Structural analysis of helically coiled once-through steam generator tubes in small modular reactors under density wave oscillation—part I: Numerical formulation

초록

This study establishes a numerical framework to evaluate the structural integrity of helically coiled once-through steam generator (H-OTSG) tubes in small modular reactors (SMRs) under density wave oscillation (DWO). The target tube features complex geometry, comprising a central helical section and arbitrarily curved transient regions. To assess structural response, spatially non-uniform thermal-hydraulic loads were derived from steady-state analysis and mapped onto the structural model using a one-way fluid-structure interaction (FSI) approach. A nonlinear finite element analysis (FEA) was developed using curved Timoshenko beam theory based on the Frenet-Serret frame. The results indicate that the tube's fundamental natural frequency provides a sufficient margin against DWO frequencies, minimizing resonance risk. The analysis identified the unsupported transient regions as the most vulnerable to displacement and stress. Furthermore, sensitivity analysis revealed that while the pressure differential governs the overall deformation magnitude, the wall temperature oscillation is the primary driver of periodic expansion and contraction. This methodology supports the rapid assessment of fatigue and wear potential during the conceptual design phase. © 2026 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license. http://creativecommons.org/licenses/by-nc-nd/4.0/

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