Advancing Neutronic‒Thermal-Hydraulic Coupling for Safety Assessment of MTR-Type Research Reactors: Application to Steady-State and Transient Analysis of OPAL Research Reactor

The paper presents the development and implementation of a neutronic–thermal-hydraulic (N/TH) coupling scheme based on a two-step Monte Carlo (MC) approach. The proposed computational strategy is applied to the benchmark OPAL research reactor to assess its capability for both steady-state conditions and a reactivity insertion accident (RIA) scenario. In the steady-state analysis, the first step of the MC-based coupling is used as an internal validation tool for the identification of power hot spots under selected critical configurations. The resulting power distributions provide valuable insights relevant to safe reactor operation under normal operating conditions. For the transient analysis, results obtained using standalone thermal-hydraulic system and subchannel codes—namely RELAP5 and PARET—are presented alongside those from the two-step MC-based approach. The comparison focuses on key parameters, including the time-dependent core power evolution and coolant outlet temperatures. The results show an overall good agreement between calculated and available experimental data for the investigated RIA transient. Minor discrepancies between measured and predicted quantities are observed and are primarily attributed to experimental uncertainties and limitations in data consistency, rather than to deficiencies in the underlying modeling approaches. These findings demonstrate that both the system and subchannel thermal-hydraulic codes, as well as the two-step Monte Carlo methodology, are capable of capturing the dominant physical phenomena governing RIA transients in the OPAL reactor. Overall, the study highlights the potential for further enhancement of transient analyses through advanced coupling strategies that combine three-dimensional space–time neutronic models with fully three-dimensional subchannel thermal-hydraulic codes.