Thermal performance evaluation of dry storage cask using hybrid 3D and 2D models under normal and fire exposure conditions

Interim storage of Used Nuclear Fuel (UNF) is increasingly important due to saturated fuel pools at Nuclear Power Plants (NPPs). Safe UNF storage requires a detailed assessment of dry cask systems, focusing on thermal, mechanical, and corrosion aspects, which are yet to be fully studied for Korean systems. This study examines the thermal performance of Korean dry cask storage, called KORAD-21 under three cases: normal, direct fire, and indirect fire exposure. Additionally, it considers beyond-design-basis conditions and the impact of radiative heat transfer in the thermal model. The first case represents normal operation while the second and third cases involve fire exposure, modeled as direct and indirect fire. Direct fire involves contact with the cask surface, with 700 °C, 800 °C, 900 °C, and 1000 °C fire for 5–50 min. All cask components stayed below safety limits except at 1000 °C for 50 min, indicating a beyond-design-basis accident. Nonetheless, these results align with U.S. Nuclear Regulatory Commission (NRC) guidelines. Similarly, for indirect fire using diesel and kerosene at radii of 0.5 m, 1 m, and 1.5 m, peak temperatures remained within safety limits. Including radiative heat transfer in the thermal model is crucial for consistent and accurate results.