FESTIM v2.0: Upgraded framework for multi-species hydrogen transport and enhanced performance

Abstract: FESTIM is an open-source finite element framework for modelling the transport of hydrogen isotopes in materials. It provides a flexible and extensible tool for simulating diffusion, trapping, surface interactions, and other processes that govern hydrogen behaviour. This paper presents FESTIM v2.0, a major release that broadens both the physical scope and the software infrastructure of the framework. On the physics side, the formulation adopts a modular structure that supports multi-species transport, advanced trapping and reaction schemes, isotope exchange, decay, and advection. Interface and boundary conditions have been generalised, and interoperability with external solvers enables multiphysics workflows, including coupling with fluid dynamics and neutron transport codes. On the software side, FESTIM v2.0 has been migrated to DOLFINx, the next-generation FEniCS platform, providing improved performance, interoperability, and long-term sustainability. Taken together, these advances position FESTIM v2.0 as a versatile platform for investigating hydrogen transport in materials across scientific and engineering applications.