<b>Impact of ENDF/B-VII.1 and ENDF/B-VIII.0 Nuclear Data Libraries on NIRR-1 LEU Core Depletion using OpenMC</b>

Abstract

The Nigeria Research Reactor-1 (NIRR-1), a 30 kW Miniature Neutron Source Reactor (MNSR), operates at low power with a highly thermalized neutron spectrum, leading to extremely low fuel burnup over extended lifetimes. Accurate depletion modeling is critical for predicting isotopic evolution, reactivity loss, core lifetime extension via beryllium shims, and safeguards compliance after the 2018 conversion from highly enriched uranium (HEU) to low-enriched uranium (LEU) fuel (~13% ²³⁵U in UO₂). This study examines the sensitivity of NIRR-1 LEU core depletion predictions to ENDF/B-VII.1 (2011) and ENDF/B-VIII.0 (2018) nuclear data libraries using the open-source Monte Carlo code OpenMC. Parallel simulations over 10 effective full power days (EFPD) at nominal power kept all inputs identical except the nuclear data library. Results reveal strong convergence in this low-burnup thermal regime. U-235 depletion shows small oscillating differences (average absolute ~0.17%, max |0.32|%), U-238 consumption is negligible (<0.1% net variation), and Pu-239 buildup exhibits a modest systematic positive bias in ENDF/B-VIII.0 (~+0.22% average, up to +0.44% early). Burnup accumulation remains linear with relative differences <0.5% (average ~+0.27% higher in ENDF/B-VIII.0). These subtle effects, stemming from CIELO-project refinements (improved U-235 fission, U-238 resonances, Pu-239 fission/nu-bar), are far smaller than discrepancies in high-burnup light-water reactor benchmarks. Nuclear data library choice has negligible short-term impact on NIRR-1 operations, supporting ENDF/B-VIII.0 adoption for enhanced long-term accuracy without altering core management or safety margins. Extrapolation indicates potential 1–3% cumulative variances over the ~50–60-year LEU lifetime, warranting extended simulations and experimental validation (flux measurements, post-irradiation examination). The work highlights open-source Monte Carlo tools for high-fidelity analysis in resource-limited settings and advances lifetime predictions, non-proliferation safeguards, and capacity building for African research reactors.