Comparison of high-fidelity transport code predictions against the NECTAR intra-pin reaction rate measurements

The advancement of high-fidelity nuclear reactor simulation methods is essential for reducing conservatism in safety margins and optimizing reactor design and operation. This work evaluates the predictive capabilities of the deterministic solvers GeN-Foam and MPACT for modeling intra-pin reaction rate distributions in heterogeneous reactor cores, using experimental data from the NECTAR campaign conducted at the CROCUS reactor. Radial and azimuthal reaction rate distributions from GeN-Foam and MPACT are compared to NECTAR measurements, with Serpent2 Monte Carlo simulations supporting the validation. GeN-Foam predictions align with measurements within the 0.4% experimental uncertainty, while MPACT results require further improvements to account for the 3D nature of the experiments, particularly in modeling the axial leakage in the vicinity of the dosimeter; and in the cross-section self-shielding treatment of dosimetry materials. To support broader validation efforts, this study provides detailed descriptions of the NECTAR experiments, which are planned to be integrated into the International Reactor Physics Evaluation Project (IRPhEP). Recommendations are made for future validation efforts, including additional measurements with alternative dosimeter materials and enhancements to solver methodologies. • NECTAR experiments provide high-resolution high-precision intra-pin reaction rates. • 3D modeling is required to capture azimuthal variations near the CROCUS reflector. • Serpent2 and GeN-Foam predictions are consistent with the NECTAR experiments. • MPACT improvements needed for dosimetery (axial leakage, nuclear reactions).