Abstract
A conceptual assessment of the conversion of the Impulse Graphite Reactor (IGR) from high-enriched uranium (HEU) fuel to high-assay low-enriched uranium (HALEU) fuel is presented. The proposed conversion concept is based on the expansion of the core and the application of uranium-impregnated graphite fuel using IG-430 graphite. The study investigates whether the key performance characteristics of the reactor can be preserved under HALEU conditions while maintaining the existing operational modes and addressing primary safety considerations. The analysis was performed using a previously validated methodology for coupled neutronic and thermal transient simulations of the IGR reactor. Five representative transient scenarios, including regulated pulses, long-duration transients, and self-terminating neutron bursts, were analyzed for both HEU and HALEU configurations. The performance criteria considered include neutron flux and fluence parameters, pulse kinetics, energy release in an experimental device (ED), reactivity margins, shutdown capability, and thermal limits of the graphite core. The results demonstrate that all required conversion criteria can be achieved for the proposed HALEU configuration without approaching limiting thermal conditions. The obtained reactivity margins and shutdown characteristics additionally provide significant tolerance to uranium concentration variations in graphite fuel blocks.