Core Deformation Reactivity Analysis Using a Neutronics–Thermal-Hydraulics–Structural Mechanics Coupled Method for FFTF LOFWOS TEST #13

Accurate evaluation of reactivity feedback caused by core deformation—including core bowing and radial expansion—during power increases in sodium-cooled fast reactors requires comprehensive modeling of the interactions among neutronics, thermal-hydraulics, and structural mechanics in the core. Conventional evaluation methods for core design studies often lack accuracy due to oversimplified modeling. To address this, Japan Atomic Energy Agency has developed a high-fidelity evaluation method that couples multiple analysis codes to simulate these complex phenomena. In this study, the evaluation method was applied to analysis of the Fast Flux Test Facility (FFTF) loss of flow without scram (LOFWOS) TEST #13 using the experimentally measured core flow rate and inlet temperature as boundary conditions. The coupled analysis successfully reproduced the non-monotonic temporal variation of net reactivity observed experimentally. Sensitivity analyses of core design parameters related to core structural mechanics revealed that the S-shaped bowing of fuel assemblies, driven by specific contact conditions, is the primary mechanism of negative feedback. Notably, initial assembly pad gaps and pad compressive stiffness significantly influence the feedback magnitude and timing, highlighting the critical impact of core structural design. These results demonstrate that the developed coupled analysis method is a valuable tool for the design and licensing of fast reactors, supporting both safety and economic competitiveness.