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 with the MELCOR code. An NGNP model was developed by modifying the GT-MHR model. Simulations under normal operation condition and pressurized conduction cooldown accident were performed and the results were compared against the data from literature. MELCOR has been demonstrated to have the ability of modeling a prismatic core VHTR. VHTR technology envisions satisfying the growing energy needs while meeting the goal of sustainability. This analysis will facilitate the development of this technology. It will be especially useful for professionals in the field of alternative or nuclear energy.