Advancing Multi-Dimensional Simulation of Nuclear Fuel Behavior: from LOCA Analysis to Fast Reactor Applications

Current research on nuclear fuel focuses on two primary objectives. The first is to optimize the fuel used in the current generation of nuclear reactors in terms of safety, efficiency, and economics. This has driven efforts to increase discharge burnup and develop new fuel designs that offer enhanced performance during accidental scenarios. The second objective is to refine the design of next-generation nuclear reactors, where fuel will be exposed to different irradiation conditions, leading to the emergence of new phenomena. For both purposes, fuel performance codes play a critical role in assisting the industrialization of innovative fuel types by supporting licensing processes, ensuring safety margin compliance, and aiding in the design of new experiments.

The overarching aim of this thesis is to provide the nuclear research community with an open-source tool capable of simulating fuel behavior with high fidelity, addressing the critical requirements of contemporary nuclear fuel research. From this premise, two specific objectives emerge. The first is the extension of OFFBEAT, an OpenFOAM-based fuel performance code developed at EPFL and PSI, to Loss of Coolant Accidental conditions and to the prediction of fuel behavior in fast reactor applications. The second objective is to develop a novel methodology for solving porosity migration in MOX fuels within a multi-dimensional framework, stepping beyond the 1D approximations used in state-of-the-art fuel performance codes.

The thesis presents the implementation in OFFBEAT of the necessary features to simulate LOCA transients. The implementation of a finite-strain mechanical framework and high-temperature models to predict cladding deformation and burst during ballooning transients are detailed. Validation against the PUZRY separate-effects experiments and the Halden IFA-650.2 LOCA test is also shown.

In addition, the manuscript details the extension of OFFBEAT to the simulation of MOX fuels for fast reactor applications, describing the material correlation for MOX and 15/15-Ti stainless steel implemented. The introduction of a new burnup model, a sodium coolant channel model and a modular framework for mass transport phenomena are also detailed. Validation against six MOX-fueled SFR rods is shown to demonstrate the extended capabilities of the code.

Finally, the thesis describes a novel dynamic-mesh methodology for the simulations of porosity migration, addressing the formation of central void in multi-dimensional scenarios and tackling some of the limitations of the standard 1D approach. The document describes the lower-scale mechanistic model used to estimate the effect of pores' motion and coalescence on the opening of the central void. Tests in asymmetric heat transfer conditions are shown, demonstrating the solver's robustness in handling complex porosity transport scenarios.