High-burnup spent nuclear fuel (HBU SNF) rods are subjected to complex mechanical loading during transportation, where irradiation-induced material degradation and interfacial interactions between fuel pellets and cladding play a critical role in their structural response. To ensure transportation safety, it is essential to evaluate the mechanical behavior of SNF rods under representative loading conditions. Detailed finite element models that explicitly account for pellet–pellet interface (PPI) and pellet–cladding interface (PCI) bonding and their breakage can capture these effects with high physical fidelity; however, their computational cost and unknown interfacial failure characteristics limit their application to assembly-level or system-level analyses. Consequently, simplified models are often employed, in which interfacial effects are idealized—commonly assuming fully debonded conditions—without quantitative assessment of the implications of neglecting interfacial bonding. In this study, the structural responses of HBU SNF rods are investigated by comparing an interfacial bonding model with a fully debonded model under quasi-static bending and dynamic impact loading conditions. The mechanical behaviors of PPI and PCI are represented using traction–separation laws, and unknown interfacial properties are identified through an optimization procedure by matching experimentally observed bending responses. Using the identified interfacial parameters, dominant interfacial damage mechanisms and the damage initiation sequences are investigated. Subsequently, the responses of the detailed interfacial bonding model are compared with those of the fully debonded simplified model. Under quasi-static bending conditions, the comparison focuses on global moment-curvature behavior, while under dynamic impact conditions, both displacement responses and energy-related quantities, including energy distribution and absorption, are evaluated. This framework allows direct assessment of how interfacial bonding influences global deformation and energy response under identical loading conditions. The results provide quantitative insight into the role of interfacial bonding in HBU SNF rod behavior and clarify the applicability and limitations of debonded simplified models for transportation safety evaluations.