Feasibility Study of History-Based Batch Method for Coupled Monte Carlo–Thermal-Hydraulics Analysis Using the iMC Code

In steady-state Monte Carlo (MC) simulations, correlations between successive fission source distributions (FSDs) result in a mathematical underestimation of the sample variance associated with tally means. The history-based batch method (HBM) addresses this issue by estimating variances from history-based batches, suppressing intercycle correlations and yielding statistically consistent uncertainty estimates. Recently, HBM capabilities have been implemented and verified in the iMC Monte Carlo code developed at the Korea Advanced Institute of Science and Technology. In this work, the HBM framework is extended to multiphysics simulations by applying it to coupled MC–thermal-hydraulics (TH) analysis using Picard iteration. This approach enables history-batch-wise evaluation of both neutronics and TH parameters at the converged solution, which is not possible with conventional single-run MC simulations. The results confirm that HBM improves variance estimation for neutronics quantities and additionally provides statistically meaningful estimates of TH-related variables, demonstrating its value as a practical tool for uncertainty quantification in coupled MC-TH analyses.