Next Generation Nuclear Plant (NGNP). A model of a 600 MW Gas Turbine-Modular Helium Reactor (GT-MHR) was developed...