Comparative Study of Helical Coil and Bayonet Tube Steam Generator Designs for Molten Salt Reactors

Abstract

High-temperature molten salt heat transfer systems are increasingly important for low-carbon energy technologies because they can couple dispatchable heat sources, thermal energy storage, and efficient power conversion. In molten salt reactors (MSRs), the steam generator (SG) is a key interface between the secondary molten salt loop and the power conversion system. This study presents a comparative thermal–hydraulic assessment of representative helical coil SG (HCSG) and bayonet tube SG (BTSG) concepts using heat exchanger analysis code developed at Korea Advanced Institute of Science and Technology (KAIST) (KAIST-HXD) version 3, which incorporates nitrate salt thermophysical properties, helical tube heat transfer and pressure drop correlations, and a three-stream nodal formulation for the bayonet configuration. The base framework has experimental support, the HCSG module reproduced published International Reactor Innovative and Secure (IRIS) design point quantities with deviations below 2%, and the integrated BTSG model has not been directly validated under representative molten salt and high-pressure steam conditions. Within the sampled design spaces, the highest feasible volumetric power densities were 35.81 MW/m³ for the HCSG and 26.97 MW/m³ for the BTSG. In the representative comparison under matched requirements, the HCSG showed lower pressure drops on the hot and cold sides, whereas the BTSG showed a smaller predicted wall temperature change across the dryout transition and a higher shell side wall temperature near the feedwater inlet. These results indicate a trade-off between compact thermal performance and thermal buffering, while the wall temperature metrics are used only as thermal-hydraulic screening indicators. Sensitivity analyses of feedwater pressure, HCSG tube count, and BTSG pitch-to-diameter ratio (P/D) further clarified the effects of key design variables. The results provide a thermal-hydraulic basis for concept screening and subsequent experimental, material, and structural assessment.