Development and verification of the COME coupling platform for full-core pin-by-pin steady-state and transient analysis of the OECD/NEA PWR MOX/UO2 core transient benchmark

High-resolution full-core neutronics/thermal-hydraulics coupling remains computationally demanding because of the large number of spatial and angular degrees of freedom, strong nonlinear feedback, and repeated physics evaluations required in steady-state and transient calculations. In this work, the COupling Multiphysics Environment (COME) is extended to full-core pin-by-pin analysis of the OECD/NEA PWR MOX/UO 2 core transient benchmark. The coupled model combines the three-dimensional multigroup discrete-ordinates transport solver comeSn with the pin-resolved thermal-hydraulics and fuel heat-conduction solver comeSC. Response-based residual construction is used to preserve the standalone solver implementations. A global Picard–Jacobian-free Newton–Krylov (JFNK) strategy is applied to the neutronics-only and coupled critical-boron-concentration calculations, whereas a nested Picard/JFNK strategy is used for the rod-ejection transient. All COME calculations directly use the benchmark-provided eight-group pin-cell-homogenized cross-section library, thereby avoiding variability introduced by independent cross-section generation. For Part 1, author-generated multigroup OpenMC calculations using the same library and geometry provide a controlled same-library verification reference. The COME–OpenMC k eff differences are 18 pcm for the all-rods-out case and 39 pcm for the all-rods-in case. The corresponding signed pin-power differences range from −2.06% to +1.41% and from −2.54% to +3.35%, respectively. Parts 2–4 are assessed against the spread of published benchmark solutions because no independent same-library three-dimensional coupled reference is available. COME predicts critical boron concentrations of 1678 ppm at hot full power and 1336 ppm at hot zero power, an effective delayed neutron fraction of 577 pcm, and rod-ejection transient metrics of 0.229 s for peak time, 441% for peak power, 1.24 dollars of peak reactivity, and 40.3%-s power integral. These results demonstrate the implementation and assessment of a three-dimensional pin-resolved transport/thermal-hydraulics workflow within COME and the problem-dependent organization of its nonlinear coupling strategies. The reported results provide a reproducible deterministic transport-based comparison dataset under a fully specified pin-cell cross-section specification and may support future code-to-code assessment and method development.