Modeling of Multi-Helical Metallic Fuel and Dual-Oxidation Impact During Severe Accident Progression Using MELCOR

There is potential in implementing advanced metallic fuel technology, namely, Lightbridge, to support power uprates in small modular reactors. The fuel consists of U-50 wt%Zr with a multi-helical geometry and nonuniform Zircaloy cladding thickness. This presents significant modeling challenges in safety analyses, specifically here, using the MELCOR code. To address these limitations, a modeling approach is introduced consisting of (1) the property-preserving area-equivalent cylindrical” transformation approach and (2) a dedicated framework to represent simultaneous fuel-cladding oxidation (dual oxidation), which may occur when steam pathways develop through the cladding. The results presented in this study represent an analytical demonstration rather than a definitive prediction. Earlier fuel failure is observed primarily because of the lower failure temperature (equivalent to the melting point). Meanwhile, dual oxidation has a more pronounced impact on overall core degradation and hydrogen generation. Increased oxidized surface area accelerates degradation and can nearly double hydrogen production, with stronger effects at higher power levels. The analysis reveals sensitivity to modeling assumptions, particularly geometry representation, failure temperature, oxidation kinetics, and dual-oxidation initiation. Variations in these parameters were shown to influence the predicted timing of fuel failure, the rate of core degradation, and the magnitude of hydrogen generation. Overall, this study provides a practical framework for modeling advanced metallic fuel using the MELCOR code, identifies key phenomena requiring further investigation, and emphasizes the need for coordinated experimental-computational research to address key knowledge gaps for a more informed safety analysis.