On a physics-based model of grain-boundary bubbles overpressurisation and its effects on fuel fragmentation

Overpressurisation of gas-filled bubbles and pores is considered the main driver of oxide fuel fragmentation and fission gas release into the rod free volume of irradiated fuel during transients. In this framework, advanced modelling of fission gas behaviour is crucial to enhance the predictive capabilities of fuel performance codes. This study develops a physics-based model for fission gas release from grain boundaries in UO₂ fuel and implements it into SCIANTIX, an open-source code developed at Politecnico di Milano to simulate fission gas behaviour in nuclear fuels. The model first describes gas release through continuous bubble networks at the grain face, exploiting data from irradiated fuel. It then focuses on gas release from damaged grain boundaries, applying fracture mechanics to predict micro-cracking induced by bubble overpressurisation. Finite elements simulations are performed using ABAQUS software, in order to assess stress intensification as a function of bubble density, shape, and size. The model is assessed against three separate-effect experiment datasets, including annealing tests and grain-face observations via scanning electron microscopy, demonstrating promising predictive capabilities for gaseous swelling and fission gas release. This work provides a valuable tool for improving fission gas behaviour modelling through a physics-based approach and lays the groundwork for future extensions to the high burn-up structure, offering a framework for a more comprehensive description of fuel fragmentation.