Characterization of Mechanical Energy Dissipation in Nuclear Fuel Rods: An Experimental and Numerical Investigation of Pellet-Cladding Interaction under Gap Evolution 

Abstract : 

Fuel rods present in the core of pressurized water reactors (PWR) contain uranium dioxide pellets initially separated from the zircaloy cladding by a given gap. The expansion of pellets due to thermo-mechanical effects leads to the phenomenon of pellet-cladding interaction (PCI), which can deform the cladding and compromise its structural integrity. Due to the extreme conditions in the core and limited space for instrumentation, the detection of the beginning of the interactions poses a significant industrial challenge. This thesis proposes a new framework for investigating PCI onset based on the nonlinear dynamics of mechanical contacts. The central hypothesis of the thesis posits that the evolution of the gaps between the pellets and the inner walls of the cladding modifies the system’s dynamic signature, with the dissipation of mechanical energy, under the vibrations to which real fuel rods are subjected, serving as an indicator for early identification of contact. In this context, a combined approach was implemented, involving the development of numerical vibro-impact models and dedicated experimental setup for validation. The methodology evolved from simplified two-degree-of-freedom (2DOF) models, incorporating Coulomb friction and contact mechanics via the Coefficient of Restitution and Hertzian methodologies, to a global structural model of the cladding using the Finite Element Method (FEM) to represent a system with multi-body impacts (40 pellets). The results of the experimental tests on a prototype manufactured via additive manufacturing revealed a 37% increase in the damping ratio between the closed and open gap configurations. Numerical results were consistent with the experimental trends, validating the developed model in terms of dynamic behavior and dissipation trends as the gaps evolve. Finally, an extrapolation with the final FEM model showed differences up to 80.88% of total dissipated mechanical energy between the simulated closed and open gaps. These results support the hypothesis that the dissipation of mechanical energy constitutes a dynamic signature that enables the detection of PCI onset. By bridging simplified experiments and vibro-impact simulations extrapolated to nuclear parameters, this work establishes a promising basis for future PCI detection strategies, contributing to increased operational safety and structural reliability in nuclear systems.

Jury

Frédéric LEBON - Professeur - Aix Marseille Université / LMA - Directeur de thèse

Guillaume RICCIARDI - Directeur de Recherche - CEA - Cadarache - Co-directeur de thèse

Stéphane BOURGEOIS - Maître de conférences - Centrale Méditerranée - Co-encadrant de thèse

David DUREISSEIX - Professeur - INSA Lyon - Rapporteur

Delphine BRANCHERIE - Professeur - Université de Technologie de Compiègne - Rapporteure

Lucie ROULEAU - Maître de conférences - Conservatoire National des Arts et Métiers - Examinatrice

Éric JACQUELIN - Professeur - Université Claude Bernard - Président

La soutenance de thèse de Matheus Versao est prévue le 26 octobre à 13h30 - amphithéâtre du LMA