Neutron transport analysis of a supercritical water small modular reactor

Abstract

A detailed model of the ECC-SMART small-modular supercritical-water reactor has been made using the OpenMC Monte Carlo code by developing three models with different levels of fidelity: a 2D infinite lattice pin-cell model, a 3D finite-length fuel assembly model including end structures, and a full-core model. An offline neutronic/thermal-hydraulic coupling was implemented by integrating the OpenMC model with an existing RELAP/SCDAPSIM model developed in the ECC-SMART framework. The temperature and density distributions from RELAP/SCDAPSIM were mapped to OpenMC, and the resulting power distribution was iteratively fed back. Convergence was reached within five iterations, yielding a neutron multiplication factor keff = 1.19338±0.00008, below the 1.22 target for a two-year cycle. The power distribution showed a bottom-skewed axial shape due to coolant density variations, with a peaking factor of PPF = 2.006±0.003. High stage peaking factors (SPF > 1.6) occurred in upper and lower stages, due to leakage and geometric asymmetries, indicating the need for better power flattening to lower peak temperatures and achieve more uniform burnup. Temperature feedback was assessed by locally perturbing the converged thermal-hydraulic distributions, updating densities using the IAPWS-IF97 standard, without re-running full coupled iterations. Moderator-to-fuel ratio effects were examined by varying the inter-assembly gap from 14 mm to 25 mm. Taking advantage of OpenMC’s Python API, all models can be easily customized to automate a wide range of parametric studies.