Modeling and simulation of the start-up of a thorium-based molten salt reactor

Previous analysis of liquid-fueled molten salt reactors (MSRs) has largely focused on the state of the reactor at an equilibrium condition after fission products have built up in the fuel salt over years of operation. Little analysis has been conducted on the way the isotopic composition of the fuel salt changes from the start-up of an MSR until this equilibrium condition. This is a particular concern when considering the start-up of a thorium-based MSR where the most likely scenario requires the first of these MSRs to be started without available U. Building upon previous work at Oak Ridge National Laboratory, a Python script known as ChemTriton has been developed to simulate MSR operation by modeling the changing isotopic composition of an irradiated fuel salt. In selecting the source of initial fissile material for starting up an MSR, the initial fuel salt compositions are made feasible after (i) removing the associated non-fissile heavy metal isotopes after much of the initial fissile isotopes have burned and (ii) adjusting the thorium concentration to maintain a critical configuration without significantly reducing breeding capability. Any non-fissile heavy metals added to the fuel salt (e.g., ²³⁸U in low-enriched uranium compositions) reside within the reactor for long periods of time, preventing an MSR from reaching a stable equilibrium state for well over 20 years. Plutonium recycled from light-water reactor spent nuclear fuel serves as the best initial fissile material alternative source to ²³³U as it contains the highest percentage of fissile isotopes and is chemically separable from the ²³³U being bred in the reactor.