Validation of ENDF/B-VIII.0 and NCrystal thermal scattering laws for graphite in criticality safety analysis using OpenMC

Accurate prediction of criticality in nuclear systems relies on high-fidelity nuclear data, particularly for thermal neutron scattering in moderating and reflecting materials. This study presents a comprehensive benchmarking analysis of two distinct thermal scattering law (TSL) treatments for crystalline graphite: the standard evaluated data from the ENDF/B-VIII.0 library and a physics-based model provided by the NCrystal library. These models were implemented within the OpenMC Monte Carlo transport code to simulate five graphite-reflected critical benchmark experiments from the International Criticality Safety Benchmark Evaluation Project (ICSBEP) handbook. The benchmarks include highly enriched uranium (HEU), intermediate-enriched uranium (IEU), and plutonium metallic core. A detailed comparison of the macroscopic cross sections revealed that the primary difference between the two models lies in the treatment of coherent elastic scattering. The NCrystal model provides a more physically rigorous treatment by explicitly calculating the sharp Bragg edges characteristic of neutron diffraction, whereas the ENDF/B-VIII.0 TSL utilizes a simplified, smoothed approximation of these crystalline effects. The calculated effective multiplication factor (keff) from both models showed good agreement with the experimental values, generally within 1%. However, systematic discrepancies of up to ∼38.10 -3 % were observed between the two models. These differences are most pronounced in benchmarks with intermediate neutron spectra, where the reactivity of the system is highly sensitive to the thermalization and reflection properties of graphite. • Graphite thermal scattering is benchmarked using ENDF/B-VIII.0 and NCrystal data. • Simulations of five ICSBEP critical experiments were performed with the OpenMC code. • The primary difference between models is NCrystal's explicit treatment of coherent elastic scattering (Bragg edges) versus the smoothed ENDF/B-VIII.0 approximation. • Systematic, statistically significant differences in keff of up to approximately 38 pcm (0.038%) are observed between the two models. • The impact on keff is most pronounced in intermediate-spectrum systems where reactivity is sensitive to the reflector's thermal scattering properties.