Progress in validation of mpact with critical experiments

Recent advancements in reactor modeling with the CASL deterministic neutronics simulator MPACT have necessitated assessment of the code's simulation capability through comparisons with critical experiment benchmarks. The benchmarks include DIMPLE and VENUS-2 from the IRPhEP benchmark handbook and OECD/NEA, respectively. Comparisons were made for criticality and fission rate distributions using 2D models with axial buckling. Results from the MPACT DIMPLE model are compared with reference results obtained from a continuous energy KENO model to allow for fission reaction rate comparisons in all fuel pins, rather than discrete foil reaction rate measurements from the DIMPLE benchmark. The VENUS-2 simulation results were compared with published benchmark data. For the DIMPLE comparison, default and fine MOC discretization parameters are used. When accounting for the experimental uncertainty of the reference axial buckling measurements, the MPACT results bound criticality. The RMS fission rate differences with KENO for each of the 2D DIMPLE models with fine discretization are below 0.5% and the criticality is calculated within 200 pcm. A fission rate bias is observed in DIMPLE S06A, which does not include the stainless-steel baffle present in S06B. The bias is likely caused by errors in the ²³⁵U thermal reaction rates. Analysis of reaction rate differences between MPACT and MCNP results show large error cancellation in thermal energy groups, leading to the observed thermal bias. For the VENUS-2 benchmark, fission rate results from MPACT and other codes show a slight underestimation in the 3.3 wt.% UO₂regions and an overestimation in MOX fuel regions. Overall, the results confirm the accuracy of MPACT's 2D MOC solver, scattering approximation, and the cross-section library for LWR problems. Future work will be focused on 3D modeling using a new cross-section library that resolves the thermal reaction rate issue at the DIMPLE core boundary.