Neutronic evolution and isotopic assessment of NIRR-1 under alternative fuel enrichment configurations: a WIMS-ANL/REBUS-ANL study

The Nigerian Research Reactor-1 (NIRR-1), a Miniature Neutron Source Reactor (MNSR), transitioned from highly enriched uranium (HEU) to low-enriched uranium (LEU) fuel to align with global non-proliferation objectives, necessitating a detailed evaluation of its neutronic behavior and isotopic evolution. This study employs the WIMS-ANL and REBUS-ANL computational framework to compare the burnup dynamics, reactivity profiles, and radionuclide inventories of NIRR-1’s HEU () and LEU core configurations under a representative operational schedule of 20 effective full-power days per year. Results reveal a steeper reactivity decline in the LEU core (17.02 pcm/EFPD) compared to the HEU core (13.97 pcm/EFPD), driven by enhanced resonance absorption, yet the LEU core’s higher initial mass extends its operational lifetime to 56.4 years versus 50.3 years for HEU. The LEU configuration produces 2.24 times more (0.767 g vs. 0.342 g at 252/282 EFPD), raising long-term waste management considerations. Both cores maintain robust safety through strongly negative temperature coefficients and low peak temperatures, supported by natural convection cooling. Comprehensive isotopic inventories and decay heat analyses (1620 W LEU vs. 1450 W HEU at shutdown) inform safety assessments and decommissioning strategies. Sensitivity studies highlight operational power and enrichment as critical lifetime influencers, with uncertainties validated against experimental data. These findings enhance NIRR-1’s operational strategy, support global MNSR conversion efforts, and provide critical data for safety and waste management planning, emphasizing proactive reactivity control and advanced fuel design exploration.