Equivalent RIA Pulse Threshold for comparative assessment of solid fuel–cladding systems

Reactivity-initiated accidents impose rapid thermo-mechanical loading on fuel–cladding systems and can compromise fuel-rod integrity through fuel heating, gap closure, pellet–cladding mechanical interaction (PCMI), and cladding stress buildup. This study presents a reduced-order framework for comparative screening of conventional and accident-tolerant fuel–cladding systems under an RIA-like power pulse. The model solves one-dimensional transient radial heat conduction in the solid fuel pellet using a lumped effective surface boundary condition, evaluates incremental thermal expansion and pulse-scaled swelling, tracks contact onset, estimates PCMI contact pressure, and calculates a normalized total hoop-stress indicator. An Equivalent RIA Pulse Threshold (ERPT) is defined from prescribed fuel-temperature, contact-pressure, and stress-utilization criteria, while gap closure is treated separately as a contact-onset indicator. The framework is applied to UO 2 , UN, U 3 Si 2 , U-Th MOX, U-Mo, and UC fuels with Zircaloy, SS316, FeCrAl, and SiC claddings. Under the baseline assumptions, finite ERPT values are 13.5 for U-Mo, 16.0 for UO 2 , 17.5 for U-Th MOX, and 20.0 for U 3 Si 2 , whereas UN and UC remain above the investigated range. All finite baseline ERPT values are temperature-controlled. Sensitivity analysis shows that fuel thermal conductivity and the prescribed temperature limit most strongly affect ERPT, while swelling, gap size, differential rod pressure, and cladding expansion mainly influence contact onset and PCMI severity. A tenfold increase in effective contact stiffness shifts UO 2 /Zircaloy and U 3 Si 2 /Zircaloy to pressure-controlled behavior. ERPT is intended as a transparent reduced-order screening metric, not an experimentally validated failure threshold.