Developed a FastFlux energy group structure within the terrapower/armi repository to support advanced fast neutron flux calculations. This work involved defining new energy groupings using the HIGH_ENERGY_EV and FAST_FLUX_THRESHOLD_EV constants, expanding the platform’s neutronics and physics calculation capabilities for fast flux scenarios. The implementation enhances model fidelity for users conducting fast flux studies and establishes a foundation for broader fast-flux modeling. Leveraging Python and domain expertise in neutronics, the developer focused on clear code traceability and maintainability, as evidenced by detailed commit documentation. No bug fixes were recorded during this period, with efforts concentrated on feature development and technical depth.