The role of the U.S. nuclear regulatory commission reactor systems codes throughout the world

In order to better understand phenomena occurring in hypothetical reactor system transients, The U.S. Nuclear Regulatory Commission (NRC) has developed an independent suite of reactor systems codes to perform confirmatory analysis. The codes are required to be flexible enough to model a wide range of scenarios in order to accurately predict the impact that any plant modifications proposed by licensees has on overall plant safety. Two of the most powerful codes currently used at the NRC, TRACE and MELCOR will be discussed as well as a brief highlight of their application both domestically and internationally. The MELCOR code is a fully integrated, engineering-level computer code whose primary purpose is to model the progression of postulated accidents in light water reactors as well as non-reactor systems (e.g., spent fuel pool and dry cask) by modeling general hydrodynamic phenomena such as heat and mass transfer as well as reactor specific phenomena important in severe accidents like core degradation and ex-vessel phenomena. The TRAC/RELAP Advanced Computational Engine (TRACE) is the latest in a series of advanced, best estimate thermal hydraulic systems analysis codes developed by the U.S. Nuclear Regulatory Commission for analyzing transient and steady-state thermal-hydraulic behavior in light water reactors. TRACE is used to analyze operational and safety transients such as small and large break loss-of coolant accidents (LOCAs), in both current and newly designed PWRs and BWRs, as well as the interactions between the related neutronic and thermal-hydraulic systems. (author)