A damping-based methodology to assess gap evolution and pellet–cladding interaction in fuel rods

Pressurized Water Reactor (PWR) cores contain fuel rods with enriched uranium pellets initially separated from the cladding by a gap. Pellet expansion during operation leads to contact, known as pellet–cladding interactions (PCIs). Extreme core conditions and the lack of instrumentation for online PCI detection limit understanding of the dynamics involved, hindering identification of these interactions. This work addresses PCI using a two-phase approach. The first phase involves designing an experimental setup to replicate a simplified fuel rod system with two degrees of freedom (2DOF), while the second phase concerns developing two numerical models to simulate the dynamics in an oscillatory system comprising a hollow cylindrical cladding and a pellet within it. The models incorporate Coulomb friction and contact mechanics via Newtonian velocity-jump and Hertz–Kuwabara–Kono approaches, enabling analysis of gap-size effects on system dynamics. Results indicate that changes in effective damping may reliably signal PCI onset. Experimental tests showed a 37% increase in damping ratio between nearly closed and widest-gap configurations. The developed models agreed well with experiments and, extrapolated to real fuel rods, predicted an 84% damping increase, confirming its potential as an online PCI indicator. These complementary methods improve understanding of PCI onset and support future PCI-detection strategies. • Damping-based approach to identify onset of pellet–cladding interactions (PCI). • Experimental 2DOF pellet–cladding system with controlled gap evolution. • 2DOF vibro-impact model including Coulomb friction and contact mechanics. • Gap closure linked to measurable increases in effective damping. • Extrapolation to real fuel rods predicts strong damping sensitivity to PCI.